SELECTION CRITERIA FOR COST EVALUATION
In summary, about WFI production, there are several factors that will influence the suitability of a particular distillation method to the desired application. An overview of design features is provided (see Table 1) (3) and should be taken into account when considering the following issues:
1. Consider the required volumes of clean utilities needed in the facility: Will primary water use be WFI or USP? If there is only a small need for USP, it may be more effective to couple a vapor compression unit with minimal pre-treatment and use the WFI for both applications. Is there a small pure steam requirement? The capability of a multi-effect still to be tapped for pure steam could be more advantageous than the additional investment of a dedicated pure steam generator.
2. What utilities are already available? If steam is not readily available, electric-driven models are available with both technologies, but only smaller volumes (75 gph) typically for multi-effect design. Desire to minimize use of cooling water could also drive selection favoring a vapor compression still. A USP system already on site could feed a multiple-effect still, thus taking advantage of the lower acquisition cost as compared to vapor compression.
3. What maintenance resources are available? The multiple-effect still requires little maintenance due to the minimal moving parts, but as part of a good preventive maintenance schedule, may undergo descaling on a yearly basis. The vapor compression still will possibly require a more frequent preventive maintenance schedule due to the moving parts, but the newer compressor advances may help minimize the time required.
4. Are there any space constraints? Multiple-effect stills in general take up less floor space than a vapor compression still. However, gravity feed to a WFI holding tank may require additional headroom than is available. Of the two multi-effect technologies, falling film type systems tend to be more compact, but taller; while the natural circulation require more floor space due to the external evaporators. In some models, a raised condenser may alleviate full height demands.
MED-VCD COMPARISON TABLE
NOTE ON WFI PRODUCTION
As a final note, a simplified equipment evaluation is based on capital costs. A more comprehensive approach would include the pretreatment for a complete WFI system evaluation with the above-mentioned considerations applied. From this perspective, one could also add operating costs to get an accurate overall view from which to guide the selection process. The process is further enhanced by considering the key features of each technology to help gauge which system may be most compatible with site operation philosophies and capabilities of staff, and then fine-tune the process by comparing the desired features with what is available from a particular manufacturer.