Acid Gas Solubility

Another important aspect in the design of an acid gas injection scheme is the solubility of the acid gas in water and brine. In the design of the surface equipment it is useful to know the amount of acid dissolved in the water removed in the interstage scrubbers. From a reservoir engineering point of view, it is important to know the solubility of the acid gas in the formation water. The water removed in the interstage scrubbers does not contain any dissolved solids because it is water of condensation. 

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When a 60 MW turbine at Hinkley A power station disintegrated in 1969 from stress corrosion cracking of a low pressure turbine disc (consequences shown in Plate 1) it was considered that Na H solutions were most probably involved (84) and it was soon found that if NaOH were the sole electrolyte present its maximum concentration (based on vapour pressure depression) was sufficient to have caused the cracking. However, it was also found that in mixtures it was only the free NaOH which led to rapid stress corrosion cracking. Considerations of acid gas solubility and solution thermodynamics showed that at the CO2 and acetate levels present it was most unlikely that free NaOH was present in sufficient quantity to be responsible for the Hinkley failure (85). 

In the literature, there are reports of work concerned with the measurement of oxoacidic properties of ionic melts by gravimetric measurements of the solubility of acidic gases in these media [76, 77], The solubility of sulfur(VI) oxide in molten sodium phosphates was determined by the gravimetric method [76], A correlation was obtained between the melt basicities and the solubility of gaseous acid S03 in them. Iwamoto reported the estimation of the basic properties of molten salt by measurements of acidic gas solubilities (carbon dioxide and water) in them [77], However, similar methods cannot be used widely, owing to two factors. The first of these consists in the fact that the solubility of any gas in a liquid phase obeys Henry s law. Let us consider the following system of reactions ... 

From these equations it follows that the acidic gas solubility in the melt consists of two terms the solubility according to the Henry law (which is constant for the same partial pressure of the gas) and the concentration of the gas fixed by oxide ions which increases with the base concentration. If KCOi is large, then the amount of gas absorbed is equal to the melt basicity, or (more correctly), to excessive concentration of O2- (as compared with acidic compounds) in the melt. If the melt contains any excess of acid then the gas solubility will be practically constant since there is no fixation of the gas by oxide-ions completely consumed after introducing the excess of acid into... 

Posey ML, Trapperson KG, Rochelle GT. A simple model for prediction of acid gas solubilities in aUcanolamines. Gas Sep Purif 1996 10 181-6. 

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

The scrubbing liquid must be chosen with specific reference to the gas being removed. The gas solubility in the liquid solvent should be high so that reasonable quantities of solvent are required. The solvent should have a low vapor pressure to reduce losses, be noncorrosive, inexpensive, nontoxic, nonflammable, chemically stable, and have a low freezing point. It is no wonder that water is the most popular solvent used in absorption devices. The water may be treated with an acid or a base to enhance removal of a specific gas. If carbon dioxide is present in the gaseous effluent and water is used as the scrubbing liquid, a solution of carbonic acid will gradually replace the water in the system. 

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. 

The physical solvent sulfolane provides the system with bulk removal capacity. Sulfolane is an excellent solvent of sulfur compounds such as H2S, COS, and CS2. Aromatic and heavy hydrocarbons and CO2 are soluble in sulfolane to a lesser degree. The relative amounts of DIPA and sulfolane are adjusted for each gas stream to custom fit each application. Sulfinol is usually used for streams with an H2S to CO2 ratio greater than 1 1 or where it is not necessary to remove the CO2 to the same levels as is required for H2S removal. The physical solvent allows much greater solution loadings of acid gas than for pure amine-based systems. Typically, a Sulfinol solution of 40% sulfolane, 40% DIPA and 20% water can remove 1.5 moles of acid gas per mole of Sulfinol solution. 

Ethyl Benzoate.—This ester has not been found, so far, to occur naturally in essential oils. It has, however, been prepared by synthetic processes, for example, by condensing ethyl alcohol with benzoic acid by means of dry hydrochloric acid gas. Its odour is very similar to that of methyl benzoate (q.v.), but not quite so strong. It is an oil of specific gravity I OfilO, refractive index 1 5055, and boiling-point 213° at 745 mm. It is soluble in two volumes of 70 per cent, alcohol. 

The hydrochloride of MP 223°-225°C is obtained by dissolving the base with alcohol and the corresponding quantity of hydrochloric acid gas in the hot with subsequent cooling of the solution. Colorless crystals are formed of MP 223°-225°C, which are easily soluble in water. 

Anthropogenic sources of fluoride include fossil fuel combustion and industrial waste. Hydrogen fluoride is water soluble and emissions are readily controlled by acid gas scrubbers. HF emission from coal combustion, that is considered to be the main anthropogenic source of HF, was estimated to be 0.18 Tg annually emission of HF from the combustion of petroleum and natural gas is almost certainly negligible [24]. Apparently only limited data are available concerning total annual emissions of HF from industrial operations however, there is evidence that emissions of fluorides have been declining [24,25]. 

Volatile boron compounds, especially boranes, are usually more toxic than boric acid or soluble borates (Table 29.9) (NAS 1980). However, there is little commercial production of synthetic boranes, except for sodium borohydride — one of the least toxic boranes (Sprague 1972). Boron trifluoride is a gas used as a catalyst in several industrial systems, but on exposure to moisture in air, it reacts to form a stable dihydride (Rusch etal. 1986). Eor boric oxide dusts, occupational exposures to 4.1 mg/m (range 1.2 to 8.5) are associated with eye irritation dryness of mouth, nose and throat sore throat and cough (Garabrant et al. 1984). 

Physical Absorption. Whereas chemical absorption relies on solvent reactions to hold acid gas components in solution, physical absorption exploits gas—liquid solubilities. The amount of absorption for these solvents is direcdy proportional to the partial pressure of the acid gas components. Thus these processes are most applicable 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. 

The solvent can be tailored to provide selective acid gas removal based on the liquid—gas solubilities. For example, the Selexol process, licensed by Union Carbide Corporation, uses the dimethyl ether of polyethylene glycol (DMPEG) to provide high hydrogen sulfide selectivity. The solubility of hydrogen sulfide in DMPEG is 8—10 times that of carbon dioxide. 

The physical solvent-based acid-gas removal systems do not rely upon the acidic properties of the acid gas. Rather, the polar acid gases have a much greater solubility in many oils and solvents, compared with fuel gas species. 

Selective Absorption in the Sweetening Process. In recent years removal of the acid gas (H2S, CO2) components from a gas stream has increasingly been by absorption in a solvent system containing amines. While non-reactive solvent sweetening processes are in use, the ability of the basic amine to react chemically with the acid gas to yield water soluble salts has favored the chemical sweetening system. Thus... 

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