Processes Combining Distillation with Permeation

Hybrid processes are combirrations of distillation with alternative separation processes. These processes rrrake rrse of the advantages of distillation (i.e., fractionation irrto prrre srrbstances) as well as of alternative processes (i.e., breaking azeotropes). In prirrciple, all alternative separation techrriques can be combined with distillation (Schweitzer 1997). Especially well suited are decantation, absorptiorr, extractiorr, stripping, adsorptiort, and merrrbrane permeation. In most cases hybrid processes corrsist of two distillation colttrrrrts and one alternative separation unit. 

A modem process is the combination of distillation with membrane permeation, often called pervaporation. This technique is often appUed to dewatering of organic compounds in industry (Rautenbach and Albrecht 1989). 

The esterification by-product, water, is removed via a process column in a continuous steady-state mode of operation. The bottom product of the column, being mainly EG, flows back into the esterification reactor. The condensed top product consists mainly of water with small traces of EG. In cases where a reverse-osmosis unit is connected to the distillate flow line, the residual EG can be separated very efficiently from the water [124], The combination of a process column with reverse osmosis saves energy cost and capital investment. The total organic carbon (TOC) value of the permeate is sufficiently low to allow its discharge into a river or the sea without any environmental impact. 

Several authors have already developed methodologies for the simulation of hybrid distillation-pervaporation processes. Short-cut methods were developed by Moganti et al. [95] and Stephan et al. [96]. Due to simplifications such as the use of constant relative volatility, one-phase sidestreams, perfect mixing on feed and permeate sides of the membrane, and simple membrane transport models, the results obtained should only be considered qualitative in nature. Verhoef et al. [97] used a quantitative approach for simulation, based on simplified calculations in Aspen Plus/Excel VBA. Hommerich and Rautenbach [98] describe the design and optimization of combined pervaporation-distillation processes, incorporating a user-written routine for pervaporation into the Aspen Plus simulation software. This is an improvement over most approaches with respect to accuracy, although the membrane model itself is still quite... 

Pervaporation have been considered an interesting alternative process for the current industrial options for aroma recovery, distillation, partial condensation, solvent extraction, adsorption, or a combination thereof. It is considered a basic unit operation with significant potential for the solution of various environmental and energetic processes (moderate temperatures). This separation process is based on a selective transport through a dense membrane (polymeric or ceramic) associated with a recovery of the permeate from the vapour phase. A feed liquid mixture contacts one side of a membrane the permeate is removed as a vapour from the other side. Transport through... 

With the advent of process simulation packages, modeling of pervaporation and vapor permeation processes in a user added subroutine allows these unit processes to be included in overall separation schemes right from the conceptual stage. This enables many different combinations of pervaporation, for example, distillation to be studied and the optimum operating parameters for the preferred configuration to be selected very quickly. Such parameters include membrane feed temperature, which strongly influences the flux rate and, therefore. 

The temperature of the feed solution decreases but the temperature of the permeate increases. A substantial portion of the heat is transferred from the feed side to the permeate side and pan of this energy can be recovered. This is shown schematically in figure VI - 54 in which a membrane distillation unit is shown combined with a heat-exchanger. The high-temperature permeate stream flows along the heat exchanger thereby increasing the temperature of the inlet feed stream. However it is also possible to carry out the same process without heat recovery. 

The objective of this study is to investigate the process configuration illustrated in figure 2c. Therefore the dehydration of the ternary mixture acetone, isopropanol and water into pure components in one distillation column combined with a hydrophilic membrane unit located in the side stream of the column is analysed. The water-depleted retentate from the permeation zone is returned back to the column while the permeate is removed out of the process. In this configuration, the operation conditions for the membrane separation is more suitable because the side stream can be placed near the maximum concentration of the most permeating component which leads to an increased driving force and consequently to smaller membrane areas. 

A hybrid process that combines membrane separation and distillation for bioethanol and biobutanol production is being worked on by MTR [88, 89]. The membrane units use either vapor permeation or PV. The BioSep processes offer more than 50% energy savings and are cost competitive with respect to conventional distillation-molecular sieve technology. They are attractive when the ethanol concentration in the fermentation step is low, such as in cellulose-to-ethanol and algae-to-ethanol. In the case of biobutanol production, the membrane systems concentrate and dehydrate the acetone, butanol and ethanol mixture, saving up to 87% of the energy required to recover biobutanol by conventional separation techniques. 

---