Patent Application
20080127676
The present invention is directed to the cryogenic separation of air by distillation for the production of primarily gaseous nitrogen.
Nitrogen is among the most heavily produced and used chemicals. It finds application in the petroleum, glass, foods, electronics, pharmaceutical, and metals industries. Cryogenic separation of air is a principal means of producing nitrogen.
Cryogenic air separation plants, chiefly for the production of gaseous nitrogen, exist in a number of configurations. These, in turn, group around single distillation column and double distillation column designs. There are many variations of these designs in each category. In most cases the objective is to produce nitrogen at the lowest energy consumption for any given delivery pressure; but aspects such as capital cost and particular features of convenience are equally important.
A simple single-column system has a relatively low nitrogen recovery, the balance of the air being discharged as an impure product containing a substantial amount of nitrogen. Means have been suggested in more complex designs for increasing the nitrogen recovery in such systems and reducing the amount of energy required per unit of product nitrogen.
Two-column systems have inherently greater nitrogen recoveries than simple single-column systems. Nevertheless, simple two-column systems do not necessarily have lower unit energy requirements than improved single column systems. Well-designed systems of either configuration compete for lowest unit energy consumption. The elements of energy consumption, capital cost, and particular convenient features remain important considerations.
Mostello (U.S. Pat. No. 6,330,812) presents a double column process for nitrogen, which because of a lower nitrogen delivery pressure, must turboexpand an intermediate stream which does not re-enter the distillation process. The example in U.S. Pat. No. 6,330,812 shows a nitrogen recovery on air of 58% at a nitrogen delivery pressure of 72 psia (4.97 bar(a)). Cheung et al. (U.S. Pat. No. 5,098,457) presents several processing alternatives utilizing a double column for nitrogen for nitrogen delivery pressures below 150 psia (10.34 bar(a)). Tables 2 and 3 in U.S. Pat. No. 5,098,457 have nitrogen recoveries on air of 56.5% at 102 psia and 54.9% at 102 psia, respectively.
It is usually expected that direct delivery of nitrogen from the distillation unit at higher pressures will result in lower nitrogen recovery. The current invention, which directs air and intermediate streams to both distillation columns (which is not practiced in either U.S. Pat. No. 6,330,812 or U.S. Pat. No. 5,098,457) achieves 62% recovery at a nitrogen delivery pressure of 164.6 psia (11.35 bar(a)).
An object of the invention is to provide a process for a two-column cryogenic distillation of air which achieves high nitrogen recovery, low unit energy consumption, and, though nitrogen is produced by each distillation column operating at different pressures, the product gaseous nitrogen may be delivered at a single pressure, a desirable and convenient feature, while maintaining high nitrogen recovery and low unit energy consumption.
Double distillation column systems which are designed to produce principally nitrogen have the following requirements:
It can be seen then that such a system described above becomes easier to effect as the pressure difference between the high pressure column and the reduced pressure derived from the bottoms product from the low pressure column becomes greater.
Another feature desirable but not essential to such processes is the recovery of all or most of the nitrogen at the pressure of the high pressure column, where part of the reflux made in the low pressure column condenser is pressurized and returned as additional reflux to the high pressure column.
The current invention allows turboexpansion of a stream, which is a product of the high pressure column, to subsequently undergo another separation process in the low pressure column. This provides high recovery of the nitrogen product at relatively high pressure in an energy-efficient process.
The low pressure column condenser coolant can operate just above atmospheric pressure. This is an advantage, since a more complete separation by distillation is the expected effect of operating the low pressure column at lowest possible pressure.
Referring to
After the air 105 leaves the main heat exchanger, it enters the bottom section of the high pressure column 13. The high pressure distillation column is composed of trays or packing to effect mass transfer between the rising vapor and the downflow of liquid. The vapor becomes richer in nitrogen as it rises. The residual oxygen content of the vapor 115 at the top of the column can be below 1 part per billion or higher.
Part of the nitrogen vapor is condensed in condenser 15 in indirect heat transfer with a coolant for return to the column as reflux stream 114, i.e. the liquid column flow which scrubs the oxygen out of the rising vapor. The balance of the nitrogen vapor 108 is removed from the high pressure column for warming in heat exchanger 11 and delivery as product 103 at pressure or to be further compressed in a product compressor.
The liquid bottoms product 111 from the high pressure column is composed of oxygen, nitrogen, and argon, and is typically termed “rich liquid” or “crude oxygen”. The rich liquid enters subcooler 19 and is divided into the coolant stream 116 which is routed to the nitrogen condenser 15 and a feed stream 124 to the low pressure column 20.
Rich liquid 116 is throttled across valve 14 to a pressure low enough to reduce its vaporization temperature below the condensing temperature of nitrogen and enters condenser 15 where it is vaporized, as nitrogen vapor is condensed to make reflux for the high pressure column. The vaporized rich liquid stream 118 is warmed in main heat exchanger 11, before being turboexpanded in turboexpander 12. The turboexpander exhaust 122 is introduced into the low pressure column 20.
The low pressure column 20 is a mass transfer device, also constructed of trays or packing, processing liquid and vapor streams, as described above. Feed stream 124 is fed to an intermediate point in the low pressure column where part of its nitrogen content is stripped out by the vapor 122 introduced at the bottom of the low pressure column. The resulting liquid 123 reaching the bottom of the low pressure column is transferred to the condenser of the low pressure column after being subcooled in subcooler 19 and reduced in pressure at valve 23. This stream serves as the coolant for condensing the nitrogen overhead vapor from the low pressure column in condenser 24. The vaporized coolant 127 is passed through subcooler 19 and main heat exchanger 11, which recover its refrigeration, and may be used for regeneration of the air purification adsorber, for instance.
Preferentially, all the nitrogen vapor 128 which is produced in the low pressure column is condensed. Part of the condensate is returned as reflux to the low pressure column; and the remainder 125 is pumped by pump 22 to the pressure of the high pressure column, passed through subcooler 19, and injected into the high pressure column as additional reflux.
A process for the recovery of substantially pure nitrogen at a rate of 1493 kg moles/hr at a pressure of 11.35 bar(a) is conducted in accordance with
A feed air flow of 2408 kg moles/hr was compressed, aftercooled to about ambient temperature, its water condensate removed, and passed to an adsorption unit for removal of water and carbon dioxide, and possibly other contaminants. The purified air 101 at 11.96 bar(a) was passed to main heat exchanger 11 where it was cooled to approximately its dew point. Air 105 entered the bottom of high pressure column 13 at −162.0° C. and 11.89 bar(a). The high pressure column is internally made up of distillation trays or structured packing for mass transfer.
Gaseous nitrogen 115 at −167.3° C. and 11.42 bar(a) exited from the top of the high pressure column, and a portion 108 was forwarded to main heat exchanger 11, where it was warmed to ambient temperature. Nitrogen product 103 exited the plant at 11.35 bar(a) with an oxygen content of 1 ppb (parts per billion by volume). The product constituted a 62% recovery based on the total air delivered to the cold box.
The balance of the gaseous nitrogen which exited from the top of the high pressure column was condensed in condenser 15 and returned to the top of the high pressure column as reflux 114.
The bottoms liquid product 111 exited from the high pressure column and had an oxygen concentration of 34.3%. This stream was subcooled to −168.0° C. in subcooler 19 and then divided. The first part 116 at a flow rate of 918 kg moles/hr was throttled in valve 14 to 6.00 bar(a) and was passed to condenser 15, where it served as coolant and was vaporized as stream 106. The second part 124 at a flow rate of 545.3 kg moles/hr was throttled via valve 21 to 3.3 bar(a) before entering an intermediate point in the low pressure column 20. Stream 106 was warmed in main heat exchanger 11 to −165.0° C. and passed to turboexpander 12 for expansion to 3.43 bar(a) and −177.2° C. The exhaust stream 122 then was introduced at the bottom of the low pressure column 20.
The bottoms liquid product 123 from the low pressure column was subcooled in subcooler 19, throttled via valve 23 to 1.135 bar(a), and introduced as coolant of condenser 24. The vaporized coolant 127 had a flow rate of 915.1 kg moles/hr and contained 55.1% oxygen. The nitrogen vapor 128 flow rate to condenser 24 was 1071 kg moles/hr and was totally condensed and a portion was returned to the low pressure column as reflux. The remaining liquid nitrogen 125 at a flow rate of 546.2 kg moles/hr was first passed to pump 22, which pumped the liquid to the pressure of the high pressure column. Stream 113 was then warmed in subcooler to −170.6° C. and added to the reflux flow of the high pressure column.
It is possible to produce a small amount of liquid product by withdrawing a liquid nitrogen stream to storage from either column, e.g. stream 132. It is also possible to add liquid nitrogen to either column, to assist in supplying the refrigeration needs of the plant, e.g. stream 133.
While particular embodiments of this invention have been described, it will be understood, of course, that the invention is not limited thereto, since many obvious modifications can be made; and it is intended to include within this invention any such modifications as will fall within the scope of the invention as defined by the appended claims.
