Revamps 2025 Issue

Reactor FCC feed riser revamps should be considered if bottoms conversion is lower than desired. This manifests itself as higher API gravities on the main column bottoms. When the residence time in the reactor is too low, there is insuf - ficient time for the multiple reactions necessary to provide lower APIs for the decant oil. This is usually caused by pre - vious revamps to increase the cracking capacity. Positive APIs are an indicator, especially those above +2.0. Pilot studies can be made to determine how much further cracking can be expected. If recycle of the bottoms is being employed to convert more bottoms, a riser revamp might be in order. Recycle of bottoms makes more coke and dry gas. Extra residence time in the reactor should reduce the need for recycle. Add another riser Adding a second riser to the process, which operates in par - allel to the main system, might be considered. Overcracking naphtha to produce more light olefins and aromatics in the cracked naphtha might be more easily controlled with an independent riser. Reactor temperature, pressure, residence time, and weight hourly space velocity can be manipulated in the added riser to achieve the best results. Control of both catalytic and thermal cracking is necessary to optimise the yields of dry gas, light olefins, and gaso - line octane and composition. Secondary reactions such as hydrogen transfer also need to be considered. Feed riser terminators Riser terminators are designed to both separate the cat - alyst and vapours, which stops the reaction, and to mini - mise the catalyst carryover to the main fractionator. It also must receive the stripping steam and hydrocarbons that are removed in the stripper. All the licensors of FCCUs offer a close-coupled system designed to minimise dilute phase cracking. Two-stage cyclone separators are marketed by several companies. The main difference between these systems is where the stripper gases are introduced into the exit gases. If it goes into the primary cyclone, then the pressure in the reactor vessel must be higher than the cyclone pressure, and the cyclone operates as a negative-pressure cyclone. If the introduction is downstream of the primary cyclone, then the primary cyclone is a positive-pressure cyclone (see Figure 1 ). Each configuration will work, but the cyclones need to be sealed. An unsealed cyclone will have more vapour underflow, which will increase delta coke and make more dry gas. Vapour quench Another technology that can be used is the quenching of the reactor vapours after the reactor temperature meas - urement. Typically, this is after the primary catalyst/vapour separator in the reactor (see Figure 2 ). This can reduce the post-reactor thermal cracking that gives dry gas and diolefins in the C₄ stream and cat gasoline. Quench added to the outlet of a rough-cut cyclone, which separates the products at the end of the feed riser, can instantly drop the

Pcn

Pcp

Hd p

Hdp

Figure 1 Negative and positive cyclones

Unit modifications: improving profitability Feed injection

The feed injection system is an area that can provide a very profitable revamp option. Benefits seen include increased conversion, lower delta cokes, reduced regenerator tem - peratures, and less dry gas. Changes to the feed system can include adding radial or additional nozzles to improve catalyst/oil contacting. Choosing the location and configu - ration of the nozzles is important, as is the instrumentation used. Reliability has to be 100%, otherwise the yields or run length will suffer. Not all feed injectors give the same performance. Earlier injectors were low-pressure drop systems and could be axi - ally or radially oriented. Every series of modern technology for previously installed injection systems showed real ben - efits over earlier designs. In one instance, a refinery looking at new feed nozzles had two offers that promised 19 and 33 cents/barrel, respectively. The higher value was chosen, and a test run indicated a 36-cent/barrel improvement. In retrospect, when asked what if it had chosen the lower bid and achieved the promised 19 cents, would they have been happy? The answer was ‘yes’. Many refiners have stopped looking for further gains and have settled for what they installed. The entire feed system needs to be reviewed if resid is to be added to the feed diet. The problem with resid is the coking that can occur at various locations of the hydrocar - bon circuit, which limits run length and adversely affects yields. This coking is often traced back directly to the feed nozzles, unit operating parameters, and/or design. Wet steam needs to be avoided.

Revamps 2025