Hydrotreater Reactor Sulfiding 101 | RefinerLink
Cor
Cor

RL Blogs

image
Hydrotreater Reactor Sulfiding 101

By Reactor Resources

Nov 27, 2017
 

An introduction to sulfiding of hydrotreating catalysts.

 
 

To reduce the air pollutants SOx and NOx that are formed by internal combustion engines, most countries strictly limit the sulfur and nitrogen content of motor fuels.  The primary process refineries use for removing these contaminants from gasoline, diesel, and jet fuel is referred to as hydrotreating or hydroprocessing.

 

The hydrotreating process involves mixing hydrogen with sour hydrocarbon feed inside a reactor filled with Cobalt/Molybdenum (CoMo) or Nickel/Molybdenum (NiMo) catalysts.  In their active form, CoMo and NiMo catalyze the reactions that convert sulfur and nitrogen contaminants into H2S and NH3. 

 

Catalyst Basics

 

Hydrotreating catalysts are produced by coating alumina extrudates with potentially active metals such as cobalt, nickel, molybdenum, and tungsten.  In the catalyst production process, these metals are deposited on the alumina substrate in their benign oxide form and must be converted to the corresponding sulfide before they will catalyze hydrodesulfurization (HDS) and hydrodenitrification (HDN) reactions. 

 

After a hydrotreater or hydrocracker reactor is loaded with fresh catalyst, the activation step, referred to as “sulfiding”, is accomplished by reacting the metal oxides with hydrogen sulfide (H2S) in the presence of hydrogen.  This process is often referred to as “presulfiding” since the sulfiding process is carried out prior to resuming operation of the hydrotreating unit.

 

 

Catalyst sulfiding involves the following chemical reactions:

 



The Sulfiding Process

 

The sulfiding process involves passing feed spiked with a sulfiding agent over the catalyst bed in a carefully controlled procedure that includes several temperature holds.  As the feed and spiking agent are heated in the presence of hydrogen, the sulfur compound will readily decompose to form the H2S required to complete the sulfiding reactions.

 

Sulfiding Agents

 

The sulfiding agents most commonly used by refiners today are dimethyl sulfide (DMDS) and tertiary-butyl polysulfide (TBPS). DMDS is used in the majority of sulfiding applications due to its higher sulfur content (68% vs. 54%) and lower cost.  It also decomposes in two steps, reducing the chance of reactor exotherms that can result from sulfiding reactions.  TBPS is used in some cases where a lower decomposition temperature is desirable.  The table below compares the two most commonly used sulfiding agents.

 


 

Each catalyst manufacturer has their own procedure for achieving maximum performance from their catalysts, but in general they all follow a process similar to the profile shown below.  This illustration represents a sulfiding regimen with DMDS as the sulfiding agent.

 

 

Temperature Profile for Sulfiding Hydrotreating Catalysts

 

The initial phase involves a drying step to remove any water that may remain in the system from cleaning during the turnaround, as well as moisture entrained within the catalyst pores.  Hydrotreating catalysts are hydroscopic and can easily absorb 2-4% water when exposed to humid air during the loading process. 

 

If the catalyst bed is heated too quickly while moisture remains in the catalyst pores, steam will rapidly form that can cause physical damage to the catalyst particles.  This phenomenon is commonly referred to as the “popcorn effect”, since rapid steam formation can fracture the catalyst similar to the way popcorn is formed by heating.

 

Catalyst Wetting

 

After the bed is thoroughly dried (typically 2-4 hours in the range of 200-250F), the “wetting step” can begin.  This may seem counter-intuitive since the catalyst was just dried in the previous step, but in this case we are referring to wetting the catalyst with warm hydrocarbon feed. 

 

A heat wave will normally be seen during the wetting step that results from the heat of absorption released when hydrocarbon wicks into the pores of the extremely dry catalyst.  Wetting ensures that all of the catalyst particles are evenly coated with hydrocarbon, leading to improved distribution once the sulfiding step begins.

 

Once wetting is complete, the temperature of the reactor can be ramped up to the level at which the sulfiding agent will be injected.  This temperature range varies from 360-420F depending on the catalyst manufacturer, the type of catalyst, and the reactor pressure. 

 

Sulfide Injection

 

After injection begins, catalyst bed temperatures must be monitored closely since DMDS decomposition and the sulfiding reaction are both exothermic.  A typical guideline will limit the delta T across each catalyst bed to 50F.  Again, this will vary depending on the size and type of unit being sulfided and the mass flux through the bed.

 

The H2S level of the gas effluent from the reactor also needs to be closely monitored once injection begins.  This is normally handled by sampling the gas stream from open sample ports with draeger tubes or an online H2S analyzer.

 

H2S Breakthrough

 

The reactor temperature is normally held below 450F for 10-12 hours until H2S “breakthrough” occurs.  This step is often referred to as the “first sulfiding plateau”.  Breakthrough is determined when the H2S concentration of the gas exiting the reactor reaches 3000-5000 ppm.  Prior to breakthrough, very little H2S will be seen in the gas effluent since the oxide catalyst is reacting with and consuming all of the sulfur injected into the feed. 

 

Breakthrough is a signal that the catalyst particles in the reactor are saturated with sulfur and indicates that the reactor temperature can be raised without harming the oxide catalyst.  If bed temperatures are raised prematurely (> 450F), reduction of the metal oxides will occur, preventing formation of the active sulfide form of the catalytic metals.

 

It should be noted that water is a by-product of the sulfiding reaction and its formation signifies that the active sulfide form of the catalyst is being produced.  The amount of byproduct water produced will be approximately 8-10 wt% of the catalyst weight.  In addition, ~2 SCF of hydrogen will be consumed per pound of catalyst loaded in the reactor.

 

Once the catalyst bed reaches 600-660F (the final hold temperature varies with catalyst manufacturer), the second sulfiding plateau can begin.  This phase will normally last for 4-8 hours.  During the second temperature hold, sulfiding reaction kinetics will initially accelerate, leading to an increased consumption of H2S in the recycle gas.  To compensate, the injection rate of the sulfiding agent is increased while the H2S content is closely monitored. 

 

Completion of the sulfiding process is normally indicated by the H2S content of the recycle gas quickly rising to levels exceeding 20,000 ppm (2.0%). Once the calculated amount of sulfur being injected to the unit equals the amount of sulfur exiting the reactor, sulfiding is complete and chemical injection should cease.

 

 

Proper sulfiding of hydrotreating catalysts is critical for obtaining optimum HDS and HDN activity from a hydroprocessing unit.  The sulfiding process must be carefully monitored to avoid reduction of the metal oxides prior to the formation of the active sulfides and to ensure that the active metal sites are properly and completely formed.

 

Having adequate information, such as sulfiding agent injection rates and the H2S content of the reactor gas effluent, allows precise control of the sulfiding process and avoids problems caused by excessively sour gas overloading the recycle compressor.

 

For more information on the optimum procedure and techniques for sulfiding, contact the pros at Reactor Resources

 
Enjoy this content? Join our Free Newsletter    
  • Brian Thompson :   Or - you can save time and the hassle of baby-sitting the unit for 24-36 hours and simply purchase pre-sulfided, pre-activated catalyst. Ask your catalyst representative about this option.

    Sep 20, 2015

  • Randy Alexander :   Catalyst preactivation and/or presulfurization is a good option for small units or those with limited furnace capabilities. Note that the cost for these ex-situ services can be exponentially higher than in-situ sulfiding with DMDS injection. As Mr. Thompson suggests, contact your catalyst representative for recommendations.

    Sep 21, 2015

  • Nelmo Fernandes :   this article is very complete and sum the main concerns in this important procedure. I would just add some aspects. First, the catalyst storage and the following loading procedure are very important phases that precede the sulfiding. Precautions should be taken to protect the catalyst from coming into direct contact with water, since it will decrease the catalyst particle strength and may affect the catalyst activity. During the dry-out we usually use nitrogen to slowly heat the catalyst bed. Second aspect is the operation pressure. To avoid reduction of the metal oxides and to avoid hydrogen embrittlement of the reactor, the pressure should never exceed metallurgical constraints and should preferably be not more than 45 bar (660 psi). Third, for the good wetting of the catalyst, the feed rate during wetting and sulfiding should be high, preferably at the design feedrate. Fourth, never use cracked feedstocks during wetting and sulfiding, because this will result in a significant lower activity of the catalyst due to premature coking. And never, never use cracked feedstocks until run the unit for, at least, four days.

    Sep 22, 2015

  • Bob Stults :   Eurecat provides catalyst presulfiding services to refinery and petrochemical customers worldwide. The Totsucat process has been applied to over 110 million pounds of catalyst going into hydrotreating/hydrocracking applications. Totsucat is a complete ex-situ sulfiding and activation of catalyst, not just a pre-sulfurizing of catalyst which provides little advantage to the customer. Totsucat can be provided in a passivated form which allows handling and loading in air. Along with standard preactivation, Eurecat provides Totsucat CFP and Totsucat HC-AP. Totsucat CFP allows customers to start-up with cracked feed, eliminating the logistic and economic issues related to storing enough straight run feed to start-up and cracked material for the 3-day waiting period. Totsucat HC-AP lets customers start-up their hydrocrackers quickly and safely without using ammonia injection or having to use a high nitrogen feed. Why do customers use Totsucat? Critical Path Units-Saves 1-2-3+ days or more of in-situ start-up time Furnace Limited Units-Cannot achieve required in-situ sulfiding temperatures Downstream Sulfur Sensitive Units-Reformer and Isom units sensitive to H2S from upstream unit in-situ sulfiding Units with problematic in-situ sulfiding issues Cannot Flare During Start-Up-New regulations do not allow a start-up flaring exemption. Flaring issues during start-up are avoided with Totsucat Hydrocracker Ammonia Injection Issues-Totsucat HC-AP eliminates need for ammonia injection Need to Process Cracked Feed Immediately-Totsucat CFP allows refinery to start processing cracked feed (coker naphtha, LCO, etc) immediately The benefits of Totsucat for the customer are significant and many customers easily justify the added cost. Some of the benefits are: Load-and-Go reactor startups Minimized flaring to meet new flare/emissions regulations No exotherms upon start-up Minimal sour water formation No additional H2 needed at startup No odors No handling of sulfiding chemicals Upset conditions will not damage the catalyst Catalyst Performance is Maximized! Totsucat + Passivation allows for loading under air Considering the advantages provided to the customer by Totsucat, for all types sizes of units, customers who evaluate the real costs of in-situ sulfiding (including stand-by time and various un-planned delays), have been able to find value in the benefits provided by Totsucat.

    Sep 25, 2015

  • Randy Alexander :   Good point, Nelmo, regarding cracked stocks. Sulfiding should be carried out using straight run feed. Catalyst manufacturers typically recommend at least a 3 day delay before gradually introducing cracked stocks to the unit. This "seasoning" time allows catalytic activity to moderate so that reactive cracked stocks can be fed to the unit without the risk of excess coke and gum formation.

    Sep 30, 2015

Do NOT follow this link or you will be banned from the site!

Blog Topics

 

People You May Know