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Gas Purifiers
 

The gas purifier system is an integrated assembly of process, chemical and control devices, which function to a programme of set requirements.

The purifier's primary role is to remove oxygen by adsorption and moisture by absorption from gas, which is forced through a chemical bed of copper catalyst and molecular sieve. This process in its simplest form is easily achieved, but the chemical bed does require reforming to initialise the process and will at some point become saturated and require regeneration, and the design requirements therefore become more complex.

Design considerations include the encapsulation of the process to exclude oxygen and moisture at a molecular level, and the application of high temperature, high vacuum, pressure, and hydrogen gas. The chemical bed is part pyrophoric in its reformed state and the whole bed is highly susceptible to deactivation by a range of gaseous, solid and liquid contaminants, one of which is water and a by-product of the reform and regeneration process, in the recombination of oxygen and hydrogen molecules.

For the gas purification system to operate efficiently over time with minimum maintenance, does require an appreciation of the factors that will impact on those qualities, for it is quite simple to "economise" on purifier design without affecting achievable purity.

This section therefore looks at aspects of gas purifier system design to assist glove box users in any selection, evaluation or specification process, gain value for money spent.

The capacity of a gas purifier, combined with the leak rate of the glove box, will determine the frequency of regeneration. Capacity has little influence on the attainable gas quality. A gas purifier capacity is therefore ideally sized relative to the volume of gas contained in the glove box, and its leak rate. However it is not commercially viable to have multiple sizes of gas purifiers, and most glove box manufacturers have only one or two sizes, but capacity does vary between glove box manufacturers.

Large volume glove boxes sometimes have multiple gas purifiers or a single oversize gas purifier, and small volume glove boxes will often have a purifier relative to its size. Glove Box Technology Ltd has two sizes of gas purifier, a model that will provide excellent to good regeneration intervals for small volume glove boxes from 125 litre up to 800 litre, and a model that will provide excellent to good regeneration intervals for medium volume glove boxes from 800 litres up to 3000 litres, and by connecting in a dual parallel configuration this can be increased to cover up to 5000 litre volume glove boxes.

A single gas purifier system needs to be taken "off line" to perform the regeneration process, and the glove box will then be subject to molecular rates of oxygen permeation causing a steady rise in indicated contamination. The rate of oxygen permeation will vary according to the glove boxes materials of construction, design, gauntlet count, condition of glove box, etc. and of course should be out of operational use when under regeneration. An average glove box should remain under 300 ppm oxygen during the time it takes to regenerate an off line gas purifier. For most glove box users this is acceptable as reactive materials can be locally protected, but for those that must have continuity of glove box gas quality and/or production, a standby change over gas purifier is specified.

Gas purifiers are taken off line using isolating valves, one each to gas inlet and gas outlet, and which should ideally be equipped with positional limit switches. These limit switches should provide positive positional reference of the isolating valves in the closed position and in the open position. Two types of isolating valves are generally used, one, which is typically used in the vacuum industry, and the other, which is used for process applications.

The vacuum industry type provides superb leak tight properties but is vulnerable to the build up of deposits on the seal faces, which over time break up leading to irregularities in the seal face and failure of the regeneration process. Their design also presents a path of high resistance to the flow of gas, due to a 90-degree change of flow and a plate valve head around which the gas has to pass.

Ball valves, as used in the process industry are preferable due to their self-cleaning properties, and well-made designs using "O" ring seals provide equally good leak tight properties. The ball rotates through 90 degrees to position a through hole, in line with the pipe, thus creating an unrestricted path for the gas flow.

Our high performance glove boxes use stainless steel ball valves with separate limit switches to the open and closed extremes of travel.

These limit switches are integrated into the microprocessor control system and provide positive functionality of system operation. Two of the important functions relate to blower operation and reform gas purge respectively. Any failure to shut down the gas blower when the isolating valves are closed will result in wasted energy at best and blower failure at worst, hence the need for positive switching when the valve moves from the open position to that of closed. Likewise, without positive interlock protection a situation could arise during the regeneration mode where reform gas (which is between 3% and 10% mixture of hydrogen in system gas [Nitrogen or Argon]), could be routed through an open valve directly into the glove box.

Limit switches also provide useful diagnostic information, which can be presented visually and audibly when there is a system failure. Isolation valves are usually electro pneumatic in operation and any failure in process control, such as slave solenoid failure, low system gas pressure, or even a stuck purifier isolation valve, could go unnoticed or difficult to locate. Limit switches become even more necessary when isolation valve electro pneumatic actuators are reviewed.

Some designs rely simply on positive pressure to open, and remain open against a spring return. This design is highly reliant upon a sustained high working gas pressure, usually above 50 psi, and any pressure drop below this due to alternate gas demand (ie anti-chamber gas purge) can cause the isolating valve to open and close, alternating with the changing pressure against the spring. This actuator design without a limit switch initiating a display alarm, will probably go unresolved with the glove box user baffled by the unexpected glove box gas pressure fluctuation.

Our isolating valve actuators, are driven by four pistons on a rack acting on the valve spindle, which rotates a ball through 90 degrees. The valve actuator is down sized relative to the valve to provide a force advantage, allowing our system to operate using low-pressure gas. The four-piston rack operates alternately, to open or close, by application of low-pressure gas to either side of the piston. Once the piston has achieved its objective the limit switch registers the valve position and isolates the gas pressure, putting no further reliance upon system gas pressure for satisfactory operation, and providing positive assurance for follow on operations.

The reader will by now appreciate that design does matter, and to be aware, is to be in a position to make choices from an informed position. The economic use of available hardware is at times not economic for the glove box purchaser, and this often does not become apparent until after the warranty period.

Gas purifiers are reliant upon some form of gas recirculation propulsion. The movement of gas presents a number of problems to the design engineer in the form of achieving variable gas flow rates without undue temperature rise within a gas tight loop.

Some designs utilise diaphragm pumps, but these are generally low in volume flow and generate heat. They are also prone to leakage in the diaphragm after relatively short periods of operation (3000 hours), requiring frequent maintenance. The low flow requires a standby diaphragm pump to be used in the event of higher flow requirements. Heat is normally dissipated by the use of a water/gas heat exchanger.

Blowers provide much improved flow rates and can be speed controlled to vary the gas flow according to the needs of the glove box user. However, most blowers compress the gas in providing the moving force, resulting in higher than desirable gas temperatures making the glove box uncomfortably warm for users. Some glove box manufacturers have countered this by adding a gas cooling system, using water or a refrigerator.

Glove Box Technology Ltd, use a specially designed blower that has a by-pass fan system, which achieves high flow at low gas pressure. This design of blower does not generate undue gas temperature rise, and therefore requires no supplementary cooling for the glove box. Speed control is user set to suit process and glove box operating conditions from the main menu of routines through a sub-menu where the blower can be speed set in incremental stages of 25% between 0 and 100%.

Our blower speed control can be linked into the oxygen monitor to automatically ramp up in speed where predetermined limits of oxygen or moisture in the glove box are exceeded, to effect rapid re-establishment of operational set limits.

Our glove box system design is integrated for the achievement of high performance at low unit operating cost, and low maintenance downtime. Key factors that need to be looked at in design detail prior to purchasing a glove box system.

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Process Scale Glove Box and Gas
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