PTQ Q2 2023 Issue

above the fractionator inlet must be kept wet, or coking will occur on the packing or bare metal. If quench for the bottom of the tower is taken from the cooled slurry that provides the cooling of the reactor vapours, care must be taken that the amount of wash oil is not reduced below the minimum necessary to prevent wash section coking (see Figure 4 ). Coking is a thermal process that depends on the feed- stock, the residence time in the hot section of the bottoms circuit, and the temperature. Paraffinic oils (high APIs) crack, and the secondary reactions can lead to the observed cok- ing. Aromatic stocks are more refractory and do not crack as easily, so higher bottoms temperatures may be possible. Modern cracking catalysts have increased FCC conversions and produced a heavy oil residue (650ºF+) with APIs that are typically zero or negative. These heavier streams may have a high viscosity and are more apt to cause coking. Fractionator-based coking The FCC unit has been referred to as a ‘gasoline machine’ since this was the main product from the process. Anything boiling over 430ºF was considered unconverted, includ - ing the LCO or diesel fraction and the bottoms (650ºF+). Economics make bottoms cracking into diesel very attrac- tive, which means the amount of bottoms has declined and its quality is worse. This is particularly true of hydropro- cessed feeds that produce volume per cent conversions in the 80s and bottoms yields of only 2-4 vol% on feed. The residence time in the fractionation section rose when this occurred, and the bottoms temperature had to be reduced from 680-700ºF to 650-670ºF. Even lower temperatures might be necessary to prevent coking if the amount of bottoms is too low. This could result in a resid that contained a large amount of diesel. If LCO is used as the flush for the slurry pumps, the bottoms could contain more than 25% diesel. The addition of a stripper to recover this valuable product may be a good solution. Coking is fairly common in the bottom of the main frac- tionator. If the temperature is too high, coke forms and the slurry circuit can plug. Many older designs had a line for cooling the bottoms that just injected the cooled decant oil through a pipe. Golden pointed out that the temperature in the bottom of the fractionator can show large variations when the material is not dispersed throughout the bottom section. Measuring the temperature at one location may not give an accurate description of the coking conditions at that location. While more than one inlet location can be used to improve the temperature variations below the main column inlet, a well-designed distributor is recommended to ensure good control. A coke trap at the bottom of the tower on the outlet tube is recommended so that pieces of coke that dislodge from the overhead line or the main fractionator do not plug the slurry lines. The screen should be sized to let catalyst through but not large coke particles. Stripping the bottom section with steam to help remove hydrogen sulphide can also help maintain a more uniform temperature. Any cata- lyst carried over to the fractionator will also be kept in sus- pension and leave with the bottoms. The slurry pumps used to circulate the main fractionator

Light cycle oil

Slurry reux

Desuperheating trays

Quench

Reactor euent

Bottoms

Figure 4 Main fractionator bottoms quench

evenly distribute the feed are the main source of this coke. Poor insulation or cooling of the product stream can also result in condensation reactions that form liquids. Pressure drop increases in the overhead vapour line cause higher pressures in the reactor and regenerator. This can reduce the capacity of the unit if it is at blower capacity and will increase costs since more horsepower will be required at the higher pressure. Higher reactor pressure can reduce olefinicity and increase delta coke. Slide valve differentials may change, and a rebalancing of the pressure balance will be needed if the excessive pressure drop is too large. Impact of transfer line design Design of the transfer line is critical to a successful operation. It should be as short as possible to minimise the pressure drop and temperature loss. Mauleon recommends a velocity of 100 ft/sec for this service with a minimum of bends. Dean and Golden say a 90-degree elbow oriented horizontally to the main fractionator inlet should never be used due to a low-pressure zone being formed that will allow any liquid that is formed to pool in the line and form coke. A short radius elbow oriented vertically will allow any liquid to be aspirated into the line. The horizontal run into the fractionator needs to be long enough to develop even flow. Velocities over 125 ft/sec can cause uneven vapour flow in the main column quench zone immediately above the inlet as well as flooding, possibly facilitating coke lay - down. Some refiners increase the size of the vapour inlet line to lower the velocity entering the column. The vapour line should be free draining and have no low points where liquid might pool. Other coking sources The blind or valve used to separate the reactor from the main column can be a source of coke build-up if not well insulated. This has been a problem for many refiners. Hangers can also act as heat sinks that lead to localised coking in the vapour line. The internals in the wash section

PTQ Q2 2023