(449h) Improving Energetic Potential of a Batch Distillation through Bottom Flashing Route

Authors: 
Jana, A. K., IIT Kharagpur
Increasing consumption of fossil fuels to meet the global energy demand has led to an exponential rise in carbon emissions. This has stimulated an intensive research in finding a renewable energy vector and in improving the energy efficiency performance of the long serving process units. In this light, distillation, a century old chemical process unit that accounts for 95% of all fluid separations, is a potential candidate for boosting its heat reversibility. It is surprising to note that a conventional distillation column (CDC) can secure a maximum thermodynamic efficiency of about 20%.

This work aims at developing a thermal integration approach for a batch distillation under the framework of heat pumping. The heat pump systems, namely vapor recompression and bottom flashing, are being used in continuous flow distillation column for a long time with showing a substantial improvement in energy savings and cost. Because of increasing use of batch separation in biochemical, fine chemicals, pharmaceutical and food processing industries, more research attention needs to pay to improve the thermodynamic efficiency of batch distillation. In this direction, we attempt to explore the feasibility of bottom flashing arrangement for a batch column that separates a binary mixture of cyclohexane and toluene.

The bottom flashing loop is connected with the conventional batch distillation through receiving the reboiler liquid and giving back a boil-up vapor. For this phase conversion, a throttling valve and a compression system are additionally involved. Actually, the reboiler content is proposed to use in this batch distillation with bottom flashing (BDBF) column as a heat sink against the overhead vapor employed as a heat source. To make it feasible, the temperature of heat sink needs to decrease so that there exists a reasonable driving force (here 150C) between the heat source and heat sink. For this, we employ a throttling valve that acts as a flasher for pressure reduction. Subsequently, a compressor is used for the recovery of that lost pressure. In between, the flashed liquid is used to replace the cooling medium in the top condenser, and this, in turn, gets vaporized and mixed with flashed vapor before subjected to compression. The compressed vapor then acts as boil-up vapor and it leads to reduce the hot utility supply to the reboiler. By this way, the BDBF scheme improves the energetic and economic potential over a conventional batch distillation (CBD).

Like the continuous batch column, the BDBF configuration operates at a transient state. Hence the constituent elements of the bottom flashing arrangement and its associates, namely flasher, top condenser, compressor and reboiler, need to be operated at a transient condition so that one can run the BDBF at a close dynamics, if not same, with its conventional counterpart. In fact, this is a prerequisite required to make a fair comparison between them for performance evaluation in terms of energy consumption.

To illustrate the BDBF scheme, we model the CBD fitted with the bottom flashing loop and simulate the resulting differential algebraic equation system. The differential equations are obtained by the application of the conservation principle and the algebraic equations represent the tray hydraulics, phase equilibrium, phase enthalpy and so on. In the present study, we assume ideal vapor phase and the Wilson activity coefficient model is employed to predict the liquid phase nonideality. The column operates at atmospheric pressure with a stage pressure drop of 0.3 kPa and a 80% tray efficiency.

It is obvious from the qualitative analysis that the BDBF column requires no external cooling medium and a reduced amount of hot utility, indicating a reduction of operating cost. At the same time, however, this thermally integrated column involves an additional operating cost for the compressor that runs with electricity, which is much more expensive than the thermal utility. Keeping these points in mind, we perform a quantitative analysis in terms of energy savings. For this, one needs to estimate the total energy consumption (QCons) made in both the startup and production phase.

For the representative binary system, the CBD consumes a total of 4.35 E6 kJ/cycle, whereas the BDBF column involves a reboiler duty (QR) of 1.37 E5 kJ/cycle and compressor duty (QComp) of 3.89 E5 kJ/cycle. So the total amount of energy consumed (QCons) by the BDBF column is 13.04 E5 kJ/cycle that is obtained from:

QCons = QR + 3 QComp

Note that the multiplication factor 3 is adopted to convert the electrical energy to thermal energy. Accordingly, it is computed that the BDBF column secures an about 70% savings in utility consumption with reference to the conventional batch distillation. The example system shows that the bottom flashing mechanism is capable of providing a substantial improvement in energy savings for batch processing.