E-learning gas purity

Membrane separation

Membrane separation technology is typically used for industrial gases where strict purity standards do not apply. In this technology, thousands of hollow fiber membranes are placed in a housing through which compressed air is passed on one side. Different gases have different diffusion rates on the fiber wall, and thus different flow rates can be used to remove contaminants moving through the systems while retaining the desired gas.

For industrial gases, membrane separation is often sufficient to achieve the desired gas purity with a purity level of 97%. The simplicity, price and lack of moving parts is the choice of this principle for industrial applications. Membrane separation is widely used in systems for refining and petrochemical plants in such applications as hydrogen purification, ammonia synthesis and carbon monoxide purification. For high and ultra-high purity applications, where purity levels greater than 99.99% or better are required, other methods are used.

Adsorption under pressure

When gases need to be purified to the level of several hundred unwanted molecules per million (PPM), Pressure Swing Adsorption (PSA) is used and is the technology in which Teesing is specialised. This technology uses pairs or series of vessels filled with adsorbents such as molecular carbon sieves, zeolites and charcoal, which are then pressurized. Van der Waals forces - relatively weak electrical forces that attract neutral molecules in gases and most organic liquids and solids - cause the supplied gas stream to be physically adsorbed onto the media. The desired gas, adsorbed at a different pressure and temperature than the impurities, then passes with impurities removed. Since impurities are adsorbed onto the media when PSA is used, the vessels must be regenerated, which can be done by reducing the pressure, releasing the trapped impurities, and then increasing the pressure in the vessel. Without this process, the impurities remain, contaminating the next batch of gas.

A key advantage of purification in the pressure swing style is the degree of purity that can be achieved with it: 5-9Ns of pure gas (99,9999999%). This makes it suitable for a wide range of applications with more stringent gas purity requirements than are possible with membrane separation, such as pharmaceuticals, laboratory research, aerospace and the semiconductor industry. A disadvantage of pressure swing adsorption is that sizing and tuning the process is complex. And the adsorbent is specific to the gas to be purified. In addition, accurate control of pressure, temperature and flow is essential. One can choose a single purifier for working in batches, or a full-continuous system with two purifiers that alternate purifying and regeneration, controlled by a PLC.

Cryogenic distillation

For the most stringent purity requirements, cryogenic distillation should be used. Cryogenic distillation removes impurities to low PPM levels and uses the unique boiling points of gases for separation. First, the gas is cooled and passes through a molecular sieve (purifier) to remove traces of impurities such as moisture, then it is fed into a distillation column where it is cooled to cryogenic temperatures against the effluent gases. Due to the different boiling temperatures, the air moves up through a series of scales in the column and cascades down as reflux fluid. Impurities are removed during this continuous process.

Cryogenic distillation is the most widely used technology for applications with ultra-high purity requirements where a single contamination can be detrimental to safety or operational results. Obviously, cryogenic distillation plant is expensive and complex. The size and design of the system varies greatly depending on the application and the amount of pure gas used.