NO124160B - - Google Patents

Description

Process for continuous production

of methanol with a low content of organics

pollutants.

The present invention relates to a method for the continuous production of methanol with a very low content of organic contaminants by allowing carbon monoxide or carbon dioxide or both to react with hydrogen at a pressure in the range of 10-150 atmospheres and at a temperature in the range of 160-300°C in the presence of a catalyst containing the oxides of copper and zinc and at least one other metal which is from groups II to IV in the periodic table and whose oxide is difficult to reduce, and the method is characterized by using a catalyst where the copper content in the catalyst is at least 20% and preferably more than 35%, and the zinc content is less than 70%, especially between 15 and 50%, and where the third metal is aluminum in an amount of at least 4%, all percentages based on the atomic percentage of the relevant metals in the catalyst, and the volume-space velocity at which the process is carried out is higher than 5000 . hour, based on a pressure corresponding to one atmosphere absolute and a temperature of 20°C.

The method according to the invention can, as a result of the special catalyst and the conditions used, be used industrially for long periods without loss of efficiency, in contrast to previous methods. This also applies at the relatively low pressures and temperatures at which it is preferred to use the method. Furthermore, from an economic point of view, it is an attractive method also for medium-scale production.

It is preferred to use a mixture of carbon monoxide and carbon dioxide in the starting material for the process, i.e. in the gas in contact with the catalyst. The percentage by volume of carbon dioxide in this starting gas is preferably between 1 and 20%, especially between 3 and 12%. The percentage share of carbon dioxide can be: equal to, greater than or less than the share of carbon monoxide. It is preferred that a primary hydrocarbon-steam reforming process is used as a source for the synthesis gas, which operates at a temperature and pressure that will give the desired gas mixture. The synthesis gas produced in this way is substantially cleaned of excess steam, which it originally contains, but is preferably supplied to the methanol synthesis plant without removal of carbon dioxide. If desired, carbon dioxide can be partially removed, and the removed carbon dioxide can be returned to the inlet of the reformer. Alternatively, the carbon dioxide can be completely removed, and part of it is supplied to the methanol synthesis and another part to the inlet of the reformer. The synthesis gas can come from any source, or a combination of sources that will give the required composition. If the reformer is used with natural gas as raw material, it will be necessary to add a considerable amount of carbon monoxide to compensate for the high hydrogen/carbon ratio in methane, which is higher than that required for methanol production. This extra carbon can conveniently be added in the form of carbon dioxide, either to the reformer or to the methanol plant, or both. Alternatively, the synthesis gas is produced by partial oxidation of methane. If the feedstock for the reformer is naphtha, only slightly more than two hydrogen atoms are present for each carbon atom, and the process is then preferably used in connection with a steam-naphtha reformer process, especially of the type described in British patent 953 877. The synthesis gas must be substantially free of sulphur, i.e. that it must not contain more than 5 dpm of sulfur by weight, and preferably less than 1 dpm. If gas with less than 0.1 dpm can be obtained, this is even more advantageous.

Such low sulfur contents can be suitably achieved by the method according to British patent 902 148, when liquid raw materials are used. The susceptibility of copper-containing catalysts to sulfur poisoning, and the usefulness of "protective catalysts" in connection therewith, is well known.

The mole ratio between carbon dioxide and hydrogen in the gas in contact with the catalyst is preferably between the stoichiometric ratio and 1:10. It appears that a faster reaction takes place when there is more hydrogen than the stoichiometric amount in the gas in contact with the catalyst. In order to make use of this greater reaction rate, the ratio is preferably between 1.3 and 3 times the stoichiometric ratio. On the other hand, the process seems satisfactory and gives good quality methanol when the ratio is the same as or not much below the stoichiometric.

A definition of the "oxides" used in the catalyst is necessary because, although it would be convenient to specify the composition of the catalyst particles fed to the synthesis converter, as the oxides present at this stage, these oxides are reduced, f .ex. of hydrogen, before the synthesis gas is passed over them. However, the reduction is incomplete, since of the oxides present, only copper oxide is readily reducible to metal. Furthermore, it is assumed that the exact state of reduction varies with the pressure of the synthesis gases, and with the content of carbon dioxide and water (which have an oxidizing effect) and of carbon monoxide and hydrogen (which have a reducing effect). Thus, the term "oxides" will be used to describe the reduction product of the oxides present under the conditions of the synthesis.

The pressure used when carrying out the method is preferably 30 to 120 atmospheres, e.g. 40 to 80 atmospheres. These are absolute pressures. These pressures are suitably equal to the pressure at the outlet of the production plant for the synthesis gas, or at most 5 times as high. The pressure can conveniently be increased to that required for the synthesis by means of a rotary compressor, and in fact one of the advantages of this invention is that it enables the use of rotary compressors in facilities for medium production, e.g. to produce from 120 tonnes of methanol per

24 hours and above.

The temperature at which the process works is preferably in the range 190 - 270°C.

The volume-space velocity at which the process is carried out is

as mentioned higher than 5000 per hour ^, and is appropriately much higher, e.g. up to 50,000 hours<-1>, especially 7,000 hours<-1>to 25,000 hours ^. These space velocities are based on a pressure corresponding to 1 atmosphere absolute and a temperature of 20°C

The process is carried out in a system with repeated circulation, in which unreacted reactants are returned to the methanol synthesis after methanol has been removed from them. Such a system may comprise more than one methanol synthesis step and removal of methanol in the return loop. If in a circuit system it is desirable to maintain the ratio between hydrogen and carbon oxides at the above-stoichiometric levels, which were mentioned above, it follows that the returned gas will generally contain a higher hydrogen-carbon oxide ratio than the synthesis gas that was supplied to the system , except, of course, when there is a local buyer, such as e.g. an ammonia synthesis, for excess hydrogen. In the method according to the invention, the temperature control in or between the synthesis converters can take place by any suitable method, e.g. in the case of supply gas preheaters, coolers or quenchers.

As mentioned, the copper content in the catalyst is preferably at least 20%, especially more than 35%. Preferably it is higher than the zinc content. The zinc content is less than 70%, especially between 15 and 50%. The content of the third metal, aluminium, is at least 4%, e.g. from 4 to 20%. thus suitable catalysts contain copper, zinc and aluminum in the ratios 30:60:10, 40.40:20, 60:30:10 and 75:20:5 and intermediate ratios. All these conditions are with respect to atoms of the relevant metals.

Other oxides, such as magnesium oxide, titanium dioxide, zirconium dioxide, cerium dioxide and thorium dioxide may be present. A certain amount of chromium oxide may also be present.

The catalyst used in the method according to the invention is preferably one in which at least copper and zinc are introduced by co-precipitation. Preferably, the largest part of aluminum is also introduced in this way. When producing the catalyst, the co-precipitation is preferably carried out by mixing

the water-soluble salts of copper, zinc and aluminium, either in mixture, or by simultaneous addition, with a carbonate or

hydrogen carbonate of an alkali metal. To avoid catalyst poisons, said salt should not be a halide or another sulphur-containing salt. Other precipitating agents include hydroxides and oxalates, and some or all of the aluminum and zinc can be added as aluminate or zincate. However, preferably all the components are present as salts in which they exist as cations, especially the nitrates. The conditions for coprecipitation must be regulated very carefully, and it is thus advantageous to mix the ingredients in a continuous manner. The pH can be between 1 unit on the acidic side of neutrality and 2 units on the alkaline side, but a better catalyst results if the pH is within 0.5 units of neutrality at the temperature used. In any case, the pH must be within the necessary limits to ensure complete precipitation. It should be noted that the numerical value for pH is temperature dependent. The temperature for the coprecipitation is preferably between 50 and 100°C. The precipitate must be washed well to remove alkali metals from the catalyst. After that, it is dried and calcined by e.g. 300°C to transfer the carbonates and other compounds to oxides and are then formed by e.g. pelletization under pressure.

The invention is illustrated by what is stated below.

Preparation of a catalyst (A) containing copper, zinc

and aluminium.

To a solution of 27.5 g of sodium aluminate in 250 ml

water, 87.5 ml of concentrated nitric acid was added. Aluminum hydroxide was precipitated first, but dissolved again on stirring. To the resulting solution was added a solution in 1.5 liters of water of 435 g of copper II nitrate trihydrate and 269 g of zinc nitrate hexahydrate, followed by sufficient water to bring the volume up to 3 liters. The solution was heated to 85°C and passed at a rate of -6.7 liters/hour through a mixing zone while a molar solution of sodium carbonate was passed through at 7.3 liters/hour. These rates produced a slurry with a constant pH of 7.0. The resulting slurry was stirred at 85°C until the pH remained constant. The final pH measured at 85°C was 7.1, which implies a weak alkalinity, corresponding to an alkali normality in the filtrate of 0.03. The slurry was filtered, washed thoroughly with water and dried overnight at 110°C and calcined at 300°C for 6 hours. Before calcination, most of the product was bright green, indicating that little, if any, degradation of the copper

the carbonates to oxide had taken place. The calcined material was pelletized with 2% graphite to give 5.4 x 5.4 mm pellets with a density of 2.2 g/cm 3 and an average vertical crushing strength of 147.4 kg. The pellets had the following composition

in

sentence:

Example 1

Use of the catalyst (A) for methanol synthesis.

A certain amount of catalyst in the form of crushed particles of size 18 to 25 B.S.S was reduced under atmospheric pressure using a mixture of hydrogen, 80%, carbon monoxide 10% and carbon dioxide 10%, the temperature being slowly raised to 250°C. The same gas mixture was then passed over it at a pressure of 50 atmospheres and a space velocity of 40,000 hours. The activity of the catalyst was measured at appropriate intervals over a period of use as the percentage of methanol in the product gas and is shown in Table 1 compared to the results for a control catalyst containing the same proportions of copper and zinc, but with chromium oxide instead of alumina.

The methanol produced was, before purification, in the form of an aqueous solution containing 76% by weight of methanol, with only a very low content of organic impurities.

The results given in Table 1 were recalculated for a catalyst having a density of 1.0. Under the process conditions described, the conversion of the reactants to methanol is low due to the high space velocity, and consequently the degree of reaction is, to a good approximation, linearly inversely proportional to the space velocity, i.e. linearly proportional to the amount of catalyst that is in contact with the gas over a small range of variation. As the reaction is not limited by diffusion under the conditions described, the degree of reaction depends on the catalyst weight, and not on the volume of the catalyst> and consequently it is permissible to divide the percentage of methanol in the product gas by the density of the catalyst. In Example 1, the density of the crushed copper oxide-zinc oxide-alumina catalyst is 1.21.

Example 2

Experiments using a catalyst (B) containing 75%

copper (atomic percentage).

Apart from the difference in composition, the catalyst was prepared by the same method as described for catalyst A. The catalyst was in pellet form and had a density of 2.14 g/cm and an average vertical crushing strength of 102.5 kg. The exact weight percentage composition of the catalyst was: Table 2 shows the activity and stability of this catalyst, as determined by the method described in example 1. The percentages have been converted to a density of 1.0 by dividing the measured values by the density (1.18 g/cm 3 )„

Example 3

Method using a catalyst in pellet form containing copper, zinc and aluminum on a larger scale The synthesis gas for this example was produced by reacting sulphur-free naphtha (boiling point range 30-170°C) with steam (steam ratio 3.0 molecules per carbon atom) over a nickel-magnesium oxide-kaolin-cement-pot ash catalyst as described in British patent 1,003,702 under a pressure of 12.7 kg/cm 2 and an outlet temperature from the catalyst bed of 780°C. The percentage composition by volume of the synthesis gas was C0215-16, CO 1-12, H265-67 and CH^6-7%, and was thus close to the stoichiometric composition for the methanol synthesis. It was used without removal of carbon dioxide.

The catalyst for this example was prepared by the method described for catalyst (A) (but on a larger scale) and had almost exactly the same composition. Two catalyst samples were prepared, (a) 5.4 mm diameter by 3.6 mm high cylindrical pellets and (b) 3.2 mm diameter x 3.2 mm high cylindrical pellets

pellets, and these were tested in parallel. The two converter systems used gas circuits and operated at a blowdown rate such that the percentage composition of the circulating gas was maintained at C0213-15, CO 7-9, H254-46, CH419.5-26. The synthesis pressure was 51.7 kg/cm 2 , Table 3 shows the production rate for methanol at a space velocity of 9600 hour 1 and at three average temperatures.

(In the converter system that was used, there was a temperature increase of 30°C between inlet and outlet. It should therefore be noted that the inlet temperature for the experiments in table 3 is approximately 15°C lower, and that the outlet temperature is around 15° C higher than the mean temperature shown).

When the space velocity was increased by about 50% more

50,000 hours"''" over the largest catalyst, the production rate of methanol increased in the same ratio, showing that over both catalysts a good approximation to equilibrium had been reached.

Then the 5.4 x 3.6 mm catalyst worked continuously in

68 days, it lost its activity only very slowly and by a

speed of no more than 2-3% per month.

In other experiments, using the same catalyst, it was found that the carbon dioxide/carbon monoxide ratio could be varied considerably with very little consequence, apart from changing the water content in the crude product's methanol. The content of organic contaminants in the methanol raw product was found to be much lower than in the methanol raw product produced at higher pressures and temperatures. Thus, e.g. at an average converter temperature of 242°C and a space velocity of 9600 h 1 , the content of organic pollutants less than 500 dpm, and at the somewhat higher temperature (250°C) and lower space velocity (5000 h "*") was the content of organic pollutants still less than 2000 dpm.

Claims (1)

Process for the continuous production of methanol with a very low content of organic impurities by allowing carbon monoxide or carbon dioxide or both to react with hydrogen at a pressure in the range of 10-150 atmospheres and at a temperature in the range of 160-300°C in the presence of a catalyst which contains the oxides of copper and zinc and at least one other metal which is from groups II to IV in the periodic table and whose oxide is difficult to reduce, characterized in that a catalyst is used where the copper content in the catalyst is at least 20% and preferably more than 35% , and the zinc content is less than 70%, especially between 15 and 50%, and where the third metal is aluminum in an amount of at least 4%, all percentages based on the atomic percentage of the relevant metals in the catalyst, and the volume-space velocity that the process is carried out at is higher than 5000 hour-, based on a pressure corresponding to one atmosphere absolute and temperature 20°C.