Note that the above Fick’s Law equation showed a differential concentration change over a differential distance. See the Figure below. To obtain the total molar flux over a given distance, integration of the equation needs to be carried out. This will be shown later.

Figure: Fick’s Law of Molecular Diffusion Similarly, for component-B:

where DBA is the diffusivity of B in a mixture of A and B. NOTE: A more recent development in molecular mass transfer in the Maxwell-Stefan approach which states that in order to achieve separation, there must exist a relative motion between the molecular species. Diffusion Through Moving Bulk Fluid Consider a bulk fluid of binary mixture A and B moving in the z-direction as shown, with an average bulk fluid velocity V m/s, as shown in Figure

Figure: Diffusion within a Moving Bulk Fluid

Z2, CA2, pA1

Concentration of A at any point in the mixture is CA kg-mole/m3. The flux of A is now given as:

Movement of A is now due to 2 contributions:

• Molecular diffusion

• Bulk movement of fluid

[This is similar to the mass flux in fluid flow: rV where r is the fluid density, e.g. mass flux = (kg/m3) x (m/s) = kg/(m2.s) ] This is the general equation describing mass transfer of component-A by diffusion through moving bulk fluid. It allows one to calculate the mass transfer rate (molar flux, e.g. in kg-mole/m2.s) between 2 points. Similarly, for component B:

Both equations are valid for diffusion in a gas, liquid or solid. Two special cases of molecular diffusion are covered in the next 2 sections. These are: (a) Equimolar counter-diffusion, and (b) Uni-molecular diffusion, or more commonly known as “diffusion of one component in another stagnant or non-diffusing component”.

Equimolar Counter-Diffusion In equimolar counter-diffusion, the molar fluxes or A and B are equal, but opposite in direction, and the total pressure is constant throughout. Hence we can write:

NA = - NB

Thus: JA = – JB [ Equimolar Counter-Diffusion ]

[Remember: pressure is caused by the collisions of molecules with the container wall. If the pressure is constant at any point in the container, then it must be implied that the number of molecules acting on the wall at any point is also constant. In other words, if certain amount of A has diffuse away, then they are replaced by the same amount of B] Under equimolar counter-diffusion, the diffusivity of A in B is the same as the diffusivity of B in A, i.e.

DAB = DBA

Fick’s Law for Steady-State Equimolar Counter-Diffusion of Ideal Gas Mixture: consider 2-component gas mixture (A and B) Ideal Gas Law: PV = nRT where P is the total pressure, and n is the total moles of gas For component-A: PAv = nART where pA is the partial pressure of A and nA is the moles of A Concentration of A:

Differentiating with respect to distance z,

Replacing into Fick’s Law :

We have:

Under constant total pressure and temperature conditions, the above equation for can be integrated over a diffusional path from z2 to z1 to as follows:

Integrate from pA1 to PA2, where pA1 is the partial pressure of A at point 1 and pA2 is the partial pressure of A at point 2:

We have, for steady-state equimolar counter-diffusion of ideal gas mixture:

Similarly, the molar flux for component-B can be written as:

Diffusion of One Component in Another Stagnant or Non-Diffusing Component

This special case of diffusion can be illustrated using 2 examples as shown in Figure below. Case (a) shows the diffusion of benzene in air. Liquid benzene is stored in a long tube, with one end of the tube exposed to air. The air inside the tube is stagnant, and benzene is evaporating through this stagnant air. The diffusion path is from point 1 at the gas-liquid interface to point 2 at the edge of the tube, i.e. across the distance ( z2 - z1 ). Case (b) shows an ammonia-air mixture exposed to liquid water. The ammonia diffuses through the gas mixture from point 1 in the bulk gas phase to point 2 at the gas-liquid interface and eventually is being absorbed into water. Air is assumed to be non-soluble in water, hence it does not diffuse, i.e. it is the non-diffusing component. Again, the diffusion path is the distance ( z2 - z1 ).

Figure: Diffusion of One Component in Another Stagnant or Non-diffusing

Component In this case, NB = 0, and the equation simplifies to

Again, using the Ideal Gas equation, we have:

where pA is the partial pressure of A, and P is the total pressure

Differentiating with respect to distance z,

Integrating from point 1 to point 2: the partial pressure of A changes from pA1 to pA2 :

We have the equation for diffusion whereby only 1 component (say component A) is diffusing:

The other component (i.e. component B) is stagnant or non-diffusing. Note: In the above equation the driving force for diffusion in the gas phase is expressed in terms of partial pressure. Other measure of concentration in the gas phase includes mole fraction, volume %, etc. Molecular Diffusion in Liquids Since the molecules in a liquid are packed very closely together, the attraction forces between the molecules are much stronger than in the gaseous phase. Molecular diffusion in liquid is much more difficult than in gases. Examples:

Air-NH3 DAB = 0.198 x 10-4 m2/s

H2O-NH3 DAB = 1.64 x 10-9 m2/s

Air-CO2 DAB = 0.142 x 10-4 m2/s

H2O-CO2 DAB = 2.00 x 10-9 m2/s

Here, we can see that the diffusion coefficient for the gas phase is much larger than that in the liquid phase, indicating that diffusion in liquid is slower than in gases. We can derive similar equation for diffusion in the liquid phase, for example, using concentration (mole/volume, such as kg-mole/m3) as the driving force for diffusion. One such equation is:

GAS ABSORPTION & DIFFUSION Gas absorption essentially involved the transfer of materials from the gas phase to the liquid phase. It is “defined” as the operation in which a gas mixture is contacted with a liquid for the purpose of preferentially dissolving one or more components of the gas mixture and to provide a solution of them in the liquid. The gaseous component is said to be absorbed by the liquid. The transferred component is known as the solute. In the simplest case, gas absorption involves at least 3 components. As an illustration, consider an ammonia-air-water system. The gas contains ammonia-air mixture. Ammonia is the solute and it is very soluble in water while air is not. Hence, by means of contacting the gas mixture with water, ammonia will dissolves preferentially in water, and a solution of ammonia in water (ammonium hydroxide) is obtained. Gas absorption processes are widely used in the industry. It can be used for removing contaminants or impurities from a gas stream. One of the most common examples of gas absorption and stripping is the amine absorption and regeneration unit whereby toxic H2S gas from a fuel gas mixture is removed by liquid amine (DEA, MEA or glycol) as will be discussed later. Other example includes the absorption of CO2 using hot potassium carbonate, or using the solvent Selexol®. Yet in other applications, it can be used for gas dehydration when an insoluble gas is dried by contact with a dehydrating liquid. An example is in the drying of chlorine using 98 wt% sulfuric acid. In air pollution control, the various oxides of nitrogen can be removed by absorption with water, sulfuric acid, and organic solutions. Gaseous ammonia can be removed by absorption with water. Equilibrium Distribution (Solubility) Curve

The gas absorption process involves the re-distribution of solute between the gas phase and the liquid phase when the 2 phases come into close contact and achieves equilibrium condition. The relationship between solute concentration in the gas phase and in the liquid phase at constant temperature and pressure is known as the equilibrium distribution curve, as shown in Figure. The presence of the solute in the liquid represents the gas solubility at the prevailing temperature and pressure.

Figure: Equilibrium Distribution Curve for a Solute

between gas and liquid phase

NOTE: The equilibrium solubility curve for gas absorption as seen here is not the same as the equilibrium curve for distillation. Some of the important differences to observe:

• the equilibrium solubility curve is plotted for a particular constant temperature. Any point on the same curve represent gas solubility at the same temperature. For the equilibrium curve in distillation, the points represent vapour-liquid equilibrium at different temperatures.

• there is no 45o diagonal for solubility curves, i.e. the solubility curve

can lie anywhere in the x-y plot (or p-y plot, etc). On the other hand, the equilibrium curve for distillation normally lies above the 45o diagonal line (except, of course, for azeotropic mixtures).

• the equilibrium solubility curve is not usually plotted over the entire

concentration range from 0.0 to 1.0 mole fractions (as in customary done for design of continuous distillation column using McCabe-Thiele Method). This is because generally most gases are soluble in a liquid only over a narrow concentration range.

Before we looked into the design of gas absorption equipment (i.e. tray or packed towers where the gas and liquid streams come into contact), we need to understand how this mass transfer actually take place between the phases. To do so, we turn to analysis using film concept. Film Concept in Mass Transfer As previously noted, gas absorption operation involves mass transfer from the gas phase to the liquid phase. That means the gas molecules must diffuse from the main body of the gas phase to the gas-liquid interface, then cross this interface into the liquid side, and finally diffuses from the interface into the main body of the liquid. Figure below showed a typical gas-liquid interface. This interface can represent any location in the gas absorption equipment where the gas contacts the liquid.

Figure: Gas-Liquid Interface in a Column

In the gas phase, 3 flow regimes can be visualized :

• Fully developed turbulent region where most of the mass transfer takes place by eddy diffusion

• A transition zone with some turbulence • A laminar film with molecular diffusion

Such phenomena are difficult to analyze. Instead, we will use a simplified TWO-FILM THEORY as a basis for analysis as well as development of various correlations of mass transfer phenomena.

Note: The Two-Film Theory mirrors closely the heat transfer process where the tube wall separating the hot fluid and the cold fluid is equivalent to the mass transfer interface. There are thin films on both sides of the interface. Heat (mass) is transferred from the hot fluid (gas-phase) to the gas film, then across the tube wall (interface), into the liquid film, and finally into the cold fluid (liquid-phase).

Two-Film Theory of Mass Transfer Once again consider the interface between the gas phase and the liquid phase, now simplified as shown in Figure below.

Figure. Two-Film Theory of Gas-Liquid Interface

This interface can represent any point in the gas absorption equipment where the gas contacts the liquid. See Figure below that shows a countercurrent gas absorption column. We will study the diffusion of solute A from the gas phase into the liquid phase, for example NH3 that is diffusing from an gaseous air-NH3 mixture into liquid phase water.

Figure: Application of Two-Film Theory to any point in Column

Assumptions of two-film theory:

• Steady-state: concentrations at any position in the tower do not change with time.

• Interface between the gas phase and the liquid phase is a sharp boundary.

• A laminar liquid film exist at the interface between the gas and liquid Equilibrium exists at the interface, thus there is negligible resistance to

• mass transfer across the interface: (xi, yi) is the equilibrium concentration.

• No chemical reaction: rate of diffusion across the gas-phase film must equal the rate of diffusion across the liquid-phase film.

Two-Film Theory and Equilibrium Solubility Curve

In the analysis of gas absorption, we are interested in the transfer of materials throughout the entire gas absorption equipment, not just a single location in the equipment. Therefore the two-film theory can be analyzed more effectively by using the equilibrium solubility curve. The concentrations at the interface in the gas ( yAi ) and in the liquid ( xAi ) is represented as a point M on the equilibrium curve. Point M thus has the coordinates (yAi, xAi ). As we move along the column along the continuous interface, we can trace out an equilibrium curve. Very often, the subscript “Ai” is dropped, and the equilibrium curve is simply a relationship between y and x; i.e. y = f(x). The concentrations in the bulk gas phase ( yAG ) and in the bulk liquid phase ( xAL ) is represented as a point P above the equilibrium curve. Point P thus has the coordinates ( xAL, yAG ). Notation: yAG = composition of A in the bulk gas phase (mole fraction) xAL = composition of A in bulk liquid phase (mole fraction) (xAi, yAi ) = equilibrium interface compositions (mole fraction)

Figure: Representation of Two-Film Theory on Equilibrium Diagram

Analysis of Mass Transfer Process using Two-Film Theory In the gas-phase, the concentration falls from yAG in the bulk gas to yAi at the interface. Thus, there is a concentration driving force for mass transfer from the bulk gas to the gas film to the interface. At the interface, the component A crossed the interface and enters the liquid side. In the liquid-phase, the concentration falls from xAi at the interface to xAL in the bulk liquid. Thus, there is a concentration driving force for mass transfer from the interface to the liquid film to the bulk liquid. NOTE : The bulk concentrations yAG, xAL are not equilibrium values, otherwise there would be no diffusion of A.

MASS TRANSFER EQUATIONS & COEFFICIENTS In commercial absorption equipment, both the liquid and the gas are usually in turbulent flow and the film thickness is not easy to determine. Therefore instead of analysis of mass transfer using Fick’s Law, it is more convenient to write the molar flux of A using mass transfer equations as follow:

NA = ky ( yAG - yAi ) = kx( xAi - xAL ) Gas Phase liquid Phase

where : NA is the molar flux of component A, mole/(area.time) ky and kx are the mass transfer coefficients in the gas phase and in

the liquid phase respectively (yAG - yAi ) and ( xAi - xAL ) are the driving force in the gas phase and liquid phase respectively Mass transfer coefficients are usually determined experimentally, or by correlations. Because there are many analogies between heat transfer and mass transfer, many correlations originally derived from heat transfer are used for the prediction of mass transfer coefficients. Analysis of Mass Transfer Equations From the 2 mass transfer equations, by rearranging and eliminating NA:

Referring back to Figure above, we see that the ratio of mass transfer coefficients is actually equal to the slope of line PM. This relationship is useful if one does not know the interface equilibrium concentrations. We can use the above equation to determine the equilibrium concentration at the interface (xAi, yAi), i.e. to locate point M, provided that kx and ky are known (or can be calculated using appropriate correlations). We do so by plotting a straight line originating from point P (xAL, yAG) with slope – kx/ky. The point of intersection of this line with the equilibrium curve gives point M which yield the values of xAi and yAi . That way we can calculate the flux NA at that particular point.

Table: Corresponding dimensionless groups of mass and heat transfer

Table. Mass Transfer Correlations for Simple Situations

Overall Mass Transfer Coefficients The previous definitions for molar flux NA require the knowledge of the interface concentrations. Since experimental sampling of the concentrations at the interface is very difficult or virtually impossible; it is more useful to define the mass transfer equation using overall mass transfer coefficients KX and KY :

xA* is the concentration (mole fraction) in liquid phase that is in equilibrium with yAG.

yA* is the concentration (mole fraction) in vapor phase that is in equilibrium with xAL.

Driving force for mass transfer: ( yAG − yA* ) in the gas phase (as indicated by line

PC) and ( xA* − xAL ) in the liquid phase (line PD). See Figure below.

Figure. Relations between Overall and Film driving forces

Relationships between kx, ky, KX, and KY :

where m” is the slope of line segment DM, and m’ is the slope of line segment MC as shown in Figure 58. If the equilibrium line is straight, then m’ = m”. kx, ky, KX, and KY all change with positions in the tower.

Figure. Relations between Overall and Film driving forces

NOTE: units for kx, ky, KX, and KY varies with the way the mass transfer equation is written (vapor phase or liquid phase) and the driving forces used, e.g. mole fractions ( y or x ), mole ratios ( X or Y ), weight fraction, partial pressures (p), or concentrations (c) etc.

Mass Transfer Resistance and Solubility

Analysis of the above 2 equations reveal a great deal about the nature of the equilibrium between the gas and the liquid, i.e. the solubility curve.

• 1 / kx represents the resistance to mass transfer in the liquid phase • 1 / ky represents the resistance to mass transfer in the gas phase

• If m’ is small (i.e. the equilibrium curve is very flat), the term m’/kx is not significant, therefore:

and the major resistance to diffusional mass transfer lies in the gas phase and the mass transfer is said to be gas-phase controlled. In this case, solute A can be interpreted as being very soluble in the liquid: at equilibrium, a small concentration of A in the gas will bring about a very large concentration in the liquid.

• If m” is large (i.e. the equilibrium curve is very steep), then the term 1/m”ky is insignificant, therefore,

and the majority of resistance to mass transfer resides in within the liquid. The mass transfer is said to be liquid-phase controlled. Solute A is relatively insoluble in the liquid: a very large concentration of A in the gas phase is required to provide even a small change of concentration in the liquid. REMINDER : The two-film theory and equilibrium curve can be expressed in

other ways, e.g. in terms of partial pressure (for the gas phase) and concentration (for the liquid phase); but the analysis for them is the same as outlined before for mole fractions ( x and y).

Refer to the Figure below which showed the two-film theory and equilibrium curve expressed in partial pressures and concentrations.

Figure: Two-Film Theory of Gas-Liquid Interface

Figure: Relations between Overall and Film driving forces

Examples of Gas Solubility Analysis

Figure below shows the solubilities of some gas-air mixtures in water (partial pressure of solute vs. liquid mole fraction of solute).

Figure: Solubilities of Gases in Water

From the solubility plot, note that: • at the same temperature (10oC) HCl is more soluble than NH3, which in

turn is more soluble than SO2. • solubility of any gas is dependent on temperature. In most cases (but not

always), the solubility of a gas decreases with increasing temperature. • NH3 is less soluble at 30

oC than at 10oC. • The solubility generally increases with partial pressure of the gas. • The relatively insoluble gas is high in concentration in the gas phase, i.e.

high partial pressure at equilibrium. Conversely, very soluble gas has low partial pressure. In many practical cases, only one component in the gas mixture is relatively soluble in the liquid, e.g. in the NH3-Air-H2O system above, since NH3 is relatively more soluble than air in water, thus, pAir >> pNH3.

[ Equilibrium solubility curves for other liquids are also available ]

HENRY’S LAW FOR NON-IDEAL SOLUTION

When the gas mixture in equilibrium with an ideal liquid solution follows the ideal gas behavior, we have – as seen previously - the Raoult’s Law:

When the solution is non-ideal, Raoult’s Law cannot be applied. For non-ideal solution, we must use Henry’s Law which states that:

where H is the Henry’s Law Constant. Since

we have

( PT = total system pressure ) Thus, an alternative form of Henry’s Law is:

Where y = mole fraction of solute in gas phase x = mole fraction of solute in liquid phase m = Henry’s Law Constant = H / PT Very often, the subscript ‘A’ is dropped. Henry’s Law predicts a linear equilibrium relationship. Still, most equilibrium relationships are actually non-linear. Henry’s Law is only applicable over a modest liquid concentration range, especially when the solution is dilute. NOTE: Several other variations of Henry’s law are available, depending on applications, e.g. membrane separation. Care must be exercised in the correct unit for the constant. For example, H has the unit of pressure/mole fraction, while m is dimensionless. m is independent of total pressure, whereas H does not.

Table below showed the values of Henry’s Law constant as a function of temperature for several gases.

Experiment:

Guidelines for Gas Absorption Separation experiment

Students will be assessed based on their in-class performance and type written report. The report is to be submitted to Prof. ______________ two weeks after the practical session.

Practical In-Class Assessment:

(a) Understanding of Objective(s)

(b) Understanding of Experimental Procedures

(c) Understanding of Theory

(d) Safety Practice (includes punctuality and attire)

(e) Data Collection and Preliminary Analysis (sample calculation)

Report Assessment:

(a) Tables and Graphs

(b) Sample Calculations

(c) Discussion of Results

Data Collection Sheet:

(a) Raw data should be well-tabulated.

(b) Any calculation must be neatly illustrated or tabulated for practical class assessment.

Report Assessment:

All reports must be type-written.

(a) Results (Tables, Graphs):

Raw data and calculated results must be neatly tabulated with consistent SI units. The tables must be numbered and titled. For example:

Table I. Back Titration Result

Graphs must be numbered and titled. For example:

Figure 1. Plot of Concentration of NaOH against Residence Time

The axes of the graph must be properly labelled and show quantities in SI units.

(b) Sample Calculations:

One sample calculation of each important variable should be shown. Results of calculated values should be tabulated.

(c) Analysis of Results:

Discuss all the questions given in Analysis of Results section. Quote all references used.

Safety Practice:

Attire:

All students must wear lab coat and goggles in Unit Operation Lab.

- Female students: Blouse/T-shirt and long slacks/jeans. Lab coat should be buttoned up. Well covered-up shoes must be worn. Long hair must be tied up. No cap.

- Male students: T-Shirt and long pants/jeans. Lab coat should be buttoned up. Well covered-up shoes must be worn. Hair must be kept short.

Operating Instructions – Gas Absorption Pilot Plant

1. Turn on the exhaust fan blower.

2. Open the gas cylinder supply for CO2 and CH4. At the

pressure regulator, check that the pressure (from the

high pressure gauge) of each gas is at least 10 barg.

3. Open the valve of compressed air supply.

4. Check that the liquid (pre-prepared by the lab

technician) level in both feed and spare tank is at

least 60% full.

5. .Turn on the main power supply.

6. The system is ready for experimental run.

UNIVERSITI TEKNOLOGI PETRONAS

Bachelor of Engineering (Hons) CHEMICAL

Syllabus No : Subject : Separation Processes I Experiment : Gas Absorption Separation

1.0 INTRODUCTION

Gas absorption is a mass transfer operation in which a gas mixture is contacted with a liquid to preferentially absorb one or more of the components of the gas stream. This operation is found in many industries for the recovery of valuable products and cleaning of exhaust or vent streams. If necessary the solute can be recovered from the absorbing liquid by distillation and the liquid can be recycled or it can be discarded completely. In some cases, a solute is removed from a liquid by contacting it with a gas. This operation is the reverse of gas absorption and is called desorption or gas stripping. This study deals with the absorption of CO2 from air by contacting the gas mixture with water in a packed tower. This system has the advantage that significant mass transfer occurs over the entire length of the column. 2.0 OBJECTIVES

• To determine the loading and flooding points for the air-water system. • To study the relationship between the pressure and flowrate for a gas

absorption operation.

3.0 EQUIPMENT Four columns filled with different sizes of packing suitable for gas absorption operation. Columns Three (3) units Borosilicate glass columns for absorption and one (1) unit Borosilicate glass Column for stripping with packing materials Column Diameter: 100mm Packing Height: approx. 1.5m Column height: 2.97m Absorption - PACKING MATERIAL

• BERL SADDLES 10 x10 m ceramic • BERL SADDLES 15 x 15 mm ceramic • RASCHIG RINGS 10 x 10 mm glass

Stripping - PACKING MATERIAL

• RASCHIG RINGS 10 x 10 mm glass

Five sets of infra red gas analyzers, each equipped with infra red sensors to detect CO2 independently, are installed at each gas Absorption inlet and outlet point. Temperature transmitters (PT100), Pressure Transmitters (Pressure Transducer) and Mass Flow Controllers as well as Mass Flow Meter are installed and assembled to work in conjunction with the online data acquisition software, NI Labview to capture real time experimental data. 4.0 PROCEDURE

4.1 Line Tracing

4.1.1 Identify the experimental equipment listed in paragraph 2.0. 4.1.2. Perform line tracing on each Absorption operation. Students must trace

and demonstrate to the lecturer the actual feed gas entry-exit path as well as liquid entry-exit routes.

4.2 Operating Column 1 – CO2 Absorption in NaOH solution

4.2.1 Refer to Figure below. Ensure all valves are closed before system start up except for V6, V4, V14 and V16. (Note: V24, V25, V26 and V34 are always OPEN).

4.2.2 Starting the system. Switch on the main power supply. At the control

panel, a Green light will light up. Turn on the main power supply to the computer and activate the National Instrument (NI) data acquisition system. Ensure that the activation procedures for the NI software are closely followed according to the steps stipulated in Appendix C.

4.2.3 Manually Open V18. Check level gauge of the Make-up Tank and ensure

that water level in the tank is more than half. If the level is below 50%,

V1

Water

Inline Mixer

Feed Tank Makeup

Tank

Spent Sol

Tank

Spare

Tank

DrainDrain

Drain Drain

Air

CO2

CH4

Steam

Sample Sample Sample Sample

Gas Analyzer

Gas Analyzer

Feed Heater

Sample

AA AA

AA

BB

BB

CC

DD

CC

DD

EE

EE

V2

V3

Fair,in

FCO2,in

FCH4,in

Cfeed

Pump spent

Pump feed Pump makeup

Pump spare

Tgas-in, 1

Fgas-in, 1

Tliq-out, 1Fliq-out, 1

Tgas-out, 1Fgas-out, 1

Cgas-out, 1

Tliq-in, 1

Fliq-in, 1

CVliq-in, 1

DD

Tgas-in, 2

Fgas-in,2

Tliq-out, 2 Fliq-out, 2

Fgas-out, 2

Cgas-out, 2

Tliq-in, 2

CVliq-in, 2

DD

Fliq-in, 2

Cgas-out, 3

Tliq-in, 3

Fgas-out, 3

Fliq-in, 3

CVliq-in, 3

AA

Tgas-in, 3

Fgas-in,3

Tliq-out, 3 Fliq-out, 3

Tsteam-in, 4

Fsteam-in,4

Tliq-out, 4

Fliq-out, 4

Cgas-out, 4

Fgas-out, 4

Tliq-in, 4

CVliq-in, 4

Fliq-in, 4

Pump strip

Tgas-out, 3Tgas-out, 2

Tgas-out, 4

V4

V5

V6

V7

V8

V9 V10 V11 V12

V13

V14

V15

V16

V17

V18

V19

V20

V21

V22V23

V24V25

V26

V27

V28

V29

V30

V31

V32

V34

Steam Control Valve Set

Cooler

V33

open V10 and shut V9, then turn on “Pump-feed” to fill then “Tankmake up”. Once the level is above 50%, shut V10 and turn off the pump.

Preparing 0.0155M NaOH solution 4.2.4 Calculate the amount of NaOH solution required (in grams) to be

added into the half full 100L Tankmake up in order to achieve a concentration of 0.0115M. Pre-mix the NaOH pellets using a beaker with small amount of water (this will dissolve the pellet before transferring this solution to the Tankmake up). Test the conductivity of the solution in the tank (it should read as 2.50). Note that there is an increase in temperature in the pre-mixed solution (why?).

Running the CO2 absorption experiment 4.2.5 Fully open V11 and V14 to prime the “Pumpmake up”. Allow the solution in

the tank to be mixed thoroughly. Turn off Pumpmake up. 4.2.6 Shut V19 and turn on the Pumpmake up from the NI control panel. (Note:

A green light will light up at the control panel) Collect the water to a constant level (at the mark) at the bottom of Column 1, and then stop the pump.

4.2.7 Wait for 15 minutes before proceeding to next step. You can allow the

air to flow through the Column 1 at a certain flowrate. (Why?) 4.2.8 Calculate the mass flow controller (MFC) settings if the following

experiments are required. Expt CO2 Flowrate

(ml/min) CH4 Flowrate (ml/min)

Total Flowrate (ml/min)

Conc CO2 Feed

1 1000 0 1000 100 2 900 100 1000 90 3 800 200 1000 80 4 700 300 1000 70 5 600 400 1000 60 6 500 500 1000 50 7 400 600 1000 40 8 300 700 1000 30 9 200 800 1000 20 10 100 900 1000 10 11 0 1000 1000 0 4.2.9 Set the MFCs for experiment 1. Open the CO2 and CH4 gas supply.

Open V2 and V3. 4.2.10 Turn on Pumpmake up and regulate the flowrate of 0.015M NaOH solution

to a feed flowrate of 2.5L/min. Stand for 2 minute for the system to achieve steady state.

4.2.11 Data of each inlet and outlet of gas as well as liquid must be recorded

as follows;

Data Sheet 1

CO2 Gas Absorption - Column Raschig Ring Packing 8 x 8mm

CO2 Flowrate (ml/min)

CH4

Flowrate (ml/min)

Total Flowrate (ml/min)

Conc CO2 Feed

Conc CO2 Exit

Conductivity Liq Feed

Conc Liq Feed

Conductivity Liq Exit

Conc Liq Exit

1000 0 1000 100

900 100 1000 90

800 200 1000 80

700 300 1000 70

600 400 1000 60

500 500 1000 50

400 600 1000 40

300 700 1000 30

200 800 1000 20

100 900 1000 10

0 1000 1000 0

4.2.12 Repeat the Steps 4.2.9 and 4.2.11. Then, close V2 and V3 completely

to stop all gas flow to the system. 4.3 Repeat procedures stipulated in 4.2 for Column 2 and 3. 4.4 Shut down the system. Turn off the main switch at the control panel.

Switch off the main power supply. Turn off the main compressed air supply.

5. QUESTIONS FOR THE PRACTICAL 5.1 Have you observed CO2 absorption during the running of each column? 5.2 Which column has the highest CO2 absorption efficiency?

5.3 Explain the reason for doing the step 4.2.7. 5.4 What is the phenomenon of “channelling”? Explain it briefly. 5.5 Define Fick’s Law. 6. ANALYSIS OF RESULTS 6.1 Convert your experimental readings of CO2 percent and gas flowrate

and perform at least one mass balances for each stream. 6.2 Plot the CO2 concentration and NaOH concentration for each column.

6.4 Analyse the behaviour of the packed column with variation in CO2 gas

concentration.

7.0 PRE-PRACTICAL CLASS WORK:

Students are encouraged to read from the following recommended text in order derive the equations necessary to understand this practical class as well as to complete the in-class assessment: 1. C.Judson King Separation Processes, 2nd edition (McGraw- Hill Inc.)

2. Robert E. Treybal Mass-Transfer Operations, 3rd edition (McGraw- Hill

Inc.)

8.0 PRECAUTIONS:

Always be sure that V24, V25, V26 and V34 are fully open. List of References

1. C.Judson King Separation Processes, 2nd edition (McGraw- Hill Inc.)

2. Robert E. Treybal Mass-Transfer Operations, 3rd edition (McGraw- Hill

Inc.)

3. Warren L. McCabe, Julian C. Smith, Peter Harriott Unit Operation of

Chemical Engineering, 4th edition (McGraw- Hill Inc.)

4. J.M. Coulson, J.F. Richardson Chemical Engineering Volume Two, 3rd

edition (Pergamon )

5. Philip A. Schweitzer Handbook of Separation Technigues for Chemical

Engineer, 2nd edition

6. Kirk -Othemer Encyclopedia of Chemical Technology Vol.1, 4th edition

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(McGraw-Hill Inc.)

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Answers to Questions for Practical

GAS ABSORPTION SEPARATION

Fick’s Law of Diffusion Fick’s Law stated that for the diffusion of component-A in a binary mixture of A and B:

[Note: the above refers only to one-directional diffusion, in the z-direction] where JA = molecular diffusion flux (kg-mole/m2.s) DAB = diffusivity @ diffusion coefficient of A in a mixture of A and B (m2/s)

= concentration gradient (kg-mole.m-3/m) The concentration gradient dCA /dz is the driving force for diffusion. The (–) showed that CA decreases as z increases. Note that the above Fick’s Law equation showed a differential concentration change over a differential distance. See the Figure below. To obtain the total molar flux over a given distance, integration of the equation needs to be carried out. This will be shown later.

Figure: Fick’s Law of Molecular Diffusion Similarly, for component-B:

Z2, CA2, pA1

where DBA is the diffusivity of B in a mixture of A and B. NOTE: A more recent development in molecular mass transfer in the Maxwell-Stefan approach which states that in order to achieve separation, there must exist a relative motion between the molecular species. Diffusion Through Moving Bulk Fluid Consider a bulk fluid of binary mixture A and B moving in the z-direction as shown, with an average bulk fluid velocity V m/s, as shown in Figure

Figure: Diffusion within a Moving Bulk Fluid Concentration of A at any point in the mixture is CA kg-mole/m3. The flux of A is now given as:

Movement of A is now due to 2 contributions:

• Molecular diffusion

• Bulk movement of fluid

[This is similar to the mass flux in fluid flow: rV where r is the fluid density, e.g. mass flux = (kg/m3) x (m/s) = kg/(m2.s) ] This is the general equation describing mass transfer of component-A by diffusion through moving bulk fluid. It allows one to calculate the mass transfer rate (molar flux, e.g. in kg-mole/m2.s) between 2 points. Similarly, for component B:

Both equations are valid for diffusion in a gas, liquid or solid.

Two special cases of molecular diffusion are covered in the next 2 sections. These are: (a) Equimolar counter-diffusion, and (b) Uni-molecular diffusion, or more commonly known as “diffusion of one component in another stagnant or non-diffusing component”.

Equimolar Counter-Diffusion In equimolar counter-diffusion, the molar fluxes or A and B are equal, but opposite in direction, and the total pressure is constant throughout. Hence we can write:

NA = - NB

Thus: JA = – JB [ Equimolar Counter-Diffusion ]

[Remember: pressure is caused by the collisions of molecules with the container wall. If the pressure is constant at any point in the container, then it must be implied that the number of molecules acting on the wall at any point is also constant. In other words, if certain amount of A has diffuse away, then they are replaced by the same amount of B] Under equimolar counter-diffusion, the diffusivity of A in B is the same as the diffusivity of B in A, i.e.

DAB = DBA

Fick’s Law for Steady-State Equimolar Counter-Diffusion of Ideal Gas Mixture: consider 2-component gas mixture (A and B) Ideal Gas Law: PV = nRT where P is the total pressure, and n is the total moles of gas For component-A: PAv = nART where pA is the partial pressure of A and nA is the moles of A Concentration of A:

Differentiating with respect to distance z,

Replacing into Fick’s Law :

We have:

Under constant total pressure and temperature conditions, the above equation for can be integrated over a diffusional path from z2 to z1 to as follows:

Integrate from pA1 to PA2, where pA1 is the partial pressure of A at point 1 and pA2 is the partial pressure of A at point 2:

We have, for steady-state equimolar counter-diffusion of ideal gas mixture:

Similarly, the molar flux for component-B can be written as:

Appendix A

Introduction to Absorption Separation Processes - Technology and Business Opportunities by G. Srikanth

The development of ion exchange Absorption some forty years ago paved the way for the Absorption separation technology. Since then due to a whole lot of technological innovations, especially in the area of new materials, Absorption technologies have been established as very effective and commercially attractive options for separation and purification processes.

What is a Absorption?

The Absorption can be defined essentially as a barrier, which separates two phases and restricts transport of various chemicals in a selective manner. A Absorption can be homogenous or heterogeneous, symmetric or asymmetric in structure, solid or liquid, can carry a positive or negative charge or be neutral or bipolar. Transport through a Absorption can be effected by convection or by diffusion of individual molecules, induced by an electric field or concentration, pressure or temperature gradient. The Absorption thickness may vary from as small as 100 micron to several millimeters.

Absorption Separation Technology

A Absorption separation system separates an influent stream into two effluent streams known as the permeate and the concentrate. The permeate is the portion of the fluid that has passed through the semi-permeable Absorption. Whereas the concentrate stream contains the constituents that have been rejected by the Absorption.

Absorption separation is employed in numerous industrial applications with the following advantages:

• Appreciable energy savings • Environmentally benign • Clean technology with operational ease • Replaces the conventional processes like filtration, distillation, ion-

exchange and chemical treatment systems • Produces high, quality products • Greater flexibility in designing systems.

Types of Absorptions

The proper choice of a Absorption should be determined by the specific application objective:

• particulate or dissolved solids removal, • hardness reduction or ultra pure water production, • removal of specific gases/chemicals etc.

The end-use may also dictate selection of Absorptions for industries such as potable water, effluent treatment, desalination or water supply for electronics or pharmaceutical manufacturing. The following section explains the types of Absorptions commonly used.

Absorption Modules

The Absorptions can be cast as flat sheets, tubes and fine hollow fibres. For accommodating such shapes and structures, different types of Absorption modules are available. The last decade of Absorption and module development has lessened the effects of physical compaction and has brought forth spiral Absorption modules capable of operating at pressures in excess of 800 psig (55.2 bar). The techno-economic factors for the selection, design and operation of Absorption modules include cost of supporting materials and enclosure (pressure vessels), power consumption in pumping and ease of replaceability.

The following Absorption modules are largely used for industrial applications:

• Plate and frame module • Spiral wound module • Tubular Absorption module • Capillary Absorption module • Hollow fibre Absorption module.

Absorption Separation Processes

Various types of Absorption separation have been developed for specific industrial applications. Some of the widely used processes are discussed hereunder:

Reverse Osmosis (RO)

Unlike water filtration, that can only remove some suspended materials larger than 1 micron, the process of reverse osmosis (RO) will eliminate the dissolved solids, bacteria, viruses and other germs contained in the water. RO is essentially a pressure driven Absorption diffusion process for separating dissolved solutes. The RO is generally used for desalination seawater for its conversion into potable water. The salient features of the process are that it involves no phase change and it is relatively a low energy process.

Almost all RO Absorptions are made of polymers, cellulose acetate and aromatic polyamide types rated at 96%-99+% NaCl rejection. RO Absorptions are generally of two types, asymmetric or skinned Absorptions and thin film composite (TFC) Absorptions. The support material is commonly polysulfones while the thin film is made from various types of polyamines, polyureas etc.

RO Absorptions have the smallest pore structure, with pore diameter ranging from approximately 5-15 A0 (0.5 nm - 1.5 nm). The extremely small size of RO pores allows only the smallest organic molecules and unchanged solutes to pass through the semi-permeable Absorption along with the water. Greater than 95-99% of inorganic salts and charged organics will also be rejected by the Absorption due to charge repulsion established at the Absorption surface.

RO finds extensive applications in the following:

• Potable water from sea or brackish water • Ultrapure water for food processing and electronic industries

• Pharmaceutical grade water • Water for chemical, pulp & paper industry • Waste treatment etc.

Future Directions for RO Applications

• Municipal and industrial waste treatment • Process water for boilers • De-watering of feed streams • Processing high-temperature feed- streams etc.

In the last six to eight years the technology has gained industry acceptance as a viable water treatment option for many different fluid separation applications. Low operating costs and the ability to remove organic contaminants and 95-99% of inorganic salts with minimal chemical requirements make RO an attractive technology for many industrial applications.

A complete RO equipment comprising pre-treatment system, Absorption modules etc. is estimated to cost around US $ 450 per m3 of flow per day. The operating cost for RO system has been estimated as Rs.24 per m3 of flow, excluding the cost of labour and chemicals used.

Nanofiltration (NF)

Nanofiltration is a form of filtration that uses Absorptions to separate different fluids or ions. NF is typically referred to as "loose" RO due to its larger Absorption pore structure as compared to the Absorptions used in RO, and allows more salt passage through the Absorption. Because it can operate at much lower pressures, and passes some of the inorganic salts, NF is used in applications where high organic removal and moderate inorganic removals are desired. NF is capable of concentrating sugars, divalent salts, bacteria, proteins, particles, dyes and other constituents that have a molecular weight greater than 1000 daltons.

Absorptions used for NF are of cellulose acetate and aromatic polyamide type having characteristics as salt rejections from 95% for divalent salts to 40% for monovalent salts and an approximate 300 molecular weight cut-off (MWCO) for organics. An advantage of NF over RO is that NF can typically operate at higher recoveries, thereby conserving total water usage due to a lower concentrate stream flow rate. NF is not effective on small molecular weight organics, such as methanol.

Ultrafiltration (UF)

Ultrafiltration is most commonly used to separate a solution that has a mixture of some desirable components and some that are not desirable. UF is somewhat dependent on charge of the particle, and is much more concerned with the size of the particle. Typical rejected species include sugars, bio-molecules, polymers and colloidal particles. The driving force for transport across the Absorption is a pressure differential. UF processes operate at 2-10

bars though in some cases up to 25-30 bars has been used. UF processes perform feed clarification, concentration of rejected solutes and fractionation of solutes. UF is typically not effective at separating organic streams.

UF Absorptions are capable of retaining species in the range of 300-500,000 daltons of molecular weight, with pore sizes ranging from 10-1000 Angstroms (103-0.1 microns). These are mostly described by their nominal molecular weight cutoff (1000-100,000 MWCO), which means, the smallest molecular weight species for which the Absorptions have more than 90% rejection.

UF usually implies separation of macromolecules such as protein from low molecular weight solvents. Pores in the support layer of the Absorption are relatively larger than those of the surface layer. Material passing into fine pores can readily be transported through the open-celled, sponge-like structure of the support layer. For example, in electrodeposition paint recovery, the paint, composed of resin, a pigment and water are separated into two streams that can he reused. The first stream includes the water and a small amount of paint resin, which can he used to rinse the parts later in the process. The paint pigment is separated from that stream and can be re-used in the paint bath, allowing the bath to be concentrated to a useable level.

It is found that, whenever the solvent of a mixture flows through the Absorption, retained species are locally concentrated at the Absorption surface, thereby resisting the flow. In the case of processing solution, this localized concentration of solute normally results in precipitation of a solute gel over the Absorption. When processing a suspension, the solids collect as a porous layer over the Absorption surface.

In view of the above, it is clear that the permeate rate can be effectively controlled by the rate of transport through the polarization layer rather than by Absorption properties. Hence, UF throughput depends on physical properties of the Absorption, such as permeability, thickness, process and system variables like feed consumption, feed concentration, system pressure, velocity and temperature.

UF has a wide range of applications as shown below:

• Oil emulsion waste treatment • Treatment of whey in dairy industries • Concentration of biological macromolecules • Electrocoat paint recovery • Concentration of textile sizing • Concentration of heat sensitive proteins for food additives ' • Concentration of gelatin • Enzyme & pharmaceutical preparations • Pulp mill waste treatment • Production of ultra pure water for electronics industry • Macromolecular separations replacing the conventional change of

phase methods.

The important characteristics for Absorption materials are porosity, morphology, surface properties, mechanical strength and chemical resistance. Polymeric materials, viz., polysulfone, polypropylene, nylon 6, Polytetrafluoroethylene (PTFE), PVC, acrylic copolymer etc. have been used successfully as UF Absorptions. Inorganic materials such as ceramics, carbon based Absorptions, zirconia etc. have been commercialized by several vendors.

UF may find wide range of applications in the near future and some of those processes important from the separation and energy savings point of view are mentioned below:

• Ultraflitration of milk • Bioprocessing: Separation and concentration of biologically active

components • Protein harvesting, useful for grass proteins, algal / plankton proteins • In food areas based on the ability to change protein and starch/ sugar,

salt and water ratios • Refining of oils.

Microfiltration (MF)

This is by far the most widely used Absorption process with total sales greater than the combined sales of all other Absorption processes. Microfiltration has numerous small applications. It is essentially a sterile filtration with pores (0.1-10.0 microns) so small that micro-organisms cannot pass through them.

Microfiltration is a process of separating material of colloidal size and larger than true solutions. A MF Absorption is generally porous enough to pass molecules of true solutions, even if they are large. Microfilters can also he used to sterilize solutions, as they are prepared with pores smaller than 0.3 microns, the diameter of the smallest bacterium, pseudomonas diminuta.

While the mechanism for conventional depth filtration is mainly adsorption and entrapment, MF Absorptions uses sieving mechanism with distinct pore sizes for retaining larger size particles than the pore diameter. Hence, this technology offers Absorptions with absolute rating, which is highly desirable for critical operations such as sterile filtration of parental fluids, sterile filtration of air and preparation of particulate,free-water for the electronics industry.

The MF Absorptions are made from natural or synthetic polymers such as cellulose nitrate or acetate, polyvinylidene difluoride (PVDF), polyamides, polysulfone, polycarbonate, polypropylene, PTFE etc. The inorganic materials such as metal oxides (alumina), glass, zirconia coated carbon etc. are also used for manufacturing the MF Absorptions.

The properties of Absorption materials are directly reflected in their end applications. Some criteria for their selection are mechanical strength, temperature resistance, chemical compatibility, hydrophobility, hydrophilicity, permeability permselectivity and the cost of Absorption material as well as manufacturing process.

MF has a wide array of applications as mentioned below:

• Preparation of parenterals and sterile water for pharmaceutical industry

• Food & beverages (concentration of fruit juices and alcoholic beverages • Chemical industry • Microelectronics industry • Fermentation • Laboratory/analytical uses etc.

The following are the likely applications of MF in the near future:

• In biotechnology for concentration of biomass, separations of soluble products

• In diatomaceous earth displacement • In non-sewage waste treatment for removing intractable particles in

oily fluids, aqueous wastes which contain particulate toxics and stack gas

• In paints for separating solvents from pigments etc.

Electrodialysis (ED)

This is an electro-Absorption process in which the ions are transported through a Absorption from one solution to another under the influence of an electrical potential. ED can be utilised to perform several general types of separations such as separation and concentration of salts, acids and bases from aqueous solutions or the separation and concentration of monovalent ions from multiple charged components or the separation of ionic compounds from uncharged molecules. ED Absorptions are usually made of cross-linked polystyrene that has been sulfonated. Anion Absorptions can he of cross-linked polystyrene containing quaternary ammoniuri groups. Usually, ED Absorptions arx fabricated as flat sheets containing about 30-50% water. Absorptions are fabricated by applying the cation and anion-selective polymer to a fabric material.

The system consists of two kinds of Absorptions: cation and anion, which are placed in an electric field. The cation-selective Absorption permits only the cations, and anion-selective Absorption only the anions. The transport of ions across the Absorptions results in ion depiction in some cells, and ion concentration in alternate ones.

Electrodialysis is used widely for production of potable water from sea or brackish water, electroplating rinse recovery, desalting of cheese whey, production of ultrapure water etc.

The present ED industry has experienced a steady growth rate of about 15% since 15 years. To ensure further growth beyond desalination and salt production, new areas of application are being exploited in the areas of chemicals and pharmaceuticals, food, industrial & municipal effluent treatment etc. This can be achieved through slight modifications in the conventional processes as well as extensive R&D work in those areas.

The following are some new business opportunities in ED separation process:

• De-ionized water from conductive spacers • Radioactive wastewater treatment by using radiation resistant

Absorptions • Deacidification of fruit juices • Heavy metal recovery • Recovery of organic acids from salts • pH control without adding acid or base • Regeneration of ion-exchange resins with improved process design • Acid recovery from etching baths etc.

The capital cost for an electrodialysis (including pre-treatment system, Absorption modules, pumps and electrical etc.) is estimated around US $ 275 per m3 of flow per day.

Gas Separation

The Absorption gas separation technology is over ten years old and is proving to he one of the most significant unit operations. These processes compete with technology alternatives such as adsorption, cryogenic distillation etc. in niche application areas. The Absorption processes enjoy certain advantages, viz., compactness and light in weight, low labour intensity, modular design permitting easy expansion or operation at partial capacity, low maintenance (no moving parts), low energy requirements and low cost especially so for small sizes. Absorptions made of polymers and copolymers in the forms of flat film or hollow fibre have been used for gas separation.

Different gases pass through certain Absorptions at significantly different rates, thus permitting a partial separation. The rate of permeation is proportional to the pressure differential across die Absorption and inversely proportional to the Absorption thickness. The rate of permeation is also proportional to the solubility of the gas in the Absorption and also to the diffusivity of gas through the Absorption

Gas separation is thus affected by three key performance attributes of Absorptions, viz., selectivity towards the gases separated, Absorption flux or permeability and the life of the Absorption, maintenance and replacement costs.

The Absorption gas separation has been used for hydrogen separation and recovery, ammonia purge gas, refinery hydrogen recovery, 'syngas' separation in petrochemicals industry, CO2 enhanced oil recovery, natural gas processing, landfill gas upgrading, air separation, nitrogen production, air dehydration, helium recovery etc.

The gas separation technology may enjoy the following applications in the near future:

• N2 enrichment of air • Low level O2 enrichment of air • H2 and acid gas separation from hydrocarbons • Helium recovery

• Natural gas dehydration

Pervaporation

This is a Absorption based process for separating miscible liquids. Here the absorption of one of the components of the liquid by the Absorption, diffusion of this component across the Absorption and evaporation, as permeate vapour, into the partial vacuum applied to the underside of the Absorption. Pervaporation differs from all other Absorption processes because of the phase change of the permeate, non-porous. Transport through these Absorptions is effected by maintaining a vapour pressure gradient across the Absorption.

Applications of pervaporation Absorptions: Pervaporation offers significant capital and energy savings in applications that are difficult to separate by more conventional techniques such as azeotropic mixtures or mixtures of close boiling components. Pervaporation has been used for separation of ethanol-water mixture, solvent recovery, separation of heat sensitive products or enrichment of organic pollutants etc.

Advantages of this process over other separation techniques are given below:

• Effective and economic separation of mixtures of substances with small difference in boiling point and azeotropic mixtures

• Modular Absorption design • No entrainers for separation of azeotropic mixtures • Reduced capital costs compared to conventional systems.

Industrially Important Absorption Separation Processes, Their Operating Principles and Applications

Table 1 represents the characteristics of Absorptions used in different Absorption separation processes, process driving forces and applications of such processes

Emerging Absorption Technologies

Absorption based solvent extraction devices, recently developed, appear to eliminate high capital, operating and maintenance costs of centrifugal devices and additionally provide very high volumetric transfer rates. Systems have been designed by using microporous Absorptions (hydrophobic or hydrophilic) for non-dispersive solvent extraction.

Hollow-fibre-contained-liquid-Absorptions (HFCLM) have been used for gas separation through a nonporous polymeric Absorption. Microporous polypropylene hollow fibres have been used as the Absorption material. Gas separations such as N2-CO2, CH4-CO2, SO2-CO2-N2 and others have been studied by HFCLM technique.

A New Class of Intelligent Ion-exchange Absorptions

A new type of ion-exchange Absorption developed by the Absorption research group at McMaster University, Canada, is based on the simple concept of taking a cheap, chemically and physically rugged microporous Absorption and filling the pores with a flexible, ionic 'jelly'. This gel fill is anchored into place and confined by the microporous substrate. These Absorptions, which in fact contain 70% water, show remarkable separation properties including very large chemical value effects. One feature of this new class of Absorptions is that they show 'intelligence' for being able to sense the nature of the solution in contact with the Absorptions and modify their properties.

These Absorptions find extensive application in electrodialysis, diffusion dialysis, nanofiltration, Absorption solvent extraction and facilitated transport applications. The prototype Absorptions have excited the interest of a series of companies in different parts of the globe.

An interfacial polymerization/coating technique has been recently developed at McMaster University. It evenly coats all the interior surfaces of a microfiltration Absorption with polymers such as polyamides, polyesters, and polysulphonamides. The coatings which cannot he removed, modify the surface chemistry and provide different types of properties for the Absorptions. Thus, the Absorption remains microporous, but, with different surface chemistry. Photochemically active groups have also been incorporated into the coated layer to modify their chemical properties.

5.5 Pervaporation Absorptions are being developed for the removal of trace organics from water using coatings derived from silicone based oligomers. Research in this area is also being carried out in collaboration with National University of Singapore, and Industrial Absorption Research Institute, University of Ottawa. The Absorption pervaporation performance test is also

carried out using a completely automated testing apparatus that analyzes feed and permeate streams online for 24 hours continuously.

Absorptions for Fuel Cells/Electrolysis

One breakthrough in the application of ionic conducting polymer Absorptions is the proton exchange Absorption fuel cell, a device that converts chemical energy directly into electrical energy without burning. As the electrochemical combination of hydrogen (the fuel) and oxygen produce water, the fuel cell is environmentally clean and is expected to replace the gasoline engine or rechargeable battery in the automobiles. Structural and electrical properties of such ion containing Absorptions were studied extensively using AC impedance spectroscopy, high- resolution X-ray scattering and solid state NMR over the past few years at the polymer lab of the Materials Science and Engineering Department, McMaster University, Canada.

Encapsulating Absorptions

Research in the area of design and preparation of encapsulating media for environmentally sensitive materials addresses phospholipid liposomes modified with synthetic polymers anchored on the Absorption outer surface, and non-phospholipid liposomes (NPL) prepared from commercially available surfactants. The aim of the above is to tailor the encapsulation medium and process to specific applications in industrially relevant areas. The liposomes structure and properties are studied by spectroscopic techniques, such as fluorescence, size exclusion chromatography and capillary electrophoresis.

G-50 Ultrafiltration systems replaces the conventional oily wastewater treatment systems which are limited to sludge generation, insufficient removal of oil, and high operating costs. The G-50 UF systems are used to treat oily waste streams to recover water and oil or to meet discharge regulations. G,50 UF systems generally designed to operate in batch mode at a net pressure of 620-825 kPa.

Advantages of G-50 UF systems

Supported Liquid Absorption (SliM) Process

Commodore's SliM is the first Absorption technology that is capable, in a single process, of selectively extracting multiple elements or compounds ftorn a mixed process stream. The conventional methods for separating metals and/or removal from solubilized process streams presently include ion exchange, RO, UF, nanofiltration, precipitation and chromatography. Most of these methods have certain drawbacks, including lack of selectivity in the separation process, inability to handle certain metals in the process streams and the creation of sludges and other harmful by-products which require further post-treatment prior to disposal.

SliM is a superior and cost-effective alternative to the existing forms of Absorption filtration technologies, as it has the following advantages:

The SliM process involves passing a contaminated aqueous or gaseous feedstream through a hollow, porous fibre Absorption. This Absorption is previously loaded with chemicals whose composition varies depending on the targeted substance in the feedstream. As the feedstream enters the module, the metal or other substance to be extracted reacts with the proprietary chemical combination in the module, and the metallic or other ions are extracted through the Absorption into a strip solution which is concentrated and collected in a separate storage container. The balance of the feedstream is either recycled or simply discharged as normal effluent.

This is an ideal system for finishers, electroplating shops, PCB manufacturers, remediation sites, landfills and mines.

Liquid Absorption Extraction

The study undertaken by Am Berends, developed a technique for in-line removal of trace impurities from solids containing process streams by combining extraction and stripping in one process step. Liquid Absorptions can be used to extract trace amounts of heavy metal ions frorn process streams in one process step. The liquid Absorption is a thin layer of organic phase, separating two aqueous phases. To achieve metal ion extraction, an extractant is dissolved in the organic Absorption phase. It acts as a shuttle, extracting the metal ion from the feed phase and releasing it again at the other side of the Absorption. Advantages of the process are the high driving force and the low organic inventory.

The second type of liquid Absorption is emulsion liquid Absorption (ELM), which is water in oil emulsion, stirred in the third water phase. This system is studied by GRM Breembroek. The main subject of the research is the investigation of the influence of various process parameters on the extraction rate in various liquid Absorption systems, and the adaption of the system for the treatment of solid-containing process streams.

Other Emerging Absorption Technologies

The other emerging Absorption technologies, which enjoy excellent applications potential are Absorption reactors, hydrogen generation, purification and degassing, hydrogen sorption in metals, Absorption based transport devices, electrostatic pseudo-liquid Absorption (ESPLIM), Absorption distillation, controlled release etc.

Technology Trends: The Patent Scenario

Patent is an important tool to assess the technology trends and directions in the future. In fact, inputs to technology forecasting can he drawn from a well-studied analysis of international patents concerning a specific technology. In order to understand the technology development efforts and future scenario for Absorption separations, a patent search was carried out on the US patents in the relevant areas. Patents related to some of the latest Absorption separation technologies are briefed as follows:

Ultrafiltration

US patent #5858238 granted in 1999, related to Baxter Research Medical Inc., Midvale, UT, discloses a method and apparatus for salvaging blood from a patient. The blood salvaging and/or blood processing circuit is coupled to a cardiopulmonary by-pass circuit, cardiotomy circuit or directly to the patient. It comprises a hemocentrator for removing water, fluids and low rnolecular weight solutes by ultrafiltration and a sorbent containing plasma separator for removing a selected solute, such as heparin.

The device is equipped with a closed plasma chamber containing a plasma solution and a hollow fiber plasma, separating Absorption for receiving blood.

US patent #5707673 (granted in 1998) belonging to PreWell Industries, Jackson, MS, describes a process for extracting lipids and organics from animal and plant matter or organics containing waste streams. It uses the A method for concentrating raw milk is invented by Hibbard C David and Raghunath Bala of Wisconsin Rapids, WI, in 1997 (US patent # 5654025). It is a process of concentrating raw milk at farm level before transportating to the market and comprises the following steps:

Microfiltration

US patent #5865899 filed by Applexion, Epone, France, in 1999, discloses a process for refining raw sugar from the sugarcane. It is an energy efficient process and finds extensive application in sugar industries. The process comprises the following steps:

US patent # 5814513 filed by Ajinomoto Co. Inc., Tokyo, Japan, in 1998, describes a method for removing cells from fermentation broth using a Absorption. This method improves the Absorption permeation rate as compared to when no polyethyleneimine is added. The process involves removing cells from a fermentation broth, which is obtained by culturing a microorganism belonging to the genus Escherichia through a Absorption and is explained in the following steps.

USF Filtration and Separations Group Inc., Timonium, MD, (US patent # 5834107) invented highly porous polyvinylidene difluoride Absorptions in 1998. It relates to the field of synthetic polymeric Absorption materials, which are formed by casting polyvinylidene difluoride (PVDF) polymer solutions and/or dispersions. Absorptions obtained in this process are highly porous and are useful in a variety of microfiltration and ultrathin applications.

Gas Separations

US patent #5873928 assigned to Enerfex Inc., Burlington, VT, granted in 1999, describes a multiple stage semi-permeable Absorption process and apparatus for gas separation. The process produce s a very high purity permeate gas by using a multiple stage Absorption. This involves a primary and a secondary stage. In primary stage, feed gas mixture is passed through a primary Absorption separator, which comprises a Absorption of relatively high intrinsic permeability. It produces an intermediate permeate gas and a retentate gas. In the secondary stage, the intermediate permeate gas is

passed through secondary Absorption separator comprising a Absorption of relatively low intrinsic permeability, to produce a very high purity permeate gas.

US patent (# 5709732) filed by Praxair Technology Inc., Danbury, CT, and granted in 1998, describes an advanced Absorption system for separating gaseous mixtures. In this system, energy requirement are lower and product purity as a function of energy requirements is improved. For example, purified oxygen gas (60-90% purity) may be derived from ambient air in an efficient manner using this technology. Systems and methods are provided by which at least three permeator stages are used while requiring less than one compressor per stage.

A method for capturing nitrogen from air using a gas separation Absorption is invented by Opus Services Inc., Post, TX, in 1998 (US patent # 5730780). It is an improved process for generating nitrogen from air. Here a vacuum is generated on the permeate side of a gas separation Absorption, usually of the polysulfone type, which results in highly enhanced flow rates and nitrogen purity. This process is useful for the grain silos and also for the oil and, gas pipelines repair works.

US patent No. 5647894 granted in 1997, related to Nitto Denko Corporation, Osaka, Japan, describes a composite gas separating Absorption and a process for separating the gases using the same. It comprises two types of polyimide resin layers having different molecular structures and each having solubility in an organic solvent. The first one comprises a porous polyimide supporting Absorption having a nitrogen gas permeation flux density of 2 Nm3/m2/atm at 250 C. The second polyimide resin layer comprises a polyimide thin film having at least three fluorine atoms in a repeating molecular structure. It has a very high gas permeation flux density while maintaining high gas permeability. It is a cost-efficient technology and is widely used in heat resistance and chemical resistance applications.

Another US patent # 5688307 filed by Absorption Technology and Research Inc., Menlo Park, CA, in 1997, discloses a process for separating low-boiling gases using super-glassy Absorptions. It involves separation of hydrocarbon gases of low boiling point, particularly methane, ethane and ethylene from nitrogen. The process is performed using a Absorption made from a super-glassy material. The gases to be separated are mixed with a condensable gas, such as C3+ hydrocarbon. In the presence of such condensable gas, improved selectivity for the low-boiling point hydrocarbon gas over nitrogen is achieved.

L’Air Liquide Society, Paris, France, patented (US patent # 5472480) a process for supplying nitrogen by means of semi-permeable Absorptions by adsorption, in 1995. The process is meant for supplying nitrogen to at least two user stations, whose requirements as to nitrogen purity are different. The following steps explains the overall process:

Pervaporation

University of Chicago, Chicago, IL, has filed a US patent (# 5723639) in 1998, on the esterification of fermentation-derived acids using pervaporation. It is a low temperature method for esterifying ammonium and amine containing

salts. In this process, the salt reacts with an alcohol in the presence of heat and a catalyst and then subjected to dehydration and deamination processes using pervaporation. The invention also provides a method for producing esters of fermentation derived organic acid salts.

US patent # 5755967 by three scientists Mr. Meagher Michael, Mr. Qureshi Nasibuddin and Mr. Hutkins Robert of Lincoln, NE, granted in 1998, reveals a silicalite Absorption and method for the selective recovery and concentration of acetone and butanol from model ABE solutions and fermentation broth. The silicalite filled polymer Absorption and process are particularly suitable for the removal of acetone and butanol from clostridium acetobutylicum fermentation media. In this, the Absorption is not fouled by the fermentation media and may be used without removing cells from the fermentation media. This Absorption shows excellent selectivity to the adsorption of acetone and butanol relative to ethanol, acetic acid and butyric acid components of fermentation media.

US patent (# 5849195) belonging to Metallgesellschaft Akitiengesell-Schaft, Frankfurt, Germany, granted in 1998, describes a composite Absorption for removing water from dilute organic or inorganic acids by pervaporation. The composite Absorption is of a porous carrier layer, a porous backing layer and a non-porous separating layer. It consists of a PVC-PCac copolymer having a PV content of 10 to 25% by weight of the copolymer.

Absorption Technology and Research Inc., Menio Park, CA, has been granted a patent (# 5670051) in 1997 on the process of separating unsaturated hydrocarbons from fluid mixtures. This differs from conventional methods, in that the feed and permeate streams both be dry, and the Absorption need not be solvent swollen or contain water. The Absorption is characterized by a selectivity for an unsaturated hydrocarbon over a

saturated hydrocarbon having the same number of carbon atoms (about 20) and a pressure-normalized flux of unsaturated hydrocarbon. The flux and selectivity can from a gas mixture containing and unsaturated hydrocarbons in a dry environment.

Reverse Osmosis

Nitto Denko Corporation, Osaka, Japan, filed a US patent (.# 5674398) in 1997, on a novel composite reverse osmosis Absorption. It comprises a thin film and a microporous substrate as a support layer. Here, the thin film comprises a crosslinked polyamide, which is obtained by the interfacial polymerisation of the following:

An amine component comprising at least one monomeric amine compound with two primary and/or secondary amino groups, and

An acid halide component which is either a benzenehexacarbonyl halide or a mixture of benzenehexacarbonyl halide with a monomeric acid halide compound having at least two acid halide groups.

This Absorption exhibits high water permeability and high desalting performance even under low operational pressure (i.e., 5 Kgf/cm2).

MacMillan Bloedel Ltd., Canada (US patent # 4995983) invented a new type of Absorption separation process in 1991. The process is particularly suited for increasing the concentration of hydrogen peroxide. Certain chemicals such as hydrogen peroxide may he concentrated in an aqueous solution by increasing the pH of feed liquor, to increase the amount of the chemical in an ionized form. This means increasing the ionized fraction and then separating the feed liquor by reverse osrnosis into a retentate having an enhanced concentration of chemical in ionized form and a permeate, having a reduced concentration of chemical in ionized form relative to the feed liquor.

US patent #4944882, filed by Bend Research Inc., Bend, OR, in 1990, describes Hybrid Absorption-based separation systems that are useful in separating solvents and solutes. The hybrid systems combine, any of the solvent removal processes like RO, nanofiltration, Absorption distillation and pervaporation with any one or more of the solute removal processes utilizing pervaporation Absorptions, supported-liquid Absorptions, coupled transport Absorptions and Absorption contactors. The solvent and solute removal processes forming part of a recycle stream that utilizes Absorption separation retentates as feeds and as recycled feed stream make-up.

Electrodialysis

A US patent (# 5792315) granted to Eka Chamicals AB, Bohus, Sweden, in 1998, describes a process for purifying aqueous effluents of pulp mills using electrochemical Absorption. The system consists of two ion permeable Absorptions as a unit cell through which an electric current is passed between anode and cathode. The effluent is fed into the system where it gets purified by removing the metal impurity cations through a cation selective Absorption on the cathode side of the compartment. The purified effluent may he utilized as raw water for washing purpose in the pulp mill.

Yeda Research and Development Company Ltd., Rehovot, Israel, filed a US patent (# 5714512) in 1998, disclosed the novel heterogenous ion exchange Absorptions useful for electrodialysis. These Absorptions have anti-polarisation and anti-fouling properties. They comprise in addition to ion-exchange particles embedded in the matrix, a layer of at least one hydrophilic cross-linked polymer having fixed electric charge.

A US patent (# 5520813) filed by two inventors viz., Korin Amos and Korin Netta of Weston, CT, in 1996, discloses a method for processing nuclear waste solutions by Absorption separation technology. It is a process for separating monovalent ions from a sodium bearing radioactive waste stream. It is achieved by passing such waste stream through a ion exchange Absorption having permselectivity for monovalent ions. Here, the monovalent ions permeate or diffuse across the ion exchange Absorption to form a permeate of monovalent ion-enriched stream. Also, a retentate is formed from the sodium bearing radioactive waste stream, which is substantially depleted of monovalent ions.

Appendix B

GAS ABSORPTION SYSTEM

The Absorption gas separation technology is over ten years old and is proving to be one of the most significant unit operations. These processes compete with technology alternatives such as adsorption, cryogenic distillation etc. in niche application areas. The Absorption processes enjoy certain advantages, viz., compactness and light in weight, low labour intensity, modular design permitting easy expansion or operation at partial capacity, low maintenance (no moving parts), low energy requirements and low cost especially so for small sizes. Absorptions made of polymers and copolymers in the forms of flat film or hollow fiber have been used for gas separation.

Different gases pass through certain Absorptions at significantly different rates, thus permitting a partial separation. The rate of permeation is proportional to the pressure differential across die Absorption and inversely proportional to the Absorption thickness. The rate of permeation is also proportional to the solubility of the gas in the Absorption and also to the diffusivity of gas through the Absorption.

Gas separation is thus affected by three key performance attributes of Absorptions, viz., selectivity towards the gases separated, Absorption flux or permeability and the life of the Absorption, maintenance and replacement costs.

The Absorption gas separation has been used for hydrogen separation and recovery, ammonia purge gas, refinery hydrogen recovery, 'syngas' separation in petrochemicals industry, CO2 enhanced oil recovery, natural gas processing, landfill gas upgrading, air separation, nitrogen production, air dehydration, helium recovery etc.

The gas separation technology may enjoy the following applications in the near future:

N2 enrichment of air Low level O2 enrichment of air H2 and acid gas separation from hydrocarbons Helium recovery Natural gas dehydration

A typical Absorption system consists of a pre-treatment skid and a series of Absorption modules. The system is highly adaptable to accommodate treatment of various gas volumes and product-gas specifications. The technology’s compactness makes it the preferred technology for offshore gas treatment.

Currently, gas separation Absorptions are most widely used in industry for:

• Hydrogen separation, for example, hydrogen/nitrogen separation in ammonia plants and hydrogen/hydrocarbon separations in petrochemical applications;

• Separating nitrogen from air; • CO2 and water removal from natural gas; • Organic vapor removal from air or nitrogen streams.

At the moment, the most widely used Absorption materials for gas separation are polymers. They are attractive as Absorptions because they can be processed into hollow fibers with high surface areas. The relatively low cost of manufacturing the fibers makes them of interest for large-scale industrial applications. Examples of such Absorptions are the MEDAL and PRISM Absorptions produced, respectively, by Air Liquide and Air Products for wide-ranging gas separation applications. Each device contains thousands of fibers.

Absorption devices for gas or vapor separation usually operate under continuous steady-state conditions with three streams. The feed stream--a high-pressure gas mixture--passes along one side of the Absorption. The molecules that permeate the Absorption are swept using a gas on the other side of the Absorption in the so-called permeate stream. The non-permeating molecules that remain on the feed-stream side exit the Absorption as the retentate stream. A pressure difference across the Absorption drives the permeation process.

The economics of a gas separation Absorption process is largely determined by the Absorption's transport properties--that is, its permeability and selectivity for a specific gas in a mixture. Ideally, Absorptions should exhibit high selectivity and high permeability. For most Absorptions, however, as selectivity increases, permeability decreases, and vice versa. That's the trade-off.

A Absorption is a selective semi-permeable barrier that allows different gases, vapors, or liquids to move through it at different rates. A Absorption is defined by what it does and not what it is. The Absorption restricts the motion of molecules passing across it so that some molecules move more slowly than others or are excluded. A wide range of mechanisms are available for this restriction; for example, size variability of the molecules, affinity for the Absorption material, and permeation driving forces--typically concentration or pressure difference.

Each gas component in a feed mixture has a characteristic permeation rate through the Absorption. The rate is determined by the ability of the component to dissolve in and diffuse through the Absorption material.

Hollow Fiber Absorption

Figure 1 illustrates an internally staged permeator which splits the pressure difference between the feed and permeate side into two steps with two Absorptions of the same type. The permeate, Lm enriched in a more permeable component emerging from the first Absorption at an intermediate pressure would then be applied as feed to the second Absorption for the second permeation step. Since the pressure arrangement follows pu > pm > pp, the internally staged permeator can be operated without interstage recompression. The main purpose for such a pressure arrangement is to optimize the use of pressure energy for the enrichment of a desired product.

Schematic diagram of an internally staged permeator

Conventionally, the internally staged permeator can be assembled using flat sheet Absorptions or hollow fiber Absorptions. In the case of hollow fiber Absorptions, two bundles of the hollow fibers are required to be housed in a permeator shell which has four openings as shown in Figure 2a. In order to provide a better flow distribution and to maximize the packing densities of the Absorption permeator, one may propose to use annular hollow fiber Absorptions for the fabrication of the internally staged permeator as shown in Figure below. To date, however, no such hollow fiber Absorptions are available commercially.

Schematic diagram of internally staged hollow fiber permeators prepared from a) two bundles of hollow fiber Absorptions, b) annular hollow fiber Absorptions. The insert illustrates hollow fiber lumen, annular space in the single fiber and the fiber potting at both ends of the permeator.

a

b

Figures 3a and 3b are SEM photomicrographs showing cross-sectional views of the annular hollow fiber Absorption at magnifications of 400 and 5,000, respectively. It can be seen from Figure 3b that the width of the annular passage is around 5 8m. The structures for both inner and outer walls of the Absorption are similar to those obtained by the conventional single orifice spinneret. It is interesting to note that stringy structures were formed from both the surfaces and they were tangled to each other within the annular passage, which binds the inner and outer hollow fibers into a stable concentric annular hollow fiber Absorption.

Figure 3 Scanning electronmicrographs of Absorption cross sections for the annular hollow fiber Absorption, (a) part, 400 and (b) annulus, 5,000.

Figure 4 shows the gas enriching performance of an annular hollow fiber permeator studied both experimentally and theoretically. It can be seen that at a given overall stage cut, the annular hollow fiber permeator offers a considerably higher enriching performance compared to the conventional single stage permeator at the same energy consumption. The experimental results, in general, can be predicted well by the theoretical models developed with incorporation of pressure drops in both the hollow fiber lumen and the annular passages. Commercial application of the annular hollow fiber permeator developed to purify a binary gas mixture can be expected as this technique holds advantages over a conventional Absorption process.

Figure 4 Permeate concentration vs. overall stage cut, 1. Conventional hollow fiber permeator operated at countercurrent flow, 2. Internally staged permeator operated at co/counter-current flow, pu = 7 bar, pm =0.75, pp =0 bar(gauge).

Hollow Fiber Absorption (HFM)—air is passed through a bundle of fibers that are made of a special polymer designed to absorb oxygen. As air moves down the tube of fibers, oxygen is absorbed into the walls of the fibers. The longer the tube of fibers, the more oxygen that can be absorbed.

Basic Morphology

Two basic morphology of hollow fiber Absorption are isotropic and anisotropic (Fig. 1). Absorption separation is achieved by use of these morphologies.

Fig. 1 Basic Absorption morphology

The anisotropic configuration is of special value. In the early 1960s, the development of anisotropic Absorptions exhibiting a dense, ultra thin skin on a porous structure provided a momentum to the progress of Absorption separation technology. The semi permeability of the porous morphology is based essentially on the spatial cross-section of the permeating species, ie, small molecules exhibit a higher permeability rate through the fiber wall. While the anisotropic morphology of the dense Absorption which exhibit the dense skin, is obtained through the solution-diffusion mechanism. The permeation species chemically interacts with the polymer matrix and selectively dissolves in it, resulting in diffusive mass transport along the chemical potential gradient, as what demonstrated in the pervaporation process.

Absorption Configurations

Types of the Absorption configuration are given in Fig. 2 as below:

Fig. 2 Absorption configuration

Advantages and Disadvantages of Hollow Fiber

Hollow fiber is one of the most popular Absorptions used in industries. It is because of its several beneficial features that make it attractive for those industries. Among them are :

• Modest energy requirement : In hollow fiber filtration process, no phase change is involed. Consequently, need no latent heat. This makes the hollow fiber Absorption have the potential to replace some

unit operation which consume heat, such as distillation or evaporation column.

• No waste products : Since the basic principal of hollow fiber is filtration, it does not create any waste from its operation except the unwanted component in the feed stream. This can help to decrease the cost of operation to handle the waste.

• Large surface per unit volume : Hollow fiber has large Absorption surface per module volume. Hence, the size of hollow fiber is smaller than other type of Absorption but can give higher performance.

• Flexible : Hollow fiber is a flexible Absorption, it can carry out the filtration by 2 ways, either is "inside-out" or "outside-in".

• Low operation cost : Hollow fiber need low operation cost compare to other types of unit operation.

However, it also have some disadvantages which lead to its application constraints. Among the disadvantages are :

• Absorption fouling : Absorption fouling of hollow fiber is more frequent than other Absorption due to is configuration. Contaminated feed will increase the rate of Absorption fouling, esapecially for hollow fiber.

• Expensive : Hollow Fiber is more expensive than other Absorption which available in market. It is because of its fabrication method and expense is higher than other Absorptions.

• Lack of research : Hollow fiber is a new tachnology and so far, research done on it is less compare to other types of Absorption. Hence, more research will be done on it in future because of its potential.

Physically and Chemically constraints : Hollow fiber which made of polymer cannot use on corrosive substances and high temperature condition

Absorption Processes

Various types of Absorption processes can be found in almost all of the literature references. In this text, we will confine ourselves to the few Absorption processes that we will encounter in the further discussion of the industrial applications.

Reverse osmosis (RO)

There is considerable confusion in the open literature as to the distinction between few Absorption separation processes, i.e., the microfiltration (MF), ultrafiltration (UF) and reverse osmosis (RO). Occasionally one will see it referred to by other names such as "hiperfiltration (HF)". In order to distinguish these separation processes clearly, Porter in his paper presented one of the useful method based on the smallest particles or molecules which can be retrained by the various Absorptions. Accordingly, RO has the separation range of 0.0001 to 0.0018m (i.e., 1 to 10 Å ) or < 300 mol wt.

RO is a liquid-driven Absorption process, with the RO Absorptions are capable of passing water whilst rejecting microsolutes, such as salts or low molecule weight organics (< 1000 daltons). Pressure driving force (1 to 10 MPa)

needed to overcome the force of osmosis that cause the water to flow from dilute permeate to concentrated feed. The principle use of this Absorption process is desalination, which show its great advantage over the conventional technique of desalination, i.e. ion exchange.

Pervaporation (PV)

In this process, liquid mixture are fed under pressure to a non-porous Absorption, where components pass through the Absorption by solution-diffusion and evaporate at the permeate side of the Absorption. This technique is able to separate an azeotropic mixture. It current usage is well know in dehydration of the organic solvents and mixtures and the removal of organics from aqueous stream. The future application of this process, which is now under the main interest of the researcher is the hydrocarbon separation,, which shows its advantages of energy require compared to the conventional distillation technique.

Gas separation

Two type of gas separation processes have been encountered: gas permeation (GP) and gas diffusion (GD). The gas separation of the industrial interest is the former process, which is a pressure driven process where vapor components pass through a non-porous Absorption by a solution-diffusion mechanism; analogous to RO. While gas diffusion process can be done for the microporous Absorptions, operating under a concentration or partial pressure gradient.

Industrial Application

Absorption processes in chemical & petrochemical industry

• Gas Separation

Gas Absorptions are now widely used in variety of application areas, as shown in Table 2. This is because of its advantages in separation, low capital cost, low energy consumption, ease of operation, cost effectiveness even at low gas volumes and good weight and space efficiency.

As the matter of fact, hollow fiber is playing a important role in gas separation. It is because of its high separation areas and selectivity. The hollow fibers have approximately 30 times the productivity of other oxygen enriching Absorptions plus excellent inertness associated with their totally flourinated chemistry. The market of the gas separation include, small and intermediate scale industrial oxygen and nitrogen at moderate purity levels(oxygen 25%-40% or nitrogrn 82%-95%), portable oxygen for respiratory care, enhanced engine power and emissions reduction and removal gases from liquid.

Hollow fibers have demonstrated stable, high flux with moderate selectivity in full scale system. The high flux from hollow fibers is due to the combination of high transfer or separatin areas and thin Absorption wall. Besides, it also has a low surface energy.

With such characters, hollow fiber is widely used in many gas separation industries. For instance, it is used in O2/N2 separation for oxygen enrichment and inert gas generation, H2 /hydrocarbons separation for refinery hydrogen recovery, H2 /CO separation for sygas ratio adjustment, H2/N2 separation for ammonia purge gas, CO2/hydrocarbons separation for acid gas treatment and landfill gas upgrading, H2O/hydrocarbons separation for natural gas dehydration, H2S/hydrocarbons separation for sour gas treating, helium separation and etc. (refer Table 3)

Apart from that, low capital cost of hollow fiber also lead to its popularity. For example, for oxoalcohol feed separation, the process cost is about 1.000 for hollow fiber Absorption. However, for crygenic(partial condensation) and PSA processes are about 1.234 and 1.133 respectively.(appen ) From here, we can see that most of the cost for hollow fiber is for compression and not for purification. It is because hollow fiber itself already provides a good medium for purification.

• Desalination

As mentioned in the above section, RO is mainly use to remove the dissolved ion in the feed water. Its current extensive use in Malaysia industry sector is found in the production of ultrapure water in the semiconductor manufacturing industry. Historically, distillation and ion exchange was first used to remove the inorganic salts, but RO Absorption processes with the combination of ion-exchange system has promised a better result in both the product requirement and a better economic view point.

Other usage of RO included the removal of organics, salts and silica ahead of deionizers in boiler feed water, removal of inorganic salts, phosphorus and nitrogen compound in the municipal waste water treatment and also the demineralization of sea water and brackish water in the production of potable water. Porter provide a good reference in the comparison of product quality and economic of the above processes.

Absorption processes in biotechnology and biochemical industry

The biotechnology industry, which originated in the late 1970s, has become one of the emerging industry that draws the attention of the world, especially with the emergence of the genetic engineering as a means of producing medically important proteins, during the 1980s. Two of the major interest applications of Absorption technology in the biotechnology industry will be the separation & purification of the biochemical product, as often known as Downstream Processing; and the Absorption bioreactor, which developed for the transformation of certain substrates by enzymes (i.e. biological catalysts). Lots of literature has been published since the last ten years for these topic which serve as a good reference is sited in the Reference.

• Downstream processing

"Downstream Processing", a new key term given a decades ago, devotes towards the science and engineering principles in separation and purification in this emerging industry, has become a key issue to enhance the quality of the biochemical product. It is particularly important because it typically accounts for nearly three-fourths of the manufacturing costs in this new industry and because reliable and effective purification can be of the utmost important to the user. Absorption separation, together with the bioaffinity chromatography, liquid extraction and selective precipitation are the few techniques in the bio-separations, which gain attention from both the industries and researcher in order to upgrade the product quality of the biochemical industry.

Lots of study has been put in this area involving the most of the recovery of the biofuels and the biochemicals. Throughout the available literature, the most useful review is presented by Stephen A Leeper (1992), which compiles a large number of the previous and current studies' data on the different types of biofuels and the biochemicals product recovery, consisting of the usage of different types of Absorption materials, Absorption processes, together with the operating parameters of the studies being carried out.

• Absorption bioreactor

Since its introduction in the 1970s, Absorption bioreactor has granted a lot of attention over the other conventional production processes is the possibility of a high enzyme density and hence high space-time yields. Whereas downstream processing is usually based on discontinuously operated microfiltration, Absorption bioreactor are operated continuously and are equipped with UF Absorptions. Two type of bioreactor designs are possible: dissolved enzymes, (as in used with the production of L-alanine from pyrurate) or immobilized enzymes Absorption.

Future Prospects

Absorption science began emerging as an independent technology only in the mid-1070s, and its engineering concepts still are being defined. Many developments that initially evolved from government-sponsored fundamental studies are now successfully gaining the interest of the industries as Absorption separation has emerged as a feasible technology.

As were noted by the US National Research Council, the technological frontiers of the Absorption technology should be concerned more in the developing of new Absorption materials and the identification of new ways of using permselective Absorptions.

New Absorption materials to be used is still a big option in the research of this brand new technology, as most of the researchers are always intend to get a better improvement for this separation process. Journal of Absorption Science serve as a good reference, where lots of the new Absorption materials research may be found.

For the latter, Absorption-based hybrid system serve as a good example, as it is a combination of conventional unit operations and Absorption separation processes, which often results in separation processes that offer significant advantages over the exclusive use of either component process. Such advantages may include more complete separation, reduced energy requirement, lower capital cost, and lower production cost. Two good example of this hybrid system are the RO / evaporator hybrid system to concentrate corn steep water, and a Absorption / vapor-recompression hybrid process to recover energy in hot, moist dryer exhaust. Studies has also been carried out and proven that these hybrid system did perform a better of the washwater purification and reuse pilot plant (HUMEF) which has been successful installed in Eindhoven pumping station, the Netherlands.

Spiral Wound Absorption

The spiral wound Absorption process combines a compression-condensation step with a Absorption separation step. The feed gas - a mixture of hydrocarbons in nitrogen, hydrogen, or methane - is compressed and cooled, condensing a portion of the hydrocarbons in the gas. The liquid hydrocarbons are recovered; the remaining gas, which still contains significant amounts of hydrocarbons, is fed to the Absorption. The Absorption separates the gas into a hydrocarbon-rich permeate stream and a hydrocarbon-depleted residue stream (the purified gas). The permeate is recycled to the compressor; the residue stream is vented or reused.

How do Absorptions work?

This Absorptions separate gas mixtures on the basis of solubility. Large hydrocarbon molecules with greater solubility in the Absorption permeate much faster than smaller, less soluble molecules such as nitrogen, hydrogen, or methane.

By comparison, conventional Absorptions separate gases on the basis of size. Small molecules are selectively permeated because they diffuse through the Absorption more rapidly than large molecules.

How are Absorptions packaged?

Absorptions are manufactured as flat sheets and rolled into spiral-wound modules. The feed gas enters the module and flows between the Absorption sheets. Spacers on the feed side and the permeate side of the Absorption sheets create flow channels. The hydrocarbon vapor that passes preferentially through the Absorption flows inward to a central permeate collection pipe. The light gas (nitrogen, hydrogen or methane) is rejected by the Absorption and exits as the residue.

Spiral-wound Absorption module

Modules are three feet long and four to eight inches in diameter. As many as four modules are placed in pressure vessels designed to meet local standards (ASME, etc.). To meet the needs of a particular application, modules are configured in series and parallel flow combinations.

Where are Absorption separation systems used?

Absorption separation systems are used in the petrochemical, refining, and natural gas processing industries. Current applications include:

• Recovery of olefins from resin degassing vent streams in polyolefin plants.

• Recovery of liquified petroleum gas (LPG) from refinery vent streams. • Fuel gas conditioning (removal of heavy hydrocarbons from fuel gas). • Recovery of natural gas liquids (NGLs) from natural gas streams.

What are the operating limits of Absorption modules?

The Absorption module can operate over a wide range of temperatures and pressures. The temperature range is -40°C to 40°C. The feed pressure can be

as high as 1,500 psi. The permeate pressure can be under vacuum if required to achieve a certain separation.

What pre-conditioning of the feed gas is required?

Several factors determine the required pretreatment. If dust or other solid particles are present, they must be removed upstream with a high-efficiency filter. If the feed gas contains entrained liquids, a mist eliminator vessel with a high-efficiency coalescing agent is required. MTR will provide detailed pretreatment requirements.

Tubular Absorption

Appendix C

(lab report format)

UNIVERSITI TEKNOLOGI PETRONAS

Bachelor of Engineering (Hons) CHEMICAL

Syllabus No : Subject : Separation Processes I Experiment : Gas Absorption Separation

2nd-year Laboratory Return Sheet

Please fill in the following details

Name:

Date:

Lab. expt: Gas Absorption Separation

Lab. group:

Marking scheme

Component Maximum Marks Comments

Report summary 20

Report introduction 10

Apparatus/experimental 10

Results 20

Discussion 20

Conclusions 10

Report presentation 5

Lab diligence 5

Total 100

General comments:

Initials of marker:

Laboratory reports should be submitted to the General Office where the report will be date-stamped.

Summary

(Some people in companies will receive only the summaries of reports so these must contain all necessary information for the reader to make sense of them. A summary should contain a statement of the objective, apparatus, procedure, results obtained and conclusions. Think of it as a mini-report but without figures or tables. The summary should not have section headings nor should it contain information not mentioned in the report itself. The length should be about half a page. )

Introduction

(The purpose of the introduction is to provide the background to the work and to 'set the scene'. In some cases, a brief account of the importance of the subject might be appropriate; in others, an account of a problem that the experiment attempts to deal with might be relevant. Include any theory which is relevant to an understanding of the work being carried out. Also include anything you will mention in the discussion or conclusions sections of the report. If the introduction from the handout is to be used as the basis please mention the handout in the references section. The length should be about half a page minimum.)

Objectives

(This section should set out clearly and concisely the objective(s) of the work

that has been carried out. {A common error in lab reports is to include educational objectives in this section. A supervisor may say to you “ The point of this experiment is to familiarise you with heat pumps. I want you to find

how refrigerant flow affects the efficiency of this one”. Do not include 'To familiarise ourselves with heat pumps' as an objective. The technical objective is along the lines of 'To find how refrigerant flow affects the efficiency of a

heat pump'. Objectives should be a few lines long.})

Apparatus

(A general description of the apparatus or plant should be given without going into too much detail on the mechanical side. A sketch or detailed drawing should supplement this description. Full details of a process or a piece of plant

should be given where applicable together with a clearly labelled flowsheet. Detailed descriptions of standard laboratory apparatus should be avoided. Length depends on the apparatus and the drawing)

Procedure

(A fairly full description of the procedure should be given. Any difficulties in operation which might lead to inaccurate results should be mentioned in passing. Fuller comment on this aspect may be left to the discussion.)

Results and calculations

(A specimen calculation of each step should be given but avoid trivial calculations -put more detailed calculations in an appendix if you wish. Try to be expansive with plenty of explanation to guide the reader rather than just presenting a mass of numbers. Experimental readings and calculated results are best given in a tabulated form. All readings should be given, including those, if any, considered inaccurate. If appropriate, the results can be - presented graphically but, in general, it is regarded as bad form to give the same results in both a table and a graph). DISCUSSION

(Discuss the shape of plot obtained and its significance regarding the distribution of particles in the talcum powder. State the mean particle size and geometric standard deviation. The standard deviation is basically the gradient of the plot and is a measure of the degree of spread of the distribution -comment. Was it valid to assume that Stokes Law prevailed? Errors/Other comments?)

CONCLUSIONS (This section should summarise the findings and inferences of the work carried out. It should be brief and consist of firm statements. One way of looking at the conclusions is that they should provide the answer to what you set out as the objective{s}). REFERENCES ("References" if referred to in the report; "Bibliography" if not referred to in the report but used by the author or may be useful for the reader. Give title, author, volume, edition, publisher, year, page if appropriate. For example, ("Mass-Transfer Operations", Treybal,R.E., 3rd. ed., McGraw-Hill, 1981 ) 1. C.Judson King Separation Processes, 2nd edition (McGraw- Hill Inc.)

2. Robert E. Treybal Mass-Transfer Operations, 3rd edition (McGraw- Hill

Inc.)

3. Warren L. McCabe, Julian C. Smith, Peter Harriott Unit Operation of

Chemical Engineering, 4th edition (McGraw- Hill Inc.)

4. J.M. Coulson, J.F. Richardson Chemical Engineering Volume Two, 3rd

edition (Pergamon )

5. Philip A. Schweitzer Handbook of Separation Technigues for Chemical

Engineer, 2nd edition

6. Kirk -Othemer Encyclopedia of Chemical Technology Vol.1, 4th edition

7. F.H.H. Valentin Absorption in gas - liquid dispersions

8. Nicholas P. Chopey Handbook of Chemical Engineering Calculations, 2nd

edition

9. Edgar, T.F. & Himmelblau, D.M. Optimisation of Chemical process,

(McGraw-Hill Inc.)

10. B. Finlayson Non-linear Analysis in Chemical Engineering, (McGraaw-Hill)

11. T.K. Sherwood, R.L. Pigford Absorption and Exraction (McGraw- Hill Inc.)

12. Kreyzig Advanced Engineering Mathematics, 7th edition (John Wiley)

13. R.H. Perry, D. Green Perry’s Chemical Engineers’ Handbook, 6th edition,

(McGraw-Hill)

14. C.R. Wilke, U.V. Stockar Industrial Engineering Chemical Fundamental,

16(2), 88, 1977

15. G.C. Coggan, J.R. Bourne Trans. Institute Chemical Engineers Vol(47)

1969

16. M. Oh, C.C. Pantelides A Modelling and Simulation Language for

Combined Lumped and Distributed Parameter Systems 1994

Appendix D

(National Instruments – NI activation procedures)

Activate National Instruments Labview Software – Screen I

Activate National Instruments Labview Software – Screen II

Open pre-programmed data acquisition file – “FILE”, “OPEN”

Select “Application”

Default Screen

To run – Go to top left hand corner – Click

Ready to run screen

Click the respective button to activate the equipment Inactive Active

General Safety Observation

HAZARD: Rotating Equipment / Machine Tools

Personal Protective Equipment: Safety Goggles; Standing Shields, Covered/safety Shoes

No: Loose clothing; Neck Ties/Scarves; Jewelry (remove); Long Hair (tie back) HAZARD: Electrical - Burns / Shock

Care with electrical connections, particularly with grounding; do not use frayed electrical cords can reduce hazard.

HAZARD: High Pressure Air-Fluid /Gas Cylinders/Vacuum Inspect before using any pressure / vacuum equipment. HAZARD: Water / Slip Hazard Any spillage must be cleaned and reported to instructor immediately!

Gas Absorption Pilot Plant

Gas absorption is a unit operation in which soluble components of a gas mixture are dissolved in a liquid. The inverse operation, called stripping or desorption, is employed when it is desired to transfer volatile components from a liquid mixture into a gas. Both absorption and stripping, in common with distillation, make use of special equipment for bringing gas and liquid phases into intimate contact. This section is concerned with the design of gas-liquid contacting equipment, as well as with the design of absorption and stripping processes.

Absorption, stripping, and distillation operations are usually carried out in vertical, cylindrical columns or towers in which devices such as plates or packing elements are placed. The gas and liquid normally flow counter-currently, and the devices serve to provide the contacting and development of interfacial surface through which mass transfer takes place.

This proposed system consists of a column of perforated plates constructed by

LENZ A/G, Germany. The gas absorption column is designed in such a area

that it permits absorption and stripping processes to be carried out either

independently or simultaneously.

Introduction

Gas absorption is a unit operation used in the chemical industry to separate gases by washing or scrubbing a gas mixture with a suitable liquid. There are some absorption systems in which the temperature rise is not large and therefore the absorption process can be treated as isothermal. However, in some case of physical absorption, thermal effects are large and heat released causes an appreciable increase in the liquid temperature. Hence, adiabatic absorption must be considered in the design of gas absorbers.

In this manual, the focus is primarily on operation of gas absorption (packed) column. Serious and relevant considerations include the heat effect and other operating variables are made. This manual is mainly divided into two parts. The first part provides some background knowledge on the theory and fundamental equations governing gas absorption operation. The second part comprises of the experimental procedures as well as results.

Gas Output

Solvent Input

Gas Input

Solvent Out

Gas absorber - Packed Column

BASIC CONCEPTS

Chemical processes may consist of widely varying sequences of steps (which may involve chemical reactions and/or physical changes), the principles of which are independent of the material(s) being operated upon and of other characteristics of the particular system. A Unit Operation involves a list of techniques that are based on principles of science that are translated into industrial applications in various fields of engineering, e.g. - fluid mechanics, heat transfer, process instrumentation and control, mass transfer, etc. Examples of unit operations: Distillation, Absorption, Humidification, Extraction, Leaching, Crystallization, Membrane Processing, Adsorption, etc. Typical chemical manufacturing process will involve a combination of several unit operations, e.g. petroleum refining, petrochemical, pharmaceutical, and other specialty chemicals. Focus on separation processes, e.g. distillation and gas absorption where mass transfer occurs from one phase to another. Types Of Separation Processes Many chemical processes materials occur as mixtures of different components in the gas, liquid or solid phase. In order to separate or remove one or more of the components from its original mixture, it must be contacted with another phase. The two phases are brought into more or less intimate contact with one another so that a solute can diffuse (i.e. transfer) from one phase to the other: hence inter-phase mass transfer. (A solute is a component that dissolves in a particular phase) Major classifications:

• Distillation (Fractionation, Fractional Distillation) • Gas Absorption & Stripping • Liquid-liquid Extraction • Leaching • Crystallization • Adsorption • Drying • Humidification • Evaporation • Screening • Filtration • Sedimentation • Centrifugation • Membrane Processing

Each of these processes is based on a fundamental principle as the basis of separation. The equipment used will also necessarily be different. Particularly,

gas absorption process and its principle of separations are described briefly in the sections below: Gas Absorption (Scrubbing)

• Transfer of one or more soluble components from a gas phase into a relatively non-volatile liquid solvent.

• Principle of separation: Solubility of a gas in a liquid. The transferred component must be preferentially more soluble in the liquid than other components.

• An example is the removal of ammonia from mixture with air using water. Ammonia is more soluble in water than air. Thus more ammonia is preferentially removed (i.e. absorbed) by water when the ammonia-air mixture is contacted with water.

• Reverse of absorption is stripping or desorption, e.g. steam stripping of hydrogen sulfide from liquid DEA (diethanolamine) solution.

THEORY OF GAS DIFFUSION & MASS TRANSFER MOLECULAR DIFFUSION Molecular diffusion or molecular transport can be defined as the transfer or movement of individual molecules through a fluid by means of random, individual movements of the molecules. The molecules travel only in straight lines and in the process, may collide with other molecules in their path. The molecules then change direction (still in a straight line) after the collision. This is sometimes referred to as a random-walk process as shown in Figure 49 below.

Figure: Random-walk molecular diffusion process

Now, consider a container with a mixture of 2 components A and B at constant pressure P and constant temperature T as shown in Figure below. A fictitious partition c-c separates the container into 2 sections. The LHS contains more molecules of component A than the RHS, and the reverse is true for the B.

Figure: Stationary Binary Mixture at constant T and P

Next consider what happens when the partition c-c is removed. The molecules will move around in all directions in a random manner. But since most of the A-molecules are to the left of c-c, more of the A-molecules will travel from the LHS to the RHS than in the opposite direction. This is the molecular diffusion of A in the direction of decreasing concentration (i.e. from the region of high concentration to the region of low concentration). At the same time there is also a net diffusion of B from the right-hand side to the left-hand side. This diffusion continues until the concentrations of A and B is uniform throughout. Fick’s Law of Diffusion

Fick’s Law stated that for the diffusion of component-A in a binary mixture of A and B:

[Note: the above refers only to one-directional diffusion, in the z-direction]

where JA = molecular diffusion flux (kg-mole/m2.s) DAB = diffusivity @ diffusion coefficient of A in a mixture of A and B (m2/s)

= concentration gradient (kg-mole.m-3/m) The concentration gradient dCA /dz is the driving force for diffusion. The (–) showed that CA decreases as z increases. See Table below for examples of the diffusion coefficient of binary gas mixtures at 101.32 kPa.

Table: Diffusion coefficients for binary gas mixtures at 101.32 kPa