DE2162652B2 - METHOD OF REGENERATING AN ADSORPTION AGENT BED USED FOR PURIFYING NATURAL GAS - Google Patents

Description

The present invention relates to a process for the regeneration of an adsorbent bed used for cleaning natural gas, in which natural gas heated to at least 200 ° C through the adsorbent bed is passed in the same direction as the natural gas to be purified from the inlet to the outflow and the adsorbent bed with it a purge gas having a temperature of less than 100 ⁰ C, which as will be passed to be purified natural gas through the adsorbent bed in the cycle in the same direction, it is again cooled to a temperature below 100 ^0C.

Such a method is known from BE-PS 7 19 964, which primarily relates to the removal of water and also the removal of other condensable impurities from natural gas, namely CO2, heavy hydrocarbons and organic compounds of sulfur. However, the removal of H ₂ S is not addressed. For regeneration, a branched stream of purified gas is used here, with which the complete desorption is carried out, as a result of which considerable amounts of gas have to be used. The bed is then cooled by means of a circulated gas.

The object of the present invention is to create a method of the type mentioned at the outset which the regeneration of a natural gas containing H2S acted upon adsorbent bed using a significantly reduced amount of purge gas in the Compared to previously common methods.

This object is achieved in that in the regeneration of a natural gas, the W 1-30000 ppm S ₁ and not more than 400 ppm H; and acted upon contains OK, a zeolitic molecular sieve as an adsorbent containing adsorbent bed on at least 200 ⁰ C. heated natural gas for a time that is at least sufficient to heat the outflow of the adsorbent bed to at least 200 ° C, is circulated through the adsorbent and that then the heated adsorbent bed before cooling in the same direction as the natural gas to be cleaned with 0, 1 to Iß kg-mol of purge gas per 100 kg of adsorbent is purged.

Natural gas is a mixture of gases consisting mainly of methane and usually in small quantities other gases including higher hydrocarbons, e.g. B. ethane and propane, carbon dioxide, nitrogen, Contains helium and sulfur compounds, the greater part of which consists of H2S and small amounts or There are trace amounts of mercaptans. The natural gases emerging from the sources also always contain Water vapor and are usually saturated with water. The respective geological environment leads to Fluctuations in the composition of a natural gas deposit. The composition also changes of the natural gas of a well during the extraction of the gas from this well. The following two Analyzes of gas samples from different deposits are representative:

I (Vol-%) II (vol .-%) CH ₄ 96 78 30 C ₂ H ₆ 1.5 7th C ₃ H ₈ 0.2 \ 2 C ₃ + 0.6 2.0 N ₂ 1.0 2.4 H ₂ O 0.1 0.2 35 CO ₂ 0.6 8.0 H ₂ S 400 (ppm) 1.2

The relative concentration of weakly adsorbable or non-adsorbable permanent gases such as nitrogen and helium is not critically important. Within the above-mentioned range of H ₂ S concentration, the concentration of any carbon dioxide that may be present in the feed gas is also reduced, but this CO _{2 is} inevitably not completely removed. The degree to which the CO ₂ concentration is lowered depends on several factors, including the concentration of H ₂ S and CO ₂ in the feed gas and the _{amount of H 2} S that is allowed to be adsorbed on the bed during the adsorption stage before the Regeneration is started. CO ₂ is less strongly adsorbed on molecular sieve _{zeolites than H 2} S. Accordingly, the CO ₂ mass transition zone precedes the H ₂ S mass transition zone as the two zones progress through the bed.

So if a feed gas with a relatively high COr content is treated, the termination of the adsorption cycle immediately before the breakthrough of the H ₂ S front has the consequence that most of the CO ₂ leaves the bed with the exiting natural gas. To a certain extent, however, CO ₂ and H ₂ S are adsorbed together in the same bed zone, J. h. a certain amount of CO _{2 is} removed with the H ₂ S. In many cases, CO ₂ in natural gas is not a disadvantage and is not objectionable.

If the CO2 concentration is to be reduced to below 50 ppm, the natural gas, by nature or as a result of previous treatment, would have to have less than about 1 ppm IKS and no more than about 30,000 ppm CO ₂

contain.

Regarding the removal of CO2 or H2S or of both impurities must be that of the mouth of the mouth incoming natural gas is usually dried before treatment. The drying requires >! gt in any conventional manner, with which a lowering of the moisture content to 400 ppm or less is possible. Advantageously, the gas is dehydrated by passing it through a bed of a zeolitic molecular sieve passed will.

The invention is illustrated by the following description in conjunction with the figures. For the purposes of the description, it is assumed that the natural gas contains 400 ppm H ₂ S, 6000 ppm CO ₂ , 15,000 ppm ethane, 2000 ppm propane, 10,000 ppm nitrogen, 20 ppm H ₂ O, Resf methane, contains The object is accordingly _{to reduce the H 2} S 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 CO ₂ from a low H ₂ S feed gas, with the same attention being paid to the CO2 mass transfer zones in the bed as the H2S mass transfer zones in the example method described below .

In the system shown in the flow diagram, the suitably dried natural gas is fed through line 10 and valve 12 to the adsorbent bed 14, which is operated alternately with the adsorbent bed 16. The two adsorbent beds 14 and 16 contain zeolitic 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 contaminating sulfur compounds and the CO _{2 are} easily adsorbed. Furthermore, since rapid transfer of these contaminating compounds and the purge gas used for regeneration into and out of the adsorbent is extremely desirable, the agglomerated zeolite particles should be relatively small. It has been found that 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 not more than about 3.2 mm is extremely effective in this regard and is therefore preferable. Calcium zeolite A (US Pat. No. 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.} A zeolite with a pore diameter of about 4 angstroms is preferred for removing CO _2. Further suitable zeolites are, for example, zeolite T (USA patent 29 50 952), zeolite X (USA patent 28 882 244), 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 feed of dry acidic natural gas which is fed to the adsorption column 14 enters a ( _{1 (} , 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 state the state of the ; Belts 14 upon entry to define the gas through the valve 12 so the Einsat / that it has a d "has temperature of minus 50 to about 100 ⁰ C., preferably from minus 30 to about 50 ⁰ C, under a pressure of about 7 up to 210 atmospheres and is essentially free of adsorbed H ₂ S at the outlet end

According to the other process parameters, the condition of the bed as defined above also means that a new H ^ mass transition zone is present in the inlet end of the bed. During the adsorption cycle in bed 14, the amount of feed gas supplied 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 absorption 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 H2S -Have a mass transition zone under the working conditions of the overall process. The term "mass transition zone" used here has the same meaning as is generally customary in the art, i.e. it means that section of the adsorbent bed in which the adsorbate loading of the adsorbent bed and the concentration of the adsorbate fraction in the gas stream both increase over time. Adsorption in bed 14 will continue for any desired time until the H2S front breaks into the purified product stream which is withdrawn from bed 14 through line 18, valve 20 and line 22 for final consumption, but not beyond that breakout carried out beyond. Subsequently, the previously regenerated bed 16 is switched to adsorption by closing the valve 12 and passing the feed gas through the valve 50, while the desorption of the bed 14 is initiated.

The overall regeneration, _{desorbing H 2} S from bed 14 and preparing the bed for new adsorption, takes place in a series of stages consisting of closed loop heating, purging and cooling. In general, it is advantageous to desorb the bed at pressures somewhat 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. Lowering the pressure at the beginning has the advantage that at the same temperature less purge gas or stripping gas is required to drive a given _{amount of H 2} S out of the bed at lower pressures. This advantage is offset, at least in part, by the low 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, work is usually carried out 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 H2S 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 made 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 carried out by removing some of the gas from the bell 14 from the system through the valve 24 into the line 26 and successively through the cooler 28, the condensate reservoir 30, the fan 34 and the line 16

and valve 38 is deflated. 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 keep the temperature of the gas supplied to the fan at about 100 ° C. or lower, so that a substantial part of any hydrocarbon or water that may be present can be condensed out. The cooler 28 is also used during the final stages of bed regeneration when the bed is being cooled to absorption temperatures. The direct current of the hot gas from top to bottom through the bed 14 is achieved by actuating the fan 34. The temperature of the bed should be finally increased as a result of this heating, in a closed circuit to at least about 200 ° C, but not more than about 375 ^C C.

The purpose of heating in the closed circuit or in the closed loop is to supply thermal energy to the adsorbent and thereby reduce the amount of flushing or displacement gas required for adequate desorption of the bed. Since the main aim of the procedure is. To save flushing or displacement gas, the heating in the closed circuit can be continued until the heat front the exit end of the bed has reached and all parts of the bed are at a temperature of at least 200 ⁰ C. However, since the subsequent cocurrent purging or displacement transports the heat through the bed, even if cold displacement gas is used, and additional heat can be added to the bed, it is not necessary that the heating be continued in the closed circuit, until the entire bed is heated to an even temperature. It is only necessary that the exit end of the bed reach a temperature of at least 200 ° C before the end of the later cocurrent flushing or displacement.

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 H2S is re-adsorbed in the unheated part of the bed. As another portion of the bed becomes hot, the total closed loop gas volume increases and the venting through line 36 and valve 38 serves to maintain the closed loop or loop pressure of the gas. As the heating stage progresses, the concentration of H ₂ S 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, there is a considerable residual load of adsorbed H? S in equilibrium with the increased partial pressure of the H2S in the Gas flow remains. This adsorbed H ₂ S is desorbed and removed from the system by opening valve 38 further and closing valve 40 and a displacement gas consisting of sweet product gas is fed from line 22 to the cocurrent. If the displacement gas is to be heated, it is passed from line 22 through line 51, valves 52 f> 5 and 42, heater 44, line 46 and valve 53 / on up and down through bed 14 when using displacement gas ambient or heated displacement gas, heater 44 is bypassed by opening valve 54 and closing valve 412. The gas exiting the bed 14 is passed through valve 24 and through lines 26 and 36 and discharged through valve 38. Closed loop heating and displacement gas desorption are carried out at approximately the same pressure for most efficient operation. In order for the bed to purify fresh feed gas to the desired level, it is not essential that the previously adsorbed H ₂ S be completely driven out of the bed will. It doesn't matter so much how much residual H2S is left in bed. More important is where it is in bed. If the exit end of the bed is essentially free of H ₂ S but H ₂ S is adsorbed in the entry part of the bed , the incoming feed gas can be purified to the required extent , although the amount of gas that is purified prior to the H ₂ S breakthrough is less is as if the bed were free of H ₂ S at the beginning of the adsorption.

It is therefore not the purpose of purging or displacement in the process according to the invention to completely remove H; S from the bed , rather the process is geared towards using a limited amount of displacement gas which is only sufficient to keep the bed in such a bed To desorb measure that a subsequent, in cocurrent in a closed loop circulation of cool gas in the process according to the invention _{to redistribute residual H 2} S 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 alteration in an amount of about 0.1 to Z5 kg-mol per 100 kg of adsorbent. Within this range, such an amount is chosen that the outlet end of the bed is at a temperature of at least 200 ° C reached The temperature of the displacement gas can be between about minus 50 ⁰ C and 375 ° C.

After the open purge or displacement, a closed circuit for a cool gas is established by closing valves 38 and 52 and opening valve 40 . By the flowing in a closed circuit cool gas remaining H2S is carefully stripped from the Austrittsabschniit the bed and deposited again in a new mass transfer zone at the inlet end of the bed 14, thereby ensuring that the product gas, the first exit at the next adsorption, no more than the permissible amount of H ₂ S contains In cases where the previous open displacement gas stream was warmer than the desired temperature of the bed for the adsorption cycle, the cool gas circulated in the closed circuit 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 manner as described above for bed 14

In the embodiment of the invention in which the 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. For this purpose, a mode of operation is preferred in which

during the heating in the closed circuit through the valve 38 more gas is blown off than for Compensation for the rise in pressure caused by the rising temperature of the gas is necessary. in the In general, the pressure increase can be repeated at any point in time after the open flushing or displacement and before returning to adsorption.

Of course, numerous modifications of the method are possible within the scope of the invention. For example it is in a closed circuit during heating and in a closed circuit during cooling Cycle during the regeneration of the bed is entirely possible, steadily small amounts of the displacement gas to feed. Small amounts of gas can also be withdrawn from the closed circuit will. However, since the larger amount of gas in the closed circuit or in the closed Loop is circulated, it is still essentially a closed-loop operation which falls under the term "closed cycle" in the sense used here. It is also possible to have a Use a single bed or three or more beds instead of the twin bed system based on which the Invention has been described above.

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

1 sheet of drawings

709525/4

Claims (1)

Claim: Process for the regeneration of an adsorbent bed used for cleaning natural gas, in which ^{natural gas heated to at least 200 0} C by the adsorbent is passed in the same direction as the natural gas to be cleaned from the inlet to the outlet end and the adsorbent bed with a purge gas at a temperature of less than 100 ⁰ C, which is circulated through the adsorbent bed in the same direction as the natural gas to be cleaned, is ^{cooled again to a temperature below 100 0} C, characterized in that; that during the regeneration of a natural gas which contains 1-30,000 ppm H2S and not more than 400 ppm H2O, an adsorbent bed containing zeolite molecular sieve as adsorbent, the ^{natural gas heated to at least 200 0} C for a time which is at least sufficient in order to heat the outflow of the adsorbent bed to at least 200 ⁰ C, is circulated through the adsorbent and that then the heated adsorbent bed before cooling in the same direction as the natural gas to be purified with 0.1 to 2.5 kg-Mo! Purge gas is purged per 100 kg of adsorbent.