Method and plant for low temperature fractionation of air
A SPECTRA process for low-temperature fractionation of air is proposed, in which bottoms liquid from an additional second rectification column used to obtain oxygen is evaporated in a second condenser evaporator arrangement. In this second condenser evaporator arrangement, gas that has been evaporated beforehand in a first condenser evaporator arrangement, which is used for condensation of tops gas from a first rectification column, is partially condensed after recompression. The invention also relates to a corresponding plant.
1 . A process for the low-temperature fractionation of air in an air fractionation plant having a first rectification column, a second rectification column, a first condenser evaporator arrangement, and a second condenser evaporator arrangement, wherein the process comprises;
feeding air into the first rectification column and feeding a stream from the first rectification column into the second rectification column, wherein the first rectification column is operated at a first pressure level and the second rectification column is operated at a second pressure level which is below the first pressure level,
obtaining a first tops gas stream from the first rectification column as a nitrogen product and discharged the nitrogen product from the air fractionation plant, and obtaining bottoms liquid from the second rectification column as an oxygen product and discharged the oxygen product from the air fractionation plant,
forming a first material stream and a second material stream, below the first pressure level, in the first condenser evaporator arrangement by evaporating liquid from the first rectification column, and condensing a further tops gas stream from the first rectification column in the first condenser evaporator arrangement and returning the condensed further tops gas stream to the first rectification column as a return flow,
wherein the first material stream has a first oxygen content, and the second material stream has a second oxygen content which is above the first oxygen content,
at least part of the first material stream to the first pressure level to form a compressed first material stream and feeding the compressed first material stream, into the first rectification column, and subjecting the at least part of the second material stream to work-performing expansion to form an expanded second material stream and discharging the expanded second material stream, from the air fractionation plant,
evaporating bottoms liquid from the second rectification column in the second condenser evaporator arrangement,
after compression, and before being fed into the first rectification column, subjecting at least a part of the compressed first material stream, to partial liquefaction to form a two-phase stream which is fed to form a two-phase stream which is fed into the first rectification column, and
supplying tops gas of the second rectification column to a work-performing expansion.
2 . The process according to claim 1 , wherein the tops gas of the second rectification column is supplied, upstream of the work-performing expansion, to the at least part of the second material stream which is subjected to the work-performing expansion, and is discharged from the air fractionation plant.
3 . The process according to claim 1 , wherein tops gas of the second rectification column is subjected to a work-performing expansion, separately from the at least part of the second material stream which is subjected to the work-performing expansion, and is discharged from the air fractionation plant.
4 . The process according to claim 1 , wherein, in the partial liquefaction in the second condenser evaporator arrangement, 5 to 30 mol %, of the at least part of the compressed first material stream is liquified.
5 . The process according to claim 1 , wherein all of the compressed first material stream is conducted through the second condenser evaporator arrangement.
6 . The process according to claim 1 , wherein a first fraction of the compressed first material stream is conducted through the second condenser evaporator arrangement, and fed into the first rectification column, and a second fraction of the compressed first material stream is fed into the first rectification column without being conducted through the second condenser evaporator arrangement.
7 . The process according to claim 1 , wherein the first rectification column is operated at a first pressure level of 7 to 14 bar, and the second rectification column is operated at a second pressure level of 3 to 7 bar.
8 . The process according to claim 7 , wherein the partial liquefaction takes place at the first pressure level.
9 . The process according to claim 7 , wherein the compressed first material stream is transferred to the first rectification column without the use of a pump.
10 . The process according to claim 1 , wherein one or more compressors are provided for compression of the at least part of the first material stream, and at least one expansion machine are provided for the work-performing expansion of the at least part of the second material stream, and wherein one or more of the expansion machines is coupled to one or more the compressors.
11 . The process according to claim 1 , wherein the tops gas of the first rectification column contains less than 1 ppb oxygen, less than 1 ppb carbon monoxide, and/or less than 1 ppb hydrogen and contains less than 10 ppm argon, on a volume basis.
12 . The process according to claim 1 , wherein the bottoms liquid of the second rectification column contains less than 10 ppb argon and/or less than 5 ppm methane, on a volume basis.
13 . The process according to claim 1 , wherein all of the air to be fractionated in the process is fed into the first rectification column in gaseous form.
14 . An air fractionation plant comprising:
a first rectification column, a second rectification column, a first condenser evaporator arrangement, and a second condenser evaporator arrangement,
means for feeding air into the first rectification column, operated at a first pressure level, and means for for feeding a stream from the first rectification column into the second rectification column, operated at a second pressure level which is below the first pressure level,
means for removing a tops gas stream from the first rectification column and means for discharging the tops gas stream from the air fractionation plant as a nitrogen product, and means for removing a bottoms liquid stream from the second rectification column and discharging the bottoms liquid stream from the air fractionation plant as an oxygen product,
a first condenser evaporator arrangement for forming a first material stream with a first oxygen content and a second material stream with a second oxygen content above the first oxygen content below the first pressure level by evaporating liquid from the first rectification column, and means for introducing a further tops gas stream from the first rectification column into the first condenser evaporator arrangement to form a condensate and means for returning the condensate to the first rectification column as a return flow,
means for subjecting the first material stream or a part thereof to compression to the first pressure level and means for feeding the compressed first material stream or the compressed part thereof into the first rectification column, and means for subjecting at least part of the second material stream to expansion to form an expanded second material stream and means for discharging the expanded second material stream from the air fractionation plant,
a second condenser evaporator arrangement for evaporating bottoms liquid of the second rectification column,
means for feeding the compressed first material stream into the second condenser evaporator arrangement located upstream of the means for feeding the compressed first material stream into the first rectification column where the compressed first material stream is configured to undergo partial liquefaction to form a two-phase stream, and
means for subjecting a tops gas stream from the second rectification column to a work-performing expansion.
15 . The process according to claim 1 , wherein, in the partial liquefaction in the second condenser evaporator arrangement, 15 to 25 mol % the at least part of the compressed first material stream, is liquified.
16 . The process according to claim 1 , wherein the first rectification column is operated at a first pressure level of 7 to 14 bar, and the second rectification column is operated at a second pressure level of 4 to 7 bar.
17 . The process according to claim 1 , wherein the first rectification column and the second rectification column are not combined with one another in the form of a double column.
18 . The process according to claim 1 , wherein air is not used to heat the second condenser evaporator arrangement.
19 . The process according to claim 1 , wherein the two-phase stream formed by the partial liquefaction has a vapor molar fraction of 0.7 to 0.95, relative to the total amount of the two-phase stream.