Project SynopsisInvestigate, simulate, and redesign a distillation train for series operation to purify titanium tetrachloride (TiCl4TICL4) and remove unwanted color bodies.
TITANIUM TETRACHLORIDE PURIFICATION SYSTEM PROCESS SIMULATION AND PHASE 2 DESIGN
The client, an engineering company, had previously contracted Process Engineering Associates, LLC (PROCESS) to execute a process design for a high purity titanium tetrachloride (TiCl4TICL4) distillation train and associated equipment. The ultimate client, a titanium pigment producer for the paints & coatings industry, had performed a series of laboratory experiments to better understand the sources and causes of color formation in the final product. As a result of these studies, the ultimate client directed the project team to revise the process design. The changes included operation of the two distillation columns in series, with pure product removal from the top of the second column. Additionally, the client desired the revised design to accommodate all previous design cases and modes of operation to maintain operating flexibility, with minimal impact on the mechanical designs that had already been developed. PROCESS was contracted to lead this redesign effort.
The focus of this redesign effort was the second column in series, since the design of the first column is not affected by this change. In series operation, the second column’s purpose is to remove color bodies that might form in the system. The causes of color are not well understood, but lab tests indicate potential contributing factors. These tests also indicate that product removal from the top of the column, with a conservative number of trays above the feed, removes the color. Because the causes have not been identified, the color cannot be modeled with simulation software, and the design was reduced to determining the appropriate hydraulic loadings for the columns. The design tasks for Phase 2 were as follows:
- Determine appropriate reflux flow rate to hydraulically load the existing column design given the new operating mode
- Develop a mass and energy balance for the new design case
- Prepare a Process Flow Diagram (PFD)
- Verify the suitability of existing reboiler and condenser designs for the new case
- Size reboiler pumps
- Size relief devices
The minimum and maximum reboiler duties and reflux flow rates were determined using PROCESS‘ licensed computer process simulation software, CHEMCAD, by iteratively changing the reboiler load and calculating the resulting reflux rate. The resulting liquid and vapor rates were then checked using CHEMCAD’s column sizing capabilities as well as a tray vendor’s hydraulic sizing software to determine appropriate upper and lower loadings. When the minimum and maximum limits were established, a mass and energy balance and PFD were generated showing the results.
Sensitivity studies were performed using CHEMCAD to determine how a suspected color-causing impurity will partition in the distillation train. These studies showed that the suspected color agent will be purged from the bottom of the second column, as was anticipated by the lab studies.
CHEMCAD was also used for additional studies to bound appropriate flow and control settings for cases when feed to the distillation train is less than the full design rate. These studies provided operating advice to the plant to allow stable operation at reduced rates while maintaining appropriate product quality.
CHEMCAD’s CCTHERM heat exchanger sizing software was used to verify if the existing exchanger designs were suitable for the new operating mode. In all cases the existing designs were acceptable.
The results of all these studies indicate that the existing process and mechanical designs for the columns, reboilers, and condensers can be used for the new operating mode in addition to the prior design modes, subject to the maximum and minimum flow rates established during this project. The tray design was adjusted in collaboration with the tray vendor to ensure proper operation in all operating modes. Some of the column controls were revised to reflect the needs of the new conditions. The column and heat exchanger detailed mechanical specifications were updated with the new information.
The client provided isometric sketches for purposes of sizing pumps and verifying hydraulic calculations. With this information, CHEMCAD was used to calculate the required pump head for the reboiler pumps. A duty specification data sheet was prepared for the pumps.
Relief devices for the new distillation train were sized using PROCESS‘ proprietary in-house relief device capacity computer program, with calculations based on guidelines published in American Petroleum Institute (API) Recommended Practices (RP) 520 and 521 and supplemented by PROCESS‘ extensive experience. PROCESS supplied the client with the relief valve sizing calculations as well as datasheets that could be used to obtain quotes from vendors for the relief valves.
In addition to these tasks, PROCESS also provided design information and physical properties from the model to allow sizing of other equipment, including pumps, control valves, and tanks. The results of these studies, as well as discussion of system controls, were submitted to the client in a final project report.
- Paints, Coatings & Pigment Manufacturing
- Process modification
- Operating scenario investigation
- Trayed distillation tower design
- Pressure relief valve (PRV) or pressure safety valve (PSV) sizing