Hydrogen sulfide selectivity factor

They also noted that hydrogen sulfide selectivity over carbon dioxide can be Improved by introducing gas gaps in a multilayer membrane because of the higher reaction rates of hydrogen sulfide. They found that catalysts increased the carbon dioxide hydration rate by a factor of 2 at low carbon dioxide partial pressure, this effect was observed to increase as carbon dioxide partial pressure decreased. Economic studies based on experimental data Indicate that cost savings on the order of 30-50% over conventional acid gas scrubbing were possible with immobilized liquid membranes. 

Gas purification processes fall into three categories the removal of gaseous impurities, the removal of particulate impurities, and ultrafine cleaning. The extra expense of the last process is only justified by the nature of the subsequent operations or the need to produce a pure gas stream. Because there are many variables in gas treating, several factors must be considered (/) the types and concentrations of contaminants in the gas (2) the degree of contaminant removal desired (J) the selectivity of acid gas removal required (4) the temperature, pressure, volume, and composition of the gas to be processed (5) the carbon dioxide-to-hydrogen sulfide ratio in the gas and (6) the desirabiUty of sulfur recovery on account of process economics or environmental issues. 

One of the most effective methods of preventing corrosion is the selection of the proper metal or alloy for a particular corrosive service. Once the conditions of service and environment have been determined that the equipment must withstand, there are several materials available commercially that can be selected to perform an effective service in a compatible environment. Some of the major problems arise from popular misconceptions for example, the use of stainless steel. Stainless steel is not stainless and is not the most corrosion-resistant material. Compatibility of material with service environment is therefore essential. For example, in a hydrogen sulfide environment, high-strength alloys (i.e., yield strength above 90,000 psi or Rc 20 to 22) should be avoided. In material selection some factors that are important to consider are material s physical and chemical properties, economics and availability. 

Another means for disposal of a sulfur product could involve reaction of hydrogen sulfide with either the ammonium or sodium sulfite solution to produce elemental sulfur. Additional processing is required, but the weight of disposable product could be reduced by a factor of about four. Moreover, under select conditions sale of sulfur could oflFset at least part of the processing costs. 

The selection of a particular process-type (Table 23.1) for a gas-cleaning operation is not a simple choice. Many factors have to be considered, not the least of which is the constitution of the gas stream that requires treatment. Indeed, process selectivity indicates the preference with which the process will remove one acid gas component relative to (or in preference to) another. For example, some processes remove both hydrogen sulfide and CO2 while other processes are designed to remove hydrogen sulfide only (Table 23.2). 

Several attempts have been made to evaluate and compare hydrogen sulfide scavenging processes based on economic analyses. Unfortunately, such studies are of limited value because they are highly dependent on the specific conditions and assumptions used in the study. Also, other factors, such as the environmental acceptability of the reactant and its waste products, operator acceptance of the process, and winterizing requirements, may be more important than cost in selecting a process. The very high rate of process modification, new process development, and removal of existing processes from the market also makes comparative evaluation difficult in this field. 

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