(182d) Improvement of Distillation Column Efficiency By Integration With Organic Rankine Power Generation Cycle

Distillation as main separation technology consumes greater than 40% of the energy used by the chemical and petroleum process industries but the energy efficiency of a distillation column is low. It is well-known that significant energy savings can be achieved if complex column configurations and heat integration are used, although these improvements withdraw the attractive advantages of conventional distillation column such as flexibility, low capital investment and low operational risk.

This theoretical research focuses on improvement of distillation column vapor generation procedure by expansion turbine installation.

The liquid stream from the column bottom is pressurized by a pump and the high pressure liquid is vaporized in the reboiler. The resultant high pressure vapor stream drives the expansion turbine, which is coupled to the electricity generator or another rotating equipments. The turbine exhaust vapor enters the column bottom.

Such distillation column with power generation cycle (DCPGC) is kind of combination of conventional distillation column and organic Rankine cycle (ORC) power plant. The ORC is a cost efficient and proven method of converting low temperature waste heat to mechanical and/or electrical energy.

Such chemical components and its mixtures as propane, i-butane, n-butane, i-pentane et al. which occurs in distillation columns bottoms are known to be utilized or considered as ORC working fluids. So DCPGC is seem to be practically feasible.

DCPGC reboiler duty is about 10-20% greater than the conventional column reboiler duty but almost whole of extra heat transforms to electricity power. The theoretical DCPGC electricity generation efficiency is more than 90% (the heat required for the distillation remains the same as in conventional column and is not included in the efficiency calculation).

Only thermodynamic properties and thermal stability of column’s bottom product  impact on DCPGC feasibility, but not the temperature difference between top and bottom of the column. This is an advantage of suggested technology to compare with heat integrated column with heat pumps where temperature difference is quite important.

This theoretical research describes practical and economical feasibility of DCPGC for separating propane/propylene, i-butane/n-butane, i-pentane/n-pentane, butane-pentane, benzene/toluene, xylene mixtures. Crude oil and naphtha fractionation are also investigated. For instance DCPGC for i-butane/n-butane binary mixtures separation with 25 tonne/h feed flowrate can produce 1 MW of electricity power.