Methanol is synthesized from carbon monoxide and hydrogen in the reaction
A process ?owchart is shown below.
The fresh feed to the system, which contains only CO and H2, is blended with a recycle stream containing the same species. The combined stream is heated and compressed to a temperature T(K) and a pressure P(kPa) and fed to the reactor. The percentage excess hydrogen in this stream is Hxs. The reactor ef?uent—also at T and P—goes to a separation unit where essentially all of the methanol produced in the reactor is condensed and removed as product. The unreacted CO and H2constitute the recycle stream blended with the fresh feed.
Provided that the reaction temperature (and hence the rate of reaction) is high enough and the ideal- gas equation of state is a reasonable approximation at the reactor outlet conditions (a questionable assumption), the ratio
In these equations, piis the partial pressure of species i in kilopascals
Suppose P = 5000 kPa, T = 500 K, and the percentage excess of hydrogen in the feed to the reactor (Hxs) = 5.0%. Calculate
- ) and the ?ow rate (SCMH) of the recycle stream.
Run the program for the following nine conditions (three of which are the same):
(c) You should ?nd that the methanol yield increases with increasing pressure and decreasing temperature. What cost is associated with increasing the pressure?
(d) Why might the yield be much lower than the calculated value if the temperature is too low?
(e) If you actually ran the reaction at the given conditions and analyzed the reactor ef?uent, why might the spreadsheet values in Columns F—M be signi?cantly different from the measured values of these quantities? (Give several reasons, including assumptions made in obtaining the spreadsheet values.)
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