METHOD OF DECARBONATING GAS STREAMS

Abstract

The present invention concerns a method of decarbonating a gas stream containing from 15% to 60% carbon dioxide, by passage of the said gas stream over a zeolitic agglomerate comprising at least one binder and at at least one zeolite, and having a mesoporous volume of between 0.02 cm.sup.3.Math.g.sup.1 and 0.15 cm.sup.3.Math.g.sup.1 and a mesoporous volume fraction of between 0.1 and 0.5, preferably between 0.15 and 0.45.

Claims

1. A method of decarbonating a gas stream, the said method comprising at least the steps of: a) supply of a gas stream containing 15% to 60% carbon dioxide, expressed as volume relative to the total volume of the gas stream, b) passage of said gas stream over a zeolitic agglomerate, and c) recovery of a gas stream depleted of CO.sub.2, method wherein the zeolitic agglomerate comprises at least one binder and at least one zeolite, and has a mesoporous volume between 0.02 cm.sup.3.Math.g.sup.1 and 0.15 cm.sup.3.Math.g.sup.1 and a mesoporous volume fraction between 0.1 and 0.5.

2. The method as claimed in claim 1, wherein the gas stream to be decarbonated contains from 15% to 60% carbon dioxide, by volume relative to the total volume of the gas stream.

3. The method as claimed in claim 1, wherein the gas stream to be decarbonated has a CO.sub.2 content ranging from 15% to 60%, by volume relative to the total volume of the gas stream, and further contains one or more of the following gases: carbon monoxide (CO), generally from 20% to 50%, by volume relative to the total volume of said gas flow, hydrogen (H.sub.2), from 15% to 25% by volume relative to the total volume of said gas flow, nitrogen (N.sub.2) and/or argon (Ar), from 5% to 15% by volume relative to the total volume of said gas flow, methane (CH.sub.4), from 0.5% to 50% by volume relative to the total volume of said gas stream, water (H.sub.2O), up to 4%, hydrogen sulphide (H.sub.2S), from 80 ppmv to 100 ppmv.

4. The method as claimed in claim 1, wherein the zeolitic agglomerate comprises crystals of zeolite(s) agglomerated with between 1% and 30% by weight, relative to the total weight of the zeolitic adsorbent, of a clay binder.

5. The method as claimed in claim 1, wherein the clay binder of the zeolitic agglomerate comprises at least one clay selected from among fibrous magnesium clays.

6. The method as claimed in claim 1, wherein the said-at least one zeolite of the zeolitic agglomerate is a zeolite in the form of Faujasite type zeolitic crystals, selected from among X, LSX, MSX and Y zeolites and mixtures thereof.

7. The method as claimed in claim 1, wherein the at least one zeolite of the zeolitic agglomerate is a zeolite in the form of crystals present in sodium form, with a sodium content expressed as sodium oxide (Na.sub.2O) greater than 9.0% by weight of oxide relative to the total weight of the agglomerate.

8. The method as claimed in claim 1, the method being carried out at a temperature of between 0 C. and 100 C.

9. The method as claimed in claim 1, the method being carried out at a pressure of between 200 kPa and 400 kPa.

10. Utilization, for the decarbonation of a gas stream, of a zeolitic agglomerate defined in claim 1.

Description

EXAMPLES

[0089] The following examples present tests of adsorption of CO.sub.2 included in a CO-rich gas stream, according to a VPSA type method, utilising different adsorbents.

[0090] The adsorbents tested are listed below:

Sample a: A2AW silica gel from the KD Corporation in the form of beads of size 2-5 mm.
Sample B: zeolitic agglomerate of mesoporous volume equal to 0.065 cm.sup.3.Math..sup.1, and the mesoporous volume fraction is equal to 0.31 comprising about 20% attapulgit binder and about 80% type FAU zeolite with a Si/AI ratio of 1.19, the zeolitic agglomerate being in the form of beads with a granulometry of 1.6-2.5 mm.
Sample C: zeolitic agglomerate similar to sample B, but in which the agglomeration binder is sepiolite

[0091] The charge in the adsorption column of a pilot installation is identical with each sample and is equal to 379 g. The column of the adsorption pilot has a diameter of 2.2 cm and a height of 2 m. The height of the charge in the column varies for each sample, depending on the density of each sample:

height for sample A: 1.45 m;
height for sample B: 1.59 m;
height for sample C: 1.49 m.

[0092] The gas mixture that feeds the adsorption column has a volume composition of 35% CO, 35% CO.sub.2, 10% N.sub.2 and 20% H.sub.2. The feed flow rate is set at 8 NL.Math.min.sup.1 and 16 NL.Math.min.sup.1 at a temperature of 40 C. and a pressure of 300 kPa.

[0093] Upon completion of the adsorption step, the regeneration is performed for 100 seconds while reducing the pressure to a vacuum level of 20-30 kPa, including a purge step of 50 seconds under vacuum by means of a gas of volume composition of around 62% CO, 2.7% CO.sub.2 and the remainder to 100% of N2.

Example 1

[0094] The total adsorption capacities of CO.sub.2 of sample A and sample B are compared in order to study the difference between a silica gel and a zeolitic agglomerate. The gaseous mixture that feeds the column is as described above. The feed flow rate is set at 8 NL.Math.min.sup.1 at a temperature of 40 C. and a pressure of 300 kPa. As previously indicated, the same quantity of adsorbent of 379 g is used for each sample.

[0095] The adsorption phase is stopped when the samples are completely saturated with CO.sub.2, i.e. the concentrations of the constituents of the incoming gas stream are identical with those of the outgoing gas stream. The adsorption time required for each sample to obtain the total saturation thereof is as follows:

Sample A: 450 seconds;
Sample B: 1055 seconds.

[0096] The breakthrough time to reach a volume concentration of 0.1% of CO.sub.2, for each sample is:

Sample A: 63 seconds;
Sample B: 747 seconds.

[0097] These results clearly show that, in equal quantity, sample B (zeolitic agglomerate according to the invention) has a total adsorption capacity of CO.sub.2 more than twice what was observed with sample A (silica gel, comparative sample). If the adsorption is stopped at the beginning of the breakthrough, the performance of sample B is about 12 times better than that of sample A.

[0098] This example clearly shows that the use of sample B (zeolitic agglomerate according to the invention) leads to a much longer use than when a silica gel is used, if the same quantity of adsorbent is used. Moreover, the adsorption unit can be made more compact by using a zeolitic agglomerate rather than a silica gel, which makes it possible to reduce both investment and operational costs.

Example 2

[0099] Under the same operating conditions as in the example 1, this example compares the dynamic adsorption capacities of CO.sub.2 of samples A and C in order to study the differences between a silica gel and a molecular sieve. The feed flow rate is set at 16 NL.Math.min.sup.1 at a temperature of 40 C. and a pressure of 300 kPa. The quantity of each sample used is 379 g.

[0100] The adsorption phase is stopped when the volume concentration of CO.sub.2 of the outgoing gas stream reaches 2.6% (breakthrough concentration).

[0101] Upon completion of the adsorption step, the regeneration is performed for 100 seconds while reducing the pressure to a vacuum level of 20 kPa, including a purge step of 50 seconds under vacuum by means of a gas of volume composition of around 62% CO, 2.7% CO.sub.2 and the remainder to 100% of N.sub.2. Fifteen (15) adsorption/desorption cycles are performed to reach a stable breakthrough time for each sample.

[0102] The breakthrough time to reach a volume concentration of 2.6% of CO.sub.2, for each sample is:

Sample A: 15 seconds;
Sample C: 38 seconds.

[0103] These results clearly show that, in equal quantity, sample C (zeolitic agglomerate according to the invention) has a performance 2.5 times better than sample A (silica gel, comparative sample) and therefore by using a zeolitic agglomerate the time of use will be longer than with a silica gel if the same quantity of the adsorbent is used, due to the possibility of reducing the number of regeneration cycles.

Example 3

[0104] Under the same operating conditions as in the example 1, this example compares the dynamic adsorption capacities of CO.sub.2 of samples A and C in order to study the differences between a silica gel and a molecular sieve. The feed flow rate is set at 16 NL.Math.min.sup.1 at a temperature of 40 C. and a pressure of 300 kPa. The quantity of each sample used is 379 g. The regeneration of each of samples A and C is compared in order to study the differences between a silica gel and zeolitic agglomerate. The gas mixture entering the adsorption column has a volume composition of 35% CO, 35% CO.sub.2, 10% N.sub.2 and 20% H.sub.2. The feed flow rate is set at 8 NL.Math.min.sup.1 at a temperature of 40 C. and a pressure of 300 kPa. The quantity of adsorbent is identical for each sample and is 379 g.

[0105] The adsorption phase is stopped when the volume concentration of CO.sub.2 reaches 2.6% (breakthrough concentration). Upon completion of the adsorption step, the regeneration is performed for 100 seconds while reducing the pressure to a specific vacuum level, including a purge step of 50 seconds under vacuum by means of a gas of volume composition of around 62% CO, 2.7% CO.sub.2 and the remainder to 100% of N.sub.2. The specific vacuum level mentioned above varies depending on the sample in order to obtain the same breakthrough time of 36 seconds (stable after a multitude of cycles) in order to obtain an output volume concentration of 2.6% CO.sub.2 for each sample. Fifteen (15) adsorption/desorption cycles are performed to reach a stable breakthrough time for each sample.

[0106] The vacuum level for each sample is:

Sample A: 20 kPa;

Sample C: 40 kPa.

[0107] These results clearly show that, with an equal quantity of adsorbent and identical adsorbent performance, sample C, zeolitic agglomerate according to the invention, requires one half the amount of vacuum required for sample A (silica gel, comparative sample) and therefore by using a zeolitic agglomerate (sample C), operational costs can be reduced during regeneration.