HK1056864B - Syngas purification process - Google Patents

Description

Process for the purification of synthesis gas

Technical collar city

The invention relates to a purified H₂/CO and/or H₂/N₂Process for forming synthesis gas by adsorption through at least one adsorption bed for removing CO₂And possibly other gaseous impurities (water, methane, ethane, NOx, etc.), said adsorbent bed comprising at least one adsorbent based on a NaLSX-type zeolite.

The impurities are adsorbed by passing the gas stream to be purified through an adsorption bed containing at least one adsorbent based on a NaLSX-type zeolite and subsequently desorbed in a regeneration step, which may be carried out by increasing the Temperature (TSA) and/or reducing the pressure (PSA or VSA).

The process is advantageously carried out before the synthesis gas thus purified is subjected to a cryogenic operation to separate hydrogen from CO and nitrogen.

Background

The generic term "synthesis gas" is used for gases consisting essentially of hydrogen and CO (about 25% by volume CO), which can be used as reaction products in certain basic chemical syntheses (methanol, acetic acid, phosgene, acrylic acid, etc.). These synthesis gases are usually water or CO by partial oxidation or hydrocarbon feed (from natural gas to heavy hydrocarbons)₂Obtained by reforming reactions to obtain H₂+CO+CO₂+H₂O + mixture of other impurities, H₂、CO、CO₂And H₂The respective proportions of O vary depending on the synthesis conditions.

Within the scope of the present invention, the term "synthesis gas" also refers to H, which is used in particular for the synthesis of ammonia₂/N₂And (3) mixing. These mixtures are usually products obtained by partial oxidation of air or reforming of hydrocarbon feeds. This step can be supplemented by a reaction called "CO shift", i.e. CO + H₂O→CO₂+H₂Conversion of CO to CO₂And thereby provide more H₂。

For example when it is desired to separate CO and H₂Or N is₂And H₂When it is generally necessary to purify the synthesis gas, said purification may be carried out cryogenically or by scrubbing with liquefied methane: it is absolutely necessary to remove all the impurities that may crystallize and thus block the cryogenically operated exchangers.

If CO is present in the synthesis gas stream to be purified₂In an amount greater than several thousand ppm, is first washed with an amine (MEA or MEDA type) to remove most of the CO₂. The gas is then sent to an adsorption column to remove residual small amounts of CO that are not removed by scrubbing with amines₂(tens of ppm) and other impurities that may be present in the synthesis gas, for example with CO in general₂Water is also present (after washing with amine, the gas is saturated with water).

In CO₂In the case of adsorption, the method for purifying synthesis gas by adsorption generally uses adsorbents based on zeolites of type 4A (NaA) or type 13X (NaX with Si/Al atomic ratio ≥ 1.25. + -. 0.05); however, these adsorbents have the disadvantage of relatively short adsorption/desorption cycle times, which require sufficiently frequent regeneration of the adsorbent material and increase the operating costs of the industrial adsorption plant.

In US5531808 and its corresponding EP718024 it is disclosed that LSX-type zeolites (low-silica or zeolite X with a low silica ratio, i.e. a Si/Al atomic ratio ≈ 1), (which is irrelevant whether or not it is exchanged with cations of groups 1A, 2A, 3B and/or lanthanides, etc.) are used for being less polar than CO₂In particular air. According to US5531808 and EP718024, the process can only be operated effectively at adsorption pressures of typically 0.02-2 MPa.

Disclosure of Invention

The process of the invention employs an adsorbent bed containing an adsorbent based on a NaLSX type zeolite having a Si/Al of 0.9 to 1.1, preferably 1 to 1.05, which is particularly advantageous compared to adsorbent beds based on 4A or NaX zeolites, since it provides a longer cycle time and therefore a reduced frequency of regeneration.

Within the scope of the present invention, the term "NaLSX-based adsorbent" is understood to mean that its zeolitic active substance consists essentially of NaLSX zeolite but may also consist of a mixture of NaLSX zeolite and NaX zeolite, as detailed in the applicant's W001/24923.

The NaLSX-based adsorbent of the process of the invention can be used in powder form (where the NaLSX zeolite is conventionally synthesized), or preferably in the form of granules, beads or extrudates, the preferred forms having the advantage of making the adsorbent easier to handle, for example in the steps of filling and emptying the adsorption column, and in particular limiting the head loss (les pertes de charge) when the gas stream passes through it during its use in operation.

For the sintering (l 'aglomerization) process, the actual LSX zeolite is first mixed with a sintering binder, which is generally in powder form per se, in the presence of water, and the mixture is subsequently converted into agglomerates by, for example, extrusion or beading, and the zeolite/binder mixture formed is heated to about 400 ℃ to 700 ℃ in order to convert the "fresh agglomerates (l' aglomerization" vert) "into compression-resistant agglomerates. Binders for the sintered zeolite include clays (particularly preferably those provided by the applicant), silicas, aluminas, metal oxides and mixtures thereof.

It is possible to prepare sintered compacts containing from 5 to 10% by weight of residual binder. One way of obtaining these agglomerates with a low binder content consists in converting the binder used for the agglomerates into a zeolitic phase. To this end, the LSX zeolite powder is first sintered with a binder capable of being converted into zeolite (for example kaolinite or metakaolinite) and subsequently converted into zeolite by alkali impregnation, for example according to the method disclosed in EP 932581. According to the invention, significantly effective particles containing at least 90% zeolite can thus be easily obtained.

Furthermore, the zeolite may also be sintered with materials such as silica/alumina, silica/magnesia, silica/zirconia, silica/thoria (thorine), silica/beryllia and silica/titania, as well as ternary compositions such as silica/alumina/thoria, silica/alumina/zirconia and clay present as a binder.

The relative proportions of the materials making up the binder and zeolite can vary over a wide range. The content of the sintering binder is usually 5 to 30 parts by weight per 100 parts of the agglomerate. The agglomerates advantageously have an average diameter of from about 0.2mm to about 5 mm.

The process for purifying synthesis gas, that is to say synthesis gas based on hydrogen and containing at least nitrogen and/or at least CO, is such that: each adsorption bed was sequentially subjected to a treatment cycle comprising the following steps:

a) passing a gas mixture based on hydrogen, carbon monoxide and/or nitrogen and containing at least carbon dioxide and one or more other impurities as impurities through an adsorption zone comprising:

at least one adsorbent capable of selectively adsorbing carbon dioxide, comprising at least one X zeolite of the faujasite type having a Si/Al ratio close to 1, preferably ranging from 0.9 to 1.1, more preferably ranging from 1 to 1.05, in which at least 70%, preferably at least 90%, of the exchangeable sites are occupied by sodium ions, the remaining cationic sites being occupied by cations of the K-or Ca-type, or by other monovalent and/or multivalent cations (magnesium ions, strontium ions, barium ions, lanthanides or rare earths, etc.),

-one or more possible other adsorbents capable of adsorbing CO₂Other impurities than water, hydrocarbons (light or heavy) and nitrogen oxides-N₂O, NO and NO₂(commonly referred to as NOx),

the adsorbents are placed layer by layer and/or in an intimate mixture;

b) the purging step consisting in recycling the partially purified gas can be supplemented to this step by desorbing carbon dioxide and possibly other one or more impurities adsorbed on the adsorbent or adsorbents described in a) by raising and/or reducing the pressure; and

c) the adsorption zone is pressurized by introducing a purified gas stream through the outlet of the adsorption zone and/or by flushing the adsorption zone with purified cold gas to cool it.

Thus, each adsorbent bed is subjected to a treatment cycle comprising a first step of producing purified synthesis gas and a second step in which the combined decompression, heating, recompression and cooling of the adsorbent can be regenerated.

The purification process of the present invention is equally well suited for purifying synthesis gas that also contains other impurities such as water, methane, ethane and other hydrocarbon compounds. The inventors have further demonstrated that the presence of other compounds contained in the synthesis gas, in particular CO, makes the adsorption of carbon dioxide more difficult.

The process of the invention is particularly suitable for CO in the gas mixture to be purified₂The concentration is not particularly high, that is to say:^*in general, for an adsorption pressure of about 3MPa (which is expressed as CO)₂Corresponding to a value less than or equal to 3 Pa), less than or equal to 1000 ppm;

^*for an adsorption pressure of about 3MPa (expressed as CO)₂Corresponding to a value less than or equal to 0.3 Pa), preferably less than or equal to 100 ppm.

When the synthesis gas to be purified also contains water, the NaLSX-based adsorbent can be used alone, but also on the basis of CO₂Adding one or more adsorbents capable of selectively adsorbing water, such as alumina, silica gel, A-type zeolite or X-type zeolite (Si/Al atomic ratio is more than or equal to 1.25 +/-0.05) into an adsorption tower of the selective NaLSX adsorbent; the selective adsorbent for water or water can be used as disclosed in EP862936 or EP904825 as a selective adsorbent for CO-based adsorption₂The adsorbent for selective NaLSX is used in the form of an intimate mixture or, preferably, as disclosed in EP862938, to be placed in an adsorption columnIn the form of a separate layer in CO₂Upstream of the selective adsorbent.

When the synthesis gas to be purified also contains heavy hydrocarbons as impurities, such as butanes, pentanes, etc., it is possible to use the NaLSX-based adsorbent alone, but it is preferred to use it in an adsorption column based on CO₂The selective NaLSX adsorbent is added with one or more adsorbents capable of selectively adsorbing heavy hydrocarbons, such as alumina, silica gel or activated carbon, or zeolites; the selective adsorbent for the heavy hydrocarbon or hydrocarbons can be used as a selective adsorbent for hydrocarbons based on CO₂The adsorbent for the selective NaLSX is used in the form of an intimate mixture or, preferably, in the form of a layer placed in the separation in an adsorption column, the layer being located in the CO₂Upstream of the selective adsorbent.

When the synthesis gas to be purified also contains light hydrocarbons such as ethane, ethylene, propylene and/or NOx as impurities, the NaLSX-based adsorbent can be used alone, but preferably in an adsorption column towards a CO-based adsorbent₂The selective NaLSX adsorbent incorporates one or more adsorbents capable of selectively adsorbing light hydrocarbons and/or NOx, such as aluminas, silica gels or activated carbon, or zeolites; the selective adsorbent for the hydrocarbon or hydrocarbons can be used as a CO-based adsorbent₂The adsorbent for the selective NaLSX is used in the form of an intimate mixture or, preferably, in the form of a layer placed in the separation in an adsorption column, the layer being located in the CO₂Downstream of the selective adsorbent.

When the synthesis gas to be purified contains water and/or heavy hydrocarbons and NOx and/or light hydrocarbons as impurities, the NaLSX-based adsorbent can be used alone, but preferably in an adsorption column towards the CO-based adsorbent₂The selective NaLSX adsorbent is added with an adsorbent that selectively adsorbs water and/or heavy hydrocarbons, which may be in the form of an intimate mixture as described in the example of EP1101521, or preferably placed in the form of separate layers:

at CO₂Upstream of the selective adsorbent, one or more adsorbents capable of selectively adsorbing water and/or heavy hydrocarbonsAn additive;

and in CO₂Downstream of the selective adsorbent, one or more adsorbents capable of selectively adsorbing light hydrocarbons and/or NOx.

Furthermore, the process of the invention may be combined with any other process for removing other impurities not mentioned above and which may also be present in the synthesis gas: for example, if the synthesis gas contains trace amounts of mercury (from the hydrocarbon feed), it can be removed by a layer of silver-exchanged zeolite placed in the adsorption zone of the present invention and desorbed during thermal regeneration. This is because it is often necessary to collect the mercury vapor before introducing the gas into the cryogenic device in order to avoid any corrosion of the exchangers. These traces of mercury can also be removed upstream or downstream of the apparatus of the present invention using iodine or sulfur-impregnated activated carbon.

The purity of the synthesis gas obtained according to the purification process of the invention is very high: can reach CO₂The residual concentration of impurities is less than 0.1vpm, and the water impurities are less than 0.1 vpm.

As a rule, within the scope of the process according to the invention, the pressure in the adsorption zone is maintained at from 0.5 to 7MPa while the mixture to be purified is brought into contact with the abovementioned adsorbent. While higher pressures do not impair the purification operation. But generally avoids the pressure higher than 7MPa from the viewpoints of energy saving and higher cost of pressure-resistant equipment. For practical reasons, pressures below 0.5MPa are not generally used for industrial synthesis gas production: this is because, in correspondence with the reaction for the preparation of synthesis gas, the operations carried out downstream of the process of the invention are generally carried out at a pressure of about 2 to 3 MPa. The pressure in the adsorption zone is preferably maintained at a level lower than or equal to 5MPa, more advantageously lower than or equal to 3 MPa. Likewise, the pressure in the adsorption zone is preferably maintained at greater than or equal to 0.5MPa, and greater than or equal to 2MPa being more advantageous.

The temperature of the gas stream entering the adsorption zone is not critical and is generally kept constant during the adsorption phase. In general, the temperature is from 0 to 80 ℃ and preferably from 20 to 50 ℃. The desorption temperature may be 100-300 deg.C, preferably between 150-250 deg.C.

The present invention is applicable to any type of PSA, VSA and/or TSA operation for purifying synthesis gas, and therefore the above-described essential features of the process of the present invention may be advantageously combined with any variation of parameters such as pressure level, cleaning rate, etc. that contribute to improved operating performance.

The invention can be used both in the context of a new plant for the purification of synthesis gas, with the same productivity, with a reduction in the size of the column (and therefore in the investment costs) compared to the industrial plants of the prior art, and also in the case of replacement of the adsorbent present in the column of the industrial plant with the adsorbent according to the invention, with a significant increase in productivity (or reduction in the number of regenerations required).

Detailed Description

Examples

In all examples, a gas stream of known composition is passed through a column filled with adsorbent until the CO₂Breakthrough (lapercoen CO)₂) And then the desorption operation, may be repeated several times.

The size of the adsorption tower used was as follows:

-diameter 2.7 cm; the height is 190 cm.

Synthesis gas having the following composition was used:

-H₂80% by volume (q.s.p);

-CO or N₂20% by volume;

-CO₂＝76vpm；

-H₂O＝2400vpm。

introducing CO₂And H₂O-analysers are placed at the outlet of the column to track their concentration changes after the circulation processAnd in particular for detecting CO₂Breakthrough point (lapercoeen CO)₂) Which is usually located before the point of water breakthrough.

The method comprises the following steps:

1. an adsorption step:

P＝2.3MPa；

T＝38℃；

total flow rate 6.7Nm³/h。

The time for the first adsorption is arbitrarily chosen (2-5h), without reaching CO₂A breakthrough point to limit the water front to the column. The following cycle is followed, continuing the adsorption until CO₂The point of breakthrough (up to 7vpm) was then automatically switched to desorption mode;

2. desorption step (counter-current run):

P＝2.3MPa；

the reaction is carried out under pure hydrogen;

flow rate H₂＝1.6Nm³/h/。

The temperature was gradually raised to 190 ℃ over 2 hours, followed by keeping the temperature at 190 ℃ for 2 hours, and then at the same flow rate (1.6 Nm)³H) with H₂And cooling the column in countercurrent for 2 hours;

3. before starting the adsorption step again, the latter step was supplemented with external cooling to reach T-45 ℃ without hydrogen flushing.

The cycle is repeated several times until CO₂The permeation time is stable.

The samples tested were made in the form of beads with a particle size of 1.6-2.5mm, consisting of 80% by weight of zeolite (active substance) and 20% of clay-based sintering binder.

Example 1 (comparative)

The gas to be treated had the following composition:

-H₂80% by volume;

-N₂20% by volume;

-CO₂＝76vpm；

-H₂O＝2400vpm。

the zeolite tested was NaX (Na exchange degree. apprxeq.100%; Si/Al. 1.23.) CO stable after several cycles₂The breakthrough time was 7.7 h.

Example 2 (comparative)

The gas to be treated had the following composition:

-H₂80% by volume;

-CO ═ 20 vol%;

-CO₂＝76vpm；

-H₂O＝2400vpm。

the zeolite tested was the same sintered NaX as in example 1.

CO Stable after several cycles₂The breakthrough time was 4.6 h.

This example clearly illustrates the effect of gas type on zeolite performance; at this time, the presence of CO is relative to the CO of the zeolite₂The effect of capacity is much greater than nitrogen.

Example 3 (comparative)

The gas to be treated had the same composition as in example 2.

The zeolite tested was a 4A zeolite (Na exchange degree ≈ 100%).

CO Stable after several cycles₂The breakthrough time was 2.7 h.

Example 4 (according to the invention)

The gas to be treated had the same composition as in example 2.

The zeolite tested was NaLSX (Na exchange 95.3%; Si/Al ═ 1.0).

CO Stable after several cycles₂The breakthrough time was 5.9 h.

Example 5 (comparative)

The difference from example 2 is that the gas to be treated is not wet. It has the following composition:

-H₂80% by volume;

-CO ═ 20 vol%;

-CO₂＝76vpm。

the zeolite tested was the same sintered NaX zeolite as in example 1.

CO Stable after several cycles₂The breakthrough time was 7.9 h.

Example 6 (comparative)

The gas to be treated had the same composition as in example 5.

The zeolite tested was the same sintered 4A zeolite as in example 3.

CO Stable after several cycles₂The breakthrough time was 3.6 h.

Example 7 (according to the invention)

The gas to be treated had the same composition as in example 5.

The zeolite tested was the same sintered NaLSX zeolite as in example 4.

CO Stable after several cycles₂The breakthrough time was 10.8 h.

As can be seen from the last six examples above, in the case of either the wet gas (examples 2-4) or the dry gas (examples 5-7), the cycle time of the NaLSX zeolite is much longer than the 4A and NaX zeolites conventionally used in such processes; the latter description corresponds to the method: wherein the zeolite is used as a second layer after the first layer of the adsorbent for removing water.

In the present plant, therefore, the NaLSX zeolite is regenerated less frequently, thus saving a lot of energy. For the new plant, it reduces the size of the column and the amount of adsorbent used.

Claims (27)

1. Process for the purification of synthesis gas based on hydrogen and carbon monoxide and/or nitrogen, contaminated with carbon dioxide and one or more possible other impurities, comprising one or more cycles comprising the following successive steps:

a) passing a gas mixture to be purified through an adsorption zone comprising:

an adsorbent capable of selectively adsorbing carbon dioxide, comprising at least one X zeolite of the faujasite type having a Si/Al ratio close to 1, at least 70% of the exchangeable sites of which are occupied by sodium ions, the remaining cationic sites being occupied by cations of the K-or Ca-type, or by other monovalent and/or multivalent cations,

-one or more adsorbents capable of selectively adsorbing various impurities,

the adsorbents are placed layer by layer in an intimate mixture or in separate beds;

b) desorbing carbon dioxide and one or more other impurities adsorbed on the one or more adsorbents described in a) by increasing the temperature and/or decreasing the pressure, optionally supplementing to this step a purging step consisting in recycling the partially purified gas;

c) the adsorption zone is pressurized by introducing a purified gas stream through the outlet of the adsorption zone and/or by cooling the adsorption zone by flushing with a purified cold gas.

2. The process for the purification of synthesis gas according to claim 1, wherein the Si/Al ratio is comprised between 0.9 and 1.1.

3. A process for the purification of synthesis gas according to claim 2, wherein the Si/Al ratio is between 1 and 1.05.

4. A process for the purification of synthesis gas according to claim 1, wherein at least 90% of the exchangeable sites are occupied by sodium ions.

5. A process for the purification of synthesis gas according to claim 1, wherein the other monovalent and/or polyvalent cations are magnesium ions, strontium ions, barium ions or ions of rare earth elements.

6. A process for the purification of synthesis gas according to claim 5, wherein the other monovalent and/or polyvalent cation is a lanthanide ion.

7. The process for purifying synthesis gas of claim 1, wherein the various impurities are water, hydrocarbons and/or NOx.

8. A process for the purification of synthesis gas according to any of claims 1 to 7, said synthesis gas being CO-depleted₂And in addition water and/or heavy hydrocarbons as impurities, characterized in that the adsorbent or adsorbents capable of adsorbing water and/or heavy hydrocarbons are/is/are capable of selectively adsorbing CO₂In the form of an intimate mixture of adsorbents, or in the form of separate beds, one or more adsorbent beds capable of selectively adsorbing water and/or heavy hydrocarbons being placed in a bed capable of selectively adsorbing CO₂Upstream of the adsorbent bed.

9. A process for the purification of synthesis gas according to claim 8, wherein the adsorbent or adsorbents capable of adsorbing water and/or heavy hydrocarbons is/are selected from alumina, silica gel or zeolites of type A or type X.

10. A process for the purification of synthesis gas according to any of claims 1 to 7, said synthesis gas being CO-depleted₂And possibly water and/or heavy hydrocarbons, and one or more light hydrocarbons and/or NOx as impurities, characterized in that the adsorbent or adsorbents capable of adsorbing light hydrocarbons and/or NOx are/is a mixture of adsorbents capable of selectively adsorbing CO₂In the form of an intimate mixture with possibly an adsorbent capable of selectively adsorbing water and/or heavy hydrocarbons, or in the form of separate beds, one or more adsorbent beds capable of selectively adsorbing light hydrocarbons and/or NOx being placed in a position capable of selectively adsorbing CO₂Downstream of the adsorbent bed.

11. A process for the purification of synthesis gas according to claim 10, wherein the adsorbent or adsorbents capable of adsorbing light hydrocarbons and/or NOx are selected from alumina, silica gel or a-type or X-type zeolites.

12. A process for the purification of synthesis gas according to any of claims 1 to 7, said synthesis gas being CO-depleted₂And possibly water and/or heavy hydrocarbons, light hydrocarbons and/or NOx, and also mercury as an impurity, characterized in that the adsorption zone comprises a zeolite based on silver exchangeThe bed of (1).

13. A process for the purification of synthesis gas according to any of claims 1 to 7, said synthesis gas being CO-depleted₂And possibly water and/or heavy hydrocarbons, light hydrocarbons and/or NOx, containing mercury as an impurity, characterized in that it comprises a supplementary step consisting in passing, upstream or downstream of the process according to any one of claims 1 to 6, a gas stream from which mercury must be removed, through activated carbon impregnated with iodine or sulphur.

14. A process for the purification of synthesis gas according to any of claims 1 to 7, characterized in that the NaLSX type zeolite is present in sintered form with a sintering binder, optionally in a quantity of 5 to 30 parts by weight per 100 parts of sintered mass.

15. The process for the purification of synthesis gas according to claim 14, wherein the sintered binder is converted to zeolite.

16. A process for the purification of synthesis gas according to claim 14, wherein the agglomerates have an average diameter of 0.2-5 mm.

17. Process for the purification of synthesis gas according to any one of claims 1 to 7, characterized in that in the adsorption step a) the pressure of the gas mixture to be purified is greater than or equal to 0.5MPa and less than or equal to 7 MPa.

18. The process for the purification of synthesis gas according to claim 17, wherein the pressure of the gas mixture to be purified is greater than or equal to 2 MPa.

19. The process for the purification of synthesis gas according to claim 17, wherein the pressure of the gas mixture to be purified is less than or equal to 5 MPa.

20. The process for the purification of synthesis gas according to claim 19, wherein the pressure of the gas mixture to be purified is less than or equal to 3 MPa.

21. Process for the purification of synthesis gas according to any one of claims 1 to 7, characterized in that the temperature of the gas stream entering the adsorption zone is between 0 and 80 ℃ and the desorption temperature is between 100 and 300 ℃.

22. A process for the purification of synthesis gas according to claim 21, wherein the temperature of the gas stream entering the adsorption zone is between 20 and 50 ℃.

23. The process for the purification of synthesis gas according to claim 21, wherein the desorption temperature is between 150 ℃ and 250 ℃.

24. Process for the purification of synthesis gas according to any of claims 1 to 7, characterized in that for an adsorption pressure of 3MPa, the CO in the gas mixture to be purified₂Is less than or equal to 1000ppm and CO₂Is less than or equal to 3 Pa.

25. The process for the purification of synthesis gas according to claim 24, wherein the gas mixture to be purified contains CO₂Is less than or equal to 100 ppm.

26. The process for purifying synthesis gas of claim 24, wherein CO₂Is less than or equal to 0.3 Pa.

27. Process for the purification of synthesis gas according to any one of claims 1 to 7, characterized in that it is of PSA, VSA and/or TSA type.