Meeting Market Demand through Deep Conversion Technologies – Fluid Catalytic Cracking Operation Modes

Introduction and Context

           The downstream industry faces a transition period where the focus of the players is changing from transportation fuels to petrochemicals aiming to ensure maximum added value to processed crude oils as well as to allow the growth of low carbon energies in the global energetic matrix.

           The growing market of petrochemicals has been lead some refiners to look for closer integration between refining and petrochemicals assets aiming to reach more adherence with the market demand, improve revenues, and reduce operating costs. In this business environment, flexible refining technologies like Fluid Catalytic Cracking (FCC) reach highlighted position in the strategy of the refiners to reach competitiveness in the market.

           Taking into account the current scenario and the forecasts are expected a great contribution of FCC units to the economic sustainability of the downstream industry.

           Despite the recent trends, some economies keep still strongly dependents of transportation fuels or specific crude oil derivatives like aromatic residues to meet their internal demand. As a flexible crude oil refining process and a consolidated technology, the fluid catalytic cracking units have a fundamental role to ensure market compliance, economic growth, and refining margins in the downstream industry.

Fluid Catalytic Cracking Technologies – An Overview

Fluid Catalytic Cracking (FCC) is one of the main processes which give higher operational flexibility and profitability to refiners. The catalytic cracking process was widely studied over the last decades and became the principal and most employed process dedicated to converting heavy oil fractions in higher economic value streams.

The installation of catalytic cracking units allows the refiners to process heavier crude oils and consequently cheaper, raising the refining margin, mainly in higher crude oil prices scenario or in geopolitics crises that can become difficult the access to light oils.  The typical Catalytic Cracking Unit feedstream is gas oils from the vacuum distillation process. However, some variations are found in some refineries, like sending heavy coke naphtha, coke gas oils, and deasphalted oils from deasphalting units to processing in the FCC unit.

The catalyst normally employed in fluid catalytic cracking units is a solid constituted by small particles of alumina (Al2O3) and silica (SiO2) (zeolite). By the catalyst characteristics and the operational conditions in the catalytic cracking process (temperature higher than 500 oC), the process is inefficient to cracking aromatic compounds, therefore, how much more paraffinic is the feedstream, higher is the unit conversion. Figure 1 presents a process scheme for a typical Fluid Catalytic Cracking Unit (FCCU). 

In a conventional scheme, the catalyst regeneration process consists of the carbon partial burning deposited over the catalyst, according to the chemical reaction below:

C + ? O2 → CO

The carbon monoxide is burned in a boiler capable of generating higher pressure steam that supplies other process units in the refinery. 

Figure 1 – Schematic Process Flow for a Typical Fluid Catalytic Cracking Process Unit (FCCU) 

The principal operational variables in a fluid catalytic cracking unit are reaction temperature, normally considered the temperature in the top of the reactor (called riser), feed stream temperature, feed stream quality (mainly carbon residue), feed stream flow rate and catalyst quality. Feedstock quality is especially relevant, but this variable is a function of the crude oil processed by the refinery, so is difficult can be changed, but for example, aromatic feedstock’s with high metals content are refractory to cracking and conducting to quick catalyst deactivation. 

An important variation of the fluid catalytic cracking technology is the residue fluid catalytic cracking unit (RFCC). In this case, the feedstock to the process is basically residue from the atmospheric distillation column, due to the high carbon residue and contaminants (metals, sulfur, nitrogen, etc.) are necessary some adaptations in the unit like a catalyst with higher resistance to metals and nitrogen and catalyst coolers furthermore, it’s necessary to apply materials with most noble metallurgy due to the higher temperatures reached in the catalyst regeneration step (due to the higher coke quantity deposited on the catalyst), that raises significantly the capital investment to the unit installation. Nitrogen is a strong contaminant to the FCC catalyst because they neutralize the acid sites of the catalyst which are responsible for the cracking reactions.  

When the residue has high contaminant content, is common the feed stream treatment in hydrotreating units to reduce the metals and heteroatoms concentration to protect the FCC catalyst.

Typically, the average yield in fluid catalytic cracking units is 55% in volume in cracked naphtha and 30 % in LPG.  Figure 2 presents a scheme for the main fractionator of the FCC unit with the principal product streams.  

Figure 2 – Main Fractionator Scheme for a Typical Fluid Catalytic Cracking Unit 

The decanted oil stream contains heavier products and has high aromatic content, is common that this product is contaminated with catalyst fines and normally this stream is directed to use as fuel oil diluent, but in some refineries, this stream can be used to produce black carbon.

Light Cycle Oil (LCO) has a distillation range close to diesel and normally this stream is directed to treatment in severe hydrotreating units (due to the high aromaticity), after this treatment the LCO is sent to the refinery diesel pool.

Heavy cracked naphtha is normally directed to refinery gasoline pool, however, in scenarios where the objective is to raise the production of middle distillates, this stream can be sent to hydrotreating units for further diesel production.

The overhead products from the main fractionator are still in the gaseous phase and are sent to the gas separation section.  The fuel gas is sent to the refinery fuel gas ring, after treatment to remove H2S, where will be burned in fired heaters while the LPG is directed to treatment (MEROX) and further commercialization.   The LPG produced by the FCC unit has a high content of light olefins (mainly Propylene) so, in some refineries, the LPG stream is processed in a Propylene separation unit to recovery the propylene that has higher added value than LPG.

Cracked naphtha is usually sent to a refinery gasoline pool which is formed by naphtha produced by other process units like straight run naphtha, naphtha from the catalytic reforming unit, etc. Due to the production process (deep conversion of residues), the cracked naphtha has high sulfur content, and to attend the current environmental legislation this stream needs to be processed to reducing the contaminant content, mainly sulfur.   

The cracked naphtha sulfur removal represents a great technology challenge because is necessary to remove the sulfur components without molecules saturation that gives high octane number for gasoline (mainly olefins).    

Over the last decades, some technology licensers had developed new processes aiming to reduce the sulfur content in the cracked naphtha with minimum octane number loss, some of the main technologies dedicated for this purpose are technology PRIME G+ ? from Axens, the processes OCTAGAIN ? and SCANfining ? from Exxon Mobil, the process S-Zorb? from ConocoPhillips and ISAL? technology from UOP.

Usually, catalytic cracking units are optimized to aiming the production of fuels (main gasoline), however, some process units are optimized to maximize the light olefins production (propylene and ethylene). Process units dedicated to this purpose have his project and operational conditions significantly changed once the process severity is strongly raised in this case.

The reaction temperature reaches 600 oC and a higher catalyst circulation rate raises the gas production, which requires a scaling up of gas separation section.  Figure 3 presents a typical scheme for a gas separation section for a fluid catalytic cracking unit. 

Figure 3 – Basic Process Flow Diagram for a Typical Gas Separation Section from FCC Unit 

In several cases, due the higher heat necessity of the unit is advantageous to operate the regenerator with the total combustion of the coke deposited on the catalyst, this arrangement changes significantly the thermal balance of the refinery once it’s no longer possible to resort the steam produced by the CO boiler. 

Over the last decades, the fluid catalytic cracking technology was intensively studied aiming mainly for the development of units capable of producing light olefins (Deep Catalytic Cracking) and to process heavier feedstocks. The main licensers for fluid catalytic cracking technology nowadays are the companies KBR, UOP, STONE & WEBSTER, AXENS, and McDermott.

Meeting the Market Demand through FCC Optimization

           According to the market demand, the FCC units can be optimized to produce the most demanded derivatives. In traditional FCC units there are normally four operating campaigns:

1 – Maximum Gasoline – In Maximum gasoline campaigns the processing unit operates under medium or high severity. The severity is limited by the octane number achieved in the cracked naphtha, in refining configurations where the refiners rely on octane boosting units like alkylation, catalytic reforming, and isomerization there is more flexibility to maximize the gasoline yield in the FCC operating in maximum gasoline mode.

           The catalyst formulation to maximum gasoline campaigns involves high zeolite and active-matrix once the presence of rare earth compounds tends to raise hydrogen transfer reactions, reducing the olefins content and consequently the octane number in the cracked naphtha. Another alternative to improve the yield of gasoline in the is operation mode changes the final boiling point of cracked naphtha to higher values, in this case, the limitation is the quality requirements, mainly the sulfur content in the final derivative, according to the operating capacity in cracked naphtha hydrodesulfurization unit.

           The main restrictions in the processing unit in maximum gasoline mode are the gas separation section capacity, especially related to the cold area compressors as well as the debutanizer columns.

2 – Maximum LPG – In this operation mode the FCC unit operates under high severity translated to high operation temperature (TRX), high catalyst/oil ratio. The catalyst formulation taking into account higher catalyst activity through the addition of ZSM-5 zeolite. There is the possibility to a reduction in the total processing capacity due to the limitations in blowers and cold area capacity.

           It’s observed an improvement in the octane number of cracked naphtha despite a lower yield, due to the higher aromatics concentration in the cracked naphtha. In some cases, the refiner can use the cracked naphtha recycle to improve even more the LPG yield.

           In the maximum LPG operation mode, the main restrictions are the cold area processing capacity, metallurgic limits in the hot section of the unit, treating section processing capacity as well as the top systems of the main fractionating column.

3 – Maximum LCO – The maximum LCO (Light Cycle Oil) operation mode is applied by refiners with great demand by middle distillates, especially diesel, and adequate hydrotreating capacity to convert the LCO in high-quality diesel. In this case, the FCC unit operates under relatively low severity conditions with low TRX, low catalyst/oil ratios, and the catalyst formulation tends to minimize the catalyst activity.

           In process units where is observed a restriction related to the cold area and blowers processing capacity, the maximum LCO operation mode can allow the raise in the total processing capacity of the unit. This fact can be positive, once allow lower time contact between catalyst and feed, improving, even more, the LCO and decanted oil (DO) yield.

           It’s important to take into account the effect of the feed stream quality over the produced LCO. Paraffinic feeds tend to produce higher quality LCO, refiners operating FCC units in maximum LCO mode tends to minimize the final boiling point of cracked naphtha and maximize this parameter in the LCO aiming to improve the LCO yield, but this action is limited by the quality of the final diesel.

4 – Maximum Aromatic Residue – This is the less common operation mode in FCC units, where the main objective is to maximize the yield of decanted oil and achieve the quality requirements of aromatic residue. The aromatic residue is normally applied to produce black carbon, these derivative presents great demand in some markets.

           The main difficulty to comply with the aromatic residue specification is regarding ash content in the decanted oil. This parameter is strictly related to the efficiency of the cyclone in the catalyst regeneration section, to achieve this objective some refiners apply additives to promote de ash decantation in the final tanks or specific filtration systems that require more capital spending.

           Another key quality parameter to meet aromatic residue specification is the BMCI (Bureau of Mines Correlation Index) that is related to the aromaticity of the decanted oil, to achieve the current specifications of black carbon it’s necessary to achieve a minimum BMCI higher than 120. The BMCI is calculated based on the viscosity of the decanted oil at the temperature of 210 oF. The metals content in the decanted oil needs to be also controlled, especially, sodium, aluminum, and silicon.

           The operating severity in maximum aromatic residue mode tends to be high with high TRX, high catalyst/Oil ratio, and high catalyst activity. As a side effect is observed the raise of octane number in cracked naphtha due to the incorporation of aromatics compounds in this intermediate.

           In maximum aromatic residue operation mode, the main restrictions are the temperature of the bottom section in the main fractionators that can lead to coke formation, metallurgic limitations in the hot sections as well as the capacity of blowers and cold area compressors.

The Petrochemical FCC Alternative – Raising Competitive Advantage

Taking into account the current scenario of the downstream industry and the last forecasts, it’s observed trend of reduction in transportation fuels demand followed of a growing market of petrochemicals, leading the refiners to optimize his FCC units to maximum LPG yield aiming to improve the capacity to produce light olefins and promote closer integration with petrochemical assets. The major part of the catalytic cracking units is optimized to maximize transportation fuels, especially gasoline, however, face to the current scenario some units have been optimized to maximize the production of light olefins (ethylene, propylene, and butenes). As aforementioned, units focused on this goal have these operational conditions severely changed, raising the cracking rate.   

The reaction temperature reaches 600 oC and a higher catalyst circulation rate raises the gas production, which requires a scaling up of gas separation section.  The higher thermal demand makes advantageous operates the catalyst regenerator in total combustion mode leading to the necessity of installation a catalyst cooler system.

Installation of catalyst cooler system raises the process unit profitability through the total conversion enhancement and selectivity to noblest products as propylene and naphtha against gases and coke production. The catalyst cooler necessary when the unit is designed to operate under total combustion mode due to the higher heat release rate as presented below.  

C + ? O2 → CO (Partial Combustion) ΔH = - 27 kcal/mol

C + O2 → CO2 (Total Combustion)      ΔH = - 94 kcal/mol

In this case, the temperature of the regeneration vessel can reach values close to 760 oC, leading to higher risks of catalyst damage which is minimized through catalyst cooler installation. The option by the total combustion mode needs to consider the refinery thermal balance, once, in this case, will not the possibility to produce steam in the CO boiler, furthermore, the higher temperatures in the regenerator requires materials with the noblest metallurgy, this raises significantly the installation costs of these units which can be prohibitive to some refiners with restricted capital access.

Conclusion

As discussed above, the FCC units offer great operation flexibility to refiners and can raise significantly the refining margins, and, according to the local market demand, the processing unit can be optimized to produce a different kinds of intermediates. Following recent trends, the synergy between refining and petrochemical processes raises the availability of raw material to petrochemical plants and makes the supply of energy to these processes more reliable at the same time ensures better refining margin to refiners due to the high added value of petrochemical intermediates when compared with transportation fuels.

Another advantage is the reduction of the risks of transportation fuels oversupply, facing the current scenario of demand reduction and restriction of fossil fuels. It’s important to consider that integrated processes lead to higher operational complexity, however, given current and middle term scenarios to the refining industry, better integration between refining and petrochemical processes is fundamental to the economic sustainability of the downstream industry. In this scenario, the FCC technologies can ensure higher added value to processed crude oils through the maximization of petrochemical intermediates, like propylene, in the refining hardware. Another important variation of the FCC technologies is the unit focused on processing residues, the Residue Fluid Catalytic Cracking (RFCC) that can allow even more added value to bottom barrel streams, especially for refiners processing heavier crudes.

        Like a flexible refining technology, the Fluid Catalytic Cracking (FCC) units have a highlighted role in the future of the downstream industry, especially considering the transitive period, where the FCC units can help to supply the demand by petrochemicals without a shortage of transportation fuels.

References:

GARY, J. H.; HANDWERK, G. E. Petroleum Refining – Technology and Economics.4th ed. Marcel Dekker., 2001.

ROBINSON, P.R.; HSU, C.S. Handbook of Petroleum Technology. 1st ed. Springer, 2017.

MYERS, R.A. Handbook of Petroleum Refining Processes. 3a ed. McGraw-Hill, 2004.

Dr. Marcio Wagner da Silva is Process Engineer and Project Manager focusing on Crude Oil Refining Industry based in S?o José dos Campos, Brazil. Bachelor in Chemical Engineering from the University of Maringa (UEM), Brazil, and PhD. in Chemical Engineering from the University of Campinas (UNICAMP), Brazil. Has extensive experience in research, design, and construction to oil and gas industry including developing and coordinating projects to operational improvements and debottlenecking to bottom barrel units, moreover Dr. Marcio Wagner have MBA in Project Management from Federal University of Rio de Janeiro (UFRJ) and is certified in Business from Getulio Vargas Foundation (FGV).