Regeneration of Hydrotreating Catalysts

Regeneration is the process of restoring the activity of the catalyst by removing coke from its surface. It also involves the recovery of the physical properties of the catalyst. Regeneration of hydrotreating catalysts is essential because of the significant rise of hydroprocessing catalyst volumes, reduced life cycle due to increased feed severity, environmental concerns related to the disposal of spent catalysts, and above all the financial benefits related to regeneration. In this blog, I have discussed general aspects of the regeneration of hydrotreating catalysts. More about pollution sources and controls in hydroprocessing unit can be found in my previous blog “ Controlling Pollution in Hydrotreating Units”

In the Hydroprocessing units, once the catalyst has completed its life, indicated by the end of run operating and reaction conditions and high differential pressure is considered a “spent catalyst” and is removed from the process. Spent catalyst, depending upon the operation severity is either sent for regeneration or treated for metals recovery.

Regeneration of hydrotreating catalysts can be in-situ (at the plant site) or ex-situ (away from the plant site) but ex-situ (off-site) regeneration is preferred because of high catalyst recovery, experienced and professional staff availability, downtime, controlled burning of coke, minimum damage to pore structure and fewer environmental issues. A spare batch of catalysts is required for sending the catalyst for off-site regeneration.

The average life cycle of hydrotreating catalysts is 3~5 years depending upon the service and operating conditions. Spent hydrotreating catalysts can be regenerated up to 3 times for less severe operations and 1~2 times for high severity conditions. The hydrotreating catalyst containing coke can easily be regenerated as compared to the catalyst that has been poisoned with metals. Spent catalyst contains up to 25 % carbon and a maximum of 5 % of metal contaminants (vanadium, iron, arsenic, silica, sodium). Metal contaminants are the precursors that restrict the maximum recovery of the catalyst.

Coke deposit is almost the same at all beds of the catalyst when an equal temperature profile is maintained across the Hydrotreating Reactor whereas with the ascending temperature profile more coke will be on the downstream beds which operate at high temperatures. More about coke formation and its mitigation on hydrotreating catalysts can be found in my previous blog ” Reducing coke formation in hydrotreating catalysts”

Comparison of regeneration and rejuvenation

Catalyst Regeneration is different from catalyst rejuvenation as regeneration only involves coke burning while in catalyst rejuvenation metals contaminants are also removed in addition to coke removal. In catalyst rejuvenation, selective leaching of contaminant metals from spent catalysts is carried out. Therefore, after regeneration 85~90 % activity of the catalyst is recovered while in rejuvenation more than 95 % of the activity of the spent catalyst can be recovered.

Methods of Regeneration of Hydrotreating Catalysts

There are various methods available in the market to regenerate Hydrotreating catalysts.

1. Oxidative Regeneration Method

The oxidative Regenerative process is the most widely used for catalyst regeneration which involves the burning of coke with diluted air or oxygen, steam, or steam air mixtures and ozone may also be applied as an oxidizing agent. But oxidative regeneration with diluted air is widely used on a commercial scale for in-situ or ex-situ. During oxidative regeneration oxygen flow and temperature are the most critical parameters that have significant effects on catalyst regeneration. Uncontrolled temperatures may lead to hot spot formation, catalyst sintering, and collapsing pores.

2. Reductive Regeneration

Another method of coke burning called reductive regeneration using hydrogen can also be applied but this method is not widely accepted commercially. This process requires higher temperatures for regeneration as compared to oxidative regeneration, but still, its coke removal efficiency is low. that regeneration In this method, coke is converted to methane CH4.

Hydrotreating Catalyst Regeneration Process

For ex-situ regeneration of spent catalyst involves the following steps are typically performed,

Unit shut down

Sweeping to remove liquid hydrocarbons and hot H2 stripping of hydrotreating catalyst at a temperature near the design temperature to remove oil, grease, and some coke from the catalyst.
Unloading and packaging

Unloading of the spent catalyst under nitrogen blanketing in standard containers. Unloading should be in sequence, mostly it is unloaded from the reactor bottom through the catalyst dumping nozzles by gravity. In addition, vacuum unloading can also be applied from unloading from the top side of the reactor.
Good quality labeling

It is essential for sampling, analyzing, and segregating a contaminated portion of the catalyst. Good labeling tells the trend of contaminants on the catalysts and helps in decision-making about the regeneration of the catalyst. Generally, spent catalyst barrels or drums should show the barrel number, reactor tag, catalyst bed number, and catalyst bed top, middle, and bottom. Shipping of the spent catalyst to the regeneration site.
Sampling and analysis

of the spent catalyst are necessary to identify the composition of the spent catalyst in order to find out its regenerable efficiency. Laboratory scale appropriate chemical and physical analysis of spent catalyst, regeneration, and activity testing helps in decision making about the percentage and specifications of the catalyst to be regenerated. A high level of metallic contaminants is present at the top layer of the bed of the catalyst.
Spent Catalyst Segregation

After sampling and analysis, the catalyst with high contaminants that can not be regenerated is segregated from the spent catalyst. Normally, the top layer should be separated during unloading. In addition, fines that are also non-regenerable must be removed.
Before regeneration of the spent catalyst oil from the catalyst is removed by the de-oiling step. In the oxidative regenerative method, in the presence of oxygen coke is burnt and mainly converted to CO2. Other gases CO, NO2, N2O, SO2, and SO2 will also produce along with CO2. Careful control and monitoring of temperature are required to recover the properties of regenerated catalyst near the fresh one. High temperature could lead to activity loss and structural changes in the regenerated catalyst. The temperature lower than the optimum design will not completely burn off the coke and higher than the design will result in loss of surface area and pore volume.
Comparison of properties of the regenerated catalyst with fresh catalyst. Compare pore volume, surface area, crushing size, and particle size distribution.
Testing of the regenerated catalyst on the pilot plant will provide reliable information and trusted decisions. Pilot plant resting can depict; 1. catalyst activity, kinetic rate constants, activation energy. 2. Catalyst response to a change in temperature pressure, space velocity, etc. 3. An estimate of relative catalyst stability
Reactivation of regenerated catalyst involves applying a chelating agent to the regenerated catalyst same as a fresh type –II catalyst. Application of chelating agents (Oxalic acid, citric acid, glycols, etc.) may result in significant enhancement of active metal dispersion. The final step of activation of regenerated catalyst is achieved by (ex-situ or in situ) sulfiding.

Industrial regeneration Processes

Various commercial processes are present in the market and these utilize mainly two processes a moving belt reactor or kiln type reactor to burn the coke. Detail of some of these can be seen on their links.

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