GB2087422A

GB2087422A - Process for removing ammonia and hydrogen sulfide from a vapor stream

Info

Publication number: GB2087422A
Application number: GB8133774A
Authority: GB
Original assignee: USS Engineers and Consultants Inc
Current assignee: USS Engineers and Consultants Inc
Priority date: 1980-11-12
Filing date: 1981-11-09
Publication date: 1982-05-26
Also published as: AU7740381A; GB2087422B; CA1155637A; SE8106662L; IT8124952A0; DE3137905A1; RO85214A; BR8107202A; FR2493723A1; RO85214B; JPS57145180A; FR2493723B1

Abstract

A process for the removal of hydrogen sulfide and ammonia from a vapor stream, and from a dilute aqueous solution containing free and fixed ammonia salts and acid gases comprises scrubbing the vapor stream in a scrubber (202) by contacting it with an aqueous ammonia solution, subjecting the dilute aqueous solution to a free and fixed ammonia still process (214, 224) passing an overhead vapor stream from the second still (224) through an indirect heat exchanger (226a) to heat the liquid in the first still (214) and then passing the overhead vapor stream to the scrubber (202) for the removal of the hydrogen sulfide. <IMAGE>

Description

SPECIFICATION Process for removing ammonia and hydrogen sulfide from a vapor stream The present invention relates to the removal of hydrogen sulfide and ammonia from a vapor stream.

Various methods have been proposed for removing hydrogen sulfide and ammonia from vapor streams such as coke oven gas. Hydrogen sulfide is generally removed from such vapor streams by passing the vapor stream through a hydrogen sulfide scrubber which is operated with aqueous ammonia as the scrubbing liquid. By maintaining a high concentration of ammonia in the scrubber relative to the concentration of hydrogen sulfide in the vapor stream, it is possible to remove high percentages such as over 98% of the hydrogen sulfide from the vapor stream.

Various ways have been suggested for removing the ammonia from the vapor stream and reusing this ammonia for the hydrogen sulfide scrubber. Some of these methods are descirbed in United States Patent Specification No. 4,009,243 which discloses distilling the wash waters from the hydrogen sulfide scrubber to produce a vapor stream of hydrogen sulfide and ammonia. This vapor stream and also the vapor stream leaving the hydrogen sulfide scrubber are then treated to separate the ammonia.

This treatment is accomplished by passing these vapor streams through one or more ammonia absorbers, such as those using aqueous ammonium phosphate solution. Ammonia vapors are then separated from the ammonia-enriched solution by distillation. These ammonia vapors are then recycled to the hydrogen sulfide scrubber.

According to the present invention, there is provided a process for removing hydrogen sulfide and ammonia from a vapor stream and from a dilute aqueous solution containing free and fixed ammonium salts and acid gases, the process comprising scrubbing said vapor stream in a hydrogen sulfide scrubber by contacting said vapor stream with an aqueous ammonia scrubbing solution to remove said hydrogen sulfide and produce an aqueous ammonia solution enriched with hydrogen sulfide, subjecting said dilute aqueous solution to a first counter-current multi-stage continuous distillation conducted by heating said solution by means of a stripping vapor, withdrawing from said first distillation an overhead vapor stream containing said acid gases in said solution and the ammonia from said free ammonium salts, withdrawing from said first distillation an aqueous bottom stream which contains substantially all of said fixed ammonium salts, adding alkali to said withdrawn bottom stream and thereafter subjecting said withdrawn bottom stream to a second counter-current multi-stage continuous distillation, withdrawing from said second distillation an aqueous bottom stream and an overhead vapor stream containing ammonia from said fixed ammonium salts, passing said overhead vapor stream from said second distillation through an indirect heat exchanger to partially condense said vapor stream, passing liquid from the first distillation through said indirect heat exchanger to produce said stripping vapor, and passing the ammonia in the second distillation overhead stream from the indirect heat exchanger into the hydrogen sulfide scrubber to provide at least in part the ammonia in said scrubbing solution.

The invention is further described, by way of example, with reference to the accompanying drawing which is a flow diagram illustrating a preferred embodiment of the invention.

Referring to the drawing, a vapor stream, such as coke oven gas, including acid gases and ammonia flow by line 201 into a hydrogen sulfide scrubber 202 where it is contacted with scrubbing liquids and vapors brought in by lines 203, 204, 205 and 206. An aqueous ammonia solution enriched with hydrogen sulfide leaves through line 217 and an ammonia-containing vapor stream passes through line 207 to an ammonia absorber 208 where the ammonia-containing vapor stream is contacted with a lean absorber solution such as an aqueous ammonium phosphate solution. The stripped vapor stream leaves through line 210 from the absorber 208. An ammonia-rich absorber solution leaves the absorber 208 through line 211 and passes on to a second ammonia absorber 212.

An aqueous solution, such as coke plant waste water, containing fixed and free ammonium salts and acid gases flows by line 213 into a first counter-current, multi-stage continuous distillation column 214. Overhead vapors containing acid gases, ammonia and water vapor leave the first distillation column 214 through line 21 5. The vapors in line 215 pass to a deacidification vessel 216 along with the aqueous ammonia solution enriched with hydrogen sulfide from line 217. Steam is added through line 21 8 to the deacidification vessel 21 6 to cause removal by distillation of ammonia and hydrogen sulfide which leaves through line 219 and passes to the second ammonia absorber 212. Bottom liquid from the vessel 21 6 passes through line 203 to the scrubber 202.A vapor stream containing acid gases leaves the second absorber 212 through line 220 and an ammonia-enriched absorber solution leaves the second absorber 212 through line 221. The ammonia can be removed from the absorber solution in line 221 by stripping.

The pH in the first distillation colum 214 is such that substantially all of the acid gases and substantially all of the ammonia from the free ammonium salts are removed from the aqueous solution passing into the column 214 from line 213. An aqueous bottom stream having a pH of less than 8 (preferably 5 to 6) and containing fixed ammonium salts passes from the column 214 through line 222.

Lime is added to this bottom stream at point 223. Precipitated solids may be removed at this point. The bottom stream, which now has a higher pH, passes into a second counter-current, multi-stage continuous distillation column 224 to which steam is added through line 226, and a vapor stream of substantially pure ammonia and steam leaves through line 225. An aqueous bottom stream with a pH of 9.5 to 12 leaves the second column 224 through line 227 where it may be passed to a biological treatment facility.

The vapor stream in line 225 is passed through an indirect heat exchanger 226a where liquid passing through lines 227 between the first distillation column 214 and heat exchanger 226a is vaporized to serve as stripping vapor for the first distillation. A partially condensed vapor stream from the heat exchanger 226a passes through line 228 and may be fed directly into the hydrogen sulfide scrubber 202 or, as shown, may be separated into liquid and vapor streams in a separator 229. An aqueous ammonia stream passes from the separator 229 through line 230. At this point, the aqueous ammonia stream may be passed through line 231 to the second distillation column 224 or may be passed through line 232 to the hydrogen sulfide scrubber 202.The ammonia vapor stream from the separator 229 passes through line 233 where it may go directly to the hydrogen sulfide scrubber 202 or, as shown, may pass through a condenser 234 where the vapor is converted to a liquid stream and passed on through line 204 to the scrubber 202.

The dilute aqueous solution supplied to the first distillation column 214 will typically contain CO2, H25, and HCN as well as the fixed and free ammonium salts. Commonly, the acid gases and free and fixed ammonium comprise up to about 0.6% by weight of the aqueous solution. Where the solution is waste water from coke plants or coal conversion plants, other components may include tars, phenols, fluorides, chlorides, sulfates, thiosulfates, and thiocyanates. In this circumstances, the tars would be removed by decanting. The waste waters from coke of other coal conversion plants are often referred to as ammoniacal liquors. The principal free and fixed salts present in the liquors are as foliows: 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 waste waters 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 1 5 grams per liter of liquor. Pyridine bases, neutral oils, and carboxylic acids are also present in much lower concentrations.

The foilowing Table illustrates the range of compositions in coke plant waste water that comprise aqueous solutions especially suitable in the practice of the invention.

TABLE 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 The inlet feed temperature in the line 213 may be in the range of about 60 265 F, the overhead temperatures in the line 21 5 may be in the range of about 1 40a265a F, and the bottom temperatures in the line 222 may be in the range of about 1600--2750F. The first distillation will be conducted at a pressure in the range of about 0.3 to 3.0 atmospheres absolute. In this first distillation, substantially all of the free ammonium salts, e.g. sulfide, carbonate and cyanide, are decomposed into ammonia and acid gases that are removed by the stripping vapor.

The lime added at the point 223 reacts both with fixed ammonium salts to liberate ammonia and with any residual acid gases.

In the second distillation, the feed temperature in the line 222 may be in the range of about 1550--2700F, the overhead vapors in the line 225 may be at a temperature of about 1400--2900F, and the bottom stream in the line 227 may be at a temperature of about 1 600--2950F. The pressure may be up to about 4 atmospheres absolute. The bottoms stream from the second distillation will have a low concentration of total ammonia and of cyanides. The pH will be in the range 9.5-12. The total ammonia may be as low as 25 ppm. The total cyanides including complexed cyanides and free cyanides expressed in terms of equivalent HCN concentration, may be as low as 2 ppm; the free cyanides, (cyanides amendable to chlorination) in some cases may even be less than 1 ppm.This bottom stream can be clarified and then treated to remove other organic materials, such as phenols.

Passing the overhead vapor stream from the second distillation through an indirect heat exchanger to produce stripping vapor for the first distillation instead of using the overhead vapor stream from the second distallation avoids the introduction of extra ammonia into the first distillation and enables the pH in the first distillation to be kept low, preferably in the mildly acid range, so as to enhance the removal of acid gases in the first distillation.

By having substantially all of the acid gases 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 minimal deposits of salts that would arise from the presence of the acid gases.

Claims

1. A process for removing hydrogen sulfide and ammonia from a vapor stream and from a dilute aqueous solution containing free and fixed ammonium salts and acid gases, the process comprising scrubbing said vapor stream in a hydrogen sulfide scrubber by contacting said vapor stream with an aqueous ammonia scrubbing solution to remove said hydrogen sulfide and produce an aqueous ammonia solution enriched with hydrogen sulfide, subjecting said dilute aqueous solution to a first counter-current multi-stage continuous distillation conducted by heating said solution by means of a stripping vapor, withdrawing from said first distillation an overhead vapor stream containing said acid gases in said solution and the ammonia from said free ammonium salts, withdrawing from said first distillation an aqueous bottom stream which contains substantially all of said fixed ammonium salts, adding alkali to said withdrawn bottom stream and thereafter subjecting said withdrawn bottom stream to a second counter-current multi-stage continuous distillation, withdrawing from said second distillation an aqueous bottom stream and an overhead vapor stream containing ammonia from said fixed ammonium salts, passing said overhead vapor stream from said second distillation through an indirect heat exchanger to partially condense said vapor stream, passing liquid from the first distillation through said indirect heat exchanger to produce said stripping vapor, and passing the ammonia in the second distillation overhead stream from the indirect heat exchanger into the hydrogen sulfide scrubber to provide at least in part the ammonia in said scrubbing solution.

2. Process as claimed in claim 1, wherein said ammonia is introduced into the hydrogen sulfide scrubber as an aqueous ammonia stream.

3. Process as claimed in claim 1, wherein said ammonia is introduced into the hydrogen sulfide scrubber as a vapor stream.

4. Process as claimed in claim 1, wherein condensate produced in the indirect heat exchanger is returned to said second continuous distillation and wherein the uncondensed portion of the vapor stream is introduced into said hydrogen sulfide scrubber.

5. Process as claimed in claim 1, wherein both the condensed and uncondensed portions of said vapor stream from the heat exchanger are introduced into said hydrogen sulfide scrubber.

6. A process for removing hydrogen sulfide and ammonia from a vapor stream and from a dilute aqueous solution containing free and fixed ammonium salts and acid gases, substantially as hereinbefore described with reference to the accompanying drawing.