- • Live steam cannot be just fed at one section of the CDU. It needs to be fed at various sections to ensure both good heat distribution and reduce relative volatilities of the hydrocarbons at various sections of the main and secondary towers.
• Therefore, live steam will enable good product quality as lighter hydrocarbons with higher relative volatilities in the bottom heavy product liquid streams will be easily stripped and carried along with the vapor.
• The only condenser in the main column is a partial condenser to facilitate the production of both gas and naptha+water stream.
• The circuitry totally avoided the existence of reboilers by introducing live steam. Therefore, much fixed costs of the column have been reduced. However, higher operating costs due to higher steam utilization rates are evident.
5.1.5 Pump-around units
- • Pump around units are most essential units in the crude distillation column.
• They are used to maintain good reflux conditions in the main column and therefore the desired product quality.
• They also provide a good heat source as the liquid streams are at higher temperatures. Therefore, they are also important units in the heat exchanger network.
• The cooled liquid is sent back to a section above.
• Usually two pump arounds are used in conventional designs. However, there are crude distillation units with even three pump around units.
The circuitry connections between primary and secondary towers along with relevant pump around units are presented in Figure 5.2. It can be seen that very complex interactions exists between the main and secondary columns.
Figure 5.2: Design architecture of main and secondary columns of the CDU.