Fluid Catalytic Cracking (FCC) Plant – Catalytic cracking breaks complex heavy hydrocarbon molecules into simpler ones to increase the quantity and quality of lighter, higher in values products such as kerosene, gasoline, LPG, heating oil, and petrochemical feedstock. Catalytic cracking is similar in objective to thermal cracking except that catalysts are used to facilitate the conversion of the heavy molecules into lighter products under much less sever operating pressures and temperatures. There are three basic steps in the catalytic cracking process, reaction, catalyst generation and fractionation.
Fluid Catalytic Cracking (FCC) Plant | Energy Efficiency Guidelines in Refinery
In general, there are three types of catalytic cracking processes; fluid catalytic cracking
(FCC) ,which is the most famous and widely used one, moving-bed catalytic cracking and
Thermoform catalytic cracking (TCC).
Catalytic cracking process is flexible wherein its operating conditions can be adjusted to allow it render changes in product demand. The FCC process shown in the above figure is the most common process, wherein gas oil, for instance, is cracked in the presence of fine catalyst kept in a fluidized bed state using the feed vapor.
FCC consists essentially of reaction, catalyst regeneration and fractionation sections. A typical FCC process involves mixing a preheated hydrocarbon charge, heavy gas oil for instance, with hot regenerated catalyst as it enters the riser leading to the reactor. The charge feed is normally combined with a recycle stream within the riser, vaporized and raised to the reactor temperature of about 900 – 1000ºF by the circulated regenerated hot catalyst. As the mixture travel up the riser, the charge is cracked at 10-30 psi. In modern FCC units all cracking occurs in the riser pipe section of the reactor.
In such units the reactor is no longer functions as a reactor; it is a vessel that holds the cyclone that separate the products from the fine catalyst particles. The resultant products stream, cracked products, is then sent to the fractionation section where it gets separated into fractions and part of the heavy oil is recycled back to the riser. Spent catalyst is continuously regenerated to get rid of coke that forms on the catalyst active sites during the cracking reaction. Spent catalyst flows through the catalyst stripper to the regenerator, where almost all of the coke formed on the catalyst particles get burned off at the bottom using preheated air/O2 stream. Fresh catalyst id added for make up to substitute any losses in catalyst due to worn-out catalyst to optimize the cracking process.
For the FCCU fractionators, as standalone distillation columns, as previously mentioned simulation, verified against the existing units, are used to benchmark those units and optimize them. The tower reflux ratio, operating pressure, and feed and side pumparounds conditions as well as tray efficiencies are all degrees of freedom. Gas compressor recycle shall be minimum and inlet cooling using chilled water system as another “design” degree of freedom that can be used to save energy in the compressor. Stripping steam should be the minimum required product specification, checked by simulation, while operating the strippers at optimum pressure.
In FCCU, convention catalytic cracking units, a strong correlation between conversion and energy consumption does exist. Feed quality is also another significant factor in the FCCU energy consumption.
The criteria for an efficient FCCU, consistent with ADU and VDU product rundown temperatures and based on 250ºF feed for gas oils and reduced crude is to ensure that all FCCU products are heat integrated down to 250ºF, including the primary fractionator overhead vapors too. Flue gas stack temperature should not be higher than 400ºF. Steam generation shall be considered in the context of the whole refinery, as like as what we mentioned before in the Hydrocracking unit. A combined heat and power optimization model (CHP) shall be then developed to optimize the best method/form for FCCU waste-heat recovery.
FCC Plant Energy TargetingAs have been mentioned several times in this part of the manual, the heat integration
initiative for the FCCU intra-process heat integration is the most important initiative in
enhancing the unit EII to push it to become closer to the best-in-class EII of that unit or in some cases to even try to beat it. This example below shows how to target for the FCCU heating and cooling utilities energy consumption using heat integration technique at
different global minimum approach temperatures among hot and cold streams (ΔT_min).
FCCU Problem Data
As shown above in the table, the FCCU hot streams to be cooled and cold streams to be heated, which are allowed to get into thermal integration, is listed with its supply temperatures; target temperatures; and stream heat capacity flowrates.
The heating and cooling utilities evaluation at different decreasing ΔT_min is presented in the table below. The table below shows that for typical FCCU at moderate ΔT_min = 30ºC, the FCCU heating utility required is about 32.2 MW and the FCCU cooling utility required is about 20 MW. The best achievable reduction in the FCCU intra-process integration is about 10%, measured from the FCCU energy targets at ΔT_min = 30ºC.
The table above shows that at ΔT_min= 1 C, the heating and cooling utilities targets are 29 MW and 16.8 MW respectively. In such case, the heating utility saving, for instance, is about 3.2 MW.
The table above shows that at ΔT_min close to zero, which is practically impossible the heating and cooling utilities targets are 28.7 MW and 16.5 MW respectively. This calculation tells us that with intra-process heat integration in FCCU we can’t reach more than 10% saving in heating utility consumption, measured from a base case that uses ΔT_min = 30ºC.
If the refinery wants to save more energy and stretch the envelope of saving in the FCCU beyond such %, inter-process integration techniques are then warranted.
10 Guidelines/Compliance Points
– Heat integration shall be applied with global ΔT_min less than or equal to 30ºF and the right use of utility level
– Process-Utilities system shall exhibit best possible synergy
– Efficiency of rotating equipment shall be more than 80-85%
– Efficiency of fired heaters shall be more than 90-92%
– Distillation columns shall be integrated with the rest of the process or among themselves and columns with long difference in its temperature profile shall be adapting inter-coolers or inter-heaters design features
– Inlet feed to compressors shall be as cold as possible and to turbines as hot as possible
– Turbines are used whenever possible and let down valves and let down drums shall be minimized
– The process products’ temperatures shall not be higher than the feed temperatures, and process temperatures to air coolers shall be lower than 200ºF
– “Source” Processes shall be integrated with adjacent ones and/or produce heating and/or cooling utilities
– Heat transfer equipment shall always exhibit high U, as high as possible.