GB1573647A - Process and apparatus for regenerating adsorbents and wastewater treatment embodying the same - Google Patents

Description

(54) PROCESS AND APPARATUS FOR REGENERATING 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 the regeneration of adsorbents and more particularly to a process for desorbing adsorbates from adsorbents by dissolving the adsorbate in an inert solvent.

In the purification and removal of impurities from fluid streams in many industrial processes an adsorbent is used to adsorb the impurities from the fluid stream. Adsorbents may also be used to separate components in a process and to isolate trace impurities for quantitative analysis. In other processes, unwanted adsorption of poisons on catalytic surfaces may occur and require removal.

Thus, for example, small amounts of organic compounds, both aliphatic and aromatic, have been removed by being adsorbed on activated carbon or polymeric adsorbents in the treatment of wastewaters from industrial processes.

Coloured bodies are adsorbed in the process of sugar refining and impurities are removed from vinyl chloride streams by adsorption.

In petroleum cracking processes the high surface area catalytic materials such as alumina or silica, with or without such metals as nickel, cobalt, molybdenum or tungsten deposited thereon, become contaminated by impurities which are adsorbed on the catalytic materials, and, in some cases, chemically react therewith. In all such cases, the adsorbates must be periodically removed from the adsorbents.

A number of inorganic adsorbents have been known and used for some time and they may generally be defined as solid phase materials having very high surface area-to-weight ratios and exhibiting the ability to concentrate ad sorbates on their surfaces. Among the more commonly used inorganic adsorbents are acti vated carbon, alumina, silica, and silicates.

(See for example Table 16-2 of "Chemical Engineers' Handbook" Robert H. Perry and Cecil H. Chilton, McGraw-Hill, New York, Fifth Edition, 1973, pp 16-5 to 16-9).

The use of such inorganic adsorbents normally includes one or more steps to effect their regeneration, i.e., the removal of all or a part of the adsorbate which adheres to the surface of the adsorbent. If the adsorbate is a volatile material, such regeneration may be accomplished by heating the adsorbent to volatilize off the adsorbate or by creating a vacuum around the adsorbent. Volatilization with heating may be accompanied by reaction with some added reactant, e.g., oxygen to oxidize adsorbed organic materials. It is, of course, apparent that the less volatile adsorbates require higher temperatures to remove them in this manner and such temperatures may contribute to the gradual thermal degradation of the adsorbent and/or adsorbate. Moreover, any reactant added, such as oxygen, may chemically degrade such adsorbents as activated carbon, causing loss of usable capacity. Such losses require that the adsorbent be periodic ally replaced. Finally, the use of high tempera tures for adsorbent regeneration requires a relatively high expenditure of energy.

Activated carbon used in removing organic impurities from wastewaters may be taken as exemplary of the type of performance now being attained in the use and regeneration of inorganic adsorbents. High surface area (1000--1300 m2/g) activated carbon has a high capacity (0.1 to 50 g/g) for most organic materials. When used as an adsorbent for treating aqueous solutions, activated carbon is usually regenerated by oxidizing the ad sorbed organic materials with air and/or steam at high temperature, e.g., 500--700"C.

Under such conditions, there is a loss of 3 to 10% of the activated carbon adsorbent for each regeneration resulting from partial oxida tion of the activated carbon. Thus, the average lifetime of activated carbon is 10 to 30 regenerations. The adsorbent loss therefore becomes a significant fraction of the total operating cost.

The use of reduced pressure to remove adsorbates from an adsorbent requires the equipment necessary to generate the required degree of evacuation and it is a technique which is limited to only certain classes of adsorbates, namely those which exhibit appreciable vapour pressure at temperatures below their decomposition point. It is known that many solids and liquids having low volatility will decompose before their vapour pressure becomes appreciable.

Although activated carbon, as well as various other inorganic adsorbents, is 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 range of applications than heretofore associated with activated carbon. In some instances, polymeric adsorbents have replaced activated carbon, silica and alumina. One of the primary reasons for the rapidly expanding use of polymeric adsorbents lies in the fact that liquids may be used to remove the adsorbate from the 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 eliminated or substantially reduced.

In regenerating the polymeric adsorbents, an organic solvent such as methanol or isopropanol may be used. If the absorbate 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.

Finally, where adsorption is from an ionic solution water may be used; and, where the adsorbate is a volatile material, hot water or steam may be used.

By far the most widely used technique for polymeric adsorbent regeneration is solvent extraction. After loading the adsorbent to the breakthrough point with the adsorbate an appropriate organic solvent is passed through the polymeric adsorbent bed to dissolve and extract the adsorbate. The cost of using solvents for the regeneration of the polymeric adsorbents 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 reuse, 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 solvent-water 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 or the 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 may require a large number of theoretical plates. It is therefore apparent that although the use of a solvent for the adsorbed species in the regeneration of a 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 regeneratingsolvent stream can be recycled or otherwise used economically in a contiguous process.

In copending patent applications 40643/75 (Serial No. 1,522,352) and 15863/77 we have described the use of supercritical fluids as adsorbate solvents to regenerate both inorganic and organic polymeric adsorbents. In using these supercritical fluids for adsorbent regeneration it is necessary during the desorbing process to maintain them in their supercritical state, i.e., at a pressure and temperature above the critical pressure and critical temperature of the solvent fluid. In the case of some adsorbents, e.g., those having extremely small pore sizes and in those adsorbateadsorbent systems in which the desorption process is highly mass-transfer limited, the supercritical fluids have distinct advantages in adsorbent regeneration. However, where the solubility characteristics of the solvent and the pore structure of the adsorbent are favourable to the use of temperatures and pressures lower than those required to maintain the solvent in the supercritical state, the use of solvents in the so-called near critical liquid state has distinct advantages. For example, the use of lower pressure materially reduces the cost of the pressure vessel in which de sorption is accomplished. Moreover, in some cases it may be easier to separate a solute from a near critical liquid than from a super critical fluid. Although it is necessary to supply the latent heat of vaporization of the solvent liquid using a near critical liquid, it is possible to operate within a near critical temperature region to keep this heat requirement to a minimum.

When compared to the use of liquid sol vents such as those now being used in re generating the polymeric adsorbents, the use of a near critical liquid has several marked advantages, among which are the possession of superior mass transfer characteristics, of higher volatilities and of lower heats of vaporization of the solvent. These advantages, in turn, give rise to the need for less energy and the the possibility of improved solute (adsorbate) recovery.

It would therefore be desirable to have a process by which adsorbates could effectively be removed or extracted from adsorbents which was efficient and economical and intermediate in operational conditions between the use of liquid solvents and supercritical fluids.

It is therefore a primary object of this invention to provide a process for regenerating 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 of this invention is to provide a process for wastewater purification using in particular polymeric adsorbents to remove organic impurities and an inert solvent in the form of a near critical liquid to extract adsorbates from the adsorbent to regenerate it.

A further object of this invention is to provide apparatus for regeneration of absorbents.

According to the present invention there is provided a process for desorbing an adsorbate from an adsorbent which comprises contacting said adsorbent having said adsorbate adhered thereto with a solvent for said adsorbate, said solvent being a near critical liquid with a temperature thereof being from 0.95 to 0.995 times its critical temperature in OK and its pressure being at least equal to its vapour pressure at the temperature at which it is used, thereby to dissolve said adsorbate in said liquid and desorb it from said adsorbent.

The process of the present invention may comprise the steps of separating the near critical liquid with the adsorbate dissolved therein from the adsorbent; vaporizing at least a portion of the near critical liquid containing the adsorbate dissolved therein to form a multi-phase system comprising solvent rich vapour and an adsorbate-rich phase; condensing the solvent-rich vapour to form a liquid; and adjusting the temperature and pressure of the condensed liquid thereby to convert it to the near critical liquid for recycling in the process. Prior to these steps the process may include the steps of contacting the adsorbent with water containing said organic material under conditions such that said organic material is adsorbed as an adsorbate on said adsorbent; and then contacting said adsorbent with said organic material adhered thereto with a near critical liquid solvent in order to dissolve said organic material in said solvent. The process is particularly well suited to the removal of organic materials such as coloured bodies, surfactants and pharmacologically active materials from wastewaters.

Apparatus according to the invention for desorbing an adsorbate from an adsorbent comprises: (a) a pressure vessel within which to effect contact between said adsorbent having said adsorbate adhering thereto and a liquid solvent for said adsorbate, said solvent being a near critical liquid with the temperature thereof being from 0.95 to 0.995 times its critical temperature in OK and its pressure being at least equal to its vapour pressure at the temperature at which it is used, said vessel being provided with means for maintaining conditions of temperature and pressure which will maintain said solvent within said conditions temperature and pressure, whereby said adsorbate can be desorbed from said adsorbent and be taken up in the near critical liquid solvent; (b) a vaporizer/separator device for vaporizing at least a portion of said near critical liquid as a solvent-rich vapour and for separating and collecting said adsorbate as an adsorbate-rich liquid phase separate from said vapour; (c) fluid conduit means connecting said pressure vessel and said vaporizer/separator device for conducting said near critical liquid solvent containing said adsorbate into said vaporizer/separator device; (d) a condenser for condensing said solvent-rich vapour, received from said vaporizer/ separator device, to a liquid; and (e) means for adjusting the temperature and pressure of said liquid from said condenser to within the said conditions of temperature and pressure for recycling.

Such apparatus may also include a vessel within which to effect contact between a fluid stream containing the material to be removed and the adsorbent contained within the vessel; and means for circulating the fluid containing the material through the vessel whereby the material is adsorbed as an adsorbate on the adsorbent The pressure vessel itself may, if desired, be employed as this vessel in which contact is effected by the fluid containing the material to be removed and the adsorbent.

In order that the invention may be more readily understood, reference will now be made to the accompanying drawings, in which: Fig. 1 is a graph of the solubility of naphthalene in carbon dioxide as a function of specific volume, illustrative of one adsorbate/ solvent system applicable to this invention; Fig. 2 is a graph of the correlation of naphthalene solubility in carbon dioxide for supercritical and near critical conditions; Fig. 3 is a graph of the solubility of naphthalene in carbon dioxide from 0 to 550 C and for a range of pressures; Fig. 4 is a graph illustrating the dimensionless co-relation of heat of vaporization with temperature; Fig. 5 is a schematic diagram of the process of this invention illustrating the regeneration of polymeric adsorbent, having naphthalene adsorbed thereon, by the use of near critic..

carbon dioxide; and Fig. 6 is a schematic diagram illustrating the application of the process of this invention to wastewater treatment.

As noted above, the inorganic adsorbents, e.g., activated carbon, alumina and silica have been used for a number of years and recently polymeric adsorbents have been widely used, particularly in wastewater treatments. The commercially available polymeric adsorbents may 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, the polymeric adsorbents may be homopolymers of styrene, copolymers of syrene and divinylbenzene, or a polymer containing an acrylic ester, trimethylolpropane trimethacrylate, or trimethylolpropane dimethacrylate. See for example Richard 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 adsorbents are given. See also German Offenlegungsschrift 1,943,807.

The polymeric adsorbents have found many varied applications in wastewater treatments.

For example, they have been used to decolorize pulp mill bleaching effluent and dye wastes and to remove pesticides from waste streams; a surfactant such as an alkylbenzene sulfonate or a linear alkyl sulfonate from wastewaters; and explosives such as TNT and DNT from effluent streams. These polymeric adsorbents have also been used in analysis procedures for determining trace amounts (as little as parts per billion) of organic contaminants in water, in chemical processing and in isolating enzymes and proteins as well as other pharmacologically 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 polymeric adsorbents are those listed in Table I 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% and 51% 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 a nominal mesh sizes 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-Endecanone 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 Table 1 (continued).

Alkyl benzenes Ethylbenzene Cumene p-Cymene Acids (acidified) Octanoic Decanoic Palmitic Oleic Benzoic Phenols Phenol o-Cresol 3,5-Xylenol o-Chlorophenol f Chlorophenol 2,4,6-Trichiorophenol 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 Benzoxazole As noted above, the 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 breakthrough point and defined as that point when 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 in the prior art by using an organic liquid 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 near critical liquid is used for adsorbent regeneration, whether the adsorbent is an inorganic material such as activated carbon or a polymeric resin.

A near critical liquid as the term implies is a liquid, the temperature and pressure of which are near the critical temperature and pressure. It is intermediate in solvation properties between a normal liquid under essentially atmospheric conditions and a supercritical fluid. In the present invention the solvent is a near critical liquid with the temperature thereof being from 0.95 to 0.995 times its critical temperature in "K and its pressure being at least equal to its vapour pressure at the temperature at which it is used.

Although pressures considerably in excess of the corersponding vapour pressuer may be used, it is preferable, from the standpoint of equipment design and energy required, to use pressures as close as possible to the corresponding vapour pressure of the liquid. Thus for example in the case of carbon dioxide, the critical temperature of which is 304.2"K (31.00 C), the near critical carbon dioxide liquid temperature should range between about 289"K and about 303"K. (between about 16 C and 30 C). At 25" C, which is within the near critical range specified, the vapour pressure of liquid carbon dioxide is 65 atmospheres. Therefore the preferable pressure range for 230 C carbon dioxide liquid lies between about 65 and 75 atmospheres.

Liquid carbon dioxide has been extensively investigated as a solvent for a large number of organic compounds of widely varying composition and structure and also for several inorganic compounds (see for example A. W.

Francis, J. Phys. Chem., volume 58, 1099 (1954) and A. W. Francis, Ind. Eng. Chem., volume 47, No. 2, 232 (1955)).

The solubility properties in the near critical liquid and supercritical fluid ranges can be illustrated using the naphthalene/carbon dioxide system as examplary. The solubility of naphthalene in carbon dioxide above and below critical temperature is reported in the literature (Yu. V. Tsekhanskaya, M. B. Iomtev, and E. V. Mushkina, Zh. Fiz. Khim., volume 36, 2187 (1962); Zh. Fiz. Khim., volume 38, 2166 (1964); Yu. V. Tsekhanskaya, N. G.

Roginskaya and E. V. Mushkina, Zh. Fiz.

Khim., volume 40, 2137 (1966); and E. L.

Quinn, J. Amer. Chem. Soc., volume 50, 672 (1928)).

The solubility data from these references are plotted as a function of specific volume in Fig. 1 for supercritical conditions (35 C, 45 C and 55" C); and points for the near critical range (200 C and 25" C) as well as for temperatures below the near critical range (100 C, 00 C and --20" C) are located on the plot of Fig. 1. From the data available for the supercritical range it is apparent that there is a trend of increasing solubility with decreasing specific volume. At constant volume, increasing the temperature brings about an increase in solubility whether in the supercritical or subcritical range.

In order to correlate these data for variabel temperature, a new parameter, represented by Xx', was defined as follows: L P vp- L where L X' = Xs where P S vpN P VPN S and P VPs are the vapour pressures of liquid and solid naphthalene at the temperature for which the naphthalene solubility, Xx, was determined.

A graph of Xx' agains specific volume is shown in Fig. 2, from which it can be seen that the factor X's materially reduces the temperature variation. In fact, all of the subcritical data fall on a smooth curve only slightly displaced from the 35 C curve.

The correlation of Fig. 2 was then used to develop a graph of naphthalene solubility in carbon dioxide ranging from subcritical to supercritical conditions. At any given temperature and pressure, the specific volume was determined from naphthalene/carbon dioxide mixture data, when available, or from pure carbon dioxide properties. (See for example, M. P. Vukalovich and V. V. Altunin, "Thermophysical Properties of Carbon Dioxide". Collet's Ltd., London, 1968). The value of X' was then determined from Fig.

2 and Xx was back calculated by multiplying S L XN' by P /P vp, vps The results of these calculations are plotted in Fig. 3. The solid lines are isobaric data; and the dashed line represents data for saturated liquid and saturated vapor below and up to the critical point It is to be noted from Fig. 3 that at high pressure (above 120 atmospheres) the solubility of the solute naphthalene decreases gradually with decreasing temperature from supercritical to subcritical conditions. At lower pressures (70 to 100 atmospheres), the solubility peaks in the near critical liquid region; and at 80 atmospheres the peak solubility occurs at 27 to 280 C. It should also be noted that on the saturation curve (dashed line), the solubility peaks in the near critical liquid region (25 to 27 C) and decreases very sharply through the critical point and on the saturated vapor line. At 25 C and 65 atmospheres, the solubilities in the saturated liquid and vapor are 0.0065 and 0.00044, respectively.

The data presented in Figs. 1-3 are illustrative of one adsorbate/near critical liquid solvent combination to which the process of this invention is applicable. As will be apparent from the published literature cited above, car bon dioxide in the near critical liquid state has been shown to be an effective solvent for many of the organic materials listed in Table 1 which can be adsorbed on polymeric adsorbents. There are, of course, a large number of other compounds, both inorganic and organic, which are suitable as near critical liquids for the practice of this invention. Selected Examples of near critical liquid solvents, their near critical temperature ranges and their enthalpies of vaporization, AHv, are listed in Table 2.

According to the process of this invention, the near critical liquid solvent is separated from the adsorbate, and, optionally recovered for recycling, by vaporizing the near critical liquid. This in turn requires that energy in the form of heat and equivalent to the latent heat of vaporization must be supplied to the system for adsorbate separation and that energy in the form of refrigeration and equivalent to the latent heat of condensation must be supplied for solvent reliquefaction. Thus the enthalpy of solvent vaporization is an important factor in the energy requirement, and hence the economics, of the adsorbent regeneration.

TABLE 2 Near Critical Properties for Selected Liquids

0.95Tc 0.995to AHv Fluid Tc, OK OC OK OC (Btu/lb) Carbon dioxide 304.2 289 16 303 30 150 Ammonia 405.5 385 112 403 130 545 Water 647.6 615 342 644 371 900 Methanol 513.7 488 215 511 238 461 Ethanol 516.6 491 218 514 241 374 Isopropanol 508.5 483 210 506 233 163 Ethane 305.6 290 17 304 31 229 Nitrous oxide 309.7 294 21 308 35 151 n-Propane 370.0 352 79 368 95 175 n-Butane 425.2 404 131 423 150 158 n-Pentane 469.8 446 173 467 194 144 n-Hexane 507.4 482 209 505 232 140 n-Hep tane 540.1 513 240 537 264 127 2,3-Dimethylbutane 500.0 475 202 498 225 Benzene 562.1 534 261 559 286 162 Dichlorodifluoromethane 384.9 366 liquid at T, and T1, respetcively, and Tc and Tc 2 1 are the reduced temperatures (T2/Tc and Tl/Te), respectively. Watson's correlation, following the above-stated relationship, is plotted as the solid line in Fig. 4. To normalize the ordinate for Fig. 4, the base value of AH 1 was taken as the enthalpy of vaporization at Tc = 6.7.

1 For many liquids, a reduced temperature of 0.65 to 0.70 is approximately equal to the normal boiling point. Thus, AH, 1 is approximately equal to the enthalpy of vaporization at the normal boiling point. Data for four exemplary solvents suitable for the practice of this invention (carbon dioxide, ammonia, propane and ethylene) are also shown Fig. 4 and these data indicate that the Watson correlation is applicable to a broad range of solvents.

The data of Fig. 4 substantiate the abovestated definition for the near critical liquid range as used herein, i.e., a temperature of from 0.95 to 0.995 times the critical temperature. Within the near critical range the enthalpy of vaporization, aH,, is equal to or less than one-half aH, at the normal boiling point of the solvent.

The process of this invention may be further described and illustrated using naphthalene as the adsorbate, a polymeric resin adsorbent and near critical carbon dioxide liquid as the solvent/adsorbent regenerating material. A schematic for this system is detailed in Fig.

5.

The adsorbent, e.g., a nonpolar polystyrene resin sold as XAD-2 by Rohm and Haas Company, is placed in a pressure vessel 10 serving ai the desorber. This polymeric adsorbent is characterized as having a porosity volume of 42By, a true wet density of 1.02, a surface area of of 300 m2/gram, an average pore diameter of 90" A, a skeletal density of 1.07 grams/cc and a nominal mesh size of 20 to 50 (U.S.

standard sieves). During the adsorbing cycle, water containing naphthalene is introduced through valve-controlled line 11 into desorber 10 constituting a vessel and clean, naphthalene-free water is discharged through valvecontrolled line 12. When the break-through point is reached in the water in line 12, lines 11 and 12 are shut off.

Liquid carbon dioxide at 25 C and 65 atmospheres, conditions which place the solvent within the required near critical liquid range, is then introduced into desorber 10, which also constitutes a pressure vessel, through valve-controlled line 13 and withdrawn, with naphthalene dissolved therein, through valvecontrolled line 14. At that point in the regeneration cycle when the liquid carbon dioxide being discharged from desorber 10 no longer contains naphthalene (detected spectrographically or by any other suitable technique), the flow of liquid carbon dioxide is stopped. As noted in Fig. 5, the solubility of naphthalene in saturated carbon dioxide liquid at 25 C and 65 atmospheres is 0.0065 mole fraction and the specific volume of the naphthalene is 62.4 cm3 per mole of carbon dioxide. This, of course, represents the upper limit of the concentration of the naphthalene in the carbon dioxide. Liquid carbon dioxide containing up to this concentration of naphthalene therefore represents the solvent which must be treated for recovery.

The liquid carbon dioxide containing the naphthalene is taken to the solute recovery vessel 15, serving as a vaporizer/separator, in which the carbon dioxide is at least partially vaporized and a multi-phase system comprising solvent-rich vapour and an adsorbate-rich phase is formed. A carbon dioxide-rich liquid may also be present in recovery vessel 15.

In a preferable embodiment of the process, a substantial portion, e.g., over 50%, of the near critical liquid is vaporized. In order to maintain the concentration of any adsorbate in the solvent vapor at a predetermined level it may be necessary to distil adsorbate out of, and thereby enrich the solvent-rich vapour.

This may be done by using one or more distillation plates 15a in or associated with the vaporizer/separator 15. If carbon dioxiderich liquid is present it may be taken with the vapor out of vaporizer/separator to become part of the recycled near critical liquid.

Vaporization of the liquid carbon dioxide is accomplished by supplying to it an amount of heat equivalent to the latent heat of vaporization of the carbon dioxide. As shown in Fig. 5, this may be done by circulating water an an appropriate temperature through coils 16 immersed in the carbon dioxide in vessel 15. The naphthalene thus separated out may be removed periodically from vessel 15 through a discharge line 17.

The carbon dioxide vapor leaving solute recovery vessel 15 is then taken, along with any carbon dioxide-rich liquid, through line 17 to condenser 18 where sufficient cooling is supplied to recondense the carbon dioxide vapour to a liquid. In this transfer to the condenser the temperature of the carbon dioxide is slightly reduced to 23 C. This slight reduction in temperature from recovery vessel 15 to condenser 18 provides a slight pressure drop to about 60 atmospheres to drive the vapour from one vessel to the other. The concentration of the naphthalene in the car bon dioxide vapor may be about 0.0003 mole fraction and it will remain at more or less this level during subsequent recycling. The condensed carbon dioxide is then directed through line 19 into recirculation pump 20 where it is repressurized to 65 atmospheres and the temperature brought back up to the desired 25 C for reintroduction by way of line 13 into desorber 10.

Normally the adsorbent will not be dried prior to desorption in desorber 10 since water can be removed by the near critical carbon dioxide liquid and subsequently separated from it in the vaporizer/separator. However, it may be desirable in some cases to remove residual water from the adsorbent.

If so, prior to regeneration of the adsorbent by desorbing with a near critical liquid, a drying gas, e.g. hot air, may be passed over the spent adsorbent to remove residual water by introducing it through line 21 and withdrawing it through line 22. Then carbon dioxide at atmospheric pressure is passed through the dried spent adsorbent to remove any air remaining in the pores of the spent adsorbent.

Based upon one pound of naphthalene recovered in the system illustrated in Fig. 5, the carbon dioxide recirculated in the system is 53 pounds, the heat transferred in the solute recovery vessel 15 and in the condenser 18 amounts to 2,710 Btu, and the work of recompression is 16 Btu. To remove an equivalent amount of naphthalene from an adsorbent using carbon dioxide in the supercritical state would require but about one-half as much supercritical fluid and less than one-half the amount of heat transferred. However, the work required in using near critical carbon dioxide is less than that for supercritical by a factor of about 20. Although the operating costs are probably comparable in the two processes, use of a near critical liquid as the adsorbate solvent requires considerably lower desorption pressure than the use of the same solvent in the supercritical state (e.g., for carbon dioxide 65 atmospheres compared with 300 atmospheres). This requirement for lower pressure will, in turn, be reflected in lower capital costs for the use of near critical liquids.

Although the mass transfer rate for desorption may generally be higher for supercritical fluids than for near critical liquid (since diffusivity increases with increasing temperature and decreasing density) this factor may be compensated for by increasing the residence time of the solvent in the desorber which, in turn, may require a larger vessel for desorption. For some systems, however, the desorption process may not be limited by mass transfer in the fluid and therefore any difference in diffusivity rates will not be a significant process factor.

The process of the invention may include the step of distilling the solvent-rich vapour to separate out an additional quantity of said adsorbate.

The incorporation of the adsorbent regeneration proces of this invention into a wastewater purificaton system such as the one detailed above is illustrated diagrammatically in Fig.

6. The apparatus of Fig. 5 is employed; and since like reference numerals have been used to describe like components the description of the circulation of the supercritical fluid need not be repeated.

Fig. 6 illustrates the use of two alternating desorbers 10a and 10b which are cycled so that while one is in use the other may be regenerated. This is, of course, a well-known arrangement and any suitable number of desorbers may be used in parallel as well as in series. The wastewater to be purified is introduced through lines 1 la and 1 lib into column 10 and 10b, depending upon which wastewater inlet line is open. Desorbers 10a and 10b are packed with the appropriate adsorbent to adsorb impurities and the treated water is dicharged through line 12 by way of either 12a or 12b. If, for example, column 10b is off stream, it can be got ready for re-use by circulating the near critical liquid therethrough in the manner described above in connection with Fig. 5. Likewise, when the stream of treated water discharged from desorber 108 has reached the breakthrough point, the desorbers are switched over.

As an alternative to performing both adsorption and desorption in columns 10 and 10b, a separate desorbing vesel 25 may be provided. In this case spent absorbent is transferred alterantely from columns 10a and 10b into desorber 25 and the near critical liquid is introduced into and withdrawn from desorber 25 rather than columns 10a and 10b.

The regenerated adsorbent is then returned to these columns through a transfer line 27.

In some cases it may be desirable to alter the chemical nature, and hence physical properties, of the adsorbate subsequent to its removal from the adsorbent. This may be done by reacting the adsorbate with a suitable reactant while it is dissolved in or mixed with the near critical liquid remaining in the solvent recovery vessel 15 at the end of a solvent recovery cycle. Any reactant used for the adsorbate must, of course, not be a reactant for the near critical liquid. Exemplary of such a reactant is oxygen to oxidize a hydrocarbon adsorbate when carbon dioxide ie the near critical liquid.

As noted previously, there is a wide range of organic materials which can be adsorbed on a number of different types of adsorbents, both inorganic and organic. There are also a number of compounds capable of serving as near critical liquid solvents for removing these organic adsorbates from the adsorbent to regenerate it. Table 2 lists, by way of example, various near critical liquids which are suit able for use in the process of this invention.

Among the other materials commonly used industrially as solvents and suitable for the practice of this invention are methane, propyl ene, haloethanes and halomethanes, sulfur di oxide, hydrogen chloride and hydrogen sul fide.

In choosing a near critical liquid for the regeneration of an adsorbent containing one or more organic species adsorbed thereon, the near critical liquid must be a solvent for the absorbate to be removed and it must be a liquid which does not react with the sur face of the adsorbent.

By using a near critical liquid to dissolve off the adsorbates from an adsorbent, the ad sorbent is not subjetced to any appreciable thermal or chemical degradation and the ad sorbate species may be recovered if desired.

Moreover, it is possible to use such near critical liquids as carbon dioxide, ethane or ethylene which require temperatures and pressures well within the capabilities of existing equipment.

Finally, these fluids (and particularly carbon dioxide) are inexpensive, a fact which con tributes materially to improving the economics of industrial processes and wastewater purifica tion. Moreover, carbon dioxide is non polluting.

WHAT WE CLAIM IS: 1. A process for desorbing an adsorbate from an adsorbent, comprising contacting said ad sorbent having said adsorbate adhered thereto with a solvent for said adsorbate, said sol vent being a near critical liquid with the temperature thereof being from 0.95 to 0.995 times its critical temperature in "K and its pressure being at least equal to its vapour pressure at the temperature at which it is used, thereby to dissolve said adsorbate in said liquid and desorb it from said adsorbent.

2. A process as claimed in claim 1, further comprising the steps of: (a) separating said near critical liquid with said adsorbate dissolved therein from said adsorbent; (b) vaporizing at least a portion of said near critical liquid containing said adsorbate dissolved therein to fonn a multi-phase system comprising solvent-rich vapour and an ad sorbate-rich phase; (c) condensing said solvent-rich vapour to form a liquid; and (d) adjusting the temperature and pressure of said liquid from step (c) thereby to con vert it to said near critical liquid for recycling ;-$e process.

~ rA process as claimed in claim 1 or claim -t ein said adsorbate comprises at least *ic material.

ss as claimed in claim 3, wherein one organic material has been deposited on said adsorbent from wastewater.

5. A process as claimed in claim 3 or 4, as dependent on claim 2, including, prior to the steps set out in claim 2, the steps of: (a) contacting the adsorbent with water containing said organic material under conditions such that said organic material is adsorbed as an adsorbate on said adsorbent; and (b) then contacting said adsorbent with said organic material adhered thereto with a near critical liquid solvent in order to dissolve said organic material in said solvent.

6. A process as claimed in claim 5, wherein said water comprises the bleaching effluent from a pulp mill and said organic material comprises coloured bodies.

7. A process as claimed in claim 5, wherein said water comprises waste dyestuffs as said organic material.

8. A process as claimed in claim 5, wherein said water contains a pesticide, a surfactant or an explosive as said organic material.

9. A process as claimed in claim 5, wherein said water contains a pharmacologically active material as said organic material.

10. A process as claimed in any one of the preceding claims, wherein said adsorbent comprises an inorganic adsorbent 11. A process as claimed in claim 10, wherein said inorganic adsorbent comprises activated carbon.

12. A process as claimed in any one of claims 1 to 9, wherein said adsorbent comprises a synthetic polymeric adsorbent.

13. A process as claimed in claim 12, 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.

14. A process as claimed in any one of the preceding claims, wherein said solvent is carbon dioxide at a temperature within the range from 160 C to 300 C.

15. A process as claimed in claim 2, or any of claims 3 to 14 as dependent on claim 2, including the step of distilling said solventrich vapour to separate out an additional quantity of said adsorbate.

16. Apparatus for desorbing an adsorbate from an adsorbent, which comprises: (a) a pressure vessel within which to effect contact between said adsorbent having ~ said adsorbate adhering thereto and a liquid solvent for said adsorbate, said solvent being a near critical liquid with the temperature thereof being from 0.95 to 0.995 times its critical temperature in "K and its pressure being at least equal to its vapour pressure at the temperature at which ir is used, said vessel being provided with

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

Claims (1)

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

   various near critical liquids which are suit able for use in the process of this invention.

   Among the other materials commonly used industrially as solvents and suitable for the practice of this invention are methane, propyl ene, haloethanes and halomethanes, sulfur di oxide, hydrogen chloride and hydrogen sul fide.

   In choosing a near critical liquid for the regeneration of an adsorbent containing one or more organic species adsorbed thereon, the near critical liquid must be a solvent for the absorbate to be removed and it must be a liquid which does not react with the sur face of the adsorbent.

   By using a near critical liquid to dissolve off the adsorbates from an adsorbent, the ad sorbent is not subjetced to any appreciable thermal or chemical degradation and the ad sorbate species may be recovered if desired.

   Moreover, it is possible to use such near critical liquids as carbon dioxide, ethane or ethylene which require temperatures and pressures well within the capabilities of existing equipment.

   Finally, these fluids (and particularly carbon dioxide) are inexpensive, a fact which con tributes materially to improving the economics of industrial processes and wastewater purifica tion. Moreover, carbon dioxide is non polluting.

   WHAT WE CLAIM IS:

   1. A process for desorbing an adsorbate from an adsorbent, comprising contacting said ad sorbent having said adsorbate adhered thereto with a solvent for said adsorbate, said sol vent being a near critical liquid with the temperature thereof being from 0.95 to 0.995 times its critical temperature in "K and its pressure being at least equal to its vapour pressure at the temperature at which it is used, thereby to dissolve said adsorbate in said liquid and desorb it from said adsorbent.

   2. A process as claimed in claim 1, further comprising the steps of: (a) separating said near critical liquid with said adsorbate dissolved therein from said adsorbent; (b) vaporizing at least a portion of said near critical liquid containing said adsorbate dissolved therein to fonn a multi-phase system comprising solvent-rich vapour and an ad sorbate-rich phase; (c) condensing said solvent-rich vapour to form a liquid; and (d) adjusting the temperature and pressure of said liquid from step (c) thereby to con vert it to said near critical liquid for recycling ;-$e process.

   ~ rA process as claimed in claim 1 or claim -t ein said adsorbate comprises at least *ic material.

   ss as claimed in claim 3, wherein one organic material has been deposited on said adsorbent from wastewater.

   5. A process as claimed in claim 3 or 4, as dependent on claim 2, including, prior to the steps set out in claim 2, the steps of: (a) contacting the adsorbent with water containing said organic material under conditions such that said organic material is adsorbed as an adsorbate on said adsorbent; and (b) then contacting said adsorbent with said organic material adhered thereto with a near critical liquid solvent in order to dissolve said organic material in said solvent.

   6. A process as claimed in claim 5, wherein said water comprises the bleaching effluent from a pulp mill and said organic material comprises coloured bodies.

   7. A process as claimed in claim 5, wherein said water comprises waste dyestuffs as said organic material.

   8. A process as claimed in claim 5, wherein said water contains a pesticide, a surfactant or an explosive as said organic material.

   9. A process as claimed in claim 5, wherein said water contains a pharmacologically active material as said organic material.

   10. A process as claimed in any one of the preceding claims, wherein said adsorbent comprises an inorganic adsorbent

   11. A process as claimed in claim 10, wherein said inorganic adsorbent comprises activated carbon.

   12. A process as claimed in any one of claims 1 to 9, wherein said adsorbent comprises a synthetic polymeric adsorbent.

   13. A process as claimed in claim 12, 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.

   14. A process as claimed in any one of the preceding claims, wherein said solvent is carbon dioxide at a temperature within the range from 160 C to 300 C.

   15. A process as claimed in claim 2, or any of claims 3 to 14 as dependent on claim 2, including the step of distilling said solventrich vapour to separate out an additional quantity of said adsorbate.

   16. Apparatus for desorbing an adsorbate from an adsorbent, which comprises: (a) a pressure vessel within which to effect contact between said adsorbent having ~ said adsorbate adhering thereto and a liquid solvent for said adsorbate, said solvent being a near critical liquid with the temperature thereof being from 0.95 to 0.995 times its critical temperature in "K and its pressure being at least equal to its vapour pressure at the temperature at which ir is used, said vessel being provided with

   means for maintaining conditions of temperature and pressure which will maintain said solvent within said conditions of temperature and pressure, whereby said adsorbate can be desorbed from said adsorbent and be taken up in the near critical liquid solvent; (b) a vaporizer/separator device for vaporizing at least a portion of said near critical liquid as a solvent-rich vapour and for separating and collecting said adsorbate as an adsorbate-rich liquid phase separate from said vapour; (c) fluid conduit means connecting said pressure vessel and said vaporizer/separator device for conducting said near critical liquid solvent containing said adsorbate into said vaporizer/separator device; (d) a condenser for condensing said solventrich vapour, received from said vaporizer/ separator device to a liquid; and (e) means for adjusting the temperature and pressure of said liquid from said condenser to within the said conditions of temperature and pressure for recycling.

   17. Apparatus as claimed in claim 16, additionally for treating a fluid stream to remove material therefrom, further comprising: (a) a vessel within which to effect contact between said fluid stream containing said material and an adsorbent contained within said vessel; and (b) means for circulating said fluid containing said material through said vessel whereby said material is adsorbed as an adsorbate on said adsorbent.

   18. Apparatus as claimed in claim 17, wherein said vessel also serves as said pressure vessel.

   19. Apparatus as claimed in claim 16, 17 or 18, including means for enriching said solventrich vapour in order to purify said vapour for recycling and recovering further quantities of said adsorbate.

   20. Apparatus as claimed in any one of claims 16 to 19, wherein said vaporizer/ separator device comprises means for effecting indirect heat exchange between said near critical liquid and an externally supplied heat transfer fluid, and means for removing said adsorbate collected therein.

   21. Apparatus as claimed in any one of claims 16 to 20, wherein said means for adjusting the temperature and pressure of said liquid from said condenser comprises a pump for liquid.

   22. A process for desorbing an adsorbate from an adsorbent, substantially as hereinbefore described with reference to Figure 4 or Figure 6 of the accompanying drawings.

   23. Apparatus for desorbing an adsorbate from an adsorbent substantially as hereinbefore described with reference to Figure 5 or Figure 6 of the accompanying drawings.