Switching from a steam stripped sidestripper to a reboiled sidestripper significantly reduces main column condenser heat load. For the basis chosen, total overhead duties dropped by approximately eight to ten percent, depending on the case. Reboiling the kerosene sidestripper had a much bigger impact than reboiling the diesel sidestripper. It is relatively easy to make flash specification on the diesel: little steam was necessary. In contrast, the steam rate required to meet the kerosene flash specification was much higher. The steam condensation load drove a revamp to using extra available stages in the kerosene rather than the diesel stripper.
Extra duty on the reboiler of a reboiled sidestripper is an effective means of shifting yield between two adjacent sidecuts. The more sensitive the product flash specification is to stripping, the greater the yield shift possible. Different operations, depending on the inter-product blending affects, can be used in selecting yield shifts. Larger yield shifts are seen when all specifications are strictly meet. Smaller yield shifts are seen when the total yield of the two products having material shifted between them is kept constant.
The most important influence on yield changes was the blending affects for the different products. The second most important affect was the increase in sidestripper effectiveness (liquid-vapor ratios) with increasing reboiled sidestripper duty. Increased liquid-vapor ratios in the main column with increased duty were third. Finally, partial pressure changes with steam rate changes in the main column were least important. Steam partial pressure changes mainly changed the split between light straight run naphtha and heavy naphtha product yields.
Practical as well as theoretical issues must be balanced when considering reboiled versus partial pressure sidestrippers. While reboiled sidestrippers have many advantages, they also suffer from two major disadvantages. First, the heat to drive the reboiler comes from either product run down or residue cooling. In either case, the heat is shifted from feed preheat duty to sidestripper reboiler duty. In a unit with sufficient heater capacity or poor heat integration this does not create any problems. In a unit with limited heater capacity, heat shifts force either lower distillate yields or lower unit charge rates.
Second, sidestripper reboilers have an established history
of fouling in many plants. Using TEMA standard fouling factors
is a sure way to make sure that the exchangers cannot achieve
the design stripping. Insufficiently stripped product cannot be
sold: it does not meet flash specifications. As required, parallel
reboilers or extra surface must be added to allow for fouling.
Extra surface should be added with caution. Oversurfaced exchangers
may drop fluid velocities so much that low velocity induced fouling
may overwhelm the added surface area: more is not always better.
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