Return to Table of Contents

XXI. SYNTHESIS OF TOLUENE FROM BENZENE AND METHANOL

The process for synthesis of toluene by reaction of an excess of Benzene with Methanol at medium pressure over an acid zinc silico-phosphate catalyst was originally developed at Leuna. A commercial plant with a design capacity of 5,000 T/month of Toluene was erected at Waldenberg in Silesia where the I.G. have a factory which includes a Methanol plant O.U.D.A. erected the plant, W.I.F.O. financed it and it was operated by the I.G. on a management-fee basis.

The Waldenberg plant started operations in mid-1942. The maximum achieved production was 3,800 T/month were made on account of transport difficulties and consequent shortage of raw materials.

A description of the process and of the Waldenberg plant was provided in the first place at Leuna by Dr. Herold, the head of the Development Department. His information was later confirmed and some additional data produced, also at Leuna, by Dr. Klopfer who was in charge of catalyst testing. A process description was also found in an I.GW.I.F.O. Ageement, a copy of which was removed from Leuna.

General Process Data

Methanol and Benzene used for the process must be as free as possible from impurities, particularly nitrogen compounds. Methanol is specially purified from amines, by means of an organic cation exchanger, down to a maximum nitrogen content equivalent to 2 mgms. NH₃/litre. Nitriles are the chief impurities in the benzene and no satisfactory method has been developed for their removal; formation of zinc chloride addition compounds is not successful. In the end, the I.G. have resorted to careful analytical control of all batches of benzene so that these can be blended to give a feed to the plant which never contains more than 0.5 mgms. NH₃/litre.

Benezene and Methanol are reacted in the molecular ration 4:1. Reaction temperature is 340-380°C and pressure 30-35 ats. Feed rate is 0.25 M³/M³ in an active form steam must be added to the reactants. Substantially all in an active form steam must be added to the reactants. Substantially all the Methanol is converted in a single pass.

The catalyst, which has a life of 4-6 weeks, was stated by Dr. Klopfer to have the following composition:

Dr. Hanish stated that the catalyst was made by mixing orthophosphoric acid, Kieselguhr and zinc oxide, and drying a 2 cm-deep layer of the resultant paste at a temperature below 250°C. The cake obtained is broken up and sieved, pieces of about 20 mm size being used as catalyst. Fine material, amounting to 30% of the charge, is recycled to the pasting stage.

Loss of activity of the catalyst is caused by carbon formation during reaction; after 4-6 weeks the catalyst contains about 40% carbon. Attempts to revivify catalyst by burning off this carbon were unsuccessful because of conversion of orthophosphoric into the inactive metaphosphoric acid.

Plant operation

The Waldenberg plant consisted of 14 units, each consisting of 1 converter 1700 mm. External diameter and 16 M long. Each converter contained 15 M³ of catalyst. Fig. XXXVIII shows the general layout of a unit. Benzene and Methanol in the weight ratio 9:1 are injected at 35 ats. And pass through a heat exchanger and gas-fired preheater where they are vaporized and heated to 280°C (D. Klopfer) or 350°C (Dr. Herold). They then enter the top of the reaction vessel which is divided into four or five catalyst beds separated by layers of Raschig rings. Cold benzol is injected in controlled quantity at the entry to the converter and to each of the Raschig ring beds in order to maintain a reaction temperature of about 370°C with a maximum of 380°C. the reaction is extremely exothermic, the heat evolution per converter being 100,000 Kg. Cals/hour. There is a layer of calcined lime about 80 cms. Thick at both the top and bottom of each converter. Lining of the converter is acid-resistant refractor brick. A 5% caustic soda solution is injected into the products immediately on leaving the converter in order to prevent corrosion by phosphoric esters. The products then pass via the interchanger and a water-cooled condenser to a separator vessel. Gas, consisting of methane, CO₂, CO dimethylether and benzene vapour, is removed from the top of this vessel and is compressed and cooled to recover benzol (800 grms/M³). The liquid product separates into water and hydrocarbon layers, the latter having the average composition:

The production of 1 Ton Toluene requires 1.45 T. Benzene and 0.85 T. Methanol.

The water layer is distilled for recovery of a small amount of unreacted Methanol which is returned to feed. The hydrocarbon product is first fractionated (2 packed columns of 1100 mm. diam.) to remove benzol and dimethylether as overheads. The bottoms pass to a second distillation stage (3 packed columns also of 1100 mm. diam.) in which most of the benzol and only a trace of Toluene is distilled over. The next distillation is for complete stripping of Benzene from the product. This is carried out in 3 to 60 plate columns of 2,400 mm. diameter and the overhead contains some 15% Toluene. These overheads are returned to the feed of the first benzene-removal column.

The benzene-free bottoms of the third distillation stage are next fed to two 3,000 mm. diameter 60 plate stills from which a nitration grade Toluene is taken overhead. The fifth stage of distillation consists of the separation of the fourth stage bottoms into Xylene and heavier alkyl benzenes.

Development work on Related Processes

Leuna have experimented with the reaction of Benzene with alcohols containing upwards of 5 carbon atoms, available as by-products of the higher alcohols synthesis. The same catalyst and general conditions as used for Toluene synthesis have been employed.

Long chain alkylates are not formed. Instead, splitting of the alcohol occurs giving simpler alkyl benzenes and some olefins which can be recycled.

Isopropyl benzene was the main product obtained. The above experiments reached the semitechnical stage and a large-scale plant was being considered for Heydebrech.

Return to Table of Contents