Gas treating

A study of gas treating can be very confusing because so many processes exist. The discussions in this chapter attempt to explain the differences among some of the most popular processes. Understanding of the different processes will aid in process selection. 

Gas treating is defined here as removal of H2S and CO2. Other sulfur compounds are discussed where applicable. Dehydration and sulfur production are not included, except for discussing sulfur production in the Stretford Process and for selective H2S removal. H2S must be removed from natural gas and process streams for health reasons and prevention of corrosion. Natural gas pipeline specifications require no more than % grain/100 SCF. This is equivalent to 4ppmv or 7ppmw (for a 0.65 specific gravity gas). By comparison, the human nose can detect 

13ppmv and the thre.shold limit value for prolonged exposure is lOppmv. 

CO2 must be removed for prevention of corrosion and because it lowers the heating value of natural gas. In some treatment situations it is desired to selectively remove H2S and slip most of the CO2. If the CO2 can be tolerated downstream, a more economical treating plant can often be provided that removes only the H2S. Sometimes, 

Economical design (capital and operating costs) H2S enrichment for sulfur recovery Special cases such as energy recovery 

GPSA Engineering Data Book, Gas Processors Suppliers Association, Vol. II, 10th Ed. 

Here are two authoritative discussions - of troubleshooting in glycol dehydration plants. 

Glycol plant troubleshooting information is presented from the article Dehydration Using TEG by Manning and Thompson. 

Troubleshooting is described by suggesting possible causes of the more common problems and discussing corrective measures. 

Change in gas flow rate, temperature, or pressure Insufficient glycol circulation (should be 1.5 to 3 gal TEG/lb water removed) 

Poor glycol reconcentration (exit gas dew point is 5-15°F higher than dew point in equilibrium with lean glycol concentration) 

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Gas solubility Gas sweetening Gas-treating Gastric acid Gastric prokinetics Gastrin... 

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. 

Methyl dietb an olamine (MDEA) and solutions of MDEA have increased in use for gas treating (150,151). Additional gas treating capacity can often be obtained with the same working equipment, because of the higher amine concentrations that can be used. 

G. Astarita, D. W. Savage, and A. Bisie, Gas Treating with ChemicalSolventSs oSm. Wiley Sons, Inc., New York, 1983. 

Other gas-treating processes involving sulfolane are (/) hydrogen selenide removal from gasification of coal, shale, or tar sands (qv) (108) (2) olefin removal from alkanes (109) (J) nitrogen, helium, and argon removal from natural gas (110) (4) atmospheric CO2 removal in nuclear submarines (5) ammonia and H2S removal from waste streams (6) H2S, HCl, N2O, and CO2 removal from various streams (111—120) and (7) H2S and SO2 removal from... 

Adsorption Processes. The processes based on adsorption of hydrogen sulfide onto a fixed bed of soHd material are among the oldest types of gas treating appHcations (4). Two common sorbent materials for low concentration gas streams are iron oxide and zinc oxide. 

Another variation of the Selectox process can be used with the Beavon process in tail gas treating. The hydrogenated Claus tail gas stream is sent to a Selectox reactor. Overall recoveries of up to 98.5% are possible. Use of Beavon/Selectox, however, typically costs more than use of Superclaus. 

A bleed from the scmbbing system is sent to a sour slurry stripper. The water is then clarified and can be recycled to minimize the volume of effluent to be biotreated and discharged or evaporated. The acid gas from the acid gas removal system and from the sour slurry stripper is fed to a Claus plant, where salable elemental sulfur (qv) is produced. For maximum sulfur recovery and minimal sulfur emissions, the Shell Claus off-gas treating process (SCOT) is used. 

In conventional treating systems using cold-gas cleanup, the small fraction of metals released to the gas phase is captured effectively in the gas cooling and gas treating steps. The combination of gas cooling and multistage gas—Hquid contacting reduces very substantially the potential for airborne emissions of volatile metals such as lead, beryUium, mercury, or arsenic. 

Gas Treating Chemicals," Technical Bulletin E-4 1335C, Union Carbide Chemicals and Plastics Co., Danbury, Conn., 1980. 

Generahzed prediction methods for fci and Hi do not apply when chemical reaction occurs in the liqmd phase, and therefore one must use ac tual operating data for the particular system in question. A discussion of the various factors to consider in designing gas absorbers and strippers when chemical reac tious are involved is presented by Astarita, Savage, and Bisio, Gas Treating with Chemical Solvents, Wuey (1983) and by Kohl and Ricseufeld, Gas Purification, 4th ed., Gulf (1985). 

A necessary prerequisite to understanding the subject of absorption with chemical reaction is the development of a thorough understanding of the principles involved in physical absorption, as discussed earlier in this section and in Section 5. There are a number of excellent references the subject, such as the book by Danckwerts Gas-Liquid Reactions, McGraw-Hill, New York, 1970) and Astarita et al. Gas Treating with Chemical Solvents, Wiley, New York, 1983). 

One of the most comprehensive economic studies was done in two phases. The first phase addressed whether the location of the treating facilities should be offshore or onshore. The second phase evaluated the process design options. The outcome of the first phase recommended onshore natural gas treating facilities the second phase recommended implementation of the turboexpander process design. The process options evaluated for this project are listed below ... 

The principal technological changes in the engineering control of air pollution were the perfection of the motor-driven fan, which allowed large-scale gas-treating systems to be built the invention of the electrostatic precipitator, which made particulate control in many processes feasible and the development of a chemical engineering capability for the design of process equipment, which made the control of gas and vapor effluents feasible. 

There are so many gas treating processes that one hardly knows where to begin. That makes selection of a process for a given situation difficult. Economics will ultimately decide, but some initial rough screening is required to eliminate inordinate study. I have attempted to condense the mass of information in the literature into some brief, user-friendly guidelines for the rough cut. 

Low acid gas partial pressures (product of system pressure and concentration of acid gases—HiS and CO2 in the feed) of roughly 50psi and below. Another way of looking at selection based upon acid gas concentration is Figure 1 in the previous section entitled Gas Treating Processes. 

Tennyson. R. N.. and Schaap, R. R, Guidelines Can Help Choose Proper Process for Gas-Treating Plants, Oil and Gas Journal, January 10, 1977. 

Provide requirements for specific processes. Every commercial process has unique requirements. One of the best examples that I am familiar with is the requirements outlined for an amine gas treating plant in Don Ballard s timeless article. Such advice as ... 

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