Neutralising amine selection for crude units

Making a proper selection of neutralising amine chemistry is a challenging task.

ProCorr Consulting Services

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Article Summary

Even though crude units have been around for a long time, maintaining long term reliable operation continues to be a challenge. The unit design, operating conditions, feedstocks, and corrosion control programme must all work together to achieve reliable operation (see Figure 1). If any part of the system is out of sync, the whole unit will suffer. With constantly changing feedstocks and operating conditions, as well as some unique challenges posed by many opportunity crudes, it can be a challenge to keep everything running properly.

The neutraliser system is a critical part of the corrosion control programme on crude tower overhead systems. This article will focus on understanding and applying some basic concepts to the design of neutraliser systems and on some more subtle aspects of neutraliser selection that are often overlooked.

Corrosion control basics
Some of the residual salts remaining in desalted crude hydrolyse to form HCl in the crude furnace. The HCl then ends up in the overhead system of the atmospheric tower. There can also be light organic acids in the crude oil as well as light organic acids formed from the thermal degradation of naphthenic acids in high TAN crudes, which also put additional acids into the overhead system. Neutraliser demand can easily increase by 50-100% when switching to a high TAN crude slate. Opportunity crudes often are difficult to desalt, which increases the amount of HCl formed. They may also contain tramp amine byproducts of H2S scavengers.

If nothing is done to neutralise these acids, the condensation of steam in the overhead system will produce a highly corrosive, very low pH steam condensate stream. To control pH and reduce corrosion, bases such as ammonia or amines (referred to commonly as neutraliser or neutralising amine) are injected into the overhead system. If these neutralising compounds are improperly employed, they can also lead to fouling and/or corrosion through the formation of ammonium chloride or amine hydrochloride salts.

To prevent formation of salts and to quickly dilute any acids present, a water wash stream is often injected into the overhead system. The water wash, which is typically water recycled from the overhead accumulator, can be injected into the overhead line or into the inlets of heat exchangers. The net water from the overhead accumulator contains a mixture of organic and inorganic acids and bases. This water is commonly used as wash water in the desalter, which provides a potential route for these acids and bases to get back to the crude tower. This recycling of amines will be discussed in more detail in the desalter section.
Neutraliser selection primer

Neutralising amine selection is typically based on a few key parameters. Basically, the neutralising amine must be a strong enough base to raise the pH at the water dew point enough to control corrosion at acceptable levels. It also must not form salts ahead of the initial point where free water is present, either from condensation or injection of water wash. It should be readily available at a reasonable price. Petersen, Lordo, and McAteer1 go into more depth on this subject.

The nature of any neutraliser salts that might form is an important consideration in neutraliser selection. Some amines, such as ethylene diamine (EDA), will form a solid salt which is a fouling and under deposit corrosion concern. Other amines, such as monoethanol amine (MEA), form liquid salts at typical process conditions. The liquid salts tend to corrode more aggressively than solid salts. Liquid salts tend to be more of a corrosion concern with some fouling potential from the corrosion products that form, whereas solid salts tend to be more of a fouling concern with some level of under deposit corrosion also occurring.

The salt point, that is the temperature where the chloride salt of the amine begins to form as a free phase, is another important consideration in neutraliser selection. The goal is to select an amine neutraliser that does not begin to form a salt ahead of the water dew point. After the bulk water dew point is reached, the water solubility of the salts precludes formation of a separate salt phase. This approach to neutraliser selection provides the pH control desired without the formation of corrosion and fouling inducing salts. In the absence of a water wash it can be challenging to find an amine that will actually accomplish its mission without forming salts.

Because the amines tend to behave independently of each other in forming salts, amine blends are sometimes used to reduce the salt formation temperature. For instance, a 50/50 molar blend of two different amines would cut the amine partial pressure in half compared to the use of 100% of either amine. Some suppliers may use five or more amines blended into a neutraliser product.
For any given amine, the salt point or sublimation point is defined by this equation:

Kp = Pamine * PHCl             [1]

Kp is the dissociation constant for the salt, which is a strong function of temperature. Pamine and PHCl are the partial pressures of the amine and HCl respectively. The value of Kp defines the maximum amount of amine and HCl that can exist in the vapour phase at that temperature. If the product of the partial pressures of the amine and chloride present is greater than the dissociation constant Kp then salts will form. If the temperature where the salts form is ahead of the bulk water dew point then a corrosive salt phase will form. Ironically, the product used to control corrosion can help cause corrosion if misapplied.

The following discussion covers a number of different crude tower design and operating parameters that can impact neutraliser selection:
- Overhead water wash vs no water wash
- Tramp amines
- Overhead temperature
- Desalting

Water wash vs no water wash
The use of an overhead water wash typically increases the flexibility the refinery has when it comes to selecting a neutraliser. The water wash forces an immediate water dew point, which greatly reduces the risk of salt formation that could otherwise occur during the more gradual cooling that happens in overhead condensers. With no perceived risk of salt formation, refiners and chemical suppliers are typically happy to use a stronger base with higher salt forming tendencies because these neutralisers are cheap and effective for the intended purpose of neutralising HCl. Water wash also reduces the risks associated with changing process conditions such as temperature and chloride concentrations, assuming that the water wash rate is adjusted to account for upward shifts in overhead temperature. When water wash is injected close to the neutraliser injection point, there is practically speaking no amount of neutraliser that could be added which would lead directly to salt formation in the overhead system.

There is a risk, however, that is often overlooked. This is the risk of salt formation in the atmospheric tower when overhead water is recycled back to the desalter. Amines are organic bases, and as such they have at least some affinity for the hydrocarbon phase. How much of the amine partitions into the crude oil rather than the water is primarily a function of the distribution coefficient and the relative rates of crude oil and wash water. Water entrainment from the desalter also increases the effective amount of partitioning to the desalted crude.

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