CA1267424A - Membrane process for separating methanol from methanol/hydrocarbon solutions - Google Patents

Abstract

Abstract A reverse osmosis process is described for removing alcohols from hydrocarbons, in the additional presence of ethers. Depending on the nature of the membrane used, the methanol can be selectively removed as the membrane permeate or retained as the membrane concentrate. The membrane may be made from cellulose esters, polyethylene, polyvinylchloride, polyvinylidene chloride-polyvinyl chloride, etc.

Description

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Membrane process ~or separating methanol from methanol/hydrocarbon solutions This invention relates to a separation process for removing alcohols from hydrocarbons, particularly in the presence of ethers, utilizing a reverse osmosis membrane.
pistillation is a common method for separating com-ponents in solution ~ased on their vapour pressure. In cases where the components have similar vapour pressures, other separation methods such as extraction and absorption must be used. These alternative methods require large capital investments and large operating costs. One such method is the separation of alcohols from hydrocarbons where azeotropes are commonly encountered. For example, the manufacture of ethers for b~ending with gasoline as an octane enhancing agent requires the separation o~
various amounts of methanol from the final product ta protect the catalyst used in subsequent eactions, to meet fuel quality specifications for vapour pressure, corrosion and miscibility, and for enhanced manufacture 20 by recycling unreacted components. Because methanol and saturated hydrocarbons form azeotropes that cannot be resolved by distillation, other methods such as liquid-liquid extraction, gas-liquid absorption and liquid-solid adsorption on resins are being used to remove 25 the alcohol~ An operational difficulty of these two separation processes is th~ir apparent ~ailure to selec tively separate methanol from solutions that contain hX~:
. i , .:, : - ' other polar solvents such as ethers when in the presence of hydrocarbons. This presents an additional burden to the separation process by requiring a preliminary separation of the ethers from the hydrocarbons, usually by distillation despite the methanol azeotrope, followed by the independent treatment of the hydrocarbon-methanol and ether-methanol mixtures.
Reverse osmosis is a widely used technique for separating components which are difficult to separate by techniques such as distillation. Osmosis occurs when two solutions of different concentrations in the same solvent are separated from one another by a membrane. If the mem-brane is ideally semi-permeable, that ;s, if it is permeable to the solvent and not to the solute, then a flow of solvent occurs from the more dilute into the more concentrated solution. This continues until the two solu-tions become equal in concentration or until the pressurein the chamber of the more concentrated solution rises to a certain well-defined value. The pressure difference at which no flow occurs is termed the osmotic pressure difEerence between the two solutions. If a pressure in excess of this osmotic pressure difference is applied to the more concentrated solution, then the solvent can be caused to flow into the dilute solution. The name "reverse osmosis" is used to describe this process. A
typical reverse osmosis system is described in U.S.
Patent 3,853,756.
Summary of the Invention This invention utilizes the reverse osmosis tech-nique for the selective removal of methanol frommethanol/hydrocarbon solutions, particularly in the presence of other polar solvents, such as ethers and aromatic hydrocarbons. Depending on the nature of the membrane used, it has been found that the methanol can be selectively removed as the membrane permeate or it may be retained as the membrane retentate.

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In the process of the invention, methanol is separated from a non-aqueous miscible solution of methanol, hydrocarbons and ethers by filtering the solution through a semi-permeable membrane having a Eeed side and a permeate side, with a higher pressure on the Eeed side o the membrane than on the permeate side. The materials permeating ~che membrane and the materials retained by the membrane on the feed side are then collected.
The reverse osmosis technique is, oE course, based upon the relative affinity o~ the components in solution for the reverse osmosis membrane surface and on their molecular size and shape. The affinity is determined by the interaction of the chemical properties o the components and the functional groups presented by the membrane surface. Because of the diverse nature of the components, a suitable membrane can be chosen that will selectively permeate the polar com~onents or the less polar components. Both cases have two product streams;
one rich in methanol and the other lean in methanol.
The methanol-rich stream can be recycled to the ether-ification reactor. The methanol lean stream can be distilled to separate the hydrocarbons and the ethers, provided the methanol content has been reduced to below the process specification limits. If ~he level remains too high, it can be passed through several membrane sepa-rations as required to make the necessary separations.
Methanol-rich straams can be recycled to the etherifica-tion reactor or to subsequent operations. The ultimate process design depends on product methanol content specifications, downstream process specifications, membrane type, and the composition and concentration of the original feed solution.
The reverse osmosis membrane is usually made from such materials as cellulose or cellulose esters such as cellulose acetate, cellulose acetate-butyrate r 74~

polye~hylene, polyvinylchloride, polyvinylidene chloride-polyvinyl chloride, etc.
Brief Description oE the Drawings For a better understanding of the invention, re~erence S may be made to the preferred embodiments exemplary of the invention, shown in the accompaying drawings, in which:
Figure l is a schematic diagram of one embodiment of the method of the invention with methanol in the permeate;
and Fiyure 2 is a schematic diagram of a urther embodi-ment of the method of this invention showing the methanol in the retentate.
Description of the Preferred Embodiments As will be seen from the drawings, a feedstock 10 typically comprising methanol, olefins and hydrocarbons is fed into an etherification rea~or ll to form ethers for blending with gasoline. The product stream 12 from the reactor ll comprises methanol, olefins, hydrocarbons and the desired ether. This product stream is fed into a reverse osmosis unit containing a selected membrane and from reverse osmosis unit 13 is obtained a retentate stream 14 and a permeate stream 15.
Example l A system of the type shown in Figure 2 was used containing a small pore polyvinyl chloride membrane.
This was used as a static reverse osmosis cell without agitation and a nominal l~ methanol in pentane solution was passed through the reverse osmosis cell. This was done at room temperature and nitrogen pressures from 1 to 6.5 MPa. The permeate and liquid on the high pressure side of the membrane (retentate~ in the static cell were collected and analyzed for methanol concentration. The permeation rate of the membrane was also measured. The results obtained are shown in Table 1 below:

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Table 1 Nominal Press~e Permeation Se~ration*
oonc. MPa ~at~ ~actor Membrane ~ g.h m x103 f~ methanol 1 1.0 1.0 19.95 0.56

2 1.0 1.5 25.~1 0.91

3 1.0 2.0 33.19 0.8g

4 5.0 1.0 11.23 0.71 1.0 4.0 1.98 0.54 6 1.0 5.0 1.27 0.36 7 1.0 5.0 1.13 0.54 8 l.G 5.0 2.33 0.78 9 1.0 6.5 11.24 0.77 *m~l. fraction methanol in permeate/total mols in Fermeate mol. fraction methanol in ~eeq/total mols in feed It will be seen from the above table that the permeate was depleted in methanol by the membrane in all tests.
Example_2 A cellulose acetate-butyrate (CAs) membrane was pre-pared from a casting solution containin~ 10 parts CAB, 20 parts acetone, 8 parts formamide and 4 parts maleic acid.
The film was cast at -10C, evaporated ~or 30 seconds, subjected to gelation in water and preshrunk in hot water at 90~C for 3 minutes.
The cellulose acetate-butyrate membrane was used in the system of Figure I to separate a nominal 1% methanol in pentane soluble. Pressures were varied from 5 to 10 MPa at ambient temperature in a static cell. The per-meate and the retentate were collected and analyzed and the permeation rate was measured. Results for several o~
these membranes are shown in Table 2 below and it will be noted that this membrane selectively permeated methanol~

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Table 2 Permeation Separation Pressure rate factor Membrane MPa g.h~l.m~2x]03 for methanol 1 5.0 1.59 1.37 2 5.0 0.19 1~17 3 6.0 2038 1.27 4 7.0 - 1.27 7.0 2.31 1033 6 8.0 3.94 1.49 7 8.0 1.31 1.67 8 8.0 1.30 1.43 9 9.0 2.80 1.48 10.0 1.67 1.42 Example 3 Thin commercial packaging films of various materials were used to separate a nominal 1~ methanol in pentane solution. The films had thicknesses in the range of about 0.01 to 0.02 mm. These membranes were used in the same type of device as shown in Figures 1 and 2 and the ope-rating conditions and results obtained are shown in Table 3 below.

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Table 3 -_ A _ _ W~iqh~ X
Permeatlon Reten~ate PermeaLe Pressure rate methanol/ meth~nol/ Sepnra~ion fac~or Membrane ~Pa ~kq h-1m-2) pentane pen~ane for ~eth~nol . ___~

Glad Wrap 6.û 1.37 0.27/99.73 0.28~99.72 1.04 Stretch & 5.S ., 0.28/99. 72 û.27/99,73 û.96 Saran Wra~ 8.0 44.26 0.34/99.66 0.11/99.~9 û.32 Seal 8.0 6.56 l 0.26/99.74 0.2 /99.73 1.02 Glad Wrap is a trademark of Union Carbide for film containing polyethylene and polyvinyl acetate.
Stretch & Seal is a trademark of Imperial Oil for film containing polyvinyl chloride and chlorinated polyethylene.
Saran Wrap is a trademark of Dow Chemical for film containing polyvinylidene chloride and polyvinyl chloride.

* not measured Example 4 For this test, an artifical etherification reactor leffluent stream was prepared. The feed solution, which was placed on the high pressure side of the membrane, con-sisted of 11% methanol, 15~ 2-methyl-2-butene, 67% pentane and 7% ether. The ether was either methyl tertiary butyl ether (MTBE) or tertiary amyl methyl ether (TAME). Pres-sures were varied from 4 to 10 MPa at ambient temperature.
Table 4 shows the separation factors obtained and it will be seen that both selective permeation and methyl rejec-tion can be obtained. Four different membranes were used, these being polyethylene (PE), polyethylene and polyvinyl acetate (PE + PVAC), cellulose acetate (CA) and cellulose ~7~

acetate-butyrate (CAB). In the case of cellulose acetate-butyrate, almost no change occurs in the ether con~ent of the feed compared with the permeate.

Table 4 _ Averaqe sëparation factor for Ether No. of Methanol/ether/
Membrane in mixLure experimen~s Pentane/2-M-2-buten~

PE MT~E 2 0.41/0.~4/1.56/1.19 rA~E 2 o.a1/o.9n/l .14/1.04 PE+PVAC MT~E 8 0.32/0.83/1.91/1.29 TAME 4 0.41/0.~3/l.79/1.21 CA MTBE 3 9.54/0.63/0.~8/0.58 TAME 6 11.27/0.~6/0.32/0.55 CA~ MTBE 5 2.~7/l.03/0.64/D.83 TAM1 ~ 2.33/0.94/0.66/0.8B

Example 5 This test was conducted in a circulated reverse osmo-sis test cell with a variety of different membranesl all membranes being tested with the same solution and for the same time period. The operating conditions and results obtained are shown in Table 5. Results are shown both for the case of methanol and hydrocarbons with only a trace of TAME and where TAME is present in quantities typical of a commercial etherification reactor product.

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Claims (5)

Claims:

1. A process for separating methanol from a non-aqueous miscible solution of methanol, hydrocarbons and ethers which comprises filtering the solution through a semi-permeable membrane having a feed side and a permeate side with a higher pressure on the feed side of the membrane than on the permeate side and collecting the materials permeating the membrane and the materials retained by the membrane.

2. A process according to claim 1 wherein a membrane is used through which the methanol passes.

3. A process according to claim 1 wherein a membrane is used through which the methanol substantially does not pass.

4. A process according to claim 2 wherein the mem-brane is made of cellulose acetate or cellulose acetate-butyrate.

5. A process according to claim 3 wherein the membrane is made of polyethylene, polyvinylchloride or polyvinylidene chloride-polyvinyl chloride.