Acid Gas Components

Process selectivity indicates the preference with which the process removes one acid gas component relative to or in preference to another. For example, some processes remove both hydrogen sulfide and carbon dioxide, whereas other processes are designed to remove hydrogen sulfide only. Thus it is important to consider the process selectivity for hydrogen sulfide removal compared to carbon dioxide removal, ie, the carbon dioxide-to-hydrogen sulfide ratio in the natural gas, in order to ensure minimal concentrations of these components in the product. 

Physica.1 Absorption. Whereas chemical absorption rehes on solvent reactions to hold acid gas components in solution, physical absorption exploits gas—hquid solubiUties. The amount of absorption for these solvents is direcdy proportional to the partial pressure of the acid gas components. Thus these processes are most appHcable in situations involving high pressure feed streams containing significant concentrations of acid gas components. To favor absorption, lower temperatures are often employed. Some processes require refrigeration. 

These processes are based on the solubility of the H2S and/or GO2 within the solvent, instead of on chemical reactions between the acid gas and the solvent. Solubility depends first and foremost on partial pressure and. secondarily on temperature. Higher acid-gas partial pressures and lower temperatures increase the solubility of H2S and CO2 in the solvent and thus decrease the acid-gas components. 

Solvent temperatures below ambient are usually used to increase the solubility of acid gas components and therefore decrease circulation rates. 

To select a process, determine flow rate, temperature, pressure, concentration of the acid gases in the inlet gas, and allowed concentration of acid gases in the outlet stream. With this information, calculate the partial pressure of the acid gas components. 

Dehydration to dew points below the temperature to which the gas will be subjected will prevent hydrate formation and corrosion from condensed water. The latter consideration is especially important in gas streams containing CO2 or H2S where the acid gas components will form an acid with the condensed water. 

The picture becomes considerably more complicated when the additional components of synthesis gas processes are added. The various types of components involved are portrayed in Figure 1 where each type of component is indicated by a circle. The components of existing oil and gas processes, shown on the right-hand portion of the figure, primarily involve the light hydrocarbons C-, Cg, Cg, C , and Cg the oil fractions Cg+ and the acid gas components composed primarily of and CO2. 

B. Sour gas treating involves the removal of the acid gas components CO2 and H2S from natural gas. Most ways of doing this involve water solutions. Treating is normally at near ambient temperatures and at pressures to 7100 kPa (70 Atm). The treating of high acid gas content natural gas is becoming more important as the value of natural gas increases. 

The major acid gas component is CO2, which is dead weight in a Claus plant but usually makes up 80 percent to 95 percent of the acid gas, while most of the remainder is H2S. Relatively more CO2 is produced from lower sulfur coals, or higher temperature gasifiers, or by shifting before acid gas extraction. 

Stability issues The many stability issues discussed for ceramic oxygen-ion conductors apply also to proton conductors. Reactions with acidic gas components... 

The hydrocarbon gases that leave the compressor are usually subjected to three additional steps (1) the removal of acid gas components, (2) the removal of acetylenic compounds, and (3) further water removal or drying. 

Although some natural gas is suitable for use as a fuel as produced, virtually all natural gas undergoes some processing to produce a merchantable gas the composition of which is principally methane and ethane. Water and acid gas components are removed to prevent freezing, corrosion, or other operating problems in transmission or utilization. Diluents that exist in significant quantities usually are removed to improve the combustion and/or... 

There are a couple of things of interest from these two charts. In the liquid phase the viscosity decreases with increasing temperature - which tends to be the common experience. On the other hand, in the gas phase the viscosity increases with increasing temperature. This was demonstrated earlier at low pressure and reaffirmed by the behavior shown in the new plots. Furthermore, these charts show the viscosity of the acid gas components over a rather wide range of pressure and temperature, and yet the viscosity only varies by about an order of magnitude (from about 0.02 to 0.2 mPa-s). 

Table 2C.2 Binary interaction parameters for acid gas components for the PR and SRK equations.

Methane, a common impurity in acid gas, tends to broaden the phase envelope because it is lighter than the acid gas components. Figure 3.6 shows two phase envelopes. The first is the phase envelope for an equimolar mixture of hydrogen sulfide and carbon dioxide. This is the same phase envelope shown in figure 3.2. The other phase envelope is for a mixture with 2 mol% methane. 

At low pressure, the solubility of the acid gas components in the vapor phase can be calculated using the simple Henry s law. 

Various units could be used for the solubilities, but the values given in the next section, the solubilities of both the acid gas components and the NaCl must be in molality (moles of salt per kilogram of solute). 

There is an interesting effect of pH on the solubility of acid gas in water. In a solution with a high pH (a basic solution), the solubility of the acid gas components is dramatically increased. This is due to the acid-base reactions that occur between the dissolved acid gas and the base in the original solution. 

Unfortunately, the acid gas components are also readily adsorbed onto most mole sieve materials. Figure 7.5 shows the adsorption isotherms for H2S on three common mole sieve materials at 25°C. Similarly, figure 7.6 show the isotherms for C02 on two mole sieve materials. These plots are based on information provide in Kohl and Nielsen (1997). 

In a typical refrigeration unit, ethylene glycol is injected into the system to prevent hydrate formation and to absorb the liquid water that forms. For acid gas applications, some have chosen to inject methanol. The acid gas components are slightly less soluble in methanol than in glycol. 

The typical hydrocarbon impurity in acid gas is methane. Methane is lighter than the acid gas components, and thus the density of an acid gas mixture is reduced by the presence of methane. Even in a liquid mixture, the density is reduced by the presence of methane. 

As was noted earlier, mineralization of the acid gas components is not necessarily a bad thing. On the contrary, it may be an excellent mechanism to sequester the acid gas safely and permanently. 

Table 4.15 compares common sulfur removal processes. Amine processes are based on the removal of an acid gas by virtue of a weak chemical bond between the acid gas component and the amine. Amine-based sulfur removal processes are generally regarded as a low capital cost option with part C02 coabsorption. However, amines do not chemically combine with COS. Only limited amounts of COS are absorbed with a physical solvent. COS can be physically removed only with very high solvent circulation rates. For syngases that contain appreciable quantities of COS, prior removal of the COS is usually required. In addition, for some of amine solvents, degradation and corrosion are also main disadvantages of the process. 

What is the major acid gas component, and what are the more minor acid gases present in flue gas that provide a low cost way to acidify aqueous alkaline waste water streams to assist recovery of pollutants, such as phenols and hydrogen sulfide ... 

To be truly competitive, the removal of the acid gas components H2S and CO2, be it trim or bulk, complete or partial, requires the optimum choice of an activator together with a carefully crafted know-how in solvent absorption/regeneration process design. The Elf Activated MDEA process developed by Total is probably the most cost-effective solution today to meet the widest range of applications from complete CO2 removal to bulk H2S and/or CO2 removal even for acid gas re-injection projects. The historical R D efforts of Elf Aquitaine coupled with current resources of Total allows this MDEA process to be credited with the most significant know-how back-up technology base on the market today. 

The selection of the liquid absorbents in membrane contactors is critical. The commonly used absorbents for CO2 removal are amine based (i.e., MEA, DEA, and TEA) [160-163], Recently, membrane contactors using ILs as alternative absorbents for the capture of acid gases have been reported [166-170], The unique properties of ILs (nonvolatile with a high affinity for the acid gas component, thermally and chemically stable [171]) make it very promising as CO2 absorbents in membrane contactors, especially for applications in harsh conditions, such as CO2 separation from precombustion flue gas at elevated temperatures and pressures [169,170]. 

Major applications of the absorption processes are the removal of acid gas components such as CO2 and H2S from gas mixes. Cases are not infrequent, however, in which other raw gas components such as higher hydrocarbons, trace impurities (metal carbonyls, NH3) or steam are eliminated either alone or in combination with acid gas components. Absorption processes, which by their very essence are gas separation processes, are as a rule used for gas purification, i.e. undesired components are eliminated fix>m a gas mix without significantly changing its pressure. Some absorption processes are, however, used to recover individual gas components. In these processes, the normally larger undesirable portion of the gas mix is left at absorption pressure while the desired gas component, after it has been desorbed, is usually obtained at ambient pressure. 

Selectivity is of interest under three aspects. For one thing, there is the capability of the absorbent to remove for instance one of two acid gas components down to a residual content of only tenths of ppm while the other is left in the... 

As already mentioned at the beginning of the chapter on gas cleaning, the oxidative processes do not eliminate the acid gas component by a reversible method and release it again as the solvent is regenerated. Rather, a reaction takes place in the wash liquor so that the absorbed gas component is obtained in some other form. This is of particular interest for selective desulfurization of raw gases and fcH H2S removal from acid gases to facilitate sulfur recovery on the one hand and. 

Acid Gas Components. The oil and gas industries have a long history of effectively removing and processing carbon dioxide, hydrogen sulfide, and carbonyl sulfide, (collectively termed acid gases because they are acidic) from fuel gas streams. It is common to remove H2S and CO2 from raw natural gas before the gas enters the natural gas pipeline system. In coal gasification, a portion of the feed coal converts to CO2. 

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