Question

Methyl tertiary butyl ether (MTBE) is used as an anti-knock additive in petrol (gasoline). It is manufactured by the reaction of isobutene (T = 50 oC, P=400 kPa) with methanol (T=25 degree C, P=400 kPa): CH3OH (l) + C4H8 (l) -> C5H12O (l) The reaction is highly selective and practically any C4 stream containing isobutene can be used as a feedstock. A 10 per cent excess of methanol is used to suppress side reactions. The mixture of the two feed materials is introduced by a pump to a heat exchanger to heat the mixture to 85 oC using low-pressure steam. In a typical process, the conversion of isobutene in the reactor stage is 97%. The product (T=127 oC, P=2915 kPa) is separated from the unreacted methanol and any isobutene by distillation. The essentially pure, liquid, MTBE leaves the base of the distillation column and is sent to storage. The methanol and isobutene leave the top of the column as vapour and pass to a column where the methanol is separated by absorption in water. The isobutene leave the top of the absorption column, saturated with water, and are used as a fuel gas. The methanol (in the methanol solution at discharge from absorption column) is separated from the water solvent by distillation and recycled (T= 56.27 oC, P=500 kPa) to the reactor stage. The water, which leaves the bottom of the distillation column, is recycled to the absorption column after cooling to 35 oC using cooling water. A purge is taken from the water recycle stream to prevent the build-up of impurities before cooling. i) Draw a block diagram for the process as described above, identifying the components of all streams, and operating conditions. [20 marks] ii) Draw the simplest possible corresponding Aspen-HYSYS diagram. numbering all streams, and unit operation. [25 marks]

Answer 1

(i) Block Diagram of the process:

(ii) Aspen flowsheet: