Retrofit of Distillation Columns in a Methanol Plant
Yasar Demirel, Chemical and Biomolecular Engineering, University of Nebraska Lincoln, Lincoln, NE 68588
Through its ‘Column Targeting Tools' the Aspen plus simulator performs the ‘Thermal Analysis' for rigorous column calculations and produce Column Grand Composite Curves (CGCC) and exergy loss profiles. The CGCC is constructed by solving the mass and energy balances for a near-reversible column operation. The stage exergy loss profiles are generated by the stage exergy balance calculations with a reference state. The CGCC displays the net enthalpies for the actual and ideal operations at each stage, and the cold and hot heat utility requirements, while the exergy loss profiles indicate the level of irreversibility at each stage including the condenser and reboiler. Therefore, the thermal analysis can identify the targets for restructuring and modifications, and may be helpful in suggesting retrofits. Some of the retrofits consist of feed conditioning (preheating or precooling), feed splitting, reflux adjustments, and adding side condensers and reboilers. These retrofits target a practical near minimum thermodynamic loss, and suggest modifications for existing distillation columns to reduce the costs of operations by increasing the efficiency in energy utilization. This study uses the CGGC and the exergy loss profiles to assess the performance of the existing distillation columns, and reduce the costs of operation by appropriate retrofits in a methanol plant. Effectiveness of the retrofits is assessed by means of thermodynamics optimum and economics. The methanol plant is based on steam reforming and utilizes two distillation columns to purify the methanol in its separation section. The first column operates with 51 stages, has a side heat stream to the last stage, a partial condenser at the top and a side condenser at stage 2, and no reboiler. The second column operates with 95 stages, has a side heat stream to stage 95, a total condenser, and high reflux ratio. The converged base case simulations have indicated the need for more profitable operations, and the required retrofits are suggested. For the first column, the retrofits consisting of a feed preheating and a second side condenser at stage 4 have reduced the total exergy loss by 21.5%. For the second column, the retrofits of two side reboilers at stages 87 and 92 have reduced the total exergy loss by 41.3%. After the retrofits, the thermodynamic efficiency has increased to 55.4% from 50.6% for the first column, while it has increased to 6.7% from 4.0% for the second. The suggested retrofits have reduced the exergy losses and hence the cost of energy considerably, and proved to be more profitable despite the increases fixed capital costs of the distillation columns of the methanol plant.