Choosing the Absorbent Fluid

There are some very general things that can be said about the absorption fluid in a PTD system:

1) It must have a very low vapour pressure.

2) It must have an affinity for the main chemical constituents in the vapour generated by the evaporator.

The second requirement suggests that different distillations will work best with different absorbent fluids. It is likely that for some challenging separations no suitable absorbent fluid can be found, making PTD ineligible for the application.

Work to date on PTD has centred mainly on aqueous systems. The vapours to be absorbed are mostly water. Absorbent fluids with an affinity for water will normally have an affinity for other polar molecules like ethanol. There are two main types of absorbent fluids that have been tried: salt solutions and high boiling organic compounds.

Salt Solutions

Solutions of three very hygroscopic and highly water soluble salts are good candidates for PTD in aqueous service. A great advantage of salt is its non-volatile nature. The initial charge of brine in a PTD system can in theory last indefinitely. Although many different salts may work, three stand out because of relatively low cost and availability in industry. These are Lithium Bromide (LiBr), Lithium Chloride (LiCl), and Calcium Chloride (CaCl).

LiBr brine is used in the absorption chiller industry. Its behaviour at low absolute pressure is well studied. All salt brines are corrosive to metals, but LiBr is least corrosive among the three listed.

LiCl brine is used in desiccant dehumidification equipment.

Calcium chloride is the cheapest of the three salts and has the added advantage of being on the GRAS list (generally regarded as safe), making it the salt of choice in environments where toxicity is an issue. Unfortunately it is also the most corrosive of the three listed.

Organic liquids

Ethylene and propylene glycols are examples of low vapour pressure hygroscopic fluids that can be used in PTD. Another example is glycerol. These substances are much less corrosive than brines but their volatility is non-zero. A portion of the absorption fluid inventory will be lost in the vacuum train during operation, so there will be a replenishment cost to the operation. Propylene glycol and glycerol are food additives, so these fluids are very suitable in the food and beverage industries.

Other absorbents

The field is wide open for studies on other candidate absorption fluids for PTD. The ideal fluid for any particular application would be cheap, have extremely low volatility, low toxicity, low corrosivity, low viscosity, low surface tension, and high chemical affinity for the substances in the vapours to be absorbed.