Evaluation of xylene recovery distillation columns for improving operating performance.
CHEMICAL FACILITY DISTILLATION OPTIMIZATION TO IMPROVE SOLVENT RECOVERY
The client owns and operates a batch chemical production facility that produces an intermediate specialty organic chemical compound. A continuous area of the process involves the recovery of xylene, an organic solvent, utilizing two distillation columns in series. The recovery operation was not operating as designed, resulting in the loss of large quantities of xylene. Furthermore, significant reboiler fouling was causing major system down time. Process Engineering Associates, LLC (PROCESS) was contracted by the client to conduct a process evaluation of these columns and make recommendations for improving operating performance. The project included these tasks:
- Evaluate current operation versus intended design
- Develop recommendations for improving on-stream time by reducing reboiler fouling
- Recommend operational control improvements
A site visit was conducted to collect all pertinent information on column design and performance. The first task involved collecting operating data and developing a computer process model to simulate columns operation utilizing PROCESS’ licensed commercial process simulation software. The model could then be used to optimize column operation to establish better operating targets for controlling top and bottom process variables. This was followed by a literature review to determine feasible options to reduce reboiler fouling. The client later expanded the scope of the project to include consideration of using existing idle equipment to condition/clean the incoming feed by flashing and removing the solids and some of the heavies prior to feeding the first column.
In the end, PROCESS developed the following findings/recommendations:
- Modeling revealed that significant solvent was lost in the first column due to azeotropic conditions that likely were not considered during the original design. A recommendation was made to develop a decant operation to recover the organic phase from the overhead aqueous distillate.
- Fouling of several key instruments (level and flow instrumentation) did not allow reliable readings to be used for control optimization. A design review of flow meters also indicated that several of the vortex meters used for control are oversized for normal process flows. New meters were sized and recommended.
- The second vacuum column should be converted from thermosyphon to forced circulation reboiling with the pump, piping, and reboiler designed to handle the heavy components and allow more organic solvent to be forced from the bottoms. The client was encouraged to perform the necessary lab work to determine the viscosity limits of the bottoms and thereby quantify the minimum amount of solvent necessary to keep the stream pumpable.
- An existing flash pot with reboiler and condenser was added to the process simulation model for the purpose of simulating the removal of solids and heavies prior to the first column. The model predicted that the existing equipment would work with the addition of a new small flash pre-heater, new reboiler recirculating pump, and conversion of the reboiler from one to two tube passes. A pre-heater provides additional duty capacity so that the otherwise marginal size of the existing reboiler is sufficient with minor modifications. The new pump and extra pass provides a tube-side velocity required to maintain a reasonable heat transfer coefficient.
- Other control improvements are to be re-evaluated after the fouling is reduced to improve instrument reliability.
The client implemented several of these changes after initial bench-scale lab testing proved the feasibility of the flash feed concept described above. These changes restored the unit to its designed capacity. The client also reported a significant reduction in reboiler fouling.
- Batch Chemical Manufacturing
- Operations evaluation
- Distillation troubleshooting
- Process improvement engineering
- Azeotropic distillation evaluation