The Fluid Catalytic Cracking Technologies as Competitive Advantage in the Downstream Industry – Maximizing Petrochemicals from Residue

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 a highlighted position in the strategy of the refiners to reach competitiveness in the market. Recent technology developments like additive manufacturing (3 D printing) can deeply change the transportation fuels demand as well as the growing practice of home office like demonstrated in the current COVID 19 crisis, facing this scenario, the look for alternatives to transportation fuels can be transformed in a survival question to refiners in middle term and the petrochemicals can offer an interesting alternative.

           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, mainly related to the maximization of petrochemicals from bottom barrel streams.

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 into 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 to 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 (mainly 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 the 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 to 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 to 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 Lummus.

Residue Fluid Catalytic Cracking (RFCC) Technologies – Dealing with Heavy Feeds

One variation of the fluid catalytic cracking that has been widely applied in the last years is the Residue Fluid Catalytic Cracking (RFCC). In this case, the feed stream to the process is basically the bottom stream from the atmospheric distillation column, called an atmospheric residue, that have high carbon residue and higher contaminants content like metals, nitrogen, and sulfur.

Due to the feed stream characteristics, the residue catalytic cracking units require design and optimization changes. The higher levels of residual carbon in the feed stream lead to higher temperatures in the catalyst regeneration step and a lower catalyst circulation rate to keep the reactor in constant temperature, this fact reduces the catalyst/oil ratio that leads to lower conversion and selectivity. To avoid these effects, the RFCC units normally rely on catalyst coolers, as presented in Figure 4. 

Figure 4 – Catalyst Cooler Process Arrangement for a Typical RFCC Unit (Handbook of Petroleum Refining Processes, 2004)

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, furthermore, helps the refinery thermal balance, once produces high-pressure steam. The use of a catalyst cooler is also necessary when the unit is designed to operate under total combustion mode, in this case, the heat release rate is higher due to the total burn of carbon to CO2, 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.

As pointed earlier, the feed streams characteristics to RFCC units require modifications when compared with the conventional fluid catalytic cracking. The presence of higher content of nitrogen compounds leads to an accelerated process of catalyst deactivation through acid sites neutralization, the presence of metals like nickel, sodium, and vanadium raise the coke deposition on the catalyst and lead to a higher production of hydrogen and gases, besides that, reduces the catalyst lifecycle through the zeolitic matrix degradation. Beyond these factors, heavier feed streams normally have high aromatics content that is refractory to the cracking reactions, leading to a higher coke deposition rate and lower conversion.   

Due to these operation conditions, the residue fluid catalytic cracking units present higher catalyst consumption when compared with the conventional process, this fact raises considerably the operational costs of the RFCC units. However, the most modern units have applied specific catalysts to process residual feed streams, in this case, the catalyst has a higher porosity aiming to allow a better adaptation to the high aromatics content, furthermore, the catalyst needs to have a higher metals tolerance.  

The control of contaminants content in the feed stream or his effects is a fundamental step to the residue fluid catalytic cracking process. Sodium content can be minimized through an adequate crude oil desalting process and the effects of nickel (dehydrogenation reactions) can be reduced by the dosage of antimony compounds that act like a neutralizing agent of the nickel dehydrogenation activity, reducing the generation of low added value gases, in its turn, the vanadium effects can be controlled through the addition of rare earth to the catalyst, like cerium compounds.  The addition of these compounds needs to be deeply studied once raises significantly the catalyst cost.

The use of visbreaking units to treat the feed streams to RFCC units is a process scheme adopted by some refiners, in these cases, the most significant effect in the reduction in the residual carbon, however, due to his higher effectiveness, the tendency in the last decades is to treat the bottom barrels streams in deep hydrotreating or hydrocracking units before to pump for RFCC units, with this processing scheme it’s possible to achieve lower contaminants content, mainly metals, leading to a higher catalyst lifecycle.  Furthermore, the hydroprocessing has the advantage of the reduction of the sulfur content in the unit intermediate streams, minimizing the necessity or severity of posterior treatments, a clear disadvantage of this refining scheme is the high hydrogen consumption that raises significantly the operational costs.

 Like to the conventional FCC units, the main operational variables to RFCC units are the reaction temperature, normally considered in the highest point in the reactor (also called riser), feed stream temperature, feed stream quality, feed stream flow rate, and catalyst quality. It’s relevant to quote that the conventional FCC units can process atmospheric residue as the feed stream, however, it’s necessary to control the content of the contaminants, mainly metals, which requires processing lighter crudes with higher costs that raise the operational costs and reduces the flexibility of the refiner in relation to the crude oil supplier.

Some of the most relevant residue fluid catalytic cracking technologies available commercially are the R2R? by Axens Company, the INDMAX? process licensed by Lummus Company, and the RxPro? process developed by the UOP Company.  

The Petrochemical FCC Alternative – Maximum Olefins Production

The major part of the catalytic cracking units is optimized to maximize transportation fuels, especially gasoline, however, face to the current scenario of the refining industry some units have been optimized to maximize the production of light olefins (ethylene, propylene, and butenes). Units focused on this goal have these operational conditions severely changed, raising the cracking rate.   

An example of FCC technology developed to maximize the production of petrochemical intermediates is the RxPRO? process by UOP Company, this process combines a petrochemical FCC and separation processes optimized to produce raw materials to the petrochemical process plants, as presented in Figure 5. Other available technologies are the HS-FCC? process commercialized by Axens Company, and INDMAX? process licensed by Lummus Company. The basic process flow diagram for HS-FCC? technology is presented in Figure 6.

Figure 5 – RxPRO? Process Technology by UOP Company. 

It’s important to take into account that both technologies presented in Figures 5 and 6 are based on Petrochemical FCC units that present special design due to the severe operating conditions.

Figure 6 – HS-FCC? Process Technology by Axens Company. 

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

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. As aforementioned, the catalyst cooler necessary when the unit is designed to operate under total combustion mode due to the higher heat release rate.

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.

Propylene Recovery Section - More Petrochemicals and Less Fuels

The growing demand for petrochemicals leads some refiners to install propylene recovery units aiming to allow the maximization of light olefins yield in his refining hardware. Among the light olefins, propylene is one of the most relevant petrochemical intermediates due to the high demand and added value.

The propylene can be applied as an intermediate to the production of some fundamental products, for example:

·        Acrylonitrile;

·        Propylene Oxide;

·        Cumene;

·        Acrylic Acid;

·        Polypropylene;

Propylene can be produced through conventional processes like Steam Cracking and Fluid Catalytic Cracking (FCC) or through directed processes like metathesis of ethylene and butane, propane dehydrogenation, olefins cracking, Methanol to Olefins processes (MTO), among others. Currently, the major part of the propylene market is supplied by steam cracking units, but close to 28 % of the global propylene demand is from the separation of LPG produced in Fluid Catalytic Cracking Units (FCC).

           Normally, the LPG produced in FCC units contains close to 30 % of propylene and the added value of the propylene is close to 2,5 times of the LPG. According to the local market, the installation of propylene separation units presents an attractive return over investment. Despite the advantage, a side effect of the propylene separation from LPG is that the fuel stays heavier leading to specifications issues, mainly in colder regions, in these cases alternatives are to segregate the butanes and send this stream to the gasoline pool, add propane to the LPG or add LPG from natural gas. It’s important to take into account that some of these alternatives reduce the LPG offer, which can be a severe restriction according to the market demand.

A great challenge in the propylene production process is the propane and propylene separation step. The separation is generally hard by simple distillation because the relative volatility between propylene and propane is close to 1.1. This fact generally conducts to distillation columns with a large number of equilibrium stages and high internal reflux flow rates.

There are two technologies normally employed in propylene-propane separation towers that are known as Heat-Pump and High-Pressure configurations.

The high-pressure technology applies a traditional separation process that uses a condenser with cooling water to promotes the condensation of top products, in this case, it’s necessary to apply sufficient pressure to promote the condensation of products in the ambient temperature. Furthermore, the reboiler uses steam or another available hot source. The adoption of a high-pressure separation route requires a great availability of low-pressure steam in the refining hardware, in some cases, this can be a restrictive characteristic and the heat pump configuration is more attractive, despite the higher capital requirements.

The application of heat pump technology allows a decrease in the operating pressure by close of 20 bar to 10 bar, this fact increases the relative volatility of propylene-propane, making the separation process easier and, consequently, reducing the number of equilibrium stages and internal reflux flow rate required for the separation.

           Normally, when the separation process by distillation is hard (with relative volatilities lower than 1.5) the uses of heat pump technology shows more attractive.

           Furthermore, some variables need to be considered during the choice of the best technology for the propylene separation process like availability of utilities, temperature gap in the column, and installation cost.

           Normally, the propylene is produced in the refineries with to specifications. The polymer grade that is most common and has higher added value with a purity of 99,5 % (minimum) this grade is directed to the polypropylene market. The chemical grade where the purity varies between 90 to 95% is normally directed to other uses. A complete process flow diagram for a typical propylene separation unit applying heat pump configuration is presented in Figure 7.

Figure 7 – Typical Process Flow Diagram for an FCC Propylene Separation Unit Applying Heat Pump Configuration.

The LPG from the FCC unit is pumped to a depropanizer column where the light fraction (essentially a mixture of propane and propylene) is recovered in the top of the column and sent to a deethanizer column while the bottom (butanes) is pumped to LPG or gasoline pool, according to the refining configuration. The top stream of the deethanizer column (lighter fraction) is sent back to FCC where is incorporated into refinery fuel gas pool, or in some cases can be directed to petrochemical plants to recover the light olefins (mainly ethylene) present in the stream while the bottom of the deethanizer column is pumped to the C3 splitter column, where the separation of propane x propylene is carried out. The propane recovered in the bottom of the C3 splitter is sent to the LPG pool where the propylene is sent to the propylene storage park. The feed stream passes through a caustic wash treating aiming to remove some contaminants that can lead to deleterious effect to petrochemical processes, an example is the carbonyl sulfide (COS) that can be produced in the FCC (through the reaction between CO and S in the Riser).

The Synergy of FCC and Hydrocracking Units – Deep Conversion Refining Scheme

           Sometimes the hydrocracking and FCC technologies are faced by competitors technologies in the refining hardware due to the similarities of feed streams that are processed in these units. In some refining schemes, the mild hydrocracking units can be applied as a pretreatment step to FCC units, especially to bottom barrel streams with high metals content that are severe poison to FCC catalysts, furthermore, the mild hydrocracking process can reduce the residual carbon to FCC feed, raising the performance of FCC unit and improving the yield of light products like naphtha, LPG, and olefins. Figure 8 presents an example of a high integrated refining configuration relying on hydrocracking and FCC technologies.

Figure 8 – Example of an Integrated Refining Focusing on Petrochemicals Scheme by UOP Company.

As presented in Figure 8, the integrated refining scheme relies on deep residue upgrading technologies as hydrocracking and fluid catalytic cracking (FCC) is capable to reach the production of high-quality petrochemicals, according to the market trends.

Considering the great flexibility of deep hydrocracking technologies that are capable to convert feed stream varying from gas oils to residue, an attractive alternative to improve the bottom barrel conversion capacity is to process in the hydrocracking units the uncracked residue in the FCC unit aiming to improve the yield of high added value derivatives in the refining hardware, mainly middle distillates like diesel and kerosene. Despite these advantages, it's important taking into account the effects of cracked feeds in the hydrocracking units, the FCC cycle oils present high aromaticity that is normally refractory to cracking reactions as well as refractory sulfur components, raising the sulfur content in the final products and reduction in diesel cetane number, on the other side, normally presents low basic nitrogen content that is a poison to the hydrocracking catalysts.

Conclusion

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.

Recent forecasts indicate that the production of petrochemical intermediates will account for most of the crude oil consumption in the medium term. In this scenario, maximizing petrochemical intermediates in the refining hardware and closer integration with petrochemical assets should be a downstream industry trend in the coming years, even refiners with capital investment constraints will be under pressure under the risk to lose market share and competitiveness, in these cases, actions with low relative cost to maximize petrochemicals can be economically attractive, again the FCC technology can maximize the added value to the downstream industry due to their operational flexibility.

References:

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

LAMBERT, N.; OGASAWARA, I.; ABBA, I.; REDHWI, H.; SANTNER, C. HS-FCC for Propylene: Concept to Commercial Operation. PTQ Magazine, 2014.

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

SPEIGHT, J.G. Heavy and Extra-Heavy Oil Upgrading Technologies. 1st ed. Elsevier Press, 2013.

VU, T.; RITCHIE, J. Naphtha Complex Optimization for Petrochemical Production, UOP Company, 2019.

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 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 the Federal University of Rio de Janeiro (UFRJ) and is certified in Business from Getulio Vargas Foundation (FGV).