separation processes, introduction

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Separation Processes, Introduction 915

Separation Processes, IntroductionC. Judson King , University of California, Berkeley CA94 720, United States

1. Introduction . . . . . . . . . . . . . . . . . . 9152. Denitions . . . . . . . . . . . . . . . . . . . 9153. Process Congurations . . . . . . . . . . . 9164. Separation Principles . . . . . . . . . . . . 9185. Advantages and Disadvantages

of Different Separation Methods . . . . . 918

6. Selection of a Separation Process . . . . 9197. Factors Inuencing Phase Equilibria . . 9208. Sources of Equilibrium Data . . . . . . . 9209. References . . . . . . . . . . . . . . . . . . . 921

1. Introduction

Separation processes are used to convert feedmixtures into two or more products differingin composition. Unit operations with diffusionalseparation processes suitable for feeds that arehomogeneous mixtures:

EvaporationDistillation and RecticationSublimationLiquid – Liquid ExtractionAbsorptionAdsorptionProcess-Scale ChromatographyBiochemical Separations (mainly diffusionalprocesses)

These contrast with mechanical separationof heterogeneous mixtures, in which the prod-uct phases are already present on a microscale.

These are discussed in the following articles:FiltrationCentrifugation, FilteringCentrifugation, SedimentingHydrocyclonesSedimentationDust SeparationScreeningElutriationAir Classication

Mineral SortingMagnetic SeparationElectrostatic SeparationGravity ConcentrationDense-Media SeparationFlotation

Separation processes constitute more thanhalf of the total equipment investment for thechemical and fuel industries. They are alsowidely used in pharmaceutical and food indus-tries, in beneciation of mineral ores and recov-ery of metals, in processing efuents, and in adiverse array of other industries. Separation pro-cesses may have a number of purposes, whichcan be loosely categorized as follows:

1) Purication: Removal of impurities, therebyenabling a desirable substance to be obtainedat a higher level of purity; e.g., rening of sugar and treatment of drinking water.

2) Concentration or Recovery: Increasing theconcentration of a desired substance in so-lution, usually by removal of a substantialfraction of solvent; e.g., production of fruit juice concentrates and recovery of metal val-ues from efuents.

3) Fractionation: Separation of desired sub-

stances from oneanother; e.g., primary distil-lation of crude oil and chromatographic sep-arations.

The rst two categories imply separation of asmall amount of one substance from a largeamount of another. The desired products are themajor and minor constituents, respectively, inthe rst and second processes.

2. Denitions

Most separation processes are based on the prin-ciple that different phases of matter have dif-ferent compositions at equilibrium. Examplesare vapor and liquid phases in distillation, and



916 Separation Processes, Introduction

immiscible liquid phases in liquid – liquid ex-traction. These processes achieve separation byallowing the phases to proceed toward equilib-rium, hence the name equilibration processes.

Another category, known as rate-governed processes , achieves separation by means of dif-ferences in rates of transport of different speciesthrough a medium or barrier. Here the prod-ucts are probably fully miscible with the feed;the separation would be destroyed by remixingthem. Examples are ultraltration and gaseousdiffusion.

A separating agent is added to achieve sep-aration. In equilibration processes, this agent

serves to create the second phase. The separat-ing agent takes the form of energy or matter.Examples of energy separating agents are re-boiler heat, which forms the vapor in distillation,and chilling or refrigeration, which causes ice toform in freeze concentration processes. Exam-ples of mass separating agents are an absorbentused to separate gases and an ion-exchange resinused to desalt water. Typically, most of the util-ities costs for a separation process are associ-ated with the separating agent, e.g., the cost of steam to drive the reboiler of a distillation col-umn or the costof regenerating a mass separatingagent for reuse. Economics and environmentalrestrictions usually dictate that a mass separat-ing agent should be regenerated and reused. Aseparation process based upon a mass separat-ing agent thereby requires a second, subsequentseparation process.

The degree of separation achieved betweentwo substances in different products relates to

the separation factor between them. For equi-libration processes, the separation factor α ij isusually dened as the ratio of the partition coef-cients ( K i and K j ) of the two species betweenthe two phases at equilibrium ( α ij = K i /K j ). Theconstants K i and K j are the partition coefcientsof species i and j, which are dened as the con-centration of a particular species in phase 2 di-vided by that in phase 1. Any concentration unitsmay be used for K i and K j in either phase, pro-vided the same units are used for both species ina given phase. Phases 1 and 2 are usually chosenso that α ij is greater than unity.

3. Process Congurations

Often, thespecies being separated is separated toan insufcient degree by a simple equilibrationor by a single passage through a barrier. In thesecases, the degree of separation between speciescan be improved by staging or countercurrency .Examples of each are shown in Figure 1. Bysending the product from one contacting or stageto another and appropriately recycling interme-diate products to previous stages (Fig. 1 A), thedegree of separation between species in the ul-timate products can be improved considerably.For a distillation operated at total reux (in-

nitesimal feed and product ow), the separa-tion factorforproducts from a singleequilibriumstage is α ij , as already noted, whereas that from N equilibrium stages joined countercurrently isα

Nij (the Fenske equation).

Continuous countercurrent contacting can beused to accomplish the same effect, as shown inFigure 1 B. Here, two phases ow countercur-rent to each other through an open material suchas a structured packing, which serves to createinterface for mass transfer between phases. Theaction is analogous to that of a countercurrentheat exchanger, where the exit temperature of the hot stream can be lower than the exit tem-perature of the cold stream. Sometimes contac-tors are built with elements of both discretelystaged and continuous countercurrent contact-ing, for example, rotating disk contactors usedfor liquid – liquid extraction.

Another way of improving the degree of sep-aration achievable in a simple contacting is the

chromatographic mode. Here a mobile phaseows along a thin, stationary phase.The thinness of the stationary phase provides

rapid rates of mass transfer, giving the effect of many stages or “transfer units” per unit length of the stationary phase. In elution chromatography,components of a mixture injected as a pulse atthe inlet end of the contactor proceed along thestationary phase at different rates, determined bythe different equilibrium distributions betweenphases, and emanate as a succession of isolatedpeaks (Fig. 2). Field-ow fractionations are rate-governed processes operated in an analogous,elution-chromatographic mode.




Figure 1. Staging and countercurrencyA) Staged mixer – settler system for solvent extraction; B) Continuous countercurrent extractor.

Figure 2. Typical output from a gas or liquid chromatograph(peak height, as sensed by any of various detection methods,vs. elution time).

A continuous staged or countercurrent con-tactor provides high feed capacity but generatesonly two products of high purity. (Intermediateproducts,or sidestreams,of lesserpuritycanalsobe present.) The elution-chromatography modecan separate a complex mixture into many purepeaks or products, but its inherent capacity islow because the feed is injected intermittentlyas pulses.

When a solid feed is used (e.g., leaching of roast and ground coffee to form coffee extract ora solid mass separating agent is employed (e.g.,an adsorbent), movement of the solids shouldgenerally be avoided because attrition and unde-

sirable mixing may result. Fixed-bed processesare commonly used (e.g., the home water soft-ener lled with ion-exchange resin or a bed of activated alumina for drying air) when a solidfeed or mass separating agent is used. A regen-erated xed-bed separation can handle a sub-




stantial feed rate but in its simplest form pro-vides only one highly puried product, the ini-tial efuent, before a second component leavesthe bed. Among the more notable innovations inindustrial separation during the past few decadeshave been effective ways of simulating counter-currency with xed beds.

Yet another fundamentally different ap-proach to separation involves migration of dif-ferent species to equilibrium positions within agradient eld. One example is isoelectric focus-ing. These processes are inherently slow but cangive separations that are not possible by othermeans.

4. Separation Principles

For a feed of any particlular phase condition,an equilibration separation process can be basedupon formation of, or contact with, any immisci-ble second phase of matter. For a liquid feed, thesecond phase may be a gas (stripping), an im-miscible liquid (extraction), or a solid (crystal-lization, adsorption). Equilibration may be with

the bulk of the second phase or with a surface.Surface-based separation processes include ad-sorption, as well as foam, bubble, and emulsionfractionation.

For rate-governed processes, differences inany form of transport can be exploited. These in-clude rates of permeation through a solid mem-brane, Knudsen diffusion in a porous medium(as used for separation of uranium isotopesin UF 6 ), thermal diffusion, electrophoresis, orpressure diffusion.

Often, two separation principles used in con-cert can operate synergistically. Examples arethe use of cross elds for rate-governed separa-tion and the enhancement of relative volatilityin a distillation by adding a substance that mod-ies the equilibrium between phases, as done inazeotropic and extractive distillation.

5. Advantages and Disadvantages of

Different Separation Methods

When the same separation factors can be at-tained, equilibration processes using energyseparating agents tend to be less costly thanthose using a mass separating agent, because

of the need for circulating and regenerating themass separating agent or else disposing of andreplacing it. Similarly, when staging or counter-currency is needed, equilibration processes tendto be more attractive economically than rate-governed processes that give an equivalent sepa-ration factor, because equipment and energy canbe utilized more efciently in the former.

Separation processes involving a solid phasecansufferdrawbacksassociated with low or van-ishingly small transport rates in the solid phaseand with the desirability of keeping the solidphase stationary.

Certain methods of separation are better

suitedforcertain rangesof feed concentration. Inparticular, xed-bed operations are most effec-tive for removing relatively dilute solutes, sincethe diluteness necessitates less bed volume andless frequent regeneration. Thus, adsorption andion-exchange processes tend to be used to re-cover solutes from relatively dilute feeds or toremove impurities. Along this spectrum of feedconcentration, extraction (with liquids) and ab-sorption (with gases) are usually considered fora middle range of concentration, because the sol-vent ow rate required tends to be relatively in-dependent of the feed solute concentration. Op-erational upper limits on feed solute concen-tration can come from the need to keep liquidphases immiscible in extraction or to avoid toolarge a percentage decrease of the ow rate of the feed phase. Distillation works well over awide range of feed concentrations but can expe-rience low stage efciencies at very low soluteconcentrations.

Membrane separation can provide high se-lectivity for removing or concentrating (ultra-ltration) high molecular mass or macromolec-ular solutes, for removing salts from water orconcentrating them (reverse osmosis, electro-dialysis), and for fractionating solutes of highand low molecular mass (dialysis). Economi-cal, high-capacity membranes that effectivelyremove polar organic solutes of lower molec-ular mass have yet to be developed, but are anarea of research. The solute throughput capac-ity and selectivity provided by membranes canbe enhanced by impregnation of the membranewith an appropriate extractant (solid-supportedliquid membranes, facilitated transport). Mem-brane processes are best suited for low molar so-lute concentrations, because providing the driv-




ing force for transport across the membrane isotherwise difcult and expensive.

Cost and Scaleup. Different methods of sep-aration have inherently different costs. An in-verse relationship tends to exist between thevalue of the product and the scale of production.For high-value substances produced on a smallscale, a much wider range of separation tech-niques can be considered than for substances of lower value. Often the cost or value of a sub-stance is inuenced strongly by the difculty of separation; therefore, for a high-value substance,utilization of a newer or less common means of separation may be necessary in order to perform

the desired separation at all. Furthermore, cer-tain separation processes (e.g., distillation, ex-traction) can be scaled up more readily than oth-ers. Those methods that rely on very thin phasesor thin ow channels (chromatography), laminarow (eld-ow processes), or ready dissipationof heat (electrophoresis)are particularly difcultto scale up to large capacities.

Thermosensitive Product. In many cases, thefeed or products are sensitive to thermal degra-dation, contamination, or changes due to achange in the chemical environment, such asdenaturation of proteins. These constraints areparticularly common in the food and pharma-ceutical industries. Here, methods of separationthat avoid these problems will have an advan-tage. To avoid thermal degradation, low temper-atures and short residence times should be used.Processes that avoid heating the feed (extraction,sorption, crystallization, etc.) are advantageous;if vaporization must be used, operation under

vacuum is helpful. Concerns about contamina-tion and chemical environment limit the typesof mass separating agents that can be used. Forinstance, in bioprocessing, many solvents havetoxic effects, so precipitation and the use of solidsorbents are relatively advantageous. Similarly,proteins may denature if they are taken into anonaqueous phase.

Processes Involving Reversible Chemical Re-actions. Most industrial separations are carriedout by processes that do not involve chemical re-actions, because of the consumption of reagentsrequired to accomplish the reaction, the cost of reagents needed to regenerate the original de-sired substance, and the need to dispose of un-wanted reaction products. However, chemicalcomplexation or association reactions (donor –

acceptor, chelation, clathration, etc.) are muchmore readily reversible and can be used advanta-geously in separations to increase the selectivityamong solutes or the capacity for a desired so-lute. Reversible chemical interactions can be im-plemented in extraction; sorption; ion exchange;azeotropic and extractive distillation; impreg-nated membranes; and foam, bubble, and emul-sion fractionation. Finally, the common pro-cesses for concentrating a solute in solution byremoval of solvent (evaporation, reverse osmo-sis, ultraltration, freeze concentration) incurexpense in proportion to the amount of solventthat must be removed. Extraction or sorption of

the solute(s) of interest can lead to concentra-tion as well, because only a limited amount of the solvent will accompany the solute into oronto the extractant or sorbent. Reversible chem-ical interactions can be used effectively here aswell.

6. Selection of a Separation Process

In seeking one or more appropriate methods of separating a particular mixture, the rst consid-eration is the size of the separation factor likelyto be attained by differentmethods of separation.Differences in volatility (distillation, evapora-tion, stripping), solubility (crystallization, ex-traction, absorption), charge-to-mass ratio (ionexchange, electrophoresis), molecular size andshape (adsorption with molecular sieves, crys-tallization, gelpermeation,clathration, dialysis),and chemical reactivity can all be used for sep-arations.

Next, whether ordinaryor extreme conditions(very high or low pressure, very high or lowtemperature, etc.) are needed to obtain attractiveseparation factors must be determined. Methodsthat require excessive temperature – time com-binations or result in contamination or chemicalchange may be ruled out by the nature of the feedor product. Also, as already noted, the value of the substance and the desired scale of operationcan determine the number and types of alterna-tives to be considered, because of the varyingcost of different separation methods and theirsuitability for scaleup. Finally, a very practicalconsideration is the amount of previous experi-ence with a particular process.

Among near equals, distillation has an advan-tage because it avoids solids, it is easy to stage




and scale up, and a vast backlog of experienceexists. Rate-governed processes are usually con-sidered seriously only when the desired separa-tion can be achieved in a single stage. Separa-tion of uranium isotopes by gaseous diffusion isa notable exception to this generalization, how-ever. With processes utilizing a mass separat-ing agent, ease of regeneration becomes a dom-inant factor. Finally, as already noted, xed-bedprocesses gain a compensating advantage whensubstances at relatively low concentration are tobe removed.

7. Factors Inuencing PhaseEquilibria

Several general factors inuence phase equilib-ria and can be utilized in the logic of selectingprocesses and choosing mass separating agents.A generalized equilibration separation processmay be regarded as having a source (feed) phaseand a second, receiving phase. For the solute(s)of interest to move in the desired direction, thechemical potential of the solute in the receiving

phase must be less than that in the source phase.The partition coefcient K i for a solute bet-

ween two bulk phases is independent of soluteconcentration when ideal solutions exist in bothphases or, in most instances, when the solute isvery dilute in both phases. The partition coef-cient is simply the ratio of the activity coefcientin the source phase to that in the receiving phase,if the activity coefcients and the partition co-efcient are based upon the same concentration

units (moles or mass per volume, weight frac-tion, etc.). If preferential interactions occur bet-ween solute and solvent [dened as the majorcomponent(s)] in either phase, the distributionof the component toward that phase will be en-hanced. Similarly, if less afnity exists betweensolute and solvent molecules than between sol-vent molecules themselves, the solute will bedriven toward the other phase.

The values of K i for many phase-distributionprocesses are dominated by the nonidealities inone of the two phases. This is particularly true forprocesses that remove nonpolar or low-polaritysolutes from aqueous solution in which theaque-ous phase is highly nonideal (very large activ-ity coefcients) and the receiving phase is muchmore nearly ideal. Thus relationships among

many organic compounds based upon a singleset of chemical parameters related to dispersionforces, polarity and polarizability, and acidityand basicity, serve to correlate such seeminglydiverse partitioning properties as solubility inwater [2], adsorption from water onto activatedcarbon [3], and extraction from water into 1-octanol [4]. All of these situations are dominatedby aqueous-phase nonidealities.

Dimerization or polymerization of thesolute,or formation of solute molecules into micelles inone of the phases, serves to increase the distribu-tion (i.e., increase K i ) of the solute in that phaseat higher solute concentrations.

Another common equilibrium feature is thesaturation of a phase. In adsorption processes,the nite amount of solid surface area presentprovides an upper limit on the amount of solutethat can be taken up within the capacity of theselective layer adjacent to the surface. The sameis true of gas – liquid surface capacity in foam orbubble fractionation. In these cases, the partitioncoefcient toward the surface phase decreasesas saturation is approached. This is another rea-son why surface-based separation processes aremore useful when the solute to be removed ispresent in low concentration.

A similar situation exists where chemical re-actions are involved (e.g., ion exchange, re-versible chemical complexation). The reactantin the receiving phase has a limited capacity,in that the stoichiometry of the underlying re-action(s) cannot be exceeded. Thus, a solventphase containing a reactive organic extractantof molecular mass 400 at 40 wt % in an organic

diluent can at most take up 5 wt % of a soluteof molecular mass 50, with which the extrac-tant forms a 1:1 complex. Complexation of addi-tional solute molecules with a reactant moleculeengenders additional uptake.

8. Sources of Equilibrium Data

Many measurements of phase-equilibriumproperties exist, but they are widely scattered inthe literature. For a particular system, a searchof Chemical Abstracts may be the most usefulapproach. Complications of data are given in [5–8] for vapor – liquid equilibria; [9] for liquid–liquid equilibria; [10] and [11] for solubilitiesof gases and solids in liquids; [12] and [13] for

separation processes, introduction

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