Strategies for gas treatment with adsorbents
Advances in regenerable, solid-bed adsorption processes for natural gas dehydration and treating can improve selectivity and cut input energy costs
Scott Northrop, ExxonMobil Upstream Research Company
Narasimhan Sundaram, ExxonMobil Research & Engineering
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Adsorbents are widely used for contaminant removal in natural gas treatment. Special materials, such as molecular sieves, are used to remove sulphur-containing impurities like mercaptans, as well as water. Such operation may be prone to problems, particularly during the regeneration of these molecular sieves. For example, unwanted byproducts like carbonyl sulphide (COS) can be created by the transient thermal waves produced during regeneration.
Improper regeneration may ultimately contribute to lowered product quality if the temperature is too low, or time is too short. On the other hand, excessive temperatures or heating times may reduce adsorbent life, or cause adsorbate decomposition (or coking) in some instances.
It is possible to take advantage of modified cycling, as well as additional adsorbents (such as alumina), to reduce and control the exposure of molecular sieves to degradation. Such additional adsorbents also allow targeting and removal of other contaminants, including oxygenates like methanol, that may promote undesirable side reactions on valuable molecular sieves. Effective compound bed configurations and process techniques to address the problems sometimes encountered with conventional molecular sieve treatment will be discussed.
The use of molecular sieve (MS) adsorbents has become widespread for contaminant removal from natural gas. Over the past three to four decades, they have proven to be suitable and economic in reducing contaminant levels during gas processing. However, with increasing treatment require-ments, including more stringent product/effluent specifications, greater plant size and increasingly challenged feedstocks, there is considerable incentive to improve MS performance. Simultaneously, decades-long experience from operating plants has begun to reveal limitations in the conventional use of these materials. With such guidance from both the past and the future, MS end users and manufacturers are compelled to develop effective strategies for enhancing the use of these materials and maximising their potential.
Key limitations that are recognised in conventional MS operating practice include deactivation via exposure to certain contaminants, sub-optimal regeneration (eg, refluxing), unwanted byproducts during regeneration, and lowered adsorbent life due to increased regeneration frequency and severity. Our strategy addresses these limitations using enhanced adsorbent combin-ations (compound beds), modified thermal and/or pressure cycles and modified partial desorption regeneration,1 or combinations thereof. Patents are being sought on some of these combinations. We are also exploring more selective adsorbents to reduce hydrocarbon loss and regeneration energy requirements.
Delaying molecular sieve degradation
The motivation to reduce adsorbent fill cost and to increase the lifetime of solid-bed dehydrators has led to the consideration of alternative adsorbents, including their use in conjunction with compound beds. Compound (or mixed) beds consist of two or more layers of different types of MS for water and mercaptan sulphurs, for example. Figure 1 shows a schematic diagram of such a compound bed of adsorbents, where A1, A2 and A3 represent different solid materials. Adsorbents can be MS such as 4A (often used for dehydration) or 13X (used for the removal of larger molecules), or other zeolitic adsorbents. Additional layers of specific adsorbents may also be used for trace mercury removal, for example, if the treated gas is to be fed to an LNG plant. Additional layers can also offer significant protection of sensitive molecular sieves from both contaminants and damaging conditions that might result from upsets during the feed cycle, or refluxing during the regeneration cycle. Compound beds are well known in the air separation prepurifier industry.2
Typically, contaminated feed gas enters from the top of the bed during the adsorption cycle. As the bed adsorbs contaminants over the course of the service cycle, it ultimately must be taken off-line prior to exceeding treated gas specifications. Usually, another bed is placed in service, and the spent bed is regenerated. During the regeneration cycle, dry, heated gas flows in from the bottom of the bed and progresses upwards through the packed bed. A thermal wave moves up through the bed as contaminants are convected out. In general, there is a lag between the arrival of the thermal front and the arrival of the mass transfer front. This can create conditions for unwanted side reactions.
The contaminant-saturated regenerated gas is usually cooled to condense water and heavy hydro-carbons. If other contaminants are present, the regeneration stream must be treated in some manner. For example, the stream may be washed with physical solvent to absorb sulphur compounds, so that the treated gas may be used for fuel. Due to the unsteady nature of the regeneration process, a peak of contaminants is generated. Generally, the solvent system must be modified to provide a fairly steady concentration of contaminants to the next process (sulphur recovery unit (SRU), for example).
The addition of activated alumina (AA) to the top of these compound beds of MS is known to protect the MS from unexpected liquid (hydrocarbon, brine, glycol or condensed water) carry-over into the bed from upstream separation facilities sometimes as aerosols. Such fouling problems can be reduced by well-designed separation equipment upstream of the MS bed. The AA can also protect the MS from refluxing, when water condenses in the upper, cooler part of the bed during the early stages of regeneration (discussed below). AA, because of its higher water capacity, also helps to ensure that the heavy compound (eg, mercaptan) removal specification will be met by reducing the chance of water breakthrough from a compound bed consisting of 4A MS and 13X MS, for example. Even a relatively small amount of water breakthrough into the 13X MS will reduce the mercaptan removal capacity substantially because of the adsorbent’s greater affinity for the more polar compound. 13X capacity cannot be restored until the water is desorbed.
There are a number of benefits to using an AA/MS compound bed, including lower overall adsorbent cost, and higher resistance to liquid upsets and liquid carry-over, as described above. AA can also have a higher equilibrium capacity than MS when the feed gas is near saturation, as shown by the isotherms in Figure 2.
Regeneration temperatures required for AA are, furthermore, lower than those of MS, thus reducing the heat requirement to desorb the water in the AA relative to MS. This lowered energy requirement translates to savings in steam or fuel gas, depending on how the regeneration gas is heated. The benefits of compound beds for natural gas treatment have been recognised by others in the industry, and compound beds of AA/MS have been used in commercial natural gas dryers at several installations for decades.2-7
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