Thermal Separation Processes

K.Sattler and H.J.Feiner, Thermal Separation Processes, VCH Publ. Inc., Weinheim, 1994. 

Besides fluid mechanics, thermal processes also include mass transfer processes (e.g. absorption or desorption of a gas in a liquid, extraction between two liquid phases, dissolution of solids in liquids) and/or heat transfer processes (energy uptake, cooling, heating, drying). In the case of thermal separation processes, such as distillation, rectification, extraction, and so on, mass transfer between the respective phases is subject to thermodynamic laws (phase equilibria) which are obviously not scale dependent. Therefore, one should not be surprised if there are no scale-up rules for the pure rectification process, unless the hydrodynamics of the mass transfer in plate and packed columns are under consideration. If a separation operation (e.g. drying of hygroscopic materials, electrophoresis, etc.) involves simultaneous mass and heat transfer, both of which are scale-dependent, the scale-up is particularly difficult because these two processes obey different laws. 

Separation processes are not only of great importance in refineries, but also in the chemical, petrochemical, gas processing, and pharmaceutical industries. Although the reactor can be regarded as the heart of a chemical plant, in most cases, 60-80% of the total cost is taken up by the separation step. This step involves one or more thermal separation processes such as distillation, extraction, absorption, crystallization, adsorption, membrane processes, c/c., which are used to obtain the products at the required purity. 

Thermal Separation Processes Using a Selective Solvent as Separating Agent... 

The development of this important thermal separation process can be dated back to ancient times. Until the end of the 18 century, however, the stmcture of the distillation apparatus had remained almost unchanged. It consisted of an evaporation unit, a distillation flask heated by an oven, and a condensation unit with an air- and later water-cooled condenser. In the early 19 century, progress in distillation techniques was spurred by the necessity to produce sugar in Europe this politically motivated development resulted in numerous patents for the production of alcohol. Depending on the different starting materials, a number of distillation units and the first rectification columns were developed in various European countries [10]. The 19 and 20 centuries saw a rapid development of distillation technology prompted by increasing applications in the petrochemical, chemical and pharmaceutical industries. 

Chromatography is part of the thermal separation processes used to separate homogeneous molecular dispersive mixtures. The separation itself can be divided into three steps (Sattler, 1995). 

Chromatography belongs to the thermal separation processes used to separate... 

Separation temology, a cornerstone of chemical technology, has a diversity that virtually no other area can match. Separation processes require about 43 % of energy consumption and 40-70% of the investment costs [Eissen 2002]. Thermal separation processes [Sattler 2001, Schonbucher 2002] such as rectification, mechanical processes such as filtration and size reduction, and chemical reactions (e.g., ion exchange) are used. Accordingly, the range of apparatus and auxiliary materials is huge, and so only a small selection can be presented here. 

Whereas about 20 years ago the design of a thermal separation process required numerous time- and cost-intensive pilot plant tests and laborious measurements of phase equilibria, modern thermodynamic models (state equations or models) allow, for the case of nonelectrolytic systems, reliable calculation of the phase-equilibrium behavior of multicomponent systems if the behavior of the two-component systems is known. Therefore, we will briefly summarize the most important relations for describing binary mixtures. 

In reality, most chemical reactions consist of numerous elementary reactions combined in reaction networks that are much more difficult to describe. An interesting elementary reaction is a 2-step sequential reaction which is relevant e.g. in modelling thermal separation processes ... 

Schwenk, W. and Raouzeos, G. (1995) Sublimation a non-fluid thermal separation process. Chemical Technology Europe, 2, (2), 14-17. 

Multiphase systems are often found in machinery and apparatuses of the processing industry because most thermal separation processes are based on the transfer of one or more components from one phase to another. 

Crystallization is the transformation of one or more substances from the amorphous solid, liquid, or gaseous phase to the crystalline phase. Above all, crystalh-zation is of great importance as a thermal separation process for the concentration or purification of substances from solutions, melts, or the vapor phase. 

Brusis, D. Synthesis and Optimisation of Thermal Separation Processes with MINLP Methods, Ph.D. Thesis TU Miinchen 2003... 

During the chemical conversion of raw materials, homogeneous and heterogeneous mixtures (Figs. 1-2 and 1-3) are generated. Both reactants and products may be found in these mixtures, according to the yield and conversion of the chemical reaction. By means of thermal separation processes these mixtures must be treated to obtain the desired products to a demanded purity and to enable the raw materials to be recycled. 

Thermal separation processes are mass transfer operations, driven by molecular forces. Mass, and often heat, is exchanged between at least two phases of different composition. The phases are the mixture phase(s) and a selective auxiliary phase. The auxiliary phase is generated by either adding heat and/or by means of an auxiliary substance. The required driving forces, concentration, and temperature gradients, are formed due to the auxiliary phase. 

In Fig. 1-4 thermal separation processes are listed and are denoted by the phases contributing to mass transfer in Table 1-2. 

All thermal separation processes follow this order of events. The basic principles of thermal separation processes Me now formulated and will be discussed in detail. 

Abbreviations s solid, 1 liquid, g gas to characterize the state of the components of the mixture to be separated, (A) thermal separation process with auxilliary component. 

Table 1-2. Characteristics of thermal separation processes by the phases in which mass and heat transfer occurs.

Knowledge of the distribution coefficient or the equilibrium relationship jc, i, = f(A , i) as a function of pressure and temperature is essential for the design of thermal separation processes. The relationship is found in three steps ... 

The phase rule is important for thermal separation processes as, if certain process parameters are choosen, it establishes which state variables are cogently fixed at an arbitrarily adjusted phase equilibrium (Table 1-5). 

Table 1-5. Selected thermal separation processes. Examples of the Gibb s phase rule. 

Thermal separation process Phases Number Type Components Number Type Degrees of freedom Process parameters Consequences for the remaining variables describing the system state, explanations ... 

In thermal separation processes sublimation is of practical interest in freeze or sublimation drying. Heat transfer into the system causes moisture to sublime directly from the frozen solid into the vapor phase (see Chapter 5.10). Desublimation is important for the separation of gas mixtures if components of the mixture change directly from the gas into the solid state when heat is removed, these components can be sepa-... 

The basic principles of mass transfer are discussed in detail in [1.95-1.97]. Thermal separation processes are actually mass transfer processes matter is transported between phases and across phase interfaces. Mass transfer is caused by differences in concentration within a phase and by disturbances of the phase equilibrium. The time taken to return to the phase equilibrium depends mainly on mass transfer, but also on heat transfer (heat is transported not only by convection and radiation at higher temperature, but also by mass). For the design of thermal separation processes, along with a knowledge of phase equilibria, it is also important to have a detailed understanding of how equilibrium is reached and the time required, taking into account restrictions in the mass transfer rate. 

Pick s first law describes equimolar diffusion, in which all components of the system may diffuse independent from each other. During thermal separation processes, matter is transported through phase boundaries. If a phase boundary is selectively permeable to one component, only one-directional diffusion is possible (an especially important case for absorption, adsorption, and drying). For one-directional diffusion, Stefan s law gives... 

General Procedure to Design Equipment for the Thermal Separation 95 Table 1-19. Planning concepts for thermal separation process units. ... 

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