Problems with Crude Column Cut Point Temperature Specifications
March 19, 2008
When modeling crude distillation columns, boiling point curve temperature specifications are often used to characterize products (e.g. ASTM D86 90 Volume % Cut Point Temperature). In the early stages of the model development it may be easier to monitor the product boiling point curve temperatures rather than converge on a particular specification. Once the column has been converged with relatively "easy" specifications such as draw rate or flow ratio for the product draw streams, those can be used as estimates and the specifications changed to the more difficult cut point temperature specifications to fine tune the simulation.
If the column will not converge to a desired temperature specification at what appears to be a reasonable operating condition, a converged solution is sometimes obtained by re-characterizing the crude to add additional components. The additional components add additional temperature resolution which makes it easier to obtain a desired temperature specification. To add additional components, you can increase the number of cuts for each temperature range on the Cut Points tab of the ProMax oil specification dialog. One option is to increase the components in the large interval. As an example, the number of cuts in the 100-825°F interval could be increased from 29 to 58. Or you could just increase the number of cuts near the problem temperature. For instance, make the interval 100-400°F 12 cuts, 400-500°F 8 cuts, 500-825°F 13 cuts. The 400-500°F originally was about 4 cuts and you could double or triple it while not changing the cuts per temperature interval for the remainder of the oil.
Authored by Lili Lyddon (BR&E Technical Support and Help Author)