MULTIPLE EFFECT DISTILLATION SYSTEM
Bram-cor’s Multiple Effect Distillers (SMPT) are designed and manufactured to meet cGMP requirements for the production of compendial Water for Injection. Each unit contains a number of boiling columns (or effects) with the first column producing pure steam, which is condensed and re-distilled in the following columns decreasing the operational costs.
MULTI EFFECT STILL DESIGN
The heating for evaporation and cooling for condensation processes are performed by double tube sheet (DTS) exchangers. The process is repeated in each column; the greater the number of columns, the hight the efficiency of the distiller. A special labyrinth-separator installed at the top of each column separates the steam generated by the evaporation process from the entrained substance in the steam. The result is a pure, “dry”, pyrogen-free steam, condensed compendial Water for Injection.
The first column of the Still may be used to produce Clean Steam (alternatively or simultaneously). Pressure vessels are designed according to ASME and PED regulation and the equipment features:
- Double tube sheet heath exchangers
- Certified AISI 316L stainless steel mirror-polished and passivated product contact surfaces
- AISI 304 frame, jackets and control board
- PTFE gaskets
- Pneumatic valves with Teflon membranes and AISI 316 L SS polished body
- ASTM C-795 – compliant insulation
Capacities range: from 50 to 15,000 pph with three to ten columns.
SMPT CONSTRUCTION CRITERIA
There are two main types of internal column designs that drive the method of evaporation: falling film and natural circulation. A column designed by the falling film principle allows feedwater in from the top where it is flash evaporated as it falls over the heated interior tubes. Natural circulation fills the column from the mid-point of a column, filling it approximately one third; then overflow is directed to the next column.
Another consideration is the type of separation that occurs after the water is evaporated within the column. Falling film relies on an initial 180 degree turn of the steam to remove heavier droplets and impurities, followed by centrifugal flow. Natural circulation relies on centrifugal flow from the outset; pure steam is then directed through a secondary separation system. The secondary separation is a select design by a particular manufacturer and will take the steam through a series of turns to remove any final impurities that may have been carried upward during the centrifugal separation. Demister pads, a tight collection of stainless steel wire mesh, may be presented as an option. However, it should be noted that the use of demister pads is not an accepted practice in the US, even though the ISPE Baseline Guide for Water & Steam Systems does not discourage their use (1).
At this point, pure steam, meeting criteria for sterile WFI (water for injection), is available to continue to the next column to provide heating for incoming feedwater or to be utilized for additional uses, such as sterilization of process lines and tanks. The level of functionality will depend on the manufacturer and should be evaluated if steam use will be desired from the still. The outlet pressure may be limited to production pressure or be designed for additional capability such as controlled steam pressure, or even operation as a pure steam generator.
PRE-HEATERS AND EVAPORATORS
Pre-heaters will make the multiple-effect still operate more efficiently in regards to steam and water consumption, but are not always a standard feature. A preheater can be installed prior to the first column for additional benefit, or prior to all columns for maximum benefit.
Evaporators will be located internal or external to the column. The evaporators bear the brunt of varying pressures and temperatures. If there is any severe failure to the still, it will most likely be with the evaporator. For this reason, the first evaporator should have a double-tube sheet design; it should be decided in the specification process if all evaporators should be of double tube-sheet design since this is not a standard with all manufacturers. Another key consideration with evaporators is location since this will impact accessibility. The falling film method of evaporation places the evaporator inside the column, while with natural circulation it is located externally. The reason it should even be a consideration is the rare possibility of stress corrosion which would dictate evaporator replacement. Likewise, evaporators internal to the column are more susceptible to such stress, and are harder to replace than external evaporators.
The design of the condenser will partially determine the amount of cooling water required. Some are designed with smaller, more tightly compacted tubes providing a greater surface area for cooling final steam and distillate. Other considerations: is it fully drainable – both shell and tube sides? Whether cooling water flow is continuous or on/off will also have an effect on overall use of utilities. Likewise, if distillate temperature needs to be lowered from 97 C, an after-cooler may be required. Finally, one should consider the de-gassing capabilities of the still. If removal of carbon dioxide is not adequately addressed through the pre-treatment system, non-condensable gases could accumulate during production and a condenser may not be able to vent these gases adequately. The more effective stills will add a de-gasser to alleviate collection and release of gases with the final product.