Heat-driven separation processes

One of the most energy-inefficient of widely used industrial processes is distillation, or heat-driven separation processes generally. The traditional distillation column, familiar to students who have had chemistry laboratory courses, has a source of heat at the bottom and a cooling fluid that runs the length of a vertical column, so that there is a temperature gradient, cooling as the... 

We can summarize the HDA example as follows. The process converts 132 kmobh of toluene. If it were possible to extract all the work contained in the reactants we could generate 1.58 MWT of electric power at standard conditions. When forced to generate power from steam we would obtain much less than 0.65 MW of electric power. When none of the reaction energy is converted to work we need to dissipate about 1.55 MW of heat to utilities. On the separation side we need 0.52 MW of mechanical power at standard conditions. If ideal heat-driven separation devices were... 

MD is one of the emerging nonisothermal membrane separation processes, known for abont 50 years but still reqniring development for its industrial implementation. MD refers to the thermally driven transport of vapor through porous hydrophobic membranes, the driving force being the vapor pressure difference between the two sides of the membrane pores. Simnltaneous heat and mass transfer occurs in this process, and different MD contignrations include direct contact MD, sweeping gas MD, vacuum MD, and air gap MD. 

They considered contact distillation, i.e. concentration-driven diffusion process, to possess a maximum at adiabatic conditions (curve 1) and thermal distillation to increase rapidly with both heat supply and heat removal (curve 2) as the result of fractional vaporization or condensation respectively. The total effect (curve 3) was regarded as a sum of the two separate effects. 

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 comparison to isothermal membrane processes, little attention has been paid to date to polarisation phenomena in non-isothermal processes. In non-isothermal processes such as membrane distillation and thermo-osmosis, transport through the membrane Occurs when a temperature difference is applied across the membrane. Temperature polarisation will occur in both membrane processes although both differ considerably in membrane structure, separation principle and practical-application. In a similar manner to concentration polarisation in pressure-driven membrane processes, coupled heat and mass transfer contribute towards temperature polarisation. 

Both expressions (10.1.21) and (10.1.22) represent what is called the net work consumption for the separation process driven by heat... 

Batch distillation (see Fig. 3) typically is used for small amounts of solvent wastes that are concentrated and consist of very volatile components that are easily separated from the nonvolatile fraction. Batch distillation is amenable to small quantities of spent solvents which allows these wastes to be recovered onsite. With batch distillation, the waste is placed in the unit and volatile components are vaporized by applying heat through a steam jacket or boiler. The vapor stream is collected overhead, cooled, and condensed. As the waste s more volatile, high vapor pressure components are driven off, the boiling point temperature of the remaining material increases. Less volatile components begin to vaporize and once their concentration in the overhead vapors becomes excessive, the batch process is terrninated. Alternatively, the process can be terrninated when the boiling point temperature reaches a certain level. The residual materials that are not vaporized are called still bottoms. 

In the noble metals and in many minerals the elements were believed to be so well combined that heat could not separate them. Other minerals, as sulphur, orpiment, asphalt, etc., when heated in the air are partly broken down, the aerial element, not being so firmly united to the earth, being driven off as vapor and mingling with the particles of the atmosphere. This process was interpreted by the Greek alchemists and their Arabian successors as the separation of the spirit from the body, and such substances as were volatilized or burned with formation of gaseous products—as sulphur, arsenic (sulphides), sal ammoniac, quicksilver—were called spirits, while the metals and minerals which, when heated in the air did not volatilize nor disappear in gaseous products, were called bodies (corpora). 

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