Low base strength neutralizers for crude unit operations were commercially introduced in the early 1990’s. This paper discusses evolution of the technology and experiences since that time. Case histories are presented, including both dry and water washed overhead systems, along with strengths and limitations of this technology which have been identified.
In the distillation section of refinery crude units, hydrochloric acid is formed from dissociation of salt in crude oil in the presence of water vapor. For example, the equation for magnesium chloride is:
(Equation in full paper)
The hydrochloric acid, being volatile, distills upwards with the lighter fractions and is absorbed into condensing water. When this happens, the pH of the condensing water is acidic, with pH’s as low as 1.5 reported in the initial condensation. Because of this low pH, basic compounds commonly known as neutralizers are injected into the process to elevate the pH of the condensate. Neutralizers used are nitrogen based, such as ammonia or low molecular weight amines.
Until recently the preferred characteristics of neutralizers included their volatility in the process, the state of the chloride salt which they formed in the process, and their pKa , or base strength. Volatility was considered helpful for distributing the neutralizer in the system the same as the vaporous HCI. It was taught that if the salt formed was a liquid, the salt would not accumulate in the system thereby preventing the salt from fouling heat exchangers and towers. And a high base strength was desirable in order to raise the pH of the initially condensing water.
Despite the application of neutralizers using these characteristics, the refining industry is still experiencing corrosion failures and fouling in crude unit overheads, and deposit analyses often report the presence of chloride. If the chloride in these deposits can be reduced or controlled, these instances should be eliminated or at least reduced.
In 1994, a NACE paper was presented by Edmondson and Lehrer1 introducing a combination of characteristics which describe the relationship behavior of neutralizers and HCI in a vapor stream. The principle was that these two components combine into a salt or “neutralize” as a function of the partial pressures of the two components.
(Equation in full paper)
Specifically, increasing the concentration of either the acid, HCI, or the base increases its partial pressure in the vapor, and increases the temperature at which a salt between the two will form as the vapors pass through the condensing system. Also, higher base strength, or pKa, increases the chemical/thermodynamic attraction between the neutralizer and chloride, which also increases the temperature at which a salt will form2.
Equilibrium constants of salt formation have been measured in the laboratory. The vapor pressures of the salts are orders of magnitude lower than the vapor pressure of the separate components. In an overhead, and in modeling, this means that once the salt forms, it can precipitate from the vapor. The temperature at which this occurs is known as the “salt point.”
