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Process Circuit Analysis 129

moved in gas-liquid separators at various points in the process. After compressing
the reformed gas in the first stage of compression, the gas then mixes with recyle
gas to form feed gas. The feed gas is compressed and then preheated by the con-
verter gas in an interchanger before entering the converter.
Because the reaction is exothermic, the synthesis gas is injected at several
points in the converter to cool the reacting gases, which prevents overheating the
catalyst. After leaving the converter, the gases are first cooled by preheating the
feed to the converter and then cooled by water to condense out crude methanol.
Then, a gas-liquid separator separates the crude methanol from the noncondensible
gases. Purging part of the recycle stream from the separator removes excess hy-
drogen and inert gases from the process. Then, the purged gases mix with natural
gas and air and finally burned to heat the reformer.
The crude methanol from the separator, containing methanol, water, low
boiling compounds, and high boiling compounds, flows to the fractionation sec-
tion. In the fractionation section, the crude methanol first flashes, and then the
vapor-liquid stream flows to a "topping" column to remove the low-boiling com-
pounds. Finally, the bottom stream from the "topping" column flows to a "refin-
ing" column to remove the high-boiling compounds, producing a purified metha-
nol product and a wastewater stream.

Process Analysis

To analyze the process circuit, consider only a small segment of the methanol
process - the synthesis loop - as indicated by the numbered lines in Figure 3.5.1.
The synthesis loop contains a recycle line, which complicates the analysis. For
simplicity, we will not consider all streams within the loop. As usual, the objective
of the analysis is to specify or calculate pressure, temperature, composition, and
flow rate in each line and the energy transferred into or out of each process unit.
We begin by noting that the energy balances are decoupled from the mass bal-
ances for the streams selected. This means that we can solve the mole balances
independent of the energy balances. If we include the determination of the flow
rates of three side streams flowing into the converter, then energy balances are
also needed.
The first step in the analysis is to determine if zero degrees of freedom exist
in any process unit. In this case, the analysis will be simplified because of the re-
duction in the number of equations requiring simultaneous solution. After analyz-
ing each process unit, we then combine the equations to determine if the process
contains zero degrees of freedom. When analyzing each unit separately, we will
repeat some variables and equations. For example, in line 3, the composition and
flow rate variables, and the mole fraction summation, are the same for the mixer
exit stream and the reactor feed stream. Later, when we combine the various
processing units to determine the process degrees of freedom, we will take the
duplication of variables and equations into account.

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