Column exergy efficiency

Example 4.25 Column Exergy efficiency Propylene-propane mixture is a close boiling mixture. A reflux ratio of 15.9 (close to minimum) and 200 equilibrium stages are necessary. Table 4.13 shows the enthalpy and entropies of the saturated feed and saturated products from the simulation results with the Redlich-Soave equation of state. The reboiler and condenser duties are 8274.72 and 8280.82 kW, respectively. The reference temperature is 294 K. The lost work ZTFis obtained from Eq. (4.198) as... 

For certain applications, such as the separation of ternary mixtures, divided wall columns may be of interest, [9] which have shown to reduce the energy requirements and increase exergy efficiency. 

The total exergy losses consist of configuration limitations due to the design of the column and transportation limitations due to the states of streams. For Exsv > o, the exergy efficiency of column i becomes... 

System Exergy input (MW) Exergy output (MW) Overall exergy loss (MW) Overall exergy efficiency (%) Column exergy losses (MW)... 

A multiobjective optimization problem is formulated for the MTBE RD column with respect to economic performance and exergy efficiency. The formulation includes the balance equations 8.1-8.5 and 8.29, the criteria definitions 8.42 and the optimization formulation 8.62. Constraints imposed by operating conditions and product specifications are included. For the sake of controlled built-up of optimization complexity the first approximation of this approach omits the response time constant as objective function. 

Figure 8.11. Entropy production rate profile for an optimal design of a MTBE RD column based on exergy efficiency (X-design). Legend ...

Table 8.6. Optimized design of a RD column for MTBE synthesis based on economic performance and exergy efficiency. Remark w =...

The analysis presented in this chapter is an example of how the principles of thermodynamics can be applied to establish efficiencies in separation units. We have shown how exergy analysis or, equivalently, lost work or availability analysis can be used to pinpoint inefficiencies in a distillation column, which in this case were the temperature-driving forces in the condenser and the reboiler. The data necessary for this analysis can easily be obtained from commonly used flow sheeters, and minimal extra effort is required to compute thermodynamic (exergetic) efficiencies of various process steps. The use of hybrid distillation has the potential to reduce column inefficiencies and reduce the number of trays. We note that for smaller propane-propene separation facilities (less than 5000bbl/day [10]), novel technologies such as adsorption and reactive distillation can be used. 

The values of efficiencies help in assessing the effectiveness of exergy usage, as the value of (1 -17) is directly related to the exergy loss. They can be used to compare the performance of columns with each other. Smaller exergy losses mean the utilization of a higher portion of available energy, and hence fewer thermodynamic imperfections in a column. 

Table 4.17 shows that column 2 operates with efficiency as low as 4.1%, while the efficiency of column 1 is 50.6%. The exergy values for the whole separation system of two columns are also low and need to be improved. The individual values of PI show that it is possible to reduce by 13.1% the total loss of 0.834 MW in column 1 and by 19.1% the total loss of 27.813 MW in column 2. Despite the heat integration, the separation section of the methanol plant performs poorly in utilizing the exergy in the distillation columns. 

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