Abstract:
Nitrogen gas at a single pressure is produced from a two-column cryogenic distillation of air. The bottoms liquid product from the high pressure column is divided into two portions, one of which is vaporized and then turboexpanded before its entry into the low pressure column as a feed stream. By these means, no stream bypasses the double distillation process, further enhancing nitrogen recovery and achieving low specific energy consumption for nitrogen product.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention is directed to the cryogenic separation of air by distillation for the production of primarily gaseous nitrogen. 
       BACKGROUND ART 
       [0002]    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. 
         [0003]    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. 
         [0004]    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. 
         [0005]    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. 
         [0006]    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. 
         [0007]    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)). 
       OBJECT OF THE INVENTION 
       [0008]    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. 
       SUMMARY OF THE INVENTION 
       [0009]    Double distillation column systems which are designed to produce principally nitrogen have the following requirements:
       1. The condenser condensing nitrogen overheads from the high pressure column must boil a stream which boils at a temperature lower than said nitrogen condensing temperature.   2. A vapor stream resulting from the aforementioned boiled stream which enters the low pressure column for further separation must be at or above the operating pressure of the low pressure column. If the pressure of the aforementioned boiled stream is sufficiently above the operating pressure of the low pressure column, a turboexpander may be inserted in said stream to capture its available energy and generate refrigeration for the process.   3. The pressure of the low pressure column must be high enough such that at least a portion of the nitrogen overheads from the low pressure column can be condensed in a condenser against a boiling stream which boils at a colder temperature than the condensing nitrogen overheads. This boiling stream can be the bottoms liquid product from the low pressure column which is reduced in pressure upon entry into the condenser.       
 
         [0013]    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. 
         [0014]    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. 
         [0015]    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. 
         [0016]    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. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0017]      FIG. 1  is a schematic representation of the preferred embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]    Referring to  FIG. 1 , air is compressed and cooled and the water condensate removed before entering typically an adsorption unit for the removal of residual water vapor, carbon dioxide, and other amounts of trace contaminants. The air  101  then enters the main heat exchanger  11 , where it is cooled to a temperature near its dew point, while products of the subsequent distillation—pure nitrogen  108  and waste nitrogen  107  streams enter as cold vapors at the opposite end and are warmed, receiving heat from the air which is being cooled. A reheat stream  106  composed of a vapor generated by boiling coolant used for condensing overhead nitrogen from the high pressure distillation column  13  also enters the cold end of the main heat exchanger and is partially warmed, before being withdrawn as  110  for expansion in turboexpander  12 . 
         [0019]    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. 
         [0020]    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. 
         [0021]    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 . 
         [0022]    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 . 
         [0023]    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. 
         [0024]    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. 
       EXAMPLE 
       [0025]    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  FIG. 1 . kg moles/hr refers to the flow rate in kilogram moles per hour. ° C. refers to temperature in degrees Celsius; bar(a) refers to absolute pressure in bars. In the specification, psia refers to pounds per square inch absolute. 
         [0026]    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. 
         [0027]    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. 
         [0028]    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 . 
         [0029]    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 . 
         [0030]    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. 
         [0031]    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 . 
         [0032]    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.