Chemical, mechanical treatment options reduce hydroprocessor fouling

Bernard C. Groce
Betz Process Chemicals Inc.
The Woodlands, Tex.

Hydroprocessing Unit Flow Diagram

The processing of opportunity crudes and the need to meet stricter environmental regulations in the production of distillates and finished fuels have increased the benefit of the hydroprocessing unit to the refiner.

With this potential for increased margins and more environmentally friendly fuel products comes increased risk of fouling in hydroprocessing units.

Increased fouling can reduce unit reliability and increase maintenance and operating costs. The refiner has several options available to help minimize the fouling and maximize the unit's profitability and flexibility.

One of the two commonly selected options is to allocate capital for a mechanical solution to address a specific cause of fouling. The other option is the use of a chemical treatment program.

Process chemicals are often the option of choice because of their ability to address several different fouling mechanisms simultaneously. Additionally, refiners maintain greater operating flexibility with chemical solutions, as they can easily be changed in rapid response to variations in feed characteristics or process conditions.

Processing trends

To remain profitable, today's refiner must maintain a tenuous balance between current market conditions, environmental compliance, and total operating costs.

Historically, hydroprocessing units have played a key role in the production of higher-quality, lower-sulfur distillate fuels and gasoline. Recently, that role has become more important because refiners are processing a variety of inexpensive opportunity crudes. These lower-gravity, higher-sulfur crudes produce feeds that require hydroprocessing to improve product quality and achieve environmental objectives.

Refining opportunity crudes improves profit margins, but also can introduce difficult processing challenges, such as increased fouling potential. Current environmental regulations and concerns for lower-sulfur fuels also have dictated the expanded use of hydroprocessing units.

To meet these environmental goals, it is now necessary to hydrotreat some previously untreated product streams. In some instances, refiners are simply increasing the severity of their existing hydrotreating process to further reduce sulfur concentrations.

Continual shifts in market demand have also had a significant impact on refiners. They now rely heavily on hydrotreating and hydrocracking units to help produce a wide variety of finished products from many different opportunity crudes. Consequently, maintaining hydroprocessing unit reliability has become a priority for most refiners.

Hydroprocessing

Hydroprocessing saturates the olefinic (unsaturated) compounds that are too unstable to be used in finished fuels or gasoline, and removes impurities such as nitrogen and sulfur compounds. A hydroprocessing unit may produce an intermediate stream or a final product.

In a hydroprocessing unit, preheat exchangers raise the temperature of the feed stream entering a furnace (Fig. 1). The furnace then heats the feed to the temperature required for the catalytic reaction with hydrogen.

Hydrotreating and hydrocracking are two similar processes that fall under the general category of hydroprocessing.

Hydrotreating is a milder process in which olefins are saturated, and nitrogen and sulfur compounds are removed without changing the boiling point range of the feed being processed.

Hydrocracking is a more severe process in which a heavier stream is hydrotreated and catalytically cracked. The hydrocracking process helps produce more-valuable, lighter products from heavier feeds by changing the boiling point range.

Fouling

Fouling in hydroprocessing units results in four major economic concerns: increased energy costs, higher maintenance costs, reduced throughput, and restricted in operating flexibility.

Fouling in either the preheat exchangers or the furnace results in a loss of heat transfer. To compensate for the heat loss, more fuel gas is used to provide increased furnace duty.

Fouling also can restrict flow through the exchangers or the reactor bed, significantly limiting unit throughput. When fouling becomes severe and degrades unit performance, a shutdown often is required to clean exchangers or remove plugged catalyst.

The loss in production during a shutdown can represent a major economic penalty to the refiner. Even if the exchangers can be cleaned on-line, throughput often is reduced.

Effective fouling control will maintain heat transfer efficiency, increase overall unit reliability, and reduce the maintenance costs associated with frequent cleanings.

Fouling mechanisms

The two predominant causes of fouling in hydroprocessing units are deposition and polymerization.

Deposition occurs when particles become too large to remain entrained in a flowing liquid or gas stream, and fall out of solution. The tendency of a particle to deposit is a function of:

• The particle's physical characteristics, including size and density

• The bulk fluid's properties

• The fluid velocity.

The smaller the particle, the less likely it is to fall out of solution and cause fouling.

As the bulk fluid is heated, its density and viscosity lessen, increasing the tendency for particles to deposit. The deposition potential also increases as the bulk fluid velocity decreases.

Deposits can comprise organic or inorganic particulates, or a combination of both. Fouling occurs when these deposits settle onto the heat-transfer surfaces of heat exchangers, onto furnace tubes, or when they become trapped on the top of the catalyst bed.

Inorganic deposits generally are iron sulfide or other corrosion products, catalyst particles, or inorganic salts entrained in the hydroprocessor feed stream. The feed may contain organic particles, or they can be formed in storage tanks or during transport by various polymerization mechanisms. These particles often contribute to the sediment found in tanks.

The formation of organic polymer is a more complex issue. Polymer may be formed by several mechanisms as the process stream is heated.

The polymerization reactions are influenced by numerous factors, including temperature, trace components, fluid type, and how a fluid is stored or handled. All of these factors need to be evaluated to determine the specific polymerization mechanism or mechanisms, and to identify effective solutions.

Polymerization

Polymerization mechanisms can be grouped into three general categories: free radical, metal catalyzed, and non-free radical. The mechanism (or mechanisms) contributing to the formation of polymer or polymer precursors depends on the reactive components in the fluid and the process conditions. It also is likely that multiple polymerization mechanisms will occur simultaneously.

Free radical

Free radical polymerization occurs when a free radical is formed and reacts with monomers to form long-chain polymers of significant molecular size.

Initiation of free-radical polymerization occurs more easily in the presence of increasing amounts of light, heat, or both. This type of polymerization can even occur in the low-temperature conditions found in storage tanks or during transport.

Highly unsaturated fluids or cracked stocks are especially susceptible to this type of polymerization. Auto-oxidation is a different type of free-radical reaction, in which its products (peroxides) can initiate the free-radical polymerization.

Metal catalyzed

There are numerous metal species that, in very low concentrations, can either catalyze polymerization reactions or initiate free-radical polymerization mechanisms through reduction-oxidation (redox) reactions. These reactions make it easier to form free radicals at lower temperatures.

The metals can take the form of metal salts or specific metal complexes. Metal salts may themselves deposit, or become catalytically active at higher temperatures and contribute to polymer formation. Some catalytically active metals are: copper, iron, nickel, vanadium, chromium, and manganese.

Non-free-radical

Non-free-radical polymerization also forms polymer but does not involve the formation of free radicals.

A classic example of this mechanism is condensation polymerization, where two molecules (or polymers) combine to form a larger molecule and water (as occurs in ester or amide formation). This new, larger compound can continue to react with other reactive species in the feed stream to make higher-molecular-weight polymers.

Another type of non-free-radical reaction is the Diels-Alder reaction. In this reaction, two unsaturated molecules combine without producing any by-products. Not only are higher-molecular-weight polymer species formed, sites for free-radical polymerization initiation are created.

Fouling potential

Fouling potential is a function of a number of variables, including feed stream characteristics, operating conditions, feed storage and handling, and unit design. A thorough understanding of how these factors interrelate and impact fouling is critical to finding the appropriate mix of chemical, mechanical, and operating solutions.

Feed characterization

A wide range of streams, from naphtha through resids, can be fed to hydroprocessing units. Cracked, heavier gas oils and resids usually have a greater fouling tendency than straight-run naphtha or kerosine streams.

The potential for a stream to foul is, in part, a function of the various trace compounds in the feed. Consequently, knowing the type and source of the various hydroprocessor feeds is an important first step in identifying potential fouling precursors.

Feed stream analyses are used to identify trace components in the feed which may contribute to fouling by deposition or polymerization. Table 1 presents examples of feed stream characterizations from hydroprocessing units where fouling has occurred.

The following tests are important tools in analyzing hydroprocessor feeds:

• The filterable solids test measures the amount of sediment or particles in a feed stream. It is often used as a preliminary screen to determine if corrosion products or organic sediment are present in feed from storage tanks.

• A feed stream's bromine number indicates its degree of unsaturation. The higher the bromine number, the more likely it is that the stream is unstable and potentially more prone to auto-oxidation and free-radical polymerization.

• The potential gum test subjects the fluid to an oxygen overpressure and determines its sensitivity to oxygen-initiated polymerization mechanisms. The more gum formed in this test, the greater the potential that oxygen will contribute to the formation of polymer.

• Neutralization or acid number tests measure the feed stream's acidity. Basicity is determined by the basic nitrogen test, which identifies the presence of organic bases such as amines. Increasing acidity or basicity may indicate a greater tendency for components in the feed to polymerize by non-free-radical mechanisms.

• Metals analysis can identify the presence of corrosion products or trace metals that may catalyze or initiate polymerization mechanisms.

Analytical testing also can help identify other stream components such as carbonyls, pyrrole nitrogen, and mercaptans. These substances are known to contribute to the formation of polymer in hydroprocessing systems.

In addition to the feed stream analysis, which determines fouling precursors, a deposit analysis can provide valuable information to help pinpoint the predominant fouling mechanism. The deposit analysis also can quantify the inorganic corrosion products, inorganic salts, or organic materials that have contributed to the deposit.

Infrared analysis of the deposit's organic fraction can identify key functional groups that have contributed to polymer formation. Numerous specialized techniques, such as pyrol ysis/mass spectrometry and nu clear magnetic residence testing, can provide additional information for more complex fouling problems.

Storage and handling

A feed stream's storage and handling before hydroprocessing can affect that stream's fouling potential. Feeds from storage tanks often have a greater fouling tendency because of the potential for oxygen contamination.

Unstable, reactive feeds readily react with oxygen to form peroxides through auto-oxidation. The resulting peroxides initiate polymerization or generate fouling precursors at higher temperatures, thus causing fouling in the unit.

Some integrated refineries have the ability to send feeds directly to the hydroprocessor from the producing unit; thereby, avoiding storage in tanks. Feeds which have bypassed tank storage typically foul less than feeds from tanks or imported feeds, because exposure to oxygen and the time available for auto-oxidation reactions to occur are decreased.

Blanketing the feed tank with inert gases or stripping oxygen out of the feed stream can reduce oxygen-initiated free-radical polymerization. Oxygen contamination in storage tanks, however, is not always the dominant factor contributing to polymerization in a hydroprocessing unit.

Many polymerization reactions occur at higher temperatures in the absence of oxygen.

Fouling control

Once the fouling mechanism has been identified from feed stream characterizations, deposit analyses, or both, a cost-effective solution to hydroprocessor unit fouling can be implemented.

Fouling control measures include process modifications, mechanical design solutions, chemical treatment programs, or a combination of the three. Chemical solutions are selected most often for several reasons.

No capital investment is required for chemical treatment programs. And such programs can provide immediate response to changing feeds or fouling conditions. This helps maximize operating flexibility and provides improved profitability.

Process modification

Process temperatures, flow rate, and the type of feed processed in a hydroprocessing unit are key factors in fouling.

Modifying process conditions or segregating feeds minimizes their fouling potential. This option often is undesirable because it may reduce the flexibility to hydrotreat or hydrocrack poorer-quality streams.

Reducing operating temperatures may minimize polymer formation in preheat exchangers, but may not provide sufficient reaction temperature to adequately desulfurize the stream.

Mechanical solutions

A number of mechanical solutions have been implemented by refiners to solve specific hydroprocessor fouling problems.

Feed filters have been installed to minimize the deposition potential of particulates in feed from storage tanks or imported streams. They often are most effective in minimizing fouling from periodic upsets.

These filter systems require regular maintenance and only capture particles of a certain size. Untrapped, smaller particles can continue to agglomerate and present a deposition potential further downstream.

These two limitations minimize the effectiveness of filters for ongoing fouling control.

Oxygen strippers minimize the potential for oxygen-initiated polymerization by reducing the dissolved oxygen concentration in the feed stream. This may not completely resolve an oxygen-related problem, however, because even very low levels of oxygen can contribute to fouling. Other limitations include high capital investment and ongoing maintenance.

Another mechanical solution involves unit design modifications that allow exchangers to be cleaned on-line. This modification can eliminate unplanned shutdowns caused by exchanger fouling, but results in reduced operating rates while selected exchangers are cleaned.

Some refineries even utilize a spare, parallel preheat train, at significant capital expense, to minimize throughput limitations caused by preheat exchanger fouling.

These mechanical solutions offer some positive results; however, high capital costs and only partial reductions in energy and maintenance penalties limit their use. While a mechanical solution may only address one variable that is causing fouling, a chemical solution can minimize the impact of several variables at once.

Chemical treatment

Effective chemical solutions can be custom-designed for specific fouling mechanisms. Chemical solutions allow the flexibility to address the changing process conditions that impact fouling.

A customized treatment program can address organic and inorganic deposition with dispersants. In addition, the formation of organic sediment or polymer in storage tanks or during processing can be minimized with polymerization inhibitors.

Dispersants are surface-active compounds that reduce particulate deposition. This is accomplished by limiting the particle size growth, increasing the surface interaction between the particle and the bulk fluid, or keeping insoluble species separated through steric stabilization.

A dispersant's effectiveness is measured by its ability to keep particles suspended, even at lower velocities.

Different dispersants have unique characteristics. Their effectiveness varies with bulk fluid composition, operating temperature range, and particulate type.

Particulate types include inorganic corrosion products, inorganic salts, organic sediment, and polymers. Each type requires a different type of dispersant. Selection of the most appropriate dispersant, or combination of dispersants, is critical to a successful chemical treatment program.

Selection of a polymerization inhibitor also is specific to each case. A variety of precursors can contribute to polymer or sediment formation by different polymerization mechanisms, and no one inhibitor will be effective in all cases.

The selection of an appropriate inhibitor is based on the identification of the dominant polymerization mechanism. An effective inhibitor program may employ one or more inhibitors in combination, because multiple mechanisms may be occurring simultaneously.

Free-radical polymerization inhibitors (chain stoppers, antioxidants, and oxygen scavengers) minimize the formation of polymers or sediment caused by unstable compounds in storage tanks.

Metal coordinators can deactivate catalytically active metal sites, thus reducing their ability to initiate polymerization.

Non-free-radical or condensation polymerization mechanisms can be addressed by specific inhibitors that block these reactions. Free-radical inhibitors are not effective at reducing non-free-radical polymerization.

Case studies

The details of three case studies reveal how chemical treatment programs can reduce hydrotreater fouling.

The first case study shows how identifying the predominant polymerization mechanism can lead to the most appropriate solution to fouling.

The second study reveals the true benefit of a polymerization inhibitor realized when the treatment program was discontinued. Increased reactor pressure drop caused by catalyst fouling forced the unit to shut down.

In the third case, specific polymerization mechanisms called for the addition of specific inhibitors to minimize fouling effectively.

Case Study No. 1

A refiner suspected nonblanketed storage was responsible for polymer formation and subsequent fouling in the hydrotreater preheat exchangers.

The refiner installed an oxygen stripper to reduce the feed's dissolved oxygen content and the associated fouling potential. As a result, unit run lengths increased from 1 month to 3 months-far short of the refiner's expectations.

Further improvement in run length was achieved by the implementation of a chemical program.

Feed stream characterizations and deposit analyses revealed that inorganics and some polymerization mechanisms, unrelated to oxygen contamination, were the predominant causes of the fouling. A dual chemical program that combined a dispersant with a free-radical polymerization inhibitor was initiated.

As a result of both the mechanical and chemical approaches, unit run length increased to more than 1 year.

Case Study No. 2

For more than a decade, a hydroprocessing unit had been successfully treated with a polymerization inhibitor that helped minimize reactor pressure drop. The inhibitor reduced the formation of polymer that could deposit and foul the reactor bed.

This treatment program was credited with limiting reactor pressure drop and extending run length, with no loss in throughput. Unit shutdowns were taken when required as a result of catalyst activity loss, and not because of fouling.

As a result of the dramatic changes in the crude and the unit feed since the start of the chemical application, the appropriateness of the inhibitor program was questioned.

The program was discontinued 14 months into a run during which the total reactor pressure drop had increased less than 2 psi.

Within 1 month of discontinuing the treatment program, the reactor bed pressure drop began increasing at a rate of 2.5 psi/month.

During the next 20 months, the reactor pressure drop increased by 60 psi. At that time, throughput limitations caused by a plugged reactor bed forced a unit shutdown.

During 15 years of chemical treatment, the unit had never been shut down for this reason. Upon start-up, the inhibitor program was reinstated and the unit has operated for 1 year with minimal increase in reactor pressure drop and no loss in throughput.

Case Study No. 3

Hydrotreater feed rates were reduced after 3 months because of severe preheat exchanger fouling. Three months later, the unit was shut down to clean the exchangers.

The economic penalties were substantial:

• Increased energy costs to make up heat transfer loss

• Higher maintenance costs related to the exchanger cleanings

• The cost of lost production during a shutdown.

Limited operating flexibility also reduced the refiner's ability to maximize production during intervals of high seasonal demand.

Over a period of several years, a variety of free-radical polymerization inhibitors were added in an attempt to solve the problem. These attempts failed.

After a detailed investigation and research effort, a non-free-radical polymerization mechanism was identified as the predominant cause of fouling.

An appropriate non-free-radical inhibitor was then added. By using this inhibitor, the refiner was able to extend the run length to 1 year, with no reduction in throughput.

This program has successfully minimized fouling in this unit for more than 5 years. In recent years, the hydrotreating severity has even been increased to produce lower-sulfur fuels. The unit has experienced no increase in fouling as a result.

Acknowledgment

The author gratefully acknowledges the contributions and technical review provided by the following members of Betz Process Chemicals' refinery antifoulant team: Dr. Alan Goliaszewski, Dr. Paul Roling, Steve Szymczak, and Bill Witzig.

The Author