THERE ARE SEVERAL METHODS for producing water-for-injection (WFI). Multiple-effect distillation (Figure 1) and vapor compression (Figure 2) are the two primary methods recognized by the FDA. Each are established methods, offering different advantages depending on the proposed application. This article provides an overview of the key features of each technology and important issues for consideration.

Key design features drive the selection of multiple-effect versus vapor compression technologies. Multiple-effect stills are noted for their multiple column design which re-uses steam energy through the process. They have minimal moving parts, but require cooling water for final distillation of product. Vapor compression units are driven by a more mechanical process involving a compressor and other moving parts to compress steam, thus reducing evaporation temperature (Boyle’s Law). This greater energy efficient design also requires minimal feedwater treatment. Both systems are powered by either steam or electric heating for operational flexibility in all capacity ranges.

In making a determination between the two technologies, the previous conventional wisdom was to base a decision on WFI capacity requirements. A break-even point of ~600 gallons/hour (gph) balanced the higher capital cost of vapor compression against the lower utility consumption costs, (such as cooling water), for multiple-effect stills. Therefore, the cost of producing WFI above 600 gph favored vapor compression technology. Vapor compression is estimated to cost $10-12 per 1000 gallons of WFI, while multiple-effect is $18-25 for the same 1000 gallons. On the other hand, the higher capital costs of vapor compression usually did not provide a desirable return on investment for capacity requirements below 600 gph. However,greater analysis of facility needs and costs compared with strengths and advantages of each technology has resulted in cross-over of this break-even point in both directions. Making an informed selection requires a better understanding ofthe main design features of each.

Figure 1. Multiple-effect Water Still
(Photo courtesy of COR srl and COR-USA)