The present invention relates to a separation method using a column with cross-corrugated structured packing for separating a mixture of gases.
Old installations for separating mixtures of carbon monoxide and hydrogen comprise only columns with plates, whereas the new generation of installations of this type uses the technology of columns with cross-corrugated structured packing without a modified interface (EP-A-0 837 031). The use of packing in these installations remains tricky, in view of the fact that physical properties that have an impact on the separation efficiency, the wettability and the foaming behavior, etc. are not comparable to those for air gases. These structured packings have a local pressure drop at the interface which may be the source of a possible foaming of the mixture to be separated. The presence of foaming impairs the correct operation of the separation of the various products to be produced.
The advantage of this invention is in preventing the formation of foam in the portion dedicated to the separation of the mixture. One of the parameters that makes it possible to control the possible formation of foaming may be summarized by the following dimensionless number: S=τm/σ where τ is the shear stress at the liquid/vapor interface (kgm−1s−2), m is the thickness of the film of liquid flowing over the packing (m) and σ is the surface tension at the liquid/vapor interface (kgs−2). This parameter therefore relates the shear stresses created by the gas over the liquid with the surface tension of the liquid as described in patent U.S. Pat. No. 5,644,932. It appears that for the applications of the present invention, the range of values of this factor S must be between 50×10−6 and 7000×10−6, preferably between 150×10−6 and 1500×10−6.
This parameter can be adjusted in several ways:
The main advantage of working in these operating ranges that operate by adjusting one or more parameters mentioned previously is an optimization of the separation capacity or more particularly a reduction in the column diameter for a given separation capacity. It is therefore possible to considerably reduce the investment costs of the columns and thus of the cold box via these adjustments. The overall reduction in the cryogenic equipment also allows an increase in the flexibility of the unit, which is a second advantage, during the startup and changeover phases.
According to one subject of the invention, a method is provided for the cryogenic separation of a gas having, as main components, at least two components chosen from one of the following groups: i) hydrogen, carbon monoxide, nitrogen and methane, ii) nitrogen, oxygen, argon and iii) carbon dioxide, hydrogen, nitrogen, oxygen, argon, the method using at least one distillation column having cross-corrugated structured packing and/or at least one absorption column having cross-corrugated structured packing with at least one section for heat and/or mass exchange between a descending liquid and an ascending gas, characterized in that the parameter S in this section is between 50×10−6 and 70 000×10−6, where S=τm/σ, τ being the shear stress at the liquid/vapor interface (kgm−1s−2), m being the thickness of the film of liquid flowing over the packing (m) and σ being the surface tension at the liquid/vapor interface (kgs−2).
According to other features of the method:
The invention will be described in greater detail and referring to the figures, in which
In the methane scrubbing systems (
The column K01 contains at least one packing body as described in WO 97/16247. The use of such a packing having a modified interface is particularly advantageous since hydrogen, having a very low pressure drop compared to other gases, allows an operation at very high gas feed without significant degradation in terms of separation efficiency. The less abrupt change at the interface between sections with packing having a modified interface makes it possible to operate with a more constant parameter S, which reduces the risk of foaming at the interface between two packing bodies and makes the operation of the column in the steady state and in the changeovers more reliable.
The dissolved hydrogen is then discharged into the medium-pressure flash column K02. The CO/CH4 binary mixture is then separated in the low-pressure distillation column K03. Gaseous CO is produced at the top, the liquid methane produced at the bottom being pumped and recycled for the scrubbing operation in K01.
The refrigerating capacity is produced in a CO cycle.
The other columns K02, K03 may also contain cross-corrugated structured packing with modified or unmodified interface(s).
All the columns operate with a factor S between 50×10−6 and 3000×10−6, more particularly between 150×10−6 and 1500×10−6.
In the systems with partial condensation (
The flash columns K12 and K13 remove the dissolved hydrogen respectively in the bottoms liquids of K11 (rich in methane) and the liquid from the pot B02 (rich in CO).
The liquids from columns K12 and K13 then feed the distillation column K14 where the CO/CH4 separation is carried out. In the CO/CH4 separation column (K14), which has at least two packing sections operating at very different refluxes, the use of packing sections having a different density and/or angle of inclination makes it possible to optimize the parameter S for the whole of the column.
The other columns K02, K03 may also contain cross-corrugated structured packing having modified or unmodified interface(s).
The refrigerating capacity is obtained by hydrogen expansion in turbines.
In the particular case where nitrogen is present, an N2/CO separation column could be added downstream of the CO/CH4 column.
All the columns operate with a factor S between 50×10−6 and 7000×10−6, more particularly between 150×10−6 and 1500×10−6.
Given below is an example of the calculation of the factor S for a methane scrubbing column.
Linear pressure drop for impure hydrogen with 1.2 mbar/m in a structured packing having a density of 650 m2/m3:
The thickness of the film for laminar flow of liquid methane at 93 K in the ascending hydrogen is:
For liquid methane at 93 K, the surface tension σ=0.018 N/m
Verification of laminar flow conditions in a falling film of liquid with ReL<2000
Legend
τ shear stress at the vapor/liquid interface (N/m2)
σ surface tension of the liquid (N/m)
γ liquid mass flow per unit of width of the exchange surface area (kg/m/s)
μL dynamic viscosity of the gas (kg/m/s)
ρL density of the liquid (kg/m3)
ρv density of the vapor (kg/m3)
Dh hydraulic diameter of the structured packing channel (m)
ΔP/ΔL linear pressure drop of the gas in the vertical direction (N/m3)
m film thickness of the liquid (m)
g gravitational constant (9.81 m/s2)
S dimensionless parameter that characterizes the internal and external forces at the vapor/liquid interface
ReL Reynolds number of the falling film of liquid (dimensionless)
Represented in
The inclination of the corrugations is defined by the angle δ formed between the wave crest 2 and the lower edge 4 in the central region C. An upper region S going from the upper edge 4a of the element to the upper limit of the central region C and in a lower region I going from the lower edge of the element to the lower limit of the central region C, each region S, I having a height h′. The angle formed between the crests of the waves and the edge 4 is δ1=90° but may have other values.
Seen in
The cooled, compressed and purified air 601 is sent to the bottom of a medium-pressure column 605 thermally coupled to a low-pressure column 609. Reflux streams 607, 603 are sent from the medium-pressure column to the low-pressure column. Streams rich in oxygen 613 and rich in nitrogen 611 are withdrawn from the low-pressure column.
Number | Date | Country | Kind |
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0752694 | Jan 2007 | FR | national |
0757533 | Sep 2007 | FR | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/FR08/50045 | 1/10/2008 | WO | 00 | 11/20/2009 |