Heat transfer duty, reduction

The final subject discussed in this chapter is the issue of reactor scaleup. Moving from a laboratory test tube in a constant temperature bath to a 20-L pilot plant reactor to a 200,000-L commercial plant reactor involves critical design and control decisions. One major problem is the reduction of the heat transfer area relative to the reactor volume (and heat transfer duty) as we move to larger reactors. This has an important effect on temperature control and reactor stability. 

Condensation of Vapors Containing Noncondensable Gases The presence of noncondensable gases in a vapor decreases the rate of heat transfer. This reduction depends upon the relative values of the gas-cooling heat load and the total cool-Ing/condensing heat duty. 

Results of process calculations of all cases are summarized in Table 16. The major variations on the thermal and C-efficiencies were lower values in Case DPOM-3 resulting mainly from a lower CH3OH selectivity. There was a small reduction in the heat-transfer duty in Case DPOM-2 due to lower recycle flow. In Case DPOM-3, higher CO, selectivity resulted in a 23% increase in O2 consumption, a 35% increase in steam generation, and a large increase in energy required for gas (mainly O2) compression. All of them contributed to substantial increases in both the heat-transfer and the gas compression duties. 

Table 18 summarizes the results of process calculations for both Cases SRM-1 and -2. 0.267 moles of CO2 per hydrocarbon C-atom was added to the reformer feed in Case SRM-2. CO2 addition to the reformer feed resulted in a 5% increase in CH3OH yield, a 22% reduction of the feed to the reformer tubes, a 19% reduction of the synthesis gas fed to the CH3OH synthesis unit, and a 13% reduction in the heat-transfer duty. 

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