Dimethyl ether (DME) is produced by the catalytic dehydration of methanol using an acid zeolite catalyst:
2CH3OH gives (CH3)2O + H2O
The fresh feed supplied to the process contains 99.15 mol% methanol and
0.85 mol% water. The feed is mixed with a recycle stream before being fed to the
reactor. This mixed reactor feed contains 98.40 mol% CH3OH, 1.14 mol% H2O and
0.46 mol% (CH3)2O. The methanol conversion per pass is 79.91%. The reactor outlet
stream is subsequently fed to a series of two separators.
The first separator recovers the dry DME product at a purity of 99.50%, with the bottoms, which is 66.92 mol% water, being fed to the second separation unit.
In the second separator waste water,
contaminated with 0.53% methanol, is recovered and the remaining unreacted methanol is recycled along with all of the DME present in the bottoms from the first separator, giving a recycle stream which contains 2.11 mol% DME.
(a) Draw a block diagram of the DME process and select a suitable basis for
your calculation.
(b) Determine the composition and flow rates of all streams in the process,
presenting your answer as a stream table.
(c) Determine the recycle ratio and the overall methanol conversion.
(d) If the annual production target is 50,000 tonnes of DME (99.5% purity)
calculate the hourly DME production and fresh feed rates in kmol/h,
assuming an operating factor of 0.956 (fractional plant availability).