In Industry the term “water consumption” comprises three main components: water that becomes part of the Product, water which becomes contaminated in the process and is sewered, and water that is evaporated in cooling towers. Distillation, per se, involves only the latter, and it does so in a very big way. Pass-through distillation can reduce or even eliminate this loss. To understand how, it is important to first understand why it exists in the first place. Cooling towers, and the water they “consume”, are part of a distillation plant’s energy flow.
Energy cannot be created or destroyed; it flows from a high temperature source to a low temperature sink. In most distillation plants it is provided to the evaporator as steam then removed from the condenser by a stream of cooling water. From the cooling water, the heat is wasted to the environment in an evaporative cooling tower where a portion of the water changes from liquid to vapour, carrying away waste heat at low temperature in the process. There are significant costs associated with the procurement and chemical treatment of the cooling water. The easiest way to mitigate these costs is to use less energy in the first place. If a conventional single effect distillation plant were retrofitted with a three-effect TIEGA process, it would produce at the same rate using half the energy input and consequently would use half the cooling water. Where there is evaporative cooling, water conservation automatically accompanies energy conservation.
A different approach to cooling is sometimes used: direct dry cooling. This operates in the same manner as a car’s radiator, and with very similar equipment. Many people know this type of equipment by the name “Fin fan”. This cooling method consumes no water at all. One of its drawbacks is higher capital cost than evaporative systems. But even when capital cost is not the most important consideration, direct dry cooling is often ruled out because it elevates operating temperatures some 20 Celsius degrees compared to evaporative cooling towers. Many distillations need to “run cool” because of the presence of temperature-sensitive materials. In some cases high temperatures cause heat exchangers to foul. In other cases high temperatures cause delicate substances to thermally degrade, imparting objectionable odor or colour to products.
Pass-through distillation is ideal under these kinds of circumstances. The final condenser is completely decoupled from the process evaporator, so that their operating temperatures may be chosen independently. Suppose a conventional distillation operated with an evaporator temperature of 70C and a condensing temperature of 30C, furnished by evaporative cooling. A retrofit for pass-through distillation could be configured to operate with those same temperatures using the same cooling system. The retrofitted plant would use half the energy of its predecessor, and would reduce the load on the cooling system to the same extent.
A second benefit of the PTD retrofit might be to reduce the process evaporation temperature from 70C to, say, 40C to eliminate fouling and improve product quality. That change would leave the triple-effect absorbent regenerator unaffected. Its first effect might operate at 190C while the water cooled condenser on the third effect would operate (as always) close to the temperature of the cooling water, 30C.
The third benefit would be to replace the evaporative cooling system with a fin-fan, and turn the plant into a “Zero Water Consumption” facility. This would raise the condensing temperature in the absorbent regeneration section to 50C, and that twenty degree increase would be felt all the way back to the first effect, raising its boiling temperature from 190C to 210C. The process evaporator however would be totally unaffected, and would continue to operate at 40C.
Zero water consumption is possible for any distillation process that can use direct dry cooling instead of evaporative cooling towers. Cooling towers, however, are more common partly because their capital cost is lower and partly because many processes cannot operate at the higher temperatures that direct dry cooling demands. A pass-through distillation plant overcomes these drawbacks. By virtue of using half the energy of a conventional plant, the capital cost premium of fin-fans is offset (a small fin-fan may even be cheaper than a large cooling tower).
A PDT plant can be configured to use “fin fan” cooling while reducing (rather than increasing) the temperatures seen by delicate process fluids.