Plate column absorber

A solvent recovery plant consists of a plate column absorber and a plate column stripper. Ninety percent of the benzene (B) in the gas stream is recovered in the absorption column. Concentration of benzene in the inlet is Yi = 0.06 mole B/mole B-free gas. The oil entering the top of the absorber contains X2 = 0.01 mole B/mole pure oil. In the leaving stream, X = 0.19 mole B/mole pure oil. Operation temperature is 77°F (250°C). 

Nonisothermal Gas Absorption. The computation of nonisothermal gas absorption processes is difficult because of all the interactions involved as described for packed columns. A computer is normally required for the enormous number of plate calculations necessary to estabUsh the correct concentration and temperature profiles through the tower. Suitable algorithms have been developed (46,105) and nonisothermal gas absorption in plate columns has been studied experimentally and the measured profiles compared to the calculated results (47,106). Figure 27 shows a typical Hquid temperature profile observed in an adiabatic bubble plate absorber (107). The close agreement between the calculated and observed profiles was obtained without adjusting parameters. The plate efficiencies required for the calculations were measured independendy on a single exact copy of the bubble cap plates installed in the five-tray absorber. 

Absorption is a commonly applied operation in chemical processing. It is used as a raw material or a product recovery technique in separation and purification of gaseous streams containing high concentrations of organics (e.g., in natural gas purification and coke by-product recovery operations). In absorption, the organics in the gas stream are dissolved in a liquid solvent. The contact between the absorbing liquid and the vent gas is accomplished in countercurrent spray towers, scrubbers, or packed or plate columns. 

In the case of plate columns, a heat balance may be performed over each plate and the resulting temperature determined. For adiabatic operation, where no heat is removed from the system, the temperature of the streams leaving the absorber will be higher than those entering, due to the heat of solution. This rise in temperature lowers the solubility of the solute gas so that a large value of Lm/Gm and a larger number of trays will be required than for isothermal operation. 

Reactive absorption can be realized in a variety of equipment types, e.g., in him absorbers, plate columns, packed units, or bubble columns. This process is characterized by independent how of both phases, which is different from distillation and permits both cocurrent (downflow and uphow) and countercurrent regimes. 

Fig. 2 shows an air-cooled condensing arrangement made hy Doulton Co., of Lambeth, London, a is the still head, the nitric acid from which passes into a preliminary stoneware receiving vessel it, and thence into a series of air-cooled vessels 11,. C is an earthenware condensing worm immersed in a cooling tank of water, while l> are condensing towers, usually packed with glass balls or cylinders, or sometimes provided with plate columns, down which trickles water to absorb the last trace of free acid. 

In designing an absorber or stripper, one mnst ascertain some index of difficnlty of separation. As in the case of distillation separations, this index is either the required number of theoretical stages or the required number of transfer units. These parameters are interchangeable, with stages often nsed for plate columns and transfer nnits for packed columns. Backgronnd on the parameters is provided in Chapter 12. For convenience in handling the associated hydranlics and mass transfer calculations, the remainder of this chapter is divided into three parts Plate Columns, Packed Columns, and Special Devices. ... 

Acetone is to be absorbed from wet air into pure water in a plate column. The laden air enters at 35°C, and the water solvent enters at 25°C. Pressure is atmospheric. The air contains 2.0 mole-% acetone and is 70% saturated with water vapor (4 vol-% water). The concentration of acetone in the air is to be reduced to 0.05 mole-% = 0.975). The solvent rate is chosen such that the operating line has a slope of 3.49 on a mole fraction-plotting basis. How many stages are needed ... 

Example t7.1. By means of a plate column, acetone is absorbed from its mixture with air in a nonvolatile absorption oil. The entering gas contains 30 mole percent acetone, and the entering oil is acetone free. Of the acetone in the air 97 percent is to be absorbed, and the concentrated liquor at the bottom of the tower is to contain 10 mole percent acetone. The equilibrium relationship is y, = 1.9X. Plot the operating line and determine the number of ideal stages. 

A straw oil used to absorb benzene from coke-oven gas is to be steam-stripped in a sieve-plate column at atmospheric pressure to recover the dissolved benzene, CgHg. Equilibrium conditions at the operating temperature are approximated by Henry s law such that when the oil phase contains 10 mol% benzene, the equilibrium benzene partial pressure above the oil is 5.07 kPa. The oil may be considered nonvolatile. It enters the stripper containing 8 mol% benzene, 75% of which is to be removed. The steam leaving contains 3 mol% C6H6. 

The exit gas from an alcohol fermenter consists of an air-C02 mixture containing 10 mole% CO2 that is to be absorbed in a 5.0-N solution of triethanolamine, which contains 0.04 moles of carbon dioxide per mole of amine. If the column operates isothermally at 25°C, if the exit liquid contains 0.8 times the maximum amount of carbon dioxide, and if the absorption is carried out in a six-theoretical-plate column, calculate ... 

These processes are carried out in a variety of equipment ranging from a bubbling absorber to a packed tower or plate column. The design of the adsorber itself requires models characterizing the operation of the process equipment and this is discussed in Chapter 14. The present chapter is concerned only with the rate of reaction between a component of a gas and a component of a liquid—it considers only a point in the reactor where the partial pressure of the reactant A in the gas phase is and the concentration of A in the liquid is C, that of B, Cg. Setting up rate equations for such a heterogeneous reaction will again require consideration of mass and eventually heat transfer rates in addition to the true chemical kinetics. Therefore we first discuss models for transport from a gas to a liquid phase. 

Consider now the absorption extraction of heavy hydrocarbons under the conditions of counter-current flow in a column absorber presented schematically in Fig. 20.7. Gas of a given composition yo = (yoi, yo2, , yon), where yoi is the molar fraction of i-th component, with the flow rate Qgo enters the bottom part of the column. At the same time, an absorbent with composition xo = (xoi 5 02, , on) and flow rate qo enters the top part of the column. The number of contact stages is equal to N. Each stage is equipped with a perforated plate operating in the ablation regime. This means that the liquid is not collected on the plate, but exists in a dispersed state in the inter-plate space. Each contact stage contains a separation device, for example, a mesh droplet catcher, in which the exhausted absorbent is separated from the gas and directed toward the next plate. 

The system of equations (20.23) complemented by the mass balance equations, has been solved numerically. As a result, equilibrium values of molar concentrations of components, and the properties of phases have been obtained. As an example, consider the calculation of the column absorber equipped with ten perforated contact plates, working in the ablation regime. The following parameter values were chosen p = 7.65 MPa T = 24 °C = 10 mill, m /day q = 217... 

Despite the wide availability of flat-sheet membranes and the impressive permeability they offer, the hollow fibre configuration is usually preferred due to its high packing density. The membrane surface area of commercial hollow fibre membrane modules varies in the contactor volume range of 1500-3000 m /m (Kumar et al, 2002), whereas in conventional contactors (bubble column, packed and plate columns) it is in the range of 100-800 mVm. Table 2.4 clearly shows that MC offers a much larger contact area per unit volume than other conventional absorbers (Yan et al, 2007). 

A very important industrial example of a plate column reactor is the so-called absorber in nitric acid production, in which NO, dissolved in dilute acid, is transformed into nitric acid by means of air oxygen. 

Ethane is absorbed by a heavy oil in a five-plate column. The feed oil (which doesn t contain ethane) enters at the rate of 50 moles for every 100 moles of gas fed. The gas stream leaving the absorber has a mole fraction of 0.03. Liquid and gas rates in the column are constant. The equilibrium relation is 0.5 mole percent ethane in the gas per mole percent ethane in the liquid. Find ethane concentration entering as well as that leaving the first tray in the tower bottom. 

Sulfur dioxide is to be absorbed into water in a plate column. The feed gas (20 mole percent sulfur dioxide) is to be scrubbed to 2 mole percent sulfur dioxide. Water flow rate is 6000 kg/hr m. The inert air flow rate is 150 kg air/hr m. Tower temperature is 293 K. Find the number of theoretical plates. Equilibrium data are... 

Distillation Columns. Distillation is by far the most common separation technique in the chemical process industries. Tray and packed columns are employed as strippers, absorbers, and their combinations in a wide range of diverse appHcations. Although the components to be separated and distillation equipment may be different, the mathematical model of the material and energy balances and of the vapor—Hquid equiUbria are similar and equally appHcable to all distillation operations. Computation of multicomponent systems are extremely complex. Computers, right from their eadiest avadabihties, have been used for making plate-to-plate calculations. 

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