Page 92 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

Consider the conversion of a mixed feed stream of methanol (88 mol%), ethanol (11 mol%), and water (1
mol%) via the following dehydration reactions:

2CH OH (CH ) O + H O ΔH reac = –11,770 kJ/kmol
3
3 2
2
methanol dimethyl ether (desired product)

2C H OH (C H ) O + H O ΔH reac = –11,670 kJ/kmol
2 5
2 5 2
2
ethanol diethyl ether (valuable by-product)

C H OH → C H + H O ΔH = –1,570 kJ/kmol
2 5 2 4 2 reac

ethanol ethylene (less valuable by-product)

The reactions take place in the gas phase, over an alumina catalyst [14, 15], and are mildly exothermic but
do not require additional diluents to control reaction temperature. The stream leaving the reactor (reactor
effluent) contains the following components, listed in order of decreasing volatility (increasing boiling
point):
1. Ethylene (C H )
2 4
2. Dimethyl Ether (DME)
3. Diethyl Ether (DEE)
4. Methanol (MeOH)
5. Ethanol (EtOH)
6. Water (H O)
2

Moreover, because these are all polar compounds, with varying degrees of hydrogen bonding, it is not
surprising that these compounds are highly non-ideal and form a variety of azeotropes with each other.

These azeotropes are as follows:
• DME – H O (but no azeotrope with significant presence of alcohol)
2
• DME – EtOH
• DEE – EtOH
• DEE – H O
2
• EtOH – H O
2

For this problem, we assume that the mixed alcohol stream is available at a relatively low price from a
local source ($0.25/kg). However, pure methanol ($0.22/kg) and/or ethanol ($0.60/kg) streams may be
purchased if necessary. The selling price for DME, DEE, and ethylene are $0.95/kg, $1.27/kg, and
$0.57/kg, respectively. Preliminary market surveys indicate that we can sell up to 15,000 tonne/y of DEE
and up to 10,000 tonne/y of ethylene.

For a proposed process to produce 50,000 tonnes/y of DME, determine the viable process alternatives.

Step 1: Batch versus Continuous