Regeneration of molecular sieves

Abstract

Molecular sieves used for dehydration are regenerated with methanol or acetone.

Description

Molecular sieves are used to dehydrate various materials including polar solvents such as, for example, methanol and ethanol, paints and plastics and saturated hydrocarbon streams. Molecular sieves are crystalline zeolites having a basic formula M.sub.2/n O.Al.sub.2 O.sub.3.xSiO.sub.2.yH.sub.2 O where M is a cation of n valence. The conventional means for regenerating the sieves is to desorb the adsorbed water with a stream of hot gas, typically at a temperature of 550.degree. F. This requires considerable energy since the entire system must be heated to a temperature at which water is desorbed. Furthermore, additional energy is required to maintain the system at the elevated temperature for the duration of the desorption cycle, which typically lasts two to four hours.

One object of this invention is to provide a method for regeneration or reactivation without the use of heat of molecular sieves that have been used for dehydration of materials.

Another object of this invention is to provide a method for regeneration of molecular sieves in a manner that permits the molecular sieves to be returned to dehydration service immediately after regeneration.

Another object of the invention is to provide a method for regeneration of molecular sieves with a solvent that can be recovered or dehydrated by distillation.

Another object of the invention is to provide a method for regeneration of molecular sieves by a process which avoids placing stresses on the sieves as results from regenerating procedures utilizing high temperature and cooling cycles.

I have now found that molecular sieves which have been used to dehydrate or remove water from substances can be regenerated with methanol or acetone. Since methanol is a chemical that can be dehydrated by molecular sieves, this finding was totally unexpected. Furthermore, Breck, D. W. and Smith, J. V., "Molecular Sieves", Sci. Amer. 200 85 (1959), state, "The crystals (zeolite molecular sieves) have a particularly strong affinity for water molecules, and will adsorb them in preference to any other substance." Thus, one would expect water to be so strongly held by the molecular sieves that methanol or acetone would not be able to dislodge the bound molecules.

In accordance with this invention, a molecular sieve which has been used for dehydration and which has become spent or which has lost some dehydrating activity because of water absorption is regenerated by contacting the molecular sieve with methanol or acetone. Regeneration efficiency is less with methanol and acetone containing substantial amounts of water and it is thus preferred to employ these regenerating agents in substantially anhydrous condition, i.e, containing not more than about 0.1% by weight water.

The regeneration procedure can be conducted in either a batch or continuous manner. For example, in a typical batch regeneration procedure, spent molecular sieves to be regenerated are placed into a vessel and methanol or acetone preferably containing not more than 0.1% by weight water is brought into contact with the molecular sieves. The amount of methanol or acetone used generally ranges from about at least 2 to 10 milliliters for each gram of the molecular sieves undergoing regeneration. A large ratio of methanol or acetone to the molecular sieve, such as 10 or more to 1, milliliters per gram, is generally preferred to desorb the maximum amount of water. Occasional stirring or other agitation is desirable to improve regeneration efficiency. The methanol or acetone is permitted to remain in contact with the molecular sieves for periods of from about 0.1 to 24 hours. The extent of water desorbed from the molecular sieves increases with extended contact times but with decreasing efficiency. A twenty-four hour contact or equilibration period removes most of the water from the spent molecular sieves, but a four-hour equilibration period usually removes over 90% of the water removed in 24 hours. Thus, from a practical standpoint, a four-hour equilibration or contact period is generally sufficient. After contact with the methanol or acetone for a desired period, the molecular sieves are separated from the bulk of the methanol or acetone by filtration or centrifugation. It is not necessary to treat the regenerated molecular sieves further and they can then be used for dehydration operations, although in some applications it may be desirable to remove residual solvent by methods known to those skilled in the art.

In a typical continuous dehydration operation, including the regeneration step, molecular sieves are packed into a vessel equipped with a bottom distributor plate and inlet and outlet openings at the bottom and top, respectively. The material to be dehydrated is made to flow through the bed until the efficiency of water adsorption by the molecular sieve decreases below a predetermined or acceptable limit. At this point, excess material undergoing dehydration by the sieves is removed therefrom. The sieves are then regenerated or reactivated by slowly pumping or percolating methanol or acetone through the bed. The regeneration efficiency is a function of the flow rate of the methanol or acetone. A relatively slow flow rate increases the contact time of the methanol or acetone with the spent molecular sieves and affords greater moisture removal per unit volume of methanol or acetone. A flow rate which provides at least a 20 minute contact time between the methanol or acetone and the molecular sieves is generally preferred. The moisture exiting from the column containing the sieves can be monitored. The methanol and acetone regeneration operation is continued until the moisture level of the column effluent falls to a low value, such as 0.2% water (weight/volume). After the methanol or acetone remaining in the sieve column is drained therefrom, the molecular sieves are again ready to be used for dehydration purposes.

EXAMPLE I

This example demonstrates the regeneration of spent molecular sieves (Linde Air Products Co. 3A potassium cation) with methanol in a batch regeneration. The molecular sieves had been loaded with water in a humid environment such that the sieves gained 21.5% weight due to moisture adsorption. The hydrated sieves contained 17.7% moisture. Weighed amounts of the sieves were placed in glass-stoppered Erlenmeyer flasks and 100 milliliters of methanol that contained 0.024% water (weight/volume) were added to each flask. The flasks were shaken for up to 24 hours at 25.degree. C. Samples of the supernatant were withdrawn periodically for moisture analysis with the following results:

______________________________________ % of Sieve Water RemovedRatio Hours EquilibratedSieve:Methanol* 0.5 1.0 2.0 4.0 24______________________________________ 1:12.2 34 46 69 76 761:6.1 29 44 50 56 601:3.6 28 41 46 47 501:2.4 33 38 41 41 43______________________________________ *Gram (as received weight):Milliliter

EXAMPLE II

The procedures used in Example I were used to determine the effect of temperature on hydrated sieve batch regeneration with methanol. A 1:6.1, gram to milliliters, ratio of sieve to methanol was used at 16.degree. C., 25.degree. C. and 60.degree. C.

______________________________________ % of Sieve Water RemovedTemperature Hours Equilibrated.degree.C. 1 2 4______________________________________16 38 50 5425 44 50 5660 49 53 68______________________________________

Regeneration efficiency was improved at the elevated temperature. Conducting the regeneration operation at an elevated temperature, such as 60.degree. C., is preferable because of the increased efficiency.

EXAMPLE III

This example demonstrates the regeneration of sieves in a stationary column in a continuous manner. The experimental conditions were as follows:

______________________________________Column dimensions 0.78 centimeter diameter .times. 22 centimeters lengthTemperature 60.degree. C.Flow rate 0.4 milliliters per minuteSieve weight 7.3 grams hydrated (1.3 grams H.sub.2 O, 6.0 grams as received sieve)Methanol contained 0.025% H.sub.2 O weight/volume______________________________________ ______________________________________Methanol Volume, ml Water Removed From Hydrated SieveAliquot Accumulative % of Original % Accumulative______________________________________5 5 37 375 10 18 555 15 11 665 20 7 7310 30 8 8110 40 5 8610 50 3 8910 60 3 92______________________________________

EXAMPLE IV

This example shows the effect of temperature when molecular sieves were regenerated in a stationary bed in a continuous manner. The experimental conditions were as follows:

______________________________________Column dimensions 0.78 centimeter diameter .times. 33 centimeters lengthTemperature as indicatedFlow rate 0.6 milliliters per minuteSieve weight 12.3-12.4 grams hydrated (1.93 to 1.95 grams H.sub.2 O)Methanol contained 0.05% H.sub.2 O weight/volume______________________________________ ______________________________________Effluent Volume, ml Accumulative Water Removed, %Aliquot Accumulative 16.degree. C. 25.degree. C. 40.degree. C. 60.degree. C.______________________________________5 5 14 14 22 255 10 24 26 37 385 15 31 35 47 485 20 37 42 53 5510 30 46 52 64 6510 40 53 60 72 7210 50 59 67 79 7810 60 64 71 84 8110 70 68 75 87 8410 80 71 78 90 8710 90 74 82 93 89______________________________________

EXAMPLE V

This example shows the effect of flow rate of the regenerating fluid on efficiency of regeneration of the hydrated molecular sieves. The experimental conditions were as follows:

______________________________________Column dimensions 0.78 centimeter diameter .times. 33 centimeters lengthTemperature 60.degree. C.Flow rate As indicatedSieve weight 12.4-13.9 grams hydrated (1.95-2.30 grams H.sub.2 O, 10.4-10.7 grams as received sieve)Methanol Contained 0.02-0.05% H.sub.2 O weight/volume______________________________________ ______________________________________ Methanol Flow Rate, ml/minMethanol, ml 2 1 0.8 0.6 0.4Aliquot Accumulative Accumulative Water Removed, %______________________________________5 5 18 22 15 25 295 10 29 34 24 38 445 15 37 43 33 48 535 20 43 50 38 55 6110 30 54 60 48 65 7010 40 60 68 54 72 7710 50 65 73 60 78 8110 60 69 78 64 81 8410 70 73 81 67 84 8710 80 76 83 69 87 9010 90 79 85 71 8910 100 81 87 73______________________________________

EXAMPLE VI

The effect of column bed depth and diameter to length ratios on regeneration efficiency was tested. The experimental conditions for these tests were as indicated:

__________________________________________________________________________ % Water Removed TotalColumn Ratio Flow Water on By No. of Column Column % ofTemperature, Column Size, cm Diameter: Rate Sieve at Volumes Volumes Water.degree.C. I.D. Length Length ml/min Start, gms 1 2 Used Removed__________________________________________________________________________25 3.18 2.80 1:0.9 0.32 2.94 22 37 6.73 67.325 0.78 33 1:41 0.6 2.3 32 46 5.42 6960 0.78 22 1:28 0.4 1.28 56 74 6.71 92.660 0.78 33 1:41 0.4 2.3 56 72 4.81 9060 0.78 55 1:70 0.4 3.73 74 91 3.80 102.460 0.78 60 1:77 0.49 3.67 59 75 5.22 93.760 0.78 90 1:115 0.5 5.87 65 79 6.26 92.3__________________________________________________________________________

EXAMPLE VII

This example demonstrates the use of molecular sieves to dehydrate ethanol, the regeneration of the sieves and subsequent use of the regenerated sieves to again dehydrate ethanol.

In Test No. 1 ethanol containing 6.12% water (weight/volume) was passed through a bed (175 grams) of molecular sieve (Linde Air Products Co. 3A potassium cation).

In the first dehydration run (Test No. 1) the bed (175 grams) of molecular sieves adsorbed 30.05 grams of water from 1400 milliliters of ethanol. After 1400 milliliters of the aqueous ethanol had been passed through the bed, the molecular sieve bed was regenerated by contacting the bed with 1440 milliliters of methanol containing 0.02% water (weight/volume). The methanol was passed through the sieve bed at a rate of 9.9 milliliters per minute, providing a contact time of 14 minutes. In the regeneration operation 1300 milliliters of the methanol desorbed 21.6 grams of water from the sieves.

After regeneration, Test No. 2 was conducted in which ethanol containing 5.15% water, weight/volume, was passed through the bed of the regenerated sieves. In the second dehydration run (Test No. 2) the bed of sieves adsorbed 22.55 grams of water from 1400 milliliters of ethanol.

The values in the following table are the percentages of water in the ethanol fed to the sieve bed that were found in successive 100 milliliter aliquots of ethanol effluent.

______________________________________ Percent of Water Level in Ethanol Feed Appearing in Effluent AliquotEthanol Aliquot, ml Test 1 Test 2______________________________________100 0.2 14.0200 0.9 20.6300 11.1 35.9400 30.7 45.4500 51.1 56.3600 65.2 67.2700 76.5 77.3800 85.9 81.4900 91.5 87.81000 96.7 91.31100 98.9 94.01200 105.1 94.81300 102.5 97.51400 99.6 98.3______________________________________

Following the procedures of this example, a molecular sieve bed is employed to dehydrate ethanol, fusel oil streams, petroleum fractions, organic solvents, purified gases, natural gas, process air streams and the like.

EXAMPLE VIII

This example shows the effect of regenerating hydrated molecular sieves with methanol that is not anhydrous. The following conditions were used in this test:

______________________________________Column dimensions 0.78 centimeter diameter .times. 33 centimeters lengthTemperature 60.degree. C.Flow rate 0.4 milliliter per minuteSieve weight 13.0 grams hydrated (2.3 grams H.sub.2 O, 10.7 grams "as received" sieve)______________________________________ ______________________________________Regenerant% H.sub.2 O % H.sub.2 O in Sieve Removed inin Methanol 1st 20 ml 1st 50 ml 1st 60 ml______________________________________0.02 61 81 841.94 49 58 584.16 41 48 48______________________________________

EXAMPLE IX

A bed of molecular sieves (Linde Air Products Co. 4A sodium cation) which had adsorbed water was regenerated in a continuous manner. The conditions used in this example were as follows:

______________________________________Column dimensions 0.78 centimeter diameter .times. 33 centimeters lengthTemperature 60.degree. C.Flow rate 0.5 milliliters per minuteSieve Weight 12.0 grams hydrated (1.7 grams H.sub.2 O, 10.3 grams "as received" sieve)Methanol Contained 0.04% H.sub.2 O weight/volume______________________________________ ______________________________________Methanol, ml % of Water on Sieve RemovedAliquot Accumulative Accumulative______________________________________5 5 175 10 345 15 465 20 5510 30 6610 40 7510 50 8210 60 8610 70 9010 80 93______________________________________

EXAMPLE X

This example demonstrates the use of acetone to remove water from hydrated sieve in a batch operation. Hydrated sieve (type 13X, W. R. Grace Co., Davison Division) was prepared by storing 13X in a humid environment. The sieve took on water to the extent that the hydrated sieve contained 17.9% moisture by weight. This hydrated sieve was equilibrated with acetone containing 0.13% weight/volume water.

______________________________________ % of Sieve Water RemovedRatio Hours EquilibratedSieve 13X:Acetone* 0.5 1.0 2.0 3.0 4.0 72______________________________________1:12.2 24 30 27 33 29 261:6.1 15 21 20 21 22 21______________________________________ *gram "as received" sieve:milliliters.

EXAMPLE XI

This example demonstrates the removal of water from hydrated sieve using acetone in a column operation. Hydrated sieve (13X, W. R. Grace Co., Davison Division) was packed into a column. Water was removed using an upflow of acetone through the column. The conditions used in this example were as follows:

______________________________________Column dimensions 0.78 .times. 38 centimetersTemperature 50.degree. C.Flow Rate 0.56 milliliters per minuteSieve Weight 13.77 grams hydrated (2.46 grams H.sub.2 O, 11.31 grams "as received" sieve)Acetone contained 0.37% H.sub.2 O weight/volume______________________________________ ______________________________________Milliliter Acetone % of Water on Sieve RemovedAliquot Accumulative Accumulative______________________________________5 5 95 10 165 15 225 20 3110 30 3810 40 3910 50 4310 60 4610 70 4810 80 4910 90 50______________________________________

EXAMPLE XII

This example demonstrates the use of acetone to regenerate a hydrated column of sieve and the subsequent use of that regenerated column for dehydration of ethanol.

Test I refers to the regeneration-with-acetone cycle. Test II refers to the dehydration-of-ethanol cycle. ______________________________________TEST I CONDITIONS______________________________________Column dimensions 0.78 .times. 38 centimetersSieve 13X, hydrated to contain 17.9% waterSieve Weight 13.48 grams hydrated 13X (2.41 grams H.sub.2 O, 11.07 grams "as received" sieve)Temperature 50.degree. C.Acetone Flow Rate 0.37 milliliters per minuteAcetone contained 0.44% H.sub.2 O weight/volume______________________________________ ______________________________________TEST I RESULTSMilliliter Acetone % of Water on Sieve RemovedAliquot Accumulative Accumulative______________________________________5 5 135 10 365 15 445 20 5010 30 6410 40 7010 50 7310 60 7410 70 7510 80 7710 90 78______________________________________ The column was air-purged at the end of Test I. ______________________________________TEST II CONDITIONS______________________________________Temperature 50.degree. C.Ethanol Flow Rate 0.4 milliliters per minuteEthanol contained 6.14% H.sub.2 O weight/volume______________________________________ ______________________________________TEST II RESULTS % of Water inMilliliter Ethanol Feed Appearing in EffluentAliquot Accumulative Accumulative______________________________________5 5 335 10 355 15 405 20 4710 30 6110 40 7010 50 76______________________________________

The advantage of the present invention include, among others: (1) the adsorptive capacity of molecular sieves can be restored to high levels in a convenient manner; (2) conventional apparatus can be used for regenerating the sieves in accordance with the invention; and (3) no preliminary treatment of the molecular sieves is required before processing.

Those modifications and equivalents which fall within the spirit of the invention are to be considered a part thereof.

Claims

1. A process for regenerating crystalline zeolitic molecular sieves containing adsorbed water which comprises contacting the molecular sieves with methanol or acetone.

2. A process in accordance with claim 1 wherein the methanol or acetone is substantially anhydrous methanol or substantially anhydrous acetone.

3. A process in accordance with claim 1 wherein contact of the molecular sieves with methanol or acetone is carried out at an elevated temperature.

4. A process for removing water from water-containing ethanol which comprises passing the water-containing ethanol through a bed of molecular sieves until the water adsorbing capacity of the molecular sieve bed falls to a predetermined level, then passing methanol or acetone through said molecular sieve bed to restore the water adsorbing capacity, and then again passing water-containing ethanol through said molecular sieve bed.

5. A process according to claim 4 wherein the methanol or acetone employed is substantially anhydrous methanol or substantially anhydrous acetone.