At today's prices, natural gas becomes more and more expensive for use in pressure maintenance of oil reservoirs. The high cost of boosting declining reservoir pressure and production can be reduced through the substitution of nitrogen for natural gas.
Nitrogen is available in unlimited quantities from the air and can be produced in an Air Separation Unit (ASU), by cryogenic distillation of air, to a purity of 99.995 Mol %.
Despite various other efforts to produce nitrogen in large quantities and high purity by other methods, the low temperature air separation process of today is still the most economical. If the market is available, it also allows for the production of the by-products of oxygen and rare gases, which are also contained in the air. In the low temperature air separation process, the air is first compressed, then purified, removing water vapour and carbon dioxide, and cooled down to liquefaction temperature. Since industrial air is also contaminated with hydrocarbons these dangerous components in an ASU have to be removed before the rectification takes place to obtain the required components. The removal of the water vapour and the carbon dioxide from the air is accomplished by one of the following processes:
Molecular Sieve Plants,
Reversing Heat Exchanger Plants (Revex Plants),
or Regenerator Plants.
ASU's differ very much from each other not only regarding type, purity, quantity, physical condition of the desired products (gaseous or liquid) but also in relation to varying conditions of energy, cost of energy, capital cost and costs of operating and maintenance personnel.
Besides this, plant owners expect to get the desired products with a minimum of energy demand, the lowest. capital investment and the most reliable plant with the minimum operating and maintenance costs. Therefore, it is not always easy to find the optimum solution in each specific application of an ASU. All these above mentioned criteria are also valid in Nitrogen-Generators when only Nitrogen for enhanced oil recovery is produced. Since the Nitrogen is produced most economically by the low pressure process the product has to be compressed to the necessary injection pressure of the oil reservoir. The required amount of Nitrogen, the injection pressure, and the injection scheme has to be determined by the reservoir engineers, and are not discussed in this paper.
In the following it is proposed to discuss various types of ASU's and their basic components for the air purification as well as the economics of a Nitrogen Generator.
All cryogenic ASU's can be subdivided independent of the type of plant into the following sections:
Air compression
Removal of H2O and CO2
Heat exchange
Refrigeration and
Air separation.
In Revex and Regenerator Plants the removal of water and carbon dioxide is combined with the heat exchange, but in a Molecular Sieve Plant these two processing steps are conducted separately.
The schematic flow diagram of a molecular sieve plant can be seen from Fig. 1.
