Grading and guard bed catalysts are the macro-porous, grading material at the inlet of the hydrotreating reactor to remove fouling agents and save the main reactive hydrotreating catalyst from fouling and poisoning. The grading and guard bed system works as a filtration system and reduces the pressure drop across the hydrotreating reactor. The grading bed helps, avoiding an unplanned shutdown and increasing catalyst cycle length.
The feedstock of the Hydrotreating unit contains numerous fouling agents that include metals, organo-metallic compounds, corrosion products, olefins, di-olefins. These compounds are a challenge for the hydrotreating catalyst and may cause premature EOR (End of Run) pressure drop conditions if not removed. They can foul the hydrotreating catalyst by depositing it on its surface and increases the pressure drop across the reactor bed thus shortening the cycle length of the hydrotreating catalyst. This can put the hydrodesulfurization process at risk hence the whole refining process will disturb.
Guard Bed Reactors
For the single bed reactors, frequent unit shutdown for catalyst replacement can not be avoided for the feeds containing, asphaltenes and heavy metals. In such cases, several fixed bed reactors are connected in series. In this case, the first reactor is designed as a “guard reactor”. The purpose of the guard reactor is to remove most of the metals with the aim to extend catalyst life in the downstream reactors. The guard reactor is filled primarily with a catalyst possessing a high metal storage capacity. The guard reactors must have high metal storage capacity and have high hydrodemetallization (HDM) activity.
In some cases, a “guard chamber” is placed upstream of the guard reactor, that removes inorganic solids dispersed in heavy feeds. The guard chamber is filled with the lower value solid materials (e.g., clays, minerals, alumina, etc.) to filter off the inorganic solids in a heavy feed stream.
Designing Guard Bed Catalyst for Hydrotreating Catalysts
The number of stages or reactors included in the process is intent on by the content of asphaltenes, metals in heavy feed, the projected daily throughput of the heavy feed, and the desired quality of liquid products. . The vacuum gas oil hydroprocessing reactor requires a graded system, either multilayer bed or multi sections reactor, particularly to increase the yield of middle distillates or to produce the feed for FCC (Fluid Catalytic cracking).
The hydrotreating reactor design determines the guard bed catalysts according to the fouling types and severity in the feedstock. Depth, type, size, and the number of layers of grading beds depend upon the severity of particulates in the feed. Generally, 1~5 layers of different grading materials can be applied. Each layer performs its function for which it is designed and applied. The grading catalyst must have sufficient capacity to remove the fouling agents for the full life span of the catalyst.
A properly designed grading bed for a hydrotreating reactor is highly effective when applied with an appropriate feed filtration system. The feedstock of the hydroprocessing unit is critically analyzed for the identification of the specific contaminants and their quantity in ppm, then the grading solution is applied by the catalyst provider. The guarding and guard materials system can be applied in naphtha hydrotreating, diesel hydrotreating, and residue hydrocracking units as well.
The general sequence of grading and guard materials system is that; the top layer is an inert one that mainly catches the bigger participles, scales, corrosion products, dust, and inorganic particles. The second layer is an active grading having a mild activity and eliminates the highly reactive compounds like Olefins, Di-Olefins through hydrogenation reaction. The third layer removes Arsenic, Silicon, and organic metals. Additional layers can also be applied according to the feed specification.
Types of Grading
1. Inert grading of hydrotreating reactor is used as a bed topping and physically entertains the fouling particles. 2. Active grading of hydrotreating reactor, stops the reactor fouling by chemically reacting. Examples include Olefins & Diolefins saturation; Gums & Polymer formation control.
Shapes of Grading Catalyst
Fig.2 Example of Catalyst Grading
Advantages of Grading or Guard bed Catalyst:
- Reduces pressure drop across the reactor by picking up contaminants and saves the bulk catalyst.
- Even flow distribution over the catalyst bed that results in low radial spread temperature and lower chances of channeling.
- Perform moderate hydrogenation reactions with highly active components to avoid sharp exotherms, hence the coking rate on the catalyst is reduced.
- Exhibits controlled Olefin saturation.
- It stops catalyst milling due to its high strength.
- Prevent crust formation by mild grading catalytic activity which usually occurs at the top of the catalyst bed. Catalyst activity can be incrementally increased from the top to the main bed.
- Removes gums precursors to reduce pressure drop.
- Saves the activity of the main catalyst as fouling agents will penetrate on the surface and block the active sites.
Fig. 03 Comparison between with and without grading in terms of Pressure drop.
Properties of grading and guard materials:
- The top layer is usually an inert material, with a high void fraction and high holding capacity for scale and particulates.
- Support (HDM) hydro-demetalization reactions on their surface. They have high porosity and the ability to trap and accumulate contaminants also called as metal traps.
- Mild reactive, other reactions like olefin saturation along with its main function of metals removal.
- Show stability in severe hydrotreating conditions.
- Are designed for the removal of specific elements like Ni, V, Si, Fe, etc. But can stop also stop other contaminants.
- They have moderate hydrogenation activity, making it appropriate for application with reactive feedstocks such as Di-Olefins and asphaltenes.
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