Acid gases, absorption

Fixed-bed desulfuri2ation is impractical and uneconomical if the natural gas contains large amounts of sulfur. In this case, bulk sulfur removal and recovery (qv) in an acid gas absorption—stripping system, followed by fixed-bed residual cleanup is usually employed. 

The conference was subdivided into four sessions, and chapters within this text are arranged according to these categories. Papers included in the first section (Thermodynamics of Electrolytes for Pollution Control) provide the reader with insights into the practical aspects of pollution control, as well as an overall appreciation of applied electrolyte phase equilibria. Other chapters include detailed descriptions of thermodynamic models that recently have been developed to describe important industrial pollution control processes with emphasis on acid gas absorption/sour water stripping and flue gas desulfurization. 

Gas Processing, acid gas absorption (Chemical and Electrol) — electrolyte NRTL. Petrochemicals, aromatics and ether production (Petchem) — Wilson, NRTL,... 

A comprehensive treatment of the thermodynamics of acid gas absorption by reactive solvents is presented by Astarila ei a ,23 These authors discuss the ihenretical approach to modeling such systems and provide equilibrium data on a number of commercially important solutes and solvents. Figures 6.1-4 and 6.1-5 show comparative equilibrium curves for CO in four different alkaline solvents ai coudilions representing absorption and stripping, respectively. 

Real-time monitoring of solvent composition for acid gas absorption processes... 

In the current work, we give an overview of the recent progress made within the field of in-line monitoring solvent compositions in post-combustion CO2 absorption. We extent this work by an assessment of the possibilities to combine the composition details with process data that is already commonly available for acid gas absorption plants. 

Combinations of in-line measurement techniques with multivariate modelling show promising properties for nse as real-time monitoring applications of the liqnid composition in acid gas absorption processes. Both spectroscopic and non-spectroscopic analytical techniques can be nsed. Althongh the first developments were mostly aimed at predicting CO2 and amine concentrations in post-combustion CO2 capture processes, recent developments are also directing into applications involving acid gas removal from natural gas and the use of solvents that inclnde mixtures of two active components. 

The stripping of diethanolamine solutions is similar to MEA regeneration. In this case, although the heat of reaction of CO2 with DEA is 20% lower than with MEA, the regeneration still requires 1.0 to 1.2 lb of steam per gallon of solvent regenerated. The 2.5-normal solution of DEA typically used requires a higher solvent flow rate for the same acid gas absorption than 3-normal MEA solvent. 

The basic flow arrangement for aU alkanolamine acid-gas absorption-process systems is shown in Figure 2-10. Gas to be purified is passed upward through the absorber, countercurrent to a stream of the solution. The rich solution from the bottom of the absorber is heated by heat exchange with lean soludon from the bottom of the stripping colunm and is dien fed to the stripping column at some point near the top. 

As previously noted, in typical 20-tray absorbers, the bulk of the add gas is absorbed in the bottom half of the column, while the top portion serves to remove the last traces of acid gas and reduce its concentradon to the required product gas specificadon. With sufddent trays and amine, the uldmate purity of the prtxiuct gas is limited by equilibrium with the lean solution at the product gas temperature. When water washing is necessary to minimize amine loss (e.g., with low-pressure MBA absorbers), two to four additional trays are commonly installed above the acid gas absorption section. A high efficiency mist eliminator is recommended for the very top of the absorber to minimize carryover of amine solution or water. 

Early experimental studies of the rate of absorption of CO2 and H2S in alkanolamines in packed towers were reported by Cryder and Maloney (1941), Gregory and Schatinann (1937), Wainwright et al. (1952), Benson et al. (1954, 1956), Teller and Ford (1958), Leibush and Shneerson (1950), Shneerson and Leibush (1946), and Eckart et al. (1967). Packed column performance data on the absorption of H2S in MDEA solutions in the presence of CO2 were presented by Frazier and Kohl (1950) and Kohl (1951). Much of the early work on acid gas absorption in alkalis and amines was reviewed by Danckwots and Sharma (1966), who proposed design procedures based on fundamental concepts. 

Allied Chemical Coiporation, Solvay Process Division, 1961, TJie Hot Potassium Carbonate Process for Acid Gas Absorption, TechVSer. Kept. No. 6.61. 

Dihydroxyacetophenone. Finely powder a mixture of 40 g. of dry hydroquinone diacetate (1) and 87 g. of anhydrous aluminium chloride in a glass mortar and introduce it into a 500 ml. round-bottomed flask, fitted with an air condenser protected by a calcium chloride tube and connected to a gas absorption trap (Fig. II, 8, 1). Immerse the flask in an oil bath and heat slowly so that the temperature reaches 110-120° at the end of about 30 minutes the evolution of hydrogen chloride then hegins. Raise the temperature slowly to 160-165° and maintain this temperature for 3 hours. Remove the flask from the oil bath and allow to cool. Add 280 g. of crushed ice followed by 20 ml. of concentrated hydrochloric acid in order to decompose the excess of aluminium chloride. Filter the resulting solid with suction and wash it with two 80 ml. portions of cold water. Recrystallise the crude product from 200 ml. of 95 per cent, ethanol. The 3 ield of pure 2 5-dihydroxyacetophenone, m.p. 202-203°, is 23 g. 

Y-Phenylbutyric acid. Prepare amalgamated zinc from 120 g. of zinc wool contained in a 1-litre rovmd-bottomed flask (Section 111,50, IS), decant the liquid as completely as possible, and add in the following order 75 ml. of water, 180 ml. of concentrated hydrochloric acid, 100 ml. of pure toluene (1) and 50 g. of p benzoylpropionic acid. Fit the flask with a reflux condenser connected to a gas absorption device (Fig. II, 8, l,c), and boil the reaction mixture vigorously for 30 hours add three or four 50 ml. portions of concentrated hydrochloric acid at approximately six hour intervals during the refluxing period in order to maintain the concentration of the acid. Allow to cool to room temperature and separate the two layers. Dilute the aqueous portion with about 200 ml. of water and extract with three 75 ml. portions of ether. Combine the toluene layer with the ether extracts, wash with water, and dry over anhydrous magnesium or calcium sulphate. Remove the solvents by distillation under diminished pressure on a water bath (compare Fig. II, 37, 1), transfer the residue to a Claisen flask, and distil imder reduced pressure (Fig. II, 19, 1). Collect the y-phenylbutyric acid at 178-181°/19 mm. this solidifies on coohng to a colourless sohd (40 g.) and melts at 47-48°. 

Method 1. Equip a 1 litre three-necked flask (or bolt-head flask) with a separatory funnel, a mechanical stirrer (Fig. II, 7, 10), a thermometer (with bulb within 2 cm. of the bottom) and an exit tube leading to a gas absorption device (Fig. II, 8, 1, c). Place 700 g. (400 ml.) of chloro-sulphonic acid in the flask and add slowly, with stirring, 156 g. (176 ml.) of pure benzene (1) maintain the temperature between 20° and 25° by immersing the flask in cold water, if necessary. After the addition is complete (about 2 5 hours), stir the mixture for 1 hour, and then pour it on to 1500 g. of crushed ice. Add 200 ml. of carbon tetrachloride, stir, and separate the oil as soon as possible (otherwise appreciable hydrolysis occurs) extract the aqueous layer with 100 ml. of carbon tetrachloride. Wash the combined extracts with dilute sodium carbonate solution, distil off most of the solvent under atmospheric pressure (2), and distil the residue under reduced pressure. Collect the benzenesulphonyl chloride at 118-120°/15 mm. it solidifies to a colourless sohd, m.p. 13-14°, when cooled in ice. The yield is 270 g. A small amount (10-20 g.) of diphen3 lsulphone, b.p. 225°/10 mm., m.p. 128°, remains in the flask. 

Method 1. In a 750 ml. three-necked flask or wide-mouthed glass bottle, equipped with a dropping funnel, a mechanical stirrer (Fig.//, 7,10) a thermometer (with bulb within 2 cm. of the bottom) and an outlet tube leading to a gas absorption device (Fig. II, 8, 1, c), place 400 g. (228 ml.) of chlorosulphonic acid and cool to 0° in a freezing mixture of ice and... 

The process options reflect the broad range of compositions and gas volumes that must be processed. Both batch processes and continuous processes are used. Batch processes are used when the daily production of sulfur is small and of the order of 10 kg. When the daily sulfur production is higher, of the order of 45 kg, continuous processes are usually more economical. Using batch processes, regeneration of the absorbant or adsorbant is carried out in the primary reactor. Using continuous processes, absorption of the acid gases occurs in one vessel and acid gas recovery and solvent regeneration occur in a separate reactor. 

Extensive work has been done on corrosion inhibitors (140), activated carbon use (141—144), multiple absorption zones and packed columns (145,146), and selective absorption and desorption of gas components (147,148). Alkan olamines can also be used for acid gas removal in ammonia plants (149). 

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