Patent Application
20130098106
The present invention relates to an apparatus and to a process for separating air by cryogenic distillation.
During shutdown of a distillation apparatus for maintenance or defrosting, it is necessary to evacuate the cryogenic liquids accumulated in the cold box, which are found essentially at the column bottoms and in the bath evaporators.
These liquids cannot be discharged to the atmosphere as they are.
More particularly in the context of an apparatus for cryogenic production of impure (<98%) oxygen intended in particular for oxy-fuel combustion, the proposal is to send all of the liquids present in the cold box to the LOX storage facility, which supplies a standby evaporator.
This also avoids the addition of specific equipment, as described in EP-A-1746374 (which is expensive and takes up a lot of ground space, and which sometimes fails since it is rarely burdened) to manage the purging of the liquids during shutdowns, and also makes it possible to retain the oxygen molecules.
The purges of the cryogenic liquids from the cold box are generally sent to a specific device for their evaporation:
In EP-A-0605262, a purge liquid is sent to a storage facility, and a liquid portion from the storage facility is sent to a heat exchanger to be mixed with gaseous oxygen. The storage facility is also supplied by liquid transported by a truck: the system is therefore one of injection of liquid oxygen with a contribution of external liquid, thereby allowing the purge to be evaporated with the reheated gaseous oxygen.
WO 2010/017968 describes the sending of an auxiliary evaporator purge to a storage facility.
U.S. Pat. No. 5,566,556 describes a process for separating air wherein the bottom liquid from the low-pressure column is sent to a storage facility as a purge stream.
It is an aim of the present invention to derive value from the refrigeration potentials of the residual streams and to reduce the investment in the apparatus.
According to one subject of the invention, a process is provided for separating air by cryogenic distillation, wherein:
According to other, optional aspects:
According to another subject of the invention, an apparatus is provided for separating air by cryogenic distillation, comprising a column system comprising a medium-pressure column and a low-pressure column, and optionally an intermediate-pressure column or argon column, a supply line for sending compressed, purified, and cooled air to the column system, a product line for withdrawing an oxygen-rich stream from the low-pressure column and for sending it to a customer, optionally after evaporation in an auxiliary evaporator, a storage facility, a standby evaporator connected to the storage facility, a line for supplying the storage facility with an oxygen-rich liquid, characterized in that it comprises a filling line connected to the storage facility and to the bottom of the medium-pressure column and/or to the bottom of the intermediate-pressure column and/or to the bottom of the argon column.
According to other, optional subjects, the apparatus comprises:
Optionally, the apparatus does not comprise a dedicated evaporator for evaporating a purge stream from the medium-pressure column and/or from the intermediate-pressure column and/or from the argon column, where appropriate.
According to an embodiment of the invention, the purge is able to compensate for the evaporation losses in the storage facility, and the excess is evaporated in the system of the standby evaporation (without recovery of the refrigeration potentials in the cold box).
According to an embodiment of the invention, provision is made to send a purge stream to a liquid oxygen storage facility, optionally by means of a pump, depending on its pressure.
An oxygen-rich stream contains at least 75 mol % of oxygen.
It will be noted that the transfer pump may also be the same as is used for the transfer of the production of liquid oxygen, where appropriate.
The liquid oxygen storage facility is filled with oxygen of “industrial merchant” quality, which is therefore “pure” (99.5 mol %). The sporadic sending of a liquid with a markedly lower average purity will slightly pollute the storage facility, which will remain:
In one embodiment, it is preferred to ensure that the storage facility is not completely filled, so as to allow the addition, when the time comes, of the quantity of purge liquid from the cold box. The quantity of purge liquid remains relatively low in relation to the volume of the storage facility.
These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, claims, and accompanying drawings. It is to be noted, however, that the drawings illustrate only several embodiments of the invention and are therefore not to be considered limiting of the invention's scope as it can admit to other equally effective embodiments.
The invention will be described in more detail with reference to the FIGURE, which illustrates an apparatus according to the invention.
In
Streams enriched in nitrogen and in oxygen are sent from the medium-pressure column to the low-pressure column in a conventional manner. In the bottom of the low-pressure column 15, liquid oxygen accumulates around the reboiler 17. An oxygen stream 19 containing less than 98% oxygen is withdrawn from the bottom of the low-pressure column 15 and is pressurized by a pump 23 to 2 bar; alternatively, the compression may be hydrostatic. A portion of the oxygen at this pressure is sent to an evaporator 24, in which the portion of the oxygen undergoes evaporation by heat exchange with a portion of the feed air (5 or B), which is then sent to the column. A purge stream 121 containing impurities is withdrawn from the evaporator 24 permanently or occasionally. The evaporated oxygen continues its reheating in the heat exchanger 3 as low-pressure oxygen stream 27.
The remainder of the oxygen is pressurized to a pressure of 10 bar in a pump 25, and is evaporated in the heat exchanger 3 as stream 29. Alternatively, all of the oxygen may be evaporated to the lower pressure, and the stream 29 will not exist.
On shut down of the system (breakdown of main compressor, maintenance or defrosting of the apparatus), a purge stream 21 is withdrawn from the bottom of the low-pressure column 15. This liquid is richer in nitrogen than the stream withdrawn at the column bottom would normally be, since all of the liquids accumulated in the packings in the column fall to the base of the column.
Accordingly, the purge stream is less pure in oxygen than the stream taken in normal operation.
The purge from the bottom of the low-pressure column may also be mixed with a purge from the medium-pressure column withdrawn via line 51, which is even richer in nitrogen than the stream withdrawn at the bottom of the low-pressure column. One or other of these purge streams, or a mixture of the two, are sent to the storage facility 31 if the liquid level measured by the element LL does not exceed a given threshold. If the level of liquid is sufficiently low, the valve 41 opens and the purge liquid is sent to the storage facility.
In one embodiment, when distillation is started up again, the sending of the liquid from the bottom of the low-pressure and/or medium-pressure column is halted.
In another embodiment, it is also possible to send the liquid accumulated in the evaporator 24 via the purge stream 121 from the evaporator to the storage facility 31, alternatively alone, mixed with the purge 21, mixed with the purge 51, or both purges 21 and 51. This liquid is less oxygen-rich than the stream 21 in normal operation.
The majority of the liquid in the storage facility comes from a tanker truck 61 or from the apparatus itself via the stream 21, which can be diverted to the storage facility as and when necessary. The liquid in the storage facility from the truck has a purity of 99.5 mol %.
The pump 23 may optionally serve to send the purge liquid 21 to the storage facility.
In order to supply the customer when the air separation apparatus is not operating or is not producing enough oxygen, a liquid oxygen stream 35 is withdrawn from the storage facility 31 and sent to a standby evaporator 37, which is heated by steam, hot water, air or another fluid.
While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims. The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. Furthermore, if there is language referring to order, such as first and second, it should be understood in an exemplary sense and not in a limiting sense. For example, it can be recognized by those skilled in the art that certain steps can be combined into a single step.
The singular forms “a”, “an” and “the” include plural referents, unless the context clearly dictates otherwise.
“Comprising” in a claim is an open transitional term which means the subsequently identified claim elements are a nonexclusive listing (i.e., anything else may be additionally included and remain within the scope of “comprising”). “Comprising” as used herein may be replaced by the more limited transitional terms “consisting essentially of” and “consisting of” unless otherwise indicated herein.
“Providing” in a claim is defined to mean furnishing, supplying, making available, or preparing something. The step may be performed by any actor in the absence of express language in the claim to the contrary a range is expressed, it is to be understood that another embodiment is from the one.
Optional or optionally means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur.
Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such particular value and/or to the other particular value, along with all combinations within said range.
All references identified herein are each hereby incorporated by reference into this application in their entireties, as well as for the specific information for which each is cited.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 55421 | Jul 2010 | FR | national |
| 10 55423 | Jul 2010 | FR | national |
This application is a §371 of International PCT Application PCT/EP2011/061278, filed Jul. 5, 2011, which claims the benefit of FR1055421, filed Jul. 5, 2010, both of which are herein incorporated by reference in their entireties.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2011/061278 | 7/5/2011 | WO | 00 | 12/24/2012 |
