ASU unit from a complete Methanol facility (IPP stock #600993)
This 150 mt/day oxygen air separation unit designed by Air Products, supplies oxygen to the ATR unit.
Oxygen consumption for ATR reformer (with all 3 reformers in operation) is .2940 ton on O2 (100%) per ton of methanol. Specific oxygen consumption per ton of methanol considering just the ATR working is 0.588
Main Equipment Include:
The air plant built under this project is a gas producing, low-pressure cycle plant with molecular sieve front-end cleanup for water and CO2 removal. Two-column distillation to produce 99.5% pure oxygen at 6 bar. Refrigeration to 40° F is furnished by a freon glycol unit used to precool the air to molecular sieve driers. Low-level refrigeration is provided by expanding an impure nitrogen stream through a blower-loaded centrifugal expander.
Front-end air compression is provided by a Demag four-stage centrifugal compressor driven by a 3500 hp Westinghouse motor.
Final pure oxygen compression is provided by a three-stage Sulzer reciprocating compressor. This oxygen plant also produces about 390 mt/day of pure nitrogen at just above atmospheric pressure.
The oxygen and nitrogen products of the plant are obtained by liquefying air and then distilling it in a double column. Before liquefaction occurs, the water and carbon dioxide are removed by molecular sieve driers. These impurities are discarded to atmosphere during reactivation. Hydro¬ carbon impurities are removed to a safe level by adsorption after liquefaction.The following subparagraph describes the methods used to cool the air to liquid temperatures. a. Isothermal Compression. Isothermal compression is accomplished by cooling the air after each stage of compression so that the temperatures of the air before and after compression are essentially the same. Heat energy is removed from the air during isothermal compression by the cooling water flowing through the compressor coolers. This heat is then discarded to atmosphere at the cooling water towers.b. Joule - Thomson Expansion. Joule - Thomson expansion occurs whenever the pressure of fluid is lowered, without extracting work from the fluid. The expansion from higher pressure to a lower pressure does not remove heat energy from the fluid. However, it does lower the temperature of the fluid so that it can be used as a coolant. If the expanded gas is returned through a heat exchanger and permitted to exchange heat with a warmer stream, it removes heat energy from the warmer stream and carries it out of the exchanger. As previously mentioned, any pressure drop taken in the plant, including friction losses in piping, contributes to the Joule - Thomson cooling. However, most of the expansion is taken across throttling valves so that the pressure drop can be controlled.
Joule - Thomson expansion occurs whenever a gas or a liquid is expanded to a lower pressure and the amount of cooling realized depends on the pressure drop taken, on the composition of the fluid, and on the temperature of the fluid before expansion. In most cases, the cooling effect increases as the temperature of the gas before expansion decreases.
In the case of liquids, expansion to a lower pressure decreases the boiling point of the liquid. If the liquid is at its boiling temperature before expansion, some of the liquid immediately flashes or vaporizes when expanded. The vaporization removes enough heat from the body of liquid to lower its temperature to the new boiling point.
c. Expansion Through Machines. Expanding pressurized gases through a machine cools a gas similarly to that described for Joule - Thomson expansion. However, in addition to lowering the temperature of the gas, expansion through a machine also extracts heat energy from the gas by making it perform work to drive the machine. As a result, the gas expanded through a machine has a greater cooling capacity than a gas expanded by Joule - Thomson expansion. This is because in warming, it will absorb an amount of heat equal to the heat it gave up by driving the machine, plus the amount of heat that it would have gained by warming if it had been expanded by Joule Thomson expansion.
d. Using Heat Exchangers to Accumulate Refrigeration During Cooldown. Cooldown of the plant progresses in stages since no single expansion of air would drop its temperature low enough to liquefy it. However, by using heat exchangers to exchange heat between the incoming air and the expanded low-pressure air, the cooling realized by each stage of expansion is retained in the plant until finally the high-pressure air gets cold enough to form liquid when it is expanded. AIR PLANT FLOW DESCRIPTION.Atmospheric air is compressed and sent through a drier system which removes water and CO2. The drier system consists of a freon cooled glycol unit, a separator, and parallel drier cylinders. The freon unit precools the saturated air from the compressor aftercooler to about 40°F, and by doing so condenses water which is dropped out in a separator before the air enters the drier.
The air then passes downwards through the heat exchanger cores and enters the high-pressure column. A small amount of the air leaves the column and passes through the guard adsorber heat pump. This air condenses by giving up its heat to boil up pure liquid oxygen in the heat pump. The boiling action provides the motive force for circulating liquid oxygen through the guard adsorber and returning it to the low-pressure column.
Most of the air entering the high-pressure column passes up through the column trays and boils up the liquid in the trays. The net effect of the boilup is that the more volatile nitrogen is boiled out of the liquid and the exchange of latent heat condenses the oxygen out of the air passing up through the trays. As a result, the liquid in the lower trays of the column becomes enriched with oxygen, while the vapors passing up to the top of the column steadily increase in nitrogen purity until finally the vapors rising from the top tray of the column are high-purity nitrogen vapors.
Some of the nitrogen vapors pass up through the 07.22 column reboiler where they are con¬densed by boiling up the pure oxygen liquid in the sump of the low-pressure column. The heat exchanged across the reboiler serves two purposes. First, by boiling up the liquid in the sump of the low-pressure column, vapor is generated for distillation in the column. Secondly, the liquid formed by the condensation provides liquid nitrogen for refluxing the high and low-pressure columns.
The reflux returned to the high-pressure column spills back down through the trays of the column to replenish the liquid nitrogen boiled away by the air. This action effectively provides a condensing medium for the oxygen in the air and keeps the oxygen from rising above the lower trays of the column.
The low-pressure column reflux is taken from the pure liquid nitrogen stream leaving the 07.22 reboiler. This stream is subcooled in the 05.51C subcooler and expanded across the HIC-604 valve and spilt into the top of the low-pressure column.
Crude liquid oxygen is withdrawn from the sump of the high-pressure column and expanded into the low-pressure column as feed for that column. (This stream is joined with the liquid formed in the 05.50 heat pump previously described). Before expansion, the stream is subcooled and passed through one of the 08.20 crude adsorber cylinders where hydro-carbons are removed. After expansion and before entering the low-pressure column, the stream is passed through the 05.51C subcooler where it is used to subcool the low-pressure column reflux stream.
An impure nitrogen gas sidestream is also taken from the high-pressure column. This stream is taken through the cold end cores of the exchangers where it is warmed to about -155°F and then expanded through the l 0.10 expander. The flow described is the major refrigeration flow of the plant. After expansion, the impure nitrogen is put back through both sets of exchangers cores and withdrawn from the plant as impure N2 product.
The distillation in the low-pressure column is essentially the same as that described for the high¬ pressure column. However, the low-pressure column yields two pure product streams and one waste stream. Pure gaseous oxygen is obtained from the bottom of the column and pure gaseous nitrogen is obtained from the top of the column. Both streams are warmed as they leave the plant in order to recover refrigeration. The oxygen goes directly to the 05.40 exchangers and then on to the 01.70 compressor. The pure nitrogen, which is at about -3l6°F, is first warmed to about -285°F in the 05.51 subcooler and then sent through the 05.40 exchangers. Upon leaving the exchangers, the pure N2 goes to the header feeding the facility 01.40 and 01.80 nitrogen compressors.
The waste N 2 stream from the low-pressure column is also prewarmed in the 05.51 subcooler and then warmed to +50° F in the exchangers before leaving the plant. This stream is used for drier reactivation with any excess being vented to atmosphere.
Passing the pure and waste nitrogen streams through the 05.51 subcooler serves two purposes. First, by subcooling the low-pressure column feed and reflux streams, there is less flashing when the liquid enters the low-pressure column. At the same time, the feed and reflux streams absorb refrigeration from the effluent streams and return or keep this refrigeration in the column. If the sub¬ cooler was not used, this 1drigeration would return to the exchangers where it would condense some of the air and open up the warm end delta T at the exchangers.
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Product Purity (%) Flowrate (*scfh) Temp. (°F) Press. (psia) **Case I **Case II
*Scf @ 68°F, 14.7 psia
** Case I Case II
Cooling Water Temp. 60°F 82°FDry Bulb Temp. 60°F 82°FRelative Humidity 60% 55%Air Flow 67,700 #/hr 66,100 #/hr.Atmospheric Press. 14.35 psia 14.35 psiaCompressor Disch. Press. 107 psia 107 psia
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