Combination of Bottom Barrel Conversion Capacity and Maximum Olefins Yield – Value Addition to Crude Oil through FCC Technologies

Introduction and Context

           In the current scenario of the downstream industry, the capacity to add value to bottom barrel streams combined to a high capacity to produce petrochemical intermediates offers a significant competitive advantage to refiners once the recent forecasts indicate a growing market of petrochemicals. The capacity to processing heavier crudes creates the opportunity to operate with low-cost raw material, allowing better refining margins.

Fluid Catalytic Cracking (FCC) is one of the main processes which give higher operational flexibility and profitability to refiners once is capable of processing bottom barrel streams and can be optimized to maximize the yield of light olefins against transportation fuels. 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.  

Fluid Catalytic Cracking Process Unit (FCCU)

The typical Catalytic Cracking Unit feed stream 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. 

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.

Residue Fluid Catalytic Cracking Units (RFCCU)

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.

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 3. 

Figure 3 – 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 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 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. 

Petrochemical FCC Units

As aforementioned, due to the current trend of reduction in transportation fuels demand, the fluid catalytic cracking units (FCC), as well as the residue fluid catalytic cracking units (RFCC), are suffering optimization actions or capital investments aiming to improve the yield of light olefins, mainly propylene, against intermediates to transportation fuels like naphtha and light cycle oil (LCO).  

According to the current market scenario, despite the high implementation cost, can be economically attractive to the installation of catalytic cracking units focused to produce petrochemical intermediates, called petrochemical FCC. Figure 4 shows a block diagram for the PetroFCC? technology developed by UOP Company aiming to maximize the production of petrochemical intermediates and promotes a closer integration of the downstream industry, another available technology is the process HS-FCC? developed by Axens Company. 

Figure 4 – PetroFCC? Process Technology by UOP Company. 

Figure 5 presents the results of a comparative study, carried out by Technip Company, showing the yields obtained by conventional FCC units, optimized to olefins (FCC to olefins), and the HS-FCC? designed to maximize the production of petrochemical intermediates. 

Figure 5 – Comparative Study between Conventional FCCs and Petrochemical FCC (HS-FCC?)

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. It’s observed a higher reaction temperature (TRX) and a cat/oil ratio five times higher when are compared the conventional process units and the petrochemical FCC (HS-FCC?), leading to a growth of the light olefins yield (Ethylene + Propylene + C4=’s) from 14 % to 40%. Figure 6 presents a typical scheme for a gas separation section for a fluid catalytic cracking unit. 

Figure 6 – 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 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.

The installation of petrochemical catalytic cracking units requires a deep economic study taking into account the high capital investment and higher operational costs, however, some forecasts indicate a growth of 4,0 % per year to the market of petrochemical intermediates until 2025. In this scenario can be attractive the capital investment aiming to raise the market share in the petrochemical sector, allowing then a favorable competitive positioning to the refiner.  In refining hardware with conventional FCC units, further, than the higher temperature and catalyst circulation rates, it’s possible to apply the addition of catalysts additives like the zeolitic material  ZSM-5 that can raise the olefins yield close to 9,0% in some cases when compared with the original catalyst. This alternative raises the operational costs, however, as aforementioned can be economically attractive considering the petrochemical market forecasts.  

Conclusion

Despite the great operational flexibility and profitability which fluid catalytic cracking technology gives for the refineries, some new projects have dismissed these units in the refining scheme, mainly when the new refinery objective is to maximize middle distillates products (Diesel and Kerosene) once this is not the focus of the fluid catalytic cracking unit, despite these cases observed in markets with high demand by transportation fuels like Brazil, closer integration between refining and petrochemical assets can represent a satisfactory alternative to both sectors in the downstream industry, ensuring higher availability of raw material to the petrochemical sector and growth of added value and market to the derivatives produced by the refiners, in this sense, the petrochemical FCC units can develop a key role. In this environment, the FCC technologies can offer an attractive combination of bottom barrel conversion capacity and olefins maximization, creating a good scenario to higher refining margins and economic sustainability in the downstream market.

References:

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

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

MALLER, A.; GBORDZOE, E. High Severity Fluidized Catalytic Cracking (HS-FCC?): From concept to commercialization – Technip Stone & Webster Technical Presentation to REFCOMM?, 2016.

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

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).