A recent unit performance workshop highlighted two important points to keep in mind when troubleshooting or analyzing unit performance. The first is to understand that the layout of both the unit and the piping can affect operation. The second is to consider how gravity head can impact fluid flow — even for systems that seemingly don’t look like gravity head should influence them.
Figure 1 illustrates a small portion of a refinery fluid catalytic cracking unit (FCCU). In an FCCU, the reactor effluent enters the main fractionator for cooling, heat recovery and initial separation. The reactor effluent may reach a temperature of 1,000°F or higher. The exact temperature depends upon factors including feed, catalyst and operating conditions. In the main column, an initial lower pumparound (usually called the slurry pumparound) removes heat and cools the feed.
Figure 1.Inability to increase the quench rate stymied efforts to decrease the boot temperature.
During cooling, some of the feed condenses to form a bottoms product stream (slurry oil). For most plants, economics favor having as little light material in the bottoms product as possible. By equilibrium, the bottoms stream gets hotter as the amount of light material in it drops. Optimum temperature of the bottoms stream might exceed 740°F in some locations.
However, the high bottoms temperature will cause extremely fast coking in the pumparound exchangers (E02 and E05 in Figure 1). The coking fouls the exchangers — reducing heat recovery, increasing pressure drop and lowering flow rate possible in the loop. To decrease coking, a slip stream (quench) of cold oil is returned directly to the tower boot. The colder boot temperature reduces available log mean temperature difference in the heat exchangers, especially E02 here. Nevertheless, the quench improves the overall heat transfer and performance of the system. In this case, the after-quench temperature was roughly 640°F.
Even at 640°F, excessive coking would occur with some feeds. So, the performance workshop explored, among other things, lowering the temperature even more. Review showed that the quench flow temperature control valve (TCVB) was fully open. Opening the flow control valve (FCVA) would enable extra flow — but all went to the pumparound return, none to the quench.
Extra flow through the pumparound return does drop the bottoms temperature. At the same time, it increases the pumparound duty. Higher duty raises the amount of light material in the bottoms — at a considerable cost in yield for this unit. The benefit of the quench system is that it lowers the bottoms temperature without changing the bottoms composition.
The inability to increase the quench rate (and decrease the boot temperature) stems from the layout. The FCVA regulates the total flow rate and heat removal duty. The TCVB adjusts the quench rate. The difference between the two rates is the pumparound rate. The pumparound line lacks a control valve. The static head generated by the height difference between the pumparound return nozzle and the quench return nozzle sets the maximum possible pressure drop across the TCVB. Once the TCVB fully opens, no more quench flows.
This returns us to the two key points — about layout and gravity head —cited earlier. Effective unit analysis and troubleshooting must keep both in mind.
Solutions to increase the flow in the quench circuit include:
• opening a bypass (if one is present) around the TCVB;
• changing the TCVB to a control valve with a lower pressure drop;
• shifting the FCVA to the pumparound return line; or
• adding pressure drop to the pumparound return line with an orifice plate.
Selecting the optimum option requirese valuating the potential benefits and costs of each possibility.
In the short term, adding a valve that could be partially closed in the pumparound return line might suffice. A block valve could serve as a deliberate pressure drop element — but, generally, using one is poor practice. Partially open block valves tend to erode, dramatically increasing their possibility of leaking when fully closed. This service boasts high erosion potential because the slurry contains catalyst particles.
For most situations, the best technical solution uses independent control valves in the quench return and the slurry pumparound return. Here, this might involveeither a new control valve or relocation of the FCVA.