GB2114555A - Improvements in the regeneration of adsorbents - Google Patents

Abstract

A water-saturated adsorbent, especially a zeolite used to dry organic solvents, having a particle size greater than 0.2 mm, e.g. in the range 1.4 to 2.0mm, is regenerated by passing a hot gas through a bed of the sieve. The gas is passed at a superficial velocity of at least 0.5 m/s, e.g. about 1.0 m/s. This is well in excess of recommended values but does not result in an unacceptably high pressure drop across the bed.

Description

SPECIFICATION Improvements in the regeneration of adsorbents This invention relates to improvements in the regeneration of adsorbents, and in particular, but not exclusively, to the regeneration of zeolite adsorbents used to dry organic solvents.

In many chemical processes the solvent becomes contaminated:wilth other solvents, water, impurities and by-prnducts.Thereforn, at the end of the process it is necessary eitherto dispose ofthe solvent orto clean it. In a cleaning process, the solvent may first be filtered and then separated from its contaminants by fractional distillation. In many cases, especially where the contaminants have a similar boiling point to the main solvent,fractional distillation only results in partial recovery of the solvent and this recovery may be further reduced by the formation of azeotropes. It is often not commercially or technically possible to recover further pure solvents from these azeotropes by distillation techniques.Typically, the azeotropescontain an organic solvent and water or two similar organic solvents, such as isopropanol/ water methanol/water, ethanoUacetone etc.

It has been proposed that further purification of solvents or azeotropes formed by distillation of contaminated solvents may be achieved using zeolite adsorbents. These adsorbents separate components in a mixture, on the basis of molecular size, shape and polarity. For instance it has been proposed to separate an isopropanol water mixture using a type 3A zeolite which retainsthe water in the pores and allows the isopropanol to pass th rough the adsorbent bed.Theadsorbentmaterialtherefore becomes saturated with water and must periodically be regenerated bydriving offthewater. This is generally done by passing a hot gasthrough the adsorbent to drive offthewater.

At present, the regeneration of zeolite adsorbents using hot gases has been carried out at low superficial gas velocities in order to avoid having excessively large pressure drops across the adsorbent bed. If the pressure drop is too large, it becomes necessary to install costly high pressure fans to drive the hot gas through the bed. The relationship between the pressure drop across a bed and the superficial gas velocity has hitherto been calculated using the following equation,which was first published in Chem. Eng. Prog. 1952,48(2), 89. It is usually known as the Ergun equation after its author.

AP 150 (1-E)2uV 1.75 (1-E) GV2 xg= + L E3 Dp2 E3 Dp where AP = pressure drop across the bed L = length of the bed g = gravitation constant E = voidage of the molecular sieve u = viscosity of regenerating gas V = superficial velocity of regenerating gas at average pressure Dp = mean particle size of molecular sieve G = density of regenerating gas Where necessary,the parameters of the equation must be calculated at the operating temperature of the regeneration.

This equation contains a number of terms which are constantfor a particulartype of zeolite adsorbent.

Therefore zeolite adsorbent manufacturers have derived a number of simplified versions of the equation for use with their particularzeolite adsorbents. One simplified version of the equation, which is applicable to a type 3A zeolite having a particle size in the range of 1 .6 to 2.5mm, is AP = 0.1157 uV + 2.107 x 10-5 GV2 L On the basis of such equations manufacturers have recommended thatthe maximum useful superficial velocity for any zeolite adsorbent regeneration is about 0.35 m/s. It is calculated that above this figure pressure drops across the bed will become u nacceptably high.

An instance of the use of low superficial velocities forthe regeneration of a zeolite adsorbent is given in British Patent No.1193127 on page3at lines 39 to 48.

The superficial velocity used is low and the gas is passed upwardly through the bed with only sufficient velocityto fluidise the zeolite adsorbent material.

It has now surprisingly been found thatthe Ergun equation, orthe above simplified derivative of it, does not accurately describe the behaviour ofzeolite adsorbents during regeneration at high superficial velocities. It has been found that even using high superficial velocities it is possible to operate at acceptable pressure drops.

Therefore, according to the present invention, there is provided an improved method of regenerating an adsorbent having a particle size greaterthan 0.2mm, comprising passing a hot gas through the adsorbent at a superficial velocity of at least 0.5 m/s.

Preferably, the gas is passed downwardly through the bed to preventfluidisation of the adsorbent. Ifthe material isfluidised, it will be subject to attrition and a fraction of it may be removed from the bed by elutriation.

The advantages of using a high superficial velocity are thatthe time taken to regenerate the bed is significantly reduced and the heating cost is also reduced. This latter advantage is due to the fact that on a commercial plantthere are many sources of heat losses and thermal inefficiency. Thus for a given gas temperature the same amount of heat per unit time will be lost from the plant at a given superficial velocity. However, usingthe method ofthe present invention, the time for which the hot gas will need to be supplied is considerably reduced. Therefore, since heat needs to be supplied for less time, the overall heat required for regeneration is reduced.

As the superficial velocity of the gas rises, the pressure drop does increase, although not according to the Ergun equation. Therefore as a practical consideration it is envisaged that the preferred superficial velocity will be in the range 0.6to 2.0 m/s.

Advantageously, the superficial velocity is about 1 mis.

The temperature ofthe regeneration gas will depend to a large extent on the material being trapped bytheadsorbent. In a typical case where the material iswaterthe gas will advantageously be dry and ata temperature of about 260"C, but may be in the range from 150 to 300"C for a gas with a significant moisture content. For a dry gas, the temperature may be as low as 1 20 C.

For adsorbed organic liquids, the preferred gas temperature is generally between 60"C and 1 00 C, although in appropriate cases it may be as low as 40"C or up to 3000C, depending upon the nature of the adsorbed molecule.

lfthetrapped material is water, it is possible to use air as the regenerating gas. However, it is necessary to flush the system first with cold air to remove any solventvapours before hot air is blown. This prevents the possibility of an explosion caused by ignition of mixed hot air/solventvapour. If the trapped material is an inflammable organic liquid, the gas should be either oxygen free or at least oxygen lean. On a large scale plant it is usual to have some form of combustion heating apparatus,andthefluegasfrom this may be used as the regenerating gas. The flue gas will normally contain some moisture.

The gas may be forced through the adsorbent by conventional fans at relatively low power consumption. Itwas previously thoughtthatthe cost of powering fans to give the expected high pressure drop at high superficial velocity would be prohibitive.

This has now been shown to be erroneous.

In some cases, it may be desirable to maintain the adsorbent material at pressures higher or lowerthan ambient pressure. In these cases the pressure drop across the adsorbent bed is the difference in pressure between the gas at the inlet and at the outlet. The whole system in such a case will be maintained under pressure or partial vacuum as desired.

It is envisaged that any known adsorbent may be regenerated at high superficial velocities, including zeolites, alumina, silica gel and chemicals such as calcium chloride, calcium sulphate and other salts which have stable forms underthe above described conditions and also have different degrees of hydration. However, the Applicants believe the method is particularly applicableto the regeneration of A-type zeolites, particularly those marketed under the de signations3A,4Aand5A. It has been proposed that these zeolites should be of especial use in the drying of organic solvents having a high water content. The method of the present invention will be of particular, but not exclusive, use in regenerating zeolites used in the above process.The invention will now be described byway of example only with reference to a process for drying a solvent stream.

Isopropanol containing 12.1% bywt.waterwas passed upwardlythrough a conventional column containing a bed of a type 3Azeolite having a particle size in the range 1 .4to 2.0mm. The bed was approximately 0.6m. in diameter and 3.4m. long, and containedabout735kg of bed material. The isopropanol was introduced to the bed at a low superficial velocity at ambienttemperature. The product from the bed was isopropanol containing 0.1% by weight of water.

After passing 1300kg of isopropanol through the bed, orwhen the water content of the product isopropanol rose to about 0.2% by weight, the flow of isopropanol was stopped and the bed was drained.

The absorbent material retained about 157 kg of waterin its pores.

The bed was regenerated as follows. Cold airwas blown downwardlythrough the column at a superficial velocity of 1 m/s for one hour. Thereafterthe air was heated to 260'C and passed downwardlythrough the bed atthe same velocity for a further six hours.

The heating was then stopped and cold air passed upwardly through the bed for one hour. Thistreatment removed substantially all the water from the zeolite. During the cold blow and the initial stages of the hot blow, gas exiting from the column was passed through a condensertotrap any solvent vapour entrained in the regenerating airflow.

Thetreatmentwith cold airatthe beginning ofthe regeneration was carried outto remove solvent vapoursfrom the column,therebyto obviate the possibility of an explosion due to ignition of hot air/ isopropanol mixtures which would beformed if hot airwere directly blown into the bed. The six hour treatmentwith the hot air effected the removal of the vast majority ofthetrapped water, andtheone hour after-treatment with cold air cooled the bed again to a temperature suitable for drying the contaminated isopropanol.The actual amount of water remaining in the adsorbentwill depend upon the humidityofthe air blown through the column and under average conditions in the British Isles, this will be about2to 7% bywt. ofthe actual capacity of the zeolite adsorbent for water. The total treatment time is therefore about 8 hours.

In a similar system operated according to conditions previously recommended, the superficial velocity used would be a maximum of about 0.3 m/s, and necessitate a treatment time with hot air of about 25.5 hours. The total treatment time would be about 30 hours including a 1 hour initial cold blow and a 3-4 hour after-treatment. It can be seen from this that operating the regeneration according to the method of the present invention reduces the regeneration time by at least 50% and concomitantly reduces the cost of heating the air. This saving in cost on down-time and heating is not offset by the extra cost of maintaining a high superficial velocity. Also the annual capacity of a given column is considerably increased because of the overall reduced cycle time.

In the example described above the Ergun equation predicts a pressure drop of about 110 MPa, whereas in fact it is only about 27 MPa. Thus there is no need to instal large and expensive fans to operate the method according to the invention.

Therefore the present invention provides an im proved method of regenerating a zeolite adsorbent which will provide commercial and operational advantages.

Claims (12)

1. An improved method of regenerating an adsor bent having a particlesizegreaterthan 0.2mm, comprising passing a hot gas through the adsorbent at a superficial velocity of at least 0.5 mls.

2. A method according to claim 1, wherein the adsorbent is a type 3A, 4A or 5A zeolite.

3. A method according to claim 1 or 2, wherein the hot gas is passed downwardlythrough the adsorbent.

4. A method according to claim 1,2 or 3, wherein the superficial velocity is in the range 0.6 to 2.0 m/s.

5. A method according to claim 4, wherein the superficial velocity is in the range 0.6 to 1.2 m/s.

6. A method according to any one ofthe preceding claims, wherein the gas is air.

7. Amethod of regenerating an adsorbent retaining water according to the method of any one of claims 1 to 5, wherein the gas is at a temperature in the range from 120 to 300"C.

8. A method according to claim 7, wherein the hot gas is a flue gas containing a significant quantity of moisture and the gas is at a temperature of at least 1 50 C.

9. A method of regenerating an adsorbent retaining an organic liquid according to the method of any of claims 1 to 5, wherein the hot gas is at a temperature in the range 40 to 1 00 C.

10. A method according to claim 9, wherein the hot gas is oxygen lean or oxygen free.

11. Amethodaccordingtoanyoneofthe preceding claims, wherein the adsorbent is maintained under pressure or under partial vacuum.

12. An improved method of regenerating an adsorbent, substantially as hereinbefore described.