Absorption liquid flowrate

The most common alternative to distillation for the separation of low molar mass materials is absorption. Liquid flowrate, temperature and pressure are important variables to be set, but no attempt should be made to carry out any optimization in the early stages of a design. 

The most common alternative to distillation for the separation of low molar mass materials is absorption. In absorption, a gas mixture is contacted with a liquid solvent that 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 one of the materials already in the process, this should be done in preference to introducing an extraneous material. Liquid flowrate, temperature and pressure are important variables to be set. 

Liquid flowrate. The liquid flowrate is determined by the absorption (L/Kfl/) factor for the key separation. The... 

In the laminar-film contactor shown in Fig. 4.116, the supporting surface has the form of a sphere held in place by a wire<3>. This arrangement has the advantage that, in the regions where the liquid runs on to the sphere, and where it leaves the sphere, the surface areas exposed to the gas are relatively small so that, even if the hydrodynamics of the liquid flow in these regions is not ideal, the effect on the rate of absorption of the gas will be small. As in the case of the cylindrical tube, the contact time for each element of liquid as it flows around the sphere can be calculated from the liquid flowrate, although the mathematics of the analytical treatment is somewhat more complicated than in the case of the tube. 

Sieve trays are used throughout the absorption column, however two distinct hydraulic designs are required. The first sieve plate design is required for trays below the weak-acid feed point. Above the weak-acid feed point, the downcoming liquid flowrate is diminished. The vapour flowrate essentially remains constant throughout the column. Different vapour to liquid ratios above and below the weak-add feed point require a second hydraulic design to be considered. 

To estimate the number of theoretical stages required, NTS, the Kremser-Sou-ders-Brown equations may be used. (The Kremser equation assumes that the operating and equilibrium lines are straight, the systems are nonreacting and that the concentrations are dilute (that is < 10%). These are reasonable assumptions for many absorptions.) The key parameter is the ratio of the product of the molar equilibrium constant m (as in y = mx ) with the molar gas flowrate divided by the molar liquid flowrate, = molar stripping factor = fnpG)/Pi = S. 

Wet scrubbers countercurrent wet packing dust diameter 0.2 to 3 pm feed concentration < 0.1 g/m temperature < 100 °C gas phase Ap = 1.25 to 6 kPa. Turbulent bed contactor [see also Section 4.8 absorber] temperature < 100 °C OK for heavy sticky particles allows high gas and liquid flowrates with high mass transfer efficiencies for gas absorption 1 to 2 pm 2.5 to 20 kW s/m. ... 

The HTU concept can also be employed for analysis of the contributions of the individual film resistances although, in general, the individual absorption coefficients are preferred for basic studies. Values of Nqg are particularly useful for expressing the performance of equipment in which the volume is not of fundamental importance. In spray chambers, for example (see Chapter 6), the effectiveness of the equipment is more a function of liquid flowrate and spray nozzle pressure than of tower volume. The use of volume-based absorption coefficients for such units is quite meaningless. 

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 operation are used. Countercurrent operation gives the highest reaction rates. Cocurrent operation is preferred if a short liquid residence time is required or if the gas flowrate is so high that countercurrent operation is difficult. 

A column 0.6 m diameter and 4 m high is, packed with 25 mm ceramic Raschig rings and used in a gas absorption process carried out at 101.3 kN/m2 and 293 K. If the liquid and gas properties approximate to those of water and air respectively and their flowrates are 2.5 and 0.6 kg/m2s, what is the pressure drop across the column In making calculations, Carman s method should be used. By how much may the liquid flow rate be increased before the column floods ... 

An acetone-air mixture containing 0.015 mole fraction of acetone has the mole fraction reduced to 1 per cent of this value by countercurrent absorption with water in a packed tower. The gas flowrate G is 1 kg/m2s of air and the water flowrate entering is 1.6 kg/m2s. For this system, Henry s law holds and ye = 1.75x, where ye is the mole fraction of acetone in the vapour in equilibrium with a mole fraction v in the liquid. How many overall transfer units are required ... 

A. Assuming (a) temperature and total pressure throughout the column to be constant, (b) no change in molar flowrates due to gas absorption, (c) plate efficiencies to be 100 per cent, (d) the equilibrium relation to be given by yn = mx + b, (e) the holdup of liquid on each plate to be constant and equal to H, and (f) the holdup of gas between plates to be negligible, show that the variations of the liquid compositions on each plate are given by ... 

In the design of an absorption tower, the most important single factor is the value of the transfer coefficient or the height of the transfer unit. Whilst the total flowrates of the gas and liquid streams are fixed by the process, it is necessary to determine the most suitable flow per unit area through the column. The gas flow is limited by the fact that the flooding rate must not be exceeded and there will be a serious drop in performance if the liquid rate is very low. It is convenient to examine the effects of flowrates of the gas and liquid on the transfer coefficients, and also to investigate the influence of variables such as temperature, pressure, and diffusivity. 

The dynamic losses can only be estimated by first sizing the pipe diameter of the line between the absorption column and the bleaching column. This is performed using recommended liquid velocities (Ref. P1, p. 163) in conjunction with the known volumetric flowrate. The area calculated can be translated into a standard pipe diameter. The dynamic losses are then estimated by two methods. The first employs Genereaux s formula (Ref. P1, p. 160) ... 

The absorption column is sized according to two key parameters, these are to design for optimum mass transfer and optimum unit cost. A column internal diameter can be estimated according to the liquid and gas flowrates by utilizing graphs and nomographs such as those contained in Ref. A3. These recommendations have been refined using a computer-based mathematical model. The model predicts the required number of trays for a specified column internal diameter. These results enable a compromise to be achieved between tower cost and tower performance. 

Vapor-liquid mass-transfer operations, such as absorption, stripping and distillation, are carried out in packed and plate columns. The key difference is that counterflowing vapor and liquid are contacted continuously with packings, and discretely with plates. The equilibrium and operating lines of packed and plate columns are identical under the same operating conditions—feed and product flowrates and compositions, temperature and pressure. Models for the design and analysis of packed columns are based on their close analogy to plate devices. 

Problem treat 500 SCFM of air containing 14 mol% acetone to remove 95% of the acetone by absorption in liquid water in a packed bed operating at 80°F and 1 atm, with 1-inch raschig rings. The feed water contains 0.02% acetone and the flowrate is... 

Analogous to the minimum reflux ratio in distillation, there exists a minimum L/G ratio in absorption. Figure 6.3 compares the operating line to the equilibrium line for absorption. Because the slope of the operating line is L/G, reducing the liquid mass-separating agent flowrate decreases the slope of the line. Compare lines aA and ab for a separation in which the gas enters with concentration of solute which must be removed to Ya and the liquid... 

For the absorption problem it is convenient to use the top of the column for specifying Xa, since one usually knows the composition of the clean absorbent (usually a pure liquid) and also the desired impurity composition that the exiting vapor stream at the top of the column has to achieve. Thus, through these compositions and flowrates all the parameters for integrating the DPE have now been specified... 

The values of interfacial area and of overall mass transfer coefficient increase with decreasing distance S between the spray nozzle and gas inlet, whatever the nozzle type, column dimensions and flowrates. Indeed the spray provides a large interfacial area in the vicinity of the nozzle, where there is intensive circulation. Then a decreases quickly away from the nozzle, as a result of both coalescence of droplets and collection of liquid on the column walls. kQa and a are approximately proportional to for absorption and desorption processes, which shows that kQ is practically independent of the column height. Moreover Mehta and Sharma indicate that a is unaffected by ionic strength and viscosity but may decrease about 20 % when solids are generated by the reaction of gas with the liquid. Thus the following correlations pay be used for design (112)... 

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