Conventional transportation and storage of industrial gases such as hydrogen takes place with the gas in the form of a liquid, the liquefied gas being referred to as a cryogen or a cryogenic liquid. In liquid form a larger quantity of the product can be stored in a smaller space than if the product were in the gaseous form. A major problem with transportation and storage of gases in liquid form is that it is impractical (i.e. excessive cost in equipment is required) to have a continuous quantity of gaseous product available to the customer at the point of use at pressures exceeding those obtainable by using a standard cryogenic storage container. Normally the product, as it is withdrawn from the storage vessel, passes through a heat exchanger to raise the temperature to that of ambient. If the gas is to be delivered to the point of use at pressures above the storage vessel pressure, the internal pressure of the gas must be raised by means of an expensive warm gas compressor.
The most common method of solving the problem of delivery of continuous quantities of product gas at high pressure is to store the product as a gas at high pressure and ambient temperature in stationary or mobile gas storage vessels. One type of vessel is the well known high pressure tube, or banks of tubes, adapted for being towed to the site in the form of a tube trailer. The storage pressure for this type of product is higher than the use pressure and is regulated to the use pressure by conventional pressure regulators. This method presents the problems of filling the stationary vessels or replacing the mobile ones as they approach use pressure. Also, the total quantity of product available "on-site" above use pressure is typically small unless a large number of costly high pressure tubes or tube trailers are used.
Another method is to store the product as liquid in the low pressure container and pump the liquid, batch-wise, to high pressure (exceeding use pressure). High pressure storage vessels are also required to store the gaseous product prior to usage. The method of liquid feed to these pumps can induce cavitation or vapor choking which is damaging to the unit, and is potentially dangerous in flammable or oxidizing gas service. Also, this type of pump tank system promotes excessive product venting due to repeated cooldowns, thermal stratification of the liquid in the tank and partial losses of insulating vacuums in the conventional cryogenic storage tank and associated plumbing insulation, these problems being especially acute where the customer has a low usage rate for the product.
Another method of solving the problem is to pressurize warm gaseous product to the required use pressure from a low pressure storage container using warm gas compressors. This method requires larger equipment, with capital costs 5 to 10 times higher than cryogenic pressurization, due to the low density of the compressor feed. Operating costs are considerably higher for maintenance and electricity and cooling water is necessary for the compressor. A much smaller pressure ratio is achieved per stage in the compressor since the feed is at ambient temperatures so more stages may be necessary to reach a desired discharge pressure versus that which would use cryogenic equipment.
One other method used to solve the problem involves constructing a special cryogenic container to hold high pressure product and regulate the product to use pressure. This method requires the use of special tanks which are extremely expensive, have a practical pressure limit (about 750 psig) for any storage advantage over warm high pressure vessels, require special filling equipment or extreme venting to enable filling thus taking the system off-line, elaborate or extensive pressure building systems all leading to problems in protecting the tank from over-pressure scenarios or situations. Also such high pressure cryogenic containers may exceed the fluid's critical pressure, raising concerns with the vessel's safety vent system for use with supercritical fluid.