Gases Absorption

Basically, a gas absorption tower is a unit in which the desirable light ends components are recovered from the gas feed by dissolving them in a liquid passing through the tower countercurrently to the gas. The liquid absorbent is called lean, oil, and it usually consists of a hydrocarbon fraction in the gasoline boiling range. After the absorption step, the liquid which now contains the desired constituents in solution is referred to as fat oil. A similarly descriptive nomenclature is applied to the gas, which is referred to as wet gas when it enters the tower and as dry gas when it leaves the absorber. 

The fat oil is fed to a splitter or stripping tower, where the absorbed tight constituents are separated from the oil by distillation. Usually the lean oil is the same material as the heavier part of the absorber feed, so that the bottoms from the stripper are split into lean oil, which is recycled to the absorber, and a stabilized gasoline product, which is passed on to subsequent processing operations. 

The absorber tower usually performs a double function. It not only recovers the desirable eonstituents from the gas, but also rejeets undesirable lighter 

The nonequilibrium model is not limited to simulating distillation operations with no fundamental change, it can be applied to absorption operations as well. A design case study is presented here by way of illustrating the model. This problem is adapted from an application discussed by Krishnamurthy and Taylor (1986). 

A hydrocarbon gas mixture is to have its propane content reduced by 45% by absorption in a heavy oil at a pressure of 4 atm. Tridecane is used to represent the oil in this illustration. Feed flows are summarized in Table 14.7. 

The important design variables in this problem are the number of trays and the oil flow rate. Seven (or more) sieve trays were needed to reach the desired reduction in the propane content of the gas mixture. A complete set of configuration and operation specifications is given in Table 14.7. Calculated product stream flows and compositions are also given. The column and tray design is summarized in Table 14.8. 

It is possible to achieve more or less the same separation achieved in the seven tray column in two equilibrium stages. Thus, the overall efficiency is about 28%. Industrial absorbers typically operate at overall efficiencies between 15 and 40%.  

In this section we present several comparisons between the predictions of a nonequilibrium model and actual experimental data. Simulations of a variety of operations are described including small and industrial scale and trayed and packed towers. 

Ordinarily, these operations are used only for solute recovery or solute removal. Separation of solutes from each other to any important extent requires the fractionation techniques of distillation. 

Liquid feed, operates at temperatures above the freezing temperature of the solute, usually in the range 18 to 35 °C. Solute formed by reaction is insoluble, key parameter is of product. Provides a sharp first cut removal of solute. 

For proteins, related topic is flocculation and coagulation. Section 9.3. 

Reactions are usually very rapid and design is based on mass transfer and mixing to distribute the reactant. For most precipitation reagents allow 5 min residence time. If secondary reagents are needed to change the oxidation state of the target species before precipitation then example residence times are arsenic 30 min, hexavalent chromium or iron 20 min. 

Either pre or post pH change allow about 0.180 kg acid or base/m water. 

For proteins, alter the hydration layer surrounding the protein by (i) electrolyte addition follow the Hoffmeister series of lyotropic ions (sulfates are preferred to phosphates because of the solubility) (ii) adding hydrophilic, non-ionic polymers, polyethylene glycol (iii) adjusting the pH to shift to zpc (but avoid low pH) (iv) lower the dielectric permeability of the continuous phase by the addition of ethanol, methanol, acetone or isopropanol (v) adding flocculant polyelectrolytes (alginate, pectate, carrogcenan, CMC). 

Familiarize yourself with the factors to be considered in designing absorbers. 

Explain the importance of exhaust gas characteristics and liquid flow. 

Compute the minimum liquid flow rate required for separation. 

Determine the diameter and the packing height of a packed-bed column. 

Find the number of theoretical plates and the height of a plate tower. 

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Fixed-bed reactors in the form of gas absorption equipment are used commonly for noncatalytic gas-liquid reactions. Here the packed bed serves only to give good contact between the gas and liquid. Both cocurrent and countercurrent operations are used. Countercurrent operation gives the highest reaction rates. Cocurrent operation is preferred if a short liquid residence time is required. 

The most common alternative to distillation for the separation of low-molecular-weight materials is absorption. In absorption, a gas mixture is contacted with a liquid solvent which preferentially dissolves one or more components of the gas. Absorption processes often require an extraneous material to be introduced into the process to act as liquid solvent. If it is possible to use the materials already in the process, this should be done in preference to introducing an extraneous material for reasons already discussed. Liquid flow rate, temperature, and pressure are important variables to be set. 

Packed columns are widely used in gas absorption, but particulates are also removed in the process (see Fig. 11.2a). The main disadvan-... 

Foams are used industrially and are important in rubber preparations (foamed-latex) and in fire fighting. The foam floats as a continuous layer across the burning surface, so preventing the evolution of inflammable vapours. Foams are also used in gas absorption and in the separation of proteins from biological fluids. See anti-foaming agents. 

Since the transfer of material between phases takes place on the plates, the degree of gas absorption, or of separation in the case of a distillation column, depends directly on their number. 

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... 

Coarse 40-60 Filtration of coarse materials. Gas dispersion, gas washing, gas absorption. Mercury filtration. For extraction apparatus. 

Absorption, or gas absorption, is a unit operation used in the chemical industry to separate gases by washing or scmbbing a gas mixture with a suitable hquid. One or more of the constituents of the gas mixture dissolves or is absorbed in the Hquid and can thus be removed from the mixture. In some systems, this gaseous constituent forms a physical solution with the Hquid or the solvent, and in other cases, it reacts with the Hquid chemically. 

Multicomponent Diffusion. In multicomponent systems, the binary diffusion coefficient has to be replaced by an effective or mean diffusivity Although its rigorous computation from the binary coefficients is difficult, it may be estimated by one of several methods (27—29). Any degree of counterdiffusion, including the two special cases "equimolar counterdiffusion" and "no counterdiffusion" treated above, may arise in multicomponent gas absorption. The influence of bulk flow of material through the films is corrected for by the film factor concept (28). It is based on a slightly different form of equation 13 ... 

Discussion of the concepts and procedures involved in designing packed gas absorption systems shall first be confined to simple gas absorption processes without compHcations isothermal absorption of a solute from a mixture containing an inert gas into a nonvolatile solvent without chemical reaction. Gas and Hquid are assumed to move through the packing in a plug-flow fashion. Deviations such as nonisotherma1 operation, multicomponent mass transfer effects, and departure from plug flow are treated in later sections. 

Fig. 5. Mass balance in gas absorption columns. The curved arrows indicate the travel path of the solute A. The upper broken curve delineates the envelope...

Equimolar Counterdiffusion. Just as unidirectional diffusion through stagnant films represents the situation in an ideally simple gas absorption process, equimolar counterdiffusion prevails as another special case in ideal distillation columns. In this case, the total molar flows and are constant, and the mass balance is given by equation 35. As shown eadier, noj/g factors have to be included in the derivation and the height of the packing is... 

General Situation. Both unidirectional diffusion through stagnant media and equimolar diffusion are idealizations that ate usually violated in real processes. In gas absorption, slight solvent evaporation may provide some counterdiffusion, and in distillation counterdiffusion may not be equimolar for a number of reasons. This is especially tme for multicomponent operation. 

It maybe noted that the above system of equations is very general and encompasses both the usual equations given for gas absorption and distillafion as well as situations with any degree of counterdiffusion. The exact derivations maybe found elsewhere (43). 

Fig. 9. Simple model of adiabatic gas absorption. A, nonisotherm a1 equihbrium line for overall gas-phase driving force y = B, nonisotherm a1...

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