It is becoming of great importance to develop processes which can utilize the “available cold” of LNG and thus recover the energy used for its liquefaction. It is possible to evaluate the amount of refrigeration that could be produced reversibly Q1, at a temperature T1, from the amount of cold available, Q2, at a lower temperature, T2, in an environment at T0. This is:
$Q1Q2≤T0−T2T0−T1·T1T2$
For LNG it would be obtained that:
$Q1Q2≤13.5$
Work is being done to develop cold multiplication cycles which would approach this ideal figure. Among the systems under development is an absorption process resembling a reversal of the well-known absorption refrigerator. An ethane-neoheptane system is presented for which the multiplication factor has been around 1.8 and the efficiency of around 17 percent. The irreversibilities of the cycle and the low enthalpy of mixing of hydrocarbons are the main reasons for the relatively low cold multiplication factor and thermodynamic efficiency.
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