The example problems in the book involve limited chemical pairs so you can compare process variables such as number of stages, reflux ratio, optimum feed stage, etc. This comes at the expense of diversity in the systems studied. My intention is to include other industrially significant systems as practice problems. This page very briefly explains why these chemicals are paired.
Benzene/Toluene
Benzene and toluene are often found together in refinery streams along with other compounds like ethyl benzene and xylene isomers. These are often named BTX or BTEX streams.
Ethanol/Water
Ethanol can be produced by fermentation or by hydration of ethylene. The concentration of ethanol in fermentation broth is limited by its toxicity to the very organisms that produce it as a metabolic byproduct. Higher concentrations require distillation.
“Proof” is an old term used to classify alcoholic beverages. In the 16th century, if it could be ignited, it was considered 100 proof. This happens around 57% alcohol by volume (ABV). In modern times, we base it on ABV – 50% is 100 proof, 100% is 200 proof.
Water/2-Pentanol
A synthetic route to aliphatic alcohols is direct hydration of an alkene using a catalyst. In this case, pentene is reacted with water to form 2-pentanol (Markovnikov addition). The unreacted water is separated from the alcohol and recycled.
Alkanes
Natural gas and petroleum fractions contain mixtures of alkanes. To completely separate such a mixture, you would need another distillation process for each additional chemical (see Chapter 10). It is not unusual for a process to feature a whole train of distillation columns to produce pure chemicals.
Styrene/Ethylbenzene
Styrene is made by the catalytic dehydration of ethylbenzene. The reactor’s product is a mixture of styrene and unreacted ethylbenzene. They are separated by vacuum distillation, which lowers the temperature and increases the relative volatility.
Methanol/Water
Methanol used to be called “wood alcohol” because prior to the early 20th century it was made by distilling wood. Now it is prepared from synthesis gas, which in turn is made by autothermal reforming of natural gas. Fun fact, because of their vast natural gas reserves, Trinidad and Tobago is the largest methanol exporter in the world! The USA actually produces more but consumes it internally.
Benzene/Chlorobenzene
Chlorobenzene is made by flowing chlorine gas through benzene in the presence of a catalyst. The reaction products will actually contain some di- and tri- substituted species because selectivity is hard to control.
Acetone/Isopropanol
Acetone can be made by the dehydrogenation or oxidation of isopropanol in a high temperature, catalytic process.
