Page 14 - Gas Purification 5E

4 GasPuri~kation

partial pressure. According to Christensen and Stupin (1978), physical absorption is generally
favored at acid gas partial pressures above 200 psia, while alkaline solution absorption is
favored at lower partial pressures. Tennyson and Schaaf (1977) place the boundary line
between physical and chemical solvents at a somewhat lower partial pressure (Le., 60-100
psia) above which physical solvents are favored. They also provide more detailed guidelines
with regard to the preferred type of alkaline solution and the effect of different acid gas
removal requirements. The absorption of hydrogen sulfide and carbon dioxide in alkaline
solutions is discussed in detail in Chapters 2,3,4, and 5. Chapter 14 covers the use of physi-
cal solvents.
Membrane permeation is particularly applicable to the removal of carbon dioxide from
high-pressure gas. The process is based on the use of relatively small modules, and an
increase in plant capacity is accomplished by simply using proportionately more modules.
As a result, the process does not realize the economies of scale and becomes less competitive
with absorption processes as the plant size is increased. McKee et al. (1991) compared
diethanolamine PEA) and membrane processes for a 1,OOO psia gas-treating plant. For their
base case, the amine plant was found to be generally more economical for plant sizes greater
than about 20 MMscfd. However, at very high acid-gas concentrations (over about 15% car-
bon dioxide), a hybrid process proved to be more economical than either type alone. The
hybrid process, which is not indicated in Table 1-2, uses the membrane process for bulk
removal of carbon dioxide and the amine process for final cleanup. Membrane processes are
described in Chapter 15.
When hydrogen sulfide and carbon dioxide are absorbed in alkaline solutions or physical
solvents, they are normally evolved during regeneration without undergoing a chemical
change. If the regenerator offgas contains more than about 10 tons per day of sulfur (as
hydrogen sulfide), it is usually economical to convert the hydrogen sulfide to elemental sul-
fur in a conventional Claus-type sulfur plant. For cases that involve smaller quantities of sul-
fur, because of either a very low concentration in the feed gas or a small quantity of feed gas,
direct oxidation may be the preferred route. Direct oxidation can be accomplished by absorp-
tion in a liquid with subsequent oxidation to form a slurry of solid sulfur particles (see Chap-
ter 9) or sorption on a solid with or without oxidation (see Chapter 16). The solid sorption
processes are particularly applicable to very small quantities of feed gas where operational
simplicity is important, and to the removal of traces of sulfur compounds for final cleanup of
synthesis gas streams. Solid sorption processes are also under development for treating high-
temperature gas streams, which cannot be handled by conventional liquid absorption
processes.
Adsorption is a viable option for hydrogen sulfide removal when the amount of sulfur is very
small and the gas contains heavier sulfur compounds (such as mercaptans and carbon disulfide)
that must also be removed. For adsorption to be the preferred process for carbon dioxide
removal, there must be a high CQ partial pressure in the feed, the need for a very low concen-
tration of carbon dioxide in the product, and the presence of other gaseous impurities that can
also be removed by the adsorbent. Typical examples are the purification of hydrogen from
steam-hydrocabon reforming, the purification of land-fill effluent gas, and the purification of
ammonia synthesis gas. Adsorption processes are described in detail in Chapter 12.
Two processes predominate for water vapor removal: absorption in glycol solution and
adsorption on solid desiccants. These two processes are quite competitive and, in many cases,
either will do an effective job. In general, a dry desiccant system will cost more, but will pm
vide more complete water removal. For large-volume; high-pressure natural gas treating, gly-
col systems are generally more economical if dew-point depressions of 40" to 60°F are suffi-