Distillation and membrane separations

Since fermentation takes place in a dilute aqueous solution, the reaction continues until the alcohol concentration approaches about 14%. At higher concentrations, the process becomes self-inhibitory. By-products from starch fermentation to ethanol can include higher-molecular-weight alcohols, glycerine, and ethers. Usually no more than 10% starch is converted to these compounds. Atmospheric distillation, vacuum distillation, and membrane separation techniques can be used to recover ethanol from the final fermented product. The distillate bottoms, called stillage, are recovered as a by-product for animal feed. A biomass fermentation flow diagram is provided in FIGURE 12-2. 

Bl. There are a number of ways distillation and membrane separators can be combined as hybrid systems. Brainstorm as many methods as you can. 

The combination of distillation and membrane separation has been explored in the literature for many years. One of the applications is to separate azeotropic mixtures. A distillation column is used to make a preliminary separation with one of the product streams being one of the pure components. The other product stream has a composition fairly close to the azeotropic composition. This stream is fed to a membrane separation unit that achieves the separation not on the basis of VLE, but based on differences in diffusion rates through a solid membrane. The fluid passing through the membrane is called the permeate. The fluid that does not pass through the membrane is called the retentate. Composition differences are the driving forces for mass transfer. 

The book may be used for a methodical study of the subject or as a reference for solving day-to-day problems. It follows a logical flow of ideas within each chapter and from one chapter to the next yet each chapter is quite self-contained for quick reference. The discussion starts with fundamental principles, prediction of thermodynamic properties, the equilibrium stage, and moves on to the different types of multistage and complex multistage and multicolumn processes, batch distillation, and membrane separation operations. Although computer simulation is a central theme of this book, no previous experience in the use of simulation software is required. 

Several techniques can be used for H2 separation from mixtures with other gases, as shown in Table 12.2. Nevertheless, pressure swing adsorption (PSA), cryogenic distillation and membrane separation techniques are the main hydrogen separation processes (Adhikari and Fernando, 2006). 

Process Concepts. Hybrid systems involving gas-phase adsorption coupled with catalytic processes and with other separations processes (especially distillation and membrane systems) will be developed to take advantage of the unique features of each. The roles of adsorption systems will be to efficiently achieve very high degrees of purification to lower fouUng contaminant concentrations to very low levels in front of membrane and other separations processes or to provide unique separations of azeotropes, close-boiling isomers, and temperature-sensitive or reactive compounds. 

Refinery product separation falls into a number of common classes namely Main fractionators gas plants classical distillation, extraction (liquid-liquid), precipitation (solvent deasphalting), solid facilitated (Parex(TM), PSA), and Membrane (PRSIM(TM)). This list has been ordered from most common to least common. Main fractionators are required in every refinery. Nearly every refinery has some type of gas plant. Most refineries have classical distillation columns. Liquid-liquid extraction is in a few places. Precipitation, solid facilitated and membrane separations are used in specific applications. 

Finally, it is possible to obtain a pure hydrogen stream through several techniques, such as PSA, cryogenic distillation or membrane separation. PSA and cryogenic distillation processes are commercially available separation techniques [14]. 

The methodology is illustrated with a further application to the design of activated sludge process systems in Section 6 and has seen further applications in multiphase reaction-separation, reactive distillation, and membrane network synthesis (79). 

List the four ways in which separating agents generate selectivity. Which of these applies to (i) distillation, (ii) membrane separations, and (iii) adsorption ... 

Stichlmair, J., J.R. Fair, J. L. Bravo, 1989, Separation of azeotropic mixture via enhanced distillation, Chem. Eng. Progress, 85(1), 63-69 Stichlmair, J., J. R. Herguijuela, 1992, Separation regions and processes of zeotropic and azeotropic ternary distillation, AIChEJ, 38, p. 1523-1535 Stichlmair, J. G., J. R. Fair, 1999, Distillation, Principles and Practice, Willey-VCH Strathmann, H., 1990, Membrane and Membrane Separation Processes, Ullmann s Encyclopaedia of Industrial Chemistry, vol. A16 Taylor, R., Krishna, R., 2000, Modelling reactive distillation, Chem. Eng. Sci., 52, 993-1005... 

In seeking the most efficient process possible, a designer will wish to explore a wide range of feasible designs. To make this possible, an efficient method for the synthesis and assessment of any hybrid separation process has been developed. Since both processes have been analyzed using similar mathematical backgrounds, it is possible to use residue curve maps and column profile maps for both distillation and membranes to design hybrid systems of the two. 

Today, RD is discussed as one part of the broader area of reactive separation, which comprises any combination of chemical reaction with separation such as distillation, stripping, absorption, extraction, adsorption, crystallization, and membrane separation. In the next decade, unifying approaches to reactive separators should be developed allowing the rigorous selection of the most suitable type of separation to be integrated into a chemical reactor. 

Caballero, J.A., Grossmann, I.E., Keyvani, M. and Lenz, E.S. (2009) Design of hybrid distillation-vapor membrane separation systems. Industrial Engineering Chemistry Research, 48 (20), 9151-9162. 

In the vinyl-chloride process, because of the significant differences in the volatilities of the three principal chemical species, distillation, absorption, and stripping are prime candidates for the separators, especially at the high production rates specified. For other processes, liquid-liquid extraction, enhanced distillation, adsorption, and membrane separators might become more attractive, in which case the design team would need to assemble data that describe the effect of solvents on species phase equilibrium, species adsorption isotherms, and the permeabilities of the species through various membranes. 

Liquid samples (filtration, dilution, pH adjustment, standard addition, derivatisation, liquid-liquid extraction, solid-phase extraction, distillation, microdiffusion, and membrane separation)... 

Gas separation, potential in membrane distillation and membrane contactors... 

Buchaly, C., Kreis, P. and Gorak, A. (2007). Hybrid separation processes combination of reactive distillation with membrane separation. Chemical Engineering and Processing, Vol. 26, pp. 790-799. 

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