GB2508234A

GB2508234A - Molecular-sieve vessel

Info

Publication number: GB2508234A
Application number: GB201221292A
Authority: GB
Inventors: Jeremy Knight
Original assignee: Endet Ltd
Current assignee: Endet Ltd
Priority date: 2012-11-27
Filing date: 2012-11-27
Publication date: 2014-05-28
Also published as: GB201221292D0

Abstract

A molecular-sieve vessel 10 comprises a housing 12 having a first gas port 14, a second gas port 16 and a desiccant chamber 18 in fluid communication with the first and second gas ports 14, 16. At least one flow guide 26 at an interior surface 28 of the desiccant chamber 18 is also provided. A flow path for treatable wet gas is defined by the first gas port 14, desiccant chamber 18 and the second gas port 16, and the flow guide 26 has a baffle surface (44, Fig 2) on a wet-gas upstream side (32, Fig 2) for directing treatable wet gas at a periphery of the said flow path inwards. The molecular sieve vessel may form part of an adsorption unit in a natural gas dehydration system. A method of extending a period between desiccant regeneration, for example, when dehydrating natural gas is also provided. Advantageously channeling of wet gas through a preferential route through a desiccant material such as silicon dioxide or salt provided in the vessel is reduced.

Description

Improvements In And Relating To A Molecular-Sieve Vessel The present invention relates to a molecular-sieve vessel, and more particularly alihough not necessarily exclusively to such a vessd used as part of an adsorption unit of a natural gas dehydration system. The invention also relates to a method of extending a period between desiccant regeneration, particularly but again not necessarily exclusively, when dehydrating natural gas.

Adsorption systems are used for the removal of water, carbon dioxide and hydrogen sulphide from pipeline gas streams. Regarding water in particular, if not removed, it can condense and potentially freeze in gathering, transmission, and distribution piping causing plugging, pressure surges, and corrosion. In particular in LNG (Liquefied Natural Gas) processing plants the moisture content needs to be at a very low level to prevent ice formation within the cooling plant. This can cause disruption of service to consumers and can be costly in terms of maintenance. In particular ice formation within a liquefaction plant shuts the process down and the whole plant has to be warmed up from its operating temperature of well below minus 100 degrees C to thaw the ice and then the plant subsequently has to be cooled down again before it can be restarted.

The adsorption systems use mo'ecular sieves having sieving beds within vessels through which the gas to be treated is passed. The contaminants, such as water vapour, contained with the natural gas preferentially react with the desiccant material of the sieve bed, thereby drying the gas. The desiccant matenal used in the sieving beds requires periodic regeneration. whereby a contaminant free stream of gas is passed over or through the loaded bed in order to remove water adsorbed by the desiccant material.

The regeneration phase is time consuming and costly, requiring the vessel housing the mo'ecular-sieve bed to be temporarily removed from drying service.

Present adsorption systems using vessels with a desiccant material therein as a drying bed suffer from channelling', whereby a higher than average amount of wet' gas travels along a preferential route through the desiccant material. This reduces the contact time of the gas with the desiccant material and overloads the desiccant material along the channelling' flow path. This therefore resuhs in gas exiting the drier which may not be suitably dehydrated, and/or requiring regeneration of the sieve bed earlier than would otherwise be necessary due to. for example. only part of the sieve bed having reached its saturation condition.

It has been noted that channelling often occurs at an interface between the desiccant material and a vessel wall containing the material. This leads to wet gas flowing along the vessel wall for prolonged periods, which reduces drier efficiency.

The present invention seeks to provide a solution to these problems.

According to a first aspect of the invention, there is provided a molecular-sieve vessel comprising a housing having a first gas port, a second gas port and a desiccant chamber in fluid communication with the first and second gas ports, a flow path for treatable wet gas being defined by the first gas port, desiccant chamber and the second gas port, and at least one flow guide at an interior surface of the desiccant chamber, the flow guide having a baffle surface on a wet-gas upstream side for directing treatable wet gas at a periphery of the said flow path inwards.

Preferable andlor optional features of the first aspect of the invention are set forth in claims 2 to ii, inclusive.

According to a second aspect of the invention, there is provided an adsorption unit for a natural gas dehydration system, the adsorption unit comprising a molecular-sieve vessel in accordance with the first aspect of the invention, and a desiccant in the desiccant chamber of the molecular-sieve vessel.

According to a third aspect of the invention, there is provided a natural gas dehydration system comprising at least one adsorption unit in accordance with the second aspect of the invention, a treatable wet natura' gas supply communicable with the first gas port of the molecular-sieve vessel, and a regenerative gas supply communicable with the second gas port of the molecular-sieve vessel.

Preferably, at least two said adsorption units are provided, a first being in a wet-gas treating condition. and a second being in a desiccant regenerative condition.

According to a fourth aspect of the invention, there is provided a method of extending a period between desiccant regeneration using a molecular-sieve vessel according to the first aspect of the invention, the method comprising a step of providing a non-linear flow path at a periphery of desiccant material, whereby the non-linear flow path causes wet gas flowing at the periphery of the desiccant material to move inwards so that saturation of peripheral desiccant material is delayed.

Preferable and/or optional features of the fourth aspect of the invention are set forth in claims 17 to 19, inclusive.

The invention will now be more particularly described, by way of example only, with reference to the accompanying drawings. in which: Figure 1 shows a diagrammatic longitudinal cross-sectional view of one embodiment of a molecular-sieve vessel, in accordance with the first aspect of the invent ion; Figure 2 is an enlarged view of a portion of a flow guide on an interior wall of the vessel, showing a lateral profile of the flow guide and schematically a turbulent flow area during a drying phase; Figure 3 is a view sinñlar to that of Figure 2, but showing the flow guide during a regeneration phase; Figure 4 is a view similar to Figure 2 showing an enlarged lateral profile of a flow guide of a second embodiment of a molecular-sieve vessel, in accordance with the first aspect of the invention; and Figure 5 shows a diagrammatic longitudinal cross-sectional view of a third embodiment of the molecubr-sieve vessel, being similar to Figure 1, but including a sampling probe for monitoring the gas flow at a typical channelling region.

Referring firstly to Figures 1 to 3 of the drawings, there is shown a first embodiment of a molecular-sieve vessel 10 which comprises a vessel housing 12 having a first gas port 14, in this case at the upper end of the housing 12, and a second gas port 16 being a the lower end of the housing 12. A desiccant chamber 18 is provided by the housing 12 between and in fluid communication with the first and second gas ports 14, 16, thereby forming a flow path F through the vessel 10. In a natural gas dehydration system, an adsorption unit is provided which typically includes at least two said molecular-sieve vessels 10, allowing one to remain operational whilst the other undergoes regeneration of its adsorbent material in the desiccant chamber 18. The system generally provides for a treatable wet natural gas supply communicable with the first gas port 14 of the molecular-sieve vessel 10, a treated natural gas conduit communicable with the second gas port 16, and a regenerative gas supply communicable with the second gas port 16 of the molecular-sieve vessel 10, dong with a regenerative gas conduit communicable with the first gas port 14.

The desiccant chamber 18 may be partitioned. for example, by a wire mesh or apertured platform 20 to provide upper and lower sub-chambers 22, 24. Each sub-chamber 22, 24 may therefore house differently sized adsorbent material, for example, in the upper chamber 22 the adsorbent materia] may be in taNet form having an average circumference of 1/8 inch 3.18 mm, and in the lower chamber 24 the adsorbent material may be in tablet form having an average circumference of 1/16 inch 1.59 mm.

Although the desiccant material is preferably an adsorbent material, such as silicon dioxide, the invention is equally applicable for use with a desiccant material which is absorbent, such as salts.

The vessel 10 includes at least one flow guide 26 on an interior surface 28 of a wall 30 of the desiccant chamber 18. Preferably, a plurality of such flow guides 26 is provided, spaced apart over the longitudinal extent of the vessel 10.

The flow guide 26 is, in this embodiment, beneficially a ring which extends circumferentially around the interior surface 28 of the wall 30 of the desiccant chamber 18. The ring is preferably metal and may be attached to the vessel wall 30 by any suitable fastening means, such as welding.

During a wet-gas drying phase, the flow guide 26 provides a wet-gas upstream side 32 and a wet-gas downstream side 34, and during a desiccant regeneration phase, the flow guide 26 provides a drying-gas upstream side 36 which is equivalent to the wet-gas downstream side 34, and a drying-gas downstream side 38 which is equivalent to the wet-gas upstream side 32.

A lateral profile of the flow guide 26 is genera'ly triangular with a rounded and/or slightly hooked and/or slightly undercut apex 40 when the base 42 is at the vessel wall 30. The wet-gas upstream side 32 of the flow guide 26 provides a baffle surface 44 which directs wet gas, denoted by X, to be treated by the desiccant material and flowing at a periphery of the desiccant material and along the vessel wall 30 inwards towards the centre of the desiccant chamber 18.

The baffle surface 44 is therefore ramped, but the apex 40 is shaped to encourage gas flow at the apex 40 to separate from the flow guide 26, so as not or substantially not to continue around the apex 40 and onto the wet-gas downstream side 34, thereby ultimately re-joining the vessel wall 30. Although shaping of the apex 40 is suggested, other gas-flow separation means for preventing or limiting the flow of treatable wet gas onto the wet-gas downstream side 34 of the flow guide 26 may be considered.

With the desiccant material, particularly at the channelling regions C, becoming saturated earlier during the drying phase. liquid Y may flow along the preferential gas flow path, in this case being on interior walls 30 of the vessel 10. It is therefore preferable that the flow guide 26 includes a drip generator 46. The drip generator 46 may be formed by the hook or undercut 48 on the flow guide 26 adjacent to the apex 40, thereby providing a stop for liquid flow which is spaced from the interior vessel wall 30. Once a sufficient mass of liquid is formed, the drip 50 detaches from the flow guide 26 and enters into the desiccant material 52. The drip generator 46 thus effectively moves liquid away from the vessel wall 30, improving drier efficiency.

To address a possible issue of the drip generator 46 saturating the desiccant material 52 adjacent thereto, the flow guide 26 may include a turbulence generator 54, which may be in the form of a vortex generator. The turbulence generator 54 is formed at or adjacent to the apex 40 of the flow guide 26, and may be incorporated as part of the drip generator 46. Conveniently, the turbulence generator 54 may be formed by the hook or undercut 48 at or adjacent to the apex 40.

With gas flow being directed by the baffle surface 44 along the wet-gas upstream side 32 of the flow guide 26, and then separating, the turbulence generator 54 causes the separating gas to swirl at or preferably just downstream of the apex 40. As the drip 50 of liquid formed by the drip generator 46 separates from the flow guide 26, it is entrained and vaporised by the swirling turbulent gas 56. The vaporised liquid is then treatable as a wet gas flowing through the desiccant chamber 18 and adsorbed by the main body of adsorbent material 52.

Additionally or alternatively, other turbulence generating means or mixing means may be considered, as required.

The flow guide 26 with one or more of the above features will improve, during the drying phase, adsorption and/or absorption of treatable wet gas tending to flow at higher volumes at or adjacent to the interior side wall 30 of the molecular-sieve vessel 10.

Such a flow guide 26 also addresses the potential issue of higher saturation rates of the desiccant material 52 at the interior surfaces 28 of the drying vessel 10, and as a result increases the drier efficiency.

However, during the regeneration phase utiising a reverse flow of drying gas D flowing through the desiccant chamber 18 from the second gas port 16 to the first gas port 14 to desorb the water from the desiccant material 52, it is important that the flow guide 26 maintains gas flow on both its drying-gas upstream and downstream sides 36, 38. As such, the surfaces of the two sides 36, 38 are preferably proffled to allow the drying gas to flow from the interior vessel wafl 30 at region A and onto the drying-gas upstream side 36. around the apex 40 without or substantially without separating, and onto the drying-gas downstream side 38 before re-joining the interior vessel wall 30 at region B. This profiling of the flow guide 26 has the benefit of not only desorbing water from the desiccant material 52 which will typicafly be on or adjacent to the drying-gas upstream and downstream sides 36, 38, but also drying the interior surfaces 28 of the vessel 10, thereby reducing corrosion.

Referring now to Figure 4, a second embodiment of a molecular-sieve vessel 10 is shown. Like references refer to parts which are the same or similar to those of the first embodiment, and therefore further detailed description is omitted.

This molecular-sieve vessd 10 includes the vessd housing 12 having the first and second gas ports 14. 16, and the desiccant chamber 18 therebetween. A flow guide 126 which is similar to that of the first embodiment is also provided within the desiccant chamber 18. The flow guide 26 of the first embodiment is permanently attached to or formed as part of the interior vessel wall 30, and this may be accomplished during manufacture of the vessel 10 or as a retro-fit to an existing vessel 10.

The flow guide 126 of this second embodiment is a retro-fit device and is non-permanently attached to the interior vessel wall 30. In this way, a base 142 of the flow guide 126 includes one or more seals 160. in this case two spaced apart seals 160, for example 0-rings, which extend in a circumferential direction around the interior vessel wall 30. The seals may not necessarily need to be of fill circumference, whereby breaks in the seals can be longitudinally staggered to provide a non-linear or tortuous path therethrough.

With the flow guide 126 in place, it is radially expanded to sealingly engage the seals IS 160 with the interior vess& wall 30. Radial expansion may be imp'emented, for example, by use of a worm screw or ratchet mechanism.

Other alternatives for engaging the flow guide 126 with the interior vessel wall 30 Expansion and contraction of the vessel 10 can be accommodated by the flow guide 26, 126 through the use of expansion plates or elements at one or more points along its longitudina' extent, or for example by the use of spreader mechanisms.

It is preferable that a plurality of flow guides 26, 126 are utilised in spaced apart configuration over a longitudinal extent of the interior vessel wall 30. The flow guides 26, 126 may be independent of each other, or may be interconnected. For example, a web. mesh or matrix of flow guides may be utilised.

Referring now to Figure 5, a third embodiment of a molecular-sieve vessel 10 is shown.

Again, like references refer to parts which are the same or similar to those of the first and second embodiments, and therefore further detailed description is omitted.

This molecular-sieve vesse' 10 includes the vessd housing 12 having the first and second gas ports 14, 16. and the desiccant chamber 18 therebetween. A flow guide 26 which is similar to that of the first and/or second embodiments is also provided within the desiccant chamber 18.

In this embodiment, one or more gas sampling probes 270 are provided for monitoring a moisture content of treatable wet gas flowing through the desiccant chamber 18. A sampling tip 272 of the probe 270 is preferably positioned adjacent to a lowermost portion 58 of the interior vessel wall 30 within the dryer bed region. As shown, it may therefore be preferable that the sampling probe 270 enters the vessel 10 partway up the interior vessel wall 30. extends laterally into the desiccant chamber 18 before then turning downwardly towards the second gas port 16. Finally, the sampling probe 270 turns back to position the sampling tip 272 at or adjacent to the interior vessel wall 30.

In this way, the treatable wet gas can be monitored prior to discharge from the second gas port 16 and as it nears the end of the desiccant chamber 18. By monitoring the gas at IS this point, and also preferably at the second gas port 1 6, it can be determined whether channelling may be occurring at the interior vessel wall 30 and whether the desiccant material 52 may be prematurely saturated.

Although treatable wet gas may be supplied to the first gas port whereby any liquid water that forms on the wall flows with gravity towards the second gas port, the first gas port may be provided at the base of the vessel with the second gas port being provided at a top of the vessel. In this way. any liquid moisture on the wall would have to move against gravity, again leading to a reduced chance of channelling and greater exposure to the desiccant material. Additionally this would make the rings more effective at removing liquid moisture from the walls.

The flow guide of the invention forms a non-linear longitudinal outer flow boundary which thereby defines a non-linear flow path through the desiccant chamber of the wet gas drying vessel at or adjacent to the interior vessel surface and at a periphery of the desiccant material. The non-linear flow path results in the prevention or limitation of channelling at a periphery of the adsorbent or absorbent material within the desiccant chamber, and also the movement of treatable wet gas flow at the periphery of the desiccant material inwards towards the centre of the vessel. Consequently, saturation rates at a periphery of the desiccant material are decreased, drier efficiency is significantly improved, and thus a period between desiccant regeneration is increased.

The embodiments described above are provided by way of examples only, and various other modifications will be apparent to persons skilled in the field without departing from the scope of the invention, as defined by the appended claims.

Claims

Claims 1. A molecular-sieve vessel comprising a housing having a first gas port, a second gas port and a desiccant chamber in fluid conimunication with the first and second gas ports, a flow path for treatable wet gas being defined by the first gas port, desiccant chamber and the second gas port, and at least one flow guide at an interior surface of the desiccant chamber, the flow guide having a baffle surface on a wet-gas upstream side for directing treatable wet gas at a periphery of the said flow path inwards.
2. A molecular-sieve vessel as claimed in claim I, wherein the baffle surface of the flow guide is ramped.
3. A molecular-sieve vessel as claimed in claim I or claim 2, wherein the flow guide is a ring which extends in a circumferential direction around the interior surface of the desiccant chamber.
4. A molecular-sieve vessel as daimed in any one of the preceding claims, wherein the flow guide is releasably engagable with the interior surface of the desiccant chamber.
5. A molecular-sieve vessel as claimed in claim 4, wherein the flow guide includes at least one fluid-tight sealing element for sealing engagement with the interior surface of the desiccant chamber.
6. A molecular-sieve vessel as chimed in any one of the preceding claims, wherein the flow guide further comprises a drip generator by which liquid moving on the baffle surface drips into the desiccant chamber, thereby being prevented or limited from re-joining the interior surface.
7. A molecular-sieve vessel as chimed in any one of the preceding claims, wherein the flow guide further comprises turbulence generating means for forming turbulence from treatable wet gas flowing on the said flow path and which is at or adjacent to the wet-gas upstream side.
8. A molecular-sieve vessel as claimed in any one of the preceding claims, whereinHthe flow guide further comprises gas-flow separation means for preventing or limiting the flow of treatable wet gas onto a wet-gas downstream side of the flow guide.
9. A molecular-sieve vessel as claimed in any one of the preceding claims, wherein the flow guide is shaped so that, during a regeneration phase, regeneration gas flows from a drying-gas upstream side of the flow guide to a drying-gas downstream side.
10. A molecular-sieve vessel as claimed in any one of the preceding claims, wherein a plurality of said flow guides is provided.
11. A molecular-sieve vessel as claimed in claim lO, wherein the flow guides are spaced apart.
12. A molecular-sieve vessel substantially as hereinbefore described with reference to Figures 1 to 3, Figure 4 or Figure 5 of the accompanying drawings.
1 3. An adsorption unit for a natural gas dehydration system, the adsorption unit comprising a molecular-sieve vessel as claimed in any one of the preceding claims, and a desiccant in the desiccant chamber of the molecular-sieve vessel.
14. A natural gas dehydration system comprising at least one adsorption unit as claimed in claim 13, a treatable wet natural gas supply communicable with the first gas port of the molecular-sieve vessel, and a regenerative gas supply communicable with the second gas port of the molecular-sieve vessel.
15. A natural gas dehydration system as claimed in claim 14, wherein at least two said adsorption units are provided, a first being in a wet-gas treating condition, and a second being in a desiccant regenerative condition.
16. A method of extending a period between desiccant regeneration using a molecular-sieve vessel as claimed in any one of claims I to 12, the method comprising a step of providing a non-linear flow path at a periphery of desiccant material, whereby the non-linear flow path causes wet gas flowing at the periphery of the desiccant material to move inwards so that saturation of peripheral desiccant material is ddayed.
17. A method as chimed in daim 16, further comprising a step of liquid-flow interruption on the non-linear flow path at the periphery of the desiccant material, whereby liquid flow on the non-linear flow path is caused to form one or more drips.
18. A method as claimed in claim 17, further comprising a step of turbulence generation at or adjacent to the non-linear flow path, whereby the said one or more drips are at least in part dispersed into the desiccant material due to at least a portion of the gas flowing on the non-linear flow path forming one or more vortices.
19. A method as claimed in any one of claims 16 to 18, further comprising a step of desiccant regeneration whereby a counter-flow of a regenerative gas is passed along the non-linear flow path, whereby the regeneration gas remains or substantially remains on the non-linear flow path as it moves along the periphery of the desiccant material.