Air distillation plants generally include adsorbers so as to subject the air, compressed on entering the plant, to a drying and decarbonation treatment before it is cooled and then introduced into the distillation column proper. For this purpose, the air to be treated flows under a pressure typically of about 5 bar absolute through a bed of an adsorbent based in the adsorber. Once the adsorption capacity of the bed has been exhausted, the adsorbent is regenerated by means of a dry and decarbonated gas. This gas may be an off-gas recovered from the distillation column and injected into the adsorber in the reverse direction. In order to increase the regeneration effectiveness, this off-gas is heated to a high temperature of about 200.degree. C. This type of adsorber, in which the adsorbent is subject to cyclic temperature variations, is known by the name "TSA (Temperature Swing Adsorption) adsorber".
Heating the gas as well as the adsorbent represents a major part of the energy consumed by the distillation plant. In order to minimize this consumption, it is therefore necessary to reduce the heat exchange between the adsorber and its environment.
Thermal insulation means consisting of an outer covering of the walls of the adsorber are known. This type of insulation proves to be very effective for the entire adsorber but does have certain drawbacks when the adsorber is subjected in a cyclic manner to temperature variations. This is because, since the walls of the adsorbers are generally made of metal, these constitute, depending on the phase of the cycle a heat store or heat sink tending to delay establishment of a steady state after a temperature change. In the regeneration phase, the walls absorb part of the heat provided by the heated gas and thus delay the heating of the adsorbent, in particular in regions close to the walls. In the adsorption phase, they restore this heat, thereby decreasing the adsorption effectiveness of the adsorbent. The walls therefore act out of step with the cycles and considerably decrease the effectiveness of the adsorber.
This phenomenon could be counteracted by increasing the size of the adsorbers and consequently by reducing, for a constant flow, the frequency of the cycles. However, recent developments in distillation plants have led to a reduction in the size of the adsorbers so as to reduce the amount of adsorbent and the cost of an adsorber. Because of the fact that the amount of adsorbent is reduced, the frequency of the cycles is considerably increased.
In order to improve the effectiveness of the known absorbers, adsorbers are known which are equipped with internal thermal insulation. Such internal thermal insulation must satisfy particular requirements:
it must not allow bypassing of gas between the wall of the adsorber and the bed of adsorbent, so that the gas that is to be treated and the gas that has been treated are always separate; PA1 it must withstand the repeated compressions and decompressions imposed by the operating cycle and be able to attain pressure variations of about ten bar; and PA1 it must easily allow periodic hydrostatic testing, this being generally required by the authorities in the operating countries. PA1 the blocking means include at least one separating wall fitted between the said wall and the shell; PA1 the adsorber comprises means for decreasing natural convection in the gas-filled cavity; PA1 the means for decreasing natural convection in the gas-filled cavity comprise a sheet, in particular a metal sheet, placed between the wall and the shell in the gas-filled cavity in order to obtain a double gas-filled cavity; PA1 the sheet is centered with respect to the said wall and to the shell; PA1 the gaps between the sheet and the wall and between the sheet and the shell are between 10 and 40 mm, and preferably equal to 20 mm; PA1 the absorber comprises elements, especially point or linear elements, forming spacers between, on the one hand, the wall and the sheet and, on the other hand, between the sheet and the shell; PA1 the elements forming spacers between the wall and the sheet are offset with respect to the elements forming spacers between the sheet and the shell; PA1 the elements forming spacers are indented portions and raised portions preferably produced by goffering in the sheet, these indented portions and raised portions having, in particular, pointed ends; PA1 the indented portions and raised portions of the sheet extend parallel to the axis of the shell, over the entire length of that dimension of the sheet which is parallel to this axis, and have a cross-section of approximately triangular shape; PA1 the indented portions and raised portions alternate on each face of the sheet; PA1 in the case of an adsorber having a shell which is extended into a part forming a dome, the adsorber includes at least one and preferably two layers of a felt of fibres applied against the inside of the dome; PA1 for an adsorber intended, in particular, to operate under a varying pressure, the felt of fibres includes ceramic fibres and has a ratio between the fluid resistivity and Young's modulus of elasticity of less than 1 s/m.sup.2 ; PA1 the thickness of the or each layer of felt is between 20 and 30 mm, and preferably equal to about 25 mm; PA1 the layers of felt are attached to the inside of the dome by means of removable attachment devices; PA1 the removable attachment devices comprise rods, fastened to the dome and intended to transpierce the layers of felt, and washers for holding the felt against the dome, the washers engaging with the rods, in particular by a clip-in mechanism; and PA1 the adsorber is used in particular for the purification of air sent into an air distillation plant, the active material being an adsorbent.
Document US-A-3,925,041 teaches a TSA adsorber equipped with internal thermal insulation. This adsorber comprises a cylindrical shell oriented vertically, the ends of which are closed by ellipsoidal parts forming, respectively, a dome and a bottom. A bed of adsorbents is placed in the cylindrical shell
In the region of the bed of adsorbent, the internal thermal insulation is produced by sheets of a rigid insulant, these being applied end to end against the shell and compressing, against this shell, a layer of a fibrous insulating material. This thermal insulation is therefore placed between the shell and the bed of adsorbent. In order to prevent bypassing of the gases through this insulation during operation, metal sheets are applied against the shell, covering the sides of the sheets of insulant so as to deflect the flow of gas towards the bed of adsorbent.
Furthermore, a fibrous insulant is applied by means of bolts against the internal parts of the dome and of the bottom of the adsorber.
Although this internal thermal insulation considerably increases the effectiveness of an adsorber, it is not suitable for undergoing a hydrostatic test. This is because, after such a test, the thermal insulation is wet and ineffective. It must therefore undergo lengthy, difficult and consequently very expensive drying. Moreover, fitting the sheets of insulant and the metal sheets in the region of the bed, of adsorbent proves to be difficult and lengthy so that only specialists are able to place this insulation against the internal walls of the adsorber. This known insulation therefore considerably increases the cost of manufacturing of the adsorber.