GB2155804A

GB2155804A - Improvements in pressure swing adsorption techniques

Info

Publication number: GB2155804A
Application number: GB08505806A
Authority: GB
Inventors: M E Garrett
Original assignee: BOC Group Ltd
Current assignee: BOC Group Ltd
Priority date: 1984-03-06
Filing date: 1985-03-06
Publication date: 1985-10-02
Also published as: GB2155804B; GB8505806D0; GB8405778D0

Abstract

A pressure swing adsorption apparatus includes a bed of adsorbent material which has been sintered and formed into a block or blocks to prevent or substantially reduce the effect of bed lifting. A plurality of blocks 8' may be directly bonded to a metallic sleeve 12 which is placed within a container 2'. <IMAGE>

Description

SPECIFICATION Improvements in pressure swing adsorption techniques The present invention relates to processes for the separation of gaseous mixtures using pressure swing adsorption (PSA) techniques and in particular to adsorption materials used in such techniques. It is known to separate a gaseous mixture such as air using a pressure swing adsorption process by passing the air during an adsorption stage of the process at a predetermined pressure through a bed of adsorbent material which preferentially adsorbs one or more components of the air. The adsorbent material comprises a large number of pellets of molecular sieve. Each pellet itself is composed of a large number of carbon particles and is of a porous structure that enables molecular sieving to take place. Assuming nitrogen is the product gas required then the adsorbent material is typically molecular sieve carbon.In operation, a bed of the adsorbent material is subjected to a cycle which includes an adsorption step during which time air is pumped through the bed and most of the oxygen, a proportion of the nitrogen and substantially all of the carbon dioxide and water vapour in the feed are adsorbed. As a consequence, a nitrogen rich product gas is supplied from the outlet of the bed. Once the adsorbent material is fully saturated with oxygen, carbon dioxide, water vapour and some nitrogen then a desorption or regeneration step is initiated during which time the outlet of the bed is closed, the bed is vented to atmospheric pressure through its inlet and/or evacuated through its inlet so that the adsorbed gases are substantially removed from the bed thereby preparing it for the next adsorption step.

There are many PSA systems employing two or more adsorbent beds. A typical system is one in which two adsorbent beds are employed and operate on similar cycles which are sequenced to be out of phase with one another by 180 degrees so that when one bed is on its adsorption step, the other bed on its regeneration step and vice versa.

It is usual to equalise the pressures in the two beds between each step by connecting the two bed inlets together and connecting the two bed outlets together. With these connections made, the gas within the void spaces of the bed which has just completed its adsorption step is sucked into the bed which has just completed its regeneration step by virtue of the pressure difference which exists between the beds at that stage and this is found to be beneficial in maximising the product output because the gas in such void spaces will have already become somewhat enriched in nitrogen.

In order to connect together the two bed inlets and the two bed outlets as aforementioned, stop valves are opened in passages connecting together the inlets and outlets respectively. Since, in practice, the initial pressure difference between the two beds when the valves are first opened is relatively large (typically in the order of several bars) there is a tendency for the upward passage of gas through the outlet of the bed at higher pressure to lift or fluidise some of the adsorbent pellets in the bed and thereby cause it to strike a wall of the container accommodating that bed. It has been found that repeated lifting of the bed in such a manner reduces the efficiency of the adsorbent, and causes attrition of the pellets. The resulting particles may then be carried in the product gas stream and thereby cause wear to associated equipment.

United Kingdom patent No. 1241065 teaches the use of flow rate limiting devices which prevent bed damage due to excessive change in pressure and fluid velocity. The flow rate limiting devices are in effect valves which control the flow of gases between the two adsorbent beds.

Other means have been employed to prevent bed lifting or fluidising, for example, in United Kingdom published patent application No. 2091121 A a pressure plate is used on the upper surface of a bed which plate is spring loaded and therefore physically prevents adsorbent material from being lifted.

All such known attempts to prevent or minimise bed lifting have been found to be only marginally effective and can result in a consequent reduction in the efficiency of the PSA system. Thus, even when restraining means are employed, it is common practice in the art to choose molecular sieve adsorbent pellets of relatively large particle size and to choose feed gas flow rates and velocities such that bed lifting is kept to tolerable limits. It is however found that a more rapid separation of the feed gas mixture occurs with pellet particle size, and therefore these choices entail operating the PSA process at less than optimum efficiency.

It is an aim of the present invention to provide an adsorbent material for use in a PSA process which by its nature is immune to or substantially inhibits any possibility of the material being lifted or fluidised.

According to the present invention, an apparatus for the separation of gases by pressure swing adsorption techniques includes a bed of adsorbent material in which said adsorbent material has been sintered and formed into at least one block. Pellets (or other bodies) of adsorbent material will normally be sintered together to form said block. The individual pellets themselves may be prepared by a process that includes sintering.

Embodiments of the invention will now be described, by way of example, reference being made to the Figures of the accompanying diagramatic drawings in which: Figure 1 is a cross-sectional elevation through a vessel containing a bed of adsorbent material; and Figure 2 is a cross sectional elevation through a different vessel containing a bed of adsorbent material.

As shown in Figure 1, a vessel 1 comprises an outer tubular container 2 fitted with upper and lower closure plates 4,6. The interior of the container 2 accommodates a bed of adsorbent material, for example, molecular sieve carbon which has been sintered and formed into a single homogeneous block 8 which fits snugly into and fills the container 2.

In a modification, it would be possible to fill the container 2 with a bed comprising two or more sintered blocks or discs capable of being stacked together.

The block 8 or blocks can be wrapped around, prior to insertion into the container 2 with fibreglass or other suitable material to prevent feed gas when passing from the inlet 9 to the outlet 10 of the vessel by-passing the block(s) and flowing between the interior of the container 2 and the exterior surface of the block(s).

In use, feed gas will pass into the vessel 1 and flow from inlet 9 towards outlet 10. Regardless of the rate of flow or pressure of the feed gas passing through the container 2, the bed of adsorbent material will not be able to lift or fluidise by virtue of its solid nature.

Referring now to Figure 2, blocks 8' of sintered adsorbent material are directly bonded to a metallic sleeve 12 which is then placed within a container 2'. 0 ring seals 15 are then placed in the annular gap between the outer surface of the sleeve 12 and the inner surface of the container 2' to avoid any by-pass flow of the feed gas. Of course a single block 8 as used with the embodiment of Figure 1 could replace the plurality of blocks 8'.

In a modification the metal sleeve 12 itself could form the outer wall of the vessel.

In both embodiments, since the adsorbent material is sintered and formed into a block or blocks, any tendency of the bed to lift by virtue of feed gas entering the vessel or for any other reason is substantially eliminated. The sintering may be performed by a conventional method. For example, conventional carbon molecular sieve pellets may be mixed with a binder (e.g. pitch) and the resulting mixture may be formed into a chosen shape and the resulting shape sintered. Alternatively, a mixture of binder and carbon particles (of the kind normally used to form conventional molecular sieve carbon) may be typically at elevated temperature extruded into a continuous rod-like body which is then cut or chopped into pellets.The pellets may be mixed with more binder, formed into a block of chosen size and defined shape, and the block then sintered The result at elevated temperature typically in a nitrogen atmosphere or a vacuum.

The use of such sintered blocks in pressure swing adsorption technology removes or lessens constraints upon the choice of pellets size, upon feed gas velocity and upon feed gas flow rate thus facilitating the selection of operating parameters to give an optimum process. Moreover, it is possible to select the proportion of void spaces in the bed, for example by choosing to form the bed molecular pellets having a range of different sizes, and such an expedient may again facilitate the design of an optimum process.

The apparatus according to the invention may be used to perform any known PSA cycle (for exam ple a cycle as described in either of the aforementioned UK patent specifications). We prefer the cycle to include a step in which one bed at relatively high pressure is placed in flow communication with another bed at relatively low pressure to reduce or eliminate the pressure difference there between.

Claims

1. An apparatus for the separation of gases by pressure swing adsorption techniques including a bed of adsorbent material in which said adsorbent material has been sintered and formed into at least one block.

2. An apparatus as claimed in claim 1, in which the or each said block comprises sintered together pellets of adsorbent.

3. An apparatus as claimed in claim 2, in which said block comprises sintered together pre-sintered pellets of adsorbent.

4. An apparatus as claimed in claim 2 or claim 3, in which the pellets are of different sizes from one another.

5. An apparatus as claimed in any one of the preceding claims in which the adsorbent material is formed into a plurality of stackable blocks.

6. An apparatus as claimed in any one of the preceding claims in which the block(s) are wrapped around by glass fibre material and accommodated in a container.

7. An apparatus as claimed in any one of claims 1 to 5 in which the block(s) are directly bonded to a metallic sleeve.

8. An apparatus as claimed in claim 7, in which the metallic sleeve is accommodated within a container and seals are arranged in the space between the outer surface of the sleeve and the inner surface of the container.

9. An apparatus as claimed in any one of the preceding claims, including a plurality of beds of molecular sieve adsorbent each comprising at least one said block.

10. An apparatus as claimed in claim 9, adapted to operate a process which includes placing one bed at relatively high pressure in flow communication with a second bed at relatively low pressure so as to reduce or eliminate the pressure difference there between.

11. An apparatus for the separation of gases by pressure swing adsorption techniques, constructed and arranged substantially as hereinbefore described, with reference to Figure 1 or Figure 2 of the accompanying drawings.

12. A process for the separation of gases by pressure swing adsorption employing an apparatus as claimed in any one of the preceding claims.

13. A method of forming a sintered block of adsorbent for use in the apparatus claimed in any one of claims 1 to 11, in which sintered pellets of adsorbent are mixed with binder, formed into a desired shape, and the resulting block.

14. A method of forming a sintered block of adsorbent for use in the apparatus claimed in any one of claims 1 to 11, in which particles of adsor bent are mixed with binder, the resulting mixture is extruded to form a continuous rod-like body, the body is cut into pellets, the pellets are mixed with binder, and the mixture is formed into a desired shape and is thus sintered.