Preventing ammonium salt fouling and corrosion
Best practices and opportunities to reduce the risk of salt deposition and damage
BERTHOLD OTZISK, FAUSTO MAGRI, JELLE ACHTEN and SANDER HALSBERGHE
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Salt deposits and corrosion can lead to damage or high energy losses when crude oils are processed. Usually, these salts are ammonium chloride (NH4Cl) or ammonium bisulphide (NH4HS). This article describes a new approach to prevent chloride corrosion or further deposition of ammonium salts in order to avoid or minimise corrosion and fouling potential.
The use of a powerful water washing system is certainly a good step in the right direction to wash out as many salts as possible. Ammonium salts are generally readily soluble in water, but can often not be completely removed in the presence of hydrocarbons. Process units suffering ammonium salt fouling or corrosion are crude distillation units, FCC units, hydrocrackers, hydrotreaters and reformer units.
Higher amounts of chlorides in residue feedstocks or low main fractionator top temperatures to produce low sulphur gasoline are two reasons for salt fouling in FCC units. Sometimes a tower washing programme can be performed to remove water soluble salts. For this purpose, water is fed into the top reflux, or the overhead temperature is lowered until enough water can condense to the top trays. This usually takes several hours or even days. The feed rate must be significantly reduced by 20-30% during this time. The produced naphtha, sometimes also the light cycle oil (LCO), goes off-specification and has to be reprocessed with increased costs. In addition to the lower throughput, these costs may be particularly high.
The addition of an oil soluble salt dispersant additive can help to prevent salt deposition and keep the salts transportable, but that practice may also lead to several disadvantages. They work by chemically binding to the salt deposits, where the lipophilic functional group keeps microcrystalline salts in the hydrocarbon phase until enough free water is present so that the salts can be solubilised later. The dispersed salts are carried out of the system with the hydrocarbon product flow. Such salt dispersants are often surface active components with filming amine properties that can stabilise emulsions. In the event that not all of the bound salts are solubilised in free water later, they may remain in the hydrocarbon phase of the product steam, to be transported to downstream processes.
Kurita’s patented ACF technology pursues a completely different approach. Liquid formulations of a very strong organic base called ACF are used. These are water soluble and do not react with hydrocarbons. The organic base ACF has a very low pKb value close to zero, which is an indicator of a very strong base. It reacts preferentially with strong acids such as â€¨hydrochloric acid (HCl) or its corresponding ammonium salts (NH4Cl). The reaction product is ACF-Cl, which is a liquid salt with a neutral pH of 7. ACF salts have very high moisture absorption characteristics (highly hygroscopic) and can be removed easily from the system with free water. One more benefit is that ACF salts have a very low corrosivity. This is a significant difference compared with conventional neutralising amine salt reactions where the formed neutralising amines are almost always very corrosive and require higher amounts of wash water for removal.
The favoured reaction of ACF with HCl is a significant advantage in process units with naturally high H2S concentrations, such as hydrotreaters or hydrocrackers. Only after the conversion into ACF-Cl can weaker acids like hydrogen sulphide (H2S) or its salts react with the ACF by forming the corresponding ACF reaction product. The reaction with ACF is shown schematically in Figure 1, which is also valid for other ammonium salts such as NH4HS or ammonium sulphate ((NH4)2SO4). In all cases, ACF displaces the weaker base ammonia by forming a liquid salt. In the absence of free water, the salts are transported at temperatures below 180°C together with the hydrocarbon stream. When they come into contact with water, the salts can be removed with the aqueous phase. This chemical programme can be applied continuously to prevent fouling and corrosion or it is used at higher dosing rates to remove already formed salt deposits.
Case study 1: FCC main fractionator
Since it is common practice for the top temperature in FCC main fractionator columns to be lowered to produce more low sulphur gasoline, negative impacts are often observed. In most cases, salt fouling is the reason and this can also lead to corrosion if the salts are present on the metal surface as a sticky, highly concentrated and viscous solution.
Negative impacts are:
• Increased pressure drop
• Plugged trays and product draws
• Increased corrosion rates
• Flooding of the main fractionator top section
• Efficiency losses between gasoline and LCO separation
• Wet gas compressor efficiency losses.
During a field trial, the online cleaning performance of ACF was tested in a Western European refinery. Chemical injection into the main fractionator overhead reflux is common to dissolve the deposited salts from the top trays. According to the refiner’s experience, naphtha circulation was also affected where salt deposits on the lower trays were assumed to be present. Therefore ACF was additionally injected into the naphtha circulation in order to remove salt deposits from the heat exchangers and from the lower trays of the fractionation column. Figure 2 shows the two dosing points where ACF was continuously injected during the online cleaning. The reaction with the chloride salts is a stoichiometric balance and, according to the expected high amounts of salt deposits, a few litres per hour of ACF were dosed at both locations to mobilise high amounts of salts.
After the dosage had been started, first successes were observed within less than 30 minutes. Figure 3 shows some recorded results from the control room with basic data before the treatment and significant changes during the online cleaning. The differential pressure of the main fractionator column as a direct key indicator for fouling, the FCC feed rate, and the heat transfer coefficient of the naphtha circulation are displayed graphically in Figure 3. The rapid increase in the heat transfer coefficient and the main fractionator differential pressure drop impressively demonstrate that higher amounts of salt deposits were dissolved and mobilised in a short period of time.
Case study 2: hydrotreater feed/effluent exchangers
In a naphtha hydrotreater operating with several feed/effluent heat exchangers, corrosion problems occurred in the space of a few years, mainly due to the formation of ammonium salt deposits. As a corrective action, ACF was injected between the second and third heat exchangers after the reactor with the aim of stopping corrosion in the third feed/effluent exchanger and airline coolers. Since the treatment was started, large amounts of chlorides were collected in the water boot of the accumulator from where they could be frequently removed. Despite the higher chloride concentration in the accumulator, the corrosion potential was significantly lowered since ACF salts have a very low corrosion potential. Figure 4 shows the location where ACF is continuously dosed with a low ppm injection rate into the effluent product stream.
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