The feed filtration system is applied in Hydroprocessing units to remove solid particles and contaminants from the feedstock before it enters the hydroprocessing reactors. It is important to remove particulates from the hydrotreater and hydrocracker feeds to protect the catalyst bed in the reactors and maintain the run-life of catalyst beds at a minimal cost. Feed filters are the last line of defence for hydroprocessing reactors because after that the particles go directly to the catalyst bed.
One of the main purposes of feed filtration is to protect the hydroprocessing catalysts used in the reactors. If these impurities are not removed before the feedstock reaches the reactors, they can foul or deactivate the catalysts, reducing their effectiveness and increasing operational costs. Particulates and metals are permanent poison for the catalyst. Once, the catalyst has plugged with these, it can not be regenerated and the affected catalyst has to be replaced.
Purpose of Feed Filtration in Hydroprocessing Units
To prevent the foulant materials like rust, scales, coke particles, asphatenic compounds etc. from entering the unit, feed filters of adequate capacity and size are installed for a desired frequency of change-out. A pair of filter banks operating in parallel will help to reduce the exposure of the unit to fouling material, as one bank can be changed while the other remains in operation.
1. Protecting Fouling in Hydroprocessing Reactors
If these impurities are not removed before the feedstock reaches the hydrotreating reactors, they can foul or deactivate the catalysts, reducing their effectiveness and increasing operational costs.
2. Preventing Fouling in the equipment
Removing contaminants and particulates from the feedstock helps prevent fouling in the feed exchangers and reactor bed. Fouling occurs when solid particles accumulate on the internal surfaces of equipment and pipelines, leading to reduced flow rates, increased pressure drop, and potential operational disruptions.
3. Enhancing Efficiency
Clean feedstock with lower levels of impurities allows the hydroprocessing unit to operate more efficiently. Reduced fouling and catalyst deactivation mean the unit can run at higher conversion rates and achieve better yields of valuable refined products, such as gasoline and diesel.
4. Extending Catalyst Life
By removing harmful contaminants from the feedstock, feed filtration helps extend the life of the hydroprocessing catalysts. A longer catalyst life translates to less frequent catalyst replacement and lower maintenance costs.
Types of Feed Filtration Systems
The selection of pre-filters depends on various factors such as types of feeds, the size and type of particles present in the feedstock, the desired level of filtration, and the operating conditions of the unit. It is essential to consider the compatibility of the pre-filters with the feedstock composition and the specific hydroprocessing unit requirements. Typical size range of feed filters in Hydroprocessing units is 10~25 microns but mainly a 25-micron size is applied.
There are various types of feed filters applied in hydroprocessing units. Their selection depends upon the types of service and the quantity of particulate matter in the feed.
- Conventional Cartridge Type Feed Filters
- Disposable Cartridge Filters
- Autobackwash Self-cleaning Feed Filters
- Feed Coalscers
1. Conventional Cartridge Type Feed Filters
Conventional cartridge-type feed filters require periodic manual intervention for cleaning and maintenance. Operators must take them offline, disassemble the filter, and manually remove accumulated contaminants to restore filtration efficiency. Despite their labour-intensive nature, they remain a reliable and cost-effective option in applications where automated filters may not be suitable due to specific process requirements or budget constraints.
These filters are applied for clean services like naphtha, kerosene and in some cases for diesel conventional manual cleaning filter system is applied. These are made of stainless steel metal fibre material and have heat, pressure and corrosion resistance. The only drawback with the conventional filters is that manual cleaning is more labour-intensive compared to auto backwash filters.
Disposable Cartridge Filters
In conventional type filters, disposable filters made of polypropylene, nylon or polyphenylene sulphide (PPS) media can also be used for clean services like naphtha. It’s only workable if its normal service life is more than a week otherwise it’s better to use metallic filter elements. Disposable cartridges are also installed in a filter housing with in-service and a second one for standby.
Working of Conventional Manual Cleaning Feed Filters
While in service during filtering mode, unlike auto backwash filters, these filters do not have an automatic cleaning mechanism and rely on manual intervention for cleaning and maintenance. The working of conventional filters involves the following steps;
Inlet and Filtration: The unfiltered feed stream enters the side of the filter station, passing through the filter cartridge walls from outside to inside and then go the outlet of the filtration vessel.
Accumulation of Contaminants: As the process stream flows through the filter medium, contaminants gradually accumulate on the surface or within the filter. As the filter becomes clogged with contaminants, the pressure drop across the filter increases and reached a maximum operating limit.
Manual Cleaning: When the pressure drop reaches a predetermined level or when the filter’s performance deteriorates, the filter undergoes the manual cleaning process. The filter set is taken offline, the assembly is disassembled, cartridges are removed and the accumulated contaminants are removed to restore the filter’s efficiency. The cleaning process of hydroprocessing feed filters is described below.
Inspection and Replacement: During manual cleaning due to physical handling, the filter element is thoroughly inspected for signs of damage or wear. If the element is damaged or worn out, it may need to be replaced.
Reassembly and Restart: After cleaning and inspection, the filter is reassembled, and the cleaned or replaced filter element is installed back into the housing. The filter is then restarted and brought back into operation.
Before taking into service, the oxygen present in the filtration assembly should be removed by purging with Nitrogen or venting to the flare by opening the inlet valve.
2. Autobackwash Self-Cleaning Feed Filters
Auto backwash filtration is employed in Hydrotreating and Hydrocracking process units for dirty service or heavy feeds like HVGO, residues, coker gas oil, etc. Filter cartridges are made of stainless steel. Unlike conventional filters, auto backwash filters are designed to perform automatic cleaning to maintain their efficiency once the predetermined differential pressure or time interval is reached.
When the differential pressure reaches a maximum operating value, the differential pressure indicator transmits a signal to the control system, which automatically triggers the backwashing process. During backflushing, the dirt particles are flushed off of the filtration surface by filtered fluid reversal of flow.
Working of Self Cleaning Auto Backwash Filter
Here’s how the auto backwash filter works;
Accumulation of Contaminants: As the process stream flows through the filter elements, solid particles, debris, and other impurities are retained by the filtering media. Over time, these contaminants accumulate on the surface of the filter, causing an increase in pressure drop across the system.
Automatic Backwashing: To prevent clogging and maintain filtration efficiency, the auto backwash filter is equipped with a backwashing mechanism that triggers at predetermined pressure differential or time intervals. When the pressure drop across the filter reaches a certain level, or after a predefined time, the automatic backwashing process is initiated.
Reversal of Flow: During the backwashing process, the direction of the flow is reversed. Instead of the regular flow direction (from outside to inside the filter element), the process stream is directed from inside to outside. This reversal of flow creates a pressure differential that dislodges and loosens the accumulated contaminants from the filter surface.
Purge and Disposal: As the contaminants are dislodged, they are flushed out of the system and collected in a separate back-washed flow collection vessel which is at low pressure.
Resume Filtration: After the backwashing cycle is completed, the auto backwash filter returns to its regular filtration mode, ready to continue removing impurities from the process stream.
Automated Control and Monitoring: Auto backwash filters are equipped with sophisticated control systems that automatically manage the backwashing cycles based on the pressure drop, flow rate, or time intervals. Normally, PLC programmed system is utilized to control the auto backwash system. Additionally, these filters may have monitoring systems that allow operators to track the performance and efficiency of the filtration process.
Types of Auto Backwash Filtration Systems
1. Internal Backwashing Filters
Internal backwashing uses <2% of the filtered process fluid to reverse flow through the cartridge and flush contaminates from the system. The backwashed fluid going to the drain system is collected and sometimes cooled and send to slop or refinery fuel oil system. Operating pressure can also be a concern, it has to be at least 45 PSI to maintain cleaning efficiencies.
2. External Backwashing Filters
External backwashing simply means that the fluid flowing in the “reversed” direction comes from elsewhere than the process fluid. For some applications, this may be necessary, such as when there’s a chemistry aspect to cleaning the element. External flushing is also required for viscous fluid applications when the process fluid is “valuable” and for systems that have operating pressure <45 PSI.
3. Gas-Assisted Auto Backwash Method
The gas-assist backwash method is used when process flow rates are high or continuous, and uninterrupted flow is required. For backwash, one vessel is isolated and the downstream side of the vessel is pressurized with a controlled quantity of filtered air or other suitable gas. The vessel drain port is rapidly opened, resulting in a hydraulic pulse that “bumps” the collected solids from the filter surface. Forward flow is restored to the vessel and the remaining filter vessels are backwashed sequentially.
During gas assist backwash, the expanding gas bubble forces the liquid through the elements in the reverse direction (inside-out) at a velocity as high as seven times the normal forward flow velocity. This effectively dislodges the accumulated cake from the elements, while significantly reducing the volume of liquid required. Thus, the concentration of the solids discharge is high.
3. Cold Feed Coalescers
For cold feed coming from intermediate storage tanks Coalescer is applied to remove water from the cold feed but Coalescer also serves to remove particulate matter from the cold feed. Usually, cartridge-type filter elements made of glass fiber are employed to remove water from cold hydrocarbon feed.
The feed stream, which contains a mixture of hydrocarbon oil and water, is introduced into the liquid coalescer. As the feedstream passes through the coalescing elements, it encounters a specialized media or surface, often made of hydrophilic materials. These materials have an affinity for water and encourage water droplets to coalesce, forming larger droplets. The larger water droplets, now coalesced, collected and removed from the boot of the vessel.
Coalescing elements are replaced when their water separation efficiency drops or differential pressure is reached to maximum operating value.
Cleaning Methods of Feed Filters
Cleaning stainless steel metallic filter cartridges is essential to maintain their efficiency and prolong their lifespan. Filters made of fiber or disposable are replaced with new ones once. While the cartridges of auto backwash filters can also be cleaned after nearly 6 months if their backwashing efficiency has been decreased.
The cleaning methods you use will depend on the type of contaminants and debris accumulated on the filter cartridges. Each site collaborating with Process Engineer can develop its own cleaning procedure depending upon the type of impurities and intensity of clogging. Here is the general cleaning method for stainless steel metallic filter cartridges;
Safety First: Before starting the cleaning process, ensure that you wear appropriate personal protective equipment (PPE) such as gloves and safety goggles. Beware of the hydrocarbon vapours while dismantling, it’s better to cool the filter and purge with inert gas to remove the vapours.
Remove from System: Dismantle the manway or top cap of the filter assembly and take the stainless steel filter cartridges out of the filtration system before cleaning.
Visual Inspection: Examine the cartridges for visible debris and damage. If there are any significant dents, cracks, or other damages, it may be best to replace the cartridge rather than attempt to clean it.
Steam and Condensate Jetting: Once the filters are removed from the filter housing or vessel, these are initially cleaned by steam or hot condensate water jetting from inside to outside.
Soak in Alkaline Solution: Soak the filter elements with 5~20% concentrated alkaline solution (sodium hydroxide solution or nitric acid) solution for 40~60 minutes. The soaking time can be appropriately extended or shortened according to the pollution degree. Then the filter elements can be blown dry with clean air or again washed with condensate or demineralized water.
Soak in Acidic Solution: After Alkaline solution cleaning the filters are soaked acidic solution (Nitric Acid) 10~15 % concentrated for 40~60 minutes.
After alkaline and acidic solution treatment, the elements can be washed with demineralized water and taken back in service or further treated in the Ultrasonic cleaning method.
Ultrasonic Cleaning Method: The cleaning liquid is acted on by longitudinal waves, which causes it to generate a certain number of small enough vacuum bubbles. These vacuum bubbles will burst when they cannot withstand the pressure, and the stainless steel filter element is washed away by the generated ultrasonic impact force.
Key Operating Parameters Related to Feed Filtration System
Following are some key operating variables of hydroprocessing unit feed filtration systems to ensure their desired performance.
- Differential Pressure: The filter differential pressure is the pressure difference between the clean side and the dirty side of the filter. This parameter indicates the level of contaminants accumulated in the filter. A higher differential pressure suggests a higher level of contamination, and it may trigger the need for cleaning or replacing the filter.
- Feed Temperature: Lower the feed temperature higher will be the differential pressure of the filters. Monitoring the temperature helps ensure that the filter operates within the recommended temperature range.
- Feed Flow Rate: Maintaining a consistent and appropriate flow rate is important for achieving the desired filtration efficiency. Too high or too low flow rates can impact the filter’s performance and may lead to inadequate removal of impurities.
- Service Interval: It is also one of the most important parameters to measure the efficiency of feed filters. If the service interval is decreasing it means that filter efficiency is also reduced.
- Viscosity: The viscosity of the feedstock impacts how easily it can pass through the filter medium. High viscosity may result in slower filtration rates, while low viscosity may lead to increased flow rates and reduced filtration efficiency.
Top References
- Cleaning methods for Stainless Steel Filters
- Filtration and Separations Technologies for Petroleum Refining by Pall Corporation
- Springer Handbook of Petroleum Refinery by Chang Samuel Hsu, Paul R. Robinson (Eds.)
- Fuji Filters Japan Catalogue
