1020
Ind. Eng. Chem. Res. 1990,29, 1020-1025
Research Needs in Liquid-Solid Separation. How Will New Systems Happen? Donald A. Dahlstrom Department of Chemical Engineering, University of Utah, Salt Lake City, Utah 84112
The dramatic changes in the practice of liquid-solid separation over the past 4+ decades are briefly reviewed in order to assist in predicting future needed improvements. Many changes have occurred in (1)materials of construction, particularly alloys and plastics; (2) basic new types of equipment; (3) size and scale of equipment; (4) strength of design; (5) increased productivity and temperature and chemical resistance; and (6) flocculants, filter media, and membranes. Further inventions and creativity in processing will still be needed because of the changes in industry, feedstocks, and products as well as completely new industries. These new developments will come from (1) new materials of construction, (2) still larger and stronger machines, (3) speed of operation increasing productivity, (4)the absolute need to reduce energy consumption, (5) special future processing requirements, (6) environmental regulations requiring better performance but a t reduced costs. By use of some very simple, old, but basic theories, technical areas are developed where these new inventions can occur in addition to the need for creative engineers. It is a distinct privilege to be asked to participate in a tribute to Hugh Hulburt. This is a man you don't forget for many reasons. His leadership, knowledge, concern, friendship, interest, and character are just a few of the words his life brings to one's mind. Unfortunately, I never had the opportunity to work with him at Northwestern University. I left that wonderful place sometime before he came. However, contacts at that university are "forever", so I did see his capabilities in action. Also, I know Hugh when he was at American Cyanamid Company because of the similar processing industries we dealt with. His work and those under him in minerals and phosphoric acid were certainly important to those industries, and his knowledge and experience were helpful to me. I t is with "sweet sorrow" to have known Hugh so well, while at the same time missing his friendship and advise. As a tribute to Hugh, I look into the future, as was his tendency, rather than presenting a rather narrow technical paper. For much of my career, liquid-solid separation was my intense interest. This was particularly on the applied side while working for the EIMCO Process Equipment Company, a large equipment manufacturer, directing research efforts. This started as a relatively small company in the process equipment business which was able to grow internationally with one or more manufacturing plants in every continent and where development was the result of applied technology, creative and hard-working people, and inspiring management. While my purpose is to look into the future of liquidsolid separation, a quick backward glance into the plast is desirable. My own work was basically 43 years long in that "unit operation", and the changes over that period were dramatic. After World War 11, the common materials of construction were cast iron, steel, and wood with lead lining for corrosive conditions. Both rubber covering and certain stainless steel alloys were just coming into usage in continuous filters and sedimentation equipment. Continuous filters were basically scraper discharge and string discharge drum type and the disk filter. A larger area drum filter was approximately 1000 ft2 and 1800 ft2 for disk filters. A 200-ft-diameter thickener was considered "big", and torque values for the driveheads were low. Filter media consisted of cotton and wool, and the operators were always fighting blinding. Synthetic fibers had not yet entered the filter market. Such things as synthetic flocculants did not exist (with the possible exception of the side effects of lime addition, whose main purpose was 0888-5885/90/ 2629-1020$02.50f 0
neutralization). Many processes were carried out with great effort because of the difficulty of dewatering of valuable solids in sludges or the clarification of liquids by sedimentation. Both problems involved very fine or colloidal solids and were aggravated by the relatively crude tools available to the operator. Liquid-solid separation was literally a "black art". Looking back, the change over the past 43 years is very dramatic. Various stainless steels and other exotic alloys are employed as materials of construction along with metals such as titanium, aluminum, and nickel. Elastomer linings are utilized in many applications. Molded plastics, particularly polyethylene and polypropylene, and other thermoplastics are employed, and filters of total synthetic materials are now manufactured, which yield a very wide application to many corrosive chemical environments. Steel is still employed when pH conditions permit, but many different grades are now available. Wood, lead, and cast iron have essentially disappeared. New types of continuous filters have been developed. Various types of filters forming their cake with gravity are available, which permit countercurrent washing of the solids with a minimum of freshwater (applicationsin the washing of digested cellulose use five stages of countercurrent washing, the amount of wash water being equal to that in the final discharge cake with greatly increased chemical recovery). This type filter has made possible the continuous manufacture of explosives because nitrated cotton linters can be handled safely with today's design and materials capability. The drum-type filter has had new discharge methods added. The continuous belt drum filter permits much better discharge of thin cakes with washing of the cloth to prevent blinding, resulting in increased filtration rates and solubles recovery. The roller discharge drum allows the filtration of fine clays (60-98% - 2 Ltm) at increased production rates per unit area. Even one of the oldest filters, the plate and frame filter, and modifications thereof, was allowed a rebirth largely due to synthetics. Practically all plates and frames are now molded plastic, which dramatically reduces weight, permitting much larger machines. In addition, automation permits downtime fm cake discharge to be less than 15 min even for the largest machines. Other machines have been developed that use the properties of flocculation to permit a hydrostatic head of only 1-2 in. to form a filter cake that then falls into a 0 1990 American Chemical Society
Ind. Eng. Chem. Res., Vol. 29, No. 6 , 1990 1021 sandwich of two high-strength special synthetic media which can be gradually increased in tension to dewater by expression. Expression forces can be greatly increased by applying pressure-loaded rollers to the sandwich. There are a large number of other batch and continuous filters that have been developed over the 43 years that represent highly specific but important applications, yielding improved cost and performance or changes to existing types of filters that permit wider applications. Equipment size and strength of design have also been dramatic. Continuous drum filters are made as large as 14 f t in diameter by 40 f t long (1760 ft2) and thickeners as large as 750 f t in diameter. Disk filters are now constructed up to 3300 ft2and with various agitation methods to make it possible to process solids of top sizes of about in. (coal). Because of the development of low-friction synthetics plus the ability to mold high-strength elastomers, continuous horizontal belt filters can be run over 60 m/min with a total filtration area of around 100 m2. With thickeners, there has been a dramatic increase in the amount of torque applied to the rotating rake mechanism. This has permitted the application of thickeners to very high dense underflow solids concentrations, permitting significant operating cost savings. Operating torque driveheads as high as 13.6 X lo6 N m (10 X lo6 f t lb) are available today. As additional tools, undoubtedly the development of synthetic or chemically modified natural flocculants have been very beneficial to liquid-solid separation. Colloidal solids can now be caused to form large flocculi that will filter far more rapidly or settle much faster. The result is that many liquid-solid separation steps are now possible that formally were too expensive, produced less than satisfactory results, or even could not be accomplished. Furthermore, this is achieved with the addition of only 0.5-1 ppm of total dilute solids feed for clarification or 0.05-0.15 kg of dry substance/metric ton of feed solids for larger solids concentrations. Flocculants are now available in anionic, cationic, and nonionic grades with molecular weights in the millions and different radicals and elements attached to the polymerized chain, permitting a very wide application of different flocculants. Various industries such as hydrometallurgy and certain chemical processes could not exist without them, and their importance in environmental controls is obvious. Filter media are practically all synthetic or special metal alloy weaves with a wide variety of finishes, weaves, etc. Temperature resistance has also been increased so that it is possible to perform liquid-solid separation steps at much higher values. Diatomaceous earth product quality has also been improved, and a greater number of grades are available to better match the problem. In addition, lower cost materials have been developed that perrmit precoat clarification at distinct savings where applicable. From this brief discussion, it should be apparent that there has been a dramatic improvement in liquid-solid separation for the process industries. There has literally been an order-or-magnitude jump in capability over the past 4+ decades. In addition, there is an improved understanding of liquid-solid separation as well as a better applied theory for continuous and batch filtration as well as sedimentation. It certainly is a distinct improvement over the highly empirical and rudimentary test methods of prior years, so that it is now out of the “black-art” accusation. Yet it is apparent that much further progress must still be achieved in liquid-solid separation. This may seem
somewhat extreme after all of the dramatic improvements claimed in the previous text. However, new products and technologies are being developed, processing requirements are increasing in complexity, and economic savings and productivity growth are needed to stay competitive within today’s global marketplace. As an example, two recent reports entitled “Separation and Purification-Critical Needs and Opportunities”, published in 1987 by the National Research Council (Judson King, Chairman), and “Frontiers in Chemical Engineering-Research Needs and Opportunities”, published in 1988 by the National Research Council (Neal R. Amundson, Chairman), indicate those areas where substantial potential growth opportunities exist and scientific and technical development is critical. These areas were stressed as high-priority research areas. From Separation and Purification, these areas were (1) generating improved selectivity among solids in separations, (2) concentrating solutes from dilute solutions, (3) understanding and controlling interfacial phenomena, (4) increasing the rate and capacity of separations, (5) developing improved process configurations for separations, and (6) improving energy efficiency in separation systems. From Frontiers in Chemical Engineering, the areas stressed were (1) biotechnology and biomedicine, (2) electronic, photonic, and recording materials and devices, (3) microstructured materials, (4) in situ processing of resources, (5) liquid fuels for the future, (6) responsible management of hazardous substances, (7) advanced computational methods and process control, and (8) surface and interfacial engineering. More than a simple majority of these areas will require improved and/or lower cost liquid-solid separation methods. Thus, they will become the “mother of necessity” for dramatic improvements in the future. Where must improvements be made? Looking backward, it is relatively easy to predict several areas. (1) Because of the emphasis on material science and engineering today, manufacturers should be able to take advantage of new alloys, composites, and other developments to either permit new application and/or reduce the cost of construction. The need to operate at higher temperatures than possible today may also be solved by improved materials. This could again result in economic savings. (2) The building of still bigger machines may be in the “law of diminishing return”, so this is not a high-priority need. Yet there are certain areas, particularly in mineral processing, where size limitations exist and larger machines could be beneficial. (3) As will be discused, speed of operation increases are necessary, which of course would increase productivity. However, new developments and understanding the limitations in the present state of technology will be necessary if this is to be achieved. (4) Thermal drying of solids is a very expensive processing step. To eliminate 1 lb of water from an input temperature at 68 O F with the water vapor exiting at 212 OF would usually take about 1800 BTU/lb of water. This is based on an 80% overall thermal efficiency with an input moisture to the dryer of 10-20%, with the solids having a specific heat of 0.25 BTU/(lb O F ) . Thus, if energy costs $5/million BTUs, a ton of water eliminated will cost $18 for thermal energy alone. When other factors such as labor, maintainance, availability, power, and overhead are considered, it is apparent that this is an expensive step. As energy costs are sure to increase in the near future, thermal energy costs of $10-20/million BTUs could be
1022 Ind. Eng. Chem. Res., Vol. 29, No. 6, 1990
predicted by the year 2000. The use of mechanical energy to replace thermal energy holds real potential for reducing liquid content in dewatering and drying and will furnish a real impetus to reduce costs in water removal. ( 5 ) Special future processing needs will require and justify new liquid-solid separation methods. One fairly obvious example is biological processing in the coming years. Even today, liquid-solid and solid-solid separation steps involving very fine solids require a high percentage of the processing costs. All must be carried out by very clean and sterile methods, and cells cannot be broken in some cases so as to prevent contamination. To realize the full potential of biological processes, liquid-solid separation steps will require a very dramatic improvement. Membrane filtration has been widely used in separations required in biological processing. This method can be divided into three categories: (1)microfiltration, (2) ultrafiltration, and (3) reverse osmosis, sometimes called hyperfiltration. Microfiltration is used down to 0.06 pm (600 A), while ultrdiltration is employed as low as 15 A. Reverse osmosis, of course, goes down to extremely small sizes. All of these three use membranes where the pores are of the required diameter for the separation required. Synthetics are employed for the membrane and must be cast into the final product which must have the proper pore size throughout and also be flexible. In biological processing, many times it is desirable to separate one type of organic material from another on the basis of particle size. Thus, membranes offer one of the best possibilities for doing this at these very small particle sizes. All of these three types of membrane filtration have had significant development over the past approximately 20 years. Unquestionably, one of the major ones has been the manufacture of new membranes of improved synthetics so that appreciably longer life would be experienced even though relatively harsh environments were involved. Consistent pore size has also been developed. Another development has been the crossflow principal of filtration wherein a relatively higher velocity is impressed on the feed stream across the membrane. By this method, the buildup of a layer of partially concentrated solids is avoided or the length of time between cleaning of the membrane surface is significantly increased. Undoubtedly, new membrane developments will be needed in the future to improve filtration rate per unit area as well as resist blinding. Other emphasis will be placed on process methods to improve performance. One important development I have seen recently is the special use of laser technology to perforate a sheet of a particular synthetic. Any hole diameter can be made of any radius, and it will be perfectly round. Furthermore, the hole can be tapered if desired so that hole diameter increases with depth. The number of holes per unit area can also be made at a wide density range. This development could well improve separation by particle diameter, resistance to blinding, and filtration rate. Another example of new processing needs in a completely different industry is the removal of liquid in dewatering of "red mud" in the processing of bauxite to produce A1203,the intermediate for the production of aluminum. These very fine and difficult to dewater solids are normally thickened to 18-35% solids (depending on the bauxite source and the process flow sheet) and must be dewatered by various forms of filtration or very excessive costs of tailings ponds, which later must be rehabilitated for final disposal. By employing a specially designed rake mechanism with 13.6 X lo6 N m (10 X lo6 f t
lb) torque drive unit on a traction thickener, the red mud solids are thickened to 50 wt % solids or higher in a 90m-diameter thickener. The thickened solids are best described as similar to toothpaste in rheology. However, the mud is pumped by a centrifugal pump due to the high shear combined with the thixotropic mud momentarily reducing the viscosity by 80%; the solids are transferred outside the periphery of the thickener tank. A highpressure membrane pump applying pressures up to 100 atm pumps the thick mud to a disposal landfill area which is located 3 km from the thickener and requires a 25-m lift. Figure 1 gives the schematic diagram of the "superthickener". The mud dries to 75 wt % solids and therefore occupies around 67 % of the original thickener underflow volume. The disposal base area is covered with sand and a drainage pipe so that any percolating liquid returns to the process. Rainfall is also collected and used as makeup. When the mud dries, large cracks occur so that further mud storage results. The storage height will be around 50 m but will be done in 5-m increments. The disposal area is 70 ha (175 acres). The use of the very high torque thickener mechanism has eliminated the entire filtration section and the oldfashioned tailings pond at a savings, while permitting an environmentally acceptable disposal system. In order that processing costs can be substantially reduced in the future, similar types of developments must occur in other unit operations as well as liquid-solid separation. (6) Environmental regulations and control will probably spawn new or improved liquid-solid separation methods. With public concerns over toxic chemicals, that same public will be demanding their removal. When these chemicals are now reported in parts per billion and even parts per trillion, the public becomes frightened even though they may not be toxic. Most pollution control processes consist of precipitating or adsorbing the pollutant with the requirement that the resultant solids must then be separated. Undoubtedly, membrane-type filtration will be one answer to this problem. In any case, it will have to be done at reasonable cost to the public; actually the global marketplace today demands a competitive price. Again, necessity will be an ally of creative engineering. Undoubtedly there will be other factors that will influence invention and improvement in performance in liquid-solid separation. Not the least w i l l be the creative and inquisitive minds of inventors who develop an interest in this field. They have been the backbone of such developments in the past and will be in the future. What technical factors will be involved in achieving new developments? This can be explained fairly easily by using some very basic and relatively old theories and equations. First considering sedimentation, Stokes' law can be employed: ut
=
@P2(P,
18P
- P)
(1)
where ut is the terminal velocity of a particle in a liquid, g is the force of gravity, D , is the particle diameter, pp. is the density of the particle, p is the density of the liquid, and p is the viscosity of the liquid. In order to increase the terminal velocity, the most obvious thing to do is to increase the particle diameter (D,). This has been done most effectively by the use of flocculants. Most of this flocculant development has occurred within the last 30 years and fortunately is still going on.
Ind. Eng. Chem. Res., Vol. 29, No. 6,1990 1023
Figure 1. Superthickener cross section.
1024 Ind. Eng. Chem. Res., Vol. 29, No. 6, 1990
However, there may well be other methods of increasing
D, not yet discovered such as electrical phenomena
(voltage, current flow, electrophoresis) that might reduce costs. Plant practices alone can always improve this factor by better process handling. I have seen several plants where particle degradation was encouraged because of excessive and unnecessary handling, abrasion, and shear. At the same time, if the particle only has to fall a small distance, definite productivity advances can occur. Another way of saying this is if the upward current separating fluid can be reduced in the required vertical distance of travel to achieve separation, this could also increase productivity. Many of the recent developments in sedimentation clarifiers have used one or both of these factors. Of course, the gravitational term (g) is what is applied in centrifugation either mechanically as in centrifuges or by fluids such as in hydrocyclones. Still, if the travel distance is also reduced, the capacity can be increased. It is expected that more attention will be given to increasing underflow density as illustrated in the earlier example. While the capital costs of the thickener may be considered high (primarily due to tank construction costs), the operating costs is low because of very low maintenance, labor, and energy costs. Thus, greater increase in solids concentration can be very cost effective. It should be noted that too little is known about the phenomena of consolidation, compaction, etc., that seem to occur in the so-called compression zone of sedimentation. Very little research has been conducted and should appear to be a fruitful area for such effort. Most of today’s knowledge is based primarily upon experience without supporting basic theory. With respect to filtration, one can start with the original Darcy’s law for flow of a liquid through a porous structure as follows: (2) where u is the velocity of the fluid, K is the proportionality term normally called permeability, AP is the pressure drop, and L is the length of the pore. Poiseuille’s law is expressed as follows: u=-
dig@
32pL
(3)
wVf
where di is the pore diameter. By making the following assumptions,
-dig =K=- 1 32 a u = - -1 dV A d8
by increasing the pressure drop or reducing the viscosity or resistance, the flow rate through the filter cake will increase. Increasing the pressure drop has been very important to batch filtration, particularly for difficult-to-filter solids (e.g., dye stuffs, clays, etc.). Normally the maximum economic feed pressure is usually around 10-20 atm except in special cases where further increases would reduce appreciably the discharged moisture content. Because of the latterlfactor, there may well be new developments possible. However, it should be remembered that the bulk of the batch process filters employ a sluice of wet cake discharge of the solids so that some very new unit types may have to be developed. Increased pressure has not been subject to much development in continuous filtration (which is still almost a monopoly of continuous vacuum filtration). The reasons are 2-fold. First, the filter shell becomes very greatly increased in cost when feed pressure exceeds 1 atm gage. ASME Unfired Pressure Vessel Codes must be employed which increases the cost substantially. Second, there has been no solution to the discharge of the dewatered solids from the pressure shell to atmospheric pressure except by either repulping or a periodic discharge by use of a dual lockhopper system. The flow rate can also be increased by decreasing viscosity. In most cases, this means conserving heat to maintain as high a feed temperature as possible or adding thermal energy to increase the temperature, which may not be economically justified. The resistance term, a , can be reduced in many cases by addition of a flocculant to increase the effective particle size. Another way, which can be particularly useful in membrane filtration, is to increase the number of pores per unit area. This is the basis of diatomaceous earth’s reduction in resistance. The diatoms have very small diameter holes in their skeleton, but there is a very large number in each diatom so that a relatively high rate filtrate flow occurs while clarifying the liquid. Equation 8 yields the instantaneous flow rate at any volume of filtrate per unit area. Thus, it must be integrated to employ in continuous filtration because of the repetitive cycles. It can be shown that the following equation results: 8fA =
(4) (5)
L = -wv A
where V is the volume of filtrate, cy is the resistance (reciprocal of permeability), A is the area of filtration, 8 is the time, and w is the weight of dry solids filter cake per unit volume of filtrate, the Poiseuille law becomes V - - KAP AP u = - 1 d- = (7) A d8 pL pawV/A The last equation in this form, 1 dV _ _ -- AP A de pcywV/A is more usable in filtration. It obviously points out that,
[
2wAPfz(60) ’ I 2 Paof
]
(9)
where V f is the volume of filtrate/hour, wVf/dfAis the weight of dry solids per unit time per unit area, APf is the pressure drop during cake formation, z is the fraction cake formation time of total cycle time, and Of is the cake formation time in minutes/cycle. The filtration rate in terms of dry solids per unit time per unit area can thus be shown to be a function of the following term:
where K’ is the proportionality constant and n is the exponent which must lie between 0 and 0.5. For the ideal situation, n would be equal to 0.5. If the solids migrate with time within the cake, n will be less than 0.5. In the majority of cases, n will lie between 0.3 and 0.5, with a substantial percentage ranging from 0.45 to 0.5. In addition to the previous comments, the rate can increase as z increases (for a conventional drum filter, z would be approximately 0.3-0.35). There are filters where
Znd. Eng. Chem. Res. 1990,29, 1025-1031 z can be as high as 0.95, but they are usually more expensive per unit area. They may still more than offset the cost by higher rates per unit area. Naturally, consideration must be given to the cake moisture content. If insufficient time is given to the dewatering of the cake in the filter cycle, this of course can be detrimental because of the excessive liquid content. Of particular interest is the influence of cake formation time. As can be seen from eq 10, as Of approaches zero, the filtration rate theoretically approaches infinity. Many attempts have been made to go to very short cycle times to increase productivity, but then cake discharge becomes a problem in a high percentage of the cases. Certain types of filters such as the continuous roller discharge drum filter can utilize short filter cycles as very thin cakes can be discharged by sticking them to the roller discharge for cutting off by a knife. For example, kaolin clay is filtered at a cycle time in as little as 15 or 20 s for 14-ft-diameter drums to maximize the rate. Undoubtedly, this principle of very short cake formation and cycle time will be very useful with some slurries and will promote increased productivity at reduced costs. However, a machine must be developed that can produce effective performance even though the total cycle time is 5 s or less. Considering the reduction of the moisture content on continuous filters, it has been shown to be a function of (t)d/ W), where dd is the dewatering time during the filter cycle, W is the cake weight in terms of weight of dry solids per unit area per cycle, and the factor e/, W is employed at constant pressure drop. Generally the moisture content as a function of (ed/ W ) , becomes asymptotic to some minimum value. Of course, pressure drop with vacuum is limited industrially to about 100 mmHg less than atmospheric pressure. However, on many materials it has been shown that each increase of 25 mmHg pressure drop reduces the moisture content between 0.2 and 0.35 percentage points of moisture. Thus,it would appear there are some definite limitations on the moisture content with continuous filters as long as vacuum is the driving force. Possibly other factors such as chemical additions (hydrocarbons have shown such
1025
potential in the past) or electrical phenomenon may offer potential for reduced moisture content. One real possibility is the combining of continuous filtration with mechanical compression. The latter is known to produce further moisture removal on compressible (squeezable) cakes with relatively little effectiveness on granular solids. In closing, the author has tried to show where possible new processes, machines, or certain techniques would yield further improvement in liquid-solid separation. Undoubtedly there will be areas not included by me that will produce increased results. The fact that we continuously must deal with finer solids, more difficult processes, and the necessity of reducing costs, makes effective creativity in the future in liquid-solid separation mandatory.
Nomenclature A = area of filtration
D, = particle diameter d i = pore diameter g = force of gravity K = proportionality term normally called permeability K’ = proportionality constant L = length of pore n = exponent which must lie between 0.5 and 1.0 AP = pressure drop hp, = pressure drop during cake formation V = volume of filtrate Vf = volume of filtrate/hour during cake formation u = velocity of fluid ut = terminal velocity of a particle in a liquid u, = weight of dry solids fiiter cake per unit volume of filtrate z =
fraction cake formation time of cycle time
Greek Symbols a = resistance (reciprocal of permeability) t9 = time
0, = cake formation time, min/cycle
density of the liquid density of the particle = viscosity of the liquid
p =
pp = p
Received for review August 28,1989 Revised manuscript received December 11, 1989 Accepted December 14, 1989
Alkylation of Substituted Phenols with Olefins and Separation of Close Boiling Phenolic Substances via Alkylation/Dealkylation Basab Chaudhuri, Ajit A. Patwardhan, and M. M. Sharma* Department of Chemical Technology, University of Bombay, Matunga, Bombay 400 019, India
The alkylation of substituted phenols, such as, m-and p-cresols and 2,4-, 2,5-, and 2,6-xylenols, with olefins such as a-methylstyrene (AMS) and diisobutylene (DIB) in the presence of homogeneous catalyst p-toluenesulfonic acid (pTSA) and heterogeneous catalyst Amberlyst 15 was studied in the temperature range 60-160 O C . The relative rates of alkylation of various substituted phenols with AMS and DIB under different operating conditions and the ortho/para product distribution were determined. A separation strategy based on alkylation, separation of the alkylated products by dissociation extraction and their subsequent decomposition, is presented for the separation of industrially important close boiling isomeric/nonisomeric substituted phenols, such as, n-and p-cresols, 2,5- and 2,4-xylenols, and p-cresol/2,6-xylenol. A new method of refining technical grade 2,6-xylenol containing p-cresol impurity through the acid-catalyzed O-alkylation of p-cresol with isobutylene is presented. A variety of industrial mixtures consist of close boiling point isomeric/nonisomeric components. In most cases,
* Author to whom correspondence should be addressed.
the separation of such mixtures cannot be economically realized by the conventional methods of separation such as those based on distillation and modified distillation processes, crystallization, etc. In such cases, the strategy
0888-5885/90/26291025$02.50/0 0 1990 American Chemical Society
