Patent Application
20130152795
The present invention relates generally to pressure swing adsorption (PSA) systems and vessels, and more particularly relates to PSA vessels having reduced void volume and uniform flow distribution during processing.
Pressure swing adsorption processes can separate selectively adsorbable components, such as carbon monoxide, carbon dioxide, methane, ammonia, hydrogen sulfide, argon, nitrogen, and water, from gas mixtures. Often, one or more of these components are adsorbed to purify a fluid stream, such as hydrogen gas. Typically, a PSA process uses an adsorber that includes a vessel surrounding an adsorbent bed formed with adsorbent particles. Generally, void volumes in the adsorber vessel include volumes within porous adsorbent particles, volumes between particles, and internal volumes defined by the walls of the vessel and the adsorbent bed.
These void volumes can decrease the efficiency of the PSA process. Specifically, the void volumes may lead to loss of recovered product such as hydrogen. Although adsorbent can be placed in the void volume to reduce the void volume, such a solution is undesirable as it adversely affects the gas flow distribution and pressure drop through the adsorbent bed. For enhanced processing performance, distribution of gases in the vessel is uniform. However, placing adsorbent in the void volume can create non-uniformity that is generally undesirable. Generally, it would be desirable to minimize the void volume in the vessel without increasing pressure drop and flow non-uniformity through the adsorbent.
Accordingly, it is desirable to provide adsorption vessels that have reduced void volumes. Also, it is desirable to provide adsorption vessels that exhibit reduced pressure drop during separation processing. Furthermore, other desirable features and characteristics will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.
Adsorption vessels having reduced void volume and uniform flow distribution are provided herein. In an exemplary embodiment, an adsorption vessel is provided for receiving a fluid mixture and for separating a component from therein. The adsorption vessel includes a vessel wall extending from a bottom end to a top end. The vessel wall defines a vessel chamber. A bottom inlet is formed in the bottom end of the vessel for introducing the fluid mixture to the vessel chamber. A support plate is positioned in the vessel chamber above the bottom end, and defines a bottom void volume between the support plate and the bottom end. Further, a filler material having a total porosity of less than about 25% is positioned in the bottom void volume and defines a channel for flow of the fluid mixture from the bottom inlet to the support plate.
In accordance with another exemplary embodiment, an adsorption vessel is formed with a vessel chamber for receiving a fluid mixture and for separating a component therein. The vessel includes a perforated support plate positioned in the vessel chamber and defining an adsorbing zone above the perforated support plate and an inlet zone below the perforated support plate. A bottom inlet is formed in the vessel for introducing the fluid mixture to the inlet zone. Further, a filler material having a total porosity of less than about 25% is positioned in the inlet zone and defines a channel for flow of the fluid mixture from the bottom inlet to the perforated support plate. The filler material fills over 50% of the inlet zone.
In accordance with another exemplary embodiment, an adsorption system is provided for separating a component from a fluid mixture. The system includes at least one vessel having a vessel wall that extends from a bottom end to a top end and that defines a vessel chamber. A bottom inlet is formed in the bottom end of the vessel for introducing the fluid mixture to the vessel chamber. The bottom inlet defines an axis. The adsorption vessel includes a support plate positioned in the vessel chamber above the bottom end. The support plate defines a bottom void volume between the support plate and the bottom end. Further, a bed of adsorbent material is positioned in the vessel chamber above the support plate to selectively adsorb the component of the fluid mixture. Also, an inner support ring is mounted to the bottom end surrounding the axis, and an outer support ring is mounted to the bottom end surrounding the inner support ring. The vessel includes a filler material having a total porosity of less than about 25% positioned in the bottom void volume between the inner support ring and the outer support ring. The filler material defines a channel for flow of the fluid mixture from the bottom inlet to the support plate.
The adsorption vessels will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and wherein:
The following Detailed Description is merely exemplary in nature and is not intended to limit the adsorbent system or adsorbent vessel or the application and uses of the adsorbent system or adsorbent vessel. Furthermore, there is no intention to be bound by any theory presented in the preceding background or the following detailed description.
The various embodiments contemplated herein relate to adsorption vessels and systems that have reduced void volume, exhibit reduced pressure drop, and provide uniform flow distribution. Further, the adsorption vessels and systems are able to reduce cycle time by about 30% to about 50%. The adsorption vessels herein utilize filler material to reduce void volume, leading to improved process performance in PSA processes. The filler material has a total porosity of less than about 25%, such as less than about 20%, less than about 15%, or less than or about 10%. As used herein, “total porosity” is a measure of the void volume, including intramaterial void volume within material particles and intermaterial void volume between material particles, as a percentage of the total volume of the filler material. The total volume, or bulk volume, of the filler material includes the solid and void components.
PSA technology is based upon the capacity of adsorbents to selectively adsorb and desorb particular gases as gas pressure is raised and lowered. Due to selective adsorption, impurities may be removed from a desired product gas. In many commercial uses of PSA systems, off gas from refineries or chemical plants is fed into a PSA system for separation. In an exemplary use, the feed is the off gas from a steam methane reformer and includes about 75 mol. % hydrogen, about 15 mol. % carbon dioxide, about 3 to 4 mol. % carbon monoxide, about 5 mol. % methane, and about 0.5 mol. % nitrogen. The PSA system is able to separate a product stream of 99.9 mol. % hydrogen from such a feed.
The PSA process involves a cyclic repetition of four basic steps: production, depressurizing, purging, and repressurizing. First, the feed gas mixture is fed under high pressure into vessels containing adsorbent material, typically alumina, silica gel, activated carbon, molecular sieves, or the like. Impurities in the feed gas adsorb onto the internal surfaces of the porous adsorbent, leaving purified product gas in the void spaces of the vessel. Product gas is then withdrawn from the top of the vessel under pressure. The pressure in the adsorption vessels is then reduced, and product gas remaining in the void spaces of the vessel is removed. The adsorbed impurities are released back into the gas phase, regenerating the adsorbent bed. The vessel is then purged with a small amount of purified product gas, to complete regeneration of the adsorbent bed. Impurities exit the PSA process in a low-pressure exhaust stream. Finally, the vessel is repressurized with a mixture of product gas from the depressurization step, feed gas, and high-purity product gas. This cycle is repeated about every 5 to 10 minutes in conventional PSA systems.
Because each cycle is essentially a batch process, multiple pressure vessels are typically used together in sequence to provide a semicontinuous flow of product gas. In addition, large surge tanks are used to dampen variations in flows of feed, product and exhaust streams. To fully utilize the adsorbent material employed, PSA systems require uniform flow of gas across the adsorbent vessel(s) throughout the PSA processing cycle. In addition, void volume and pressure drops in the PSA vessel entrance and exit regions (i.e., the inlets and outlets and their associated headers) have adverse effects on the process performance of a PSA system and must be minimized in practical commercial operations.
In accordance with an exemplary embodiment contemplated herein, an apparatus 10 for performing selective adsorption is illustrated in
As shown, the feed stream 12 is delivered to the vessels 20 through feed lines 22. Further, the feed lines 22 are connected to a pressure source 24 for pressurization to an upper adsorbent pressure. During the high pressure product producing step in the PSA cycle, the product stream 14 exits the vessels 20 through outlet lines 26. Further, the apparatus 10 includes impurities lines 28 for removal of the impurities during regeneration steps in the PSA cycle. As shown, the impurities lines 28 may be connected to a low pressure sink 30 for removal of the impurities from the vessels 20.
Referring now to
The vessel 20 is provided with a perforated support plate 60. The support plate 60 defines a plane 62 and can be considered to divide the vessel chamber 46 into an inlet zone 64 and an adsorbing zone 66. As shown, the support plate 60 sits on, and is connected to, such as by a bolted connection, an inner support ring 68 and an outer support ring 70. Each of the support rings 68, 70 is cylindrical and is perforated near its respective top. As shown, the inner support ring 68 is centered about the axis 50 and the outer support ring 70 is centered about the inner support ring 68. The vessel 20 may also include a perforated deflector 72 for deflecting gas flow.
In
In order to reduce void volume in the vessel 20, a filler material 80 is positioned in the inlet zone 64 below the support plate 60. The filler material 80 may be, for example, polymeric closed cell foams, liquid, concrete, refractory insulation, plastic blocks, granite blocks, ceramic balls, sand, paraffin wax, or combinations thereof. As stated above, the total porosity of the filler material, whether a single material or combination of materials, is less than about 25%, such as less than 20%, less than 15%, or less than 10%. As shown, the filler material 80 forms an annular or ring shape, and abuts an outer face 82 of the inner support ring 68. Further, the filler material 80 abuts an inner face 84 of the outer support ring 70. The filler material 80 extends along the bottom end 42 of the vessel 20 between the inner support ring 68 and outer support ring 70. The vessel 20 may also include a cover 86 for the filler material 80. The cover 86 may be a membrane bag, or a structural element such as sheet metal, for holding the filler material 80 in place, particularly during shipping. As shown, in the volume between the outer support ring 70 and the vessel wall 40, a plurality of ceramic balls 88 may be positioned to further reduce void volume, to prevent seepage of adsorbent material 74 below the support plate 60 along the vessel wall 40, and to aid in flow distribution.
The filler material 80 is utilized to reduce void volume in the vessel 20 and to define channels or flow paths for the feed mixture (arrows 92). The flow paths pass through the perforated upper portions of the support rings 68. 70. As shown, the flow paths are bounded by the filler material 80 and/or cover 86, and by the vessel wall 40 below the perforated support plate 60. As shown, vessel 20 has a vessel height 100, a chamber inner diameter 102, an adsorbent bed height 104, an inlet inner diameter 106, and a support plate height 108.
In another exemplary embodiment, vessel 20 has a volume of about 15.857 cubic meters (or about 560 cubic feet) and a total inlet zone volume of between about 3% and about 15% of the vessel volume, for example about 6% or about 8.5% of the vessel volume. Within the inlet zone 64, the filler material fills about 50% of the inlet zone volume. As a result, the remaining void volume in the inlet zone is about 4% of the vessel volume, and the filler material volume is between about 2% and about 10% of the vessel volume, such as about 3% or about 4.5% of the vessel volume.
As a result of the placement, design and volume of the filler material 80, as well as the material properties including low total porosity, the void volume of the vessel 20 is reduced without disrupting uniform flow distribution of the feed gas mixture and without increasing pressure drop across the vessel. As a result, process efficiency is increased. For example, the decreased amount of void volume results in decreased product gas (for example, hydrogen) lost to the impurities stream 16 during depressurization of the vessel in the PSA processing cycle. As a result, the cycle time itself can be reduced, resulting in a shorter necessary bed height 104 without decreasing the fractional recovery of product stream 14 from feed gas stream.
Accordingly, adsorbent systems and vessels for separating impurities from a product gas have been described. The adsorbent vessels are provided with filler material for reducing void volume to improve processing efficiency. While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or embodiments described herein are not intended to limit the scope, applicability, or configuration of the claimed subject matter in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing the described embodiment or embodiments. It should be understood that various changes can be made in the processes without departing from the scope defined by the claims, which includes known equivalents and foreseeable equivalents at the time of filing this patent application.
