The present invention relates to an integrated method and an integrated unit for separating air by cryogenic distillation and for cooling a gas. The cooling makes it possible to cool, or even at least partially condense, a gas originating from a store which contains a liquid; the liquid is produced by a unit supplied with a gas originating from the air separation unit.
It is often necessary to store liquids at low temperature, i.e. at temperatures below ambient temperature. The stores are insulated to prevent the ambient heat from evaporating the liquid but nonetheless defects in the insulation give rise to ingresses of heat and gas is formed at the top of the store. This gas must be either discharged to the atmosphere or recondensed and sent back to the store.
Another possibility, illustrated in
In order to condense the gas, another possibility is to use water cooled by an electric refrigerator to a temperature of 10° C. which exchanges frigories with the gas in a heat exchanger via indirect exchange.
In many cases, a cryogenic distillation air separation unit produces oxygen at the request of a customer, but there is no customer for the nitrogen, or even all the nitrogen, which is inevitably co-produced.
According to the invention, the nitrogen produced in excess is used at the outlet of the air separation unit for cooling water in a direct-contact tower fed at the bottom with cold nitrogen gas and at the top by the water to be cooled.
The cooled water is then used to cool, or even to condense a gas originating from a store of a liquid at a subambient temperature, make it possible to reduce the electric power consumption.
A process according to the preamble of claim 1 is known from EP-A-0 748 763.
According to one subject of the invention, a process as claimed in claim 1 is provided.
According to other optional aspects of the invention:
According to another aspect of the invention, an integrated unit as claimed in claim 8 is provided.
According to other optional aspects:
The store may for example be a store of liquid methanol or of liquid propylene.
The liquid contained in the store preferably has a boiling point at the pressure inside the store of below 50° C. or below 15° C., or even below 10° C., or even below 0° C.
For a store of methanol at 1.1 bar abs, the saturation temperature is 11° C. and for a store of propylene at 16 bar, the saturation temperature is 40° C.
The process will be described in greater detail with reference to
The air separation unit ASU also produces nitrogen gas 17 which is heated by heat exchange with the air to be separated. This nitrogen gas 17 is sent to the bottom of a cooling tower 19 fed at the top by water 21 to be cooled. The nitrogen gas contacts the water to be cooled and the reheated nitrogen leaves at the top of the tower 19 whilst the cooled water 25 leaves at the bottom of the tower 19. The cooled water 25 is sent to the heat exchanger 23 in order to cool or even to condense, at least partially, the gas 9.
The feed air sent to the air separation unit ASU in order to be separated therein is preferably not cooled by heat exchange with the cooled water in the tower 19. Preferably, no element of the air separation unit is cooled by heat exchange with the cooled water in the tower 19. Thus, the cooling tower 19 provides cooled water not for the cooling of the air separation unit but for, preferably solely for, cooling the evaporated gas 9 originating from the store 7 or from other stores that are not used to store a fluid originating from the air separation unit.
According to another variant, cooled water from the bottom of the tower 19 is used to cool air intended for the distillation in the ASU unit. In this case, the cooling may be carried out by means of a second tower fed at the top with cooled water and at the bottom with the air to be cooled. Only a portion of the cooled water will be available for cooling the gas 9 originating from the store 7.
It can be envisaged to collect nitrogen from several air separation units, at least one of which supplies oxygen-enriched gas to the production unit MEOH which produces the methanol. The nitrogen collected may be sent to a common cooling tower for all the distillation units and then be sent to one or more stores for cooling the gases produced by reheating.
The methanol is often converted into other alkenes which are also stored at subambient temperature and are liable to evaporate. The gases originating from at least one store of alkene produced from the methanol from the production unit MEOH may also be cooled by the nitrogen from the air separation unit, using water from the cooling tower 19 or from a cooling tower common to several air separation units. The alkene may for example be propylene, stored at 16 bar abs with a saturation temperature of 40° C.
Number | Date | Country | Kind |
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1661936 | Dec 2016 | FR | national |
Filing Document | Filing Date | Country | Kind |
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PCT/FR2017/052315 | 8/31/2017 | WO | 00 |