PROCESS FOR SEPARATING CARBON DIOXIDE FROM A GAS STREAM AND USE

Abstract

The present invention addresses to a CO.sub.2/CH.sub.4 separation process using enzyme carbonic anhydrase, by means of a system with contactor membranes and a vessel containing absorbent liquids that have high salinity, which maintains a specific pH range to promote the formation of carbonate salts, in a way integrated into the CO.sub.2 capture process. Such a process results in a more efficient separation with conversion of CO.sub.2 into products with greater added value or, alternatively, sequestering CO.sub.2 more permanently, thus avoiding its emission into the atmosphere. The present invention is applied to natural gas streams with CO.sub.2 contents, more particularly in offshore oil fields or onshore natural gas processing units, as well as biogas streams.

Claims

1. A process for separation of carbon dioxide from a gaseous stream comprising: a. passing a continuous flow gaseous stream containing carbon dioxide and methane through a contactor module containing membranes; b. adding the enzyme carbonic anhydrase to the absorber liquid; c. passing the absorber liquid solution from step (b) through the contactor module by a loop recirculation system, wherein the absorber liquid solution and the gaseous stream operate in a countercurrent direction; and d. adjusting the pH of the absorber liquid solution with a NaOH solution, to maintain the environment alkaline when the pH is less than 9.

2. The process of claim 1, wherein the gaseous stream is natural gas or biogas.

3. The process of claim 1, wherein the gaseous stream contains comprises between 2% and 70% of carbon dioxide.

4. The process of claim 1, wherein the contactor module comprises two contactor modules in series.

5. The process of claim 1, wherein the absorber liquid is industrial water, sea water or production water.

6. The process of claim 5, wherein the production water is synthetic or natural.

7. The process of claim 1, wherein the absorber liquid is used with or without pre-treatment and conditioning, with or without the addition of a promoter, amine, hydroxide or inorganic carbonate.

8. The process of claim 1, wherein the absorber liquid solution in step (c) passes in continuous mode.

9. The process of claim 1, wherein the pH is adjusted to the range of 9.5 to 12.

10. The process of claim 1, wherein the membrane of the contactor is chosen from ptfe, pvdf, pdms, pfa, pp or ceramic.

11. The process of claim 1, further comprising directing the liquid stream containing absorbed CO.sub.2 to a second unity for recovery of CO.sub.2 in gaseous form.

12. The process of claim 11, wherein the CO.sub.2 recovered in gaseous form is destined to conversion processes of this gas into other molecules, or is directed to geological storage.

13. (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The present invention will be described in more detail below, with reference to the attached figures which, in a schematic way and not limiting the inventive scope, represent examples of its embodiment. In the drawings, there are:

[0026] FIG. 1 illustrating a schematic of the process of the present invention in a laboratory scale, where there are represented: (1) cylinder of high purity CO.sub.2; (2) cylinder of high purity CH.sub.4; (3) box of gas flow rate controller and flow rate, temperature and pressure recorder of all streams; 4) gas mixing box; (5) gas-liquid contactor module; (6) liquid phase vessel, provided with magnetic stifling; (7) liquid phase pH indicator; (8) gas chromatograph;

[0027] FIG. 2 illustrating CO.sub.2/CH.sub.4 separation time courses in a gas-liquid contactor with different gas compositions, 10 mM NaOH+0.1 g/L CA as absorber solution, and without pH adjustment. FIG. 2a shows the time course profiles of pH reduction under each condition; FIG. 2b shows the time course profiles of methane purity in the product stream (retentate) at each condition; FIG. 2c shows time course profiles of percent CO.sub.2 removal under each condition;

[0028] FIG. 3 illustrating infrared spectra of test samples with NaOH without pH adjustment, where they are represented in (a) condition without addition of CA and (b) condition with addition of CA. The bands show the formation of carbonates and bicarbonates, much more intense in the condition in the presence of the enzyme;

[0029] FIG. 4 illustrating time courses of 50% CO.sub.2/50% CH.sub.4 stream separation in a gas-liquid contactor, using sea water+0.1 g/L CA as absorber solution, and without pH adjustment. FIG. 4a shows the pH reduction time course profile; FIG. 4b shows the time course profile of methane purity in the product stream (retentate); FIG. 4c shows the time course profile of percent CO.sub.2 removal; FIG. 4d shows the time course profile of the system selectivity to the gases;

[0030] FIG. 5 illustrating time courses of 50% CO.sub.2/50% CH.sub.4 stream separation in a gas-liquid contactor, using sea water+0.1 g/L of CA as absorber solution, and with pH adjustment. FIG. 5a shows the time course profile of pH reduction and added total NaOH concentration; FIG. 5b shows the time course profile of methane purity in the product stream (retentate); FIG. 5c shows the time course profile of percent CO.sub.2 removal; FIG. 5d shows the time course profile of the system selectivity to the gases;

[0031] FIG. 6 illustrating infrared spectra of test samples with sea water and pH adjustment;

[0032] FIG. 7 illustrating (a) SEM image, (b) with ion detection by EDS, proving the formation of inorganic carbonates in the test with sea water;

[0033] FIG. 8 illustrating time courses of 50% CO.sub.2/50% CH.sub.4 stream separation in a gas-liquid contactor, using production water+0.1 g/L CA as absorber solution, and with pH adjustment;

[0034] FIG. 8a showing the time course profile of pH reduction and added total NaOH concentration; FIG. 8b showing the time course profile of methane purity in the product stream (retentate); FIG. 8c showing the time course profile of percent CO.sub.2 removal; FIG. 8d showing the time course profile of the system selectivity to the gases;

[0035] FIG. 9 illustrating infrared spectra of test samples with production water and pH adjustment;

[0036] FIG. 10 illustrating (a) SEM image, (b) with ion detection by EDS, proving the formation of inorganic carbonates in the test with production water;

[0037] FIG. 11 illustrating time courses of 50% CO.sub.2/50% CH.sub.4 stream separation in a gas-liquid contactor, using 10 mM NaOH+0.1 g/L CA solution as absorber solution, and with pH adjustment. FIG. 11a shows the time course profile of pH reduction and added total NaOH concentration; FIG. 11b shows the time course profile of methane purity in the product stream (retentate); FIG. 11c shows the time course profile of percent CO.sub.2 removal; FIG. 11d shows the time course profile of the system selectivity to the gases.

DETAILED DESCRIPTION OF THE INVENTION

[0038] The process for separating carbon dioxide from a gaseous stream according to the present invention comprises the following steps: [0039] a. passing a continuous flow gaseous stream containing carbon dioxide and methane, or carbon dioxide, methane and heavier hydrocarbons, or carbon dioxide, methane, heavier hydrocarbons and water through a contactor module containing a membrane or two serial modules; [0040] b. adding to the absorber liquid the enzyme carbonic anhydrase, in pure form, formulation or peptides associated with the enzyme; [0041] c. passing the absorber liquid solution from step (b) through the contactor module through a recirculation system in a loop or in continuous mode, wherein the absorber liquid solution and the gaseous stream operate in a countercurrent direction; [0042] d. adjusting the pH of the absorbent liquid solution to the range of 9.5 to 12 with a NaOH solution to maintain the environment alkaline when the pH is less than 9.

[0043] The gaseous stream is a stream of natural gas or biogas, containing from 2% to 70% carbon dioxide.

[0044] The absorber liquid can be chosen from industrial water, sea water and synthetic or natural production water, with or without any type of pre-treatment and conditioning. Absorption promoters, amines, hydroxides, inorganic carbonates, among others, can be added to the absorber liquid.

[0045] The type of contactor membrane is chosen from poly(tetrafluoroethylene) (PTFE), poly(vinylidene fluoride) (PVDF), poly(dimethyl siloxane) (PDMS), poly(tetrafluoroethylene-co-perfluorinated alkyl vinyl ether) (PFA), polypropylene (PP), ceramics and others.

[0046] The gas inlet flow rate can be from 5 to 300 cm.sup.3/min/m.sup.2 membrane.

[0047] The gas inlet pressure can be between 0.9 and 70 Bar (90 kPa and 7 MPa).

[0048] The liquid flow rate can be between 0.5 and 20 mL/s/m.sup.2 membrane.

[0049] The liquid stream containing absorbed CO.sub.2 must be directed to a second unit, for recovery of CO.sub.2 in gaseous form. And after the CO.sub.2 is recovered in gaseous form, it must be destined for processes of converting this gas into other molecules, or be directed to geological storage.

EXAMPLES

[0050] The examples presented below are intended to illustrate some ways of implementing the invention, as well as to prove the practical feasibility of its application, not constituting any form of limitation of the invention.

[0051] As illustrated in FIG. 1, from the pure gas cylinders, different gas compositions were inserted into the contactor module, after passing through a flow rate control box and a gas mixing box.

[0052] The mixed gaseous phase was inserted into the contactor in countercurrent with the liquid phase, coming from a glass vessel provided with stirring (magnetic or mechanical). The liquid passed through the loop system, returning to the vessel after leaving the contactor. The gas, on the other hand, passes in continuous flow, going again to the control box after leaving the contactor, to record its conditions (flow rate, temperature, pressure). This stream of gas leaving the contactor was called retentate. After having its properties recorded, the gaseous phase proceeded for analysis of its composition in a gaseous chromatograph.

Example 1

Capture of CO.SUB.2 .from Mixture with CH.SUB.4., using 10 mM NaOH Solution as Absorber

[0053] Different gas stream compositions (CO.sub.2/CH.sub.4) were passed continuously through the shell side (outside the fibers) of a contactor module containing hollow hydrophobic fibers of poly(tetrafluoroethylene) (PTFE) with a total area of 1 m.sup.2 as shown in FIG. 1 (item 5).

[0054] The total flow rate of the gaseous stream was maintained at 40 cm.sup.3/min (STP). As a liquid phase, a 10 mM NaOH solution containing 0.1 g/L of CA was used, recirculated in a loop at a flow rate of 3.3 mL/s between the glass vessel, as shown in FIG. 1 (item 6), and the inside of the contactor fibers. The absorber liquid solution had an initial pH of about 11.5, which was not adjusted during the process.

[0055] The liquid and gas passed in a countercurrent direction. FIG. 2 shows the results throughout the test, in which the addition of the enzyme to the absorber solution increases its CO.sub.2 capture capacity from 0.48 to 0.56 g/L, with more intense formation of bicarbonate ions, compared to the test control (without enzyme), as shown in FIG. 3.

Example 2

Capture of CO.SUB.2 .from Mixing with CH.SUB.4., using Sea Water as Absorber Solution, Without pH Adjustment

[0056] A gaseous stream containing 50% CO.sub.2/50% CH.sub.4 was passed continuously through the shell side (outside the fibers) of a contactor module containing hollow hydrophobic PTFE fibers with a total area of 1 m.sup.2 as shown in FIG. 1 (item 5), at a total flow rate of 40 cm.sup.3/min (STP). As a liquid phase, sea water containing 0.1 g/L of CA was used, recirculated in a loop at a flow rate of 3.3 mL/s between the glass vessel, as shown in FIG. 1 (item 6), and the inside the contactor fibers. The absorber liquid solution had an initial pH of about 10, which was not adjusted during the process.

[0057] The liquid and gas passed in a countercurrent direction. Sea water was preserved under refrigeration since its collection, and its content of divalent ions is shown in Table 1. FIG. 4 shows the results throughout the test, in which an enrichment of the retentate stream from 50% to 69% CH.sub.4, with CO.sub.2 removal of up to 43% and CH.sub.4/CO.sub.2 selectivity in retentate of up to 2.2.

TABLE-US-00001 TABLE 1 Divalent ion composition of the sea water used in the tests. Component Concentration (mg/L) Ca.sup.+2 555.6 Mg.sup.+2 1369.3 Fe.sup.+2 0.035

Example 3

Capture of CO.SUB.2 .from Mixing with CH.SUB.4., Using Sea Water as Absorber Solution, with pH Adjustment

[0058] A gaseous stream containing 50% CO.sub.2/50% CH.sub.4 was passed continuously through the shell side (outside the fibers) of a contactor module containing PTFE hydrophobic hollow fibers with a total area of 1 m.sup.2, as shown in FIG. 1 (item 5), at a total flow rate of 40 cm.sup.3/min (STP). As a liquid phase, sea water containing 0.1 g/L of CA was used, recirculated in a loop at a flow rate of 3.3 mL/s between the glass vessel, as shown in FIG. 1 (item 6), and the inside the contactor fibers. The absorber liquid solution had an initial pH of about 10, which was adjusted intermittently during the process by adding 1 M NaOH solution when the pH reached values below 9, totaling an equivalent addition of 0.1 M NaOH to the liquid phase.

[0059] The liquid and gas passed in a countercurrent direction. FIG. 5 shows the results throughout the test, in which the enrichment of the retentate stream from 50% to 98.8% of CH.sub.4 is observed, with CO.sub.2 removal of up to 98.7% and CH.sub.4/CO.sub.2 selectivity in the retentate of up to 81.

[0060] The infrared spectra (FT-IR) indicate the presence of carbonate bands, increasing over the test time as shown in FIG. 6. Scanning electron microscopy (SEM), shown in FIG. 7, proves the presence of crystals of inorganic carbonates, formed by the enzymatic reaction.

[0061] Thus, with this condition, it is shown that the use of sea water as an absorber solution, combined with pH control of the solution, to maintain an alkaline environment, is very efficient, leading to the formation of a product that has a value of market or, in the case of reinjection into a reservoir, it may represent a more permanent carbon sequestration.

Example 4

Capture of CO.SUB.2 .From mixing with CH.SUB.4., Using Production Water as Absorber Solution, with pH Adjustment

[0062] A gaseous stream containing 50% CO.sub.2/50% CH.sub.4 was passed continuously through the shell side (outside the fibers) of a contactor module containing PTFE hollow hydrophobic fibers with a total area of 1 m.sup.2, as shown in FIG. 1 (item 5), at a total flow rate of 40 cm.sup.3/min (STP). As liquid phase, synthetic production water containing 0.1 g/L of CA was used, recirculated in a loop at a flow rate of 3.3 mL/s between the glass vessel, as shown in FIG. 1 (item 6) and the inside of the contactor fibers. The absorber liquid solution had an initial pH of about 9, which was adjusted intermittently during the process by adding 1 M NaOH solution when the pH reached values below 8.5-9, totaling an equivalent addition of 0.09 M NaOH to the liquid phase.

[0063] The liquid and gas passed in a countercurrent direction. Table 2 shows the content of divalent cations in the used production water. FIG. 8 shows the results throughout the test, in which an enrichment of the retentate stream from 50% to 89.1% of CH.sub.4 was observed, with CO.sub.2 removal of up to 86.2% and CH.sub.4/CO.sub.2 selectivity in the retentate of up to 8.1.

[0064] The infrared spectra indicate the presence of carbonate bands, increasing over the test time, as shown in FIG. 9. SEM analysis, shown in FIG. 10, prove the presence of inorganic carbonate crystals, formed by enzymatic reaction. Furthermore, it was quantified that the salinity of the production water was reduced from 68.9 to 62.8 g/L with the test, also indicating that the process promotes a partial treatment of this stream, which is an effluent from the oil and gas sector.

[0065] Therefore, the use of an effluent from the E&P area, combined with pH control of the solution, also proved to be technically feasible for the CO.sub.2 capture process.

TABLE-US-00002 TABLE 2 Divalent ion composition of the production water used in the tests. Ion Concentration (mg/L) Ca.sup.2+ 2530 Mg.sup.2+ 530 Sr.sup.2+ 7

Example 5

Capture of CO.SUB.2 .from Mixture with CH.SUB.4., using NaOH Solution as Absorber Solution, with pH Adjustment

[0066] A gaseous stream containing 50% CO.sub.2/50% CH.sub.4 was passed continuously through the shell side (outside the fibers) of a contactor module containing hollow hydrophobic PTFE fibers with a total area of 1 m.sup.2, as shown in FIG. 1 (item 5), at a total flow rate of 40 cm.sup.3/min (STP). As a liquid phase, a 10 mM NaOH solution containing 0.1 g/L of CA was used, recirculated in a loop at a flow rate of 3.3 mL/s between the glass vessel, as shown in FIG. 1 (item 6), and the inside of the contactor fibers. The absorber liquid solution had an initial pH of about 11.5, which was adjusted intermittently during the process by adding 1 M NaOH solution when the pH reached values below 9, totaling an equivalent addition of 0.1 M NaOH to the phase liquid.

[0067] The liquid and gas passed in a countercurrent direction. FIG. 11 shows the results throughout the test, in which an enrichment of the retentate stream from 50% to 99.2% of CH.sub.4 was observed, with CO.sub.2 removal of up to 99.2% and CH.sub.4/CO.sub.2 selectivity in the retentate up to 124 times.

[0068] Therefore, this example demonstrates a strategy for efficiently using NaOH solution for the separation of CO.sub.2/CH.sub.4, under appropriate conditions for the enzyme to act, since with the batch fed with NaOH solution (for the intermittent control of pH) having very high pH conditions in the liquid is avoided, which could cause the loss of CA activity.

[0069] It should be noted that, although the present invention has been described in relation to the attached drawings, it may undergo modifications and adaptations by technicians skilled on the subject, depending on the specific situation, but provided that within the inventive scope defined herein.