DE2162652A1 - Purification of sour natural gas - Google Patents

Description

PATENT ADVOCATE DR. HANS-GUNTHER EGGERT, DIPLO'MCHEMIKER

5 COLOGNE-LINDENTHAL PETER-KINTGEN-STRASSE 2

Cologne, December 16, 1971 Eg / Ax / Wf

Union Carbide Corporation, 27o Park Avenue, New York, N.Y. 1QQ17, U.S.A.

Purification of sour natural gas

The invention relates to the purification of acidic natural gas, in particular the removal of H _g S and COp from natural gas by a selective adsorption process in which the need for purge gas for regenerating the adsorbent bed is reduced to a minimum.

The demand for natural gas as a fuel has reached such proportions that periods are already inadequate Delivery occurred. Even more serious shortages have been predicted. To improve this situation, Natural gas producers are increasingly dependent on fields of acid gas. There are numerous Reasons why the sour natural gas from these fields must be sweetened, however, are two of those most compelling reasons are the corrosive effects of the polluting sulfur compounds on the pipes and valve systems and the pollution of the atmosphere that occurs when the acid gas is burned, especially in areas with heavy natural gas consumption, e.g. B. in large industrial cities.

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As a means of sweetening natural gas, the selective adsorption of sulfur compounds, mainly HpS, on zeolitic molecular sieves, which serve as adsorbents, proposed. This general procedure enables the sweetening of these gases to a high degree of purity and the reliable and fast regeneration of the adsorbent used to remove the impurities, especially in comparison with the known ones Process of chemisorption.

Largely due to the fact that these sweetening processes are inevitably carried out in places that far from petroleum refineries and the chemical industry in general, cheap purge gases are for the desorption of the accumulated sulfur compounds from the adsorbent bed is seldom available. It is therefore It is common to use part of the previously purified natural gas product for this purpose. Because of the strong Affinity of HgS to used as adsorbent Molecular sieves and other factors, e.g. B. the applied working pressures and temperatures had to be known in these Method a relatively large amount of purge gas can be used. Paradoxically, increase when you look Turns to natural gas sources with higher concentrations of HgS, in order to meet the additional demand, the loss of purified natural gas due to its use as a purge gas.

The economic and technical circumstances involved in removing CO ₂ from natural gas and producing liquefied natural gas are similar to those involved in sweetening natural gas. During the process of cooling and liquefying the natural gas used, carbon dioxide, which is present in amounts of more than about 50 ppm, causes troublesome and unpleasant clogging of valves and lines, so that it must be removed. The selective adsorption of the carbon dioxide on a zeolitic molecular sieve using a portion of the purified raffinate is a common purification process.

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The invention relates to a method for reducing the HpS and COp content of natural gas by selective adsorption, the amount of purge gas required to regenerate the adsorbent bed compared to the previous ones conventional procedures is significantly reduced.

The objects set by the invention are achieved by a method in which a dry natural gas that as impurities, hydrogen sulfide or carbon dioxide or a mixture of hydrogen sulfide or carbon dioxide contains, passes through an adsorbent bed consisting of a zeolitic molecular sieve, the purified Withdraws product gas from the exit end of the bed while selectively adsorbing the contaminants in the bed, and then ceasing the supply of feed gas to the adsorbent bed and treating the bed regenerated. · The process is characterized in that one

a) natural gas heated to at least 200 ^° C. passes through the bed in cocurrent (in the same direction as the feed gas), the gas emerging from the bed is reheated and circulated through the bed for at least a sufficient time in order to bring the outlet end of the bed to a temperature of at least 200 ^° C. before the end of step (b),

b) the bed cocurrently with about 0.1 to 2.5 kg-moles, preferably 0.5 to 2.5 kg-moles, of purified Purge gas, preferably natural gas, is purged and flushed per 100 kg of adsorbent in the bed

c) then a purge gas at a temperature of less than 100 ° C in cocurrent through the bed and the exiting gas is cooled at least as long and returned in cocurrent to the bed until the outlet end of the bed is ^{cooled to a temperature of 100 0} C or below is.

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A typical embodiment of the process for sweetening Natural gas is shown in the flow diagram.

The term used here: "Natural gas" refers to a gas mixture which consists mainly of methane and usually small amounts of other gases including higher hydrocarbons, e.g. B. ethane and propane, carbon dioxide, nitrogen, Helium and sulfur compounds consists, the greater part of HpS and of small amounts or trace amounts consists of mercaptans. The natural gases emerging from the sources also always contain water vapor and are usually saturated with water. As the geological environment Fluctuations in the composition of a natural gas deposit are not a precise indication of the composition possible. Furthermore, the composition of the natural gas from a source changes during the removal of the gas from the same Drilling. The following two analyzes of gas samples from different deposits are representative:

II

CH ^ 96 78 ^C 2 ^H 6 1.5 7th ^C 3 ^H 8 0.2 1.2 0.6 2.0 N ₂ 1.0 2.4 H ₂ O 0.1 0.2 co ₂ 0.6 8.0 H ₀ S 400 (ppm) . 1.2

The natural gases which are expediently used in the sweetening process according to the invention can contain 1 to about 30,000 parts by volume of HgS per million parts by volume and up to about 400 ppm of HgO for reasons which will be discussed in more detail below. The relative concentration of weakly adsorbed or non-adsorbed permanent gases such as nitrogen and helium is not critically important. Within the above-mentioned range of the H ₀ S concentration, the

C.

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The concentration of any carbon dioxide that may be present in the feed gas is also reduced, but this COp is inevitably not completely removed. The degree to which the COp concentration is lowered depends on several factors including the concentration of HpS and COp in the feed gas and the amount of HLS that is allowed to be adsorbed on the bed during the adsorption stage before regeneration begins. COp is less adsorbed on molecular sieve zeolites than H _p S. Accordingly, the COp mass transition zone precedes the HpS mass transition zone as the two zones progress through the bed. So if a feed gas with a relatively high COp content is treated, the termination of the adsorption cycle immediately before the breakthrough of the HLS front has the consequence that most of the COp leaves the bed with the exiting natural gas. To a certain extent, however, COp and HpS are adsorbed together in the same bed zone, ie a certain amount of COp is removed with the HpS. In many cases, CO ₂ in natural gas is not disadvantageous and not objectionable.

If the COp concentration is to be reduced below 50 ppm, the natural gas must, by nature or as a result of prior treatment, _{contain less than about 1 ppm HpS and no more than about 30,000 ppm CO 2 in} order to be processed in accordance with the invention.

With regard to the removal of CO ₂ or H ₂ S or both impurities, the natural gas coming from the mouth of the well must usually be dried prior to treatment by the method according to the invention. Drying is done in any conventional manner that allows the moisture content to be reduced to 400 ppm or less. Advantageously, the gas is dehydrated by passing it through a bed of zeolitic molecular sieve.

The invention is illustrated by the following description in conjunction with the figures. For the purpose of Description is assumed that the natural gas 400 ppm HgS,

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6,000 ppm CO ₂ , 15,000 ppm ethane, 2,000 ppm propane, 10,000 ppm nitrogen, 20 ppm H ₂ O, remainder methane. The object is accordingly to reduce the HpS content to less than about 4 ppm _{regardless of the concentration of CO 2 in the sweetened product gas.} Of course, the process would also be carried out in the same manner for removing COp from a low HpS feed gas, with the _{same attention being paid to the C0 2} mass transition zones in the bed as the HpS mass transition zones in the method described below as an example.

L This is useful for the system shown in the flow diagram dried natural gas fed through line 10 and valve 12 to the adsorbent bed 14, which alternately is operated with the adsorbent bed l6. The two adsorbent beds 14 and 16 contain zeolitic ones Molecular sieves. The type of molecular sieve used is not critically important, but it is advantageous that the zeolite has a pore diameter such that the polluting sulfur compounds and that COp easily are adsorbed. There is also a rapid transition of these polluting compounds and that to regeneration purge gas used in the adsorbent and out of the adsorbent is extremely desirable, the

"agglomerated zeolite particles can be relatively small. A divalent cation-exchanged form of zeolite A having a pore diameter of about 5 angstroms in agglomerate form with an average particle diameter of no more than about 3.2 mm has been found to be extremely effective in this regard Calcium zeolite A (USA Patent 2,882,243) in the form of a clay-bonded cylindrical extrudate with a diameter of 1.6 mm is particularly preferred for the removal of H _{2 S. To remove CO 2} , a zeolite is used with a pore diameter of about 4 Angstroms is preferred. Further suitable zeolites are, for example, Zeolite T (USA Patent 2,950,952), Zeolite X (USA Patent 2,882,244),

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natural and synthetic mordenite and naturally occurring chabazite, erionite, clinoptilolite and gmelinite.

For the purposes of this description, it will be assumed that the operation of the system has stabilized, that is, the dry sour natural gas feed to adsorption column 14 enters a bed which has been regenerated immediately beforehand. The description of the regeneration stages is saved for a later suitable place. It is sufficient here to define the state of the bed 14 when the feed gas enters through the valve 12 so that it has a temperature of minus 50 to about 100 ^° C., preferably of minus 30 to about 50 ^° C., under a pressure of about 7 to 210 atmospheres and is essentially free of adsorbed H ₂ S in the outlet end.

In accordance with the other process parameters which are essential to the invention, the condition of the bed as defined above further implies that a new HpS mass transition zone is present in the inlet end of the bed. During the adsorption cycle in bed 14, the amount of feed gas added per unit of time is not critically important. The pressure is also not a critically important factor, but the increased pressures are favorable for the effective capacity of the zeolitic adsorbent for the contaminating H ₂ S. The shape of the bed is theoretically not very important, but in practice it should be at least the length of an H _{Have 2} S mass transition zone under the working conditions of the overall process. The term "mass transition zone" as used herein has the same meaning as is generally used in the art, that is, it means that portion of the adsorbent bed in which the adsorbate loading, the adsorbent bed and the concentration of the adsorbate fraction in the gas stream both increase over time. Adsorption in bed 14 is continued for any desired time until the HgS front breaks into the purified product stream exiting bed 14 through line 18, valve 20 and line 22 for the final

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valid consumption is deducted, but not carried out beyond this breakthrough. Then the previously regenerated bed 16 by closing the valve and passing the feed gas through valve 50 for adsorption switched while the desorption of the bed 14 is initiated.

The overall regeneration, in which H ₂ S is desorbed from bed 14 and the bed is prepared for a new adsorption, takes place in a succession of stages consisting of closed-circuit heating, purging and cooling. In general, it is advantageous to desorb the bed at pressures slightly lower than the pressures used during adsorption. The successive measures for regeneration can therefore optionally include a pressure decrease at the beginning and a pressure increase at the end. The initial pressure reduction has the advantage that, at the same temperature, less purge gas or stripping gas is required to drive a given _{amount of H g} S from the bed at lower pressures. This advantage is offset, at least in part, by the lower adsorptive capacity of the bed at the beginning of the following adsorption cycle as a result of the heat generated by the pressure increase. To compensate for this, it is common practice to reduce the pressure in the bed by up to about 80 % before the irrigation or abortion, but a greater pressure reduction is also possible. In this regard it should be noted that the reduction in pressure in the bed produces a cooling effect which, if it occurs after heating in the closed circuit and before the rinsing and stripping stages, makes the HpS more difficult to desorb from the zeolitic adsorbent is, so that disadvantageously larger amounts of purge gas are required for cleaning the bed. Thus, if the pressure reduction is carried out during desorption, it must take place before the completion of the heating in the closed circuit. In the case of the bed 14, a pressure reduction is expediently made by a

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Part of the gas from the bed 14 through the valve 24 into the line 26 and successively through the cooler 28, the condensate receiver 30, the fan. 34, line 36 and valve 38 is drained. A closed loop consisting of bed 14, line 18, valve 24, line 26, cooler 28, condensate receiver 30, fan 34, valves 40 and 42, furnace 44, line 46 and valve 53 is then formed by suitable valve actuation. The cooler 28 serves to maintain the temperature of gas supplied to the blower at about 100 ⁰ C or lower, so that a substantial part can be condensed out of about existing Cp ⁺ hydrocarbon, or about water present. The cooler 28 is also used during the final stages of bed regeneration when the bed is being cooled to adsorption temperatures. The direct current of the hot gas from top to bottom through the bed 14 is achieved by actuating the fan 34. The bed temperature should eventually be at least slightly as a result of this closed loop heating
increase.

at least about 200 ⁰ C, but not more than about 375 ° C

The purpose of heating in the closed circuit or in the closed loop is to supply thermal energy to the adsorbent and thereby to reduce the amount of flushing or displacement gas required for adequate desorption of the bed. Since the main goal of the method is to save flushing or displacement gas, the heating may be continued in the closed circuit until the heat front the exit end of the bed has reached and all parts of the bed is at a temperature of at least 200 ⁰ C. However, since the subsequent cocurrent purging or displacement transports the V / poor through the bed even when cold displacement gas is used and additional heat can be added to the bed, it is not necessary that the heating continue in the closed circuit will until the entire bed is at an even

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Temperature is heated. ^{It is only necessary that the outlet end of the bed reaches a temperature of at least 200 °} C. before the flushing or displacement, which later takes place in cocurrent, is completed.

The heated closed loop gas flow increases the bed temperature in an advancing heating zone which causes the desorption of the part of the bed that is being heated and the desorbed HpS is re-adsorbed in the unheated part of the bed. As another portion of the bed becomes hot, the total volume of gas in the closed loop increases and the venting through the line and valve JQ is used to maintain the pressure of the closed loop or loop gas. As the heating stage progresses, the concentration of HpS in the circulating gas increases with the result that at the point in time at which the bed temperature has reached the desired desorption temperature, a considerable residual load of adsorbed HpS remains in equilibrium with the increased partial pressure of the HpS in the gas flow. This adsorbed HpS is desorbed and removed from the system by opening valve 38 further and closing valve 40 and adding a displacement gas consisting of sweet product gas from line 22 to the cocurrent. When the displacement gas is to be heated, it is passed from line 22 through line 51 *, valves 52 and 42, heater 44, line 46 and valve 53 from top to bottom through bed 14. When using ambient displacement gas or non-heated displacement gas, heater 44 is bypassed by opening valve 54 and closing valve 42. The gas emerging from bed 14 is passed through valve 24 and through lines 26 and 36 and discharged through valve 38. The closed circuit heating and the desorption with the displacement gas are carried out at approximately the same pressure to achieve the most efficient operation. In order for the bed to purify fresh feed gas to the desired level, it is not essential that it do so beforehand

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adsorbed HpS is completely driven out of the bed. It doesn't matter how much residual HgS is left in bed. More important is where it is in bed. If the exit end of the bed is essentially free of HpS, but HpS is adsorbed in the entry portion of the bed, the incoming feed gas can be purged to the extent required, although the amount of gas purged prior to HpS breakthrough is less than when the bed would be _{free of H 2} S at the start of adsorption.

It is therefore not the purpose of the purging or displacement in the process according to the invention to _{completely remove H 3} S from the bed, rather the process is aimed at using a limited amount of displacement gas which is only sufficient to make the bed in one to desorb to such an extent that a subsequent cocurrent circulation of cool gas in the process according to the invention redistributes residual HpS adsorbed in the outlet end of the bed to the inlet end of the bed before a new adsorption cycle. Natural gas is therefore used for purging or displacement in an amount of about 0.1 to 2.5 kg-moles per 100 kg of adsorbent. Within this range, such an amount is selected which results in that the outlet end of the bed reaches a temperature of at least 200 ⁰ C. The temperature of the displacement gas can be between about minus 50 ⁰ C and are 575 ° C.

After the open purging or displacement, a closed circuit for a cool gas is established by closing the valves 38 and 52 and opening the valve 40. The cool gas flowing in the closed circuit _{carefully drives off residual H 2} S from the exit section of the bed and precipitates it again in a new mass transition zone at the entry end of the bed 14, which ensures that the product gas which emerges first during the next adsorption does not more

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62652

ale contains the permissible amount of H ₂ S. In cases where the previous open displacement gas stream was warmer than the bed desired temperature for the adsorption cycle, the closed loop cool gas also serves to cool the bed to an acceptable temperature for adsorption. Adsorption in bed 14 is initiated by opening valves 12 and 20 and closing valves 24 and 53. At the same time, bed 16 is switched to regeneration in the same way as described above for bed 14.

St.

In the embodiment of the invention in which the L regeneration of the bed is carried out at least partially at a pressure which is lower than the pressure During the subsequent adsorption, the pressure reduction can be achieved in various ways. Preferred this is done using a mode of operation in which more gas is added through valve J58 during heating in a closed circuit is blown off than necessary to compensate for the rise in pressure caused by the rising temperature of the gas is. In general, the renewed pressure increase can take place at any time after the open flush or displacement and before return to adsorption.

Of course, numerous modifications of the method in and scope of the invention are possible. For example, it is entirely possible during the heating in the closed circuit and during the cooling in the closed circuit during the regeneration of the bed to continuously supply small amounts of the displacement gas. Small amounts of gas can also be withdrawn from the closed circuit. However, since the larger amount of the gas is circulated in the closed circuit or in the closed loop, it is nevertheless essentially a closed circuit operation which falls under the term "closed circuit" as used here. It is also possible to use a single bed or three or more beds instead of the two-bed system on the basis of which the invention is based

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has been described above.

The term "purge gas" or "displacement gas" as used herein refers to all gases known in the art are used, are not reactive with the hydrocarbons in the bed and the zeolitic adsorbent, not damage by destroying the crystal lattice or take up a large part of an adsorptive capacity to take. Suitable purging or displacement gases are, for example, nitrogen, hydrogen, methane, ethane, Natural gas, helium and other gases from the group of noble gases.

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Claims (1)

Claims 1. Process for sweetening sour natural gas, which 1 to Contains 50,000 ppm HS and no more than 400 ppm HgO, passing the gas through a bed containing a zeolitic molecular sieve as an adsorbent, withdraws the sweetened product gas from the outflow end of the bed, while the adsorption bed removes the HpS selectively adsorbs and holds, then interrupts the supply of feed gas to this bed and that , Bed regenerated by a treatment that is characterized is that one a) natural gas heated to at least 200 ° C is passed cocurrently (in the same direction as the feed gas) through the bed, the gas flow emerging from the bed is reheated and passed cocurrently and in circulation through the bed for a time which is at least sufficient in order to heat the outflow end of the bed to at least 200 ^° C. before the stage b) is completed, which consists in the fact that the heated bed in cocurrent with about 0.1 to b 2 * 5 kg-moles of purge gas or displacement gas per 100 kg Flushes adsorbent in bed and c) then a flushing or displacement gas in cocurrent at a temperature of less than about 100 ⁰ C through the bed leads the emerging Gas in cocurrent and circulating at a temperature of less than 100 C through the Bet1 until the outflow of the bed has reached a temp is. Chilled temperature of less than about 100 ° C 209828/0967 one 2. method naoh claim 1, characterized in that the Carrying out regeneration treatment by a) a gas heated to a temperature between 250 and 375 ° C passes in cocurrent through the adsorption bed, the gas emerging from the bed is reheated and circulated through the bed for a time which is at least sufficient to remove the outflow to heat the bed to at least 250 ⁰ C before the stage b) is finished in which the heated bed is co-current with about 0.5 to 2.5 kg-moles of purified gas per 100 kg of adsorbent in bed rinses and c) then passes natural gas cocurrently through the bed at a temperature of less than 100 ^° C. and the gas flowing out of the bed passes through the bed cocurrently and in circulation at a temperature of less than 100 ^° C. until the outflow end of the bed is cooled to a temperature of less than 100 ^{° C.} j5. Process for lowering the COp concentration of natural gas that up to 50,000 ppm COp, less than about 1 ppm HpS, and Contains less than 400 ppm HpO, whereby the gas is passed through a The bed, which contains a zeolitic molecular sieve as adsorbent, carries the product gas from the outlet end of the bed Betts withdraws while the COg selectively adsorbs in the bed and held back, then cutting off the supply of feed gas to the bed and treating the bed regenerated, which is characterized in that one a) a gas heated to at least 200 ° C in cocurrent through the bed, the gas emerging from the bed reheated and circulated and in cocurrent through the bed for a time which is at least sufficient to at least the outlet end of the bed Heat 200 ⁰ C before the stage 209828 / 09G7 b) is finished, in which the heated bed is purified cocurrently with 0.1 to 2.5 kg-mol of purified Flushes and natural gas per 100 kg of adsorbent in bed c) then natural gas at a temperature of less than 100 ⁰ C in cocurrent through the bed and here the outflowing gas in cocurrent and in circulation at a temperature of less than 100 C through the bed until the outflow of the bed to a temperature is cooled by less than 100 ^{° C.} 209828 / Ü967