GB2080274A

GB2080274A - Method of recovering ammonia from process streams

Info

Publication number: GB2080274A
Application number: GB8122494A
Authority: GB
Original assignee: USS Engineers and Consultants Inc
Current assignee: USS Engineers and Consultants Inc
Priority date: 1980-07-23
Filing date: 1981-07-21
Publication date: 1982-02-03
Also published as: SE8104436L; AU7314181A; BR8104633A; DE3128591A1; JPS5747715A

Abstract

A method is described for recovering ammonia from a gas stream (101) and a waste water stream (115), both streams containing ammonia and acid gases, comprising adding alkali to the waste water stream (116), subjecting the thus treated stream to distillation with steam (118), withdrawing a hot ammonia-containing vapor stream from the distillation, passing said gas stream through an aqueous absorbing solution of ammonium phosphate (102), passing said hot ammonia containing vapor stream (120) through said absorbing solution to strip acid gases from said absorbing solution (104), and heating the absorbing solution to strip ammonia from said solution (106). <IMAGE>

Description

SPECIFICATION Method of recovering ammonia from process streams This invention relates to a method of recovering ammonia from process streams containing ammonia and acid gases.

Ammonia has been recovered from gaseous streams containing ammonia and acid gases by absorbing the ammonia in an aqueous solution of ammonium phosphate. The solution absorbs a preponderance of the ammonia and a small part of the acid gases. The solution is stripped of ammonia, and recycled. The ammonia and water vapor leaving the ammonia stripper are condensed and fed to an ammonia enriching column. Such processes are described in United States Patent Specification Nos.

2,797,148; 3,024,090; 3,186,795; 3,718,731 and 4,060,591, and are suitable for treating such process streams as coke oven gas and overhead vapors from ammonia distillation stills used in the recovery of ammonia from waste water. When both of these process streams are to be treated, it has been proposed to first contact the ammonium phosphate solution with the coke-oven gas stream, and then to contact the solution with the ammonia-rich ammonia still vapors. This procedure is disclosed in United States Patent Specification Nos. 3,024,090 and 3,186,795. One of the objects was to remove acid gases from the ammonium phosphate solution.

However, in general the acid gases were not removed from the absorbing solution. Therefore, in most commercial facilities today, the ammonia still vapors are simply combined with the coke-oven gas stream prior to the absorption step. Heat from a separate source is applied to the ammonia-rich ammonium phosphate solution to strip the acid gases.

According to the present invention, there is provided a method of recovering ammonia from a gas stream and a waste water stream, both streams containing ammonia and acid gases, comprising adding alkali to the waste water stream, subjecting the thus treated stream to distillation with steam, withdrawing a hot ammonia-containing vapor stream from the distillation, passing said gas stream through an aqueous absorbing solution of ammonium phosphate, passing said hot ammonia containing vapor stream through said absorbing solution to strip acid gases from said absorbing solution, and heating the absorbing solution to strip ammonia from said solution.

The invention is further described, by way of example, with reference to the accompanying drawing which is a diagrammatic representation of a system for carrying out the method of this invention.

A gas stream, such as coke-oven gas, containing ammonia and acid gases is introduced by line 101 to the bottom of an absorber 102. The gases ascend through the absorber 102 counter-current to descending sprays of absorbing solution inside the absorber 102. The absorbing solution is an aqueous solution of ammonium phosphate.

The gases leave the absorber 102 through line 103, substantially free of ammonia. The solution, enriched in ammonia, overflows from the bottom of the absorber into a chamber 104 for contact with a hot ammonia-containing vapor stream. A portion of the ammonia-containing vapor stream condenses and is absorbed into the solution, further enriching the solution in ammonia and heating it to its boiling point. Acid gases absorbed in the solution are driven off by this heating and carried off by the uncondensed portion of the vapor. If necessary to achieve boiling, steam or another source of heat may be used to supplement that from the hot ammonia containing vapor stream.The chamber 104 may be built inside the absorber 102 as shown, or may be a separate vessel to which the rich solution from the bottom stage of absorber 102 is pumped.

From chamber 104, the rich solution passes through line 105 to an ammonia stripper 106 in which the solution descends counter-current to vapor such as steam introduced at the bottom through line 108, and is thereby stripped of its absorbed ammonia. Water vapor and ammonia leave the top of the stripper 106 through line 107.

The hot, lean regenerated solution flows from the bottom of stripper 106thorough line 109 for reuse in absorber 102.

Waste water comprising a dilute solution of ammonia, acid gases, fixed ammonium salts and organic materials, such as from a coke-oven plant or other coal conversion plant flows by line 110 into a free-ammonia still 111 in which an upflowing stripping medium becomes enriched in the more volatile components of the solution. The stripping medium may be a condensable gas or a non-condensable gas, for example; steam, air, hydrogen, nitrogen or methane; or a combination of these. The stripping medium may be generated by vaporization of the liquid or by injecting a gas or vapor into the still or by a combination of these. The stripping medium will generally be steam. The overhead vapor leaving the free still 111 by line 112 will have a major proportion of stripping medium such as steam, and substantially all of the acid gases and free ammonia from the waste water.A line 113 conducts the overhead vapor to the line 101. The bottom liquid from free still 111 passes through line 115 and is mixed with alkali such as lime or sodium hydroxide in a vessel 116. This alkali-treated liquid is then passed through line 117 to a fixed-ammonia still 118. In this still, ammonia, freed by the alkali, is stripped out of the liquid by steam and an overhead injected through line 119 vapor of ammonia and water vapor is removed by line 120. Bottom liquid is removed from fixed still 118 through line 126. The overhead vapor which is substantially free of acid gases passes through line 120 to the chamber 104 in where it is used to heat the bottom liquid from the absorber 102 to boiling as described previously. optionally, some of the overhead vapors from the free still 111 pass through 114 to mix with the fixed still overhead vapors.Optionally, a portion of the fixed still overhead vapors may pass through line 121 and the heat contained therein may be used either by directly adding the vapors to free still 111 or by being passed through a heat exchanger 122. In the event that the heat exchanger is used, liquid from the bottom of the free still passes through line 123 through the heat exchanger 122 and back to the free still 111 through line 124. Any uncondensed fixed still vapors from the heat exchanger 122 pass through line 125 to combine with the fixed still vapors flowing in line 120.It is essential that a major portion of the vapors passing to chamber 104 be fixed ammonia still vapors which have not passed through the free still 111 so that the concentration of acid gases in the ammonia-still vapors contacting the rich liquid in chamber 104 is sufficiently low to enable absorbed acid gases in the rich solution in chamber 104 to be stripped from the solution.

The fixed ammonia still vapors have a significantly higher concentration of ammonia than the gas stream and a low content of acid gases, preferably below about 200 ppm. The ammonia content of these fixed ammonia still vapors is typically about 10 to 40 percent by volume.

For the purpose of this invention, a fixed ammonia still is defined as a still in which the feed stream is made alkaline with a strong and non-volatile alkali such as soduim hydroxide or calcium hydroxide such that the pH of the liquid is maintained above about 9.5 throughout the still. Sufficient alkali (other than ammonia) is present in the feed in order that the bottoms stream preferably has a pH of 9.5-12.0, and more preferably in the range of 10.0-11.0. The fixed ammonia still may be the only still in those processes where the feed is made alkaline before distillation, or it may be the second still in those processes where free ammonia is first distilled off with acid gases and the bottoms from that free ammonia still is made alkaline and redistilled in a second, or fixed ammonia still.

By utilizing the invention of this application, it is now possible to strip off the acid gases from the ammonia-enriched aqueous solution of ammonium phosphate solution from the absorber without requiring a special heat exchanger or using vapors from the ammonia stripper. The fixed still operates under strongly alkaline conditions which strongly inhibit the volatility of acid gases and, therefore, fixed still vapors will have a low content of acid gases. Therefore, for the purpose of this invention, fixed still vapors are vapors from any still receiving a stongly alkaline feed, whether or not the free still operates on a separate heat source in whole or in part.Even if there is no free still and the total feed stream containing free and fixed ammonia is made alkaline, for example with sodium hydroxide or lime, the overhead vapor will be low in acid gases and useful in this invention.

It is preferred to have substantially all of the hot ammonia containing vapor stream come from the fixed ammonia still. However, depending upon the relative quantities of fixed still vapor and ammoniarich phosphate solution to be heated and purged of acid gases, it may be necessary to supplement the fixed still vapor with free still vapor or with steam. It is, however, important that the concentration of acid gases be maintained sufficiently low in the vapor stream contacting the rich solution of ammonium phosphate to allow stripping of acid gases from the solution to occur.

The absorbing solution in the absorber is preferably at a temperature of about 35 to 60"C, and more preferably, about 45 to 60on. The temperature may, however, go up to above tOO,,C.

The absorption and stripping steps may be performed at different pressures. In general, the absorp- tion is conducted at the available pressure of the gas.

The stripping is preferably conducted at a pressure between about 100 and about 300 psia.

The aqueous ammonium phosphate solution gn- erally has a concentration of ammonium phosphate of from about 10 percent up to saturation. A concent- ration of about 25 to 40 percent is preferred.

Claims

1 A method of recovering ammonia from a gas stream and a waste water stream, both streams containing ammonia and acid gases, comprising adding alkali to the waste water stream, subjecting the thus treated stream to distillation with steam, withdrawing a hot ammonia-containing vapor stream from the distillation, passing said gas stream through an aqueous absorbing solution of ammonium phosphate, passing said hot ammonium containing vapor stream through said absorbing solution to strip acid gases from said absorbing solution, and heating the absorbing solution to strip ammonia from said solution.

2. A method as claimed in claim 1, in which, prior to the addition of said alkali, the waste water stream is subjected to steam distillation to remove acid gases and free ammonia.

3. A method as claimed in claim 1 or claim 2, in which said hot ammonia-containing vapor stream has a higher ammonia content than said gas stream.

4. A method of recovering ammonia from a gas stream and a waste water stream, both streams containing ammonia and acid gases, substantially as hereinbefore described with reference to the accompanying drawing.