CA1036724A - Separation process - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

The instant invention relates to a method for removing dissolved compounds from solution by decomposing them and removing the decomposition products continuously and simultaneously by at least two different mechanisms. The decomposition is an equilibrium reaction, thus simultaneous removal from the solution of two or more products of decomposition results in efficient and complete removal. Preferably, the dissolved compound is the hydrolyzable salt of a weak acid and a weak base in an aqueous solution. The simultaneous removal processes for the hydrolysis products include stripping, distillation, extraction, adsorption, and liquid membrane separations. Preferably, more than one process is utilized to remove the separate decomposition products; that is, the method utilized herein relates to a process wherein one decomposition product is removed by adsorption on or reaction with a solid sorbent, while the other decomposition product is simultaneously stripped with steam.
The removal processes are further selected so as to leave behind a solution which is substantially free from the starting compound and any of its undesirable decomposition products.

Description

-` ~o36~7z4 FIELD OF THE INVENTION
BACKGROUND OF THE INVENTION
The instant invention relates to a method for removing dissolved compounds from solution by decomposing them and removing the decomposition products continuously and simultaneously by at least two different mechanisms. The decomposition is an equilibrium reaction, thus simultaneous removal from the solution of two or more products of decom-position results in efficient and complete removal. Pre~
ferably, the dissolved compound is the hydrolyzable salt of a weak acid and a weak base in an aqueous solution. The simultaneous removal processes for the hydrolysis products include stripping, distillation, extraction, adsorption, and liquid membrane separations. Preferably, more than one process is utilized to remove the separate decomposition products; that is, the method utilized herein relates to a process wherein one decomposition product is removed by adsorption on or reaction with a solid sorbent, while the other decomposition product is simultaneously stripped with steam. The removal processes are further selected so as to leave behind a solution which is substantially free from the starting compound and any of its undesirable decomposition products.
DESCRIPTION OF THE PRIOR ART
A process for separating volatile bases, e.g.
ammonia, and weak volatile acids, e.g. H2S, from their salts or the liquid or gaseous dissociation products thereof, which comprises contacting these salts in a first step with a nonvolatile acid, e.g., benzoic acid, in the presence of at least one liquid phase at a temperature and pressure at which the volatile acid is evolved in the form of a gas, and then in a second step increasing the temperature or reducing

2.

~0367;Z4 the pressure to liberate the volatile base and regenerate the acid, is taught in U.S. Patent No~ 3,649,190. This process requires two steps since the same reagent is utilized to remove both the volatile acid, i.e., by neutralization, and the volatile base, i.e., by decomposition. The process further requires two steps since the second step, wherein the base is removed from its reaction product with said nonvolatile acid, is necessarily carried out at a higher temperature or lower pressure than the first step. This process is generally 10 more inefficient than a one-step process wherein said weak acid and weak base may be removed simultaneously.
U.S. Patent No. 3,620,674 teaches the reverse of the above process; that is, a two-stage process for regenerating volatile bases and weak volatile acids from the salts which comprises contacting in a first stage the salt with an organic base of low volatility to liberate the volatile base in a gaseous form and liberating the weak volatile acid in a gaseous form in a second stage while regenerating the base of low volatility in the liquid phase. Once again, it is a two-step process which is taught which is necessarily less efficientthan a one-step process which removes the same weak acids and weak bases. Furthermore, in both cases it is apparent that the non-volatile acid or base which is utilized to remove the volatile acid or base from solution is necessarily left behind and would cause further problems in that the nonvolatile acid or base is a contaminant of the aqueous solution and may have to be separated therefrom. It is apparent from this discussion that the processes of U.S. 3,620,674 and 3,649,190 are eminently unsuitable to the treatment of dilute aqueous solutions of said weak acid/
weak base salts, and this is precisely the field in which 1 the process of the present application is most applicable, 2 i.e. water pollution abatement.

3 Other processes for the removal of H2S and ammonia

4 ~rom aqueous solutions are known in the art. See, for example~ U.S. patent 3,518,166 which describes the difficul-6 ties which are encountered in a proce3s wherein H2S and 7 ammonia are separated bly means o~ the prior art methods.
8 The patentee deals with t~ese difficulttes by utilizi~g 9 a multi-step process wherein H2S is stripped out of an aqueous solution in a first distillation column to obtain 11 an H2S rich overhead stream and an aqueous bottom stream l2 of reduced H2S content. Ammonia is stripped out of said 13 aqueous bottom stream in a second distillati3n column to 14 obtain an ammonia-rich vapor overhead which is partially condensed to obtain an ammonia-rich vapor and an ammonia-l6 rich overhead condensate. The ammonia rich condensate is 17 combined with the aqueous solution containing H2S and 18 ammonia, which is passed to the first distillation column.
19 This reference is cited to show the dif~iculties enco~ntered in removing ammonium sulfide from aqueous solution and the 21 complicated methods employed at present.
22 Also, in all the above processes, pH adjustment 23 of the feed to the various stages is frequently necessary 24 to obtain satisfactory operation. This, of course9 leads to high chemical costs, especially when large volumes of 26 dilute waste water streams are involved, and further results 27 in contamination of the treated waste water with the ions 28 introduced with the pH ad~usting agents.
29 Other processes are known wherein the salts of weak bases with strong acids are separated by precipitation 31 methods. See, for example, U.S. Patent NoO 3,321,275.

1 Water softening processes are known where-2 in cations and anions are removed from aqueous solution 3 by contacting with a mixture of two ion exchange resins 4 wherein both anionic and cationic exchange sites are available. This process differs from the instant inven-6 tion in that the ions which are removed irom solution are 7 not in equilibrium, i.e., usually they are salts of strong 8 acids and strong bases, and thus the removal o one; that 9 is, the removal of either the cation or the anion, does not affect the removal of the other. Consequently, the 11 rate of removal nor the complèteness of removal of either 12 ion by the proper ion exchange resin is not affected by 13 the absence or presence of the opposite ion exchange 14 resin for removal of the other ion. In the instant in-vention, as will be clearly shown, such simultaneous removal is not only convenient from an engineering and 17 economic point of view, but is absolutely essential 18 to achieve high removal rates and complete removal of 19 the original compound and its decomposition products.
SUMM~RY OF THE INVENTION
21 The instant invention relates to a method 22 for removing a dissolved compound from its solution by 23 allowing the compound to decompose and removing the de-24 composition products continuously and simultaneously by two different mechanisms. The decomposition of the com-26 pound is an equilibrium reaction and thus the simultaneous 27 removal from the solution of the decomposition products is 28 required to achieve rapid and complete removal. Thus, 29 this invention is generally drawn toward a process for re val of decomposition products of a compound from solution ~036q24 1 wherein the removal of one decomposition product without 2 the other hinders total re val of the compound and the 3 decomposition products.
4 The process of the instant invention specifically S excludes removal of decomposition products by addition of 6 chemical reactants which would leave behind soluble pro-7 ducts, since these soluble products would have to be 8 removed subsequently.
9 A preferred commercial embodiment of the instant invention is the removal of a salt of a weak acid and a 11 weak base from an aqueous solution. In the prior art 12 processes, removal of either the weak acid or weak base 13 generally results in changing the pH of the solution 14 and thus hinders the subsequent removal of the weak acid or weak base species left behind. This phenomenon may be 16 more fully understood by reference to the following 17 formulas.
18 Consider the weak acid HA and weak base BOH and 19 `their salt BA. When salt BA is dissolved in water, it ionizes completely according to the equation 21 (1) BA ~ B+ + A , 22 but then the ions interact with the H+ and OH- present due 23 to the water equilibrium 24 (2) H2O ~ H+ + OH-where kW ~ ~H~ 10H-~ ~ 10 14 at 25C and ~] represents 26 molar concentrations of the particular ions. The inter-27 action of the salt ions with the water decomposition ions 28 is governed by the acid and base equ~libria 29 (3) HA ~ H+ + A-~0367'~4 1 where Ka ~ ~ for the acid 2 and (4) BOH ~ B+ + OH
3 where Kb ' ~B~ rOH-l for the base 4 L~oH] ~
S Normally, the rate of removal of the acid, HA, 6 or base, BOH, is a direct function of the concentration 7 of the undissociated species HA or BOH in the aqueous 8 solution. The reason is that it is the undissociated form 9 which exerts a vapor pressure, or establishes solubility distribution between a solvent and the aqueous phase, 11 or an adsorption equilibrium with a solid phase. Con-12 sequently, the higher ~HA] and tBOH] are at any given con-13 dition in the aqueous solution, the higher their rate of 14 removal by whatever extraction, stripping, adsorption, etc. process is being èmployed. But from (3), 16 (5) ~H] - ~H~lrA 1 18 and from (4), (6) 1BOH] - lB+l~oH 1 - ~ X RW
21 Therefore, as 1H+] increases, i.e. the solution becomes more 22 acidic, [HA] tends to increase, which favors the removal 23 rate of the acid, HA, while lBOH] tends to decrease, making 24 ~he base BOH harder to take ou~ of solution. Therefore, if base BOH is removed by some technique $rom the solution, 26 since the solution becomes more acidic, BOH removal will 27 become increasingly hard not only because the total "B"
28 concentration (B+ + BOH) decreases, but also because of the 29 effect of the ionic equilibria. The result is that BOH
removal ceases rapidly.

~036~24 1 By the same argument, it can be shown that as HA
~ ls removed, since the solution becomes more basic, HA
3 removal becomes rapidly more and more diffi^ult and stops 4 long before complete removal can be achieved.
The answer is therefore to remove both decomposi-6 tion or hydrolysis product simultaneously~ since the 7 removal of one enhances the ease o~ removal o~ the other.
8 While the desire to achieve such a simultaneous removal is 9 apparent from the above mechanistic explanation9 the means o~ àchieving it e~fectively is the obJect of this invention.
11 It is to be noted that it is important that the 12 removal of A and B both be irreversible so as not to make 13 the removal of either the decomposition products or the 14 compounds complicated by further equilibrium processes.
The method o~ the instant inven+ion is a general 16 process; it is not strictly limi+ed to the removal of the 17 salts of weak acids and weak bases from aqueous solution.
18 Further, the ~eneral process is not restric+ed to removal 19 of compounds which break down to yield only two decomposi-tion products; that is, ary compound which ~reaks down to 21 yield two or more decomposition products, which decomposi-22 tion products in equilibrium with the origin~l compound may 23 be successfully treated by the instant process. More than 24 one process must be utilized to effectively remove each individual decomposltion product to prevent recomb~nation 26 of the decomposltion products after removal9 but where mor-e 27 than two decomposition products are in equilibrium with the 28 compound, one removal mecharlsm may, of course9 be utillzed 29 to remove more than one decomposition product. For example9 when it is desired to remove ammonium hydrosulfide from 31 dilute aqueous solutlon, the ammonia can be removed ~y per-32 meation into a liquid mem~rane emulsion in whish an aqueous ~036~24 acid solution is emulsified into an oil phase, while the H2S
is removed at the same time by stripping the solution with an inert gas or steam. With ammonium carbonate, the same procedure can be used, except that'it is CO2 which is gas-stripped out. When both salts are present, the hydrolysis products are NH3, H2S and CO2, and the ammonia can be taken out via the acidic liquid membrane, while both the H2S and C2 are steam-stripped out. If the contaminants are a mixture of amine salts of weak volatile acids such as HCN, H2S, CO2 and the like, there will be many simultaneous hydrolysis products, but all the basic products (i.e. amines) will be removed via the liquid membrane, while all the acids can then be stripped out by means of air, inert gas or steam.
Of course, the liquid membrane can be replaced by an appropriate ion exchange resin, such as an acidic material of the sulfonate or acrylate type which will combine with the ammonium ion, methylamine ion or the cation of whatever basic compound is present in the aqueous solution. The other, acidic decomposition product, e.g. H2S, can again be stripped out simultaneously by the vapor route, but it may also be extracted out using a hydrocarbon solvent.
It is important to point out here why the simultaneous removal of both the acidic and basic decomposition products is important even if one of the constituents is removed by means of an ion exchange resin. Since the latter clearly removes ions, the argument presented previously in regard to maintaining a high concentration of undissociated speaies by the simultaneous removal clearly does not apply to that species which is removed by ion exchange. It does, however, still apply to the other species, which is not removed by ion exchange~such as the H2S which is taken out by vapor stripping.

9.

~036~Z4 1 Ion exchange, such as the removal of NH4+ ion 2 by the acid form of a sul~onate resin or polyacrylic acid 3 resin, is a competitive process between the NH4+ ions and 4 the H~ ions present in the solution. At any given total ammonia concentration(sum o~ NH3 ànd NH4~ concentrations), 6 the capacity of the ion exchange resin for NH4~ lon is a 7 direct function of the ~NH4+]/~H+] ratio, being greater 8 the higher this ratio. Now, ~t the pH's ordinarlly encoun-9 tered in waste waters, i.e. pH 5-9~ the NH3 is over 90%
in the NH4+ form. Consequently, o~er this pH range, 11 ~NH4 ~/~H+] varies nearly lO fold, lnversely as ~H+], or l2 directly wlth pH. This means the higher pH, the higher 13 ~NH~+~/~H ] and the greater the tendency for NH4~ to go 14 onto the resin, and the lower pH, the less likel~ for NH4 to be picked up by the resin.
16 Since this trend is exactly the same as that in 17 permeation or vaporization, the same argument holds and 18 simultaneous removal of the two decomposition fragments is 19 beneficial to both the removal processes9 including the ion exchange route.
21 It is apparent that by the use o~ a basic ion 22 exchange resin, or of a liquld membrane with encapsulated 23 ~aslc solutlon, it ls possible to extract or otherwise 24 remove the acidlc constituent formed by the decomposition of the original salt, allowing the base, e.g. NH3 or amine, 26 to be vapor stripped or solvent extracted. Neither the 27 liquid membrane extraction nor vapor stripping alone could 28 proceed with any degree of completion if carried out alone 29 for reasons discussed earlier.
Other examples of processes which would be within 31 the scope of the instant invention include the removal of 32 (l) ammonium salts of ~eak volatlle acids such as CO23 HCN9 1036~724 1 S02, from aqueous streams by liquid membrane extraction of 2 NH3 and stripping the acid; (2) ammonillm salts of weak, 3 nonvolatile organic acids as acetic, benzoic, naphthenic 4 by ion exchange resin extraction of NH3, and solvent extraction of the organic acid; (3) amine salts of volatile 6 acids by li~uid membrane or ion exchange extraction of the 7 amines while stripping the acid; and (4) metal salts o~
8 organic acids by liquid membrane or ion exchange extraction 9 o~ the metal while solvent extracting or adsorbing of the orgànic acid~
11 In general, the removal mechanisms which are l2 utilized in the instant invention are know~ to the skilled 13 artisan and may be chosen according to ~pecific problems 14 ~aced. The novelty o~ the instant disclosure relates to the use o~ two or more removal processes to simultaneously 16 remove decomposition products which are in equilibrium 17 with a compound. Thus, the instant invention contemplates 18 that liquid membrane permeation, gas stripping, distilla-19 tion, extraction, adsorptiong ion exchange, chemical reaction to ~orm a separate phase, and solid membrane dif~usion may 21 all be utilized in the instant invention. Two or more of 22 these processes will be combined in the method of ~he 23 instant invention. Preferably the instant invention 24 relates to the removal of compounds which break down to yield cations and anions which are in equilibrium with the 26 original compound. Preferably the compound is dlssolved 27 ln an aqueous solution. In these pre~erred embodiments9 28 the cation, for example, may be removed by adsorption on a 29 solid support whlch lrreversibly adsorbs cations9 while the anlon may be removed simultaneously by permeation through a 31 solid membrane. Examples of combinatlons which are within 32 the scope of the invention in^lude the remova~ o~ ammonium 1036~7Z4 sulfide from dilute aqueous solution. The cation hydrolysis product (ammonia) may be extracted with an acid-emulsion using liquid membrane permeation, while the anion hydrolysis product (hydrogen sulfide) is stripped out using steam at reduced pressure. See our copending Canadian Application Serial No. 204,067 filed July 4, 1974.
A similar result can be achieved by using ion exchange beads dispersed in the dilute aqueous solution and vapor stripping this suspension in a countercurrent tower.
If a cation exchange resin such as polyacrylic acid is used, it will absorb the ammonia as the ammonium ion while the H2S is stripped out by the rising vapors. In place of a synthe-tic ion exchange resin, a natural regenerably ammonia adsorbent such as clinoptilolite clay can be employed.
A process achieving the removal of dilute ammonium sulfide or carbonate or both from waste water using dispersed ion exchange beads combined with counter-current vapor stripping may be carried out in equipment similar to the vapor-liquid contacting column described in the above-cited copending patent application. Into the sour water feed containing up to 15,000 ppm of ammonium sulfide is slurried an acid-type ion exchange xesin, such as Permutit*~-70 (cross-linked acrylic acid) or Dowex*-50 (cross-linked sulfonated polystyrene). Particle sizes ranging from about 20~200 mesh are employed to allow easy separation of the treated water from the spent resin, yet prevent settling out of the slurry on the trays of the column.
The capacity of the resins for ammonia is 3-6% by weight, which æts the resin to waste water ratio on the basis of feed water composition, allowing for a 10-25%

*Trade Mark 1036!724 1 excess o~ resin. Thus, if the waste water contained 2 2000 ppm of NH3, the resin/water weight ratio would be 3 about 0.08-0.16/1.
4 This ion exchange/water slurry is fed to the top tray of a perforated plate column which is capable of 6 handling without plugging the countercurrent ~low of slurry 7 and vapor. A stripping vapor comprising steam in the 8 amount of 0.02-0.2 lb steam/gal of waste water is introduced 9 into the bottom o~ this tower which operates at a pressure o~ 5-10 psia and a temperature o~ 150-200F. Under these 11 conditions, H2S ls effectively stripped from the descending l2 slurry and will leave the top of the column together with 13 the spent stripping steam. At the same time, NH3 is 14 effectively picked up by the ion exchange resin which leaves the tower with the treated water -at the bottom.
16 The bottoms slurry is settled and the treated 17 water taken off. Thespent resin is then regenerated by 18 treating it with an aqueous solution of a strong mineral 19 acid, or with a concentrated NaCl solution.
The H2S in the overhead ls separated from the 21 stripping steam by condensing out the latter, ~ollowed by 22 conversion of this H2S to elemental sulfur in a Claus 23 plant. The ammonia content o~ the overhead vapor is minor.
24 If desired, several stages of slurrying-vapor stripping can be installed in sequence to permit counter-26 current flow of water and ion exchange resina Thls allows 27 more complete removal of ammonia from the waste water with 28 maximum loadlng of the spent resin with ammonia.
29 A very similar process can be used if the cation combined with the hydrogen sulfide or carbon dioxide 31 in the waste water is an amine such as methylamine, 32 diethylamine, hexamethyldiamine, etc. I~ the cation is an 10367~4 1 alkali metal, such as sodium, a strong acid ion exchange 2 resin like Dowex-50 is very suitable, and regeneration is 3 achieved with a strong mineral acid solution. Thus, sodium 4 sul~ide or carbonate can be ef~ectively removed from dilute aqueous solutions 6 Similarly, an oil soluble cation exchange resin 7 can be dissolved i~ a hydrocarbon solvent, such as kerosene, 8 and used ~or treating an amlne carbonate or sul~ide solution 9 by solvent extraction to remove the amine while C02 or H2S
are vapor stripped. A similar mechanlsm can be used to 11 remove metal salts, uslng an ion exchange resin to complex l2 with cation, while solvent extraction, ~or example, is 13 used to remove the anion.
14 The removal of sodium or ammonium acetate ~rom a dilute aqueous solution can be achieved by slurrying the l6 cation exchan~e resin into the feed water, as explained in 17 the previous example and again feeding this slurrg to the 18 top tray o~ a multistage plate tower. However, instead of 19 contacting the descending water-solid slurry with an ascending vapor, acetic acid is removed from the water by 21 extraction into an ascending stream of liquid hydrocarbon, 22 such as hexane.
23 The water leaving with the spent solid at the 24 bottom o~ the tower wlll be essentially free o~ acetate salt. The spent ion exchange resin is æettled out and can 26 be regenerated as previously described.
27 The hexane stream leaving the top of the tower 28 will contain the acetic acid removed from the waste water, 29 and can be separated by distillation, washing with caustic, or other means, and recycled for further treating.
31 An identical process can be employed i~ sodium 32 phenate, pota~sium ben70ate, or ammonlum salicylate for 1 example, are the contaminants. Again, several such 2 treating stages can be arranged in a sequence to allow 3 countercurrent flow of waste water and ion exchange resin.
4 The removal of amines from dilute aqueous 801u-tions also containing hydrogen sulfide or carbon dioxide 6 can also be achieved by solid adsorbents, such as charcoal 7 provided the acid is vapor stripped out at the same time.
8 Charcoal has good adsorptive capacity ~or amines only if 9 the amines are in the non-ionic state. Therefore, a treating process, pre~erably multi-stage, similar to slurry-11 ing ion exchange resin into an ammonium sulfide solution l2 and vapor stripping in a countèrcurrent tower, is very 13 effective for amine sulfide removal when charcoal is used 1~ in place of the ion exchange resin. In order to minimize charcoal circulation and maximize amine removal, several l6 stages of countercurrent adsorbent-waste water flow are 17 preferred.
18 By slurrying or dissolving the acid form of an 19 ion exchange resin into the hydrocarbon solvent, counter-current contacting of the waste water with both the resin 21 and the solvent can be achieved. The aqueous stream con-22 taining ammonium sulfide or sodium acetate is fed to the 23 top of the countercurrent extraction tower and the slurry 2~ of the acid ~orm of the ion-exchange resin and hydrocarbon solvent is introduced into the bottom of the column. As 26 the slurry rises up, the resin becomes loaded with cation 27 from the waste water, while the solvent picks up the hydro-28 carbon soluble acid, i.e. hydrogen sulfide or acetic acid.
29 The water leaving the bottom of the tower is free of dlssolved salt. The slurry leaving the top is first heated 31 to distill off the dissolved acid, such as H?S or acetlc 32 acid, or phenol. The slurry may then be treated with a 103672~
1 strong acid to remove the exchanged cation (ammonium or 2 sodium, for example) be~ore returning the regenerated 3 slurry to the treating tower.

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- 16 - ~

Claims (8)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR PRIVILEGE
IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for the removal of a salt of a weak acid and a weak base from an aqueous solution wherein said salt is in equilibrium with said acid and said base which comprises simultaneously removing said acid and said base by separate mechanisms wherein said mechanisms are vapour stripping, distillation, solvent extraction, adsorption on or reaction with a solid adsorbent or permeation into a liquid membrane.

2. A process according to claim 1 wherein said acid and said base are removed by vapour stripping either the acid or base while removing the base or acid respectively by adsorption on, or reaction with, a solid adsorbent.

3. A process according to either of claims 1 or 2 wherein said salt is ammonium sulphide, ammonium hydrosulphide or ammonium carbonate.

4. A process according to either of claims 1 or 2 wherein said salt is an ammonium salt of a weak volatile acid or an ammonium salt of a weak, nonvolatile organic acid.

5. A process according to claim 4 wherein said volatile weak acid is HCN, H2S or CO2 and said weak, nonvolatile organic acid is acetic, benzoic or naphthenic acid.

6. A process according to either of claim 5 wherein the dissolved salt is an amine salt and said amine is removed by contacting with an ion-exchange resin or by adsorption on charcoal while the volatile weak acid is vapour stripped from the aqueous solution with an inert gas.

7. A process according to either of claim 5 wherein the acid is removed by solvent extraction while the amine or ammonia is removed by ion-exchange.

8. A process according to either of claims 1 or 2 wherein said salt is a metal salt of an organic acid wherein said metal is removed by permeation into a liquid membrane or ion-exchange extraction while the organic acid is removed by solvent extraction or adsorbing on, or reaction with, a solid adsorbent.