GB1563512A

GB1563512A - Process for separating acid gas and ammonia from dilute aqueous solutions thereof

Info

Publication number: GB1563512A
Application number: GB48152/76A
Authority: GB
Original assignee: USS Engineers and Consultants Inc
Current assignee: USS Engineers and Consultants Inc
Priority date: 1975-11-19
Filing date: 1976-11-18
Publication date: 1980-03-26
Also published as: SE426162B; JPS5262958A; BR7607656A; SE7612804L; DE2652524C2; AU513320B2; JPS621796B2; CA1090281A; DE2652524A1; AU1917876A

Description

(54) A PROCESS FOR SEPARATING ACID GAS AND AMMONIA FROM DILUTE AQUEOUS SOLUTIONS THEREOF (71) We, USS ENGINEERS AND CONSULTANTS, INC., a corporation organised and existing under the laws of the State of Delaware, United States of America, of 600 Grant Street, Pittsburgh, State of Pennsylvania, 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 separation of acid gas and free and fixed ammonia from an aqueous solution containing them.

Aqueous solutions of acid gas and ammonia also having fixed and free ammonia salts can be obtained from the washing of coal carbonization gases, e.g.

the by-products from coke ovens. The separation of the ammonia and acid gas is practiced by using distillation apparatus with a free and a fixed ammonia section, see "Industrial Chemistry", E. R. Riegel, Reinhold Publishing Corporation, N. Y., 1942, pp. 265-268. This system, however, has had problems in reaching consistent low levels of ammonia and acid gas in the effluent aqueous solution, efficiency of separation of both acid gas and ammonia, economically satisfactory energy consumption per gallon of aqueous solution treated, and stable operation, and suffers from deposits of solids or fouling in the apparatus, and large quantities of sludge formed when lime is used to free the fixed ammonia.

The present invention provides a process for removing acid gas and free and fixed ammonia as herein defined from a dilute aqueous solution thereof, the process comprising subjecting the solution to a first continuous distillationconducted by heating the solution by means of a stripping vapour flowing countercurrent thereto, withdrawing from this first distillation an overhead vapour stream containing substantially all of the acid gas and substantially all of the free ammonia and an aqueous bottoms liquid which contains substantially all of the fixed ammonia, adding alkali to a part of the withdrawn bottoms liquid in an amount sufficient to evolve ammonia from the fixed ammonium salts during subsequent distillation and subjecting it to a second counter-current continuous distillation by heating it by means of a stripping vapour flowing counter-current thereto, withdrawing from this second distillation an overhead vapour stream containing the fixed ammonia and an aqueous bottoms stream, and vaporising another part of the aqueous bottoms liquid from the first distillation to form part of the stripping vapour used in the first distillation by means of indirect heat exchanged with an overhead vapour stream withdrawn from the second distillation, the first distillation being conducted at a pressure lower than that used in the second distillation.

Dilute aqueous solutions treated by this invention are those having acid gas in the solution together with fixed and free ammonia, i.e. solutions having water as the major component, with the total of dissolved acid gas and free and fixed ammonia being for example up to 10 percent by weight.

"Free ammonia" as herein defined is ammonia which can be released from the solution simply by boiling it, and so includes ammonia in the form of "free" ammonium salts i.e. salt(s) decomposing to ammonia purely on boiling the solution. "Fixed ammonia" as herein defined is ammonia in the form of "fixed" ammonium salt(s), i.e. salt(s) yielding ammonia only on reaction with alkali.

The acid gas may be selected from, for example, CO2, SO2 HCN, and H2S. Any one of these may be present by itself or in combination with one or more of the others. The simultaneous low concentration of acid gas and ammonia in the treated solution is an important aspect of this invention. Where the acid gas includes CO2, the simultaneous reduction of CO2 and NH3 concentrations in the bottoms liquid from the first distillation tends to reduce the amount of sludge produce after lime addition. Where the acid gas includes HCN, the reduced concentrations of ammonia and cyanide in the exit water are very useful when activated sludge plants are subsequently used to remove biodegradable materials from the water.

The most common dilute aqueous solutions will contain CO2, H2S, and HCN with fixed and free ammonia; Van Krevelan et al, Recueil 68 (1949) pp. 191-216 describes the vapor pressures of such solutions as well as the ionic species of acid gas salts and ammonium compounds in such solutions, which would be representative of the aqueous solutions upon which the invention may be practiced.

Where the solution is a waste water of streams collected from coke plants and coal conversion plants, other components may include tars, phenols, fluorides, chlorides, sulfates, thiosulfates, and thiocyanates. In these circumstances, the tars would be removed by decanting and then the ammonia and acid gases would be removed according to the invention.

The collected waste waters from coke plants are often referred to as ammoniacal liquors. Most of the ammonia is present in the form of free and fixed ammonium salts. Some ammonia may be present as ammonium hydroxide. "Free" salts are those which can be decomposed to give ammonia purely by boiling the solution, for example, ammonium sulfide: (NH4)2S(+steam)=2NH3+H2S.

The "fixed" salts as herein defined are not decomposed unless reacted with an alkali, such as lime; for example ammonium chloride: 2NH4Cl+Ca(OH)2=2NH3+CaCl2+2H2O The principal free and fixed salts present in coke plant waste liquors are as follows: Free Salts Fixed Salts ammonium carbonate ammonium chloride ammonium bicarbonate ammonium thiocyanate ammonium sulfide ammonium ferrocyanide ammonium cyanide ammonium thiosulfate ammonium sulfate In addition to ammonia and ammonium salts, the liquors may contain low concentrations of suspended and dissolved tarry compounds. The most important of these compounds are the phenols or "tar acids", the concentration of which usually ranges from about 0.3 to about 15 grams per liter of liquor. Pyridine bases, neutral oils, and carboxylic acids may also be present but in much lower concentrations.

Typical compositions of liquors from various sections of the coke oven operation are: TABLE I Composition of Weak Ammonia Liquors From Several Coke Plants Ammonia-Recovery Process Semidirect Indirect Plant A B C D Ammonia, total, gpl 7.60 6.20 4.65 3.59 Free, gpl 4.20 4.76 3.37 2.70 Fixed, gpl 3.40 1.44 1.28 0.89 Carbon dioxide as CO2, glp 2.35 3.94 2.78 1.74 Hydrogen sulfide as H2S, gpl 0.86 0.34 1.26 1.13 Thiosulfate as H2S203, gpl 0.022 0.51 Sulfite as H2SO3, gpl 2.84 Sulfate as H2SO4, gpl 0.15 TABLE I (cont.) Composition of Weak Ammonia Liquors From Several Coke Plants Chloride as HCI, gpl 6.75 1.85 Cyanide as HCN, gpl 0.062 0.05 Thiocyanate as HCNS, gpl 0.36 0.42 Ferrocyanide as (NH4)Fe(CN), gpl 0.014 0.039 Total sulfur, gpl 1.014 0.57 Phenols as C6H5OH, gpl 0.66 3.07 Pyridine bases as C5H5N,gpl 0.48 0.16 1.27 0.98 Organic number, cc N/50 KMnO4 4856 3368 per liter TABLE II Typical Compositions of Flushing and Primary Cooler Liquors Flushing Primary-Cooler Liquor Liquor, gpl Condensate, gpl Total ammonia 4.20 6.94 "Free" ammonia 1.65 6.36 "Fixed" ammonia 2.55 0.58 Total sulfur 0.668 Sulfate as sulfur trioxide 0.212 Sulfide as hydrogen sulfide 0.003 Ammonium thiosulfate 0.229 0.29 Carbonate as carbon dioxide 0.374 Cyanide as hydrogen cyanide 0.002 Chloride as chlorine 8.13 1.05 Ammonium thiocyanate 0.82 Phenols 3.55 3.20 A fuller discussion for the recovery of ammonia from coke oven gases and the origin of the various ammonia salt species in the various sections of coke oven plants is given in the book, Coal, Coke and Coal Chemicals, P. J. Wilson and J. H.

Wells, McGraw-Hill Book Company, Inc., N. Y., 1950, particularly Chapter 10, pp.

3s325.

The following Table 3 illustrates a range of compositions in coke plant waste water that constitute aqueous solutions especially suitable for the practice of this invention: TABLE III Typical Composition Ranges for Coke Plant Waste Water Typical Compositions, ppm* Composition Waste Waste Waste Range, Water Water Water Component ppm* No. 1 No. 2 No. 3 Free ammonia 450 to 10000 1900 770 1350 Fixed ammonia 700 to 4000 1900 1190 2440 Cyanide 2 to 1000 210 35 65 Sulfide o to 1300 500 1 10 Carbonate 150 to 4000 2180 190 350 Chloride 750 to 8500 2300 1920 4460 Sulfate/Sulfite 150 to 3000 310 325 415 Thiosulfate 90 to 600 440 115 300 Thiocyanate 100 to 1000 700 150 310 Total sulfur 200 to 2000 1300 250 550 Fluoride 30 to 150 60 40 75 Phenols **300 to 3600 1500 400 725 pH 7 to 9.1 9.0 7.5 7.6 *Parts per million by weight.

**Dephenolized coke plant waste water could contain as little as 0.1 ppm phenols.

Because of its desirable physical properties and ready availability, the preferred stripping vapor consists substantially of steam in which minor amounts of non-condensables may be present. In the hereinafter specifically described preferred practice of this invention the stripping vapor is steam, it being understood that due allowance is to be made for variations in operating conditions where the stripping vapor contains a significant proportion of a non-steam component. For the first distillation, the inlet feed temperature may be in the range of 60"--212"F, e.g. 1600 to l850F, the overhead temperature in the range of 140"-- 265"F, and the liquid bottoms temperature in the range of l600-2750F. The first distillation may be conducted at a pressure in the range of 0.3 to 3.0 atmos abs. The low part of the pressure range allows for efficient use of low pressure steam while the higher pressure give more efficient removal of cyanides. In this first distillation, the gradient of ammonia concentration is usually controlled to be decreasing towards the bottom of the column. Thus, substantially all of the free ammonium salt(s), e.g., sulfide, carbonate and cyanide, are decomposed into ammonia and acid gas that are removed by the stripping vapor. The decreasing ammonia concentration results in decreasing the pH which enhances the strippability of the acid gas in the lower section of the distillation tower. This results in the solution at the bottom of the column becoming mildly acid, e.g. at a pH of about 5 to 6 and acid gas can thus be removed substantially completely from the solution with the result that the still bottoms is also very low in acid gas content. The preferred concentration of ammonia in the first distillation bottom stream is 40 to 200 ppm.

As another aspect of these conditions, if direct injection of steam is used to supply heat required for vaporization, then this steam should be substantially free of ammonia so that the pH range in the range distillation can be maintained as mentioned above in the mildly acid range.

The ratio of liquid to vapor flow per unit time in the distillation tower should be as high as possible to achieve good economy of operation; this L/V ratio may for example be in the range of 10/1 to 2/1 for this first distillation.

Part of the bottom liquid from this first distillation is treated by addition of an alkali, preferably lime, to increase its pH e.g. to 9.5-12 when measured at 500C.

The line reacts with both fixed ammonia salt(s) and any residual acid gas.

Amongst the ammonia salts normally present, the principal reaction is usually with ammonium chloride; if ammonium thiocyanide and sulfate are present theq also react according to the following equations: 2NH4Cl+Ca(OH)2 )CaCI2+2NH3+2H20 2NH4SCN+Ca(OH)2 < Ca(SCN)2+2NH3+2H20 (NH4)2SO4+Ca(OH)2CaSO4+2NH3+2H2O The reactions between lime and any residual acid gas are usually according to the following equations: CO2+Ca(OH)2oCaCO3+H20 H2S+Ca(OH)2CaS+2H2O 2HCN+Ca(OH)2eCa(CN)2+2H20 The major portion of any residual acid gas is usually CO2 and the calcium carbonate thus formed tends to consume lime and form additional lime sludge and thus causes scaling or fouling in distillation equipment. The alkali material used may be sodium hydroxide or potassium hydroxide in place of the calcium hydroxide or lime.

The treated bottoms part is subjected to a second distillation. As noted above, the high pH and heat cause the "fixed" ammonium salts to decompose with liberation of the ammonia. The resulting overhead vapors are essentially ammonia and water. The feed temperature may be in the range of 1550-2700F, the overhead vapors may be at a temperature in the range of 140 290 or 295"F, and the bottoms stream may be at a temperature in the range of 1600-2950F. The pressure may be up to 60 psia. The bottoms stream from the second distillation will have a low concentration of total ammonia. The pH will usually be in the range of 9.5-12. The total ammonia may be as low as 25 ppm. The total cyanides when present, including fixed (complexed) cyanides and free cyanides expressed in terms of equivalent HCN concentration IHCN (total)], may be as low as 2 ppm: the free cyanides, e.g. cyanides amenable to chlorination as opposed to the fixed cx anides which are not, in some cases may even be less than 1 ppm. This bottoms stream can be clarified and then treated to remove other organic materials, such as phenols.

By having the acid gas and free ammonia removed in the first distillation, the second distillation can be conducted for the optimum removal of ammonia from the alkaline fixed ammonium salt solution. Also, the fixed ammonium salt solution will have minimum deposits of salts that arise from the presence of acid gas. It is another benefit of this invention that the various streams may be utilized as heat sources in the distillations to minimize the consumption of energy for achieving effluents with low concentrations of ammonia and cyanide. The overheads stream from the first distillation may be treated to separate the ammonia and acid gas; the overhead stream from the second distillation consists substantially of water and ammonia from which the ammonia may be recovered.

The single Figure of the accompanying drawing illustrates an embodiment of the invention in which the process is used to achieve low concentrations of ammonia and cyanide in the effluent water while reducing energy consumption.

A waste water is collected from a coke oven plant and has a composition similar to that set forth in Example 1 hereafter.

It is at a temperature which may vary from about 85"F in cold weather to about 1600F in the summer. The feed may be preheated by going through line 201 to heat exchanger 227 to provide a feed temperature of about 1600 F. Also, the bypass of line 201a and valve 201b with line 201c permits control over the fraction of feed water which is preheated. At about 1600F the feed is introduced into a first distillation column 202 at a position in the upper half of the column preferably near the top. This stream should be hot enough so that substantial rectification of the liquid will occur in the column and excessive steam condensation in the distillation should be avoided. This column is at a pressure of about I atmos. abs. and at a temperature and pH to remove substantially all of the free ammonia and acid gas from the feed. The overhead vapors are at a temperature of about 204"F and are mainly water vapor, ammonia, hydrogen cyanide, carbon dioxide, and hydrogen sulfide. These vapors have about 95 percent of the free ammonia available and substantially all of the acid gas. The vapors contain about 90 percent steam. The liquid to vapor flow rates below the feed plate are at a ratio of 10:1. The bottoms liquid is at a temperature of about 218"F and has a pH in the range of 5-6. It has fixed ammonia salts, fixed cyanide salts, and organic matter, such as phenols.

The overhead vapors from the first distillation leave by line 203 where a fraction may be diverted by line 220 through valve 221 and lines 221a and 226 directly to the heat exchanger 227. The remaining fraction may go by the line 222 through a valve 223 to an ammonia recovery unit 224. A preferred type of unit may be one in which aqueous ammonium phosphate is used to recover ammonia from a mixture having acid gas. Thereafter, the gas goes by lines 225 and 226 to the heat exchanger 227. The fraction which is treated to remove ammonia may be the whole of the overhead or a portion thereof. Especially where distillation facilities are available for recovery of the ammonia from the ammonia-water vapor overhead of the second distillation, it is desirable to also recover ammonia from the overhead vapors of the first distillation so that resultant ammonia water vapor mixtures can be combined and treated in a single distillation column for recovery of anhydrous ammonia and water.

In the heat exchanger 227, the vapors are cooled to preheat the feed stream.

Because of the temperature of the vapors and the condensation of the vapors, a substantial reduction in the heat required for the overall first distillation is obtained by this heat exchanging to preheat the feed stream. The first heat exchanger 227 may condense from one-half to the whole of the vapor stream with the second heat exchanger 229, via line 228, providing additional condensation capacity. The combination of heat exchangers 227 and 229 gives good control over the degree of vapor condensation and the temperature range of the feed stream to the distillation column. The condensate and vapors, if any, flow by line 230 to a separator 231. In the separator 231, the volatile acid gas leaves as vapor by line 232 while the condensate, a water stream depleted in ammonia and acid gas, returns as reflux by line 233 to the top of column 202. This reflux helps to remove entrained chlorides from the overhead vapors leaving column 202. The vapors leaving the separator have a high ratio of acid gas to ammonia.

The first distillation column is heated by firect injection of fresh steam and of vapour obtained from the first bottoms liquid. A preferred arrangement is shown where low pressure steam is directly injected into the column by line 205, preferably at the bottom of the column, and where overhead vapors from the second distillation are used for indirect heat exchange with liquid from the bottom of the first column. As shown, liquid leaves the bottom of the column 202 by line 240 to enter a reboiler 241. In the reboiler 241, this liquid is vaporized and returned to the column 202 by line 242. The heat for this vaporization is obtained by indirect exchange with the ammonia-rich vapors from line 212. The reboiling of the liquid bottoms having a low pH from the first column helps to achieve substantially complete removal of the free ammonia and acid gas from the liquid that is to be treated in the second distillation.

From the first column, a bottoms stream is withdrawn by line 204 and is sent to a mixing tank 208 where the pH of the stream is increased. This pH adjustment may be accomplished by adding a strong alkali in the form of dry solids, such as lime, through line 209 or it may be achieved by using a liquid slurry of the alkali in an aqueous vehicle, such as water or clarified bottoms liquid resulting from the second distillation. Lime is preferred in that it is relatively inexpensive and forms low solubility calcium salts with fixed cyanides that can be partially removed with the lime sludge.

The treated stream then flows by line 210 to a second column 211 where the second distillation is performed. In this column, the heat and high pH cause the fixed ammonium salts to decompose to release ammonia. The overhead vapors are essentially water and ammonia; these leave by line 212 and are condensed in the reboiler 241 to provide heat for the first column 202. The fluid leaves the reboiler by line 250 to a separator 251. Vapors from the separator are enriched in the more volatile component ammonia compared to the liquid in the separator. These vapors are withdrawn by line 252 through valve 253 to line 254. This valve can be used to control the pressure in the separator 251, the reboiler 241, and the second distillation column 211. Thus, the valve can be used as a primary control for the process. Liquid from the separator 251 is water with a small amount of ammonia. It goes by line 255 as reflux to the top of the second distillation column 211.

Bottoms from the second column have a low concentration of ammonia and acid gas, especially cyanide. They may leave by line 213 through valve 260 to a flashing vessel 261. Becuase the liquid is at its boiling point under the temperature and pressure of the second column, the liquids may be flashed to a pressure slightly greater than the bottom pressure of the first distillation column 202 in the flashing vessel. Overhead vapor from the flashing vessel is essentially water vapor and in this condition can be used as low pressure steam. This steam can leave the vessel by line 262. A portion can flow through line 263 and valve 264 into line 265 where it is mixed preferably by steam ejectors with high pressure steam from line 266 for direct injection into the second distillation column through line 214. Another portion can flow by line 267 as make-up steam to be mixed with other low pressure steam from line 268 for direct injection into the first distillation column by line 205.

From the flashing vessef, the processed water may be withdrawn by line 270. A small amount may be recycled by line 271 through valve 272 and line 273 for the slurry vehicle in mixer 208. The remainder of the processed water may be sent by line 274 to a clarifier 275 where flocculation of the suspended insoluble salts will reduce the solids content of the stream. The water is withdrawn by line 276 and may be further treated to reduce its biologically degradable components.

The following experiment and examples will help understanding of the invention.

Experiment Samples of waste water collected from a coke plant was subjected to distillations in a 20-tray, l-inch-diameter glass Oldershaw column. The absolute pressure levels were 388mm Hg in the first distillation and 760 mm Hg in the second. The waste water was fed to the top of the column. Bottoms liquids were collected.

TABLE A Effect of Pressure on Cyanide Removal Steam Type. 4NH3 0 0 Operating Pressure, mm Hg 760 388 Feed Still Bottoms, Cn, ppm Total CN Content Test No. Total Fixed Sample ppm Sample ppm 276 3.2 1 9.4 1 15.3 2 10.7 2 19.1 3 10.9 3 14.6 Avg. 10.3 Avg. 16.3 2 430 3.2 1 10.0 1 16.2 2 14.6 2 20.5 3 11.7 3 18.8 Avg. 12.1 Avg. 18.5 3 '335 1.7 1 7.9 1 15.3 2 7.3 2 15.2 3 6.6 3 14.1 Avg. 7.3 Avg. 14.9 4 332 1.6 1 5.9 1 7.5 2 6.2 2 - 3 5.7 3 6.9 Avg. 5.9 Avg. 7.2 The data in Table A illustrate that more cyanide is removed by conducting the distillation at one atmos. abs. rather than 0.5 atmos. abs.

TABLE B Effect of Ammonia Content in Steam on Cyanide Removal Steam Type, NH3 6 6 0 0 Operating Pressure, mm Hg 760 388 760 388 Liquid/ Feed Test Vapor Cn, ppm Still Bottoms Total CN Content No. Ratio Total Fixed Sample ppm Sample ppm Sample ppm Sample ppm 5 10 327 4.9 1 20.5 1 13.5 2 20.5 2 11.8 3 21.7 3 13.1 Avg. 20.9 Avg. 12.8 6 10 262 3.3 1 24.7 1 21.5 2 26.6 2 17.5 3 27.0 3 26.3 Avg. 26.1 Avg. 21.7 7 10 392 3.1 1 19.6 1 4.5 2 16.1 2 6.2 3 20.1 3 10.0 Avg. 18.6 Avg. 6,9 8 12 194 1.8 1 7.4 1 2.4 2 9.0 2 2.1 3 - 3 - Avg. 8.2 Avg. 2.3 The data in Table B illustrate that the presence of the additional ammonia causes a substantial increase in the cyanide content of the still bottoms and that operation at one atmosphere absolute provided substantially better cyanide removal than operation at 0.5 atmospheres absolute EXAMPLE 1 In an apparatus essentially as described with reference to the drawing, a waste water collected from a coke plant was treated according to this invention.

The waste water was heated to a temperature of 160"F in a heat exchanger and then admitted near the top of a first distillation column. The liquid to vapor flow ratio was about 10:1. The overhead vapors were at 2040F and 13 psia. The bottoms liquid was at 2190F and 16.5 psia.

The concentrations of acid gas and ammonia (free) in the first column were as follows: Feed Overhead, Bottoms, Wt. % Vol. % Wt. % H2O 99.56 93.7 99.99 CO2 0.163 1.19 ~~~~ NH3 (free) 0.200 3.90 0.0061 H2S 0.0520 0.56 0.0001 HCN (total) 0.0220 0.604 0.0012 The overhead vapors were treated to remove the ammonia, and about 95 percent of the ammonia was recovered. Then, these vapors were sent to the heat exchanger to preheat the feed and to the second exchanger cooled with water.

From these heat exchangers, the condensate was separated into an acid gas stream and a reflux stream for the first column. These streams had the following compositions: Acid Gas Reflux Vol. Vn Wt. % H2O 69.3 98.4 CO2 17.2 0.28 NH3 2.96 0.53 H2S 6.96 0.23 HCN 3.50 0.54 The thermal requirement for the first distillation was supplied by both direct injection of fresh steam (approximately 400/, of the required BTUs) and vapours obtained by indirect exchange in the bottoms reboiler 241 (approximately 60 n of the required BTUs).

The bottoms stream withdrawn through 204 was treated with a slurry of lime in an aqueous vehicle so that its pH was increased to 9.5-12. Thereafter, the treated stream was fed to near the top of the second distillation column. The overhead vapors from this column were at 2680F and 42.4 psia, the bottoms stream was at 275"F and 45.8 psia. Their compositions were as follows: Overhead Bottoms Vol. % Wt. V0 H2O 96.08 99.998 CO2 0.0005 NH3 (free) 3.91 0.0001 H2S HCN (total) 0 0.0011 This overhead vapor was partially condensed in the reboiler of the first column to supply heat thereto. The vapor condensate mixture was separated under a pressure of 35 psia into a vapor and a liquid with compositions as indicated below.

The liquid was utilized for reflux to the second column.

Vapor Liquid Vol. % Wt. V, H2O 83.2 98.44 CO2 NH3 (free) 16.8 1.55 H2S HCN The bottoms fraction from the second column was flashed to a pressure of about 17.4 psia. The vapor was essentially pure steam and a part of it was used with high pressure steam for direct injection into the second distillation column. The remainder, approximately 85 percent of the flashed vapor, was directly injecting into the first column to supply both heat and ammonia-free stripping steam thereto.

About 0.48 lb. steam per gallon of feed was recovered by this flashing.

The bottoms stream from the flasher was clarified with flocculating agents. A portion was used as the slurry vehicle in the addition of lime to the bottoms from the first distillation column while the remainder was sent to a treatment where aerobic bacteria under the influence of oxygen caused biological degradation of the reactive organic matter remaining in the water.

The effects upon purification of the waste water are illustrated by the following compositions for the initial feed water and for the effluent water sent to the biological treatment system.

Feed, ppm Effluent, ppm Phenols 1500 1270 Oil and Tar 50 9 HCN (total)* 220 12 Total Ammonia 3900 24 Free Ammonia 2000 3 Fixed Ammonia 1900 21 Thiocyanates 700 610 Hydrogen Sulfide 520 Sulfates 1700 1130 Chlorides 2300 2000 Carbon Dioxide 1630 0 Calcium 0 2080 Dissolved Solids 6000 5170 BOD 3420 Suspended Solids 88 This example represents the achievement of a reduction in ammonia content of more than 99 percent and in cyanide content of about 94 percent. Only about one pound of steam is required per gallon of feed as opposed to conventional ammonia stills which require about 2.0 to 3 pounds of steam per gallon of feed.

EXAMPLE 2 In an apparatus similar to that described in the Figure, a waste water collected from a coke plant was treated according to another embodiment of this invention.

This waste water was a flushing liquor combined with other coke-plant waste water.

The combined flushing liquor and other waste water were subjected to a preliminary separation of tar, oil, and grit. The waste water had a temperature in the range of Acid Gas Reflux Vol. S Wt. "o H2O 78.9 98.1 CO2 5.1 0.4 NH3 12.6 1.1 H3S 2.3 0.3 HCN 1.1 0.1 The thermal requirement for the first distillation was supplied bv direct injection of both steam (approximately 33 percent of the required BTUs) and of vapor obtained by indirect heat exchange in a bottoms reboiler 241 (approximatelv 67 percent of the required BTUs).

The bottoms liquid was withdrawn. An aqueous slurry of lime was added to bottoms liquid to raise its pH to 9.5-12. Thereafter, the treated bottoms liquid entered as feed near the top of the second distillation column. The overhead vapors from this column were at 2420F and 26.6 psia: the bottoms stream from the column was at 2460F and 28 psia and a pH in the range of 9.5--12. Their compositions were as follows: Overhead Bottoms Vol. n Wt H2O 97.23 99.99 CO2 NH3 (free) 2.76 0.0088 H2S --- 0.0001 HCN (total) 0.0007 The overhead vapor was condensed in the reboiler of the first column to supply heat thereto. At 25.7 psia, the condensate was separated into a vapor and a liquid for reflux to the second column. These had the following compositions: Vapor Reflux Vol. % Wt. 0/, H2O 87.78 98.97 CO2 NH3 12.22 1.03 H3S HCN This vapor was combined with the vapors from condensation of overheads from the first column and the resulting mixture was sent to ammonium sulfate saturators for recovery of the ammonia.

The bottom fraction from the second distillation was flashed to 17.0 psia. In the flashing vessel, a small amount of water was admitted to wash out salts entrained in the flashing. Alkali salts, especially lime, should be removed to avoid corrosion and fouling in the heat exchange equipment. The vapor leaving the flashing vessel was combined with make-up steam and directly injected into the first distillation to supply heat and stripping steam. About 0.26 lb. steam per gallon of feed was recovered by using this flashing.

The bottoms liquid from the flash vessel contained about 90 ppm total ammonia. It was subjected to clarification in which suspended solids were removed. Thereafter, it was sent to treatment with aerobic bacteria in an activated sludge system for reduction in organic matter; the concentrations of cyanide amenable to chlorination, phenol, and thiosulphate may thus be reduced to below I ppm each.

The following comparison of inlet feed and effluent stream illustrates the effect of the invention: Feed, 0.78 mgd* Effluent, 0.95 mgd* ppm ppm Oil and Tars 50 9 Phenols 1280 1050 Free Ammonia 1330 53 Fixed Ammonia 1700 34 Feed, 0.78 mgd* Effluent, 0.95 mgd* ppm (cont.) ppm (cont.) CO2 1375 0 HCN (total) 185 7 H3S 490 Thiocvanate 620 500

Sulfités andUl Chloride 2020 1650 *Million gallons per day.

This example illustrates the achievement of more than 97 percent reduction in ammonia and about a 95 percent reduction in cyanide. This also uses about 1.2 pounds of steam per gallon of feed and has about 60 percent of its steam requirement satisfied by low pressure steam, ca. 5.5 psig.

It will be appreciated that the treated waters obtained according to the invention are in a suitable condition for further processing by biological degradation of the phenols. The consistent low concentrations of ammonia and cyanide avoid inactivation of the bacteria so that smooth operation of the activated sludge system is achieved.

It is within the practice of this invention to make use of additional materials and processes as would be familiar to those skilled in this art. For example. the first and second distillations may be performed in one or more vessels. Also, additional reboilers and other heat exchangers may be used at intermediate locations in both the first and the second distillations.

WHAT WE CLAIM IS: 1. A process for removing acid gas and free and fixed ammonia as herein defined from a dilute aqueous solution thereof, the process comprising subjecting the solution to a first continuous distillation conducted by heating the solution by means of a stripping vapour flowing counter-current thereto, withdrawing from this first distillation an overhead vapour stream containing substantially all of the acid gas and substantially all of the free ammonia and an aqueous bottoms liquid which contains substantially all of the fixed ammonia, adding alkali to a part of the withdrawn bottoms liquid in an amount sufficient to evolve ammonia from the fixed ammonium salts during subsequent distillation and subjecting it to a second counter-current continuous distillation by heating it by means of a stripping vapour flowing counter-current thereto, withdrawing from this second distillation an overhead vapour stream containing the fixed ammonia and an aqueous bottoms stream, and vaporising another part of the aqueous bottoms liquid from the first distillation to form part of the stripping vapour used in the first distillation by means of indirect heat exchange with the overhead vapour stream withdrawn from the second distillation, the first distillation being conducted at a pressure lower than that used in the seconnd distillation.

2. A process according to claim 1 wherein the indirect heat exchange includes the condensation of the overhead vapours from the second distillation into a liquid fraction and a vapour fraction.

3. A process according to claim 2 wherein the liquid fraction is returned as reflux to the second distillation.

4. A process according to any preceding claim including heating the aqueous solution by indirect heat exchange with at least a part of the overhead vapour stream withdrawn from the first distillation.

5. A process according to claim 4 wherein a liquid fraction condensed during the heat exchange with the overhead vapour stream withdrawn from the first distillation is returned as reflux to the first distillation.

6. A process according to any preceding claim wherein the aqueous solution fed to the first distillation is at a temperature of 60 to 212"F.

7. A process according to claim 6 wherein the aqueous solution fed to the first distillation is at a temperature of 160"F.

8. A process according to claim 6 wherein the aqueous solution fed to the first distillation is at a temperature of 185"F.

9. A process according to any preceding claim wherein the first distillation is conducted at a pressure of 0.3 to 3 atmospheres absolute.

10. A process according to any preceding claim wherein the bottoms stream from the second distillation is subjected to biological degradation to reduce its content of biodegradable substances.

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. Feed, 0.78 mgd* Effluent, 0.95 mgd* ppm (cont.) ppm (cont.) CO2 1375 0 HCN (total) 185 7 H3S 490 Thiocvanate 620 500 Sulfités andUl Chloride 2020 1650 *Million gallons per day. This example illustrates the achievement of more than 97 percent reduction in ammonia and about a 95 percent reduction in cyanide. This also uses about 1.2 pounds of steam per gallon of feed and has about 60 percent of its steam requirement satisfied by low pressure steam, ca. 5.5 psig. It will be appreciated that the treated waters obtained according to the invention are in a suitable condition for further processing by biological degradation of the phenols. The consistent low concentrations of ammonia and cyanide avoid inactivation of the bacteria so that smooth operation of the activated sludge system is achieved. It is within the practice of this invention to make use of additional materials and processes as would be familiar to those skilled in this art. For example. the first and second distillations may be performed in one or more vessels. Also, additional reboilers and other heat exchangers may be used at intermediate locations in both the first and the second distillations. WHAT WE CLAIM IS:

1. A process for removing acid gas and free and fixed ammonia as herein defined from a dilute aqueous solution thereof, the process comprising subjecting the solution to a first continuous distillation conducted by heating the solution by means of a stripping vapour flowing counter-current thereto, withdrawing from this first distillation an overhead vapour stream containing substantially all of the acid gas and substantially all of the free ammonia and an aqueous bottoms liquid which contains substantially all of the fixed ammonia, adding alkali to a part of the withdrawn bottoms liquid in an amount sufficient to evolve ammonia from the fixed ammonium salts during subsequent distillation and subjecting it to a second counter-current continuous distillation by heating it by means of a stripping vapour flowing counter-current thereto, withdrawing from this second distillation an overhead vapour stream containing the fixed ammonia and an aqueous bottoms stream, and vaporising another part of the aqueous bottoms liquid from the first distillation to form part of the stripping vapour used in the first distillation by means of indirect heat exchange with the overhead vapour stream withdrawn from the second distillation, the first distillation being conducted at a pressure lower than that used in the seconnd distillation.

11. A process according to any preceding claim wherein heat contained in the

aqueous liquid bottoms stream from the second distillation is transferred to the first distillation by flashing said liquid into a vapour fraction and a liquid fraction, and adding a portion of the resulting vapour fraction as a stripping vapour to the aqueous solution undergoing the first distillation.

12. A process according to any preceding claim wherein the acid gas comprise at least one of CO2, SO2, H2S, and HCN.

13. A process according to any preceding claim wherein the aqueous solution comprises waste liquor from coke or coal conversion plant operation.

14. A process according to any preceding claim wherein the acid gas and free and fixed ammonia as herein defined constitute in total up to 10 percent by weight of the aqueous solution.

15. A process according to any preceding claim wherein the aqueous solution contains tars which are removed by decanting before the first distillation.

16. A process according to any preceding claim wherein the stripping vapour for each distillation consists substantially of steam.

17. A process according to claim 16 wherein the overhead vapours withdrawn from the first distillation are in the temperature range of 140 to 2650 F.

18. A process according to claim 16 or 17 wherein the liquid bottoms withdrawn from the first distillation is in the temperature range of 1600 to 275"F.

19. A process according to any of claims 16 to 18 wherein the bottoms from the first distillation is fed to the second distillation at a temperature in the range of 155 to 2700F.

20. A process according to any of claims 16 to 19 wherein the overhead vapours are withdrawn from the second distillation at a temperature in the range of 140 to 2950F.

21. A process according to any preceding claim wherein the liquid bottoms from the first distillation has a pH of 5 to 6.

22. A process according to any preceding claim wherein the bottoms stream withdrawn from the second distillation has a pH in the range of 9.5 to 12.

23. A process according to any preceding claim wherein the second distillation is conducted at a pressure of up to 60 psia.

24. A process according to any preceding claim wherein the alkali is selected from calcium hydroxide, sodium hydroxide, and potassium hydroxide.

25. A process for separating acid gas and free and fixed ammonia as herein defined from a dilute aqueous solution thereof, the process being substantially as hereinbefore described with reference to the accompanying drawings.

26. A process for separating acid gas and free and fixed ammonia as herein defined from a dilute aqueous solution thereof, the process being substantially as hereinbefore described in Example 1 or 2.

27. Acid gas or ammonia obtained by a process according to any preceding claim.