Minimise corrosion while maximising distillate

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 refin-ing 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, maximis-ing the production of these fuel streams requires a continual assess-ment 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 temper-atures in an effort to increase product draws in the distillate sections of the column, the condi-tions 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 maximisa-tion 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 opera-tion while maximising distillate production.

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

BRandon Payne GE Water & Process Technologies

overhead salt point and lower tower top temperaturesCrude unit overhead corrosion deals with corrosion affecting the upper sections of the crude unit atmos-pheric fractionation column, including the top tower trays, over-head 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 corro-sion of the system’s metal surfaces. Secondary corrosion mechanisms in the tower top and overhead are typi-cally 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 neutralisa-tion 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 neutral-iser), the water partition coefficient (the rate at which it will enter the first water droplets formed in the overhead system) and the neutralis-er’s salt point.

The salt point is defined as the temperature at which the first neutralisation salts begin to precipi-tate from the vapour phase. These salts can be very corrosive them-selves and can also give rise to under-deposit corrosion at certain points in the system. In order to control the deposition and corrosiv-ity 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 addi-tional material into the distillate draw section of the column, the loca-tion 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, corro-sion can be severe and equipment failure rapid. Therefore, it is critical to continuously re-evaluate the neutraliser being used to determine

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Corrosion in the crude unit overhead system is primarily due to acid attack at the initial water condensation point

Reprinted from PTQ, 3rd Quarter issue, 2012, pp 75-81

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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 under-stand 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 back-grounds, surges and sources is necessary to enable targets and inter-vention 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.

The tendency for all these amines to cycle up in the system is largely driven by overhead receiver pH and desalter effluent brine pH. As the pH rises above 5.5, the tendency for these species to cycle up is substan-tially increased. This is because the partitioning rate at which amines migrate from the hydrocarbon to the water phase are strongly influenced

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 depo-sition 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 aminesChloride 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 tempera-tures. The lower chloride levels entering the distillation column are, the greater the degree of corrosion control that is possible from a treat-ment 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 inten-tionally 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 over-head and tower top than the injected neutraliser amines. Tramp amines are broadly defined as any amines, other than the appropriate neutral-iser being used, found cycling in the system. Tramp amines that are enter-ing 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

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Figure 1 Typical amine recycle loops

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by pH. Once cycled up, the rate at which amines will blow down is dependent on the pH of both the desalter effluent brine and the over-head receiver waters, as well as the relative rate of amine input. A lower pH will cause a faster blow-down at a constant input of amines to the recycling system. The dynamic aspects of amines having reduced partitioning rates, reduced recycling rates and increased blow-down rates as pH is lowered has important consequences.

GE Water & Process Technologies (GE) has developed a comprehensive methodology to address the drivers of system salt points, salt deposition rates and subsequent corrosion rates in systems with significant salt foul-ing problems through a systematic review of amine inputs. By properly controlling these inputs and system pH, the most rapid reduction in deposition and corrosion potential can be achieved. Although lowering pH offers positive benefits, as described above, iron should be monitored in the overhead waters. The effective minimum pH will be dictated by the onset of increased iron production due to ICP corrosion at the water dew point. The pH should never be lowered beyond this level in order to keep ICP corrosion under control.

overhead water wash systemsWater washing is used as a means of forcing the water dew point temper-ature to higher levels and to physically dilute and wash salts. This is accomplished by raising the amount of free liquid water in the system. This free water will then dilute corrosive species at the ICP and also wash away any neutralisa-tion salts formed at temperatures below the wash water injection temperature, or mixed exit tempera-ture. However, it should be noted that an inadequate water wash can be worse than no water wash. Water wash should be injected in two stages using high-efficiency nozzles in a co-current configuration to

provide a small droplet size with a large surface area and dispersal pattern. This will impact both the wall wetting capability of the spray, as well as the vapour scrubbing efficiency.

The first injection stage should be a single point injected into the over-head vapour line near the top of the column, while the second stage needs to be multiple points injected in parallel just prior to the exchanger inlets. In a well-controlled unit, the

first-stage wash injection should provide just enough water to form 20% of total liquid water and prima-rily saturate the overhead vapour. The second injection stage should then inject the remainder of the total water needed to achieve the wash water target. Enough water should be added to achieve a minimum of 5% free water. While 5% is a mini-mum value, 10-15% water wash can be even more effective. However,

care must be taken to make sure that the overhead receiver can handle the additional amount of water flux and still provide acceptable water separa-tion. Water carry-over in the overhead reflux can be a very signifi-cant problem, as high levels of water-soluble amine salts will be carried back to the tower. This can lead to a large cycling effect that will dramatically increase salt points, as well as place salts directly on tower internals. Additionally, the water can cause wetting of already existing salts and increase their corrosion potential considerably. Care should also be taken to ensure that overhead vapour velocity stays within a range of 30-80 ft/s.

Caustic usageA target of <15 ppm chlorides, with <5 ppm upside variation, is an ideal target. While effective programmes can be maintained with higher levels of chlorides, it generally becomes more difficult and expensive if either chlorides or variation increase. A large variation in chloride levels can be very detrimental, as either low pH acid attack or high pH salt depo-sition can occur. Therefore, efforts should be prioritised to maintain effective chloride control within control ranges. The injection of caus-tic (NaOH) into the desalted crude oil can be used as a polisher to further reduce chlorides after the

Figure 2 LoSalt ionic equilibrium model input

Selection of the proper treatment chemistry is critical to the programme’s success and the unit’s equipment reliability

allowed the refinery to decrease average overhead corrosion rates by 80% to a corrosion rate of <5 mpy (verified by consecutive UT thickness readings).

Filming inhibitor chemistries are the mainstay of the overhead corro-sion inhibitor programme. Filmers work by coating the metal surface with a hydrophobic barrier, which prevents corrosive species from reach-ing and reacting with the metal surface. While a neutraliser has a maximum theoretical limit to the overall reduction of corrosion rates, filmer chemistries are not constrained by such a limit. The level of corrosion protection from a filming corrosion inhibitor can approach 100%, given adequate dosage and the proper conditions. However, the practical aspects of the filmer application generally do not allow such a degree of protection. It is, however, relatively easy to realise 90-95% protection for coated surfaces at reasonable dosages. Traditional filmers require a pH

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best performance is obtained from the desalters. However, caustic usage must be carefully evaluated and monitored to determine accurately the downstream impacts and the critical threshold concentrations. It should not be used as a replacement for optimising desalter operation.

Proper selection of filming corrosion inhibitors and neutralisersIn order to properly control corro-sion in a crude unit overhead system, a three-pronged strategy comprised of an organic neutralising amine, a filming inhibitor and a water wash should be implemented. Every corro-sion control programme will utilise these three elements to varying degrees, based upon the unit design, crude diet and operating envelope. Therefore, the selection of the proper treatment chemistry is critical to the programme’s success and the unit’s equipment reliability.

Under normal conditions, the ideal neutraliser utilised will form its

amine-chloride salt at a temperature that is at least 15°F (8°C) lower than the water dew point in the system. Calculating the ICP and salt point are critical to controlling corrosion. The use of modelling software, such as GE Water & Process Technologies’ proprietary LoSalt Ionic Equilibrium model (see Figures 2 and 3), allows for the rapid and efficient prediction of the overhead system salt point. These modelling tools can be used to quickly determine the best neutralis-ing amine and/or operating parameters by predicting salt points for various amines and the ICP for various operating conditions.

The LoSalt Ionic Equilibrium model has been effectively used to assist refiners in optimising tower top temperatures by establishing the operating limits they must not exceed in order to prevent salt formation and deposition in the system. In one case, the operational changes made, based on the information provided by the ionic modelling analysis,

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Figure 3 LoSalt ionic equilibrium model output

above 4 to maintain optimal film stability. The newest filmers in the GE Water & Process Technologies pHilm-Plus line can offer film stability down to a pH of 2. This is especially impor-tant during desalter upsets and high-chloride events, which can drive overhead system pH to very low levels. In addition to providing a significant further reduction in corro-sion rates, over and above that available with a neutraliser, certain filmers can act as salt dispersants. As such, they help reduce the risks asso-ciated with salt fouling due to amine-based neutralisers or tramp amines present in the system. Filmers have been used with success in the dispersion of deposited salts in areas where no water wash is present, such as pumparound circuits and tower trays.

Unlike neutralisers, filming inhibi-tors will not vaporise in the overhead system and will remain liquid. Therefore, filmers must be atomised into the overhead at the point of injection. Nozzles and dilution streams must be used to achieve proper distribution. Since the filmer injection exists as an entrained drop-let distribution in the overhead vapour stream, it follows a two-phase flow profile. Liquid droplets will impact with and coat the wall through direct interaction. Both the choice of application point and the droplet size distribution of the injected liquid play a large role in

the overall effectiveness of a filmer. Larger droplets in the flow field have a tendency to impinge onto the outside wall of any bends or turns in the line. A loss of filmer coverage can be caused by 90-degree elbows or U-bends due to the momentum of the travelling droplets forcing them to move to the outside wall of the curves via centripetal forces. Also, vapour-liquid route preferencing and maldistribution can cause poor filmer coverage downstream of splits, T-s and in manifolds. Droplet sizes larger than 50 microns will have a greater tendency to be lost prema-turely to the wall for systems with complex bends. For this reason, high-quality hydraulic nozzles that are properly designed for the system flows and pressures should be utilised to achieve a minimum drop-let size distribution. Also, at least two stages of filmer distribution are recommended for most systems, similar to a water wash system.

The first stage should be near the beginning of the overhead vapour line, preferable into the 90-degree elbow pointing down the vertical pipe near the side of the tower. The second stage should be distributed over parallel legs, just upstream of the individual exchangers. The injec-tion direction should be co-current with the flow, and each injection point should use its own carrier stream that is metered and control-led. Filmer injection points should

be at least 10 pipe diameters away from other injection points. The first filmer injection is preferred downstream of the neutraliser injec-tion point, and both should be significantly upstream of the water dew point and salt point locations.

ConclusionAs refiners reduce atmospheric tower top temperatures to maximise diesel and middle distillate production, a thorough understanding of the ICP, salt point and control of amine recy-cle loops is critical to maintaining plant reliability in changing plant operational conditions. By practising good operational diligence, treatment programme stewardship and utilis-ing predictive diagnostic tools, such as GE’s proprietary LoSalt Ionic Equilibrium model to predict amine salt points, refiners can establish safe operating regimes for diesel and middle distillate maximisation and ensure the long-term safety and prof-itability of refinery assets.

LoSALT and pHilmPLUS are trademarks of General Electric Company and may be registered in one or more countries.

Brandon Payne is a Product Applications Specialist with GE Water & Process Technologies’ Refinery Corrosion Center of Excellence. He is responsible for global support of refinery corrosion treatment programmes, and has over 14 years of refinery engineering and process treatment experience.

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