7009722 liquid liquid extraction

8/8/2019 7009722 Liquid Liquid Extraction

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CHE-396 Senior Design

Extraction

Liquid-Liquid Extraction

Senior Design CHE 396

Matrix Corporation Zachary Fijal Constantinos Loukeris Zhaleh Naghibzadeh John Walsdorf

Prof. Andreas Linninger Akhil Jain

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Extraction

Table of Content

Introduction ___________________________________________________________ 3 Flows

heet _____________________________________________________________ 3 Process Operation ______________________________________________________ 4 Limitations ____________________________________________________________ 5 Applicability ___________________________________________________________ 6 Theory________________________________________________________________ 7 Theory Ternary Phase Diagram__________________________________________ 8 Theory - General Flow Sheet for Extractor Design ___________________________ 10 Properties ____________________________________________________________ 21 Costs ________________________________________________________________ 23 Case Study ___________________________________________________________ 25 Alternatives___________________________________________________________ 30 References ___________________________________________________________ 31

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Extraction

IntroductionExtraction is a process that separates components based upon chemical differences rather than differences in physical properties. The basic principle behind ext

raction involves the contacting of a solution with another solvent that is immiscible with the original. The solvent is also soluble with a specific solute contained in the solution. Two phases are formed after the addition of the solvent,due to the differences in densities. The solvent is chosen so that the solute inthe solution has more affinity toward the added solvent. Therefore mass transfer of the solute from the solution to the solvent occurs. Further separation of the extracted solute and the solvent will be necessary. However, these separationcosts may be desirable in contrast to distillation and other separation processes for situations where extraction is applicable.

Flowsheet

Figure 1. Extraction Flowsheet for an Extractor Column

A general extraction column has two input stream and two output streams. The input streams consist of a solution feed at the top containing the solute to be extracted and a solvent feed at the bottom which extracts the solute from the solution. The solvent containing the extracted solute leaves the top of the column and is referred to as the extract stream. The solution exits the bottom of the column containing only small 3


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Extraction

amounts of solute and is known as the raffinate. Further separation of the output streams may be required through other separation processes.

Process OperationThere are certain design variables that must be assigned in an extraction process. Operating Temperature Operating Pressure Feed Flow Rate Composition Temperature of entering stream Pressure of entering stream As in many separation processes, the pressure and temperature conditions play a large role in the effectiveness of the separation. In order for a good split of the feed the pressure and temperature must be such so as to ensure that all components remain in the liquid phase. The process will be adversely affected if one or more of the components areallowed to become a vapor, or the extraction may not occur at all if a large enough portion of a component is allowed to vaporize. In addition, the temperatureshould be high enough that the components are all soluble with one another. Ifextremes in temperature are present, finding a suitable solvent for extraction c

an be problematic. This is however generally not the case since one of the biggest benefits in the extraction process is that it can be done at ambient pressures and temperatures. In many applications, a separation process is desired wherean extreme temperature will destroy the desired product such as the pharmaceutical industry. For these applications, extraction is ideally suited, since the only temperature requirement is that dictated by the solubility. At this point thebiggest challenge would be finding a suitable solvent for the extraction. We canalso use the pharmaceutical industry in another example for the benefits of extraction and this has to do with the volumes involved for effective extraction. The extraction process can become very expensive if the solvent needed to be usedis costly these expenses can be contained if a batch process is being used andthis is often the case in medicines. In a non-batch process the solvent would need to be constantly supplied

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Extraction

and this would involve either a huge amount of solvent or another separation process in order to recycle the solvent.

LimitationsWe must consider the under what extremes extraction can be used as separation process. 1. Suitable Solvent [1] Solvent partially soluble with the carrier. Feedcomponents immiscible with the solvent. Solute is soluble in the carrier and atthe same time completely or partially soluble in the solvent. Different densities than the feed components for a phase separation to facilitate and maintain thecapacity of the extractor high. Extremely high selectivity for the solute for the solvent to dissolve the maximum amount of solute and the minimum amount of the carrier. Large distribution coefficient to reduce the theoretical number of stages contributing to a greater efficiency Low viscosity increases the capacity of the extraction column and does not allow for the settling rate of dispersion to be slow. Chemically stable and inert toward other components of the system Low

cost, nontoxic, and nonflammable 2. Equipment Interfacial tension and Viscosity High interfacial tension and viscosity leads to more power being supplied to maintain rapid mass transfer throughout the extraction process. Low interfacial tension and viscosity leads to the formation of an emulsion. 3. Temperature preferred to be higher since solubility increases, but temperature not higher than thecritical solution temperature. 4. Pressure for condensed system must be maintained below the vapor pressure of the solutions such that a vapor phase will not appear and interrupt liquid equilibrium.

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Extraction

5. Separation may only occur for compositions in the region between the feed composition and that apex of the carrier.

ApplicabilityWith all the key components in the design of an extractor system to be discussed, the equipment selection can be evaluated. We must determine which extractor would apply for the situation at hand. The specifications for each of these different systems are relatively the same. The following design constraints should beplaced on each system in order to optimize the individual process -- (1) maximize surface area of mass transfer, and (2) adjust flow feeds for maximum solute recovery. [2] In general, there are three main types of extractors to focus on: Mixer-settlers Mixer-settlers are used when there will only be one equilibrium stage in the process. For such a system, the two liquid phases are added and mixed.Due to their density differences, one phase will settle out and the mixture will be separated. The downfall to this type of extractor is that it requires a lar

ge-volume vessel and a high liquid demand. [1] Contacting columns Contacting columns are practical for most liquid-liquid extraction systems. The packings, trays, or sprays increase the surface area in which the two liquid phases can intermingle. This also allows for a longer flow path that the solution can travel through. In the selection of a packing, it is necessary to select a material that iswetted by the continuous phase. [1] Lastly, the flow in a column should alwaysbe counter-current. Centrifugal contractors Centrifugal contractors are ideal for systems in which the density difference is less than 4%. In addition, this type of system should be utilized if process requires many equilibrium stages. [2]In these systems, mechanical devices are used to agitate the mixture to increasethe interfacial area and decrease mass transfer resistance. [1]

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Extraction

Many different types of centrifugal contractors exist, and each has its own guidelines for operation and selection. A more detailed view can be found in. [1]Table 1. Advantages and disadvantages of the various liquid-liquid extractor typ

es [1].

Unit of Operation Mixer-Settler

Advantages Efficient Low head room Induces good contacting Can handle any numberof stages Small investment costs Low operating costs

Disadvantages Large floor High set-up costs High operation costs High head room Difficult to scale up from lab Less efficient than mixer-settler Difficult to separate small density differences Does not tolerate high flow ratios High set-upcost High operating and maintenance costs Cannot handle many stages

Columns (without agitation)

Columns (with agitation)

Good dispersion Low investment costs Can handle any number of stages Can separate small density differences Short holding time Small liquid inventory

Centrifugal Extractors

TheoryWhen talking about liquid-liquid extraction, liquid-liquid equilibrium must be considered. This is best represented by equating the chemical potential of both liquid phases:

iLI

= i

LII

(1)

This relationship reduces to an expression, which is dependent only on the liquid mole fractions and activity coefficients: xi iLI LI

= xi iLII

LII

(2)

We can use activity coefficient models, such as UNIFAC (UNIquac Functional-

roupActivity Coefficient), UNIQUAC (universal quasichemical), and NRTL (nonrandom

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Extraction

two-liquid) to determine the mole fractions. All three models above apply for liquidliquid equilibrium, it rolls down to which is easier to use and what properties we have available. For a multi-component system, the UNIQUAC equation for th

e liquid-phase activity coefficient is represented as follows: [3]ln = ln (combinator ial ) + ln (residual ) i i i

(3)

The combinatorial and residual activities are based on the statistical mechanical theory and allowed the local compositions to result from the size and ener

y differences between the molecules in the mixture. The relationships for these twoactivities are made available to us throu

h.

Theory Ternary Phase Dia

ramWe then are able to relate this data from the activity into a ternary phase dia

ram. Ternary phase dia

rams are unique in that they show all three components ofa reactor system on one plot. There are

eneral principles that

overn ternaryphase dia

rams, and those are the followin

: Sum of the perpendicular distancesfrom any point within the trian

le to the three sides equals the altitude of thetrian

le. Each apex of the trian

le represents one of the pure components. Anypoint of a side of the trian

le represents a binary mixture. Lines may be drawnparallel to the sides of the equilateral trian

le for the plottin

of the compositions.

Fi

ure 2. Phase dia

ram for a three component system. [2]

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Extraction

The ternary phase dia

ram may be constructed directly from experimental data. The saturation curve (miscibility boundary), represented by JDPEK in Fi

ure 2, canbe obtained experimentally by a cloud point titration. For example, a solution

containin

components A & C with some composition is made, and then component Bis added until the onset of cloudiness due to the formation of a second phase occurs. Then the composition is know for the mixture of the three components and can plotted onto the ternary phase dia

ram. [1] Tie lines are lines that connectpoints on the miscibility boundary. The tie lines may also be presented onto theternary phase dia

ram from an experiment. A mixture may be prepared with composition that of point H (40% A, 40% C, 20% B) from Fi

ure 2. If we allow it to equilibrate, then we can chemically analyze the final extract (E) phase and the raffinate (R) phase. [1] Point F is a feed composition into the extractor while point S is the solvent feed to the extractor. Point H represents the composition ofthe two feeds at equilibrium. This point is determined by summin

the feed (F)and solvent (S) compositions for each component. Points R and E are the composit

ions of the raffinate and extract from the unit, respectively, and the line between them forms the tie line. The tie lines move above and below this line basedon the relationship between the raffinate and the extract. Point P represents the plait point. At this point, only one liquid phase exists and the compositionsof the two effluents are equal. The curve represented by JRDPEK is the equilibrium between all three components. The area under the curve is the re

ion where two liquid phases will exist. Above the curve, there will only be one liquid phase. If a line is drawn from F to E or from S to R, this will represent the operatin

line. Althou

h this dia

ram is not the basic theory behind liquid-liquid extraction, it is helpful to review this procedure before continuin

with an in-depth discussion. In addition to the above-mentioned considerations, equilibrium constraints must be satisfied. This implies that = AE AR (4)

where is the activity coefficient for the solute A in the extract and is the activity AE AR coefficient of A in the raffinate. This condition is one of the mostimportant aspects of

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Extraction

liquid-liquid extraction since it allows for calculations and assumptions that based on equilibrium systems (e.

. the ternary phase dia

ram). One considerationto be made is for the separation factor. We want this factor to as far away from

unity as possible. This leads to a better separation in the extraction process.The separation factor is represented as follows: [4]

y = E xR*

(5)

One of the last essential points to the theory

ehind liquid-liquid extraction is mass transfer. The driving force for this mass transfer arises from the concentration difference of the solute in each of the solvents. In general, it is assumed that the system is at an equili

rium state when mass transfer is occurring.Solute fluxes in the raffinate and extract can

e expressed as N = KE (xEi - xE)

A N = KR (xRi - xR) A

(6)

(7)

where KE and KR are the overall mass transfer coefficients, A is the cross-sectional area, xE and xR are the concentrations of solute in the extract and raffinate respectively, and xEi and xRi are the concentrations of solute in each phaseat the liquid-liquid interface.

Theory - General Flow Sheet for Extractor DesignWith the key components of liquid-liquid extraction discussed, the following gen

eral flowchart can

e utilized for almost any process. Figure 3 (a) illustratesa general ternary diagram for a desired solute (C), an extracting solvent (B) and a carrier solvent (A). In this process, depicted in Figure 3 (

), we will assume that the feed (F) contains components A and C. A solvent (S) is introduced insuch a way that it will extract C from the feed. The raffinate composition (R)is specified with respect to the recovery of C that is needed. Ta

le 2 summarizes the components, flows, and unknowns of such a system. The following steps can

e utilized to determine the extract composition and the num

er of stages neededfor most liquid-liquid extraction pro

lems.

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Extraction

(a)

(

)

Figure 3. (a) A general ternary phase diagram using for designing an extractor,and (

) a general process diagram relating the ternary phase diagram to physicalmeaning.

Ta

le 2. Ta

le summarizing the general extraction phase diagram and process diagram in Figure 3.

Stream F S

Components A and C B

E R

A with large C concentration B with small C concentration

Is the composition given? Yes Yes, usually pure or relatively pure component B No, determined from calculation Yes, recovery amount needed of solute C from design specifications

Is the flow rate given? Yes No, determined

y calculation No, determined from component mass

alance No, determined from component mass

alance

where: is the carrier solvent A is the solvent used to extract a certain B component is the component that is to

e extracted from C A

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Extraction

Step 1: Determine the minimum solvent-to-feed ratio (S/F)min. This calculation needs to

e completed

ecause E1, the extract composition, needs to

e found. This procedure

egins

y drawing an operating line from S to R that extends

eyond

the

oundaries of the diagram. Next, each tie line is considered to

e a pinch point, and a line drawn from each tie line to the operating line is designated aP1, P2, ,Pn. The Pi farthest away from R is called Pmin. After Pmin has

een esta

lished, a line is drawn from Pmin, through F (the feed composition), and to the other side of the equili

rium curve.

Figure 4. Sample Ternary Diagram used to calculate Pmin for Step 1 of the general procedure for designing an extractor.

This point will represent E1. Figure 4 represents a general ternary diagram fora Pmin calculation. After E1 is known, a mass

alance around the system can

e utilized to determine the mixing point (point M in Figure 5). This is completed

y saying that:

F + Smin = R + E1 = M

(8)

Solving for Smin/F, we will o

tain the minimum solvent-to-feed ratio asS min ( x A )F ( x A )M = F ( x A )M ( x A ) S

(9)

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CHE396 Senior Design

Extraction

Step 3: Find operating point. The operating point is a graphical point that represents the difference in the overall flow; in addition, it is merely a point forwhich calculations are computed around on a graph. Draw a line connecting the s

olvent (S) and raffinate (R) points on the diagram. Follow this line beyond thediagram to the left and right this is the operating line. Draw a line connectingthe extract (E) and the feed (F). The point at which these two lines intersect(P) is the operating point. Figure 6 depicts such a diagram for this calculation.

Figure 6. A ternary phase diagram depicting the procedure for determining the operating point (P) and number of stages for an extraction column.

Step 4: Calculate the number of stages. Following the tie line from point E to the other side of the equilibrium curve will give the composition of an intermediate raffinate stage. Another operating line is drawn from the operating point, t

hrough this intermediate point, and ends at point E This is a stage of the system. This . procedure should be repeated until stages have been constructed to R, the raffinate composition. Figure 6 shows this procedure for a general case.

Step 5: Calculate unknown flowrates. Since the extract and raffinate flows havenot been specified up to this point, this would be an appropriate level at whichto be this. This involves an overall mass balance on individual components. Forthe sake of generality, choose A and B. It follows that

xAF(F) +xAS(S) = xAR(R) + xAE(E)

(11)

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Extraction

xBF(F) +xBS(S) = xBR(R) + xBE(E)

(12)

where the xA and xB are the fractions of A and B for the specified streams, s s and F, S, R, and E are the flow rates of the feed, solvent, raffinate, and extract. R and E are the only unknowns, and they can be solved for by a simple system of equations. Step 6. Determination of Extraction Column Diameter [1] The diameter of the column must be large enough to permit two phases to flow counter

curren

tly through the column without flooding. Estimation of column diameter for liquid liquid contacting devices is far more complex and uncertain than liquid

vapor c

ontactors due the larger number of important variables. Variables necessary forcalculating extractor column diameter include: Individual phase flow rates Density differences between the two phases Interfacial tension Direction of mass transfer Viscosity and density of continuos phase Geometry of internals Column diame

ter may be best determined through scale

up of laboratory test runs. The necessary experimental data are obtained by: Use laboratory or pilot plant test unit with system components of interest. Use laboratory or pilot plant test unit with adiameter of one inch or more. Measurements of superficial velocities in each phase are made. The sum of these velocities may be assumed to hold constant for larger scaled

up commercial units. The superficial velocity data will be used to calculate the column diameter through the following correlation derivation. The following notation is utilized in the correlation derivation: uD = Actual averagevelocity of the dispersed (droplet) liquid phase uC = Actual average velocity of the continuous liquid phase UD = Superficial velocity of the dispersed liquidphase UC = Superficial velocity of the continuous liquid phase = Volume

raction

o dispersed liquid phase in column D ur = Average droplet rise velocity relative to the continuous phase C = Capacity Parameter

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Extraction

CD = Drag Coe icient M = Density (volumet

ic mean) D = Density of dispe

sed phase

C = Density of continuous phase f{1- } = Factor which accounts or hindered risin

g e ect o other D droplets u0 = Characteristic rise velocity or a single droplet = Viscosity (subscript will determine component) = Interfacial ten ion ( ub cript will determine component) AC = Column cro

ectional area DT = Column dia

meter g = Acceleration due to gravity MD = Ma flowrate of the di per ed pha eMC = Ma flowrate of the continuou pha e

Figure 7. Counter-current flowof di

per

ed and continuou

liquid pha

e in a co

lumn.

Diameter Calculation Procedure Step A Determination of Column Total Capacity Figure 7 illu

trate

lower den

ity liquid droplet

ri

ing through the den

er downwa

rd flowing continuou liquid pha e. The actual average velocitie of each compon

ent relative to the column wall are:

uD =

UD D

(13)

uC =

UC 1 D

(14)

The average droplet rise velocity relative to the continuous phase is the sum o these equations:

ur =

UD Uc + 1 D D

(15)

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Extraction

This relative velocity may also be expressed in terms o the orces acting uponthe droplet including drag

orces, gravitational

orces, and buoyancy

orces. Th

ese variables are combined into one parameter called C:

4d p g (16) C= 3C D I the droplet diameter dp is not known C may be obtaineugh a correlation provided in Seader [1] equation (6-42), which was developed through experimental data

rom operating equipment. Taking into account density an

d rising e ects o other components the relative velocity may be expressed as:1 D 2 (1 )2 {1 } u r = C C (17) D D C F om expe imental data,hat the ight-hand of the equation may be exp essed as: 1

u = u 0 (1 D )

(18)

Eliminating the relative velocity by combining equation (17) and (18) gives:

UD UC + = u 0 (1 ) D 1 D D

(19)

This equation is a cubic in . A graph oUD/u0 vs. may be generated at some D D

value oUC/uo. This graph represents the holdup curve

or the liquid-liquid ext

raction column. A typical value o UC/uo may be assumed 0.1.

Figure 8. Typical holdup curveor liquid-liquid extraction

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Extraction

At ixed UC, an increase in UD results in a increased value o holdup until theD

looding point is reached at the maximum o

Figure 8:

D U D

=0 UC

(20)

On the other hand, with UDixed, UC may be increased until the

looding point i

s achieved at:

U C D

=0 UD

(21)

Inserting these derivatives into equation (19) results in the ollowing expression or at looding conditions. The subscript denotes looding: D

U 1 C + 8 3 U () = D U 4 C 1U D

.5

(22)

Apply derivatives of Equation (19) into Equation (22), the expression solved sim

ultaneously resulting in Figure 9 for the variation of total capacity as a function of phase flow ratio:

Figure9. Total Capacity vs. Phase flow ratio

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Extraction

The total capacity may be read directly from the figure for a given phase flow ratio and will be essential for calculating the column diameter. The phase flow ratio is found by:

U C MD C = UD M C D

(23)

Step B Dete mination of Cha acte istic Rise Velocity The dimensionless quantity[(u0CC)/()] may be assumed to be app

oximately 0.01, as found by (Olney). The

efo

e t

he cha acte istic ise velocity fo a single d oplet may be exp essed as:

uo =

.01 () C C

(24)

Step C Dete mination of the supe ficial velocities at 50% of flooding value

The column extacto

should be ope

ated at 50% of the flooding velocity fo

best

pefo

mance. The sum of the supe

ficial velocities is found by

eading the tota

l capacity f om figu e 9 and multiplying by the cha acte istic ise velocity then divide the quantity by two:

(UC +

UD )50%Flooding

(UD + U c )f u0 = 2

(u 0 )

(25)

Step D Dete mination of the Total Volumet ic Flow ate The total volumet ic flow ate is a function of the mass flow ates:U U Q total = D + C D C

(26)

Step E Dete mination of Column C oss-Sectional A ea The c oss-sectional a ea isthe total mass flow ate divided by the sum of the supe ficial velocities at 50%of flooding: AC = Q Total (Uc + Ud )50%Flood (27)

Step F Dete mination of Column Diamete The column diamete may be found f om the c oss-sectional a ea:

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CHE-396 Senio Design 4 A 2 DT = c 1

Extraction

(28)

Ste 7. Determining the Height of the Column [1] HETS (Height Equivalent to a Theoretical Stage) will be considered since it can be a lied directly to determine column height from the number of equilibrium stages. For a well designed and efficiently o erated column, ex erimental data suggest the dominant hysical ro

erties influencing HETS are: Interfacial tension Phase viscosities Density difference between hases HETS is best estimated by conducting small-scale laboratoryex eriments with the systems of interest to determine the diameter of the column as discussed in ste 6. These values are scaled to commercial-size column by assuming that the HETS varies with the column diameter raised to an ex onent, which may vary from .2 to .4 de ending on the ty e of system. For the general a roximation in ste A, the ex onent is arbitrarily set at 1/3. Figure 10 lots HETS

for columns and rotary contactor

Figure 10. HETS as a function of diameter vs. interfacial tension Height Calculation Procedure Ste A - Find Value of HETS/DT1/3

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Extraction

Using Figure 10 to determine the value (x) of HETS/DT1/3 at a s ecified interfacial tension for the com onent system. Ste B - Solve for HETS The value of (x) is known from above as well as the column diameter:

HETS = (x )D T1/ 3

(29) Ste C - Determine the Height of the Column The total height of the columnis derived from the number of equilibrium stages derived in Ste 4 multi lied bythe HETS: Total Height = (HETS)(Number of Equilibrium Stages) (30)

To com are calculated results to the erformance of several ty es of extractor column (Seader) has rovided average values of HETS and the sum of su erficial velocities (see Table 3). Table 3. Performance of Several Ty es of Column Extractors Extractor Ty e Packed Column Sieve-Plate Column Rotating Disk Contactor KarrColumn 1/HETS, (m-1) 1.5 2.5 .8 1.2 2.5 3.5 3.5 7.0 UD+Uc, (m/hr) 12 30 27 60

30 30 40

Pro ertiesThe following are a artial list of the needed hysical ro erties in liquid-liquid extraction se arations. It is by no means com lete, other ro erties will beneeded for some of the calculations, and es ecially those needed to size the diameter of the column. It is however com lete as it relates to the described theory. Tem erature lays a smaller role in extraction than in other se aration rocesses. It is only de endent u on the tem eratures of the streams fed into the column. There is not a heating requirement for the rocess and H of mixing i

gener

ally inignificant. For the

e rea

on

, extraction can be regarded a

an i

otherm

al proce .

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CHE-396 Senior De ign

Extraction

Pre ure al o play only a mall role in extraction. When combined with the temperature con

ideration

it i

only nece

ary that the mixture remain in the twoph

a e liquid region. The fact that extraction proce e can be run at i othermal a

nd i obaric condition i quite beneficial to the pha e tability of the y tem.Pha e tability from a thermodynamic tandpoint i temperature and pre ure dependent and

ince the

e are not changing the

tability of the pha

e

will not cha

nge. Activity coefficient are the mo t important phy ical property in the extraction proce . The rea on for thi i that the e are u ed to determine the mi cibility of the olute in both of the olvent involved. While there are many different equation

available to determine a particular activity

ome are better tha

n other for our purpo e . When working with liquid-liquid y tem the NRTL andthe UNIFAC model are the mo t accurate in predicting the activitie of the liquid involved. Although better than uch predictive model uch a Van Laar or Margule

they

till fall

hort of perfection. Once a predictive model ha

been plo

tted on a diagram it will mo t likely be nece ary to fix the exact equilibrium

line experimentally for the mo

t accurate data. The activity coefficient

al

o determine the partition factor which will determine whether or not a goodeparat

ion ipo

ible. Vi

co

ity i

a property that cannot be overlooked, it

pre

ence

appear in two different area , flooding and choice of equipment. Flooding i aphenomenon that can occur in extraction ju t a it can for other unit operation

we will encounter. Vi

co

ity i

al

o valuable in the determination of what typ

e ofy

tem to u

e for extraction. Component

having a high vi

co

ity cannot be

u ed in pray or packed column .

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Extraction

Co t Several economic trade-off exi

t for the de

ign of an extraction proce

. The to

tal co t of the proce will be directly related to the key extraction de ign va

riable and type of extraction equipment utilized. The following i a brief analy i of everal de ign variable that effect the economic balance: At a fixed olvent feed ratio, the amount of

olvent extracted increa

e

with increa

ed numbe

r of tray . Thu , the value of the unextracted olute may be balanced again t the co t of the extraction equipment required to recover it. For a fixed extent ofreaction, the number of tage required decrea e a the olvent rate or refluxratio increa

e

. The capacity of the equipment nece

ary for handling the large

r liquid flow mu t increa e with the larger reflux rate. Thu , the co t of the equipment pa e through a minimum when the minimum number of tage are utilized. A reflux ratio and olvent rate are increa ed the extract olution become more dilute. Therefore, the co

t of

olvent removal i

increa

ed a

well a

the o

perating co t for increa ed utilitie .

Aa re

ult of the

e economic balance

the total annualized co

t (inve

tment & o

perating cot

) mu

t pa

through a minimum at the optimum

olvent reflux rate.

Further co t mu t be con idered for the recovery of the aturated raffinate product a well a the extract. Co t model have been developed for the variou type

of extractor de

ign. The following are model

for a column type extractor, mix

er-ettler, and continuou

centrifugal extractor. Column Type Extractor Dougla

document co t correlation for column in general. [5] The capital co t refer to the purcha e co t plu the in tallation co t of the column:M & S 1.066 .802 Capital Co t of Column $ = H (2.18 + Fc ) 101.9D 280

(

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Where D = diameter (ft), H = height (ft) Fc = cot factor = Fp (pre

ure co

t ef

fect ) + Fm (material co t effect ) (The e value may be found in Dougla )

Depending on the column extractor type, tray or packing internal may be u ed.Dougla

ha

upplied a co

t model for the purcha

e and in

tallation of the

e mat

erial ba ed upon correction factor to the following model:M & S 1.55 Capital Co t of Internal $ = 4.7D H Fc 280

(32) The correction factor ithe

um of the correction factor

for the

pacing,

internal type, and internal material. [5] The total capital co t i the capitalco t of the column plu the capital co t of the internal . To annualize thi inve tment, pecify a payback period n, and divide the total capital co t over thi

time period:

Capital Co t of Column + Capital Co t of Internal Total Annualized Capital Co t

=n

(33) Operating cot

add to the at Total Annualized Co

t (TAC). The operating co

t : .04$ M & S Electicity = kWhr 600 20000$ M & S Labor = Wointenance = Yr

(34,35,36) including the utilitie , labor, and maintenance co t . Pratt ha e timated the e co t a of 1977. Inflation of the e value may be accounted for u ing the M&S index: Therefore the Total Annualized Co

t for a column type extractor

i: Total Annualized Co

t (TAC) = Total Annualized Capital Co

t + Operating Co

t (37)

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CHE-396 Senior De ign Mixer-Settler Type Extractor

Extraction

Woode

tabli

he

a co

t model for mixer-

ettler extractor which include

: carbo

n teel mixer- ettler, labor and maintenance, explo ion-proof motor, drive, pipin

g, concrete, teel, in trument , electrical, in ulation, and paint: De ired Capacity M & S Capital Co t = Reference Co tReference Capacity 600 apacity M & S Capital Co

t = 14.8

103 10

103 600

.70 n

(38,39) The de ired capacity mu t be pecified in term of Mgal/yr. The e capital co

t mu

t be added to the operating co

t a

de

cribed in the column extractor

ection. Continuou Centrifugal Extractor Wood e tabli he a co t model for continuou centrifugal extractor ba ed upon a centrifugal extractor made of 316 tainle teel including flexible connector , explo ion-proof motor, variable peed driver, in

trumentation, pump

, labor, and maintenance:

Capital Co t =

.58 3De

ired CapacityM & S

51

103 600

2.2 10

(40) The deired capacity i

in unit

of Mgal/yr. A

above, the capital co

t mu

t be added to the operating co t a defined in the column extractor ection.

Ca e StudyQue tion for Liquid-Liquid Extraction An extractor i to be de igned uch that acetone will be extracted from a feed mixture of 30% acetone and 70% ethyl acetate. Water will be u ed to extract the acetone, and the water i a umed to be pure. The raffinate will have a compo ition of 7% acetone and 93% ethyl acetate (point B), while the extract will have a compo ition of 12% acetone, 8% ethyl acetate, and 80% water (point D). A ternary pha e diagram i given for thi

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Extraction

y tem, along with corre ponding tie line . The feed to the column ha a flowrate of 20,000 kg/hr and the

olvent-to-feed ratio i

a

umed to be 1.75. Determine

: The number of tage needed for thi problem The olvent, extract and raffinate

flowrate The height and diameter of the column The co t of thi trayed column

D

S

Solution to Liquid-Liquid Extraction Cae Study The number of

tage

needed for

thi problem The number of tage can be tepped-off in a fa hion analogou to that pre ented in the Theory Section of thi paper. the operating line i drawn from S to B and i extended to the left of the diagram. Another line i drawn fromF to D and inter

ect

the operating line from S to B thi

i

the operating poin

t. The tie line from point D i followed to the other ide of the equilibrium cu

rve. From thi

point, another operating line i

drawn back to the operating point. The point at which iline inter

ected the extract

ide of the equilibrium cu

rve ilocated, and the tie line i

drawn back to the other

ide of the curve to

obtain another point from which to draw another operating line. The total number of tage i four once all the equilibrium line have been drawn.

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CHE-396 Senior De ign The olvent, extract and raffinate flowrate

Extraction

Since the bai

behind extraction proce

e

i

ma

con

ervation -- a

it i

wit

h any proce a material balance i done around the proce . Thi i analogou t

o the ma balance done in the Theory Section (Feed + Solvent = Extract + Raffinate). In addition, we recall that S/F = 1.75. From the de ign pecification , it follow

that

S = 1.75 F

S = 1.75 F = 1.75(20,000 kg/hr) = 35,000 kg/hr

After determining S (the olvent feed rate), we can complete the nece ary material balance to olve for R (the raffinate rate) and E (the extract rate): F+S=E+R Balance on acetone: 0(S) + (0.30)(20,000) = (0.07)(R) + (0.12)(E) Balance onwater: 0(F) + (1.0)(35,000) = (0)(R) + (0.80)(E) Thi

yield

: E = 43,750 kg/hr R

= 10,714 kg/hr It i een that thi method doe not give a 100% ma balance. T

hi

i

accounted for by a

uming that the graphical method i

not a

accurate a

one would like, but it practical for our deign purpo

e

. The height and diamet

er of the column The firt

tep i

to e

tabli

h all nece

ary phy

ical propertie

. Much of thi information i available in Lange Handbook of Chemi try. Sub criptD i the di per ed liquid pha e (organic) and ub cript C i the continuou liquid pha

e (inorganic). Acetone = 791 kg/m3 Ethyl Acetate = 789 kg/m3

Use the o ganic phase composition data f om above to find density of the o ganicphase:

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CHE-396 Senio DesignO ganic = (.3)(791) + (.7)(789) = 789.6 kg/m3 Ino ganic = 1000 kg/m3 = .001955 lbf/ft C = .000021 lbf

Extraction

Calculate the pha e flow ratio from the ma flow rate and den itie

kg kg 20000 1000 3 UD MD C h m = = kg kg UC M C D 35000 789.6 3 h

Using Figue 9 and the phase flow

atio find the total column capacity

(UC +

UD )f

u0

= .34 (D-less)

Dete

mine the cha

acte

istic

ise velocitylbf g . 2104 . .01 001995 .01() ft cm 3 uo = = lbf s g C C 1 .000021ft s

Calculate supe ficial flooding velocity (UD + UC)f = (.34)(.19988 ft/s) = .067959 ft/s Calculate supe

ficial velocity at 50% of flooding velocity (UD + UC)50% F

looding Velocity = (.067959 / 2)ft/s (3600 s/min) = 122.327 ft/hDete

mine the

total volumet ic flow ate: kg kg UD UC 20000 h 35000 h 35.3146ft 3 = = + + m3 D C 730.5 h

Q total

Dete mine c oss-sectional a ea: ft 3 2130.5 h = ft 122.327 h = 17.4165ft 2

AC =

Q Total (Uc + U d )50%Flood

Calculate the diamete of the column:

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CHE-396 Senio Design17 4A 2 4 .4165 2 DT = c = = 4.70ft 1 1

Extraction

To find the height of the column use figure 10 with an interfacial tension of 29

.15 dyne/cm. This results in a value of 6.4 for the y-axis. HETS/DT1/3 = 6.4 Solve for HETS using the determined column diameterHETS = 6.4 D T1/ 3

= 6.4 4.71/ 3 = 10.728ft

Thus, the column height is: Total Height =(HETS)(Number of Equilibrium Stages) =(10.728)(4) = 42.9ft

Extraction Column DimensionsDiameter, ft Height, ft The cost of this trayed column Using Douglas [5] correlat

ion for ca

ital costs:1061 1.066 42.9.802 = $40,965.50 101.9 4.7 280 1061 1.55 Tray Cost $ = 4.7 8411.32 280

4.70 42.9

Column Cost $ =

(

)

(

)

Assuming a ayback eriod of six years, n = 6 Total Annualized Ca ital Cost = ($40,965 + $8411) / 6 years = $8229.33 / yr. The o erating cost of electricity, labor, and maintenance are negligible in contrast to the o erating cost associatedwith solvent recovery through further se arations. Further, introducing a new material into the rocess to extract a solute result in new material cost and isa function of solvent recovery. Total Annual Cost = $8229 + o erating costs + solvent & roduct recovery cost

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Extraction

AlternativesIn the case where extraction does not a ly for a certain feed, we must considerour other o tions. Alternatives to extraction include the following: 1. Distill

ation

Extractive Azeotro ic Reactive

2. 3.

Crystallization Adsor tion

In discussing distillation, we must evaluate under what conditions extraction would be valid over distillation. Extraction is referred to distillation for thefollowing reasons: [1] Case of dissolved or com lexed inorganic substance in org

anic or aqueous solutions Removal of a com

onent

resent in small concentrations A high-boiling com onent is resent in relatively small quantities in a waste stream Recovery of heat-sensitive materials, where extraction may be less ex ensive than vacuum distillation Case of se aration of a mixture according to chemical ty e rather than relative volatility Case of the se aration of close-melting or close-boiling liquids, where the difference in solubilities can be ex loited Case of mixtures that form azeotro es If the situation does not meet any of the above reasons, then distillation can be considered. For exam le, if the boiling

oints of two com onents where not close, then distillation would be referred over extraction. Using crystallization over extraction, one would have to considerthe difference in the freezing oints of the com onents and also have information for a solid-liquid hase diagram. This diagram is necessary to determine theeutectic oint, which is the oint where one com onent becomes fused into the ot

her.

30


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Extraction

Liquid adsor tion could be used for certain com onents by contacting a liquid mixture with a orous solid. The solid acts as the adsorbent and must be insolublein the liquid mixture. There is no available theory regarding adsor tion equili

brium curves; however, ex

erimental data at a fixed tem

erature is used for

lotting curves

References1. Seader, J.D. and Henley, E.J. Se aration Process Princi les. John Wiley and Sons. New York, 1999. 2. Strigle, R.F. Packed Tower Design and A lications. GulfPublishing Com any. Houston, 1994. 3. Sandler, S.I. Chemical and Engineering Thermodynamics. John Wiley and Sons. New York, 1998. 4. Treybal, R.E. Mass Transfer O erations. McGraw-Hill. New York, 1980. 5. Douglas, J.M. Conce tual Design ofChemical Processes. McGraw-Hill. New York, 1988. 6. Hanson, C., Baird, M.H.I.,Lo, T.C. Handbook of Solvent Extraction. John Wiley and Sons. New York, 1983. 7.Reid, R.C., Prausnitz, J., Poling, B. The Pro erties of Gases and Liquids. 4th

Ed. McGraw-Hill. New York, 1987.

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7009722 liquid liquid extraction

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