GB2067591A - Low sodium molecular sieve catalyst for removing olefins from paraffinic hydrocarbons - Google Patents

Abstract

A low sodium molecular sieve is used to remove odour-forming olefinic material from a paraffinic hydrocarbon e.g., propane, isobutane, pentane. The hydrocarbon may be dried by passage through a high sodium molecular sieve.

Description

SPECIFICATION Low sodium molecular sieve catalyst for removing olefins from paraffinic hydrocarbons This invention relates to the removal of an olefin from a paraffinic hydrocarbon with which it is an admixture. In one of its aspects the invention relates to the removal of an olefin from a paraffinic hydrocarbon feedstock to render the same useful as a propellant. In another of its aspects the invention relates to a combination operation in which the feedstock is pre-dried prior to the olefin removal.

In one of its concepts, the invention provides a process for the removal of an olefin from its admixture with a paraffinic hydrocarbon by bringing the same into contact under olefin-removal conditions, with a low sodium content molecular sieve. Another of its concepts, the invention provides a method as herein described wherein a paraffinic feedstock containing a small amount of olefin, which can cause odor, is percolated through a solid catalyst bed comprising a molecular sieve having a low sodium content. Further in a concept of the invention, the paraffinic feedstock is pre-dried by bringing the same into contact under drying conditions with a high sodium molecular sieve.

One of the most popular methods heretofore used for removing olefins from paraffinic hydrocarbon fractions has been the treatment of the olefin-containing paraffinic hydrocarbon fractions with strong sulfuric acid. Although effective, the method has waste disposal problems as well as process problems such as pumping and corrosion.

Catalytic removal of olefins by passage over a solid bed is another effective method of removing olefins but this method also has disadvantages. U.S. Patent 3,445,542 is such a catalytic method and employs two types of molecular sieves in treating C4 to C7 isomerizable hydrocarbons; 4A molecular sieves to remove catalyst poisons such as hydrogen sulfide, water, and ammonia and 1 3X molecular sieve to remove unsaturated hydrocarbons. This method produces gaseous products requiring subsequent separations.

U.S. Patent 3,629,356 describes a process wherein ethylene and propylene are absorbed from a gas stream onto molecular sieves and desorbed with hot isobutane. Apparently olefins dissolved in isobutane and passed over the same molecular sieve absorbent would not be separated or absorbed by the molecular sieve as in the case of the current invention. U.S.

Patent 3,078,321 describes the use of molecular sieves to separate straight-chain paraffins and olefins from branched-chain paraffins and olefins. Thus, some olefins are absorbed and some are not.

The current invention provides improvements over processes of the art employing molecular sieves by providing a process which, in one embodiment, is continuous wherein olefins are converted to non-odorous saturated hydrocarbon derivatives that can be easily separated by distillation or can if desired be left in the product effluent, thus, eliminating any subsequent disposal problems of absorbed olefins.

According to the present invention there is provided a process for the removal of an olefin from a paraffin hydrocarbon containing the same which comprises contacting the mixture with a low sodium molecular sieve.

The invention finds particular applicability in provision of aerosol propellants e.g., low boiling paraffinic hydrocarbons, such as those which are replacing fluorocarbons for example propane, isobutane, and pentane.

Small amounts of impurities e.g., olefinic material, impart undesirable odors and must be substantially completely removed. Yet, these materials are present in such small quantity that their effective removal can be quite difficult.

Olefin impurities which are present, as stated, in amounts as low as 10-50 parts per million (0.001-0.005 wt. %) are known to cause odor when used as propellant.

Hydrocarbon feedstocks being purified by the process of this invention can be any paraffinic hydrocarbon generally containing less than 20 wt. %, preferably less than 5 wt. % of an olefin, indeed very small amounts of an olefin, or other odor-forming impurity. Of specific importance are paraffinic hydrocarbon feedstocks potentially useful as aerosols such as propane, 2 methyl propane (referred to herein as isobutane), n-butane, and other pentanes and mixtures of the foregoing. Most preferred hydrocarbons are those considered to be readily alkylated such as isobutane. Olefins being removed from these hydrocarbon feedstocks include, but are not limited to, such compounds as propylene, isobutylene, n-butenes (including cis and trans isomers), pentenes, and the like and mixtures thereof.Other impurities like sulfides, mercaptans and other such sulfur-based compounds or oxygenated compounds that contribute to odor are believed to be removed by the catalyst disclosed herein.

Other applications requiring an essentially olefin-free product such as polymerization solvents are also within the scope of this invention.

The catalyst used in this invention for the removal of olefins, which is believed to be by alkylation, is a low sodium molecular sieve. One such material which can be employed in this invention is LZ-Y82 available from Union Carbide. The chemical composition and some physical properties of LZ-Y82 are listed below.

LZ-Y82, Low Sodium Molecular Sieve Typical Chemical Composition Wt. % SiO2 65.6 Al203 33.6 Na2O 0.15 Fe203 0.18 CaO 0.03 Typical Physical Properties Bulk Density 39.5 pounds/cubic foot (632 kg/m3) Surface Area, B.E.T. 625 m2/g Unit Cell Size 24.45 angstroms Any similar type molecular sieve can be used in this invention providing the alkali metal oxide content, particularly sodium and potassium oxide, is between 0.01 to to 5.0 wt. %, preferably between 0.05 to 1.5 wt. %. Such type molecular sieve is to be distinguished from a molecular sieve such as 1 3X which has a 14.5 wt. % sodium, as later more fully described.

Pre-drying the feedstock before the olefin removal step is not absolutely essential in the current invention. However, as the data herein discloses, water can be detrimental to the catalyst lift and efficiency and for this reason pre-drying the hydrocarbon feedstock is recommended. Any recognized drying agent is, thus, within the scope of this invention.

Preferred drying agents are those solid materials that are adaptable to a continuous flow-through type operation, namely, those materials that are pelletized, granular or powder having a particle size greater than about 10 microns. The preferred drying agent is 1 3X molecular sieve, an alkali metal alumino-silicate having the chemical formula Na6[(Al02)86(Si02)106xH2O.

The catalyst used for olefin removal can be regenerated or activated by any suitable method.

One such method is to pass air or nitrogen or a mixture of air/nitrogen through the catalyst bed at elevated temperature (e.g. 343"C). Another method is to pass the treated liquid (e.g.

isobutane) counter-current to the flow used in the olefin removal step at ambient room temperature at a rate of about 0.6 LHSV (herein defined) and about 10 psig (0.069 MPa).

Any type equipment can be used to carry out the current invention, however, a vertical tubular reactor is now preferred. The hydrocarbon feedstock flow can be either downward or upward, or otherwise, as desired. Upflow is now preferred. The paraffinic hydrocarbon feed rate will depend in part on the type of paraffinic hydrocarbon being treated as well as the amount of odor-forming impurities such as olefins being removed. The acceptable olefin content in a final paraffinic hydrocarbon material depends on the particular application in which the paraffinic hydrocarbon is employed. For aerosol propellants requiring low odor, a level of less than 10 ppm (0.001 wt. %) is considered satisfactory. For most other applications requiring low odor, less than 50 ppm (0.005 wt. %) is generally satisfactory.

A typical feed rate is about 10 liquid volumes of hydrocarbon feedstock per volume of catalyst bed used per hour. This is expressed as 10 LHSV or 10 Liquid Hourly Space Velocity. The same feed rate can also be expressed as 7.72 WHSV or 7.72 Weight Hourly Spaced Velocity which is 7.72 parts by weight of paraffinic hydrocarbon feedstock per 1 part by weight of catalyst bed used per hour. Expressing the same rate in the vapor phase, a value of 2460 GHSV (Gas Hourly Space Velocity) would be obtained. The following flow rates are considered with the scope of this invention.

0.5- 200 LHSV 0.3- 150 WHSV 100 -50,000 GHSV The process described herein can be operated preferably at ambient room temperature but it also can be conducted at elevated temperatures if desired. However, temperatures above about 150to are not recommended.

The process described herein can be carried out in either liquid or vapor phase but liquid phase is preferred. Thus, to conduct the process at liquid phase it may be necessary to operate under a slight pressure particularly with the lower boiling paraffinic hydrocarbons. For example, when propane is being treated a pressure of 130-150 psig (0.896-.034 MPa) may be necessary to maintain a liquid phase. When isobutane is the feedstock, a pressure of about 40-60 psig (0.27-0.41 MPa) may be necessary to maintain liquid phase.

Example I This example is an inventive run illustrating the effectiveness of a low sodium molecular sieve type catalyst in removing olefins from a paraffinic hydrocarbon feedstock. The example also illustrates the ease of regenerating this catalyst and of the effectiveness after regeneration in continuing to remove olefins.

Into a 63.5 cm. (25 inch) X 1.27 cm. (0.5 inch) stainless steel tube was place 52 grams (72 milliliters) of freshly activated 1 3X molecular sieves used for pre-drying of the hydrocarbon feedstock material. Into another similar stainless steel tube connected to the drying tube was charged 35 grams (50 milliliters) of a low sodium Linde Molecular Sieve (LZ-Y82 from Union Carbide) and the tube heated for 4 hours at 121 C (250"F) and then 5.5 hours at 427"C ((800"F) while a slow stream of nitrogen passed through the drying tube and the activated molecular sieve heated tube.After cooling to ambient room temperature, a paraffinic hydrocarbon feedstock containing Wt. % ethane 0.03 propane 0.490 isobutane 96.950 n-butane 2.450 isobutylene 0.0205 trans 2-butene 0.0024 cis 2-butene < 0.001 Total olefins 0.0229 wt. % (229 ppm) was passed upwardly first through the 1 3X molecular sieve drying tube and then upwardly through the tube containing the low sodium molecular sieves at a rate of about 5 LHSV using a column pressure of 50 psig (0.344 MPa). Analysis of the paraffinic hydrocarbon before and after treatment was determined by GLC using a 914.4 cm. (30 ft.) x 0.635 cm. (0.25 in.) column packed with 1 9 wt. % bis[2-(2-methylethoxy)ethyl]ether plus 1 wt. % squalene on 45-60 mesh Chromosorb P-NAW and operated at ambient room temperature using a 30cc/min. helium flow.Analyses were periodically made to determine the olefin content. A level of 10 ppm (0.001 wt. %) olefin was considered to be the maximum acceptable level for an odorless paraffinic hydrocarbon. Initially no olefin was detected in the treated paraffinic hydrocarbon. Eventually a trace of olefin began to appear but the level was so low that the exact amount could not be determined. The run was continued until about 10 ppm of olefin was observed in the treated effluent. The volume of feed per volume of catalyst at this point was 10,172.4 calculated by dividing the total feed by the density at 20"C (0.563), then dividing this total volume by the catalyst volume, 50. During the run the 1 3X molecular sieves used for drying was changed about every 500 to 1000 volumes of feed/catalyst bed volume.The extended length of the run necessitated using different feedstock lots. However, each lot represented the same paraffinic hydrocarbon feedstock and only varied slightly (+ 25 ppm) in olefin content.

The catalyst was regenerated by passing a slow stream of treated paraffinic hydrocarbon feedstock in a reverse flow (downwardly) through the catalyst bed at 340"C for 1 8 hours. After this regeneration, the column was cooled and the paraffinic hydrocarbon feedstock passage resumed at ambient room temperature. The pressure of the column was increased to 1 80 psig (1.241 MPa) at about 10 LHSV. After a feed volume/catalyst volume reached an additional 6843.2 (17,015.6 total), the colmn was again regenerated as herein described. The feedstock passage was again resumed till the feed volume/catalyst volume reached 2023.6 (19,039.2 total). The run was terminated at this point.

A typical analysis of treated paraffinic hydrocarbon, taken when the feed volume/catalyst volume was 1043 is shown in Table I.

Table I. Typical Analysis of Treated Hydrocarbon Wt. % ethane 0.0002 propane 0.170 isobutane 98 50a n-butane 1.52 isobutylene < 0.0002 trans 2-butene < 0.0002 cis 2-butene < 0.0002 C6+ heavies 0.029 a. Depends on feedstock. Isobutane content can vary from 95-99 wt. %.

An aliquot of treated paraffinic hydrocarbon was subjected to fractional distillation to remove the lower boiling materials from the C6+ heavies. The heavies were then analyzed by GLC and found to be saturated hydrocarbons indicating that the olefin impurities originally present in the isobutane feedstock are removed by the low sodium molecular sieve catalyst via an alkylation process probably between the olefin and the isobutane. Analysis of the heavies is shown in Table II Table 11. Analysis of C6 + Heavies Wt. % hexanes 2.70 heptanes 57.74 octanes 24.79 nonanes 1.16 decanes 4.94 undecanes 8.37 dodecanes 0.30 olefins < 1.00 Example II This example is an inventive run wherein the feedstock contains more olefin initially than in Example I.The procedure described in Example I was repeated using an isobutane (98.3 wt. %) feedstock containing 840 ppm (0.0840 wt. %) olefin. No olefin was present in the treated effluent after a volume of feed/catalyst bed volume value of 2,500.

Example 111 This runs shows the inability of a high sodium molecular sieve effectively to remove olefins from a hydrocarbon feedstock. A typical isobutane feedstock was passed through a column of 1 3X molecular sieve at ambient room temperature and at less than 100 psig (0.689 MPa) pressure. 1 3X Molecular sieve is an alkali metal aluminosilicate noted for its ability to remove water and carbon dioxide. It has a calculated 1 4.5 wt. % sodium with an assigned chemical formula of Na86[(AlO2)86(SiO2)106].xH2O. The results shown in Table III indicate the ineffectiveness of 13X molecular sieves in removing olefins.

Table 111. Treatment of an Olefin-Containing Isobutane Feedstock With 1 3X Molecular Sieve Wt. % by GLC Component Before Treatment After Treatment ethane < 0.001 < 0.001 ethylene < 0.001 < 0.001 carbon dioxide 0.008 < 0.001 propane 0.173 0.122 propylene 0.0002 < 0.001 isobutane 96.560 96.336 n-butane 3.229 3.537 isobutylene 0.026 O.023 1 -butene trans2-butene 0.002 0.004 cis 2-butene < 0.001 < 0.002 isopentane 0.002 0.002 Total sulfur 0.0001 0.00001 Total olefin 0.0280 0.0270 Example IV This example is an inventive run employing the low sodium molecular sieve LZ-Y82 and a hydrocarbon feedstock containing an exceptionally high level of olefin. In addition, the hydrocarbon feedstock contains mostly n-butane rather than isobutane as in the case of Examples I, II, and III.A hydrocarbon feedstock containing 93.2 wt. % n-butane, 5.82 wt. % isobutane and 0.21 wt. % (2090 ppm) olefins was treated in the same manner as described in Example I. The volume of feed/catalyst bed volume was merely 200 indicating that although the low sodium molecular sieve removes olefins the best results are obtained with isobutane feedstocks with < 0.1 wt. % olefins than with n-butane feedstocks with > 0.1 wt. % olefins. It is believed that the olefin removal mechanism is via an alkylation process between isobutane and the olefins (e.g. isobutylene) aided with the low sodium molecular sieve catalyst. Therefore, the process of the invention would be less efficient when treating feedstocks low in isobutane content.

This example was repeated but using a feedstock containing more isobutane (40.6 wt. %) and more olefin (0.62 wt. %-6200 ppm). The volume of feed/catalyst bed volume was an even lower value, namely 50.

Example V This is a control run employing the high n-butane (93.2 wt. %) feedstock material of Example IV and a higher sodium content molecular sieves. The procedure described in Example IV was repeated using a 93.2 wt. % n-butane, 5.82 wt. % isobutane feedstock and a molecular sieve LX-Y62 (Union Carbide) having 2.3 wt. % No20. The volume of feed/catalyst bed volume was 80. When an even high sodium content molecular sieve (LZ-Y52, 10.4 wt. % Na2O) was employed the volume of feed/catalyst bed volume was zero. These results plus those in Example IV using the same feed are listed in Table IV and show that low sodium content molecular sieves favor olefin removal.

Table IV. Effect of Sodium Content of Molecular Sieve on Olefin Removal(Hydrocarbon Feedstock: 5.82 wt. % isobutane/93.2 wt. % n-butane) Initial Catalyst %Na20 Olefin, ppm Vol. Feed/Cat. Vol.

LZ-Y82 0.15 2090 200 LZ-Y62 2.3 2090 80 LZ-Y52 10.4 2090 0 Example VI This is a control run using an acid clay as the catalyst. The procedure described in Example I was essentially repeated except without catalyst regenerating using an isobutane feedstock having 0.0169 wt. % (169 ppm) isobutylene and an acid-treated montomorillonite sub bentonite clay catalyst having an acidity value of 12-20 mg KOH/gram sample (FiltrolR 24 Filtrol Corporation). Montmorillonite sub-bentonite clays have the idealized formula Al 2Si40,0(0H2) nH20. The actual mineral, however, has every sixth aluminum ion replaced by a magnesium ion.A typical analysis shows about 1 wt. %K20 and Na2O. The isobutane feedstock was treated until the olefin content of the effluent increased above 0.001 wt. % (10 ppm). The volume of feed/catalyst bed volume at this point was 534. This value is significantly less than those obtained in the inventive runs in Examples I and il.

Example Vll This example is another control run using the acid clay catalyst Filtrol 24 but without the 1 3X molecular sieve drying step preceeding the olefin removal step. Although not absolutely essential, the run does demonstrate the slight advantage of using a molecular sieve drying step, namely an increased volume of feed/catalyst bed volume value. The procedure described in Example VI was repeated except the molecular sieve drying step was omitted. The volume of feed/catalyst bed volume value was 322.

Example VIII This example is a control run illustrating the effect of water on the volume of feed/catalyst bed volume value and hence the need for a pre-drying step. The procedure described in Example VII was repeated except 200ppm (0.020 wt. %) water was intentionally added to the isobutane feedstock. The volume of feed/catalyst bed volume was found to be 140.

Summary The data herein disclosed is summarized in Table V wherein it is shown that low sodium content molecular sieves, particularly those with less than 1.0 wt. % Na2O (e.g. Examples I and II) are very effective in removing olefins from hydrocarbon feeds, especially those containing mostly isobutane (compare examples I and II with Example IV). The data also shows that molecular sieves high in sodium content (e.g. > 5.0 wt. %), Examples ill and V, are not effective in removing olefins regardless of the amount of isobutane present. The data shows that other type acid clay catalysts Table I. Summary of Olefin Removal Data Catalyst Feedstock Example Feed Volume/ No.Type % Na2O iso-C4, Wt. % n-C4, Wt. % Olefin, ppm Catalyst Volume I Molecular Sieve, LZ-Y82 0.15 96.5 2.45 230 10,172 (6843)(2023+) II Molecular Sieve, LZ-Y82 0.15 98.3 0.27 840 2,500+ III Molecular Sieve, 13X() 14.5 96.3 3.5 282 0 IVa Molecular Sieve, LZ-Y82 0.15 5.82 93.24 2090 200 IVb Molecular Sieve, LZ-Y82 0.15 40.60 48.00 6200 50 Va Molecular Sieve, LZ-Y62 2.3 5.82 93.24 2090 80 Vb Molecular Sieve, LZ-Y52 104 5.82 93.24 2090 0 VI Montmorillonite Clay, Filtrol 24 plus 13X molecular sieve < 1.0 98.5 1.36 169 534 VII Montmorillonite Clay, Filtrol 24 < 1.0 98.5 1.36 169 322 VIII Montmorillonite Clay, Filtrol 24 plus 200 ppm H2O in feed < 1.0 96.5 2.45 230 140 (1) Calculated as 14.5 wt. % sodium metal.

(2) After catalyst regeneration.

(3) Catalyst had no effect on olefin removal.

Example VI, are not as effective in removing olefins from an isobutane feedstock as the inventive catalyst, Example I. Although not absolutely essential, it is helpful to remove water from the feedstock before olefin removal since water is detrimental to the process, Example Villi. High sodium 1 3X molecular sieves are useful in removing water, Examples VI and VIII. Finally, olefins are believed to be removed from the hydrocarbon feedstock by low sodium molecular sieves via an alkylation mechanism. For this reason, olefins are more easily removed from feedstocks high in isobutane content since isobutane is readily alkylated with olefins to give a saturated derivative whereas n-butane is not readily alkylated and thus olefins are more difficult to remove.

Claims (9)

1. A process for the removal of odour-forming olefinic material from admixture with a paraffin hydrocarbon which comprises contacting the mixture with a low sodium content molecular sieve.

2. A process according to claim 1, wherein the alkali metal oxide content of the sieve is in the range 0.01 to 5 wt %.

3. A process according to claim 1, wherein the alkali metal oxide content of the sieve is in the range 0.05 to 1.5 wt %.

4. A process according to any one of claims 1-3 as applied to paraffinic hydrocarbons containing as a contaminant as olefinic material having a boiling point close to that of the paraffinic hydrocarbon.

5. A process according to any one of claims 1-4, wherein the hydrocarbon comprises propane, isobutane or pentane.

6. A process according to any one of claims 1-4, wherein the mixture contains at least one of ethane, propane, isobutane and n-butane, and at least one of isobutylene, trans 2-butene, and cis 2-butene.

7. A process according to one of the preceding claims, wherein the mixture containing the paraffinic hydrocarbon and the olefin is pre-dried using a high sodium content molecular sieve prior to contact with the low sodium content molecular sieve.

8. A process according to one of claims 1 to 7, wherein the olefinic material is present in an amount of from 10 to 50 ppm, based on the total mixture.

9. A process according to one of claims 1 to 8, wherein said process is conducted in liquid phase.