Hydrocracking Technologies: Producing High Quality Derivates From Residues


One of the biggest challenges for the oil refining industry is raising the profitability or the so-called refining margin face to a scenario with environmental legislations increasingly restrictive, which requires high costly processes and the volatility of the crude barrel price. 
Despite the high investment for hydrocracking units construction, this process is what gives more flexibility to refineries to processing heavy oils, so with lower cost, on the other hand, these oils produces a high quantity of derivates with lower value added and with restricted markets like fuel oils and asphalt. 
The hydrocracking process is normally conducted under severe reaction conditions with temperatures that vary to 300 to 480 oC and pressures between 35 to 260 bar. Due to process severity, hydrocracking units can process a large variety of feed streams, which can vary from gas oils to residues that can be converted into light and medium derivates, with high value added. 
Among the feed streams normally processed in hydrocracking units are the vacuum gas oils, Light Cycle Oil (LCO), decanted oil, coke gas oils, etc. Some of these streams would be hard to process in Fluid Catalytic Cracking Units (FCCU) because of the high contaminants content and the higher carbon residue, wich quickly deactivates the catalyst, in the hydrocracking process the presence of hydrogen minimize these effects. 
According to the catalyst applied in the process and the reaction conditions, the hydrocracking can maximize the feed stream conversion in middle derivates (Diesel and Kerosene), high-quality lubricant production (lower severity process). 
Catalysts applied in hydrocracking processes can be amorphous (alumina and silica-alumina) and crystallines (zeolites) and have bifunctional characterístics, once the cracking reactions (in the acid sites) and hydrogenation (in the metals sites) occurs simultaneously. The active metals used to this process are normally Ni, Co, Mo and W in combination with noble metals like Pt and Pd. 
It’s necessary a synergic effect between the catalyst and the hydrogen because the cracking reactions are exothermic and the hydrogenation reactions are endothermic, so the reaction is conducted under high partial hydrogen pressures and the temperature is controlled in the minimum necessary to convert the feed stream. Despite these characteristic, the hydrocracking global process is exothermic and the reaction temperature control is normally made through cold hydrogen injection between the catalytic beds. 
Figure 1 shows a typical arrangement for hydrocracking process unit with to reactions stages, dedicated to producing medium distilled products (diesel and kerosene). 
Figure 1 – Basic Process Flow Diagram for Two stages Hydrocracking Units 
According to the feed stream quality (contaminant content), is necessary hydrotreating reactors installation upstream of the hydrocracking reactors, these reactors act like guard bed to protect the hydrocracking catalyst. 
The principal contaminant of hydrocracking catalyst is nitrogen, which can be present in two forms: Ammonia and organic nitrogen. 
Ammonia (NH3), produced during the hydrotreating step, have temporary effect reducing the activity of the acid sites, mainly damaging the cracking reactions. In some cases, the increase of ammonia concentration in the catalytic bed is used like an operational variable to control the hydrocracking catalyst activity. The organic nitrogen has permanent effect blocking the catalytic sites and leading to coke deposits on the catalyst. 
As in the hydrotreating cases (HDS, HDN, etc.), the most important operational variables are temperature, hydrogen partial pressure, space velocity and hydrogen/feed ratio. 
Depending on feed stream characteristics (mainly contaminants content) and the process objective (maximize middle distillates or lubricant production) the hydrocracking units can assume different configurations. 
For feed streams with low nitrogen content where the objective is to produce lubricants (partial conversion) is possible adopt a single stage configuration and without the intermediate gas separation, produced during the hydrotreating step, this configuration is presented in Figure 2. The main disadvantage of this configuration is the reduction of the hydrocracking catalyst activity caused by the high concentration of ammonia in the reactor, but this configuration requires lower capital investment.
Figure 2 – Typical Arrangement for Single Stage Hydrocracking Units without Intermediate Gas Separation
Normally for feed streams with low nitrogen content where the objective is to produce middle distillates (diesel and kerosene), the configuration with two reaction stages without intermediate gas separation is the most common. This configuration is showed in Figure 3. 
Figure 3 – Typical Arrangement for Two Stage Hydrocracking Units without Intermediate Gas Separation
Like aforementioned, the disadvantage, in this case, is the high concentration of ammonia and H2S in the hydrocracking reactors, which reduces the catalyst activity. 
The higher costly units are the plants with double stages and intermediate gás separation. These units are employed when the feed stream has high contaminant content (mainly nitrogen) and the refinery looks for the total conversion (to produce middle distillates), this configuration is presented in Figure 4. 
Figure 4 – Typical Arrangement for Two Stage Hydrocracking Units with Intermediate Gas Separation
In this case, the catalytic deactivation process is minimized by the reduction in the NH3 and H2S concentration in the hydrocracking reactor. 
Like cited earlier, the hydrocracking units demand high capital investments, mainly to operate under high hydrogen partial pressures, it’s necessary to install larger hydrogen production units, which is another high costly process. However, face of the crescent demand for high-quality derivates, the investment can be economically attractive. 
The Residue Hydrocracking Units have severity even greater than units dedicated to treating lighter feed streams (gas oils). These units aim to improve the residues quality either by reducing the contaminant content (mainly metals) like an upstream step to other processes, as Residue Fluid Catalytic Cracking (RFCC) or to produce derivates like fuel oil with low sulfur content. 
Residue hydrocracking demand even greater capital investment than gas oils hydrocrackers because these units operate under more severe conditions and furthermore, the operational costs are so higher, mainly due to the high hydrogen consumption and the frequent catalyst replacement. 
Hydrocracking technologies have been widely studied over the years, mainly by countries with large heavy oil reserves like Mexico and Venezuela. The main difference between the available technologies is the reactor characteristics.
Among the hydrocracking Technologies which applies fixed bed reactors, it can be highlighted the RHU technology, licensed by Shell company, Hyvahl technology developed by Axens and the UnionFining Process, developed by UOP. These processes normally operate with low conversion rates with temperatures higher than 400 oC and pressures above 150 bar.
Technologies that use ebullated bed reactors and continuum catalyst replacement allow higher campaign period and higher conversion rates, among these technologies the most known are the H-Oil technology developed by Axens and the LC-Fining Process by Chevron-Lummus. These reactors operate at temperatures above of 450 oC and pressures until 250 bar.
An improvement in relation of ebullated bed technologies is the slurry phase reactors, which can achieve conversions higher than 95 %. In this case, the main available technologies are the HDH process (Hydrocracking-Distillation-Hydrotreatment), developed by PDVSA-Intevep, VEBA-Combicracking Process (VCC) developed by VEBA oil and the EST process (EniSlurry Technology) developed by Italian state oil company ENI.
Despite the high capital investment and the high operational cost, hydrocracking Technologies produces high-quality derivates and can make feasible the production of added value product from residues, which is extremely attractive, mainly for countries that have difficult access to light oils with low contaminants.
 In countries, with a high dependency of middle distillates like Brazil (because his dimensions and the high dependency for road transport), the high-quality middle distillate production from oils with high nitrogen content, indicate that the hydrocracking technology can be a good way to reduce the external dependency of these products.

Dr. Marcio Wagner da Silva

Commenti

Posta un commento