Debottlenecking Crude Units

The first unit for which we will discuss different debottlenecking options is the first unit in the refinery- the crude unit. Depending upon the unit configuration, operating conditions, and crude slate, the crude unit can have a variety of bottlenecks. A typical bottleneck is crude charge pressure drop through the preheat train. Sometimes, booster pumps are required to overcome the preheat train pressure drop. Before additional pumps are installed, other options should be considered. Oftentimes, pressure drop can be alleviated inexpensively by depassing the crude side exchanger bundles. Depending on the exchanger design, this may require relatively minor exchanger modifications, but with large payouts. Depassing a 4 tube pass exchanger to 2 passes reduces the pressure drop by approximately a factor of 8. Of course, suitable exchanger velocities must be maintained to prevent excessive exchanger fouling.

Another common limit is the exchanger train design pressure. Bigger impellers cannot be installed in the charge pumps to increase the charge rate because the increased pressure will exceed the exchanger design pressure. Downstream booster pumps can alleviate this problem. An even better solution is to move or install a flash drum upstream in the exchanger train. Some exchanger trains do not have a flash drum, or it is located at the end of the train. Moving or adding a flash drum earlier in the exchanger train has several advantages. First, vapor generated does not have to flow through the rest of the exchanger train reducing the significant two phase pressure drop. Second, moving the flash drum from the end of the exchanger train to the beginning allows the exchanger train to run at the lowest possible pressure above the crude tower pressure. The lower pressure minimizes exchanger design pressure limitations. Sometimes a high pressure is maintained on the flash drum at the end of the exchanger train to minimize flashing, but this is not required. Third, a flash drum reduces the vapor traffic load on the crude tower. For even more debottlenecking, a preflash tower can be installed.