A chemical heat transformer uses a chemically reacting system for the internal cycle. Chemical conversions in a chemical heat transformer produce entropy due to irreversibilities, which are taken into account in our derivation of a new, more realistic value for the thermal efficiency of this heat transformer. We studied the example of the endothermic dehydrogenation of 2-propanol, yielding acetone and hydrogen, versus the exothermic hydrogenation of acetone yielding 2-propanol. Different from other papers we allow the dehydrogenation temperature Tm to be higher than the boiling point Tb of the 2-propanol and we allow the isolation of 2-propanol from the mixture to be incomplete. The internal entropy production in the dehydrogenation and hydrogenation reactor is calculated as a function of the hydrogenation temperature Th with three different values of Th-Tm and several incomplete separation values. The thermal efficiency is much lower than Carnot's efficiency for low-temperature lifts because of the high irreversibilities that are already present for these temperatures and because of the low efficiency of the heat engine. A higher, but still low, efficiency is obtained for Tm>Tb and for the incomplete separation of 2-propanol from the mixture.
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