Performance Monitoring of Hydrotreating Catalyst
Hydrotreating is one of the most important catalytic processes in the oil refinery to produce valuable products to meet stringent environmental regulations. The hydrotreating process is utilized to remove Sulfur, Nitrogen, and metals from Naphtha, Diesel, Kerosene, etc.
With increased emphasis on ultra-low sulfur in the refinery products, it becomes extremely critical to know the performance of the hydrotreater, particularly from a catalyst stability or deactivation point of view, so that unit can be optimized timely or a replacement schedule can be established.
Following but not limited to parameters are examined to find the performance of the reactor. Initially, WABT and catalyst deactivation rates should be considered. If the deactivation rates are higher then a detailed analysis of all other related parameters is required to find the root cause.
Parameters Affecting performance of the Hydrotreating Reactor
Following are the key performance indicators of the hydrotreating catalyst;
1. Weight Average Bed Temperature (WABT)
This is the average temperature of the catalyst based on the weight of the catalyst. From the WABT we can easily examine the current activity of the catalyst and can predict the remaining life.
2. Deactivation Rate
It is the average deactivation temperature per month of the operation. It shows the remaining operating window for the catalyst temperature. We can also use the reactor inlet temperature for finding the deactivation rate but at the same time, we have to consider reactor exotherms as well. It can be calculated as follows; Deactivation Rate= (Current Average Temperature – SOR Temperature) / Total nos. of months in service
3. Radial Spread
This temperature is the indication of process fluid distribution across the catalyst bed. Greater than allowed limit shows mall distribution of process fluid hence channeling in the catalyst. This in the end will result in unequal utilization of the catalyst and a smaller life span.
4. Impact of Feed Contaminants on Reactor Performance
This is probably the most important factor for measuring the performance of the catalyst. If it remains within the design then the catalyst will show maximum efficiency and higher than the design limit will increase reaction severity.
- Increased rates of Sulfur, Nitrogen, Aromatics, and Olefins.
- High cracked stock or VGO flow because it contains heavy precursors.
- Feed with high distillation endpoints or high specific gravity will cause more coke formation on the catalyst along with difficult desired reactions.
- Heavy feeds also contain more metals that will increase differential pressure across the reactor.
5. Product Specifications
Reactor temperatures needed to be controlled according to the final requirement of the diesel pool. For example, if your requirement is to control the Sulfur less than 350 ppm then it is better to control near 300~350 ppm. But if it is maintained at 250 ppm average then you will compromise catalyst activity.
6. Recycle Gas Rate or Gas to Oil ratio
Every plant is designed with minimum gas to oil ratio. The gas flow provides the Hydrogen for reaction and also serves as a carrier of the heat of reaction. The low flow of gas will result in a higher temperature of the catalyst, resulting in a speedy coking rate. The greater the gas rate greater will be the life span of the catalyst.
7. Recycle Gas Purity
Greater the purity of the recycle gas greater will be the Hydrogen partial pressure, hence lower deactivation rates of the catalyst. If the H2S concentration has been increased in the system then it will have adverse effects on the catalyst activity. Amine scrubber should always be in service to remove H2S from the recycle gas.
8. Operating Pressure
The operating pressure of the system must be at the designed set point. Lower system pressure results in the lower partial pressure of Hydrogen, hence a higher deactivation rate.
9. Feed Rate or LHSV
The greater the feed rates greater will be the deactivation rate of the catalyst. It is better to always monitor the feed rates per unit volume of the catalyst and maintain as recommended. High rates will cause a high coking rate, high temperatures to meet the product quality, and high differential pressure across the reactor.
10. Catalyst Temperature
Catalyst temperature is the variable that is controlled according to the product quality requirement. The general rule is to maintain the catalyst temperature as minimum as possible while achieving the required product Sulfur. The greater the catalyst temperature greater will be the deactivation rate of the catalyst.
11. Bed differential temperature
It also shows the activity of the catalyst. Greater DT shows a greater rate of reaction, greater exotherms across the reactor hence greater activity. But if the differential temperature decreases, this shows that the activity of the catalyst has been reduced.
12. Differential Pressure of the Reactor
This indicates that the reactor has been loaded with metals or particles. Metals are the permanent poison for the catalyst which comes with the feed. Another reason for high differential pressure is the coking on the catalyst. If the reactor has reached the designed deferential pressure then complete or partial catalyst replacement would be required.
13 Coke Formation
Coke is a carbonaceous deposition on the active sites of the catalyst. It can be formed with almost all feeds and the coke rate increases as the molecular weight and boiling range of the feed goes up. Polymerization or polycondensation are the main reactions that lead to coking. Heavy feeds, high temperatures, and low gas to oil ratio contribute to the coke formation. Please visit my blog “Reducing Coke Formation on Hydrotreating Catalysts“.