Technologies
Since the commercialization of the Linde-Frankle process in the 1920s, oxygen supply has been dominated by cryogenic technology.
The principle features of cryogenic air separation are shown in Figure 8-1. Air is compressed, dried in a prepurification unit, and then cooled to its liquefaction temperature. The liquid air is then distilled into its two main constituents, oxygen and nitrogen. These separated products are then heated and vaporized. This basic flow scheme has formed the basis for all processes, or cycles as they are known, to this day, although many detail improvements have been made over the years to decrease costs and improve efficiency. In addition to these improvements, the size of air separation units has risen dramatically over the last 40 years. The 1400t/d (40,000 Nm3/h) ASU supporting a gasification plant with 65 t/h visbreaker residue
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feed that came on stream in 1972 was at that time the largest in the world (Butzert 1976), but there are several plants now running with a capacity of about 90,000 Nm3/h (3200 t/d), including for instance that at Rozenburg, The Netherlands, which among others provides the oxygen for the Shell Pernis gasifiers.
The operating pressure of modern plants varies with the application. Low-pressure (LP) cycles, which supply by-product gaseous nitrogen at only atmospheric pressure, operate at about 5-7 bar, depending on oxygen pressure and required energy efficiency. Where much of the nitrogen is required at higher pressures, it can be advantageous to operate the ASU at elevated pressure (EP) above this level. This is particularly appropriate in an IGCC with air and N2 integration, since the air is in any case available from the gas turbine compressor at the higher pressure. Operating at the higher pressure also has the advantage of tending to reduce equipment size and cost.
The prepurification unit (PPU) has seen substantial changes since the original Frankie regenerators. After a period when most plants were equipped with reversing exchangers, in which water and C02 were frozen onto the surfaces of the main heat exchangers and evaporated off in the regeneration part of the cycle with waste nitrogen, the 1970s saw the introduction of molecular sieves for prepurification. Most modern PPUs have twin beds of silica gel and molecular sieve. They hold back not only water and C02 but also potentially dangerous hydrocarbons that may be in the atmosphere.
The partially liquefied air enters the lower, high-pressure column where pure nitrogen is drawn off as overhead product. The bottoms, an oxygen-rich liquid, is expanded through a turbine into the upper, low-pressure column, where in the simplest cycles, pure oxygen is drawn off as bottoms product. The overhead is an impure nitrogen stream, which like the main products is used for chilling the incoming air before being used as regeneration gas in the PPU and then being discharged to the atmosphere. Where large quantities of pure nitrogen (<10vppm 02) are required as in ammonia plants, a reflux can be added to the top of the LP column and some of the impure nitrogen also recovered as pure product. Typically, the maximum amount of pure product obtainable is about 70% of the incoming air.
Production of argon is possible by tapping the middle of the LP column, where the argon concentration is highest, and adding a further distilling stage. Final purification of the argon takes place by the catalytic reduction of final traces of oxygen with hydrogen. This additional processing stage requires a higher operation pressure of the ASU at the upper range of LP cycles.
For the production of pressurized oxygen, two cycles are used: the compression cycle, and the pumped liquid cycle. In the former gaseous oxygen leaves the cold box at slightly above atmospheric pressure and is compressed in a compressor. Alternatively, the liquid oxygen can be pumped up to the required pressure and the vaporized under pressure. This latter cycle, also known as internal compression, required when introduced about 5-7% more energy, but cycle development has reached the point where there is little difference between compression and pumped liquid cycles.
Air separation has a widely recognized reputation for reliability. This is important for the gasification process, since oxygen production is at the beginning of the flowsheet, and loss of oxygen brings the whole downstream facility to a standstill. Traces of water or C02 can slip past the PPU and over a period of time freeze out on the heat exchangers. If this goes beyond a tolerable limit for the operation, then the coldbox must be reheated to ambient temperature and derimed. Typically, under normal operation this may be necessary every two years. The deriming itself may take about one or two days.
The vulnerability of a gasification plant to interruptions of oxygen supply makes the consideration of building some liquid storage capacity an important issue for the planning of any gasification project. The only economic method of oxygen storage is as a liquid at low temperature. Typically, under such conditions a boil-off of some 0.2-0.5%/d, depending on the size of tank, must be expected. The principle aspects to be considered are the response time in which oxygen from the storage is available at the gasifier (seconds), and the size of the storage. For the latter consideration, some hours worth of storage to cover a compressor trip and restart would be a minimum. A storage volume to cover a coldbox deriming period is unrealistic.
Alternative Processes for Small Quantities
For small units, other processes are available. They cannot, however, reach the purity obtainable with a cryogenic unit. Pressure swing adsorption units are available up to a capacity of about 140 t/d (4000Nm3/h), but they can only reach a purity of about 95% 02. The product purity obtainable with polymer membrane technology is much less—about 40% 02—and such units are available for capacities of 20 t/d (600Nm3/h). The by-product capability of both these technologies is poor, but they both have the advantage of quick start-up compared with cryogenic units (Smith and Klosek 2001). Given that most small gasification facilities are for chemical applications where even 5% nitrogen in the oxygen is unacceptable, their use in connection with gasification is likely to be limited. The most probable gasification application could be with biomass power applications where sizes are also at the lower end of the scale.
Oxygen production by means of ion transport membranes is the subject of intense research and development (Allam, Foster, and Stein 2002).
The principle of these devices is based on the use of nonporous ceramic membranes that have both electronic and oxygen ionic conductivity when operated at high temperatures, typically 800 to 900°C (Figure 8-2).
Oxygen from the feed side of the membrane adsorbs onto the surface of the membrane, where it dissociates and ionizes by electron transfer from the membrane. The oxygen diffuses through the lattice of the membrane, which is stoichiometrically deficient in oxygen. The driving force is the differential partial pressure of oxygen across the membrane. Oxygen ions arriving at the product side of the membrane release their electrons, recombining and desorbing from the surface as molecular oxygen. Electrons flow in counter current from product to the feed side of the membrane.
This ion-transport mechanism is specific to oxygen so that, discounting leakage through any cracks or seals and so on, the product is 100% oxygen.
Considerable work has been performed in scale-up and demonstration of production facilities (Armstrong etal. 2002). Integration with both the gasification and/or the combustor of the gas turbine is mandatory for its success. Commercialization is currently expected “in the 2006 to 2008 time frame.”