Fluid Separation

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Process Engineering Guide: GBHE-PEG-MAS-600

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Process Engineering Guide: Fluid Separation CONTENTS Page 0 INTRODUCTION/PURPOSE 3 1 SCOPE 3 2 FIELD OF APPLICATION 3 3 DEFINITIONS 3 4 A SEPARATION LOGIC TREE 3 5 METHODS OF DISTILLATION 4

5.1 Fractional Distillation 4 5.2 Azeotropic Distillation 7 5.3 Extractive Distillation 8

6 LIQUID-LIQUID EXTRACTION 9 7 OTHER COMMERCIAL METHODS OF SEPARATION 11

7.1 Adsorption 11 7.2 Fractional Crystallization 12 7.3 Ion Exchange 12 7.4 Membrane Processes 13

7.4.1 Ultrafiltration 13 7.4.2 Reverse Osmosis 13 7.4.3 Pervaporation 14 7.4.4 Liquid Membranes 15 7.4.5 Gas Permeation 15 7.4.6 Dialysis 16 7.4.7 Electrodialysis 16

7.5 Supercritical Fluid Extraction 16 7.6 Dissociation Extraction 17 7.7 Foam Fractionation 18 7.8 Clathration 18 7.9 Chromatography 19



8 OTHER METHODS OF SEPARATION 19

8.1 Precipitation 19 8.2 Paper Chromatography 19 8.3 Ligand Specific Chromatography 19 8.4 Electrophoresis 19 8.5 Isoelectric Focusing 20 8.6 Thermal Diffusion 20 8.7 Sedimentation Ultracentrifugation 20 8.8 Isopycnic Ultracentrifugation 20 8.9 Molecular Distillation 20 8.10 Gel Filtration 20



APPENDICES A AT A GLANCE CHART BASED ON FENSKE, UNDERWOOD 21 B A GENERALIZED y - x DIAGRAM 22 C TEMPERATURE - COMPOSITION DIAGRAMS FOR

AZEOTROPIC MIXTURES 23 D A TYPICAL y - x DIAGRAM FOR EXTRACTIVE DISTILLATION

(SOLVENT FREE BASIS) 24 E RAPID ESTIMATION OF LIQUID-LIQUID EXTRACTION

REQUIREMENTS 25 F LIQUID - LIQUID EXTRACTION - THE USE OF EXTRACT 26 REFLUX G SELECTIVITIES REQUIRED FOR EQUAL PLANT COSTS 27 FIGURE 1 SEPARATION LOGIC TREE 4 DOCUMENTS REFERRED TO IN THIS PROCESS ENGINEERING GUIDE 28



0 INTRODUCTION / PURPOSE

A beginner in the field of fluid separation can be overwhelmed by the apparent wealth of choice available. In reality that choice is limited. This Engineering Guide presents the options available with the intention of giving an overview of methods of separation. Knowledge of the basic concepts is assumed.

1 SCOPE

This Engineering Guide describes each method of separation and outlines the basic theory involved. For distillation and solvent extraction (the most widely used fluid separation techniques) a short cut method of design is given. One or two commercial processes for gas separation are arbitrarily included. A conscious effort is made, however unapparent to the casual reader, to call on experience to highlight important points and areas where caution should be exercised. Where appropriate, the implication of the separation technique on the total flowsheet is discussed. A bibliography of further useful reading material is given at the end of each Clause for the serious advocate. This Engineering Guide does not cover the process engineering design of fluid separation equipment.

2 FIELD OF APPLICATION

This Guide applies to the process engineering community in GBH Enterprises worldwide.

3 DEFINITIONS

For the purposes of this Guide no specific definitions apply.



4 A SEPARATION LOGIC TREE

Based on an appreciation that simple is best and what is best understood usually prevails, a separation logic tree can be proposed. This presents a stepwise procedure for selecting the separation method most likely to be accepted and is shown in Figure 1. Realistically, if distillation can be used then this is the preferred technique; the simpler the type of distillation the better. After distillation think of liquid-liquid extraction. Any other method is specialized and should be considered with an GBHE expert or the company offering the system. The purist will maintain that the separation method chosen will depend on feasibility and cost. However, a large monetary carrot is required to change from a conventional, totally satisfactory, established technique to any other method. Fractional distillation is favored because it is tried and trusted. It can be applied over a wide range of conditions (i.e. where vapor and liquid co-exist) provided that there is a difference in volatilities. Azeotropic distillation, extractive distillation and liquid-liquid extraction are more complex: another component is added to enhance the non-ideality of the mixture to be separated. These methods are usually employed when classes of components (e.g. paraffins from aromatics) have to be separated, or the system is heat sensitive, or maybe the operating pressure for fractional distillation would be very high or very low. The other commercial methods of separation have often been developed for a specific application. This has necessarily involved a large expenditure in development time and money. Not unreasonably their propagators try to widen their scope and applicability. In line with the aforementioned, this document concerns itself mainly with distillation in terms of how to reach the most appropriate system, with attention also being given to liquid-liquid extraction. An outline of other methods of separation, together with their general areas of applicability, is also given.



5 METHODS OF DISTILLATION

5.1.1 Fractional Distillation

Distillation involves the separation of the components of a liquid mixture by partial vaporization of the mixture and separate recovery of vapor and residue. To refresh memories relative volatility (a) is a direct measure of the ease of separation by a distillation procedure. Using normal nomenclature for a binary mixture A-B:



where K = vapor-liquid equilibrium constant

x = liquid mole fraction y = vapor mole fraction

and for ideal solutions:

where P° = vapor pressure of pure components The closer the value of a is to unity the more difficult the separation. In a simple system a knowledge of the boiling points shows whether the mixture would be easily separable. Boiling Point Difference Approximate

°C Relative Volatility 2 1.05 5 1.11 10 1.25 20 1.6 30 2.0 50 3.1 100 8.7

Knowing α, a feel for still requirements can be readily obtained using Fenske, Underwood, Gilliland or for α = 1.2 to 2.0 by use of Appendix A. For the operating optimum, remember to take Nmin x 2 (and Rmin x 1.3). This approach can be adopted for mixtures containing more than two components by using the key components concept. These only apply strictly to ideal systems (i.e. relative volatility does not change with composition, constant molal overflow). In practice many systems are non-ideal. To allow for deviations from ideality in the liquid phase the concept of activity coefficients was introduced, thus:

, where ال = activity coefficient



The majority of non-ideal systems (greater than 90%) exhibit positive deviations from Raoult’s Law. Systems in which the components are strongly associated, e.g. mixtures containing basic and acidic components, may exhibit negative deviations. Qualitatively the differences displayed between ideal systems and those exhibiting positive and negative deviations are apparent on considering a generalized y-x diagram, see Appendix B. The curves in Appendix B show that in non-ideal mixtures the greatest deviations from ideal behavior occur at high dilution. In practice it will, for example, be more difficult to obtain pure A, (the more volatile component) and easier to obtain pure B (the less volatile component), for a system exhibiting positive deviations than would be the case if the system behaved ideally. As on most occasions the concern is to produce a pure tops product, care should be taken with the design of columns embracing non-ideal systems. The Nmin requirement can often be 1.5 to 2 times that calculated assuming ideal behavior, and on occasions very much higher. Vapor phase non-idealities can usually be neglected at atmospheric and sub-atmospheric pressures. The aforementioned considers a single fractionation column, in real life multi-component systems and separation trains have to be considered. In general terms the distillation train should be designed to give the lowest total vapor boil-up rate. Two rules of thumb are of help in sequencing columns to arrive at this desired state of affairs, viz: (a) Favor the scheme in which 25 to 50% of the feed is removed as distillate; and, less importantly: (b) Do difficult separations last. In practice, another possibility which should be considered when addressing multi-component systems is the suitability of including side-stream operation. This is a very useful way of minimizing the total vapor rate required, and hence saving capital and energy, providing a pure product is not required. Normally, if it is more important to minimize heavy-ends content in the side stream product a liquid side stream would be removed above the feed. If it is more desirable to minimize light-ends in the side stream product a vapor (or liquid) side stream would be removed below the feed.



Another choice that has to be made is whether to use batch or continuous fractionation columns. Separation by batch distillation is widely used, especially in the Fine Chemicals area. Continuous distillation is the accepted operation in the petrochemicals and other large tonnage areas. Batch operation is usually confined to low rates of production (say, 3000 te/annum), where adjacent processing stages are batch operation and where there is the need for operational flexibility in a multi product unit. Attention may have to be given to components undergoing chemical reaction during the distillation operation. This can be allowed for if the reaction is an intention of the process. Difficulties can occur if reaction occurs at fractionation conditions and this likelihood was not recognized at the design stage. Other system properties which may detract from the performance of distillation columns are the deposition of solids or the tendency to foam. The addition of an anti-foam agent may offer a solution to the latter problem, though it is usually possible to design for a foaming system so that anti-foam is not needed. Behavior such as foaming is often not evident or is difficult to recognize in a laboratory or semi-technical simulation of the system. Although not strictly within the scope of this Engineering Guide, absorption is a technique widely used in separation trains. It is akin to distillation in that the absorption column is similar to that used in distillation, although not usually including a condenser or reboiler. Absorption is the removal of one or more selected components from a mixture of gases by absorption into a suitable liquid. The process is dependent on the differential solubility of the gas phase components in the liquid. It is usually necessary to remove the gas from the solvent by stripping in another column, either by pressure swing and/or increasing temperature. The following documentation may prove to be of further value:

(1) GBHE-PEG-MAS-607



(4) Distillation systems design procedure, GBHE Engineering Group.



5.1.2 Azeotropic Distillation

An azeotrope is a mixture of two or more liquid components which boils at constant temperature and distils over completely without change of composition. The ease of formation of a binary azeotrope is determined by:

(a) the magnitude of the deviations from Raoult’s Law;

and

(b) the difference in boiling points of the two components.

The smaller the difference in boiling points the smaller the deviations from Raoult’s Law (i.e. from ideality) required for azeotrope formation. Positive deviations from Raoult’s Law (1 < ال), by far the most common, can give rise to minimum boiling azeotropes. Negative deviations (1> ال) can result in the formation of maximum boiling azeotropes. Azeotropes can be classified as homogeneous (those which exist in one liquid phase and include minimum and maximum boiling azeotropes) and heterogeneous (those which exist as two liquid phases in equilibrium and are always minimum boiling). Heterogeneous azeotropes are characterized by large positive deviations from Raoult’s Law. Most azeotropic systems are of the minimum boiling type. Typical temperature-composition diagrams are given in Appendix C. By definition, Azeotropic systems are non-ideal and should from a design viewpoint be treated with care. However, there is nothing mystical about these systems and the azeotrope can in the most simple interpretation be considered as a pseudo-component. For example, there may be a requirement to separate a complex mixture, two of the components of which form an azeotrope. The azeotrope may be a key component in the system. Considering the azeotrope as a pseudo-component would allow Fenske-Underwood-Gilliland to be used. This would at least be a safe approach giving an over design (although sometimes a grossly over designed system). This approach can be significantly refined if account is taken of the azeotrope composition (see reference asterisked!*). The principal applications of azeotropic distillation, ie where an azeotropic agent is added to the system, are:



(1) In the separation of close boiling components, where the azeotropic agent forms a minimum boiling azeotrope with only one component, or if it forms binary azeotropes with two of the components in the system, one of the binary azeotropes boils sufficiently lower than the other;

(2) To facilitate separation of the two components in an already

existing binary azeotrope by formation of a ternary azeotrope. The ternary azeotrope should boil sufficiently below any binary azeotrope, and the ratio of the original components in the ternary azeotrope should be different from their ratio before the azeotropic agent was added.

The introduction of an azeotropic agent to facilitate separation often necessitates additional equipment and as a result a more complex separation train. Thus, if an ester is added to make the separation of water from acetic acid easier the condensed overheads will split into two phases. The ester-rich phase will be returned to the column as reflux. The water phase will contain solvent ester which should be recovered in an additional distillation column. Remember the composition of an azeotrope can be changed with increase or decrease in pressure. In general, the added azeotroping agent is best returned as reflux to the top of the column, usually in about 30 to 50% excess over that required to form the azeotrope. The following documentation may prove to be of further value: Swietoslawski, Azeotropy and Polyazeotropy, Pergamon Press, 1963. Kirk-Othmer, Encyclopedia of Chemical Technology, Wiley-Interscience, 3rd Edition, Vol 3, 352 (1978). Horsley, For azeotropic data, see: Azeotropic Data -III, Amer Chem. Soc., Advances in Chemistry Series 116 (1973).



5.1.3 Extractive Distillation

Extractive distillation is based upon the addition of an extractive agent to a mixture preferably containing two components of different chemical structure. The added agent enhances the deviation from ideality or activity coefficient of one component relative to the other, ie. it enhances the relative volatility. The resulting difference in volatility permits fractionation which may not have been economically attractive using fractional distillation. In some cases extractive distillation is used where classes of components are to be separated (eg paraffins from aromatics) where the boiling point spread could be such that fractional distillation in one column would be impossible. The extractive agent is chosen such that its volatility is low relative to those of the feed components. The solvent is always introduced above the fresh feed stage in order to maintain a high solvent concentration, which can be assumed to be constant throughout most of the column. Usually in extractive distillation, 40 to 90 mole % solvent is required in the liquid phase to maximize the difference in volatilities between the feed components. A typical y-x diagram, on a solvent free basis, is given in Appendix D. Vapor-liquid equilibria data are essential for design of an extractive distillation column. Solvent is not fed to the top stage because a few plates should be provided above the solvent entry point to reduce the concentration of solvent in the overheads to an acceptable level. Feed is usually introduced to the column in vapor form as liquid feed dilutes the descending solvent and reduces the solvent concentration in the bottom section. Reflux at the top of the column also dilutes the solvent, and increased reflux is not always synonymous with increased separation. In addition to being easily separable from the feed components the solvent should be:

(a) Completely miscible with the top product under top plate conditions, ie the solvent should not be too selective. The appearance of a second liquid phase gives an unwanted decrease in relative volatility.

(b) Incapable of forming azeotropes with the feed components in the extractive distillation zone.



The extractive distillation system requires at least two columns - the main extractive column and a second column to separate the extracted components from the solvent. The overall separation train may be even more complex if solvent has to be recovered from the overheads leaving the extractive column. This may require a further distillation column or a water wash system. Plate efficiency is often low in extractive distillation columns (25 to 35%). The following documentation may prove to be of further value: Perry, Chemical Engineers Handbook, 6th Edition, McGraw-Hill Book Company, 13-53 (1984).

6 LIQUID-LIQUID EXTRACTION

Liquid-liquid (or solvent) extraction involves the addition of an extractive solvent to the mixture to be separated, the solvent being partially miscible with at least one component, or class of components, in the mixture. The solvent is such that it is selective toward one component, that is it enhances the deviation from ideality or the activity coefficient of one component relative to another. In general the solvent is required to have a selectivity factor β, greater than 2 and a capacity or solubility for the component(s) to be extracted of not less than 10%.

Where الA, xA = mole fraction of component A in co-existing equilibrium

phases.

B, xB = mole fraction of component B in co-existing equilibriumالphases.

As β decreases, the number of extraction stages required for a given separation increases; as capacity decreases, the amount of solvent required increases. In practice, the choice of solvent is always a compromise, as β increases capacity normally decreases. Solvent selection is a critical design step that depends on the properties of the solutes to be recovered; there is no universally applicable solvent. A common approach in solvent selection is to carry out a literature survey of solvents used in similar applications. More erudite approaches may be based on hydrogen bonding tendencies or the determination of activity coefficients at infinite dilution by gas-liquid chromatography.



In liquid-liquid extraction, the partition (or distribution) coefficient gives an indication of the ease of separation. The partition coefficient is defined as:

where C is the concentration of solute in phase I and phase II, respectively. Ideally, K is independent of the concentration of solute and of the ratio of the two immiscible phases. If the required separation cannot be achieved using one extraction stage the generally favored mode of operation is in a counter-current system, where the solvent and feed travel in opposite directions. For such a system Appendix E allows a rapid means of estimating the number of theoretical extraction stages at a given solvent to feed ratio to achieve a required separation. However, even with an infinite number of stages the richest extract layer is that in equilibrium with feed. As a rule the richer the feed in extractable components the richer will be the equilibrium extract layer in these substances and the sharper the separation. The shortcomings of liquid-liquid extraction without reflux are therefore obvious, this applies particularly to a feed lean in extractable components. What cannot be achieved by increasing the number of stages can be accomplished by means of reflux. In a feed lean in extractable components the use of extract reflux would give a sharper separation. Extract reflux involves returning part of the extract phase from which the solvent has been completely removed (the solute) or partly removed. The concentration of solute in the extract layer is then greater than that in equilibrium with the feed. For example, Appendix F illustrates a solvent extraction process using a high boiling solvent for the separation of aromatics from non-aromatics. Part of the aromatic extract phase is returned to the extractor as reflux. In extractors operating with reflux the feed enters an intermediate point in the system. Reflux return should not give a completely miscible system. The use of reflux results in an increased energy requirement.



Three basic types of unit are available. (a) Mixer-settlers:

This is the name given to a type of extractor made up of a number of mixing and settling chambers connected alternately in series. These are normally only used when a few extraction stages are required. Scale up is good and they have a good turn down ratio.

(b) Packed and plate columns: Probably the most widely used for simple systems requiring a small number of stages and at low throughput. The system is good in that there are no moving parts. Some caution should be exercised in the scale up process.

(c) Mechanically agitated columns: As throughput and a larger number of stages becomes important, mechanically agitated equipment should be used. A variety of proprietary devices are available, including rotating disc contactors (RDC’s), the Kuhni extractor etc. These systems offer a lower height per theoretical extraction stage and also flexibility in terms of throughput and phase ratio.

In practice, liquid-liquid extraction is often used in conjunction with extractive distillation; for example in the recovery of aromatics from hydrocarbon mixtures. In this system the overheads from the extractive distillation column, consisting mainly of light paraffins and naphthenes, are returned as reflux to the solvent extraction column. They act as a backwash to remove heavier non-aromatics, which would be more difficult to remove in the extractive distillation stage. As in azeotropic and extractive distillation, the use of liquid-liquid extraction leads to a more complex separation train. A distillation column is required to remove the extracted components from the solvent. The raffinate may require water washing to recover small amounts of solvent present. Quite often a further small distillation column is required for clean up of a solvent purge. Ideally, the system would use as solvent a component already present in the process. The presence of minor contaminants, in particular surfactants, can have a major influence on the process. Before establishing final design a laboratory or semi-technical scale simulation should be carried out using the selected equipment, actual process streams and at the proposed operating conditions.



The following documentation may prove to be of further value: Treybal, Liquid Extraction, McGraw-Hill, New York, 1963. Reissinger, Schroeter, Modern Liquid-Liquid Extractors: Review and Selection Criteria, I Chem E Symposium Series No 54. The following Separation Processes Service reports (Harwell, Warren Spring): SAR1 Liquid-liquid extraction, June 1974. DR6 Selection of solvents for liquid-liquid extraction processes. Part 1 Organic systems, October 1978. An interesting paper of general applicability compares the selectivities in fractional distillation, extractive distillation and solvent extraction which are required to give equal plant cost. (Souders, Mott, Chem Eng Prog, 60, No 2, 75 (1964)). Although published in 1964 the findings presented in Appendix G are still of interest. The comparison assumes 67% solvent concentration and four times as much liquid in extractive distillation and in solvent extraction as in fractional distillation. Thus, for example, for the cost of separation to be the same the relative volatility in fractional distillation would be 1.5, that in extractive distillation 2.0 and the selectivity factor for liquid-liquid extraction would be 6.0.

7 OTHER COMMERCIAL METHODS OF SEPARATION

The following sub clauses 7.1 to 7.9 (inclusive) outline other methods of separation that have been used, or have obvious potential to be used, commercially. They may be worth considering for specialized purposes. The methods are not listed in any order of merit.

7.1 Adsorption

Adsorption is a physical phenomenon which occurs when gas or liquid molecules are brought into contact with a solid surface. There are two main categories of adsorption. The first type, and generally of primary interest, is known as physical or van der Waals adsorption where the interaction between the solid and the condensed molecule is relatively weak.



In this process the equilibrium between solid and condensed molecule is reversible and is rapidly attained when the temperature and pressure are changed. The second type is called activated adsorption or chemisorption where interaction is strong, the bonds formed being almost as strong as those in chemical compounds. This type of adsorption is often irreversible. Important adsorbents include activated carbon, aluminium oxide, silica gel and molecular sieves. The latter are widely used in commercial applications. Molecular sieves function as physical adsorbents, they are highly efficient, easily regenerable, crystalline silica-aluminas. They can facilitate separation by two mechanisms :

(a) conventional adsorption where the sieve shows a strong preference for polar compounds, particularly water;

and

(b) separation by size where only those molecules able to migrate through the sieve’s pore or window opening are retained. The pore size of a particular molecular sieve can be controlled accurately within a small range of molecular dimensions.

The adsorption process requires cyclic operation, adsorption being followed by desorption to recover the adsorbed species. The length of operating cycle determines the number of beds which should be used to allow continuous operation. Desorption is usually the most inefficient step in the cycle and can be accomplished by means of thermal swing, pressure swing, purge gas stripping or displacement cycles. Adsorption is a technique widely used to remove impurities from various process streams, for example in drying, sweetening and color removing operations. However, in the last quarter of a century, largely due to the availability of synthetic molecular sieves, adsorption has become established for specific bulk separations difficult to achieve by other means. In particular the separation of normal paraffins from admixture with other hydrocarbons (the vapor phase IsoSiv process and the liquid phase Molex process), the separation of p-xylene from other C8 aromatics (the Parex process) and the recovery of hydrogen from process gas streams.



The following documentation may prove to be of further value: Kirk-Othmer, Encyclopedia of Chemical Technology, Wiley-Interscience, 3rd Edition, Vol 1, 531, 563 (1978). Perry, Chemical Engineers Handbook, 6th Edition, McGraw-Hill Book Company, 16-5 (1984).

7.1.1 Fractional Crystallization

Crystallization processes make use of solid-liquid equilibria to effect a separation. A liquid mixture cooled past its freezing point produces a solid phase, different in composition from the mother liquor. Separating the two phases and remelting the solid phase gives a purified product. Numerous types of solid-liquid equilibria are known, the eutectic type is the most important industrially. In a eutectic system, if the solid could be perfectly separated from the liquid phase, 100% pure product could be produced in one stage. This is not achieved in practice because of mother liquor adhering to the crystal surface, or held in the crystal mass by surface tension and capillary forces, or being occluded in crystal imperfections. A conventional crystallization process would involve three stages:

(a) A crystallization stage where crystals are formed either in cooling tanks with agitation for long periods or in scraped surface chillers.

(b) A separation stage, usually effected by centrifuges or filters.

(c) A purification stage. Purification is usually carried out by using a wash liquor (chosen to be easily separable from the required component) or by continuous countercurrent treatment of the impure crystal mass with some of the melted crystal product (eg the Phillips pulsed column). Various continuous fractional crystallization devices have been proposed which claim to produce high purity product in a single piece of equipment. Although apparently proven on the laboratory and pilot plant scale they have not been used on large tonnage commercial plant.



Although heats of fusion are much lower than heats of vaporization, crystallization is an expensive technique and its use is consequently limited. It is applied where components having very close boiling points and similar chemical structure (usually isomers) are to be separated. The following documentation may prove to be of further value: Kirk-Othmer, Encyclopedia of Chemical Technology, Wiley-Interscience, 3rd Edition, Vol 7, 243 (1978). Separation Processes Service report (Harwell, Warren Spring), SAR2 (Reviewed), Industrial crystallization, March 1986.

7.1.2 Ion Exchange

Ion exchange is exactly what the name implies, an exchange of one ion for another. Ion exchange resins are insoluble electrolytes, consisting of a high concentration of polar or functional groups incorporated in a synthetic, resinous polymer. The polar groups may be acidic (cation exchange resins) or basic (anion exchange resins). The strongly acidic cation exchange resins contain, for example, the sulphonic group -S03H; the weakly acidic resins contain the carboxyl group -COOH. The latter only has a useful capacity in neutral or alkaline solutions. The strongly basic anion exchange resins contain the quaternary ammonium group - NR+30H-; the weakly basic resins contain amino (NH2), mono – and di-substituted amino groups. The latter can only usefully be used in neutral or acid solutions. Important factors which influence the rate and extent of an ion exchange process are the nature of the resin and the molecular size, valency, and concentration of the ions to be absorbed. Reduction in the particle size of the resin results in an increased rate of exchange, consistent with a diffusion controlled process. Ion exchange allows the removal of one or more ionic species from a liquid phase by means of an exchange, or transfer, for another ion. This transfer may be required to purify or modify the liquid phase, to concentrate, isolate and/or purify one or more of the ionic components, or to separate mixed ionic species into two or more fractions. The use of ion exchange resins for water treatment is widely recognized. Provided ions are present ion exchange processes can be carried out in aqueous-organic systems and in non-aqueous solvents.



The following documentation may prove to be of further value: Schweitzer, Handbook of Separation Techniques for Chemical Engineers, McGraw-Hill Book Company, 1979. Li (Editor), Recent Advances in Separation Techniques -II, A.I.Ch.E Symposium Series, Vol 76, No 192, 60 (1980). 7.4 Membrane Processes The use of membrane separation processes has been mooted for some considerable time. Advances have been made in some areas (noticeably gas separation) but wider application still awaits the development of membranes capable of giving high throughputs at high selectivities, whilst maintaining good chemical and physical stability. 7.4.1 Ultrafiltration Molecular filtration or ultrafiltration involves a sieving mechanism. The solvent permeates the membrane by Poiseuille type flow down microcapillaries within the membrane, and solute molecules are rejected because they are larger than the pores through which the solvent molecules can pass. Ultrafiltration can reject materials of molecular weight down to about 500. A range of membranes is available with molecular weight retentions from 500 to 300,000. Because of the high molecular weights osmotic pressures are low and operating pressures of only 5 to 60 psi g are required. In general, this technique is excellent for concentration and may also be used to separate two solutes (one able to pass through, the other held back by the appropriate membrane). The major problem with ultrafiltration is membrane fouling. Ultrafiltration is widely used commercially. For example, in biological applications, in the food industry (dewatering), the dairy product industry (cheese whey treatment), the pharmaceutical industry (concentration and separation on molecular weight basis) and in the purification and recovery of electrophoretic paints.



7.4.2 Reverse Osmosis Osmosis occurs whenever a solution is separated from its solvent, or from a more dilute solution, by a membrane and is the phenomenon of spontaneous flow of solvent through the membrane and into the more concentrated solution. In osmosis the membrane is normally considered to be semipermeable, that is it allows the passage of solvent but not the solute. The osmotic pressure of a solution is that pressure which has to be applied to the solution to stop the osmotic flow of solvent. Application of a hydrostatic pressure in excess of the osmotic pressure results in flow of solvent from the solution into the pure solvent. This process is called reverse osmosis (and on occasions hyperfiltration). Transport of material through membranes under reverse osmosis conditions is via a sorption diffusion process. The criterion for solubility is that like dissolves like. Rejection of solute occurs largely because of its low solubility in the membrane. Thus, it is the chemical similarity between diffusing species and membrane that determine the direction of separation. The rate of permeation is inversely proportional to the membrane thickness and is directly proportional to the difference between applied and osmotic pressure. As pressure has little effect on the solute rate, selectivity appears to improve as the applied pressure is increased. The process is usually operated at 400 to 600 psi (25 to 40 bar) pressure. Reverse osmosis is used commercially for desalination and for brackish water treatment (75% to 85% treated water recovery is normal in the latter). Salt rejection is high at 95 to 99% in water treatment. However, selectivity factors for, say, isopropanol-water separation using a cellulose acetate membrane under reverse osmosis conditions tumble to less than 2. In practice, there is still not a suitable membrane for the separation of low molecular weight organic mixtures. The technique is used by Organics Division for the concentration of Procion T-dyes. In the long term this technique may be developed such that it will find application in:

(a) Removing the bulk of a material away from trace impurities which may have unwanted effects, eg odor, color.

(b) Removing solvent from a homogeneous catalyst stream to allow catalyst recycle.



(c) Processing industrial aqueous effluents to renovate the water.

7.4.3 Pervaporation In the pervaporation process a liquid mixture is brought into contact with one side of a polymeric film, the downstream side being maintained at a low pressure. Separation is affected if one component permeates through the membrane at a relatively faster rate. The temperature at which the process can be operated is limited by the nature of the membrane material. It is preferred to heat the liquid feed mixture to increase the vapor pressure and thus the rate of permeation. The permeate is removed as a vapor. A variation on this technique is Perstraction in which the permeate is dissolved and carried away in a fluid diluent which does not interact with the membrane. Like reverse osmosis the transport mechanism in pervaporation is a sorption-diffusion process. Very simply it proceeds via:

(a) Solution of the permeating molecules in the membrane;

(b) Diffusion through the membrane;

(c) Evaporation from the downstream surface of the membrane.

Selectivity is, therefore, due principally to preferential sorption. The rate of permeation is inversely proportional to membrane thickness and is also dependent on temperature. Pervaporation can be considered complementary to reverse osmosis in that it is best suited to permeation of the component present at low levels in a mixture rather than the major component, as in reverse osmosis. The use of pervaporation has been proposed for more than a quarter of a century and small, 6m3/day, commercial units are now in operation. The main reported usages of pervaporation are in the dehydration of alcohols, ketones and ethers.



7.4.4 Liquid Membranes A liquid membrane, or more accurately a liquid surfactant membrane, is a film formed at an oil-water interface by a surfactant solution. Such films are formed by dispersing the solution to be separated in the form of very small droplets in a surfactant solution. The droplets covered with liquid membrane are then contacted with an organic solvent. One of the components of the mixture permeates through the liquid membrane at a faster rate than the other. The solvent therefore becomes richer in this component whilst the droplets become richer in the less permeable component. To increase drop stability and permeation rate the drop diameter is normally reduced by emulsifying the feed in the surfactant solution. The emulsion is then mixed with the solvent which is the continuous phase. Separation is achieved by selective diffusion of one component through the liquid membrane into the liquid of lower concentration. Once separation is effected, the three phases can be separated by first settling the emulsion and continuous phase and then breaking the emulsion. Liquid membranes are purported to have several advantages over solid polymeric membranes. They do not have pin holes, do not have to be replaced or repaired and require no mechanical support. However, as yet, there is no commercial exploitation of this technique. This is also true for the proposed supported liquid membrane configuration. In this system the liquid membrane is incorporated within an inert microporous support. Possible applications may be found in:

(a) Separation of species which are chemically different;

(b) Recovery of products from low conversion processes by permeation of components present in least amount;

(c) Removal of trace impurities, especially in waste water treatment.

7.4.5 Gas Permeation One area in which membrane processes are competitive at the present time is in the separation of gases. The most widely reported use is that of the Monsanto Prism separator for hydrogen purification. High pressure feed gas is supplied to one side of the membrane. Permeate accumulates on the membrane low pressure side.



Again mechanisms describing gas transport generally involve solubilization and diffusion. The diffusion rate depends on the size of the gas molecule and the gas solubility in the polymer, with gas partial pressure as the mass transfer driving force. For the Monsanto system where the active membrane is a polysulfone, the following is a list of 'fast’ and ’slow’ permeating gases: Fast gas Slow gas Hydrogen Oxygen Helium Methane, ethane etc Hydrogen sulfide Carbon monoxide Carbon dioxide Nitrogen Water vapor Ethylene, propylene etc In general, gas separation processes are good for enriching; say from 50% to 90%. The feed pressure should be greater than 150 psi (10 bar) and temperature in the range 10 to 50°C. They are not so good for obtaining a high purity product, say 99%, or at feed pressures below 150 psi (10 bar) and temperatures greater than 100°C. The polysulfone membrane is susceptible to certain aggressive gases, eg, methanol, ammonia, acid gases and aromatics. Gas permeation for hydrogen purification applications has successfully competed with established processes - cryogenics, pressure swing adsorption. The latter still has the edge where pure hydrogen is required. With industry gaining confidence from the commercial ventures, gas permeation technology could develop rapidly and other separations may become a reality on the large scale. 7.4.6 Dialysis Dialysis involves the use of a membrane which selectively separates the solutes in a solution by allowing the low molecular weight solutes to permeate through the membrane into the pure solvent. The membrane restricts the passage of high molecular weight solutes. At the same time, solvent will diffuse by osmosis in the opposite direction. By periodically replacing the fresh solvent, complete extraction of the diffusing solute can be achieved. Concentration gradients provide the driving force and the nature of the membrane establishes the selectivity in this diffusion process. The type of membrane determines whether dialysis, a two way flow, or osmosis, flow of solvent only into the concentrated solution, occurs.



The most widely known use of dialysis is in artificial kidney machines. The relative slowness of the dialysis process gives it little scope for industrial usage. 7.4.7 Electrodialysis Electrodialysis involves the transport of ionic species across a permselective membrane under the driving force of an electric gradient. Normally alternating anion and cation selective membranes are used. This allows the separation of ionic substances present in the feed stream. The largest commercial application for electrodialysis is in the treatment of brackish water to produce a potable water and in desalination generally. It is more economic than reverse osmosis at high salt concentrations. It is also used in the dairy and pharmaceutical industries. The following documentation may prove to be of further value: Torrey, Membrane and Ultrafiltration Technology, Noyes Data Corporation, 1984. Hwang, Kammermeyer, Membranes in Separation, Vol VII, Techniques of Chemistry, Wiley and Sons, 1984. 7.5 Supercritical Fluid Extraction Supercritical fluid extraction refers to the use of fluids which are gases at ambient temperature and pressure, but which become good solvents when compressed to supercritical fluids (at pressures above the critical pressure). The supercritical fluid region is loosely defined as being in the range of reduced temperatures (actual temperature divided by critical temperature) of 0.9<Tr<1.4, and reduced pressures of 1.0<Pr<5.0. The properties of a supercritical fluid are between those of a liquid and a gas. Fluids possess high solubilities similar to liquid solvents because of high densities (specific gravities of 0.2 to 0.9). Viscosities and diffusivities of fluids are intermediate to those properties for liquids and gases, this enables highly efficient penetration and rapid mass transfer compared to liquid solvents. In the supercritical fluid region relatively small changes in temperature and pressure produce large changes in density and hence in solvent power.



Carbon dioxide has been widely used as a supercritical fluid extractant as it is non-toxic, nonflammable, inexpensive and has a conveniently low critical temperature of 31°C. Other gases, or combination of gases, can be used depending on the extraction requirement. Supercritical fluid extraction is similar to liquid-liquid extraction and can be carried out using a countercurrent extraction system. The highly volatile solvent can be recovered by letting down the pressure and/or by distillation before recompressing and returning to the extraction system. The process requires capital-intensive, high pressure equipment which should be evaluated against any potential energy savings. Probably the two most quoted commercial applications of supercritical fluid extraction are:

(a) The Residual Oil Supercritical Extraction (ROSE) process used for deasphalting oil residues with pentane in the 1950’s;

And

(b) The decaffeination of green coffee beans using carbon dioxide introduced in the late seventies.

Other uses have been claimed including the separation of organic chemicals from water, oils from natural products and in polymer processing. The following documentation may prove to be of further value: Separation Processes Service report (Harwell, Warren Spring), SAR48, Supercritical extraction and other high pressure extraction processes, September 1983. Paulaitis et al, MIT Industrial Liaison Program, Report 9-33-82, Supercritical Fluid Extraction, April 1982. 7.6 Dissociation Extraction Dissociation extraction exploits the differences in the dissociation constants of the components of a mixture in order to effect a separation.



Typically, if a mixture of two weak organic bases (differing in their dissociation constants) in an organic solvent is contacted with an aqueous phase containing a stoichiometric deficiency of a strong acid, relative to the bases, then the bases will compete for the available acid. The stronger base, ie that with the higher dissociation constant, will react preferentially with the strong acid forming a salt in the aqueous phase. The weaker base will be consequently enriched in the organic phase. Products of high purity can be obtained if a multi-stage counter-current process is adopted, as in liquid-liquid extraction. A mixture of weak acids can be separated in similar fashion using a strong base. This separation technique has been used to separate isomeric mixtures which could not be practically achieved using distillation or crystallization methods. Such isomers often exhibit considerable differences in their dissociation constants, eg 3- and 4-picoline and meta- and paracresol. The following documentation may prove to be of further value: Hanson (Editor), Recent Advances in Liquid-Liquid Extraction, Pergamon, New York, Chapter 4 (1971). 7.7 Foam Fractionation Foam fractionation is dependent on the preferential concentration at the liquid-gas interface of a naturally surface-active molecule. This species can be separated from the bulk simply by providing sufficient interface and collecting the resultant foam. A surface-inactive material can be removed by complex formation with a suitable surfactant. Separation at the normal air-water foam interface is affected by numerous factors including:

(a) Concentration of the surfactant.

(b) pH.

(c) Temperature.

(d) Viscosity.

(e) Flow rates.



(f) Bubble size etc.

Foam fractionation is particularly useful when the concentration of the material to be removed is low. The technique has been considered for the removal/recovery of detergents, alcohols and phenol from waste water streams. The following documentation may prove to be of further value: Schweitzer, Handbook of Separation Techniques for Chemical Engineers, McGraw-Hill Book Company, 1979. 7.8 Clathration Clathrates (or adducts) are inclusion or cage-like compounds which can be considered as organic molecular sieves. Organic clathrates can trap other molecules in the cavities of their regular geometric structure. The resultant crystalline molecular complex is stable although normal chemical bonding is not present. The formation and dissociation of the complex can therefore be achieved with small changes in temperature and pressure. Different types of clathration agents have been identified. They exist in several forms ranging from spherical cavities, layer complexes, crystals with interconnecting chambers and tubular structures. The ability of clathrates to separate molecules on the basis of their shape has been used commercially. Urea adducts were used to separate normal from branched-chain paraffins. This separation is now carried out using synthetic zeolites (inorganic molecular sieves) which afford a cleaner, simpler to operate total system. The following documentation may prove to be of further value: Bhatnagar, Clathrate Compounds, Chand and Company (1968).



7.9 Chromatography Separations by chromatographic methods depend upon the different attraction which two alternative phases have for the components in a feed mixture. The two phases can be gas-solid, liquid-solid, liquid-liquid or gas-liquid. The latter is the system commonly used in analytical applications. In gas-liquid chromatography the mixture to be separated is vaporized and passed together with a continuous stream of nitrogen or other inert gas through the column. The column is packed with a solid impregnated with a non-volatile liquid. The stronger the attraction between this stationary liquid phase and the feed components, the more slowly is a given component swept through the column by the inert gas (or carrier gas). The mixture therefore separates into pure components as discrete bands within the column. These bands are eluted from the column by the carrier gas. Although widely used in the laboratory for analytical and preparative purposes, chromatography has found limited commercial application. This is largely due to the difficulty of obtaining good resolution in large diameter columns. The use of chromatographic techniques on an industrial scale has been proposed for the separation of close boiling mixtures (particularly isomers), the fractionation of natural products and the purification of pharmaceutical intermediates and products. The following documentation may prove to be of further value: Schupp, Gas Chromatography, Technique of Organic Chemistry, Vol XIII, Interscience Publishers (1968).

8 OTHER METHODS OF SEPARATION

The details contained in sub clauses 8.1 to 8.10 (inclusive) are included more for completeness than practical usefulness. Some of the methods mentioned are, and are likely to remain, laboratory oddities. Others have extremely limited usage and are not strictly within the scope of this Engineering Guide.



8.1.1 Precipitation

A chemical reactant is added to a liquid mixture and reacts with one of the components to form an insoluble precipitate.

8.1.2 Paper Chromatography

A liquid mixture is separated by differences in solubilities and adsorption potentials on paper (or gel phase).

8.1.3 Ligand Specific Chromatography

A liquid mixture is contacted with an immobilized ligand which forms a reversible chemical interaction with one of the components. 8.4 Electrophoresis An electrical potential applied to colloidal systems dispersed in buffered solutions in a cell causes the colloidal particles to migrate toward the electrodes according to their charge. 8.5 Isoelectric Focusing Carrier ampholyte mixtures are electrophoresed to establish a stable pH gradient. The amphoteric macromolecules eg proteins, to be separated migrate until they reach their isoelectric point; the pH at which the positive and negative charges balance. The separation is therefore on the basis of composition rather than size. 8.6 Thermal Diffusion Components of a homogeneous solution (gas or liquid) are separated by means of a temperature gradient. In a gas mixture the heavier molecules concentrate in the cold region, in liquids molecular shape determines the separation.



8.7 Sedimentation Ultracentrifugation The use of centrifuges rotated at high speeds that cause the rapid sedimentation of macromolecules and allow, for example, the separation of large polymeric substances according to molecular weight. 8.8 Isopycnic Ultracentrifugation Biological substances are separated in high rotation centrifuges in which a density gradient has been established. Use of the proper gradient material allows particulates to be banded together isopycnically in the density gradient. 8.9 Molecular Distillation In molecular distillation, the distance between evaporating and condensing surfaces is less than the mean free path of the molecules involved at the pressure used, normally high vacuum. 8.10 Gel Filtration The separation of components is effected by the difference in their molecular size and hence their ability to penetrate a swollen gel matrix.



DOCUMENTS REFERRED TO IN THIS PROCESS ENGINEERING GUIDE This Process Engineering Guide makes reference to the following documents: ENGINEERING GUIDES GBHE-PEG-MAS-601 VLE Data : Selection and Use (referred to in 5.1) GBHE-PEG-MAS-603 Shortcut Methods of Distillation Design (referred to in 5.1) GBHE-PEG-MAS-607 Batch Distillation (referred to in 5.1) .

Fluid Separation

Technology

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