Reactor and Process for the Hydrogenation of Carbon Dioxide

Abstract

The present invention is directed to a membrane reactor for the hydrogenation of carbon dioxide, said membrane reactor comprising a reaction compartment (2) comprising a catalyst bed, a permeate compartment (4) and a membrane separating the reaction compartment and the permeate compartment, wherein said permeate compartment comprises a condensing surface.

Claims

1. A membrane reactor for the hydrogenation of carbon dioxide, said membrane reactor comprising a reaction compartment comprising a catalyst bed, a permeate compartment and a water-permeable membrane separating the reaction compartment and the permeate compartment, wherein said permeate compartment comprises a condensing surface and a means for cooling that is connected to said condensing surface such that during operation of the reactor, water can condense on the condensing surface.

2. The membrane reactor in accordance with claim 1, wherein the means for cooling comprises an active-cooling device.

3. The membrane reactor in accordance with claim 1, wherein said reactor comprises an inner wall bounding an inner space that defines the reaction compartment; an outer wall that is arranged around said inner wall, wherein said outer wall and inner wall bound an outer space that defines the permeate compartment; wherein said inner wall comprises the water-permeable membrane, and wherein said condensing surface is connected to the outer wall.

4. The membrane reactor in accordance with claim 1, wherein said reactor comprises an inner wall bounding an inner space that defines the permeate compartment; an outer wall that is arranged around said inner wall, wherein said outer wall and inner wall bound an outer space that defines the reaction compartment; wherein said inner wall comprises the water-permeable membrane, and wherein the condensing surface is located away from the inner wall.

5. The membrane reactor in accordance with claim 1, wherein said permeate compartment and means for cooling comprise an active-cooling device which active-cooling device is disconnected from the inner wall.

6. The membrane reactor in accordance with claim 3, wherein said inner wall and outer wall are tubular and the outer wall is at least partially co-axially arranged around said inner wall.

7. The membrane reactor in accordance with claim 1, wherein the condensing surface comprises protruding surface elements.

8. The membrane reactor in accordance with claim 1, wherein said water-permeable membrane comprises a hydrophilic membrane.

9. The membrane reactor in accordance with claim 1, wherein said catalyst bed comprises copper, zinc oxide, zirconia, palladium, cerium(IV) oxide or combinations thereof.

10. The process for the hydrogenation of carbon dioxide, wherein said process comprises reacting carbon dioxide with hydrogen to form methanol and/or dimethyl ether, and water as a side product, and wherein said process further comprises removing said water from the process by a combination of permeation of said water through a water-permeable membrane and condensation of the water.

11. The process for the hydrogenation of carbon dioxide carried out in a membrane reactor in accordance with claim 1, wherein the condensing surface has a temperature of less than 150 C.

12. The process in accordance with claim 11, wherein reacting carbon dioxide with hydrogen is carried out at a temperature in the range of 150-400 C. and/or at a pressure in the range of 1-10 MPa.

13. The process in accordance with claim 10, wherein said carbon dioxide and/or said hydrogen originate from biogas.

14. The membrane reactor of claim 1 wherein the means for cooling comprises a passage through which a cooling fluid can flow.

15. The membrane reactor of claim 1 wherein said water-permeable membrane comprises a zeolite membrane, an amorphous membrane or a polymer membrane.

16. The process of claim 11 wherein the condensing surface has a temperature of less than 50 C.

17. The process of claim 11 wherein the condensing surface has a temperature of less than 10 C.

18. The process of claim 12 wherein reacting carbon dioxide with hydrogen is carried out at a temperature in the range of 200-300 C. and/or at a pressure in the range of 2-8 MPa.

19. The process of claim 12 wherein reacting carbon dioxide with hydrogen is carried out at a temperature of about 250 C. and/or at a pressure of about 5 MPa.

Description

EXAMPLE 1EFFECT OF WATER REMOVAL ON DME PRODUCTION

[0031] The following reaction and reactor is analysed in silico. In a membrane reactor as illustrated in FIG. 5, at the feed side (i.e. in the reaction compartment) a carbon dioxide hydrogenation reaction takes place to produce methanol and subsequent conversion to dimethyl ether, at 250 C. and 50 bar, according to the reactions 1-4. Reaction 3 is the combination of reaction 1 and 2. By removing water from reaction 4, the reaction can be shifted to the right-hand side to produce more dimethyl ether. Therefore, in-situ removal of water from the mixture of H.sub.2, H.sub.2O, CO, CH.sub.3OH, CH.sub.3OCH.sub.3 will drive the reactions towards more DME production. To show the effect of water removal, two models were created.

CO+2H.sub.2.Math.CH.sub.3OH(1)

CO.sub.2+H.sub.2.Math.CO+H.sub.2O(2)

CO.sub.2+3H.sub.2.Math.CH.sub.3OH+H.sub.2O(3)

2CH.sub.3OH .Math.CH.sub.3OCH.sub.3+H.sub.2O(4)

[0032] In the first model kinetic equations and equilibrium constants for reaction 1-3 were used from Portha et al., Erena et al. and Alharbi et al., to show the establishment of chemical equilibrium in DME production. (see also Portha et al., Ind. Eng. Chem. Res., 56 (2017) 13133-13145, Erea et al., Chem. Eng. J. 174 (2011) 660-667 and Alharbi et al. ACS Catal 5 (2015) 7186-7193. This equilibrium limits the amount of DME produced. By modelling the chemical equilibrium with and without water removal, it is shown that in-situ water removal with membrane reactor leads to an increase in DME production. Table 1 contains the starting conditions and pressures for the equilibrium model.

TABLE-US-00001 TABLE 1 Input values for equilibrium model Input values: T [ C.] 250 P [bar] 50 Start pressure CO.sub.2 [bar] 7.5 Start pressure CO [bar] 7.5 Start pressure H.sub.2 [bar] 35

[0033] A second model calculates the steady state water removal by the membrane reactor, driven by the pressure difference between feed and permeate side. The calculated water removal from the feed side is used as an input for the first model in the graph, to show the increase in DME production. Additionally, variations in air gap and temperature of the cooling element show the effect of different parameter on the membrane reactor's performance

[0034] For modelling a steady state in-situ water removal during conversion from CO.sub.2 to dimethyl ether (DME), theory was used that is commonly applied for air gap membrane distillation processes used in water treatment. FIG. 6 shows the temperature and pressure profiles for the membrane reactor from FIG. 5. Water removal through the membrane is driven by a pressure difference between the water gas pressure on the feed side P.sub.F and the water pressure just above the liquid film on the condenser surface P.sub.C.

[0035] The water gas pressure on the feed side is calculated by multiplying its calculated vapor fraction with the total pressure on the feed side of the reaction.

P.sub.F=y.sub.i*P.sub.total(5)

[0036] On the other side of the membrane, the vapor pressure just above the liquid film on the condenser surface is described by the Antoine equation (see equation 6).

[00001]$\begin{array}{cc}{P}_C={10}^{4.6543-\frac{1435.264}{T_{\mathrm{cool}}-64.848}.Math.\left(1.0.Math.E+5\right)}& \left(6\right)\end{array}$

[0037] The dominant mechanism for the water vapor mass flux is indicated by the Knudsen number. Equation 7 is used to calculate the Knudsen number, with k.sub.B,T,r, d.sub.H2O and P as Boltzmann constant, temperature, membrane thickness, diameter water molecule and pressure.

[00002]$\begin{array}{cc}{K}_n=\frac{k_B.Math.T}{2.Math.L.Math..Math..Math..Math.d_{H.Math..Math.2.Math.O}^2.Math.P.Math.2}& \left(7\right)\end{array}$

[0038] A membrane thickness of 1 mm gives a Knudsen number smaller than 0.01 indicating that molecular diffusion through the air gap will be the mass transfer limiting step at 250 C. and 50 bar. Molecular diffusion through the air gap is usually the limitation in mass transfer in air gap membrane distillation.

[0039] Equation 8 gives the water flux for transition flow. Based on membrane properties in Table 2, a flux of 0.097 kg/m.sup.2/s was calculated.

[00003]$\begin{array}{cc}J=\frac{\mathrm{.Math.}.Math..Math.\mathrm{PDM}.Math..Math..Math..Math.p}{\left(+b\right).Math.\mathrm{RT}\left(P_a\right)}& \left(8\right)\end{array}$

TABLE-US-00002 TABLE 2 properties used in flux calculations Properties Values used in calculation Temperature condenser surface [ C.] 8 Radius condenser element [mm] 1 Air gap [mm] 2 Vapor fraction H.sub.2O in feed gases yi 2.94E4 Length membrane [m] 0.1 Membrane thickness [mm] 1 Membrane pore radius r [m] 1 Porosity 0.56 Tortuosity 2 Diffusion coefficient water-air [m.sup.2/s] 2.8E5 Flow rate H.sub.2O feed side [l/s] 3

[0040] Equation 8 represents the one-dimensional flux, whereas the experimental setup is cylindrical. As a simplification, the surface of the cylindrical membrane area was multiplied with the one-dimensional flux to get the total flux to be deposited on the inner cylindrical cooling element.

[0041] The result of the in silico reaction are illustrated in FIGS. 7 to 9.

[0042] FIG. 7 shows the model for establishing chemical equilibrium for reactions 1-4. This represents the chemical reactions that occur on the feed side of the membrane reactor, without water removal, starting from the pressures in Table 1. This can be compared to the situation where the cooling element at the permeate side of the membrane reactor is at the same temperature as the feed side and there is no in-situ water removal. Water is produced in reaction 3, but consumed in the water gas shift reaction (reaction 2 to the left hand side). Overall, without water removal, the end concentration of DME is 6.21 vol %.

[0043] FIG. 8 shows the same chemical equilibrium model with a constant water removal where 33% of the created water is removed. This amount of water removal matches with the water flux calculated in the model of the in-situ water removal membrane reactor with the standard parameters in Table 2. An equilibrium model with an added constant water removal corresponds to the situation where the reactions 2 and 3 shift more to the right-hand side of the equations due to the water removal, giving a higher DME yield. With water removal matching to the flux of water removal calculated in the membrane reactor model, the yield of DME rises with 51 vol %. FIG. 9 shows the 51 vol % increase in DME yield due to water removal.

EXAMPLE 2EFFECT OF CONDENSING SURFACE TEMPERATURE ON THE FLUX OF WATER ACROSS THE MEMBRANE

[0044] Based on the models described in Example 1, the water mass flux over the membrane can be calculated, depending on the temperature of the condensing surface. The results are depicted in FIG. 10, from which it can be deduced that at condensing surface temperatures of >50 C., essentially no water flux is expected. The operating temperatures of membrane is close to 200 C. at which the condensation on/inside the membranes is not taking place. The results further show that active cooling with condensing temperature of <10 C. results in positive flux for water flux across the membrane.