Reduced to its essentials, distillation is a two-step process, evaporation and condensation. These two steps are “coupled” because they are carried out at the same pressure.
Pass-through distillation is a four step process, similar to simple distillation in that it begins by evaporating some feed liquid and ends by condensing it. These steps however are decoupled by an absorption step (step 2) and a desorption step (step 3) which involve a recirculating inventory of absorbent fluid. In step 2 this fluid absorbs the gases evaporated in the first step. In step 3 the absorbed material is boiled out of the absorbent fluid.
Decoupling permits the evaporator to operate at very low pressure (and a consequent low temperature) while the condenser operates at higher pressure (with consequent low cost cooling).
The four steps lend themselves to interesting heat economies. The absorption step runs at a higher temperature than the evaporator. This means that the heat released in the absorber may be used in the evaporator, permitting the first step to operate without an external energy source. The material absorbed by the absorption fluid must be boiled out in step 3 (desorption) through externally supplied heat. However if all the temperature sensitive material was left behind in step 1, step 3 may safely involve high temperatures, making it possible the use of low-energy multiple effect distillation (MED).