The third article in the series will discuss pitfalls associated with design, operation, maintenance, and other important factors for distillation and pure steam generation in pharmaceutical water applications.
By William V. Collentro
1. Multiple Effect Distillation Units – Feed Water Quality
Multiple Effect Distillation Units (MES) generally consist of 3-6 evaporator sections. Pressure decrease from the first to the last effect allows the latent heat from steam produced from a “previous” effect to be employed for Pure Steam generation in a subsequent effect. As a result, MES operate at higher pressures than single effect stills or Vapor Compression (VC) stills. The higher pressure and associated higher temperature increase the possibility of corrosion such as chloride stress and pitting attack. To minimize long term corrosive effects, pretreatment of the feed water to a MES is required.
While the Unites States Pharmacopeia (USP)
Official Monograph states that feed water must meet “Drinking Water” Standards, including a generally accepted 500 cfu/ml total viable bacteria “Action Limit” stated in the
General Information Section, the actual feed water quality to a MES should be high purity water. Items of particular concern relate to Bacterial Endotoxin (BET) levels in raw water supplies, ionic concentration determined by conductivity, total hardness and silica (reactive and colloidal). Generally, conductivity, total hardness and total silica limits are specified by the MES manufacturer. It is suggested that MES feed water quality should exhibit a conductivity < 1.0 µS/cm @ 25°C, Total Hardness <1 mg/l as Calcium Carbonate, Total Viable Bacteria < 10 cfu/ml, Total Silica < 1 mg/l, and BET < 1 EU/ml. The use of pretreatment, employing reverse osmosis with polishing ion exchange is required. Quite often, inadequate pretreatment, particularly of water from surface supplies, is not employed.
2. Multiple Effect distillation Units – Blowdown Rate
The evaporative process in the first effect of a MES increases the concentration of all impurities in the feed water to the unit. Obviously, higher quality pretreated feed water will reduce the required blowdown rate to maintain an acceptable concentration of impurities in the evaporator of the first effect. Generally a blowdown rate of 5% is common. However, a higher blow rate may be required with poorer quality feed water.
High concentration of impurities within the first effect of a MES unit can produce highly undesirable results. As an example, high concentrations of Total Suspended or Dissolved Solids increase the surface tension of the water in the first effect evaporator. This results in entrainment of water with steam during the evaporative process. This produces two undesirable results. The entrapped material in Pure Steam will precipitate on the surfaces of vapor-liquid disengaging mechanisms/devices. In addition, the presence of entrapped water can result in the presence of unacceptable levels of BET in product water since the principle of distillation, a phase change from liquid to gas, is not completely achieved.
3. Multiple Effect Distillation Units – Vapor-Liquid Disengaging Section Inspection and Maintenance
The three primary sections of a MES are the evaporators, the condenser, and the individual vapor-liquid disengaging devices. The vapor-liquid separation devices for each evaporator insure complete separation of water (liquid) from steam (gas). The principle of distillation is based on total absence of water in the Pure Steam from the evaporator section(s). The vapor-liquid disengaging devices can employ “impingement” or “centrifugal” principles. Impingement devices would employ a wire mesh type surface as a barrier to water, allowing Pure Steam to pass. The water falls back into the evaporator section by gravity. Centrifugal devices physically change the flow direction of the Pure Steam to create a centrifugal force, separating the water from Pure Steam. The water is physically directed to the outside of the device where it returns to the evaporator section.
Both of the indicated techniques as well as similar techniques may accumulate material with MES operation. It is suggested that the devices be inspected at least annually and that replacement be performed if a build-up of material is noted. Failure to inspect and replace these devices will result in a reduction in the BET removal capability. Ironically, excellent pretreatment to a MES may mask “carryover” of material due to the low BET levels in feed water. To complicate this situation, the temperature associated with distillation will destroy all bacteria. As a result, failure to maintain vapor-liquid disengaging devices may only be noted by a slow but steady increase in USP Water for Injection Total Organic Carbon and Conductivity values.
4. Multiple Effect Distillation Units – Individual Effect Level Sensors
Water level sensing and control in MES evaporator sections is critical to successful unit operation. An annual calibration check is suggested. Some level sensors are equipped with indicating devices positioned exterior to the evaporator columns. The indicators should not be used during calibration. The calibrating technique should be independent of any component employed by the level sensor/indicator/controller.
Level sensing is critical to long term successful operation of a MES. The physical distance between the water level in the evaporator and the vapor-liquid disengaging device allows gravity to remove any entrapped water in the Pure Steam. Operation at higher water levels can result in carryover of water and precipitation of impurities in water on the vapor-liquid disengaging device. Inadequate water levels can result in undesirable “superheating” of the Pure Steam (to a temperature greater than the saturated steam value at the effect operation pressure) as well as increased corrosion of the sections of tubes that are not immersed in water.
5. Multiple Effect Distillation Units – Condenser Hydrophobic Vent Filtration System
The selection, sizing, heating provisions, and maintenance of the Hydrophobic Vent Filtration System for the MES condenser are important. One gallon of Pure Steam (saturated at atmospheric pressure) occupies a volume of >200 cubic feet. A pound of water at water at the same conditions occupies a volume of < 0.1 cubic feet. During the condensing process, all Pure Steam is converted to liquid. The reduction in volume is significant. The quantity of displacement air required for the condensing product must be considered during the selection/specification of the Hydrophobic Vent Filtration System. A simple 10” long hydrophobic vent filter in a sanitary stainless steel housing may not be appropriate for all applications.
Many MES Units are provided with Hydrophobic Vent Filtration Systems without provisions for heating. Steam with associated water vapor can be present in the vent filter element. Heating to a temperature of about 225°C (3-5 psig steam or regulated electrical heater) can remove any liquid by converting it to steam at atmospheric pressure. This will avoid condensate from interfering with displacement air flow through the hydrophobic vent cartridge.
It is suggested that MES Hydrophobic Vent Filter Cartridges be replaced every 6 months. In addition, it is suggested that laboratory filter/housing integrity testing be performed every 6 months. While “in-situ” membrane filter integrity testing has certain advantages, it is not only cumbersome but adds additional valves and fittings that can provide undesirable “dead legs”.
Finally, hydrophobic vent filter membrane selection should consider the high operating temperature. Filters for low temperature (ambient) operating conditions are not appropriate.
6. Multiple Effect Distillation Units – Distillate Tubing
Generally, distillate product from an MES flows through distillate tubing to a USP Water for Injection Storage Tank. There are several items that must be considered during this gravity driven process. The slope of the distillate tubing should be adequate to insure flow without back pressure and associated condensate “flooding” of the condenser. A suggested minimum slope of ½” per linear feet of tubing should provide adequate delivery unless there are elbows or other obstruction in the delivery tubing.
The MES distillate tubing should be equipped with conductivity monitoring provisions and a divert-to-drain arrangement. The tubing arrangement for the conductivity cell and divert-to-drain valve should not allow unfiltered air to enter the delivery tubing during pre make-up or below quality divert-to-drain operations. Distillate sampling provisions should also be included.
7. Vapor Compression Distillation Units – Feed Water Quality
While Vapor Compression Distillation Units (VC) are capable of operating with feed water that has been filtered, softened, and residual disinfecting agent removed, the secret to long term successful operation of any distillation unit is excellent pretreatment. It is strongly suggested that the feed water to a VC Still include reverse osmosis which removes/reduces inorganic material, organic material, bacteria, BET, and colloids. If the raw water supply to the pretreatment system employs chloramines as the disinfecting agent, ammonia removal in the feed water will be required. Cation units operating in the sodium form are generally employed for removal of ammonia as the ammonium ion. Do to the monovalent charge and light molecular weight of the ammonium ion, countercurrent brine regeneration is strongly suggested.
8. Vapor Compression Units – Blowdown Rate
With softened pretreated water, the suggested minimum blowdown rate for a VC Still is 10%. With high quality pretreated water employing reverse osmosis, the suggested blowdown rate may be reduced to 5%. While VC Stills operate a lower pressure and temperature than MES, frequent chemical cleaning of the evaporator section can be required if the blowdown rate is too low.
9. Vapor Compression Units – Evaporator Cleaning
Periodic Cleaning of the evaporator section of a VC Still is required. The cleaning operation should be part of a preventative maintenance program. An annual chemical cleaning frequency (minimum) is recommended. It may be appropriate to employ the use of service organization familiar with cleaning of VC Stills. Further, since precipitation of iron in the evaporator section is a concern, cation resin employed in water softening units and ammonia removal pretreatment units should be periodically tested for iron fouling and replaced as appropriate.
10. Vapor Compression Distillation Units – Vapor-Liquid Disengaging Provisions
VC Stills have a large cross sectional area evaporator. This results in low Pure Steam velocity from liquid in the evaporator to the vapor-liquid disengaging section. While water entrainment in Pure Steam is decreased by the lower velocity, it is suggested that this is offset by the relatively low temperature differential between heating steam (compressed Pure Steam) and the water in the evaporator section. Subsequently, concerns associated with vapor-liquid disengaging discussed in Item 3 above apply.
11. Distillation – Chloramine Concerns
As discussed earlier, many municipal treatment facilities using raw water from a surface source, inject ammonia and chlorine into potable water. The resulting chloramine compounds provide disinfection while producing reduced amounts of disinfection byproducts, particularly trihalomethanes. Unfortunately, pretreatment system unit operations required to remove residual disinfection from distillation unit feed water produce ammonia. Ammonia, a gas, is very soluble in water. Further, it reacts with water, in an equilibrium reaction producing the hydroxyl and ammonium ions. If not removed, the ammonia (gas) is “carried over” with Pure Steam in distillations units. The conductivity of the distillate product is increased by the presence of the ammonium ion. The conductivity value may exceed the specification set forth in USP Physical Tests Section <645>.
For VC stills with water softening pretreatment systems, ammonia removal is required as discussed in Section 7 above. For MES the pretreatment should include reverse osmosis with polishing ion exchange (mixed bed or continuous electrodeionization) that will remove ammonia as the ammonium ion.
12. Ammonia – USP 23 Reference
Considering the reference to ammonia from chloramines, it should be indicated that the Fifth and Eighth Supplements to USP 23 (1996) deleted pass/fail wet chemistry test and provided TOC and Conductivity testing. The current Conductivity requirements set forth in USP
Physical Tests Section <645> are based on a chloride concentration of 0.5 mg/l and an ammonia concentration of 0.3 mg/l. Subsequently, ammonia control to achieve the USP Specification is very important.
13. Distillation – Bacterial Endotoxin Quantitative Monitoring
As indicated earlier, sampling provisions should be provided in the distillate tubing from the still to the downstream USP Water for Injection Storage Tank. Periodic quantitative analysis for distillate BET levels should be performed. These results should be tabulated as a function of time. Generally, a new distillation unit (MES or VC) will exhibit > 3 log reduction in BET level. However, changes in feed water parameters, unit maintenance, vapor-liquid disengaging section performance, etc. may result in a decreased reduction of BET. Quantitative measurement allows personnel to identify (and react to) this situation before qualitative measurement of point-of-use Water for Injection samples indicates a problem.
14. Distillation – Presence of Disinfection Byproducts
Item No. 11 above discusses disinfection byproducts, mentioning trihalomethanes. These byproducts are produced from the reaction of disinfecting agents with heavy molecular weight naturally occurring organic material present in surface water supplies. Trihalomethanes are light molecular weight volatile chlorinated organic compounds. The principal trihalomethane compound is chloroform. Trihalomethane compounds are not effectively removed by conventional pretreatment techniques. They will be present in still feed water, “carried over” with Pure Steam and present in Water for Injection. Generally, the presence of compounds such as chloroform in Water for Injection is unknown because testing to identify its presence is not conducted. However, with both seasonal and climatic changes in the feed water to the pretreatment system, the concentration of chloroform (regulated by the United States Environmental Protection Agency at 80 µg/l Total Trihalomethanes) may increase. Further, in hot recirculating Water for Injection, decomposition of chloroform may occur. This situation coupled with “enhanced” response of many TOC Analyzers to chlorinated organic compounds, may inhibit production of Water for Injection meeting the TOC specification set for in USP Physical Tests Section <643>.
15. Pure Steam Generation – Feed Water Quality
The feed water quality to a Pure Steam Generator (PSG) should be similar to that of a MES. While operating pressure and temperature may be lower than a MES, a PSG is essentially the first effect of a MES unit without a condenser. For facilities employing VC Stills with softened pretreatment, PSG feed should be from a separate source, of polished RO quality. It is important to note that many facilities with VC Stills employ Water for Injection as feed water to a PSG. It is suggested that PSG feed from a Water for Injection recirculating loop (hot, ambient, or cold) utilize a feed tank with air break between the tank water and the Water for Injection supply valve to avoid back contamination.
16. Pure Steam Generation - General
PSG design, operating, maintenance and calibration criteria should be established. Quite often these parameters are neglected or ignored. Design should consider the desired delivery pressure and pressure drop, at maximum demand, to the delivery points. Annual maintenance is recommended by a PSG manufacturer’s service individual. Yearly calibration of critical instrumentation, including level and pressure devices, should be performed.
17. Pure Steam Generation – “Delivered” Quality
Pure Steam should not contain any water (liquid). Generally, PSG produce steam without water. Unfortunately, steam quality at delivery points is a function of delivery tubing design, the number of steam traps, and the location of the steam traps. Delivery systems with dead legs will accumulate water (condensed Pure Steam). Systems with inadequate steam traps or improperly positioned steam traps will also allow condensation. Unfortunately, point-of-use periodic sampling for Pure Steam microbial and chemical attributes is seldom performed. This inhibits the ability to determine the quality of delivered Pure Steam. A periodic point-of-use sampling program for Pure Steam Systems should be established.
18. Pure Steam Generation - Gaskets
PSG gaskets and gaskets in the Pure Steam Distribution System should be periodically inspected and replaced. Steam traps should also be periodically inspected and cleaned. Gasket material may be Teflon, Teflon encapsulated EPDM or various propriety materials of stainless steel and Teflon construction.
19. Distillation Unit and Pure Steam TOC Monitoring
TOC monitoring of distillate product water and condensate Pure Steam is suggested. Monitoring provisions can be obtained by online or periodic “grab” samples. This program will detect organic impurities not detected by online conductivity monitoring. TOC monitoring programs can detect a failure of a heat transfer surface that could introduce facility steam into the Pure Steam of a distillation unit or PSG. It should be noted that certain volatile boiler water treatment chemicals may actually suppress TOC measurements for some TOC analyzers. While this would appear to negate the suggested TOC monitoring program, trending of TOC data as a function of time allows observation of both gradual and rapid increases/decreases in TOC.
20. “Supplemental” Monitoring Provisions – Distillation Units and Pure Steam Generators
As indicated earlier, pretreated water quality is the key to long term successful operation of MES, VC Stills, and PSG. A responsive periodic monitoring program should be established. Parameters to be considered for both feed and product are indicated in the following table.