GB1574871A - Process and apparatus for regenerating polymeric adsorbents and wastewater treatment embodying the same - Google Patents

Description

(54) PROCESS AND APPARATUS FOR REGENERATING POLYMERIC ADSORBENTS AND WASTEWATER TREATMENT EMBODYING THE SAME (71) We, ARTHUR D. LITTLE, INC., a corporation organised under the Laws of the Commonwealth of Massachusetts, United States of America, of 25 Acorn Park, Cambridge, Massachusetts 02140, United States of America, do hereby declare the invention for which we pray that a patent may be granted to us and the method by which it is to be performed to be particularly described in and by the following statement:- This invention relates to a process for the regeneration of adsorbents and more particularly to a process for desorbing adsorbates from polymeric adsorbents bv dissolving the adsorbate in an inert solvent maintained in a supercritical condition. It is a modification in or improvement upon the process described and claimed in our prior application No. 40643/75, now serial No.

1522352.

In our prior application No. 1522352 there is described and claimed a process for regenerating an adsorbent by extracting adsorbate therefrom, comprising the steps of (a) contacting an adsorbent with adsorbate adsorbed thereon with a supercritical fluid which is a solvent for said adsorbate thereby to desorb a substantial amount of said adsorb ate and to dissolve it in said supercritical fluid, (b) separating said supercritical fluid with said dissolved adsorbate from said adsorbent, thereby to render said adsorbent capable of adsorbing an additional quantity of said adsorbate.

The said application also describes and claims apparatus for regenerating an adsorbent by extracting adsorb ate therefrom, comprising in combination (a) fluid contacting means for contacting adsorbent with adsorbate adhered thereto with a supercritical fluid which is a solvent for said adsorbate; (b) fluid separating means for separating said supercritical fluid containing said adsorbate dissolved therein from said adsorbent; (c) first physical treating means for changing the physical conditions of said supercritical fluid thereby to convert it into a nonsolvent state for said adsorbate whereby there is formed a twophase system comprising said adsorbate and said nonsolvent state supercritical fluid; (d) phase separation means to separate said nonsolvent state supercritical fluid; and (e) second physical treating means for converting said nonsolvent state supercritical fluid to its solvent state for use in said fluid contacting means.

Although activated carbon, as well as various inorganic adsorbents are still widely used for many purposes, the development of synthetic polymeric adsorbents in recent years has extended the use of adsorbents in industrial processes to a much wider ranRe of applications than heretofore associated with activated carbon. For some applications, synthetic polymeric adsorbents have replaced activated carbon, silica and alumina. One of the primary reasons for the rapidly expanding use of synthetic polymeric adsorbents lies in the fact that liquids may be used to remove the adsorbate from the synthetic polymeric adsorbent through the mechanism of solvation or reaction. Since this liquid removal is normally carried out under ambient conditions, many of the disadvantages inherent in the regeneration of activated carbon, for example, can be ei;minated.

In regenerating the synthetic polymeric adsorbents, an organic solvent such as methanol or isopropanol may be used. If the adsorbate is a weak acid, a base may be used to react with it to remove it; and, if the adsorbate is a weak base, an acid may be used as a reactant. Where adsorption is from an ionic solution, water may be used as a solvent; and, where the adsorbate is a volatile material, hot water or stream may be used.

By far, the most widely used technique for synthetic polymeric adsorbent regeneration is solvent extraction. After loading the adsorbent to the saturation point with the adsorbed substance, an appropriate organic solvent is passed through the synthetic polymeric adsorbent bed to dissolve and extract the adsorbate.

The cost of solvents used for the regeneration of the synthetic polymeric adsorbates requires that a high percentage of the solvent be recovered. Moreover, many such solvents, whether in bulk or in small quantities, cannot be disposed of without raising serious pollution problems.

In recovering and purifying such solvents for re-use, operational factors are encountered which add considerably to the cost of such recovery.

In solvent regeneration the solvent is used to displace water (or other liquid from which the impurity is removed) from the adsorbent bed. This means that a solventwater mixture is obtained which must be separated in the solvent recovery process.

Since some of the more common and inexpensive solvents which are most effective for the regeneration of the synthetic polymeric adsorbents form azeotropes with water, such azeotropes must be dealt with in solvent recovery. In the distillation of a mixture which forms an azeotrope one column is used to recover one component and the azeotrope. The azeotrope must then be sent to a second column operating at either higher or lower pressures in order to recover the other component in a purified form. Each of such columns requires a large number of theoretical plates. It is therefore apparent that although the use of a solvent for the adsorbate in the regeneration of a synthetic polymeric adsorbent involves no new art, it presents a serious economic problem.

Indeed, the severity of the solvent recovery problem often rules out the use of synthetic polymeric resin adsorption unless the unpurified regenerating-solvent stream can be recycled or otherwise used economically in a contiguous process.

It would therefore be desirable to have a process by which adsorbates can effectively be removed or extracted from synthetic polymeric adsorbents efficiently and in which the solvent cun be purifed for recycling more economically than presently possible.

It is therefore a primary object of this invention to provide an improved process for regenerating synthetic polymeric adsorbents. It is another object to provide a process of the character described based on the dissolution of adsorbates which makes possible the efficient and economical recovery of the solvent used and, if desired, of the adsorbate.

Another principal object is to provide an improved process for wastewater purification using a synthetic polymeric adsorbent to remove organic impurities and an inert solvent in the form of a supercritical fluid to desorb adsorbates from the adsorbent and to regenerate it.

The present invention provides a process for regenerating a synthetic polymeric adsorbent by extracting adsorbent therefrom, which process comprises (a) contacting a synthetic polymeric adsorbent having said adsorbate adsorbed thereon with a supercritical fluid which is a solvent for said adsorbate to desorb a substantial amount of said adsorbate and to dissolve it in said supercritical fluid, and (b) separating said supercritical fluid having said dissolved adsorbate from said adsorbent, so as to render said synthetic polymeric adsorbent capable of adsorbing an additional quantity of said adsorbate.

In the process of the invention the adsorbent, with the adsorbate adsorbed thereon, is contacted with a suitable supercritical fluid and the supercritical fluid containing the dissolved adsorbate is then subjected to a physical treatment which renders it a nonsolvent for at least a portion of the adsorbate, thus separating the adsorbate and the supercritical fluid into two phases. Subsequent to the separation of the adsorbate from the supercritical fluid, the supercritical fluid, which may be described as being in a nonsolvent state, is subjected to another physical treatment to restore it to the condition in which it is a solvent for the adsorbate so that it may be recycled. Thus it may be said to be in a solvent state prior to recycling. The physical treatments may constitute altering the temperature and/or the pressure of the supercritical fluid. As an optional step, the adsorbate may be reacted with a reactant while dissolved in or mixed with the supercritical fluid.

In one embodiment the present process comprises (a) contacting the synthetic polymeric adsorbent with water containing at least one organic material under conditions such that said organic material is adsorbed on said synthetic polymeric adsorbent; (b) then contacting said synthetic polymeric adsorbent with said organic material adsorbed thereon with a supercritical fluid which is a solvent for said organic material in order to dissolve at least part of said organic material in said supercritical fluid; (c) separating said supercritical fluid having said dissolved organic material in solution therein; (d) subjecting said supercritical fluid having said organic material dissolved therein to physical treatment which renders said supercritical fluid a nonsolvent for said organic material and thus to form a twophase system comprising, as one phase, said supercritical fluid in a nonsolvent state and, as the other phase, said organic material; (e) separating the resulting two-phase system into nonsolvent state supercritical fluid and organic material; and (f) subjecting said nonsolvent state supercritical fluid subsequent to said separating step to a physical treatment which converts it to a solvent state supercritical fluid thus making it a solvent for said organic material.

The physical treatment of the supercritical liquid containing the organic material may then consist in reducing the pressure thereover: this may be accomplished by expansion which leads to a concomitant fall in the temperature of the supercritical liquid. When the supercritical liquid is in the nonsolvent state the physical treatment thereof may involve compressing it to a pressure which is at least equal to its critical pressure. When sufficient compression is then applied the fluid becomes heated and it is raised to a pressure at least equal to its critical pressure.

Indirect heat exchange between supercritical liquid in the nonsolvent state and the resulting compressed fluid may be brought about which will reduce the temperature of the latter and form the supercritical liquid in the solvent state.

Alternatively the physical treatment of the supercritical liquid containing the organic material dissolved therein may involve changing the temperature thereof in one direction and the subsequent physical treatment of the nonsolvent state fluid involves changing the temperature thereof in the opposite direction to that of the previous change.

Yet a further alternative is to carry out the physical treatment of the supercritical fluid in stages in order to bring about the separation of more than one fraction from the adsorbate.

A still further alternative is to bring about a chemical reaction between adsorbate dissolved in supercritical liquid and a reactant specifically introduced for that purpose. Such a reaction may be carried out prior to subjecting the supercritical liquid containing organic material dissolved therein to the physical treatment.

Alternatively such a chemical reaction may be brought about subsequent to applying the physical treatment to the supercritical fluid containing organic material.

In this specification the term "nonsolvent state" is applied to the supercritical fluid to indicate that it has a relatively low solubility for one or more adsorbates; and the term "solvent state" is applied to the supercritical fluid to indicate that it has a relatively high solubility for one or more adsorbates. Thus these terms are not used in the absolute sense, but in the relative sense.

The invention will now be described with reference to the accompanying drawings, in which: Fig. 1 is a graph of naphthalene solubility in carbon dioxide over a temperature range from 35"C to 550C for selected pressures; Fig. 2 shows a modification of the apparatus described in copending application No. 4064375 Serial No. 1522352.

Commercially available synthetic polymeric adsorbents may generally be described as hard, insoluble, high surface area, porous polymers.

Typically, they are provided in spherical form with a nominal mesh size of about 16 to 50 U.S. Standard sieves. They are available with both polarities and a variety of surface characteristics thus making it possible to use them as adsorbents in a wide range of applications. For example, synthetic polymeric adsorbent may be a homopolymer of styrene, a copolymer of styrene and divinylbenzene, or a polymer containing an acrylic ester, trimethylolpropane trimethacrylate or trimethylolpropane dimethacrylate. (See for example R. M. Simpson, "The separation of Organic Chemicals from Water" presented at the Third Symposium of the Institute of Advanced Sanitation Research, International on April 13, 1972, wherein exemplary chemical structures for polymeric adsorbants are given. See also German Offenlegungsschrift 1,943,807).

The synthetic polymeric adsorbents have found many varied applications in wastewater treatments. For example, they have been used to decolorize pulp mill bleaching effluent and waste dyestuffs and to remove a pesticide from waste streams, a surfactant such as an alkylbenzene sulfonate or linear alkyl sulfonate-type surfactants from wastewaters; or an explosive such as TNT and DNT from effluent streams. These synthetic polymeric adsorbents have also been used in analysis procedures for determining trace amounts (as litte as parts per billion) of organic contaminants in water, in chemical processing and in isolating enzymes and proteins as well as other organic pild;macologically active materials such as Vitamin B-12, tetracycline, oxytetracycline and oleandomycin.

Examples of the pesticides which can be removed by adsorption on a polymeric adsorbent from a waste stream are Lindane, DDT and Malathion and pesticide ingredients such as endrin, heptachlor and other chlorinated hydrocarbon intermediates.

Examples of the organic materials which may be removed from a water stream using synthetic polymeric adsorbents are those listed in Table 1 as reported by Junk et al Journal of Chromatography, volume 99, 745-762 (1974). The resins used were two different polystyrenes characterized as having 42 /n and 51 /n helium porosity, surface areas of 330 and 750 m2/gram, average pore diameters of 90 and 50 A, skeletal densities of 1.08 and 1.09 grams/cc, (respectively) and nominal mesh size of 20 to 50 (U.S. Standard Sieves). Sold as XAD-2 and XAD-4 by Rohm and Haas Company).

TABLE 1 Organic Materials Removable From a Water Stream by Adsorption on Polymeric Adsorbents Alcohols Hexyl 2-Ethylhexanol 2-Octanol Decyl Dodecyl Benzyl Cinnamyl 2-Phenoxyethanol Aldehydes and ketones 2,6-Dimethyl-4-heptanone 2-Undecanone Acetophenone Benzophenone Benzil Benzaldehyde Salicylaldehyde Esters Benzyl Acetate Dimethoxyethyl phthalate Dimethyl phthalate Diethyl phthalate Dibutyl phthalate Di-2-ethylhexyl phthalate Diethyl fumarate Dibutyl fumarate Di-2-ethylhexyl fumarate Diethyl malonate Methyl benzoate Methyl decanoate Methyl octanoate Methyl palmitate Methyl salicylate Methyl methacrylate Polynuclear aromatic materials Naphthalene 2-Methylnaphthalene 1-Methylnaphthalene Biphenyl Fluorene Anthracene Acenaphthene Tetrahydronaphthalene Alkyl benzenes Ethylbenzene Cumene p-Cymene Acids (acidified) Octanoic Decanoic Palmitic Oleic B enzoic Phenols Phenol o-Cresol 3,5-Xylenol o-Chlorophenol p-Chlorophenol 2,4,6-Trichlorophenol 1 -Naphthol Ethers Hexyl Benzyl Anisole 2-Methoxynaphthalene Phenyl Halogen compounds Benzyl chloride Chlorobenzene Iodobenzene o-Dichlorobenzene m-Dichlorobenzene 1,2,4,5-Tetrachlorobenzene a-o-Dichlorotoluene m-Chlorotoluene 2,4-Dichlorotoluene 1,2,4-Trichlorobenzene Nitrogen-containing compounds Hexadecylamine Nitrobenzene Indole o-Nitrotoluene N-Methylaniline Benzothiazole Quinoline Isoquinoline Benzonitrile B enzoxazole As noted above, the synthetic polymeric adsorbents are regenerated by dissolving out the adsorbate when the adsorbent bed has reached a predetermined point of saturation, normally referred to as the saturation point and defined as that point at which the stream discharged from the bed contains a preset level of the adsorbate. As also previously noted, this removal of the adsorbate has previously been accomplished by using liquid organic solvent, such as methanol or isopropanol, under ambient temperature and pressure, and has included a costly solvent recovery procedure.

According to the process of this invention, a supercritical fluid is used for adsorbent regeneration, a process which requires no phase changes of the solvent and hence requires low expenditure of energy. This process also provides the possibility of attaining efficient recovery of the adsorbate if this is desired.

It is a well-known phenomenon that when certain gases are subjected to a specified pressure and maintained above a certain temperature they reach a supercritical state.

Broadly, this supercritical state as the term is used herein may be defined as the region of temperature and pressure above the critical temperature and critical pressure of the compound. Although the term "supercritical" is applied generally to all temperatures above the critical state, it will be used hereinafter to refer to a gas which is at a pressure above its critical pressure and at a temperature up to about 1.3 times the critical temperature in K and preferably 1.01 to 1.3 times the critical temperature. In most cases, however, it will be preferable to use temperatures no greater than about 1.1 times the critical temperature in "K and most preferably within the range of 1.01 to 1.1 times the critical temperature in "K.

In the process of this invention, carbon dioxide has been found to be particularly suitable for removing organic adsorbates from synthetic polymeric adsorbents. The critical temperature of carbon dioxide is 304.2"K (31.00C) and the critical pressure is 72.9 atmospheres. The temperature range in which the carbon dioxide is in its most advantageous condition for use in this invention is between about 1.01 and 1.1 times the critical temperature or between about 308"K and 335"K (34cm and 62"C).

This temperature range is not far above ambient temperatures. Moreover, carbon dioxide has several other advantages among which is the fact that it is does not pollute the atmosphere and that it is inexpensive.

In choosing a supercritical fluid for the regeneration of a synthetic polymeric adsorbent containing one or more organic adsorbates adsorbed thereon, the supercritical fluid must be a solvent for the adsorbate to be removed and it must be a fluid adsorbate which does not react with the surface of the adsorbent.

For any one adsorbate/supercritical fluid system, the physical changes to which the supercritical fluid must be subjected to render it first a solvent for the adsorbate and then a nonsolvent for it will, of course, depend upon the solubility of the adsorbate in the supercritical fluid under different temperatures and pressures. An example of such a system is naphthalene/supercritical carbon dioxide. Fig. 1 is a graph of naphthalene solubility in carbon dioxide over a temperature range from 350C to 55"C for selected pressures (taken from Yu.

V. Tsekhanskaya, M. B. Iomtev and E. V.

Mushkina, Zh. Fiz. Khim. volume 38, 2166 (1964)). The graph shows that naphthalene can be separated from the supercritical carbon dioxide solvent by either: (a) temperature-cycling at constant pressure (e.g., at 300 atmospheres, absorbing at 550C and precipitating at 350C; or at 80 atmospheres absorbing at 35"C and precipitating at 450C); (b) pressure-cycling at constant temperature (e.g., at 550C, absorbing at 300 atmospheres and precipitating at 80 atmospheres); or (c) a combination of temperature/pressure cycling.

In the specification of our copending application No. 40643/75 (Serial No.

1522352) various embodiments of the method and apparatus in accordance with the invention of that application are illustrated therein in somewhat diagrammatic form in Figs. 1--4. Such embodiments of method and apparatus may be modified to illustrate the present invention. Carbon dioxide is again used as illustrative of the supercritical fluid to remove hydrocarbon impurities, whilst a synthetic polymeric adsorbent is used in place of the activated carbon which is used as an adsorbent in the method and apparatus of Figs. 1-4 of Application No.

40643/75 (Serial No. 1522352). The spent synthetic polymeric adsorbent to be regenerated in accordance with the present invention is in the form of a transportable finely divided particulate material derived from an adsorbent storage tank; and it is introduced at substantially ambient temperature and pressure into a desorption column and the in-flow of spent synthetic polymeric adsorbent is controlled by means of a highpressure on-off valve. The supercritical fluid for regeneration, e.g., carbon dioxide at 300 atmospheres pressure and 35"C is employed in the same manner as described in Application No. 40643/75 (Serial No. 1522 352).

Normally the synthetic polymeric adsorbent will not be dried prior to desorption since water can be removed by the solvent supercritical carbon dioxide and subsequently separated from it by appropriate variations in temperature and/or pressure. However, it may be desirable in some cases to remove residual water from the adsorbent prior to regeneration with supercritical fluid. If so, prior to the extraction of adsorbate from the adsorbent a drying gas, e.g., hot air, is allowed to pass over the spent adsorbent to remove residual water. Then carbon dioxide at atmospheric pressure is passed up through the dried spent adsorbent to remove any air remaining in the pores of the spent adsorbent. Regeneration can then be effected as previously described. The regenerated synthetic polymeric adsorbent can then be transferred into an adsorbent storage tank from which it may be further transferred into an off-stream column.

Fig. 2 illustrates a modification of the apparatus shown in Fig. 4 of Application No. 40643/75. (Serial No. 1522352). In the arrangement of Fig. 2, a separate desorption column 10, along with the conduit means for transferring spent adsorbent into and withdrawing regenerated synthetic absorbent therefrom is eliminated. Instead, desorption is accomplished directly in columns 70 and 71, in which case these columns are vessels capable of withstanding the pressure required to maintain the supercritical fluid supplied from storage tank 20 in its solvent state. Fluid conduit 23, through which the supercritical fluid containing the adsorbate is alternately withdrawn from columns 70 and 71, is fed by conduits 23a and 23b having valves 24a and 24b, respectively. Likewise, fluid conduit 21, through which supercritical fluid is supplied alternately to columns 70 and 71, feeds into conduits 21a and 21b having valves 22a and 22b, respectively.

The process of this invention may be further illustrated using phenol and 2,4,6trichlorophenol as the adsorbates, a synthetic polymeric resin as the adsorbent and supercritical carbon dioxide as the solvent. An example of such a synthetic polymeric resin is a polystyrene having a porosity volume of 51%, a true wet density of 1.02 grams/cc, a surface area of 780 m2/gram, an average pore diameter of 50 A, a skeletal density of 1.08 grams/cc and a nominal mesh size of 20two 50 (U.S. Standard Sieves). Using a water flow rate of 0.5 gallons per minute per cubic foot this synthetic polymeric adsorbent adsorbs about 0.78 pounds per cubic foot of phenol with zero percent leakage from a concentration of 250 ppm in the water feed.

This adsorbent also adsorbs up to 12 pounds per cubic foot of trichlorophenol with zero percent leakage at the same flow rate.

Regeneration of the adsorbent can be carried out by using supercritical carbon dioxide under the conditions described above when the saturation point in adsorption is detected. The supercritical carbon dioxide can be used at 350C and 300 atmospheres to dissolve the phenol and the trichlorophenol from the synthetic polymeric adsorbent and it can then be converted to the nonsolvent state for the adsorbates by reducing the pressure to 80 atmospheres and/or increasing the temperature to 450C. The presence of phenol in the carbon dioxide may be detected through ultraviolet detection means. The resulting reactivated adsorbent can then again be contacted with additional aqueous phenol/trichlorophenol solution.

By using a supercritical fluid to dissolve out the adsorbates from a synthetic polymeric adsorbent, the adsorbent is not subjected to any appreciable thermal or chemical degradation and the adsorbate may be recovered if desired. Moreover, it is possible to use such supercritical fluids as carbon dioxide, ethane or ethylene which require temperatures and pressures well within the capabilities of existing equipment. Finally, these fluids (and Darticularlv carbon dioxide) are inexpensive, a fact which contributes materially to the economics of industrial processes and wastewater purification.

WHAT WE CLAIM IS: 1. A process for regenerating a synthetic polymeric adsorbent by extracting adsorbent therefrom, which comprises (a) contacting a synthetic polymeric adsorbent having said adsorbate adsorbed thereon with a supercritical fluid which is a solvent for said adsorbate to desorb a substantial amount of said adsorbate and to dissolve it in said supercritical fluid, and (b) separating said supercritical fluid having said dissolved adsorbate from said adsorbent, so as to render said synthetic polymeric adsorbent capable of adsorbing an additional quantity of said adsorbate.

2. A process as claimed in claim 1, wherein said supercritical fluid solvent for said adsorbate is at a temperature of up to 1.3 times the critical temperature is degrees K of said fluid.

3. A process as claimed in claim 2, wherein said supercritical fluid is at a temperature within the range from 1.01 to 1.3 times the critical temperature in degrees K of said fluid.

4. A process as claimed in claim 3, wherein said supercritical fluid is at a temperature within the range from 1.01 to 1.1 times the critical temperature in degrees K of said fluid.

5. A process as claimed in any one of the preceding claims, wherein said synthetic polymeric adsorbent is a homopolymer of styrene, a copolymer of styrene and divinylbenzene, or a polymer containing an acrylic ester, trimethylolpropane trimethacrylate or trimethylolpropane dimethacrylate.

6. A process as claimed in any one of the preceding claims, wherein said supercritical fluid is carbon dioxide.

7. A process as claimed in claim 6, wherein said carbon dioxide is at a temperature within the range from 340C to 62"C.

8. A process as claimed in any one of the preceding claims, which comprises: (a) contacting the synthetic polymeric adsorbent with water containing at least one organic material under conditions such that

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (24)

**WARNING** start of CLMS field may overlap end of DESC **. can then be transferred into an adsorbent storage tank from which it may be further transferred into an off-stream column. Fig. 2 illustrates a modification of the apparatus shown in Fig. 4 of Application No. 40643/75. (Serial No. 1522352). In the arrangement of Fig. 2, a separate desorption column 10, along with the conduit means for transferring spent adsorbent into and withdrawing regenerated synthetic absorbent therefrom is eliminated. Instead, desorption is accomplished directly in columns 70 and 71, in which case these columns are vessels capable of withstanding the pressure required to maintain the supercritical fluid supplied from storage tank 20 in its solvent state. Fluid conduit 23, through which the supercritical fluid containing the adsorbate is alternately withdrawn from columns 70 and 71, is fed by conduits 23a and 23b having valves 24a and 24b, respectively. Likewise, fluid conduit 21, through which supercritical fluid is supplied alternately to columns 70 and 71, feeds into conduits 21a and 21b having valves 22a and 22b, respectively. The process of this invention may be further illustrated using phenol and 2,4,6trichlorophenol as the adsorbates, a synthetic polymeric resin as the adsorbent and supercritical carbon dioxide as the solvent. An example of such a synthetic polymeric resin is a polystyrene having a porosity volume of 51%, a true wet density of 1.02 grams/cc, a surface area of 780 m2/gram, an average pore diameter of 50 A, a skeletal density of 1.08 grams/cc and a nominal mesh size of 20two 50 (U.S. Standard Sieves). Using a water flow rate of 0.5 gallons per minute per cubic foot this synthetic polymeric adsorbent adsorbs about 0.78 pounds per cubic foot of phenol with zero percent leakage from a concentration of 250 ppm in the water feed. This adsorbent also adsorbs up to 12 pounds per cubic foot of trichlorophenol with zero percent leakage at the same flow rate. Regeneration of the adsorbent can be carried out by using supercritical carbon dioxide under the conditions described above when the saturation point in adsorption is detected. The supercritical carbon dioxide can be used at 350C and 300 atmospheres to dissolve the phenol and the trichlorophenol from the synthetic polymeric adsorbent and it can then be converted to the nonsolvent state for the adsorbates by reducing the pressure to 80 atmospheres and/or increasing the temperature to 450C. The presence of phenol in the carbon dioxide may be detected through ultraviolet detection means. The resulting reactivated adsorbent can then again be contacted with additional aqueous phenol/trichlorophenol solution. By using a supercritical fluid to dissolve out the adsorbates from a synthetic polymeric adsorbent, the adsorbent is not subjected to any appreciable thermal or chemical degradation and the adsorbate may be recovered if desired. Moreover, it is possible to use such supercritical fluids as carbon dioxide, ethane or ethylene which require temperatures and pressures well within the capabilities of existing equipment. Finally, these fluids (and Darticularlv carbon dioxide) are inexpensive, a fact which contributes materially to the economics of industrial processes and wastewater purification. WHAT WE CLAIM IS:

1. A process for regenerating a synthetic polymeric adsorbent by extracting adsorbent therefrom, which comprises (a) contacting a synthetic polymeric adsorbent having said adsorbate adsorbed thereon with a supercritical fluid which is a solvent for said adsorbate to desorb a substantial amount of said adsorbate and to dissolve it in said supercritical fluid, and (b) separating said supercritical fluid having said dissolved adsorbate from said adsorbent, so as to render said synthetic polymeric adsorbent capable of adsorbing an additional quantity of said adsorbate.

2. A process as claimed in claim 1, wherein said supercritical fluid solvent for said adsorbate is at a temperature of up to

1.3 times the critical temperature is degrees K of said fluid.

3. A process as claimed in claim 2, wherein said supercritical fluid is at a temperature within the range from 1.01 to

1.3 times the critical temperature in degrees K of said fluid.

4. A process as claimed in claim 3, wherein said supercritical fluid is at a temperature within the range from 1.01 to

1.1 times the critical temperature in degrees K of said fluid.

5. A process as claimed in any one of the preceding claims, wherein said synthetic polymeric adsorbent is a homopolymer of styrene, a copolymer of styrene and divinylbenzene, or a polymer containing an acrylic ester, trimethylolpropane trimethacrylate or trimethylolpropane dimethacrylate.

6. A process as claimed in any one of the preceding claims, wherein said supercritical fluid is carbon dioxide.

7. A process as claimed in claim 6, wherein said carbon dioxide is at a temperature within the range from 340C to 62"C.

8. A process as claimed in any one of the preceding claims, which comprises: (a) contacting the synthetic polymeric adsorbent with water containing at least one organic material under conditions such that

said organic material is adsorbed on said synthetic polymeric adsorbent; (b) then contacting said synthetic polymeric adsorbent with said organic material adsorbed thereon with a supercritical fluid which is a solvent for said organic material in order to dissolve at least a part of said organic material in said supercritical fluid; (c) separating said supercritical fluid having said dissolved organic material in solution therein; (d) subjecting said supercritical fluid having said organic material dissolved therein to physical treatment which renders said supercritical fluid a nonsolvent for said organic material and thus to form a twophase system comprising, as one phase, said supercritical fluid in a nonsolvent state and, as the other phase, said organic material; (e) separating the resulting two-phase system into nonsolvent state supercritical fluid and organic material; and (f) subjecting said nonsolvent state supercritical fluid subsequent to said separating step to a physical treatment which converts it to a solvent state supercritical fluid thus making it a solvent for said organic material.

9. A process as claimed in claim 8, wherein said physical treatment of said supercritical fluid containing the organic material comprises decreasing the pressure of said supercritical fluid.

10. A process as claimed in claim 9, wherein said decrease in pressure is accomplished by expansion accompanied by a concomitant fall in the temperature of said supercritical fluid.

11. A process as claimed in either of claims 9 or 10, wherein said physical treatment of said nonsolvent state fluid comprises compressing said nonsolvent state supercritical fluid to a pressure which is at least equal to its critical pressure.

12. A process as claimed in claim 11, wherein said nonsolvent state fluid is compressed sufficiently to heat said fluid and to raise it to a pressure which is at least equal to its critical pressure and effecting indirect heat exchange between said nonsolvent state supercritical fluid and the resulting compressed fluid so as to decrease the temperature of said compressed fluid and to form said solvent state supercritical fluid.

13. A process as claimed in claim 8, wherein said physical treatment of said supercritical fluid containing the organic material dissolved therein comprises changing the temperature of said supercritical fluid in one direction and said physical treatment of said nonsolvent state fluid comprises changing the temperature of said nonsolvent supercritical fluid in a direction opposite to that of the first mentioned step.

14. A process as claimed in claim 8, wherein the physical treatment of said supercritical fluid containing the organic material dissolved therein and the separation of the resulting two-phase systems are carried out in stages in order to effect the separation of more than one fraction of said adsorbate.

15. A process as claimed in claim 8, which includes chemically reacting said adsorbate dissolved in said supercritical fluid with a reactant introduced into said supercritical fluid.

16. A process as claimed in claim 15, wherein said chemical reaction of said adsorbate with a reactant is carried out prior to subjecting said supercritical fluid containing the organic material to physical treatment.

17. A process as claimed in claim 15, wherein said chemical reaction of said adsorbate with a reactant is carried out subsequent to subjecting said supercritical fluid containing the organic material to physical treatment.

18. A process as claimed in any one of claims 8 to 17, wherein said water comprises bleaching effluent from a pulp mill and said organic material comprises coloured bodies.

19. A process as claimed in any one of claims 8 to 17, wherein said water contains waste dyestuffs as said organic material.

20. A process as claimed in any one of claims 8 to 17, wherein said water contains a pesticide, a surfactant or an explosive as said organic material.

21. A process as claimed in any one of claims 8 to 17, wherein said water contains a pharmacologically active material as said organic material.

22. A process for regenerating a synthetic polymeric adsorbent according to claim 1 and substantially as hereinbefore described.

23. A synthetic polymeric adsorbent which has been regenerated by the process claimed in any of the preceding claims.

24. Apparatus for regenerating a synthetic polymeric adsorbent by extracting adsorbate therefrom substantially as hereinbefore described with reference to Figure 2 of the accompanying drawings.