Separator for Heterogeneous Mixtures

Having made an initial specification for the reactor, attention is turned to separation of the reactor effluent. In some circumstances, it might be necessary to carry out separation before the reactor to purify the feed. Whether before or after the reactor, the overall separation task might need to be broken down into a number of intermediate separation tasks. Consider now the choice of separator for the separation tasks. Later in Chapters 11 to 14, consideration will be given as to how separation tasks should be connected together and connected to the reactor. As with reactors, emphasis will be placed on the choice of separator, together with its preliminary specifications, rather than its detailed design. 

When choosing between different types of reactors, both continuous and batch reactors were considered from the point of view of the performance of the reactor (continuous plug-flow and ideal batch being equivalent in terms of residence time). If a batch reactor is chosen, it will often lead to a choice of separator for the reactor effluent that also operates in batch mode, although this is not always the case as intermediate storage can be used to overcome the variations with time. Batch separations will be dealt with in Chapter 14. 

If a heterogeneous or multiphase mixture needs to be separated, then separation can be done physically by exploiting 

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The four principal methods for the separation of heterogeneous mixtures are... 

Microfiltration. Microfiltration is a pressure-driven membrane filtration process and has already been discussed in Chapter 8 for the separation of heterogeneous mixtures. Microfiltration retains particles down to a size of around 0.05 xm. Salts and large molecules pass through the membrane but particles of the size of bacteria and fat globules are rejected. A pressure difference of 0.5 to 4 bar is used across the membrane. Typical applications include ... 

Capillary electrophoresis has become extremely important in several application areas (a) enantiomeric separations can be achieved by the simple addition of a chiral additive to the run buffer (b) rapid and distinctive profiles for different isoform groups can be obtained by reversed-charge CE separations of heterogeneous mixtures and (c) inorganic and organic ion determination has improved significantly with new detection systems. 

Which technique—distillation, crystallization, or filtration— is most useful for separating a heterogeneous mixture composed of a solid and a liquid ... 

Choice of separator. For a heterogeneous mixture, separation usually can be achieved by phase separation. Such phase separation normally should be carried out before any homogeneous separation. Phase separation tends to be easier and usually should be done first. 

Benzoylpropionitrile. To a mixture of 21 4 g. of p dimethylamino propiophenone hydrochloride, 13 0 g. of potassium cyanide in a 500 ml. flask, add 260 ml. of boiling water heat the heterogeneous mixture under reflux for 30 minutes. Part of the dimethylamine, which is eliminated in the reaction, distils collect this in dilute hydrochloric acid. Cool the reaction mixture in ice the oil sohdifies and crystals form from the aqueous layer. Collect the solid (crude p benzoylpropiouitrile, 10-5 g.) by suction filtration and recrystallise it from benzene - light petroleum (b.p. 40-60°) it separates as almost colourless blades, m.p. 76°. 

In practice, the taffy process is generally employed for only medium molecular-weight resins (1) (n = 1-4). The polymerization reaction results in a highly viscous product (emulsion of water and resin) and the condensation reaction becomes dependent on agitation. At the completion of the reaction, the heterogeneous mixture consists of an alkaline brine solution and a water—resin emulsion and recovery of the product is accompHshed by separation of phases, washing of the taffy resin with water, and removal of water under vacuum. 

Metals are insoluble in common liquid solvents but can dissolve in each other (like dissolves like). A mixture of substances with metallic properties is called an alloy. Some alloys are true solutions, but microscopic views show that others are heterogeneous mixtures. Brass, for instance, is a homogeneous solution of copper (20 to 97%) and zinc (80 to 3%), but common plumber s solder is a heterogeneous alloy of lead (67%) and tin (33%). When solder is examined under a microscope, separate regions of solid lead and solid tin can be seen. When brass is examined, no such regions can be detected. 

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