Multicomponent strippers

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. 

Occasionally separating multicomponent solutions requires designing a sequence of fractionators. Henley and Seader [31] discuss some aspects of this problem. Once the sequence has been established, then estimate the size of each fractionator. Table 6.27 lists the equations for a short cut method for calculating the height and diameter of fractionators and Table 6.28 outlines the calculation procedure. Like rotary drum filtration, absorbers, and strippers, discussed earlier, the final design may require testing to support the calculations. 

Several of the commercial simulation programs offer preconfigured complex column rigorous models for petroleum fractionation. These models include charge heaters, several side strippers, and one or two pump-around loops. These fractionation column models can be used to model refinery distillation operations such as crude oil distillation, vacuum distillation of atmospheric residue oil, fluidized catalytic cracking (FCC) process main columns, and hydrocracker or coker main columns. Aspen Plus also has a shortcut fractionation model, SCFrac, which can be used to configure fractionation columns in the same way that shortcut distillation models are used to initialize multicomponent rigorous distillation models. 

Earlier papers on the continuous membrane column (28,29) have discussed the separation of CO2-N2, CO2-O2 and O2-N2 (air) mixtures in stripper, enricher and total column units composed of 35 silicone rubber capillaries. A characterization of the membrane column using a membrane unit concept (analogous to transfer unit concept — HTU, NTU) has also been presented. The purpose of this paper is to present some new data and discussions on the extended study of continuous membrane column. Specifically, the topics of multicomponent separations, Inherent simulation difficulties, composition minima in the enriching section, variation of experimental parameters, and local HMU variation along the column will be covered. 

One of the next steps in developing the continuous membrane column will be to obtain extensive data on multicomponent systems. Some preliminary experiments with a C02 CH -N2 mixture using a stripper have already been conducted. The results of two such experiments are presented in Figures 3 and 4. The agreement between experiment and model is excellent. 

All procedures for executing the numerical simulation of a multicomponent separation are similar to those described for a binary system, except for evaluating the Initial permeate composition at the residue end of the stripper. Again, the Initial permeate composition will be that of the mixture which permeates through the endmost Increment of the membrane. The appropriate relations are ... 

The existence of a methane peak is not considered a phenomenon that will always occur with intermediately permeable gases in multicomponent mixtures. Rather, the peak is thought to be the result of a combination of factors. These factors include composition of the feed mixture, pure-gas permeabilities, and the internal reflux ratio. For instance. Figure 3 indicates that the intermediate-gas composition profile will steadily decrease in a stripper 1.0 m long, but otherwise identical to the column used in this study, fed with a 63.6% N2 - 32.3% CH - 4.1% CO2 mixture under similar total reflux conditions. The presence of an intermediate peak, however, is reminiscent of multicomponent distillation profiles and raises the possibility of withdrawing a side stream enriched with an intermediate gas. 

The BP and SR methods for vapor-liquid contacting converge only with difficulty or not at all for separations involving very nonideal liquid mixtures (e.g., in extractive distillation) or for cases where the separator is like an absorber or stripper in one section and a fractionator in another section (e.g., a reboiled absorber). Furthermore, BP and SR methods are generally restricted to the very limited specifications stated above. More general procedures capable of solving ail types of multicomponent, multistage separation problems are based on the solution of all the MESH equations, or combinations thereof, by simultaneous correction (SC) techniques. 

Due to the tremendous costs associated to distillative separations, many alternate schemes to the simple column shown above have been proposed over the past several years both to improve on some of its inherent costs. Traditionally, when purifying a multicomponent mixture, an entire series of distillation columns are used in series, and the way in which these columns are sequenced may make a tremendous difference in the eventual process costs. However, due to the large energy requirements of even the most optimal sequence, more complex column arrangements have been proposed and subsequently utilized. These arrangements include thermally coupled columns such as side rectifiers and strippers, the fully thermally coupled columns (often referred to as the Petlyuk and Kaibel columns). 

Generally, techniques for designing absorbers and strippers for multicomponent systems have been aimed at the recovery of light hydrocarbons from natural or refinery gas streams usir trqr towers. Numerous shortcut calculation methods have been developed to obviate the need for tedious tray-lqr-ttay calculations however, the importance of these techniques has declined sonrewhat because of (1) the advent of small powerful computers and (2) the use of low-temperature separation instead of oil absorption to recover light hydrocaitwns. As a result, the cahwlational techniques for multicomponent hydrocarbon absorbers are treated rather briefly in this section. 

Multicomponent Absorbers and Strippers. The performance of absorbers can be most eadly understood by means of a mathematical study. In an equilibrium plate the relation between the composition of the vapor leaving the plate and the liquid overflowing from the plate may be expressed as follows ... 

---