#VaporCompression (VC) distillation has been used for the production of #WFI and other #pharmaceuticalwaters for decades, especially for large capacity requirements. While the USP Purified Water (PW) market has been dominated by reverse osmosis and deionization based systems, VC has been competitive in certain PW applications. These are usually limited to instances where the VC can be run on softened feed water without of the need for reverse osmosis (RO) and/or deionization (DI). Sensible water recoveries on these softened fed VC systems of 85% or higher have been observed.
Unlike multiple effect distillation systems that operate a higher temperatures and pressures, VC systems are capable of handling feed waters with relatively high total dissolved solids (TDS) levels. This generally includes any brackish feed water meeting potable water standards. Hence, the concern for VC is not the feed water TDS levels, but the presence of certain contaminants that challenge VC’s ability to meet pharmaceutical water quality standards, or those that may oxidize or scale the units. These six categories of impurities are the principal feed water concerns for VC systems designed to meet pharmaceutical water quality standards and that operate without RO, DI, and Electrodeionization (#EDI) as pretreatment.
The concern for VC is not the feed water TDS levels, but the presence of certain contaminants that challenge VC’s ability to meet pharmaceutical water quality standards, or those that may oxidize or scale the units.
1. Chlorine / Chloramine
Chloramines are a class of compounds which include monochloramine (NH2Cl), dichloramine (NHCl2), and nitrogen trichloride (NCl3). Chloramines are formed by a reaction of chlorine with ammonia. The former is a common disinfect added by drinking water municipalities, and the latter which may be naturally occurring or added by municipalities to reduce the propensity of chlorine to form unwanted disinfection byproducts. The most common species of chloramines found in feed waters is monochloramine – which is the predominant species for neutral pH or slightly basic feed waters.
Chlorine and chloramines are both oxidizing agents. When the feed water is heated in the distillation unit, the conditions become more conducive to stress corrosion and pitting attack. All chlorine species should be removed prior to any distillation unit to prevent this from occurring. Activated carbon is the preferred method of chorine or chloramine removal for softened fed VC systems due to its reliability and flexibility handle fluctuations in flow and impurity levels. Given the variable concentrations of these chlorine species in the feed water, a volumetric flow rate of less than 1.0 gpm per cubic foot of activated carbon and a bed change of six to twelve months is recommended. For concentrations of total chlorine (chlorine plus chloramine) greater than 2 or 3 ppm, the use of catalytic activated carbon should be considered. These carbons have a modified surface which increases the rate of removal as well as the capacity of the activated carbon.
Activated carbon is the preferred method of chorine or chloramine removal for softened fed VC systems due to its reliability and flexibility handle fluctuations in flow and impurity levels.
Ammonia may naturally occur in feed waters, generally at lower concentrations. The presence of higher concentrations (>0.1 ppm) of ammonia is typically the consequence of chloramine removal; either activated carbon or sodium sulfite. The result of which is the production of ammonia gas which in water will exist in equilibrium with ammonium ion (NH4+). The ratio of the two is dependent on the pH and the temperature of the solution.
In a distillation column the ammonia, like other dissolved gases, will be carried over with the steam and contaminate the distillate. As equilibrium is established with ammonium ion, an increase in distillate conductivity will occur. Since ammonium ion is more conductive compared to the ionized carbon dioxide species, trace concentrations of feed water ammonium as low as 0.1 ppm can inhibit distillation units from consistently meeting the USP Stage 1 conductivity limit of 2.7μS/cm at 80∞C.
Absent of any deionization process employed as pretreatment to the VC still, a sodium cycle ion-exchange is one practical option for ammonia/ammonium removal. To ensure successful removal of the ammonia species, the softener must be positioned downstream of the chlorine/chloramine removal step to ensure that any ammonia species that are present are liberated. Throughout the softener bed, the ammonium ion is exchanged for sodium competes with any cations (e.g. ions such as calcium and magnesium associated with water hardness) for absorption sites. Therefore, two banks of softeners are generally required; initial softening for hardness removal and polishing softening for ammonium removal. These will typically be positioned upstream and downstream of the activated carbon de-chlorination step to maximize efficiency. Acid injection may be required to lower the pH, converting ammonia gas to ammonium ion, to optimize ammonium removal in the polishing softener. A process design featuring softener-carbon-softener is a common pretreatment configuration for softened fed VC systems operating in the U.S.
(For ammonia/ammonium), two banks of softeners are generally required; initial softening for hardness removal and polishing softening for ammonium removal.
An endotoxin specification of 0.25 EU/ml is mandated for WFI - and is also a typical limit for high-purity pharmaceutical grade waters. Properly designed VC units will generally provide a 3 to 4 log reduction in endotoxin concentrations. This equates to a feed water endotoxin limit of 250-2500 EU/ml, levels that are not common for most municipal sources. However, feed waters from a surface water source may experience endotoxin levels in this range during upset conditions. For lakes or reservoirs, these may include turnover or disruption of the thermal stratification, excessive precipitation introducing contamination, or flushing of new or existing distribution pipes. Additionally, endotoxin spikes can be observed in the product of activated carbon units notorious for their contribution to increased total viable bacteria levels.
The most practical method for endotoxin removal in pharmaceutical water systems is filtration, preferably via membranes rated at the #ultrafiltration (UF) range or tighter. Pretreatment systems to VC stills, in particular those devoid of RO, could be supplemented by a UF system to ensure acceptable concentrations of endotoxin even in upset conditions.
The most practical method for endotoxin removal in pharmaceutical water systems is filtration, preferably via membranes rated at the #ultrafiltration (UF) range or tighter.
Silica is often reported as total silica, which is the sum of both the reactive (ionic) and non-reactive (colloidal). Reactive silica is in equilibrium in water with bisilicate (HSiO3-), which would be removed by ion-exchange or RO. Colloidal silica exists as long polymeric chains and can be removed by UF or more efficiently by RO. When the concentration of silica exceeds the saturation level, it can precipitate on the internal surfaces of the VC units and disrupt thermal transfer in the unit. The saturation limit of silica varies based on the silica species, temperature, and pH of the water. It is often reported that silica concentrations in the range of 100 ppm will produce precipitated or amorphous silica.
For softened fed VC systems operating on high silica levels, the silica can either be reduced in the feed water - or the unit may be operated at lower recoveries. Like reverse osmosis, distillation units concentrate the feed water and concentrate impurities during operation. Maximizing water recovery and improves the efficiency of the process, but risks exceeding solubility limits of feed water impurities. At 80% recovery, the nominal concentration factor would be 5X. However, the concentration factor jumps to 10X concentration factor for a system operating at 90% recovery. Even with silica levels thought to be quite manageable at 10-15 ppm in the feed water, high operating recoveries for softened fed VC systems may not be practical without feed water silica management.
Maximizing water recovery and improves the efficiency of the process (of removing silica), but risks exceeding solubility limits of feed water impurities.
Another class of impurities that is concentrated during the VC process is the calcium and magnesium associated with water hardness. Properly designed water softeners will produce water with hardness levels than 5 ppm which is suitable for VC feed. As with any deionization process, any feed water hardness will be removed, but excessive hardness that can lead to precipitation on the still internals can also lead to an undesirable cleaning or descaling requirement for the still.
For the softener-carbon-softener process mentioned above, the operation of the primary softener is critical to ensure that no hardness ions are competing with ammonium ions in the polishing softener. This can lead to short run times and frequent regeneration of the polishing softener. Ammonia breakthrough from the polishing softener will cause an immediate effect on the distillate quality.
The operation of the primary softener is critical to ensure that no hardness ions are competing with ammonium ions in the polishing softener.
6. Carbon Dioxide
Carbon dioxide is a dissolved gas which will not be specifically removed in the VC process. Most feed waters fed have a slightly basic pH as municipalities increase the pH to minimize the corrosion in their piping network.
Complete carbon dioxide removal is not required to meet pharmaceutical water quality standards, but it may help the performance of the VC unit itself. Carbon dioxide is denser than air (primarily nitrogen) and may accumulate in the VC units and limit heat transfer. The presence of carbon dioxide may also lead to localized corrosion or rouge due to the formation of carbonic acid in water. While we have seen no published data to confirm the latter occurs in VC units, the implementation of a carbon dioxide management program for VC feed waters is recommended. For softened fed VC systems, a feed water degasser (steam stripper) integral to the VC unit is recommended as compared to a VC degasser positioned downstream of the unit. This may assist in the removal of other non-condensable gases from the systems as well.
Complete carbon dioxide removal is not required to meet pharmaceutical water quality standards, but it may help the performance of the VC unit itself.
Not unlike other primary deionization techniques, they key to successful, long term VC operation for production of pharmaceutical grade waters is to understand and monitor the feed water impurities and to design and operate a robust pretreatment system. For VC pretreatment systems absent of RO and DI (or EDI), this is absolutely imperative.
There are many softened fed VC systems that have been reliably producing pharmaceutical grade water, including #WFI, for years. Like any high purity water system, the feed water design should be based on a worst case scenario and the system designed for upset conditions. Specifically for surface water supplies, the endotoxin concentrations can vary significantly and they are likely to be treated with chloramines and consequently contain ammonia species. Implementing sound pretreatment techniques – and monitoring the concentrations of these impurities to ensure properly conditioned water is fed to VC units – are both critical to reliably maintain the validated state of a pharmaceutical water treatment system.