Absorbers and Distillation Columns

Recall that when we designed distillation columns with the graphical McCabe-Thiele method, we specified the relative flow rates of liquid and vapor to obtain the operating lines. What diameter of column is needed to accommodate the absolute flow rates If the column is too narrow le., the sieve plate area is too small), the liquid will pass over the sieve plate too quickly and not equilibrate with the vapor. If the colunm is too wide, the liquid will not cover the tray completely and the vapor will blow through without equilibrating. 

Another design tool allows one to determine the consequences of operating a column near flooding conditions. An important parameter is the fractional entrainment -the amount of liquid entrained in the gas phase and carried to the plate above. Ideally, no liquid should be carried to the plate above. If one defines a dimensionless group for flooding to be 

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The treatment presented below is restricted to systems of distillation columns. Systems containing both absorbers, strippers, or reboiled absorbers and distillation columns may also be solved by use of the capital 0 method of convergence as demonstrated in Chap. 4. 

The 2N Newton-Raphson method may be applied to any type of distillation column or to any system of interconnected columns. Absorbers, strippers, reboiled absorbers, and distillation columns are treated in Sec. 4-1. Selected numerical methods for solving the 2N Newton-Raphson equations are presented in Sec. 4-2. In Sec. 4-3, two methods for solving problems involving systems of columns interconnected by recycle streams are presented. 

The enthalpy balance functions are formulated for any stage j in a manner analogous to that for absorbers and distillation columns. The solution values of the variables (the temperatures, vapor rates, and product flow rates) are presented in Tables 4-14 and 4-15. 

Reboiled absorber and distillation column 12 24 See Table 4-11 2N Newton-Raphson method. Procedure 3 for reboiled absorber, 6 method for distillation column and 0 method for the system. Procedure 1 17.8 s, 15 trials, Procedure 2 14.62 s, 18 trials, Procedure 3 14.46 s, 18 trials. FORTRAN H... 

Many separations which would be difficult to achieve by conventional distillation processes may be effected by a distillation process in which a solvent is introduced which reacts chemically with one or more of the components to be separated. Three methods are presented for solving problems of this type. In Sec. 8-1, the 0 method of convergence is applied to conventional and complex distillation columns. In Sec. 8-2, the 2N Newton-Raphson method is applied to absorbers and distillation columns in which one or more chemical reactions occur per stage. The first two methods are recommended for mixtures which do not deviate too widely from ideal solutions. For mixtures which form highly nonideal solutions and one or more chemical reactions occur per stage, a formulation of the Almost Band Algorithm such as the one presented in Sec. 8-3 is recommended. 

The equations presented f r the formulation of the N(r + 2) Newton-Raphson method for absorbers may e applied to distillation columns in a manner similar to that demonstrated in (Thap. 4 for the 2N Newton-Raphson method for absorbers and distillation columns. 

The simulation of Ammonia process required time and effort to converge with acceptable results. The main parts of simulation that required many trials were the Acid Gas Sweetening and the CO2 Removal units because of the recycle stream material balance and the required purity of gas streams. Absorbers and distillation columns are usually sensitive to composition and flow rates (L. Oi, 2007 I. Halim and R. Srinivasan,... 

In the previous chapter we found that an effective design for absorbers and distillation columns is a series of equilibrium stages, as shown in Figure 4.70. Liquid flows down the column and vapor permeates up the column. At each stage the liquid and vapor mix and components transfer from one phase to the other to approach equilibrium. For efficient performance, we want equilibrium to be approached rapidly at each stage. To improve the rate of mass transfer between the liquid and vapor phases, one... 

There are many more design tools for absorbers and distillation columns. These may be found in the text by King (1971) and in Perry s Chemical Engineers Handbook. 

This balance is no different from similar mass balances used in packed-gas absorbers and distillation columns (see Chapter 8) and takes the form... 

Data Sources m the Handbook Sources of data for the analysis or design of absorbers, strippers, and distillation columns are mani-... 

Mass Transfer Relationships for calculating rates of mass transfer between gas and liquid in packed absorbers, strippers, and distillation columns may be found in Sec. 5 and are summarized in Table, 5-28. The two-resistance approach is used, with rates expressed as transfer units ... 

Data Sources in the Handbook Sources of data for the analysis or design of absorbers, strippers, and distillation columns are manifold, and a detailed listing of them is outside the scope of the presentation in this section. Some key sources within the handbook are shown in Table 14-1. 

RadFrac - rigorous two phase and three phase, absorber, stripper, distillation columns using stages... 

Next, the 2N Newton-Raphson method is applied to reboiled absorbers, conventional distillation columns, and complex distillation columns, and then a procedure which makes use of the calculus of matrices for solving these equations is presented. 

For the case where one or more reactions occur on each stage of an absorber or distillation column and the vapor and liquid phases form highly nonideal mixtures, a formulation of the Almost Band Algorithm is recommended. In the present formulation for the case where one or more chemical reactions occur on each stage of an absorber, the following choice of N(2c + 1 + r) independent variables and N(2c 4-1 + r) independent functions are made. In particular, for the case of one chemical reaction per stage, the independent variables and functions are taken to be... 

I decided to write a textbook for a first course on mass-transfer operations with a level of presentation that was easy to follow by the reader, but with enough depth of coverage to guarantee that students using the book will, upon successful completion of the course, be able to specify preliminary designs of the most common mass-transfer equipment (such as absorbers, strippers, distillation columns, liquid extractors, etc.). I decided also to incorporate, from the very beginning of the book, the use of Mathcad, a computational tool that is, in my opinion, very helpful and friendly. The first edition of this book was the result of that effort. 

The variation in Hq can be determined from Eq. fl6-371 and Figure 16-5. which are valid for both absorbers and distillation. The term that varies the most in Eq. H 6-371 is the weight mass flux of liquid, Wl. Hq depends on Wq to the -0.5 to -0.6 power. In a single section of an absorber, a 20% change in liquid flow rate would be quite large. This will cause at most a 10% change in Hq. kya is independent of concentration, since the concentration effect was included in k y in Eq. f 16-421. Since the variation in Hq over the column section is relatively small, we will treat Hq as a constant. Then Eq. 116-501 becomes... 

Alternately, the cell may be designed similarly to a shell-and-tube heat exchanger, with flow inside the tubes and on the outside or shell side. The shell-side flow may be strictly parallel to the tubes or also across the tubes, or tube bundle, and directed by the use of baffles and baffle cuts. Such a layout is illustrated in Figure 6.3, with more information about the intricacies provided by Hoffman. There is an analogy with the treatment of absorbers, strippers, and distillation columns as a continuum, described in terms of the rate of mass transfer."... 

Fig. 3. Schematic of toluene diamine phosgenation process A, cold phosgenator B, hot phosgenator C, wash column D, solvent distillation E, preflasher F, evaporator G, TDI distillation H, phosgene removal I, HCl absorber and K, phosgene decomposition.

The HCl gas is absorbed in water to produce 30—40% HCl solution. If the HCl must meet a very low organic content specification, a charcoal bed is used ahead of the HCl absorber, or the aqueous HCl solution product is treated with charcoal. Alternatively, the reactor gas can be compressed and passed to a distillation column with anhydrous 100% Hquid HCl as the distillate the organic materials are the bottoms and are recirculated to the process. Any noncondensible gas present in the HCl feed stream is vented from the distillation system and scmbbed with water. 

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. 

Absorption recovers valuable light components such as propane/propylene and butane/ butylene as vapors from fractionating columns. These vapors are bubbled through an absorption fluid, such as kerosene or heavy naphtha, in a fractionating-like column to dissolve in the oil while gases, such as hydrogen, methane, ethane, and ethylene, pass through. Absorption is effectively performed at 100 to 150 psi with absorber heated and distilled. The gas fraction is condensed as liquefied petroleum gas (LPG). The liquid fraction is reused in the absorption tower. 

Select design pressure drop for operations. Suggested values of below 1.0 in. water/ft. Low-pressure, atmospheric, and pressure columns usually require 0.5 to 0.7 in. water/ft, with absorbers and strippers around 0.2-0.6 in. water/ft. For vacuum distillation low values of 0.05-0.6 in. water/ft are often necessary, usually depending on the required boiling point of the bottoms. 

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