PTQ Q2 2023 Issue

Main fractionator vapour

Sponge absorber

Debutanised gasoline

LIQUID to absorber

Primary absorber

Wet gas compressor

Stripper

VAPOUR to absorber

High pressure receiver

Overhead receiver

Interstage receiver

LCO PA

Preheat

To sour water stripper

Debut feed

Figure 4 WGC through absorber/stripper

MC overhead system MC overhead system performance affects both the WGFR and WGC capacity. 4,5,6 Pressure drop in the pip- ing and equipment system along with condenser perfor- mance set overhead receiver temperature and pressure. Raising pressure and decreasing temperature reduces the WGFR and potentially allows driver power to be reduced. Many units have both fin-fan and CW condensers where summer ambient air and CW temperatures cause up to 20-40°F increase. Fan performance and CW flow rates are other variables that should be considered when try- ing to reduce WGFR. A good rule of thumb is that a 1°F variation in overhead receiver temperature changes the WGFR by 1%. Simple changes like adding a CW booster pump to increase flow rate can materially reduce receiver temperature. Raising overhead receiver operating pressure decreases the WGFR, because more liquid condenses at a given temperature. The overhead system piping, fin-fan, CW exchanger, wet gas flow meter, and pressure control valve all impact the WGFR and WGC capacity. Some examples are discussed in the following sections that show how pres- sure increases and temperature reductions can be used to lower the WGFR to minimise WGC system changes. WGC - managing gas flow rate and head Understanding how WGC performance is impacted by complex hydraulic and condensing systems is essential to develop cost-effective solutions to increase LPG yield. Both WGC driver power and WGFR increase when reactor LPG yield goes up unless operating pressure and/or temperature are manipulated through equipment modifications. The effectiveness of equipment changes depends on the spe- cific process and equipment design along with the magni - tude of the reactor LPG and dry gas yield shifts. Figure 5 PFD shows a typical two-stage WGC that may have three or four wheels (sometimes called stages)

sometimes inter-stage condensers and after-condenser pressure drops are high, and bundle changes allow higher absorber operating pressure. In other cases, relatively short runs of piping and flow meters generate high pressure drop. Increasing absorber operating pressure by 20 psi improves recovery, especially with low operating pressure absorbers. MC through absorber/stripper This transcript reviews flow from the MC through the absorber/stripper bottoms. Other systems, including down- stream distillation and treating systems, are all affected by higher LPG yield and must be considered during scope development. It is not unusual to limit LPG production to the ultimate capacity of the debutaniser, C3/C4 splitter, C3 split- ter, or treating system vessels. Main column The MC itself is not materially affected by reactor yield shifts, and it is relatively simple to overcome small load increases caused by higher LPG yield by making tray modifications or replacing trays with packing. Only in cases where FCC feed rate is being increased or the MC is already fully packed and highly loaded is it necessary to be concerned about higher LPG yield. Most MCs still use some trays that generate 3-5 pounds per square inch (psi) pressure drop, whereas a fully packed MC generates about 1 psi. Packing reduces pressure drop, allowing higher WGC suction pressure, and is a widely accepted debottlenecking tool. There are three and as many as six MC pumparounds (PAs), which are often connected to gas plant reboilers and may have higher duties when producing more LPG. MC heat balance must be shifted to allow more reboiler heat as required. PA circulation rates must be increased, which may require pump modifications or replacement. Occasionally, MC capacity is limited by the slurry pumparound section capac- ity, which requires innovative solutions such as split beds and proprietary distributor designs to maximise shell capacity.

PTQ Q2 2023