Why Is Conventional Distillation Hot?

Some processes, like the baking of bread, are hot by nature. You can’t bake bread in a cold oven; baking is inherently a high temperature activity. Distillation on the other hand may be a hot operation – if carried out at or above atmospheric pressure – or it may be a cold operation if carried out under vacuum conditions. A very popular high school science lab experiment is making a glass of room temperature water boil in a bell jar. A vacuum pump connected to the bell jar reduces its internal pressure. When it gets down to 20 Torr (one fortieth of atmospheric pressure), the water starts to boil. So why then is distillation widely considered a hot process? More importantly, why is that the reality in most industrial settings?

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Chemists view distillation in terms of the diagram above. In the lab they are concerned exclusively with the chemical separation taking place. They pay scant attention to the bunsen burner supplying heat to the evaporation flask or to the cooling water removing heat from the condenser. 

But the Chemical Engineers responsible for designing an industrial plant must take a wider view of the process, as shown below. The economics of the process depend heavily on the heat flowing from the steam plant to the evaporator and from the condenser to the cooling plant.

The first law of thermodynamics tells us that energy can neither be created or destroyed. A portion of the heat energy supplied by the boiler is converted into chemical potential energy through the separation that occurs; however this amount is almost negligible in comparison to the amount involved in boiling and condensing. Practically speaking then, the heating and cooling loads are the same. The heating and cooling subsystems are equally important in making distillation happen.

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Here is the same diagram showing the major ancillary components pictorially.

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People generally understand the equipment normally used to provide heat. A steam boiler has much in common with the kettle found in every kitchen. Less familiar to most people is the evaporative cooling tower used almost universally to disperse waste heat into the natural environment. The mechanism of cooling is similar to that of the human body: water evaporating carries away heat. In an industrial cooling tower electrically-driven fans blow ambient air through streams of cooling water to force the evaporation to occur rapidly.

Small cooling towers like the one shown above have a modest capacity to dissipate heat into the atmosphere. Larger units have larger capacities with no upper limit. However there is a very definite limit to the temperature these units can deliver. The cooled water can never be cooler than the wet bulb temperature of the ambient air.

When the air has 100% relative humidity (RH) the wet bulb temperature is equal to the the temperature one would measure with an ordinary thermometer (known as the dry bulb temperature). When the air is dryer than 100% RH the wet bulb temperature is lower than the dry bulb temperature. While these values vary from day to day and location to location, most industrial plants worldwide are designed to operate with cooling water around 28 degrees Celsuis (82 Fahrenheit).

Most industrial distillation plants pump that 28C cooling water directly to the condensers.The water picks up heat in the condenser causing its exit temperature to rise above 30C. There must be a temperature difference between the water side and the process side of the condenser tubes in order to induce heat to flow. There is also a need to sub-cool the process liquid below its condensation temperature. The minimum practical condensation temperature then is on the order of 40C. If a meaningful separation is taking place the temperature of the liquid boiling in the evaporator must exceed 50C. More often evaporator temperatures are above 60C due in part to the inevitable fouling that occurs on heat exchange surfaces.

That’s a hot process. It is hot not because of the way heat is applied but by the way heat is removed. The evaporative cooling tower is the culprit. While mechanical chillers can and are being used to overcome the temperature limitation of the cooling tower, this technique adds cost that only small scale production of high value products (such as pharmaceuticals) can bear.

Pass-through Distillation is one way that low temperature distillation can be economically implemented, as shown elsewhere in this site.