The BP Grangemouth Fluidised Catalytic Cracker Unit (FCCU or Cat Cracker) operates within the Oil Refinery on the North Side of the Complex. The FCCU converts heavy oils into lighter, more volatile materials suitable for use in motor fuels and as chemical feedstocks. The plant was designed for steady state continuous operation and limited start-up and shutdowns were originally anticipated.
The FCCU handles a variety of flammable materials which will readily ignite and therefore careful consideration is given to the elimination of potential ignition sources on and in the vicinity of plant and equipment containing flammable substances by the appropriate design of electrical equipment and the elimination of hot surfaces by insulation of pipework and vessel surfaces.
Prevention of a loss of containment of the chemicals from the process vessels and pipework is a fundamental requirement of the design, operation and maintenance of the FCCU in order to minimise the likelihood of a major accident
In its current configuration the FCCU can process approximately 128 tonnes/hour of "wax" fed from the Vapour Distillation Unit located elsewhere in the Oil Refinery.
The FCCU comprises two main sections:
the "front end" where "cracking" is carried out in the reactor/catalyst section. Cracking is the process whereby long chain hydrocarbon molecules (large molecules) are broken down into shorter chain molecules (smaller molecules); and
the "back end" (or "light ends") where separation of cracked materials is achieved by passing cracked materials through a series of columns. This process is also known as fractionation.
The FCCU unit has a total inventory of approximately 120 tonnes of flammable materials with most of the inventory being contained within the fractionator train and various separation and process vessels taking the overheads from the fractionators. Due to the inventory of materials the FCCU meets the definition of a "top tier" installation under the COMAH Regulations and therefore a safety report is required to be prepared.
A "simplified" description of FCCU operations is given below. FCCU operations are complex and involve many different recycle and heat recovery systems within the processes described below.
Heavy oils and steam are fed into a "riser" where they contact an upflowing stream of hot catalyst particles (zeolite). The oils are "cracked" (i.e. broken down) into smaller more volatile compounds and coke (basically carbon) is deposited onto the catalyst particles. There is a significant heat loss from this part of the reaction. The mixture of hydrocarbon gases, steam and catalyst pass from the "riser" to the cyclones where the catalyst is separated from the gases. The catalyst drops into the stripping section of the reactor where residual hydrocarbons are stripped off the catalyst by steam. The catalyst then flows to the Regenerator where the carbon is burnt off by a stream of air and the carbon is oxidised to give carbon monoxide (CO), which is then further oxidised to give carbon dioxide in the new CO-boiler. The overall coke oxidation process is highly exothermic and produces adequate heat to counteract the heat loss in the cracking reaction, and to provide a source of heat for the columns in the "back end" section of the plant.
The "front end" of the FCCU process is represented schematically in the figure below.
From the cyclones and the Reactor the hydrocarbon gases and steam pass to the "light ends" section consisting of a number of fractionation columns. The purpose of this section is to separate the newly formed reactor products into various distillation fractions such as fuel gas, LPG and components for blending into motor spirit, diesel, gas oil and fuel oil. A number of different columns are present in this section for the separation of individual products.
The Main Fractionator operates at high temperature (about 500oC), low pressure (0.4 barg) and separates the incoming feed from the "front end" section of the plant into a number of different product streams including gasoline, light gas oil, heavy gas oil and decanted oils. The overhead vapours which come off the top of the fractionating column are partially condensed, the liquid collected in a reflux drum, the gases compressed and cooled and the condensed liquids are combined with the liquids in the reflux drum. The wet gases are routed via a compressor into the Vapour Recovery Unit which consists of a number of additional columns identified below.
The overhead product from the Main Fractionator is contacted with a stream of wash oil. The unabsorbed gases pass from the top of the column into the Oil Refinery fuel gas system or to the flare system. The liquid from the base of the column is pumped to the next column.
The column which operates at about 12 barg and 190oC takes feed from E4. The LPG components (propane/butane) are distilled off the top of the column as a mixture and fed into the Depropaniser column (E7) – see below. The heavier material (Debutaniser bottoms) which is similar to petrol is removed from the base of the column and routed into the Re-run column (E6).
Debutaniser bottoms are fed into the rerun column via a heat exchanger (C25) before being "flashed" across a control valve. ("flashing" refers to the instantaneous change of state from liquid to gas that occurs when liquid is fed through a control valve which drops the pressure of the liquid and the temperature of the liquid on the downstream side of the control valve is then greater than the boiling point of the liquid at that new pressure). In the Re-run column the feed is separated into light and heavy gasoline products and a residue which is recycled from the base of the tower. This is a low pressure column operating typically at about 2 barg.
The feed from the Debutaniser column (E5) is separated and used as either chemical feedstocks or blended into LPG products from other parts of the Complex. This column is also operated at around 12 barg.
BP has classified the whole of the Vapour Recovery Unit (VRU) ("back end" of the FCCU) as Zone 2 for electrical classification purposes with a few Zone 1 areas. There are no Zone 0 areas. The electrical equipment is appropriately certified for use in such areas. (Zone 2 indicates a zone in which a flammable atmosphere is not likely to occur in normal operation and if it does occur only exist for a short time. Zone 1 indicates a zone in which a flammable atmosphere is likely to occur for a short period of time during normal operation. Zone 0 indicates a zone in which a flammable atmosphere is likely to be continuously present or present for long periods).
The "back end" of the FCCU process is represented schematically in the figure below.
The plant is controlled by a "distributed control system" (DCS) located in the Central Control Building (CCB) sited 400m to the south east of the plant. Significant manual intervention is also required on the plant particularly during start up and shut down of the plant.
Since it was designed, constructed and commissioned in 1951 various modifications have been carried out on the plant. The original FCCU was designed to process approximately 60 tonnes/hour.
The unit was changed to incorporate "riser cracking", where the reaction takes place in the reactor riser rather than in the main reactor vessel itself. This allowed the plant to take advantage of more active catalysts.
The unit was modified further to increase its throughput to the present capacity (approximately128 tonnes/hr). This project included modification of the pipeline between the Debutaniser (E5) and Re-run (E6) columns with the provision of a preheater for the E6 column.
The plant was re-instrumented with a DCS system as part of a process instrumentation project.
A Stage 1 revamp project was carried out to improve the environmental performance of the unit in response to new legislation covering particulate emissions. This involved the installation of 3rd and 4th stage cyclones to the Regenerator for enhanced catalyst recovery and the installation of a carbon monoxide (CO) boiler for energy conservation and associated improved steam generation capacity. As part of this project the drainage of the unit was improved and improvements and repairs to fire resistant insulation on the structural steel also took place.
|A Stage 2
revamp project was undertaken in order to enable the unit to address
future feedstock requirements (lower quality feed/heavier
feedstock). This required major changes to the reaction section of
the unit, including a new reactor and riser assembly, new air
blowers and extensive modifications to the waste heat recovery
systems and the main fractionator overhead system. The Stage 2
revamp project also installed remote isolation valves in those
sections of the plant where significant modifications were
At the same time improvements to the plant’s relief systems were also carried out as part of an Oil Refinery Relief Project.
At the time of the Stage 2 revamp a Stage 3 revamp was envisaged for 2003 which would have resulted in extensive modification and improvements to the Fractionation section. Many proposed changes were deferred for later consideration and possible inclusion in the Stage 3 revamp project. At this stage significant changes to the inventory isolation systems for the VRU were expected to take place.
Overall the "front end" section of the FCCU had been significantly revamped since commissioning (especially in 1998) but the "back end" however had not been subject to large-scale modifications and was largely the original design/installation with the exception of the pre-heater which had been installed on the E5/E6 transfer line in 1987.