Separation by Condensation

Let s start with an important separations problem How does one separate pollutants from an effluent to be discharged to the air Note that this is a poor statement of the real problem. The real problem is How does one reduce air pollution This broader definition invites creative approaches such as Don t create the pollutant in the first place. Chemical engineers examine processes and modify the chemistry to avoid pollutant by-products. Producing pollutants may also be avoided by changing solvents or catalysts. Such changes may increase the cost of purifying the product, but the overall cost - which includes pollution control - may be less. 

Assume that we don t have the option of modifying the chemical process. Let s further assume that our pollutant is a volatile organic compound. Volatile organics are often used as solvents in chemical processes, although much of the air pollution by volatile organics comes from oil-based paints and fugitive fumes at automotive service stations. Common pollutants are benzene, xylenes, methanol, and acetone. 

There are two ways to deal with pollutants in an effluent to be discharged into the air Capture the pollutants or convert the pollutants. The four chief processes for reducing pollution by volatile organics are reaction (such as incineration), condensation, adsorption, and absorption. In this chapter we will analyze condensation and absorption. For simplicity we will assume we have only one pollutant benzene in air. 

We use this device to observe the phase(s) present at various temperatures, pressures, and volumes. We arbitrarily decide to systematically decrease the volume while holding the temperature constant, and we record the pressure and the phase(s) present. Table 4.1 contains the results of one such experiment. 

For safety, we dare not exceed 100 atm in our device. At this limit, the benzene is still entirely liquid. The experiment is repeated at different temperatures. At lower temperatures, the first evidence of liquid is detected at higher volumes at 50°C, a minuscule liquid film forms at 8.134 m /mol and 0.357 atm. At even lower temperatures, solid benzene is detected in the liquid at high pressures. And at even lower temperatures, the first phase to condense is solid benzene. 

---

Refrigerated condensation. Separation by condensation relies on differences in volatility between the condensing components. Refrigeration or a combination of high pressure and refrigeration is needed. 

When the gas available for regeneration is in short supply, the regeneration steps ate often carried out in a closed loop. This recycle of the bed effluent back to the inlet has the advantage of concentrating the impurity and making it easier to separate by condensation or other recovery means. 

Oxyfuel-combustion. The fuel is converted to heat in a combustion process. This is achieved with pure oxygen as an oxidiser. Mostly C02 and water vapour are produced in the flue gases, and thus C02 can be easily separated by condensing the water vapour. 

Liquid ammonia is separated by condensation from the synthesis loop and is either subcooled and routed to storage, or conveyed at moderate temperature to subsequent consumers. 

The off-gas passes scrubber containing NaOH solution and filters with silver-impregnated zeolite or charcoal in which Ru and I are retained. Kr may be separated by condensation or adsorption on charcoal at liquid-nitrogen temperature. 

These gases are often of higher calorific value than the low BTU gas obtained with gas generators, but include higher hydrocarbons. These products must be separated by condensation, in order that the engine burn only the incondensable part. 

The reaction is rapid and exothermic (and may be explosive at too high a temperature), and the COFj and COBrF are readily separated by condensing out the COBrF into a trap cooled to -78 "C, and subsequently trapping the COFj into a trap cooled to -196 C. To remove the final traces of bromine from the COFj, it may be passed over antimony powder and recondensed [1193],... 

Heuristic Separation by condensation may be considered when the relative volatility between the key components is greater than 7, or boiling point difference bigger as 40 °C. 

The helium from which the water has been separated by condensation is withdrawn from the system through the line 35 and passed to the purifying system previously described in connection with FIG. 1. 

Phenol is vaporized with recycled and make-up hydrogen and hydrogenated by excess hydrogen at temperatures of 120 to 200 °C and 20 bar on silica or aluminum oxide catalysts, which are modified with nickel. The cyclohexanol is separated by condensation. The yield of cyclohexanol is almost quantitative. 

Oxy-fuel combustion, whereby the fuel is burned in oxygen + recycled CO2 (instead of air) in order to produce an exhaust consisting primarily of CO2 and steam, from which CO2 is readily separated by condensing out the water. 

Hydrogen separation membranes can be used to produce H2-rich streams for either H2 production or power generation with pre-combustion CO2 capture. In the latter case, high purity H2 is not required since it is used to fuel a combustion turbine in fact, dilution with membrane sweep gases such as steam and N2 actually reduces NO, formation by reducing the stoichiometric combustion temperature. In H2 production, high purity H2 (>99.9%) is generally required and only steam, which is easily separated by condensation, can be used as a sweep gas. 

The upper sections of the absorber are used for the recovery of the CS2. When they are saturated, the activated carbon is regenerated with steam. The steam-CS2 mixture is separated by condensation and the CS2 is directly recycled back to the tank farm. The remaining CS2 in the water is stripped off. 

Separation by membranes is selective, which can limit its efficiency since VOCs are made up of a mixture of gases. Improving membranes involves developing materials that can separate a range of organic compounds [173]. VOCs can also be separated by condensation techniques where the VOCs are cooled to low temperatures. The various remediation techniques for VOCs are summarised in Table 6. 

The hydrogen must be separated from the methane, oxygen, water, and carbon dioxide. As before we first consider separation by condensing some of the gases to liquids. We augment our table with more boiling points. 

In laboratory tests it was found that this method is unprofitable for large-scale destruction of Adamsite. Information in the literature [10] suggests that the process yield and efficiency may be improved when reaction conditions are more severe. Process should proceed in the special shunting kiln in steam - gas phase in the presence water steam and of hydrogen at temperature of 900 - lOOO C. Arsenic trichloride, arsenic, hydrogen chloride and soot, which spring up in the Adamsite destruction, are separated by condensation, filtration and absorption processes. 

---