Jul-2012

Minimise corrosion while maximising distillate

Reducing atmospheric fractionator overhead temperatures to maximise middle distillate production requires a full understanding of resulting corrosion mechanisms

Brandon Payne
GE Water & Process Technologies

Article Summary

Sustained growth in the demand for jet fuel, diesel and other middle distillate products is expected to have a continuing impact on unit operations, product pricing, product selection and refining margins into the foreseeable future. As more and more new facilities come online to supply the demand for tighter product sulphur specifications, refiners will continue to maximise distillate production in their atmospheric distillation units to take advantage of favourable product pricing. However, maximising the production of these fuel streams requires a continual assessment of the entire processing system beyond the mechanical capability of pumps, piping and valves to ensure reliable operation of the unit in a market environment that favours distillate production. As refineries continue to lower tower top temperatures in an effort to increase product draws in the distillate sections of the column, the conditions for introducing salt fouling and corrosion mechanisms into areas that previously were not affected come to the forefront.

Refiners must address the hazards of unmonitored distillate maximisation on corrosion in the crude distillation column top section and overhead system. In this article, overhead corrosion control strategies and guidelines are discussed to help refiners maintain reliable unit operation while maximising distillate production.

Overhead salt point and lower tower top temperatures
Crude unit overhead corrosion deals with corrosion affecting the upper sections of the crude unit atmospheric fractionation column, including the top tower trays, overhead condenser system and top pumparound circuits. Corrosion in the crude unit overhead system is primarily due to acid attack at the initial water condensation point (ICP), resulting in low pH conditions and the associated aggressive corrosion of the system’s metal surfaces. Secondary corrosion mechanisms in the tower top and overhead are typically due to amine-chloride salt deposition driving under-deposit corrosion.

Neutralisers are used to control the pH of condensing overhead waters within an optimal range to maximise the reduction of corrosion rates while minimising the tendency for salt deposition caused by the neutralisation reaction with the acidic species. The type of neutraliser used in an overhead system is selected based on three primary factors: neutralisation capacity (the strength of the neutraliser), the water partition coefficient (the rate at which it will enter the first water droplets formed in the overhead system) and the neutraliser’s salt point.

The salt point is defined as the temperature at which the first neutralisation salts begin to precipitate from the vapour phase. These salts can be very corrosive themselves and can also give rise to under-deposit corrosion at certain points in the system. In order to control the deposition and corrosivity of these salts, a water wash is often used to provide a means of diluting and washing the corrosive salts from the overhead system. In these cases, the salts are scrubbed from the overhead vapour, washed from the overhead piping and condenser system, and flow into the overhead receiver. However, as the overhead process temperature is lowered in an effort to force additional material into the distillate draw section of the column, the location of the salt point temperature moves further upstream into the overhead line, pumparound circuits and tower top internals where there is no water wash.

Without the means of removing deposited salts in these areas, corrosion can be severe and equipment failure rapid. Therefore, it is critical to continuously re-evaluate the neutraliser being used to determine if it is still appropriate for changes in overhead and operating conditions. The ideal neutraliser for the system will form its amine chloride salt at a temperature that is 15°F (8°C) lower than the water dew point in the system. To protect against the deposition of precipitated amine-chloride salts inside of the distillation column, the neutraliser salt point temperature must also be 25°F (14°C) lower than the tower top temperature.

Role of excess chlorides and tramp amines
Chloride control in the overhead system is one of the most important aspects of a good corrosion control programme. This is because altering chloride levels has the largest overall impact on the corrosion potential by dramatically affecting both pH and the salt point deposition temperatures. The lower chloride levels entering the distillation column are, the greater the degree of corrosion control that is possible from a treatment programme. Therefore, with the desalter having the greatest impact on the condition of the charge to the distillation tower, all efforts should be made to ensure optimal desalter performance, reducing desalted crude chlorides to the lowest possible levels. However, maintaining low chlorides alone is not sufficient to guarantee good overhead corrosion control. The amines present in the system are equally important to the overhead system’s fouling and corrosion potential.

Neutralising amines that are intentionally added to control overhead pH conditions are not the only amine species that play a role in overhead salt formation. The presence of tramp amines may play a larger role in undesired salt formation in the overhead and tower top than the injected neutraliser amines. Tramp amines are broadly defined as any amines, other than the appropriate neutraliser being used, found cycling in the system. Tramp amines that are entering and recycling in the system will strongly affect overhead pH and typically have very high salt points. Sources of these tramp amines include incoming crude and slop oils, steam neutralisers, alkanol amine units, sour water strippers, H2S scavangers and cold wet reflux. Such amines can make it virtually impossible to either keep salt points below the water dew point or to drop pH to desirable levels. The most desirable condition is to have overall tramp amines in the system low enough to enable the usage of a quality neutraliser with a low salt point. If tramp amine levels are high enough, the net system salt point can negate the impact of a quality neutraliser. This situation can cause salt point temperatures to exceed the tower top temperature and cause various deposition problems that can become quite severe and affect tower operation and charge rates. Efforts should always be taken to understand total amine loading.

Both elevated chloride levels and amine levels will negatively impact overhead corrosion due to salt point effects. While chloride control is a relatively direct and straightforward effort, lowering levels of tramp amines can be much more difficult. This is often because operational practices prevailing in the refinery will give rise to high levels of tramp amines cycling up in the crude unit overhead. These practices are often caused by units outside the crude unit boundary. Four primary sources of tramp amine entry are the sour water stripper, steam production, alkanolamine scrubbing units, and amines entering the refinery with the incoming crude oil. An overall understanding of tramp amine backgrounds, surges and sources is necessary to enable targets and intervention for control of these species. Levels as low as 5 ppm of certain tramp amines can have a dramatic impact on salt points and associated corrosion. Figure 1 illustrates typical tramp amine cycles.