WFI equipment cost evaluation: a simplified and incomplete equipment evaluation for WFI production is based on capital costs. A more helpful and comprehensive approach would include a custom water pretreatment system for a complete evaluation of WFI production plant, with some considerations applied. From this point of view, 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 both the technologies to help gauge which system may be perfectly compatible with site operation concepts and operational tasks, and then fine-tune the selection process by comparing the desired features with what is available from the manufacturer.
In summary, there are several factors that will influence the suitability of a particular distillation method to the desired application. An overview of design features (see the comparison table above) should be taken into account when considering the following issues:
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 VCD unit with minimal pretreatment 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.
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. If it is necessary to minimize the use of cooling water could also drive selection favoring a Vapor Compression Distiller. 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.
The Multiple Effect Distiller 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.
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.
This still technology is easily recognized by its multiple columns (i.e. effects) which re-use heat energy through the additional effects. First column is heated by an external source such as plant steam (electric heating is available from some suppliers in smaller models). Plant steam is replaced in remaining effects by the pure steam produced from the initial evaporation process. This pure steam is utilized as a both a heating source for additional columns as well as a preheating source for incoming feedwater. In turn, the feedwater helps condense the pure steam so that the remaining pure steam produced during the process requires minimal cooling water in the final condenser.
Vapor Compression stills have been a dominant method of WFI production for LVPs (Large Volume Parenterals) over the years due to the cost efficiencies provided with the higher capacity WFI production. However, recent improvements with Vapor Compression technology as well as rethinking of WFI usage make it a compelling choice for select smaller-scale operations, allowing them to break under the 600 gallon per hour barrier. The units operate at lower temperatures and larger tubing, allowing them to bypass the more exacting pretreatment requirements of a Multiple Effect still. Thus, the Vapor Compression unit can possibly be the more cost-effective choice when evaluated as a full system. Likewise, without a need for cooling water, utility consumption is further enhanced.
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