Page 215 - Chemical engineering design

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CHEMICAL ENGINEERING
4.5. Ammonia is synthesised from hydrogen and nitrogen. The synthesis gas is usually
produced from hydrocarbons. The most common raw materials are oil or natural
gas; though coal, and even peat can be used.
When produced from natural gas the synthesis gas will be impure, containing up
to 5 per cent inerts, mainly methane and argon. The reaction equilibrium and rate
are favoured by high pressure. The conversion is low, about 15 per cent and so,
after removal of the ammonia produced, the gas is recycled to the converter inlet.
A typical process would consist of: a converter (reactor) operating at 350 bar; a
refrigerated system to condense out the ammonia product from the recycle loop;
and compressors to compress the feed and recycle gas. A purge is taken from the
recycle loop to keep the inert concentration in the recycle gas at an acceptable
level.
Using the data given below, draw an information flow diagram of the process
and calculate the process stream flow-rates and compositions for the production of
600 t/d ammonia. Use either the ‘Nagiev’ split fraction method, with any suitable
spreadsheet; or manual calculations.
Data:
Composition of synthesis gas, mol fraction:
A
N 2 H 2 CH 4
24.5 73.5 1.7 0.3
Temperature and operating pressure of liquid ammonia gas separator, 340 bar
Ž
and 28 C.
Inert gas concentration in recycle gas, not greater than 15 per cent mol per cent.
4.6. Methyl ethyl ketone (MEK) is manufactured by the dehydrogenation of 2-butanol.
A simplified description of the processes listing the various units used is given
below:
1. A reactor in which the butanol is dehydrated to produce MEK and hydrogen,
according to the reaction:

CH 3 CH 2 CH 3 CHOH ! CH 3 CH 2 CH 3 CO C H 2
The conversion of alcohol to MEK is 88 per cent and the yield can be taken
as 100 per cent.
2. A cooler-condenser, in which the reactor off-gases are cooled and most of the
MEK and unreacted alcohol are condensed. Two exchangers are used but they
can be modelled as one unit. Of the MEK entering the unit 84 per cent is
condensed, together with 92 per cent of the alcohol. The hydrogen is non-
condensable. The condensate is fed forward to the final purification column.
3. An absorption column, in which the uncondensed MEK and alcohol are
absorbed in water.
Around 98 per cent of the MEK and alcohol can be considered to be absorbed
in this unit, giving a 10 per cent w/w solution of MEK. The water feed to the
absorber is recycled from the next unit, the extractor. The vent stream from the
absorber, containing mainly hydrogen, is sent to a flare stack.
4. An extraction column, in which the MEK and alcohol in the solution from
the absorber are extracted into trichloroethylane (TCE). The raffinate, water