Separation Method Using A Column With A Corrugated Cross Structure Packing For Separating A Gaseous Mixture

Abstract

A separation method using a column with cross-corrugated structured packing for separating a gas mixture is presented.

Description

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:

• • by playing on the geometrical parameters of the packing (within one and the same section or between two different sections) that is to say:
    • the angle of inclination δ of the channels relative to the horizontal (preferably between 30° and) 70°;
    • the crimp angle γ of the corrugations (preferably between 40° and 150°;
    • the density of the packing at least in its central region (preferably between 300 m²/m³ and 1000 m²/m³);
    • the degree of perforation of the packing (preferably between 3% and 20%);
    • the diameter of the perforations of the packing (preferably between 1 mm and 4 mm); and
    • the radius of curvature of the corrugation on going from a central region with corrugations inclined with a given angle relative to the vertical to a side region where the corrugations are inclined with a shallower angle relative to the vertical, or even become vertical (preferably between 0.5 mm and 3 mm);
  • by gradually modifying the angle of inclination of the channels so as to have channels that are more and more vertical at the high and low extremities of this angle as described in patent WO 97/16247.

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:

• • at least one packing body in the section for heat and/or mass exchange comprises a central region and a lower region and optionally an upper region, the lower region and optionally the upper region being modified relative to the central region so that the resistance to the rise of liquid is reduced therein relative to that in the central region;
  • the gas has, as main components, hydrogen, carbon monoxide, methane and optionally nitrogen;
  • the gas has, as main components, hydrogen, carbon monoxide and optionally methane and also nitrogen;
  • the packing is operated with a gas feed F between 0.2-2.5 (Nm⁻²)^0.5 where F=(ρ_vv_gas)^0.5 and/or the liquid flow rate in the packing in L/dm²/h may vary from 50 up to 600 L/dm²/h;
  • the gas that has, as main components, at least two components chosen from the group: hydrogen, carbon monoxide, nitrogen and methane in which the parameter S in this section is between 50×10⁻⁶ and 7000×10⁻⁶, more particularly between 150×10⁻⁶ and 1500×10⁻⁶;
  • the gas that has, as main components, at least two components chosen from the group: nitrogen, oxygen, argon as claimed in one of the preceding claims 1 to 5, in which the parameter S in this section is between 500×10⁻⁶ and 70 000×10⁻⁶;
  • at least one packing body in the section for heat and/or mass exchange comprises a central region and a lower region, the lower region being modified relative to the central region so that the resistance to the rise of liquid is reduced therein relative to that in the central region;
  • the internal diameter of the column varies from one section to another as a function of the gas and liquid feed for the gas to be separated;
  • the packing has, at least in one central region, a density of 300 m²/m³ to 1000 m²/m³;
  • at least one characteristic of the packing varies within one and the same section, the characteristic(s) being chosen from the group:
    • packing density;
    • angle of inclination;
    • crimp angle;
    • radius of curvature; and
    • degree of perforation,
  • at least one characteristic of the packing varies from one section to the other, the characteristic(s) being chosen from the group:
    • packing density;
    • angle of inclination;
    • crimp angle;
    • radius of curvature; and
    • degree of perforation.

The invention will be described in greater detail and referring to the figures, in which

FIG. 1 illustrates a methane scrubbing process according to the invention and

FIG. 2 illustrates a partial condensation process according to the invention,

FIG. 3 schematically represents a view of the corrugations in the axis of the waves,

FIG. 4 represents a schematic top view of the corrugated lamella,

FIG. 5 represents a method for separating carbon dioxide according to the invention and

FIG. 6 represents a method for separating air according to the invention.

In the methane scrubbing systems (FIG. 1), the syngas which is under pressure and cooled to −180° C., is scrubbed with liquid methane in the column K01 operating at high pressure (between 12 and 50 bara) and a temperature as low as possible, carbon monoxide is entrained in the bottom of the column and hydrogen is produced at the top.

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 (FIG. 2), the syngas, scrubbed of its methane in K11 by the liquid CO originating from B01, is cooled to the lowest possible temperature, the limitation being the solidification temperature of the CO; the liquid condensed in B02 is predominantly CO.

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:

$\tau =\frac{D_h}{4}·\frac{\Delta P}{\Delta L}=\frac{0.0062m}{4}·120\frac{N}{m^3}=0.186\frac{N}{m^2}$

The thickness of the film for laminar flow of liquid methane at 93 K in the ascending hydrogen is:

$\begin{array}{c}m={\left[\frac{3·{\mu }_L·\Gamma }{{\rho }_L·\left({\rho }_L-{\rho }_V\right)·g}\right]}^{1/3}\\ ={\left[\frac{3·0.0021\frac{\mathrm{kg}}{m·s}·0.0052\frac{\mathrm{kg}}{m·s}}{460\frac{\mathrm{kg}}{m^3}·\left(460\frac{\mathrm{kg}}{m^3}-7.7\frac{\mathrm{kg}}{m^3}\right)·9.81\frac{m}{s^2}}\right]}^{1/3}\\ =0.000117m\end{array}$

For liquid methane at 93 K, the surface tension σ=0.018 N/m

$S=\frac{\tau ·m}{\sigma }=\frac{0.186\frac{N}{m^2}·0.000117m}{0.018\frac{N}{m}}=1209*{10}^{-6}$

Verification of laminar flow conditions in a falling film of liquid with Re_L<2000

${\mathrm{Re}}_L=\frac{4·\Gamma }{{\mu }_L}=\frac{4·0.0052\frac{\mathrm{kg}}{m·s}}{0.00021\frac{\mathrm{kg}}{m·s}}=99$

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)

μ_L dynamic viscosity of the gas (kg/m/s)

ρ_L density of the liquid (kg/m³)

ρ_v density of the vapor (kg/m³)

D_h 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

Re_L Reynolds number of the falling film of liquid (dimensionless)

FIG. 3 shows a packing lamella having a corrugation of height H with a crimp angle γ.

Represented in FIG. 4 is a lamella 1, having oblique parallel corrugations, of which the crests 2 are represented as thick lines and the troughs 3 as thin lines.

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 FIG. 5 is the process for separating carbon dioxide by distillation in accordance with the invention. A stream 1 of carbon dioxide mixed with nitrogen, oxygen and argon is cooled in an exchanger 3, separated in a distillation column 5 having an overhead condenser and a bottoms reboiler. The bottoms liquid 7 from the column 5 is sent to a column 9 having an overhead condenser and a bottoms reboiler. The column 9 produces at the top a stream 11 and at the bottom a stream 13. The stream 13 is the liquid product that is rich in carbon dioxide and the stream 11 contains nitrogen, argon and oxygen. This process is described in detail in EP-A-503910. Other examples of units that can be operated according to the method of the invention are given in patent applications U.S. 60/890233, U.S. Ser. Nos. 11/695,422, 11/695,446, 11/695,455 and 11/695,471.

FIG. 6 illustrates a double air separation column that operates according to the method of the invention. Other types of columns may also function according to the invention, such as single columns, mixing columns, triple columns, argon separation columns, etc.

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.

Claims

1-6. (canceled)

7. A method for the cryogenic separation of a gas having, at least two components selected from one of the groups consisting of: the method using at least wherein

i) hydrogen, carbon monoxide, nitrogen and methane;

ii) ii) nitrogen, oxygen, argon; and

iii) iii) carbon dioxide, hydrogen, nitrogen, oxygen, argon;

one distillation column having cross-corrugated structured packing,

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, or

a combination thereof,

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), ρm 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).

8. The method of claim 7, further comprising at least one packing body in the section for heat and/or mass exchange, and wherein said at least one packing body comprises a central region and a lower region and an upper region, the lower region and the upper region being modified relative to the central region so that the resistance to the rise of liquid is reduced therein relative to that in the central region.

9. The method of claim 7, wherein the gas comprises at least hydrogen and carbon monoxide.

10. The method of claim 9, wherein the gas further comprises methane and nitrogen.

11. The method of claim 7, wherein the packing is operated with a gas feed F between 0.2-2.5 (Nm−2)0.5 where F=(ρvvgas)0.5 and/or the liquid flow rate in the packing in L/dm2/h may vary from 50 up to 600 L/dm2/h.

12. The method of claim 7, wherein the gas that has at least two components selected from the group consisting of hydrogen, carbon monoxide, nitrogen and methane, wherein the parameter S in this section is between 50×10−6 and 7000×10−6,

13. The method of claim 12, wherein the parameter S in this section is between 150×10−6 and 1500×10−6.

14. The method of claim 7, wherein the gas that has at least two components selected from the group consisting of nitrogen, oxygen, argon, wherein the parameter S in this section is between 500×10−6 and 70 000×10−6.