Patent Application
20050210916
The present invention relates generally to the cryogenic separation of air into gaseous nitrogen (“GAN”) and gaseous oxygen (“GOX”) and, in particular but not necessarily exclusively, it relates to the production of GOX at low pressure and low purity.
There is a considerable market, particularly in the glass and metallurgical industries, for low purity, e.g. from 80 to 98 vol %, low pressure, e.g. from 1.5 to 5.0 bar absolute (0.15 to 0.5 MPa), GOX. The GOX is used in processes requiring oxygen-enriched combustion in which the required pressure of the oxygen at the point of use is near atmospheric.
An O2 vacuum swing absorption (“VSA”) process is commonly used for applications requiring 90 to 93 vol % O2. However, up to 98 vol % O2 GOX is often required and such high purities cannot be supplied economically by the VSA process. Cryogenic distillation processes are economic for both low and higher purity oxygen requirements. There are many prior public disclosures of processes using cryogenic distillation of air to produce a GOX product. A number of the disclosed processes use a liquid cryogen from an external source to provide at least part of the refrigeration duty for the process.
U.S. Pat. No. 5,408,831 (Guillard et al) and U.S. Pat. No. 5,505,052 (Ekins et al) both disclose single reboiler/condenser cycles for the production of GOX in which at least a portion of the refrigeration duty for the processes is provided by at least one refrigerant from an external source. For example, in Guillard et al, both liquid oxygen (“LOX”) and liquid nitrogen (“LIN”) are used and, in Ekins et al, both LOX and liquid argon (“LAR”) are used. A further single reboiler/condenser cycle is disclosed in US-A-2003/0110796.
U.S. Pat. No. 6,539,748 (Prentice et al) discloses the use of LOX from an external source to provide refrigeration in a single reboiler/condenser cycle process for the production of low purity GOX. A stream of LOX refrigerant from an external source is injected into the main heat exchanger at a pressure greater than that of the LOX entering the main heat exchanger from the distillation system. In the exemplified embodiment, the resultant vaporised LOX refrigerant is then combined with the vaporised LOX taken from the distillation system to provide a combined GOX product.
Low pressure GOX may be produced in a double cryogenic distillation column system using a dual reboiler/condenser cycle. Examples of existing dual reboiler/condenser cycle processes are disclosed in U.S. Pat. No. 3,210,951 (Gaumer), U.S. Pat. No. 4,410,343 (Ziemer) and U.S. Pat. No. 4,702,757 (Kleinberg). The processes disclosed in each of these references produce refrigeration by expansion of a process stream.
An example of an existing system is depicted in
At an intermediate point within the main heat exchanger 22, stream 34 is divided into two parts, the first part 36 is divided into two substreams. The first substream 38 is fed to the higher pressure (“HP”) column 40 of the distillation column system at a high pressure for separation into nitrogen-rich overhead vapour and crude liquid oxygen (“CLOX”). The second substream 42 is partially condensed in a first reboiler/condenser 44 located in the sump of the LP column 26 to produce a stream 46 of partially condensed air which is then fed to the bottom of the HP column 40.
A stream 48 of nitrogen-rich overhead vapour from the HP column 40 is condensed in a second reboiler/condenser 50 against vaporising CLOX removed via stream 52 from the HP column 40. The condensed nitrogen-rich overhead is fed as streams 54 and 56 to the HP column 40 and LP column 26 respectively to provide reflux for the separations. Vaporised CLOX is fed via stream 58 to the LP column 26.
Refrigeration for the process is provided by an expansion turbine 60. The second part 62 of the cooled compressed air is fed to the expander 60 where it is work expanded to produce a stream 64 of expanded air that is then fed to the LP column 26. As the expander sends part of the medium pressure air directly to the LP column 26, it reduces the air flow available to the HP column 40 and the second reboiler/condenser 50. Consequentially, distillation is impaired with reduced boil-up of LOX in the sump of the LP column 26 and reduced reflux down the LP column 26.
An example of a dual reboiler/condenser cycle using external refrigerant and an expansion turbine to generate refrigeration in a cryogenic air separation process to produce high purity nitrogen is disclosed in U.S. Pat. No. 4,668,260.
The benefit of the dual reboiler/condenser cycle is that high pressure air is at a lower pressure than in a conventional cycle as LOX in the LP column 26 is vaporised by condensing air rather than nitrogen. This difference in air pressure results in reduced power unless the air flow to produce a given flow of GOX increases. The effect of impaired distillation is to increase the air flow and hence the power advantage of the dual reboiler/condenser cycle with an expander is small when compared to a conventional cycle and unlikely to be sufficient to justify the additional complexity and cost of this cycle. The Inventors are unaware of any teaching in the prior art that the substantial power benefits of a dual reboiler/condenser cycle process can be almost fully realised if external refrigerant is employed instead of an expander to meet the refrigeration requirements of the process.
One objective of preferred embodiments of the present invention is to provide a process and apparatus for the production of low pressure GOX, in particular at low purity, with reduced power consumption (and, thus, with reduced operating cost) without any significant increase in capital cost.
The present invention provides a process for cryogenically separating air, in particular for the production of low purity GOX, in a distillation system comprising a higher pressure (“HP”) distillation column and a lower pressure (“LP”) distillation column, a first reboiler/condenser, a second reboiler/condenser and heat exchange means. The process comprises separating feed air in the HP column into nitrogen-rich overhead vapour and crude liquid oxygen (“CLOX”). At least a portion of the CLOX or crude oxygen vapour derived therefrom is separated in the LP column to produce nitrogen overhead vapour and liquid oxygen (“LOX”). Air is at least partially condensed by indirect heat exchange against LOX in the first reboiler/condenser to produce oxygen vapour and at least partially condensed air. At least a portion of the nitrogen-rich overhead vapour is at least partially condensed by indirect heat exchange against an oxygen-rich liquid in the second reboiler/condenser to produce oxygen-rich vapour and at least partially condensed nitrogen-rich overhead vapour. LOX from the LP column is vaporised by indirect heat exchange against compressed air in the heat exchange means to produce gaseous oxygen (“GOX”) and cooled compressed air. At least a portion of the at least partially condensed nitrogen-rich overhead vapour and/or at least a portion of the at least partially condensed air is used as reflux in the distillation system. The required refrigeration duty for the process is provided by introducing into the distillation system from an external source at least one refrigerant. The or at least refrigerant is preferably selected from liquid nitrogen (“LIN”) or LOX.
The invention also provides apparatus for carrying out the process. The apparatus comprises an HP distillation column for separating feed air into nitrogen-rich overhead vapour and CLOX, together with an LP distillation column in fluid flow communication with the HP column for separating at least a portion of the CLOX or crude oxygen vapour derived therefrom to produce nitrogen overhead vapour and LOX. A first reboiler/condenser for at least partially condensing air by indirect heat exchange against LOX to produce oxygen vapour and at least partially condensed air is also provided, together with a second reboiler/condenser in fluid flow communication with the HP column for at least partially condensing at least a portion of said nitrogen-rich overhead vapour by indirect heat exchange against an oxygen-rich liquid to produce oxygen-rich vapour and at least partially condensed nitrogen-rich overhead vapour. The apparatus further comprises heat exchange means in fluid flow communication with the LP column for vaporising LOX from the LP column by indirect heat exchange against compressed air to produce GOX and cooled compressed air. At least one reflux conduit means is provided in fluid flow communication with the HP column and/or the LP column for feeding at least a portion of the at least partially condensed air and/or at least a portion of the at least partially condensed nitrogen-rich overhead vapour as reflux to the distillation system. The apparatus also has at least one refrigerant conduit means in fluid flow communication with the distillation system for introducing into the distillation system from an external source at least one refrigerant. The apparatus is without an expansion turbine to expand a process stream to provide refrigeration.
According to a first aspect of the present invention, there is provided a process for cryogenically separating air in a cryogenic distillation system comprising a higher pressure (“HP”) distillation column and a lower pressure (“LP”) distillation column, a first reboiler/condenser, a second reboiler/condenser and heat exchange means, said process comprising:
One advantage of the present invention is that it is not necessary to provide refrigeration for the process by expansion of a process stream. The use of an expander incurs significant capital and operational cost and can increase energy consumption. The overall refrigeration requirement of the process is met without expansion of a process stream thereby eliminating these penalties in cost and energy consumption. The liquid and vapour flow in the HP column is increased, compared to prior art processes having expanders. For a given flow of GOX, the increase in air flow in the present invention is substantially zero when compared to the air flow in a conventional single reboiler/condenser cycle. Therefore, the power advantage of dual reboiler/condenser cycles using imported refrigerant such as LIN and/or LOX to provide refrigeration is large when compared to conventional dual reboiler/condenser cycles using expansion to provide refrigeration. This power advantage is sufficient to justify the additional complexity and cost of this cycle.
The or each refrigerant may be introduced into any suitable location in the cryogenic section of the plant. Where the or at least one refrigerant is LOX, at least a portion of the required refrigeration duty for the process may be provided by feeding LOX from an external source to the sump of the LP column. Alternatively, at least a portion of the required refrigeration duty for the process may be provided by vaporising LOX from an external source by indirect heat exchange against compressed air in the heat exchange means to produce cooled compressed air and vaporised refrigerant. It would also be possible to provide at least a portion of the refrigeration duty by carrying out both of these steps in combination. Where LOX refrigerant is vaporised in the main exchange means, the resultant GOX may be combined with the GOX produced by vaporising LOX from the LP column to produce combined GOX product.
Where the or at least one refrigerant is LIN, LIN from an external source may be introduced into the distillation column system at a location having a high nitrogen concentration. For example, LIN from an external source may be fed to the top of the LP column, the top of the HP column or to the top of both columns.
Both LIN and LOX from external sources may be used simultaneously to provide at least a portion of the refrigeration duty required by the process. However, in preferred embodiments, only one refrigerant is used.
The first reboiler/condenser is usually located in the sump of the LP column. In such embodiments, the process comprises at least partially condensing air by indirect heat exchange against LOX produced in the LP column to produce said oxygen vapour and said at least partially condensed air.
The second reboiler/condenser may be located at an intermediate location in the LP column in which case the process comprises at least partially condensing the nitrogen-rich overhead vapour by indirect heat exchange against oxygen-rich liquid descending the LP column to produce said oxygen-rich vapour and said at least partially condensed nitrogen-rich overhead vapour. Alternatively, the second reboiler/condenser may be located outside the LP column. In such cases, the process may comprise at least partially condensing the nitrogen-rich overhead vapour by indirect heat exchange against CLOX produced in the HP column to produce said crude oxygen vapour and said at least partially condensed nitrogen-rich overhead vapour.
In processes of the present invention, there is no expansion of a process stream to provide refrigeration. An advantage of these processes is that the capital and operational costs of dual reboiler/condenser cycles may be reduced if there are no expansion turbines present in the system.
The feed air may comprise at least a portion of the cooled compressed air. Alternatively or additionally, the feed air may comprise at least a portion of the at least partially condensed air. Feed air to the HP column is preferably cooled compressed air with LAIR being fed to the LP column. However, in other embodiments, the all of the cooled compressed feed air is at least partially condensed in the first reboiler/condenser and then fed to the HP column. Also, LAIR can be fed to the HP column or split between the HP and LP columns.
Reflux for the LP and HP columns may be provided by any suitable liquid stream in the process. In particular, both the HP column and the LP column may be refluxed with at least partially condensed nitrogen-rich overhead vapour.
In preferred embodiments, low purity GOX, e.g. GOX having an oxygen concentration from about 80 to about 98 vol %, preferably about 95 vol %, is produced. The pressure of the GOX is preferably from about 1.5 to about 5.0 bar absolute (0.15 to 0.5 MPa). Preferably, the pressure is from about 1.7 to about 2.3 bar absolute (0.17 to 0.23 MPa). Nitrogen overhead vapour may be removed from the HP column, warmed in the main heat exchanger and collected as a GAN product.
According to a second aspect of the present invention, there is provided apparatus for cryogenically separating air comprising:
The apparatus may be adapted and/or constructed to operate any of the preferred processes described above.
In particular, where the or at least one refrigerant is LOX, the or at least one refrigerant conduit means is adapted to carry LOX and, preferably, is in fluid flow communication with the sump of the LP column. Additionally or alternatively, the or at least one refrigerant conduit means may be in fluid flow communication with the cold end of the heat exchange means. In such embodiments, the apparatus may further comprise GOX conduit means for combining the GOX produced by vaporising LOX from the LP column and GOX produced by vaporising the LOX refrigerant.
Where the or at least one refrigerant is LIN, the or at least one refrigerant conduit means is adapted to carry LIN and, preferably, is in fluid flow communication with a location of the distillation system having high nitrogen concentration. Suitable examples of such locations include the top of the LP column and the top of the HP column.
The first reboiler/condenser is usually located in the sump of the LP column. The second reboiler/condenser may be located at an intermediate location in the LP column or may be located outside the LP column. In the latter case, the apparatus may further comprise conduit means for feeding crude oxygen vapour from the second reboiler/condenser to the LP column.
The apparatus may further comprise conduit means for feeding at least a portion of the partially condensed air as feed air to the HP column. The apparatus may further comprise, either additionally or alternatively, conduit means for feeding at least a portion of the cooled compressed air as feed air to the HP column.
The following is a description, by way of example only and with reference to the accompanying drawings, of presently preferred embodiments of the invention.
The process depicted in
The flow diagrams depicted in FIGS. 2 to 6 have many features in common with the flow diagram depicted in
In the flow diagram depicted in
In the flow diagram depicted in
In the flow diagram depicted in
In the flow diagram depicted in
In the flow diagram depicted in
The variations regarding the external refrigerant depicted in FIGS. 4 to 6 can also be applied to the embodiment of the process depicted in
Computer simulations have been carried out to compare the energy consumption of known single reboiler/condenser cycles (S1-S4) with dual reboiler/condenser cycles (D1-D3). The simulations were run on the basis of a GOX production of 3500 Nm3/h (contained) at 95 vol % O2. The results are depicted in Table 1.
(*Partial cond. means partial condensation)
Cycle S2 is a single reboiler cycle with imported LOX for refrigeration (i.e. no expander). Cycle S3 is the same cycle as cycle S2 except it uses an expander to provide refrigeration instead of imported LOX. There is a MAC/booster power increase of over 4%.
Cycle D2 is a conventional dual reboiler cycle using an expander for refrigeration. The MAC/booster power for this cycle is about 5% lower than observed in cycle S2. Cycle D1 is a dual reboiler cycle according to the present invention (i.e. uses imported LOX rather than an expander to provide refrigeration). The MAC/booster power for this cycle is about 11% lower than that for cycle S2 and about 6% lower than that for about conventional dual reboiler cycle D2.
The results indicate that the use of a dual reboiler/condenser cycle with LOX refrigeration (D1) reduces MAC/booster power consumption when compared to a standard single reboiler/condenser cycle (S1) by about 13%. This reduction in power consumption is significant as it reduces the overall cost of GOX production considerably. The estimated capital cost for the D2 and D3 processes is approximately the same as that for the standard single reboiler/condenser cycle S1. However, the estimated capital cost for the D1 process is about 2% less than that for S1. Therefore, preferred embodiments of the present invention not only reduce the cost of GOX production but also reduce the capital cost.
Throughout the specification, the term “means” in the context of means for carrying out a function, is intended to refer to at least one device adapted and/or constructed to carry out that function.
It will be appreciated that the invention is not restricted to the details described above with reference to the preferred embodiments but that numerous modifications and variations can be made without departing from the spirit and scope of the invention as defined in the following claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 04251844.9 | Mar 2004 | EP | regional |
