Abstract:
Refinery waste waters containing dissolved hydrogen sulfide and ammonia are stripped of hydrogen sulfide by a countercurrent stream of refinery absorber gas which removes substantially all of the hydrogen sulfide and none of the ammonia. The dissolved ammonia is subsequently removed by steam stripping, leaving a waste water sufficiently clean to be non-polluting.

Description:
NATURE OF INVENTION 
     The removal of hydrogen sulfide and ammonia from foul process water prior to disposal of the warter has long been commonplace in petroleum refineries. The customary disposal technique has been to strip the foul water with steam by introducing steam at the bottom of one or more contacting columns and flowing it upwards through a descending stream of foul process water. Subsequently, the ammonia and hydrogen sulfide gases removed overhead are incinerated. 
     In numerous instances within the past several years, however, the mixture of hydrogen sulfide and ammonia gases removed has not been directly incinerated because of air pollution problems. Instead, the gases have been directed to a sulfur recovery system such as a Claus plant wherein the hydrogen sulfide is converted to free sulfur and water vapor. Most of these systems utilize a bauxite catalyst. The quantity of ammonia in the gas mixture fed to the sulfur recovery system originally was not considered to be a problem but ammonia (in the form of ammonium salts) has been determined to be a catalyst poison causing decreased recoveries of free sulfur. Reduced efficiency of the sulfur recovery system creates further problems in that the effluent gases contain hydrogen sulfide and/or sulfur dioxide in quantities sufficient to create air pollution problems. The presence of ammonia in feed gas flowing to a sulfur recovery plant also reduces its hydrogen sulfide handling capacity. 
     An example of a foul water treating system is described in Hydrocarbon Processing, Volume 51, No. 10, October 1972. In this process, water degassed of dissolved hydrogen, methane, and other light hydrocarbons is pumped into a reboiler stripping column where hydrogen sulfide is stripped overhead while water and ammonia flow downwards and out of the column as the bottoms product. The overhead product is a high purity hydrogen sulfide directly suitable as sulfur or sulfuric acid plant feed. The bottoms product from the first column is fed directly to a second reboiler stripper column which produces a clean water bottoms substantially free of hydrogen sulfide and ammonia. The overhead is taken to an ammonia purification system. This publication notes the importance of removing ammonia from a hydrogen sulfide gas stream before the latter is fed to a sulfur or sulfuric acid conversion unit. 
     Most of the current processes for removing ammonia and hydrogen sulfide from foul refinery waste waters require the use of steam as a stripping source. A process which will reduce the need for a stem stripping source is thus highly desirable because of energy shortages and resultant high steam costs. Petroleum refineries usually produce a quantity of residual gas which is available at pressures of between 100 and 220 psig. Ordinarily the energy contained in this residual gas is not utilized because the gas is released to a header operating at 40-80 psig. This gas subsequently is burned in various furnaces. 
     OBJECTS OF THE INVENTION 
     A primary object of this invention is to provide an improved process for removing ammonia and hydrogen sulfide separately from refinery waste waters. 
     Another object of this invention is to provide a method of removing hydrogen sulfide and ammonia from refinery waste waters wherein the steam consumed in such treatment is minimized. 
     Another object of this invention is to maximize the ultimate recovery of hydrogen sulfide as a liquid sulfur product rather than allowing it to be discharged into the air in the form of sulfur dioxide. 
     Another object of this invention is to treat refinery foul waters so that they will be non-polluting when released to natural waterways. 
     SUMMARY OF THE INVENTION 
     Briefly stated, this invention in its broadest aspect comprises a process for removing dissolved hydrogen sulfide and ammonia from a refinery waste water wherein: 
     a. a stream of a refinery absorber gas is introduced into one end of a contacting zone; 
     b. a stream of the refinery waste water to be treated is introduced into the other end of said zone; 
     c. the two streams are flowed countercurrently through the contacting zone and are intimately mixed; 
     d. an effluent stream of absorber gas is removed from one end of said zone, this gas stream being enriched with hydrogen sulfide gas and containing only neglible quantities of ammonia; and 
     e. simultaneously a waste water stream containing most of the original ammonia present but only neglible quantities of hydrogen sulfide is removed from the other end of said zone. 
     In a less broad aspect, our invention includes the additional step of stripping or removing the dissolved ammonia from the effluent waste water stream removed from the contacting zone. 
    
    
     DESCRIPTION OF THE DRAWING 
     The attached drawing is a process flow diagram illustrating the flow path and major pieces of equipment necessary to carrying out the method of this invention. To avoid undue complication of the drawing, auxiliary equipment such as pumps, exchangers, refluxing equipment and control valves have been omitted. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The terms &#34;refinery waste water&#34; or &#34;foul water&#34; used in this specification refer to water streams derived from petroleum refining operations which contain dissolved hydrogen sulfide and ammonia. These streams must be treated to remove the dissolved ammonia and hydrogen sulfide before they can be recycled or released. 
     The term &#34;absorber gas&#34; is a term well known to those skilled in the art of petroleum refining and is available at most typical refineries. It includes the uncondensible overhead gas derived primarily from catalytic cracking, but may include light gas from other refinery processing units such as crude oil topping units, reformers, hydrotreaters, etc. Ordinarily this gas contains hydrocarbons such as methane, ethane, ethylene, and small amounts of propane and propylene. Other constituents include hydrogen as well as inert gases such as nitrogen, carbon dioxide, carbon monoxide and oxygen. A relatively small amount of hydrogen sulfide is usually present. The gas typically has little value other than as fuel. The absorber gas, if it is burned, is usually released from some higher pressure, for example 100 psig to 220 psig, to 40-80 psig when used as a fuel and the energy available from expansion of the gas is lost. 
     The process of our invention may be better understood by reference to the attached drawing which depicts a preferred embodiment of the invention. In the embodiment depicted in the drawing, the refinery waste water containing hydrogen sulfide and ammonia is introduced through line 2 into liquid-gas contacting column 4 which can be filled with packing, distillation plates or other means that will effect intimate contact between countercurrent flowing streams of gas and liquid. At a point near the bottom of column 2 absorber gas is admitted through expansion valve 24 in line 6. Line 8 conducts steam into the bottom of column 4 if this is desirable. Effluent gas from column 4 is removed through line 10 to a process for concentrating the hydrogen sulfide such as by amine absorption. The gas removed in line 10 is substantially free of ammonia, that is, the concentration of ammonia will not be more than about 0.1 mole percent. The extracted hydrogen sulfide, free of ammonia, is then ready for diversion to an SO 2  or free sulfur plant. 
     The effluent water stream from column 4 is removed through line 14 and admitted to the upper part of a second column 16 which can be of a design the same as or similar to column 4. This water stream in line 14 is substantially free of hydrogen sulfide, that is, the concentration of hydrogen sulfide will not be more than about 2000 ppm. Steam is introduced through line 18 near the bottom of column 16 in sufficient quantity to heat the water to a temperature of between about 200° and about 300°F., depending on pressure. Effluent water substantially free of ammonia is removed through line 22 and can be released to a sewer, waterway, etc. Stripping steam and ammonia are removed overhead through line 20 to an incinerator or are otherwise disposed of. 
     As for operating conditions, the water coming through line 2 into column 4 will be at a temperature of between 100°F. and 250°F. and will contain up to between 2,000 and 40,000 ppm of hydrogen sulfide and between 1,000 and 40,000 ppm of ammonia. In column 4 the operating pressure will be between 40 psig and 80 psig and the absorber gas and hydrogen sulfide in line 10 will be removed at a temperature of about 100°F. The gas will contain a minimum of 0.1 mole percent of ammonia. The incoming absorber gas will be expanded from a pressure of between 100 psig and 200 psig across expansion valve 24 to the bottom of column 4. The water in line 14 containing dissolved ammonia will leave at a temperature between 100° and 300°F. and will contain a maximum amount of H 2  S of 2,000 ppm. The ratio of the volume of water to the volume of absorber gas flowed through column 4 should be between 25 and 150 ft 3  of gas (measured at 60°F. and 1 atmosphere) per cubic foot of water. The amount of steam admitted through line 8 should be that needed to maintain a temperature of between 100° and 300°F. in the bottom of column 4. The temperature in column 4 where the gases and liquids are mingled, will thus be between about 100° and about 300°F. 
     In column 16 the operating pressure will be between 0 and 25 psig and the temperature between 210° and 270°F. The ratio of steam injected through line 18 to the water injected through line 14 should be between about 0.5 and about 2.0 pounds of steam per gallon of water. 
     EXAMPLE 
     Refinery foul water containing 14,000 ppm of sulfur in the form of dissolved hydrogen sulfide and 4,700 ppm of dissolved ammonia are flowed into the top of a stripper column at the rate of 40 gallons per minute. The temperature of the incoming water is approximately 100°F. Into the bottom of the column an absorber gas is introduced at the rate of 1 million scf per day (measured at 1 atmosphere and 60°F). This is equivalent to an hourly injection rate of approximately 41,500 scf. The gas is expanded from a line pressure of 220 psig to a pressure of 70 psig inside the bottom of the stripper column. The gas is a mixture primarily of methane, ethane, nitrogen, carbon dioxide and carbon monoxide and contains approximately 2 mole percent of hydrogen sulfide.  Steam is introduced at a rate of 2,500 lb/hr to maintain the bottom temperature of the column at about 225-260°F. The steam does not function as a stripping agent since the top of the column and the vapors leaving the top of the column have a temperature of only about 100°F. The overhead vapors which are a mixture of absorber gas, water vapor and 8.0 mole percent hydrogen sulfide leave the column at a pressure of 70 psig and are carried to an amine absorption unit for extraction of the hydrogen sulfide. The effluent water from the bottom of column 4 is removed at a rate of slightly in excess of 40 gallons per minute, the excess consisting of condensed steam. The effluent water contains 1,600 ppm of sulfur in the form of dissolved hydrogen sulfide and 4,700 ppm of dissolved ammonia. It should be noted that all of the ammonia has remained in solution in the waste water and the amount of hydrogen sulfide retained is considered minimal. The effluent is carried to a conventional steam stripper operated at an internal pressure of 20 psig and a temperature of 250°F. The vapors leaving the stripper are a mixture of ammonia, water vapor and traces of hydrogen sulfide and are subsequently combusted. The sulfur dioxide emitted from the combustion is sufficiently low as to have neglible impact on the environment. The effluent water from the bottom of this second stripper has no detectable hydrogen sulfide and only a slight ammonia content. It is thus acceptable for disposal into sewers, natural waterways, etc. 
     The advantages of our invention are readily apparent. The substitution of absorber gas in the first stage or first column diverts hydrogen sulfide from air pollution to a recovered product. The amount of steam required is reduced substantially. Energy contained in the compressed absorber gas formerly wasted in many refineries is now fully utilized.