PROCESS FOR THE REMOVAL OF H2S FROM NATURAL GAS AT HIGH PRESSURES BY MEANS OF A PSA PROCESS

Abstract

The present invention addresses to the use of NaY zeolite with a Si/Al ratio >2.6 as a solid adsorbent in the process of removing H.sub.2S from natural gas through a PSA process. The described adsorbent has the capacity of removing H.sub.2S from natural gas from offshore exploration platforms, enabling in situ regeneration. The experimental development proved the high capacity of capturing H.sub.2S by the NaY zeolite in consecutive cycles of pressurization, adsorption, depressurization and purging. This capture capacity remains at 74.2% of the initial capacity, remaining stable in subsequent cycles. The structure of the material maintained crystallinity above 95% in use, in 15 consecutive cycles, allowing the reuse of the adsorbent for a prolonged period of operation, preventing the solid from being constantly changed, which is quite common in a non-regenerative process.

Claims

1. A PROCESS FOR THE REMOVAL OF H.sub.2S FROM NATURAL GAS AT HIGH PRESSURES BY MEANS OF A PSA PROCESS, characterized in that it comprises the following steps: a) Promoting the contact of a stream of natural gas containing H.sub.2S with particles of the NaY zeolite adsorbent; b) Pressurization at a pressure of 20 to 80 bar (2 to 8 MPa) and a temperature of 25 to 70° C.; c) Adsorption under constant pressure between 20 and 80 bar (2 and 8 MPa) and at a temperature of 25 to 70° C.; d) Depressurization from 0.9 to 1.1 bar (0.09 to 0.11 MPa), at a temperature from 25 to 70° C.; e) Purge using H.sub.2S-free gas at a temperature of 25 to 70° C.

2. THE PROCESS according to claim 1, characterized in that the NaY zeolite has a Si/Al ratio equal to or greater than 2.6.

3. THE PROCESS according to claim 1, characterized in that the NaY zeolite has an average particle diameter equal to 0.1810 mm.

4. THE PROCESS according to claim 1, characterized in that steps b) and c) are conducted preferably at a pressure of 51 bar (5.1 MPa) and a temperature of 30° C.

5. THE PROCESS according to claim 1, characterized in that step d) is conducted preferably at a pressure of 1 bar (0.1 MPa) and a temperature of 30° C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0040] 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, wherein:

[0041] FIG. 1 shows the flowchart of the adsorption equipment at high pressures, which was used to evaluate the potential application of NaY zeolite with a Si/Al ratio greater than 2.6 in the adsorption of H.sub.2S, where there are represented: pressurization gas (1) and adsorbate (2); syringe pump (3); micrometric valve at the inlet (4) and outlet (8) of the adsorption bed; adsorption bed (5); mass flow rate meter (6); vacuum pump (7); 6-way electric valve (9); gas chromatograph (10); and extractor (11);

[0042] FIG. 2 illustrates the breakdown curves of H.sub.2S in NaY zeolite obtained in the adsorption/desorption cycles;

[0043] FIG. 3 shows a graph containing the amounts of H.sub.2S adsorbed in each adsorption cycle;

[0044] FIG. 4 illustrates (a) the X-ray diffractogram of the NaY zeolite, measured before the adsorption and desorption cycles, and (b) the X-ray diffractogram measured after the adsorption/desorption cycles;

[0045] FIG. 5 illustrates adsorption tests carried out in the first stage showing the breakdown curves of cycles 1 to 5;

[0046] FIG. 6 shows the absorbed amount of H.sub.2S/kg zeolite per cycle;

[0047] FIG. 7 shows the adsorption of the CH.sub.4+CO.sub.2+H.sub.2S+He mixture.

DETAILED DESCRIPTION OF THE INVENTION

[0048] The process for removing H.sub.2S from natural gas at high pressures by means of the PSA process, as described by the invention, comprises the following steps: [0049] a) Promoting the contact of a stream of natural gas containing H.sub.2S with particles of the NaY zeolite adsorbent with Si/Al ratio >2.6; [0050] b) Pressurization at a pressure of 20 to 80 bar (2 to 8 MPa) and a temperature of 25 to 70° C.; [0051] c) Adsorption under constant pressure between 20 and 80 bar (2 and 8 MPa) and at a temperature of 25 to 70° C.; [0052] d) Depressurization from 0.9 to 1.1 bar (0.09 to 0.11 MPa), at a temperature from 25 to 70° C.; [0053] e) Purge using H.sub.2S-free gas at a temperature of 25 to 70° C.

[0054] Steps b and c are preferably conducted at a pressure of 51 bar (5.1 MPa) and a temperature of 30° C. Step d is preferably conducted at a pressure of 1 bar (0.1 MPa) and a temperature of 30° C. The natural gas stream for the PSA process has a content of up to 50,000 ppmv of H.sub.2S (5.0% mol.Math.mol.sup.−1 H.sub.2S), highlighting that the outlet natural gas has a H.sub.2S content of less than 5 ppmv.

[0055] Consecutive adsorption and desorption tests were carried out in a high-pressure adsorption module shown in FIG. 1. This module allows pressurization gases (1) or adsorbate (2) to be fed into a syringe pump (3), which controls the gas pressure at the inlet of the bed (5). The module also has a micrometric valve (4), which allows pressurization control at the bed inlet (5), when necessary; an adsorption bed (5), which allows accommodation of the adsorbent solid; a mass flow rate meter (6); a vacuum pump (7); a micrometric valve at the bed outlet (8) allowing the control of the volumetric flow rate at the outlet; an electric 6-way valve (9) allowing the periodic injection of portions of the effluent gas from the adsorption bed; a gas chromatograph (10), equipped with a thermal conductivity detector (TCD). The effluent gases are led to the exhaustion (11).

[0056] The invention presents a new application of NaY zeolite with Si/Al ratio >2.6, that is, the removal of hydrogen sulfide from a stream of natural gas at pressures from 20 to 80 bar (2 to 8 MPa), for the production of gas practically free of H.sub.2S at high pressures. The material shows partial regenerability in this process, when the total operating pressure is reduced to the atmospheric pressure. In this way, there is the possibility of using the material in a fixed bed for an extended period of time. Thus, its main advantage is the possibility of regenerating the NaY zeolite in the process equipment, that is, in situ, eliminating or reducing the changing of solid material, a process that is very costly in offshore operations.

EXAMPLES

[0057] The following examples are presented in order to illustrate some particular embodiments of the present invention, and should not be interpreted as limiting the same.

[0058] To prove the use of NaY zeolite with Si/Al ratio >2.6 as a regenerable material in face of the adsorption of H.sub.2S in a PSA process, the following steps of preparation of NaY zeolite particles without a binder, in adsorption/desorption cycles of H.sub.2S, were carried out as described below.

Example 1: Preparation of NaY Zeolite Particles without Binder

[0059] The NaY zeolite used, with Si/Al ratio=2.8, in powder form, was pelletized in a press at 8 ton for 5 minutes. The formed pellet was crushed and classified in sieves. The content retained between 65 and 100 mesh sieves was collected, making the mean particle diameter equal to 0.1810 mm. The solid was inserted into the adsorption bed (5) of FIG. 1.

Example 2: Adsorption of He+H.SUB.2.S

[0060] Consecutive adsorption and desorption tests were carried out in an adsorption module at high pressures, which is illustrated in FIG. 1. This module allows pressurization gases (1) or adsorbate (2) to be fed into a syringe pump (3), which controls the gas pressure at the inlet of the bed (5). The module also has a micrometric valve (4), which allows pressurization control at the inlet of the bed (5), when necessary; an adsorption bed (5), which allows accommodation of the adsorbent solid; a mass flow rate meter (6); a vacuum pump (7); a micrometric valve at the bed outlet (8), allowing the control of the volumetric flow at the outlet; an electric 6-way valve (9), allowing the periodic injection of portions of the effluent gas from the adsorption bed; a gas chromatograph (10), equipped with a thermal conductivity detector (TCD). The effluent gases are led to the exhaustion (11). The tests followed the steps described below: [0061] a) Prior to adsorption, a heat treatment aimed at removing water was employed. Initially, helium was drained at ambient pressure with a flow rate of 50 mL min.sup.−1, at a temperature of 300° C., achieved through a heating ramp of 10° C. min.sup.−1. Then, a vacuum was created in the system by the vacuum pump (7). This step lasted 6 hours and was performed just before the first adsorption cycle; [0062] b) After the activation procedure, the consecutive adsorption/desorption cycles were carried out following the sequence of events: Pressurization, Adsorption, Depressurization and Purging. [0063] I. In the pressurization step, the bed was pressurized to an absolute pressure of 51 bar (5.1 MPa) and a temperature of 30° C. [0064] II. In the adsorption step, a mixture of 4.96% mol.Math.mol.sup.−1 of H.sub.2S in 95.04% of He was passed through the bed (5), at a flow rate of 100 NmL.Math.min.sup.−1; [0065] III. In the desorption step, the bed was depressurized from an absolute pressure of 51 bar (5.1 MPa) to an absolute pressure of 1 bar (0.1 MPa); [0066] IV. In the purge step, He was fed at 100 NmL.Math.min.sup.−1, at a total pressure of 1 bar (0.1 MPa), for 30 min; [0067] c) At the end of the purge step, a new pressurization was performed and the cycle was repeated. In total, the procedure was repeated 15 times; [0068] d) The composition of the gas at the bed outlet was calculated by means of the integration of the peaks detected by the TCD and subsequent application of the external standard method for quantification.

[0069] As shown in FIG. 2, it appears that the breakdown curves of cycles 2 to 15 are displaced from the curve of cycle 1, for shorter retention times, indicating a decrease in the capacity to capture H.sub.2S.

[0070] The calculation of the absolute amount adsorbed in each cycle, performed by means of the material balance applied in each adsorption step, discloses that in the first adsorption run the material has the capacity to remove 6.59 mol of H.sub.2S per kg of NaY zeolite. In later cycles, this amount reduced, on average, by 25.8%, to 4.89 mol of H.sub.2S per kg of NaY zeolite. Despite the reduction, this value remained stable in subsequent cycles, as indicated by FIG. 3.

[0071] The X-ray diffractometry performed on the material before (FIG. 4 (a)) and after (FIG. 4 (b)) cycles of adsorption/desorption of H.sub.2S showed no significant difference. The characteristic peaks of NaY zeolite (12 to 19) do not show horizontal displacement and practically have the same shape and intensity. In fact, the application of the method described by ASTM D396-19—“Standard Test Method for Determination of Relative X-ray Diffraction Intensities of Faujasite-Type Zeolite-Containing Materials”, indicates low crystallinity reduction. The zeolite subjected to adsorption and desorption cycles showed 96.2% of the crystallinity of the starting zeolite. Such a fact discloses that the structure of the material was not severely affected by continuous contact with H.sub.2S, which proves the stability of the adsorbent solid in face of the consecutive adsorption of H.sub.2S.

Example 3: Adsorption of CH.SUB.4.+CO.SUB.2.+H.SUB.2.S

[0072] Another adsorption test was conducted, with the objective of verifying whether the adsorbent material was selective to H.sub.2S even in the presence of CH.sub.4 and CO.sub.2 gases, the main components of natural gas. This test was carried out in two stages. In Step 1, 5 cycles of adsorption and desorption of H.sub.2S+He gas were performed, allowing the chemisorption of H.sub.2S. In Step 2, 2 cycles of adsorption and desorption of gas containing CH.sub.4+CO.sub.2+H.sub.2S+He were carried out, allowing to evaluate the selectivity of adsorption of H.sub.2S in relation to CH.sub.4 and CO.sub.2.

[0073] Step 1 was conducted in accordance with the procedures described below: [0074] a) A thermal treatment was used to remove the water contained in the adsorbent material. Initially, helium was drained at ambient pressure with a flow rate of 50 mL.Math.min.sup.−1, at a temperature of 300° C., achieved through a heating ramp of 10° C..Math.min.sup.−1. Then, a vacuum was created in the system by the vacuum pump (7). This procedure lasted 6 hours and was performed just before the first adsorption cycle; [0075] b) After the activation procedure, the consecutive adsorption/desorption cycles were carried out following the sequence of events: Pressurization, Adsorption, Depressurization and Purging. [0076] I. In the pressurization process, the bed was pressurized to an absolute pressure of 51 bar (5.1 MPa) and a temperature of 30° C.; [0077] II. In the adsorption process, a mixture of 4.96% mol mol.sup.−1 H.sub.2S in 95.04% He was passed through the bed (5) at a flow rate of 300 NmL.Math.min.sup.−1; [0078] III. In the desorption process, the bed was depressurized from an absolute pressure of 51 bar (5.1 MPa) to an absolute pressure of 1 bar (0.1 MPa); [0079] IV. In the purging process, He was fed at 300 NmL.Math.min.sup.−1, at a total pressure of 1 bar (0.1 MPa), for 30 min. [0080] c) At the end of the purging process, a new pressurization was performed and the cycle was repeated. This procedure was repeated 5 times.

[0081] In Step 2, the CH.sub.4+CO.sub.2+H.sub.2S+He adsorption/desorption tests were conducted following the sequence of events: Pressurization, Adsorption, Depressurization and Purging. [0082] a) In the pressurization process, the bed was pressurized to an absolute pressure of 51 bar (5.1 MPa) and a temperature of 30° C.; [0083] b) In the adsorption process, a mixture was passed through the bed (5) which contained 27.6% mol.Math.mol.sup.−1 of CH.sub.4, 16.9% mol.Math.mol.sup.−1 of CO.sub.2, 1.98% mol.Math.mol.sup.−1 of H.sub.2S and 53.5% mol mol.sup.−1 of He at a flow rate of 400 NmL.Math.min.sup.−1; [0084] c) In the desorption process, the bed was depressurized from an absolute pressure of 51 bar (5.1 MPa) to the absolute pressure of 1 bar (0.1 MPa); [0085] d) In the purge process, He was fed at 400 NmL.Math.min.sup.−1 at a total pressure of 1 bar (0.1 MPa) for 30 min.

[0086] At the end of the purge step, a new pressurization was performed and the cycle was repeated.

[0087] The gas composition at the bed outlet was calculated by integrating the peaks detected by the TCD and subsequent application of the external standard method for quantification.

[0088] Regarding the adsorption tests performed in the first stage, as shown in FIG. 5, the breakdown curves of cycles 1 to 5 practically coincide, suggesting that there is little change in the amount adsorbed from one cycle to the next.

[0089] The calculation of the absolute amount adsorbed in each cycle, performed by means of the material balance applied in each adsorption step, discloses that in the first adsorption run, the material has the capacity to remove 5.92 mol of H.sub.2S per kg of NaY zeolite. In later cycles, this amount reduced, on average, by 25.5%, to 4.47 mol of H.sub.2S per kg of NaY zeolite. Despite the reduction, this value remained stable in subsequent cycles, as shown in FIG. 6.

[0090] With regard to the adsorption of the mixture CH.sub.4+CO.sub.2+H.sub.2S+He, it is shown in FIG. 7 that the breakdown curves of CH.sub.4 and CO.sub.2 are prior to the breakdown curve of H.sub.2S, disclosing that CH.sub.4 and CO.sub.2 are effluent from the adsorption bed before H.sub.2S. It is observed that a free stream of H.sub.2S is produced for a time of 32.2 minutes in the first cycle and 37.7 minutes in the second cycle.

[0091] The breakdown curves of CH.sub.4 and CO.sub.2 show the roll-up effect, that is, their concentrations at the outlet exceed the respective initial concentrations (Li, G., Xiao, P., Xu, D., & Webley, P. A. (2011). Dual mode roll-up effect in multicomponent non-isothermal adsorption processes with multilayered bed packing. In Chemical Engineering Science (Vol. 66, Issue 9, pp. 1825-1834). Elsevier BV. https://doi.org/10.1016/j.ces.2011.01.023). This phenomenon is generally related to the displacement of one component by the other due to different affinities with the solid surface. In this context, it is suggested that both CH.sub.4 and CO.sub.2 are displaced by H.sub.2S.

[0092] The high amount of capture verified, the stability of the amount captured in subsequent cycles, even with prolonged contact with H.sub.2S in very high concentration, combined with the high stability of the structure in the adsorption of this corrosive compound, make the NaY zeolite a promising adsorbent for removal of H.sub.2S contained in natural gas, at high pressures, through a PSA process.

[0093] 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.