CCB 3024

PROCESS PLANT DESIGNConceptual Process Design

Synthesis of Separation Systems

Dr. Rajashekhar PendyalaChemical Engineering DepartmentUniversity Teknologi PETRONAS

1. Able to sequence appropriately the separator (homo and heterogeneous)

2. Identify the suitable heterogeneous separator(s) for a specified separation task.

At the end of this lecture, student should be able to

SYNTHESIS OF SEPARATION SYSTEM – HETEROGENOUS MIXTURE SEPARATION

Some Review

WHY SEPARATORS ARE NEEDED ?

- SEPARATION OF DESIRED PRODUCTS FROM BYPRODUCTS AND UNREACTED FEED UP TO THE REQUIRED SPECIFICATION.

- PURIFICATION OF FEEDS.

IS IT AVAILABLE AT ANY PLANT ?

.........

TYPES OF SEPARATOR

DISTILLATION

STRIPPER

ABSORBER

MEMBRANE

ADSORBER

ETC...

Extraction

Liquid-Liquid

Solid-Liquid

WHERE ARE SEPARATORS NORMALLY LOCATED ?

REACTOR

SEPARATION

& RECYCLE

SYSTEM

HEAT EXCHANGER

NETWORK

UTILITIES

reactor

Normally, form the major part of any chemical plant.

Prior and after reactor !

Separator System Synthesis – Heuristic Approach

During the conceptual design of a chemical process, among the related decisions that have to be made concerning separator system ;

Separator type

What type of

suitable separator ?

ABSORBER

MEMBRANE

DISTILLATION

Sequencing of Separator

DISTILLATION

DISTILLATION

MEMBRANE

How are the

separators arranged ?

Operating Conditions

TEMPERATURE

PRESSURE

CONCENTRATION

ADDITION OF EXTRA COMPS.

SUGGESTED PROCEDURE….HEURISTIC

1. Decide on type of separator that will be likely used for the required separation.

Off course, we have to start with gathering the related information !

What type of mixture ?

Homogenous

Require addition or creation of another phase to perform separation.

eg. Distillation (liquid mixture)

Extraction (solid, liquid mixture)

Condensation (gas mixture)

Absorption (gas mixture)

Adsorption (gas mixture)

Heterogeneous

Exploiting differences in certain physical properties (eg. density, molecular size, surface tension etc.) in order to perform separation.

eg. Membrane Separation

Filtration

Gravity settler

Centrifugal Separator

Phase Separation

Flotation

2. Decide on the sequencing of the separator to achieve the process requirement.

1st Golden Rule : Perform the heterogeneous mixture separation first or as soon as

the mixture is formed during any stage of the process.

The phase separation involves :

Vapour/Liquid

Eg. flash drum

Liquid/Liquid (immiscible)

Eg. phase separator

Solid/Liquid

Eg. filtration, membrane separation, gravity settler

Solid/Vapour (gas)

Eg. Venturi Scrubber

Solid/Solid

Eg. shaking, vibrating screen

It is easiest to conduct !

SUGGESTED PROCEDURE….

Gravity Settler Separators

a. Vapour Liquid Separation - Flash Drum

Liquid drops separated from vapour/gas flow based on the settling velocity(terminal velocity) of the liquid drop. It is a function of the droplets size. Ageneral eqn. that can be used is;

The maximum velocity of the vapour set within the vessel must be less than thesettling velocity.In the event where droplets particle is very small, the use of baffles of mesh padcould promote coalescence between the particles to form larger size thus easierto separate.The method is not practical to separate particle diameter less than 10micrometer.

Gravity Settler Separators

b. Liquid-Liquid Separation - Decanter

Separation of dispersed liquid phase from a continuous phase. The continuousphase velocity must be low enough to allow low density droplets to rise frombottom of the vessel to the interface and coalesce, and the high density dropletsto settle down to the interface and coalesce. The velocity of the continuousphase must be lower than the terminal velocity for the particles calculated usingequation above.

c. Solid Liquid Separation - sedimentation

Solid particles settle to the lower part of the vessel as it travels with the liquid flow across the vessel. When the prime function of the equipment is to produce more concentrated slurry, it is called thickener while if the function is to remove the solid particles from the liquid, it is called clarifier.

Inertial and Centrifugal Separators

In the event when gravity settler is deemed to be too slow due to factor such as closeness in the density between the droplets or particles and the continuous phase, inertial or momentum separators could improve the efficiency by giving the momentum force in addition to the gravity force. The additional force provided is normally in the form of centrifugal force.

Particle Size (mm)

The design of the inertial separators normally follows a collection efficiency curve supplied by equipment manufacturers from sets of experiments conducted.Cyclone (solid gas) and hydro cyclone (solid liquid or liquid liquid) are examples of equipment exploiting the principle.

Electrostatic Precipitation

Electrostatic precipitators are commonly used to separate particulate matter that is easily ionized from a gas stream. Electrostatic field is produced between wires or grids and collection plates by applying high voltage between the two. A corona is established around the negatively charged electrode. The corona ionizes the molecules of gases such as O2 or CO2 which in turn attach themselves to the particles and charging them at the same time. The opposite charge electrode (collection plates) will attract these particles and later dislodged them by mechanical forces.

The method is most effective when separating particles with high resistivity. The operating voltage typically range from 25 and 45 kV or more depending on the design and operating temperature.

The application is typically restricted to the separation of fine particles of solid or liquid from a large volume of gas. Again the design could be based on the collection efficiency curve as provided by manufacturer.

Filtration

Suspended solid particles in a gas, vapour or liquid are removed by passing the mixture through a porous medium that retains the particles and passes the fluid (filtrate). The solid can be retained on the surface of the filter medium (cake filtration) or captured within the filter medium (depth filtration).

The filter media for cake filtration can be made of cloth, ceramic or even metal. There are various arrangement for the filter media such as plate & frame, bag, conveyor belt, rotating filter etc. Conventional filter media can be used up to temperature of 250 C. Higher temperatures require ceramic or metallic.

Filtration

For the depth filtration, granular medium consisting of layers of particulate solid (eg stones, pebbles etc.) placed on a support grid is used as the filter medium. Downward flow of the mixtures causes the solid to be captured within the medium. Such arrangement is normally used for removing small amount of solids from large quantities of liquids. This filtration mechanism can removed particles with sizes down to 10 microns.

For smaller sizes particles down to 0.05 microns, microfiltration using polymer membrane is used. Two most arrangements used consist of spiral wound and hollow fiber.

Settling and sedimentation

In settling processes, particles are separated from a fluid by gravitational forces acting on the particles. The particles can be liquid drops or solid particles. The fluid can be a gas, vapor or liquid

Gravity settler for the separation of gas-liquid and vapor-liquid mixtures Gravity settler for the separation of

liquid-liquid mixtures

Gravity settler for the separation of fluid-solid mixtures

When a particle falls under the influence of gravity, it will accelerate until the combination of the frictional drag in the fluid and buoyancy force balances the opposing gravitational force

If the particle is assumed to be a rigid sphere, at this terminal velocity, a force balance gives

Settling and sedimentation

Settling and sedimentation

Rearranging equation gives

More generally, Equation can be written as

By applying the correlations for cD

Settling and sedimentation

Flow is in the laminar region (Stoke’s Law region)

The terminal velocity will be

Settling and sedimentation

Empirical expression for Re > 2

The terminal velocity will be

For higher values of Re

The terminal velocity will be

Settling and sedimentation

When designing a settling device, the maximum allowable velocity in the device must be less than the terminal settling velocity. For this the particle diameter must be known.

For gas-liquid and vapor-liquid separations, there will be a range of particle droplet sizes. It is normally not practical to separate droplets less than 100 μm diameter in such a simple device

Settling and sedimentation

• A simple gravity settler or decanter is used for removing a dispersed liquid phase from another liquid phase

• The horizontal velocity must be low enough to allow the low-density droplets to rise from the bottom of the vessel to the interface and coalesce and for the high density droplets to settle down to the interface and coalesce.

• The decanter is sized on the basis that the velocity of the continuous phase should be less than the terminal settling velocity of the droplets of the dispersed phase.

• The velocity of the continuous phase can be estimated from the area of the interface between the settled phases

• The velocity of the continuous phase must be less than the terminal settling velocity

Settling and sedimentation

• Decanters are normally designed for a droplet size of 150 μm

• Dispersions of droplets smaller than 20 μm tend to be very stable can’t be separated by this equipment

• The band of droplets that collect at the interface before coalescing should not extend to the bottom of the vessel. A minimum of 10% of the decanter height is normally taken for this.

• An empty vessel may be employed, but horizontal baffles can be used to reduce turbulence and assist the coalescence through preferential wetting of the solid surface by the disperse phase.

• More elaborate methods to assist the coalescence include the use of mesh pads in the vessel or the use of an electric field to promote coalescence.

• Chemical additives can also be used to promote coalescence

Settling and sedimentation

• A mixture of gas, vapor or liquid and solid particles enters at one end of a large chamber, particles settle toward the base

• Again the device is specified on the basis of the terminal settling velocity of the particles

• The efficiency with which the particles of a given size will be collected from the simple setting devices is given by

Settling and sedimentation

• When high concentrations of particles are to be settled, the surrounding particles interfere with individual particles.

• This is particularly important when settling high concentrations of solid particles in liquids.

• When separating a mixture of water and fine solid particles in a gravity settling device, it is common in such operations to add a flocculating agent to the mixture to assist the settling process.

• This agent has the effect of neutralizing electric charges on the particles that cause them to repel each other and remain dispersed.

• The effect is to form aggregates or flocs, which, because they are larger in size, settle more rapidly.

Settling and sedimentation

• The separation of suspended solid particles from a liquid by gravity settling into a clear fluid and a slurry of higher solids content is called sedimentation

A thickener for liquid-solid separation

• A thickener, the prime function of which is to produce a more concentrated slurry

Settling and sedimentation

• When the prime function of the sedimentation is to remove solids from a liquid rather than to produce a more concentrated solid–liquid mixture, the device is known as a clarifier

Clarifiers are often similar in design to thickeners

Settling and sedimentationSimple gravity settling classifier

• A large tank is subdivided into several sections. A size range of solid particles suspended in gas, vapor or liquid enters the tank.

• The larger, faster-settling particles settle to the bottom close to the entrance, and the slower-settling particles settle to the bottom close to the exit.

• The vertical baffles in the tank allow the collection of several fractions

This type of classification device can be used to carry out solid–solid separation in mixtures of different solids. The mixture of particles is first suspended in a fluid and then separated into fractions of different size or density in a device similar to classifier

Settling and sedimentation

Example:Solid particles with a size greater than 100 μm are to be separated from larger particles in a settling chamber. The flowrate of gas is 8.5 m3/s. The density of the gas is 0.94 kg/m3 and its viscosity 2.18×10-5kg/m/s. The density of the particles is 2780 kg/m3.

a. Calculate the settling velocity, assuming the particles are spherical

b. The settling chamber is to be box-shaped, with a rectangular cross section for the gas flow. If the length and breadth of the settling chamber are equal, what should the dimensions of the chamber be for 100% removal of particles greater than 100 μm?

Assume initially that the settling is in the Stoke’s Law region

Solution

Settling and sedimentation

Check the Reynolds number

Since outside the range of validity of Stoke’s Law

Example:

Settling and sedimentation

For 100% separation of particles

and

Example:

Settling and sedimentation

Assuming L = B, thenExample:

Inertial and Centrifugal Separation

• Sometimes gravity separation may be too slow because of the closeness of the densities of the particles and the fluid, because of small particle size leading to low settling velocity or, in the case of liquid–liquid separations, because of the formation of a stable emulsion

• Inertial or momentum separators improve the efficiency of gas–solid settling devices by giving the particles downward momentum, in addition to the gravitational force.

Inertial and Centrifugal Separation

Impingement separator

Inertial and Centrifugal Separation

• Use of centrifugal forces increases the force acting on the particles. • Particles that do not settle readily in gravity settlers often can be

separated from fluids by centrifugal force.

• The simplest type of centrifugal device is the cyclone separator for the separation of solid particles or liquid droplets from a gas or vapor

• This consists of a vertical cylinder with a conical bottom. Centrifugal force is generated by the motion of the fluid. The mixture enters through a tangential inlet near the top, and the rotating motion so created develops centrifugal force that throws the dense particles radially toward the wall. The entering fluid flows downward in a spiral adjacent to the wall. When the fluid reaches the bottom of the cone, it spirals upward in a smaller spiral at the center of the cone and cylinder

Inertial and Centrifugal Separation

• The design of cyclones is normally based on collection efficiency curves

• The same principle can be used for the separation of solids from a liquid in a hydrocyclone. Although the principle is the same, whether a gas or vapor is being separated from a liquid, the geometry of the cyclone will change accordingly. Hydrocyclones can also be used to separate mixtures of immiscible liquids, such as mixtures of oil and water.

Inertial and Centrifugal Separation

Separation of liquid-solid mixture Separation of liquid−liquid mixture

Electrostatic precipitators

• Electrostatic precipitators are generally used to separate particulate matter that is easily ionized from a gas stream

• This is accomplished by an electrostatic field produced between wires or grids and collection plates by applying a high voltage between the two

Electrostatic precipitation

Filtration

• Suspended solid particles in a gas, vapor or liquid are removed by passing the mixture through a porous medium that retains the particles and passes the fluid (filtrate).

Plate-and-flame filter Bag filter

Filtration

Belt vacuum filter Rotary vacuum filter

Scrubbing

Packed bed scrubber Spray scrubber

• Scrubbing with liquid (usually water) can enhance the collection of particles when separating gas–solid mixtures

Scrubbing

Venturi scrubber

Flotation is a gravity separation process that exploits the differences in the surface properties of particles. Gas bubbles are generated in a liquid and become attached to solid particles or immiscible liquid droplets, causing the particles or droplets to rise to the surface. This is used to separate mixtures of solid-solid particles after dispersion in a liquid or liquid-liquid mixtures of finely divided immiscible droplets. The liquid normally used is water and the particles of solid or immiscible liquid will attach to the gas bubbles if they are hydrophobic.

Gas bubbles are generated by means of dispersion, dissolution (changing of pressure from high to low in the vessel) and electrolysis.

Widely used in the mineral ore processing.

Flotation

Flotation

A typical flotation cell for solid separation Dissolved air flotation (DAF)

Removal of water moisture from a solids into a gas stream (normally air) by heat. Four commonly used type of dryer are tunnel dryers, rotary dryers, drum dryers and spray dryers. Another important class of dryers is the fluidized bed dryers.

Choice between dryers is normally made based on practicalities such as material handling characteristics, product decomposition, product physical form etc.

Dryer efficiency is measured by heat of vaporization / total heat consumed.

Drying

Drying

Common types of thermal dryer

Common types of thermal dryer

Drying

Questions

References

1. Smith, R. Chemical Process: Design and Integration. Wiley, 2005.

2. Sinnott R. K. Chemical Engineering Design, Coulson & Richardson’s Chemical

Engineering Volume 6. 4th Ed. Oxford, UK: Elsevier, 2006.

3. Peters M. S. and Timmerhaus K. D. Plant Design and Economics for Chemical

Engineers. 5th Ed. New York: McGraw-Hill, 2003.