January 2025

Process Optimization

Ebullated bed hydrocracking: A process and mechanical upset scenario

The purpose of this article is to describe, analyze and suggest some technical peculiarities and criticalities that can affect the operation and safety of ebullated bed hydrocracking technology units—a process and mechanical upset scenario during emergency shutdown.

IP: 18.118.142.122

Ebullated bed hydrocracking (EB-HCU) technology is adopted in refining to process vacuum residue feedstocks, around 70 wt% at a 550°C conversion, maximizing the yields in distillates like naphtha, diesel, and light and heavy vacuum gasoils (VGOs), and producing a stable fuel oil with a sulfur content of 1 wt%. 

EB-HCU technology can basically promote and boost the development of converting crude into chemical products. In fact, it can be integrated in a refinery scheme to enhance the conversion of the bottom-of-the-barrel into chemical naphtha and used as feed for downstream mixed feed steam cracker complexes. On the market, the EB-HCU technology is licensed by two companies: Axens with H-Oil and Chevron Lummus Global with LC-Fining. 

The purpose of this article is to describe, analyze and suggest some technical peculiarities and criticalities that can affect the operation and safety of EB-HCU technology units—a process and mechanical upset scenario during emergency shutdown. The content is applicable to reaction section technologies, regardless of the selected licensing company, and both EB-HCU licensed technologies processing schemes include the following main subsystems: 

• A reaction section 

• An atmospheric and vacuum fractionation section 

• A catalyst handling section. 

The reaction section is typically designed with single or multiple trains in parallel, with the dimensions depending on the unit capacity. Given the severe reaction pressure and temperature operative and design conditions, mechanical limitations impose the maximum diameter of a single reactor, and consequently, the single train maximum feed capacity. A single train contains two reactors and two interstage separators to maximize the conversion and optimize the hydrogen (H2) consumption. 

In the reaction section, the feed and H2 are preheated and react in two ebullated bed reactors in series. The reaction section also includes the facilities for feed and H2 preheating, to separate reactors liquid and vapor effluents and treat and recycle H2. 

The reactors are upflow gas/liquid/solid vessels maintained with a minimum temperature differential by recycling liquid product to feed the ebullating pumps. Ebullating pumps can be external or flanged to the reactor bottom, depending on the selected technology. Typical operating conditions are about 420°C and 170 barg. FIG. 1 shows the schematic representation of a typical reaction section of an EB-HCU. 

FIG. 1. A schematic representation of the EB-HCU typical reaction section. 

Typical catalysts use a nickel-molybdenum active phase supported on a porous alumina substrate that is designed to maximize hydrocracking reactions that convert heavy molecules into lighter molecules; therefore, EB-HCU catalysts are compositionally very similar to fixed-bed hydroprocessing catalysts. Catalyst particles are cylindrical extrudates with nominal diameters of about 1 mm. Shaped extrudates (trilobes, etc.) are not typically used in EB-HCU reactors. EB-HCU catalysts must meet stringent physical criteria to ensure good ebullation characteristics and particle integrity in the turbulent reactor fluids. 

The vacuum residue feed to EB-HCU reactor units accounts for several different organic items. It would be impractical to list these components individually—nonetheless, it is possible to identify the main sections reported below: 

• Hydrocracking reactions 

• Desulfurization reactions 

• Nitrogen removal 

• Metal removal 

• Coking. 

The overall reactions are exothermic, and reactor temperatures are typically controlled by operating the upstream preheating section (by the liquid feed heater) and on liquid quench streams (feed quench and heavy vacuum gasoil quench). Adding and withdrawing catalysts during operation is one of the technical peculiarities of the EB-HCU plant that maximizes and ensures constant catalyst activity, allowing the reactor to operate at a constant temperature. Operating at constant reactor temperature and constant catalyst activity minimizes the risk of undesirable side and secondary reactions. 

This is one of the key design benefits that EB-HCU technology grants process and crack feeds containing solids or high asphaltene levels, which would normally plug the fixed-bed reactor systems. The catalyst handling section is designed to operate within daily catalyst withdraw/addition cycles. The catalyst addition rate depends on several aspects and is typically about one catalyst kilogram (kg) for each fresh supplied liquid feed ton. 

Spent catalysts are normally stored in drums or bins. The offsite rejuvenation of hydroprocessing catalysts in the petroleum industry is widely accepted and recommended for the spent EB-HCU catalyst, promoting safety, sparing production time and improving catalyst activity recovery. 

The purging systems are additional specific facilities, typically characterized by a diffused and extended small-bore piping network, serving and integrating the reaction section, preserving the mechanical performance and allowing continuous smooth operation. 

Heavy, dirty and plugging material circulates within the whole reaction section due to the presence of vacuum residue together with reactor products with coke particles, asphaltenes and the potential presence of catalyst particles, given by carryover phenomena from reactors. 

To avoid and prevent the undesirable plugging for instrument hook-ups [pressure transmitter (PT), differential pressure transmitter (DPT), level transmitter (LT) and flow transmitter (FT)], pressure safeties, manual and control valve seats and three purging systems with different purging fluids are present and usually implemented in the reaction section services, directly fluxing and streaming the critical fouling points. The purging systems are: 

• High-pressure H2: Serves the reaction section operating in the vapor phase 

• High-pressure cold vacuum gasoil: Serves the reaction section instruments in contact with the liquid phase 

• High-pressure hot vacuum gasoil: Serves the reaction section control and manual valves in contact with the liquid phase. 

This article will focus on the following items, listed within the specific design of EB-HCU plants with the main related potential issues and upsets: 

• Reactor cutbacks, depressurization systems and overfilling risks 

• Relevant purge system overfilling criticalities and performance. 

In the following sections, the topics are technically described, followed by the main aspects related to design, operation and safety. The study is based on the authors’ plant experience gained on a project during startup and operations. 

MAIN CONTENT AND RESULTS 

The reactor cutback and depressurization system. During normal EB-HCU reactor operations, gas and liquid enter the catalyst bed from the bottom of the reactor and are homogeneously mixed with the ebullated catalyst. Once above the ebullated bed, the gas is separated from a large portion of the liquid. Most of the liquid flows down the recycle loop, through the ebullating pump and back into the bottom of the reactor. In the bottom of the reactor, the liquid combines with the fresh feed and gas flows to re-enter the reactor bed. The upward velocity of the gas and liquid maintains the catalyst bed in ebullition. 

The remaining liquid and gas exit the top of the reactor for separation. This description is applicable to both reactors. Through a series of separators and exchangers, the H-Oil product is separated into various gas and liquid products. 

Process upsets can result in the misdistribution of gas, liquid and catalysts within the reactor and create temperature excursions. The purpose of the reactor cutback and depressurization system is an automatic emergency shutdown (ESD) system that performs the desired actions to provide a controlled reduction in the reaction activity within the reactor and to protect reactors from significant temperature excursion once an emergency process condition has been detected. The process upsets are also called cutback initiators. The typical initiators for cutback and depressurization are: 

• Loss of ebullating pump 

• Loss of ebullation 

• Loss of H2 flow 

• Loss of reactor feed oil 

• High-reactor temperature 

• Manual cutback 

• Manual depressurization. 

Cutback and depressurization actions relieve the reaction in the reactor by reducing the heat input into the reactor and reducing the H2 partial pressure to reduce the reaction rate. The automatic cutback actions are not intended to take all the required steps to control an upset, but rather to place the reactor in safe conditions. The reported typical automatic actions are: 

• Reactor feed oil heater cutback 

• Reactor H2 heater cutback 

• Makeup H2 cutback 

• Reactor depressurization. 

All actions have three final purposes: 

1. Reducing the heat input to reactors operating in fired heaters 

1. Reducing the H2 reaction rate to the reactor 

1. Maximizing the reactors cooling operating on quench streams and separation section air coolers. 

The liquid feed is not paused during these emergency scenarios. For the typical fixed-bed hydrocracker plant, vacuum residue feed pumps are kept in operation and the vacuum gasoil quench streams sent to the reactors are maximized. 

The reactors’ pressures are ramped down simultaneously in a controlled way, opening the depressurization automatic valves to flare. Usually, these valves are located downstream of the high-pressure/low-temperature separator. The pressure is typically ramped down to half of the normal operating pressure within a 10-min to 15-min timespan. 

To depressurize the first reactor in FIG. 1, the differential pressure control valve on the vapor outlet is forced into an open position. This valve opening could lead to severe critical upsets if the action is not properly designed, and the relevant effects are not properly evaluated during the unit design. 

Typically, the first and second reactor pressures are equalized immediately; consequently, the driving force pushing the bottom liquid from the first separator into the second reactor is reduced at zero and the liquid level inside the system of the first reactor/separator increases. Based on the incoming liquid feed rate, the system can overfill and the hot heavy product inside the reactor begins to flow into the first separator outlet vapor line.  

Usually, the vapor line from the first separator up to the first heat exchanger of the heat recovery system is designed for the gas phase only, and the hold-up of the warm high-pressure separator is designed only considering the condensed liquid from the heat recovery system. During this upset scenario, the liquid flow can potentially reach the total liquid feed plus the additional quench liquid streams. Moreover, the increase of liquid flow into the vapor system is also enhanced by the concurrent depressurization. In the worst-case scenario, vapor lines from the warm high-pressure separator and the cold separator can also be affected by liquid overfilling. The ultimate consequence is to have the entire cold reaction section from the warm high-pressure separators reach the H2 recovery section (i.e., high-pressure amine absorber) affected by heavy material entrainment. 

As a consequence, before the plant restarts, a preliminary removal of the reactor material from all cold sections—including air cooler, instruments and lines—is required. During this scenario, the overfilling could potentially affect the high-pressure H2 purging system, as well. 

H2 purges are delivered by the recycle gas compressor at conditions of about 120°C and 180 barg. As per normal operation, H2 flows into the process system due to its higher-pressure level. However, in a transient scenario, when the compressor trips the purging network, pressure can decrease below the reaction pressure, allowing the overfilled heavy and clogging liquid from the reactors to improperly backflow and fill the purging system network. During plant upsets, this critical scenario has been experienced and registered in running the EB-HCU, especially during cutback events. 

Process systems and purging networks are generally connected by a piping arrangement of manual block valves, normally opened in series with piston check valves, installed according to the normal purging mean flow direction. The process/purge interface arrangements are provided for each single purging connection or user; however, no mechanical devices are usually foreseen on the main purging headers to prevent backflow.   

If the process fluid backflow into the purging system occurs, this could expose the purging system network to pressure/temperature conditions, mechanical stresses and liquids for which it is not designed. The main upsets registered in the past during cutbacks were: 

• Mechanical stresses on piping and reaction section vapor system equipment due to two-phase flow and slug flow phenomena, joined by excessive vibration, huge hammering and thermal shock  

• Potential backflow of heavy hot material into the high-pressure H2 purge system. 

The recommended preventive and corrective actions and specific engineering topics to develop and focus on are: 

• Engaging the plant licensors to assess all potential side effects linked to the cutback scenario 

• Engaging the plant licensors and design contractors to assess all potential scenarios leading to purging pressure lower than the reaction section and potential backflow 

• Engaging the plant licensors and design contractors to study the described critical scenario and set up the system to ensure the recycle gas compressor is in operation and maintains continuous flowing of purges into the reaction system (e.g., introduce and adopt a prompt process control system that can bring the compressor itself running on full spillback during a cutback event, reducing the risk of compressor trip when the H2 make-up is stopped) 

• Engaging the engineering contractor to perform piping and equipment design using the licensor’s recommendation, following the cutback or overflow/backfill scenario detailed analysis 

• Engaging the licensors and engineering contractors to design a double dissimilar check valve installation as backflow protection for the H2 purge system’s main distribution header. 

The recommended actions should be taken during the process design and detailed design phases. 

REACTION SECTION PURGING SYSTEMS: HIGH-PRESSURE H2 PURGES BACKFLOW PREVENTION 

Moving forward, the authors analyze a specific critical issue related to the reaction section purging systems that could be experienced during operation, transient scenarios or upsets if not properly studied and engineered during the design phase. Moreover, specific mechanical recommendations to mitigate the associated risks are contextually listed. 

Undesirable process conditions/upsets from the hot fluid in the reaction section could back flow into the H2 purging network. As previously described, the main purging header can be protected from hot heavy fluid backflow by introducing adequate design safeguards. Similarly, the process/purge interface piping arrangements can be consolidated to increase reliability. 

Typically, the check valves specified on the purging piping arrangement by EB-HCU licensors are a piston-check type, which could experience fail close at the backflow for ≤ 50 barg differential pressure and < 50 barg for low flowrate, or coke and foul deposits and buildup that is typical of these heavy dirty services. 

Therefore, detailed and dedicated engineering studies are recommended to determine the proper mitigation/preventive measures (e.g., the double-check valves installation on the piping purge connection to user) that could significantly increase the process/purge interface arrangements reliability (FIG. 2). 

FIG. 2. A schematic representation of a typical H2 purging network distribution to the user. 

Moreover, the piston-check valves specified by EB-HCU licensors could experience disc chattering at the usual low flowrate of the purging fluid. It is recommended to conduct a specific study of the phenomena during the detailed engineering phase. 

Takeaways. Considering the process parameters and the specific operating upset risks, vacuum residue EB-HCUs are one of the most complicated process units in refinery schemes, marked by the typical high pour point and high viscosity fluid, and further complicated by coke particles and fine catalysts that can be carried over into the separation section.  

Additionally, at the operative conditions, the treated feed well is prone to cracking, so reactor parameters and hydraulic conditions must be carefully monitored to avoid massive coke formations or overfilling events that could cause an extended unplanned shutdown, extraordinary maintenance works and undesirable production losses. 

This article has highlighted some potential process upsets and their effects during the initial EB-HCU operation, which can run into unstable process parameters with instrumentation still not fully reliable and operators not completely confident with the unit peculiarities and criticalities. 

Moreover, capitalizing on the authors’ personal experience in previous EB-HCU EPC projects, the article has proposed and listed preventive actions and recommended specific engineering topics to investigate and study during the project design phase to mitigate and prevent cutbacks and related overfilling risks. 

The Authors

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