The addition of a process material or substance to a process to allow for its gradual loss. The loss, which may or may not be intended, may be due to leakage, through purging, evaporation, or accompanying a product as an impurity. Additional solvent make-up is added to extractive distillation processes, which lose solvent as an impurity to the product. Cooling towers that cool process water for reuse require make-up water for the water lost as vapour.