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 portions, at least one of which does not enter the low pressure column as a feed stream. By these means, a portion of an oxygen-rich stream is removed from the distillation, further enhancing nitrogen recovery and achieving low specific energy consumption for nitrogen product.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    This application is entitled to the benefit of Provisional Patent Application Ser. No. 60/186,572 filed Mar. 2, 2000.  
         [0002]    This is a Continuation-in-part of Ser. No. 09/775,362, Feb. 1, 2001, now abandoned. 
     
    
     
       FIELD OF THE INVENTION  
         [0003]    The present invention is directed to the cryogenic separation of air by distillation for the production of primarily gaseous nitrogen.  
         BACKGROUND ART  
         [0004]    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.  
           [0005]    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.  
         OBJECT OF THE INVENTION  
         [0006]    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 is delivered at a single pressure, a desirable and convenient feature, while maintaining high nitrogen recovery and low unit energy consumption.  
         SUMMARY OF THE INVENTION  
         [0007]    Double distillation column systems which are designed to produce principally nitrogen have the following requirements:  
           [0008]    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.  
           [0009]    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.  
           [0010]    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.  
           [0011]    It can be seen then that such a system described above becomes easier to effect as the pressure ratio of the pressure of the high pressure column to the pressure of the stream vaporizing in the condenser of the low pressure column becomes greater. This pressure ratio, when coupled with the quantity of nitrogen actually recovered, has a direct impact on the requisite energy to produce a nitrogen product at a given delivery pressure. A greater pressure ratio indicates a higher energy consumption for a given product delivery pressure than other processes which have lower corresponding pressure ratios. For energy reduction, improvements in this process strive to reduce this pressure ratio and the related pressure ratio of the pressure of the high pressure column to the pressure of the low pressure column.  
           [0012]    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.  
           [0013]    The current invention improves on this process by conducting the condensation of vapors at the pressure of the high pressure column, all of which may be the overhead vapor from the high pressure column, in at least two stages of coolant vaporization in series. The composition of the boiling stream becomes richer in oxygen as the extent of vaporization increases. At essentially a constant temperature of vaporization, the first stage of vaporization occurs at a higher pressure of the vaporizing stream and the second stage at a lower pressure of the vaporizing stream. The vapor from the first stage is both richer in nitrogen and higher in pressure than the vapor from the second stage, and constitutes a feed to the low pressure column. Therefore, the pressure of the low pressure column is maximized—a desirable effect for a given high pressure column pressure, and oxygen is preferentially rejected from the column system from the second stage condenser. Because the composition of the liquid bottoms from the low pressure column are related to the composition of the vapor feed to the bottom of the low pressure column, these bottoms are richer in nitrogen and vaporize at a colder temperature when transferred to the low pressure column condenser and reduced in pressure, which reduces the ratio of the pressure of the high pressure column to the pressure of the low pressure column. The low pressure column condenser coolant can operate just above atmospheric pressure; but in alternative designs may operate at higher pressure, retaining the energy-reduction benefits of the invention. The effects of reducing the pressure ratio of the operating pressures of the two distillation columns, and rejecting an oxygen-rich mixture from the second or last stage of the high pressure column condenser lead to lower compression energy and higher nitrogen recovery, which minimize unit energy expenditure for the nitrogen produced at a specified delivery pressure. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 is a schematic representation of one embodiment of the invention.  
         [0015]    [0015]FIG. 2 is a schematic of another embodiment of the invention which has the capability to generate more refrigeration and entails more capital cost than the embodiment of FIG. 1. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]    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. In some cases a small part of the air  105  may be liquefied and may be removed separately from the balance of the air which remains in vapor state. A reheat stream  106  composed of a second waste nitrogen stream also enters the cold end of the main heat exchanger and is partially warmed, before being withdrawn as  110  for expansion in turboexpander  12 .  
         [0017]    After the air 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 at the top of the column can be below 1 part per billion or 0.5% or higher. Part of the nitrogen vapor is condensed in condensers  15  and  18  in indirect heat transfer with a coolant for return to the column as reflux streams  114  and  115 , i.e. the liquid column flow which scrubs the oxygen out of the rising vapor. The balance of the nitrogen vapor  129  is removed from the high pressure column for warming in heat exchangers  19  and  11  and delivery as product  103  at pressure or to be further compressed in a product compressor.  
         [0018]    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 condensers  15  and  18  and a feed stream  124  to the low pressure column  20  after further subcooling in subcooler  19 .  
         [0019]    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 partially vaporized, as nitrogen vapor is condensed to make reflux for the high pressure column. Rich liquid  116  is partially boiled in condenser  15  and liquid and vapor phases are separated in separator  16 . The residual liquid from condenser  15  has a higher oxygen content than the rich liquid feed to condenser  15 . In order to vaporize the balance of this residual rich liquid, its pressure and temperature must be lowered still by throttling valve  17  which passes the residual rich liquid to condenser  18 , where it is all or nearly all vaporized. Nitrogen vapor from the high pressure column is also condensed in condenser  18  and becomes part of the reflux to the high pressure column.  
         [0020]    The vaporized rich liquid from separator  16  is fed to the bottom of the low pressure column  20 . This rich liquid vapor was vaporized at essentially the operating pressure of the low pressure column. The balance of the rich liquid which was passed to condenser  18  is vaporized, is partially warmed in subcooler  19  and main heat exchanger  11  and turboexpanded in  12  to produce refrigeration. The turboexpander exhaust gas  109  is warmed in subcooler  19  and main heat exchanger  11  and may be used elsewhere or vented to atmosphere. This is a stream of elevated oxygen content; and therefore, its disposition in this manner assists in the separation of the air to make the nitrogen product.  
         [0021]    The low pressure column  20  is a mass transfer device, also constructed of trays or packing, and processing liquid and vapor streams, as described above. The part of the rich liquid stream  124  fed to an intermediate point in the low pressure column has part of its nitrogen content stripped out by the vapor rising from the bottom of the low pressure column. The resulting liquid reaching the bottom of the low pressure column  123  is transferred to the condenser for 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 subcoolers  19  and main heat exchanger  11 , which recover its refrigeration, and may be used for regeneration of the air purification adsorber, for instance.  
         [0022]    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.  
         [0023]    Another embodiment of the invention is shown in FIG. 2. In this embodiment three condensers are employed for condensing reflux liquids primarily for the high pressure column. The purpose of such an arrangement is to vaporize the last portion of the rich liquid coolant  116  utilizing air as the heating medium in condenser  31 . In so doing, since air at approximately the pressure of the high pressure column  33  condenses at a higher temperature than nitrogen at the pressure at the top of the high pressure column, the last portion of rich liquid  209  which vaporizes in condenser  33  can vaporize at a higher pressure by being heated against air than against nitrogen. A higher pressure stream  208 , composed of streams  206  from condenser  34  and  207  from condenser  31 , is available for turboexpansion and production of additional refrigeration, for instance, for achieving a greater production of liquid nitrogen product, if desired.  
         [0024]    Liquid air  203  produced in condenser  31  is routed principally to the high pressure column  33  for assisting the distillation there. Depending on overall distillation requirements, some liquid air may be routed to low pressure column  20 .  
         [0025]    In other respects the process embodiment in FIG. 2 is similar to that of FIG. 1. Still another embodiment of the invention (not shown) achieves elevation of the low pressure column pressure and the high pressure column pressure by means of elevation of the vaporization pressure of the low pressure column condenser coolant. In these cases said vaporized coolant may also be turboexpanded to produce refrigeration. Another advantage of such operation is that the delivery pressure of the nitrogen product from the high pressure column can be efficiently raised to meet a specified product delivery pressure, while maintaining low energy requirements inherent in the process invention.  
       EXAMPLE  
       [0026]    A process for the recovery of substantially pure nitrogen at a rate of 2687 Nm3/hr at a pressure of 4.9 atma is conducted in accordance with FIG. 1. Nm3/hr refers to the flow rate of a substance measured as a gas at 0 C. and 1 atma. C. refers to temperature in degrees Celsius; atma refers to pressure in absolute atmospheres. K refers to temperature in degrees Kelvin.  
         [0027]    A feed air flow of 4632 Nm3/hr was compressed to a pressure of 5.2 atma, 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  was passed to main heat exchanger  11  where it was cooled to approximately its dew point, producing a small amount of liquid. Air  105  entered the bottom of high pressure column  13  at 98.6 K and 5.05 atma. The high pressure column is internally made up of structured packing for mass transfer.  
         [0028]    Gaseous nitrogen at a 94.1 K and 5.0 atma exited from the top of the high pressure column, and a portion was forwarded to subcooler  19  where it was warmed to 95.4 K, and further warmed in main heat exchanger  11  to ambient temperature. Nitrogen product exited the plant at 4.9 atma with an oxygen content of 5 vpm (parts per million by volume). The product constituted a 58% recovery based on the total air delivered to the cold box.  
         [0029]    The balance of the gaseous nitrogen which exited from the top of the high pressure column was condensed in condensers  15  and  18  and returned to the top of the high pressure column as reflux.  
         [0030]    The bottoms liquid product  111  exited from the high pressure column and had an oxygen concentration of 40%. This stream was subcooled to 96 K in subcooler  19  and then divided. The first part  116  at a flow rate of 1830 Nm3/hr was throttled in valve  14  to 3.05 atma and was passed to condenser  15 . 1058 Nm3/hr was vaporized and sent to the bottom of the low pressure column as stream  122 . The remaining liquid was throttled via valve  17  to 2.1 atma before entering condenser  18  as coolant. This remaining liquid was not totally vaporized in order to limit the concentrations of non-volatile contaminants. Stream  119  had a composition of about 51.5% oxygen. Stream  119  was warmed to 95.4 K in subcooler  19  and further warmed in main heat exchanger  11  to 120 K and passed to turboexpander  12  for expansion to 1.04 atma and 101.7 K. The exhaust stream  109  then was passed to the main heat exchanger where it was warmed to about ambient temperature.  
         [0031]    The second part of rich liquid stream  111  was further subcooled to 91.9 K and stream  124  was reduced in pressure by valve  21  and fed to the low pressure column  20 .  
         [0032]    The bottoms liquid product  123  from the low pressure column was subcooled in  19 , throttled via valve  23  to 1.2 atma, and introduced as coolant of condenser  24 . The vaporized coolant  127  had a flow rate of 888 Nm3/hr and contained 49.7% oxygen. This stream was not totally vaporized in order to limit the concentration of non-volatile contaminants. The nitrogen vapor  128  flow rate to condenser  24  was 1013 Nm3/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 482 Nm3/hr was first passed to pump  22 , which pumped the liquid to the pressure of the high pressure column. Stream  125  was then warmed in subcooler to 93.9 K and added to the reflux flow of the high pressure column.  
         [0033]    It is possible to produce a small amount of liquid product by withdrawing to storage liquid nitrogen at  132 , for instance. It is also possible to add liquid nitrogen at, for instance,  132 , to assist in supplying the refrigeration needs of the plant.  
         [0034]    It is also possible to recover more than 60% of the air as nitrogen at the same pressure of feed air by modification of the operating and plant design conditions, requiring somewhat larger heat transfer equipment.  
         [0035]    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.