Preparing lower alkenes from methanol
FIELD OF THE INVENTION 
     Modified zeolite is used as catalyst in methanol dehydration to produce low molecular weight alkenes. 
     BACKGROUND 
     Methanol has been converted to olefins and aromatics by catalytic dehydration of methanol, using ZSM-5 acidic zeolite. The obtained product has a reported distribution as follows: 
     
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Light Gas (C.sub.1, C.sub.2)                                              
                     1.3                                                  
LPG (C.sub.3, C.sub.4)                                                    
                    17.8                                                  
Gasoline (C.sub.5 -C.sub.12)                                              
                    80.9                                                  
Aromatics (% of gasoline)                                                 
                    38.6                                                  
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     SUMMARY OF THE INVENTION 
     A high yield of ethylene and propylene are obtained by catalytically dehydrating methanol in admixture with steam or an inert gas (which does not interfere with the reaction, e.g. nitrogen and methane) without forming higher alkenes or aromatics. The catalyst is a type-A, type-X or a type-Y molecular sieve zeolite in the aluminum and hydrogen form. 
     An object of this invention is to produce ethylene and propylene in high yield from methanol. A further object is to provide a non-polluting process for such production. A still further object is to devise a process wherein the proportion of ethylene to propylene produced is readily controlled. Another object is to produce ethylene and propylene economically. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The sole figure of drawing is a schematic flow diagram of the disclosed process. 
    
    
     DETAILS 
     Catalytic dehydration of methanol to form ethylene and propylene proceeds according to the following reaction schemes: 
     
         2 CH.sub.3 OH→C.sub.2 H.sub.4 +2 H.sub.2 O 
    
     
         3 CH.sub.3 OH→C.sub.3 H.sub.6 +3 H.sub.2 O 
    
     Both of these reactions are exothermic. 
     The reactions are conducted continuously in a column reactor and in vapor phase, generally at temperatures from about 250° to about 320° C. and at a pressure ranging from atmospheric pressure to 20 bar. 
     With reference to the drawings, methanol from storage tank 1 is pumped through pump 2 to line 3 at a pressure of from atmospheric pressure to 10 bar. Water is pumped through pump 4 at a pressure of up to 10 bar. (Alternatively, inert gas, such as nitrogen or methane, is fed directly into the feed superheater 8.) 
     Line 5 receives methanol and diluting water, and delivers the resulting solution to feed vaporizer 6. Vaporized feed is transmitted through line 7 to feed superheater 8, in which the temperature is raised to 180° C. Superheated vapors from the feed superheater are passed through line 9 to tubular reactor 10, which is filled with catalyst inside tubes. Converted methanol exhausting from the reactor at 300° C. is passed through line 11. Gases containing converted methanol subsequently serve to preheat feed in the feed superheater and subsequently exhaust through line 12. They are then cooled and condensed by condensers 13 and 14. 
     Optionally, converted methanol exhausting through line 11 is passed though an auxillary heater 15 prior to being used for preheating in the feed superheater 8. 
     Condenser 14 eliminates practically all of the water and slight amounts of unconverted methanol. Dry gases from condenser 14 are conducted through line 16 to condenser 17, in which they are cooled to -40° C. prior to introduction through line 18 to splitter 19. Ethylene and propylene are conventionally separated in the splitter, from which propylene is conveyed through line 20 to storage tank 21, and ethylene evaporation heat is used to condense reflux through condenser 22. 
     Water from condensers 13 and 14 is returned to pump 4 for recycling to the system. Excess water is discharged through valve 23. 
     The ratio between ethylene and propylene produced is determined by the volume ratio between pump 4 and pump 2. Residence time for methanol in reactor 10 is determined in accord with well-established chemical engineering principles, as are methods for recovering produced ethylene and propylene. 
     PRODUCT CONTROL 
     The ratio of ethylene to propylene in the reaction product is readily varied widely from 5% ethylene and 95% propylene to 95% ethylene and 5% propylene by diluting the methanol feed with steam or an inert gas prior to passing the methanol through the catalyst. The higher the methanol dilution, the higher the proportion of ethylene to propylene obtained. The effect of dilution is shown in the following table. 
     
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Feed                 Product                                              
Inert  Methanol      Ethylene Propylene                                   
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95      5            79.98    20.02                                       
85     15            20.44    79.56                                       
80     20             7.93    92.07                                       
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     Passage of pure methanol through the catalyst at the involved temperatures leads to oligomerization of ethylene and propylene, producing higher molecular weight alkenes. The molar ratio of methanol to steam or inert gas is advantageously from 2 to 50 percent. 
     CATALYST 
     The catalyst is prepared by ion exchange of the sodium ions of a molecular sieve zeolite for aluminum and ammonium ions and subsequently driving off the ammonia, leaving the structure in the aluminum and hydrogen form. The zeolite is a type-A, a type-X or a type-Y molecular sieve zeolite which advantageously has an ammonium ion exchange capacity of from 4 to 7 equivalents per kilo. 
     Although practically all types (A, X, Y) are useful, type-A molecular sieve zeolites are preferred. Particularly those having a molar composition in the range of: 
     
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       Al.sub. 2 O.sub.3                                                  
                     1 ± 10%                                           
       Na.sub. 2 O   1 ± 10%                                           
       SiO.sub.2     2 ± 10%                                           
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     The optimum pore size of the zeolite is between 3 and 5 angstroms. 
     The composition or ratio between the aluminum sulfate and ammonium sulfate solution used to exchange sodium in the zeolite is optionally varied widely between a ratio of one equivalent of aluminum to 20 of ammonium and a ratio of 20 equivalents of aluminum to 1 of ammonium. The optimum is the proportion equivalent to ammonium alum. The concentration of the solution used to exchange the sodium optionally varies from 1% to saturated. Other soluble salts of ammonia and aluminum are optionally used in the exchange. 
     EXAMPLE 
     1 kilo of Baylith TE-144 (manufactured by Bayer in Germany) or its equivalent molecular sieve zeolite is placed in a column having a height to diameter ratio of 4 to 1. Water is passed downflow through the column in an amount sufficient to wet the zeolite spheres thoroughly. 
     A solution of ammonium alum [NH 4  A1 (SO 4 ) 2  ·12 H 2  O], 5% by weight, is passed downwardly through the column at a low rate (1 cm. per second) until the outlet composition confirms that all sodium has been replaced. Completion of the exchange is normally assured by passing an excess amount of alum through the column. 
     The column filled with zeolite is then washed with pure water until the pH of the outlet is about 7. Once washed, the zeolite spheres are heated to about 300° C. until no more ammonia is detected. The catalyst is now ready. 
     To assure good heat transfer, the diameter of the reactor tubes should not exceed two inches. At a space velocity of 80 cm. per minute, with a bed height of 22 cm., and a temperature of 300° C., 94% conversion is obtained. 
     The invention and its advantages are readily understood from the preceding description. Various changes may be made in the compositions and in the process without departing from the spirit and scope of the invention or sacrificing its material advantages. The process and compositions hereinbefore described are merely illustrative of preferred embodiments of the invention.