Pure steam is used in sterilization chambers as a common method to sterilize pharmaceutical products, such as equipment parts, instruments, containers and materials for sterile environments. Although regional Pharmacopeias in Europe, Canada, Japan, India and China have minor differences in limits and measurement techniques, they each have similar specifications on pure steam sterilization and Water for Injection (WFI).
The US Pharmacopeia (USP) defines pure steam as steam that meets the criteria for WFI when condensed. The European and British Pharmacopeias have similar standards with additional specifications for dryness and the noncondensible gas content allowance. The UK National Health document HTM 2010 and the European Standard EN 285 set limits for noncondensible gasses at 3.5% v/v and moisture content < 5.0%.
The USP does not include a requirement for noncondensible gasses but does state “The level of steam saturation or dryness, and the amount of noncondensible gases are to be determined by the pure steam application”. This places the responsibility of producing the proper quality of steam for a particular application on the user. In many cases pure steam generators adopt the European and British pharmacopeia requirements for noncondensible gasses and dryness to assure the steam meets or exceeds the required quality.
Whenever water and gas come into contact gas has the tendency to dissolve into the water. The amount of gas that dissolves into the water is governed by Henry’s Law, which dictates that the amount of gas dissolved into a liquid is proportional to the partial pressure of the gas. The partial pressure value of the gas is dependent on temperature and atmospheric pressure. Henry’s Law shows us that noncondensible gasses, such as nitrogen, oxygen and carbon dioxide, will be present in water that comes into contact with the atmosphere.
Any noncondensible gasses present in water that is used to produce steam will carryover with the steam when the water is boiled. These noncondensible gasses mixed into the steam decreases the steam temperature at a given pressure. This reduces the sterilizer’s ability to properly transfer the required heat to sterilize its contents, so the removal of dissolved gasses is a necessary step in the steam generation process.
The conductivity specification for WFI water is 1.1 μS/cm. Dissolved carbon dioxide can contribute to the conductivity of water. Carbon dioxide will freely pass through an RO membrane and dissociate in the permeate water to increase the conductivity.
Meeting the Specification
In order to remove the dissolved gas, many steam generators will preheat the feed water. Heating lowers the solubility of dissolved gasses in the water, thereby lowering any noncondensible gas carried over into the steam. However, this process requires that the hot water is stored to allow time for the gas to be released. This process is energy intensive.
Carbon dioxide removal is often done using a double pass RO with pH adjustment. This requires a higher initial investment and operating cost.
Liqui-Cel® Membrane Contactors offer a reliable, in-line solution that is far more economical for removing dissolved gas. The feedwater enters the membrane contactor on the outside (shellside) of the microporous hollow fiber. Since the hollow fiber is a hydrophobic membrane, water cannot pass through the small pores in the membrane wall. However, the dissolved gases will pass though the pores to the inside (lumenside) of the hollow fiber and exit the lumenside port.
A vacuum is applied to the lumenside of the hollow fiber. By lowering the gas pressure in contact with water a driving force is generated to remove the dissolved gas from the water. (See figure 1.).