Impact of chlorides on fluid catalytic cracking
Fluid catalytic cracking (FCC) is an important conversion process in many refineries. It produces transportation fuels, including gasoline and diesel precursors, as well as feedstocks for many chemical processes, including propylene and ethylene.
MELISSA CLOUGH MASTRY, CORBETT SENTER, FERNANDO SANCHEZ and BILGE YILMAZ
BASF Refining Catalysts
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Impact of chlorides on fluid catalytic crackinThe FCC process uses a fluidised solid catalyst to crack feeds with a variety of properties. The need for a sustainable FCC process is becoming increasingly important as fuel regulations become more stringent and environmental regulations become increasingly tight. In light of this, the refining industry aspires to make the most of every barrel of crude oil processed. This often means avoiding any byproducts that reduce the amount of valuable products (gasoline, diesel, propylene) produced by the FCC process.
Refineries that process residue- containing feedstocks (resid feed) face significant challenges to their sustainability and even to reaching their economic targets. Resid feeds often introduce contaminants that cause unwanted side reactions in an FCC unit. Common contaminants in resid feeds include vanadium, iron, and nickel, each of which offers obstacles by promoting unwanted side reactions in an FCC unit. Vanadium, for example, primarily deactivates the fluidised catalyst, leading to lower conversion and unwanted heavy liquid products (bottoms or slurry). Iron can cause physical blockage on the surface of catalyst particles, leading to a similar outcome.
Nickel, on the other hand, catalyses the dehydrogenation of hydrocarbon molecules. Nickel is a well known dehydrogenation catalyst that is often used in the petrochemical industry, and it can promote unwanted dehydrogenation reactions in the FCC unit as well. In the FCC unit, nickel increases hydrogen and coke yields, which decreases valuable product outputs (LPG, gasoline, and/or light cycle oil will be decreased). Over time, as nickel circulates through the FCC unit, it loses its activity and becomes relatively inactive as a dehydrogenation catalyst by becoming oxidised to the +2 oxidation state. In this oxidation state, nickel is essentially immobile. Nickel in the Ni(0) oxidation state is responsible for unwanted dehydrogenation reactions and is considered relatively more mobile.1,2 Once nickel is rendered relatively inactive through oxidation, it remains in this state until it is physically removed from the unit. However, in some cases, nickel can be reactivated from its relatively inactive state to its Ni(0) oxidation state. One element in particular, chlorine, can reactivate nickel.
In addition to their chemical effects, chlorides can pose challenges to refiners by introducing operational challenges. A prominent example of this is the formation of chloride based deposits that manifest downstream of the FCC unit. The reactivation of nickel by chlorides and operational challenges posed by chlorides will be the focus of this article.
Reactivation of nickel
In other (non-FCC) processes, chloride-containing materials have been shown to not only reactivate nickel’s dehydrogenation activity but also increase its mobility on various solid supports.1 Considering that chloride-containing materials can also enter an FCC unit, the chemistry of these reactions in an FCC setting are of interest since this would mean an increase in hydrogen and coke upon nickel reactivation. For most refineries, this is an unwanted side reaction and can cause significant constraints to operation. Therefore any chlorides entering the FCC unit should be scrutinised. Chlorides can enter an FCC unit by various methods: poor desalter operations, opportunity feeds, residue or bio feeds, or slops processing.3 Yet another avenue for chlorides to enter the FCC unit is through the fresh catalyst itself, specifically through fresh catalyst manufactured via an ‘incorporated’ technique that involves chloride-containing ingredients. In this manufacturing route, the use of a binder, often an alumina-sol based binder, is needed to provide the catalyst its structural integrity (its attrition resistance). Many of these binders include chlorides either as an integral component or as a byproduct during the manufacturing step. Therefore some incorporated catalysts often come with a significant amount of chlorides; industrial reports detail up to 1.2 wt% chloride content. Regardless of source, once in the FCC, chlorides can have detrimental effects relating to the reactivation of nickel contaminant, among other issues.
Operational impact of chlorides in the FCC unit
Aside from the chemical effects of chlorides, which will be discussed in more detail in the next section, chlorides can also lead to unwanted and unfavourable operational situations in an FCC unit. For example, there have been many industrial reports of main fractionator deposits that are heavy in chlorides. This phenomenon is a result of the formation of solid ammonium chloride (NH4Cl) which forms according to the following reaction:
NH3 (gas) + HCl (gas) ⇋ NH4Cl (solid)
Deposition takes place at the top of the fractionator (downstream of the FCC reactor) where temperatures are relatively low compared to the reactor itself. When both ammonia (NH3) and hydrochloric acid (HCl) are present in the vapour phase, the lower temperature leads to their condensation in the cold areas of the main fractionator and their reaction to form NH4Cl dissolved in water. When water is later boiled off downstream, NH4Cl is deposited onto the trays in the fractionator. NH3 is formed upstream in the FCC unit, especially in partial burn regenerators. Less NH3 is formed in a full burn regenerator, as most N is oxidised to NOx. But NH3 is rarely ever the limiting reagent in the above reaction mechanism so this condensation mechanism is seen in both partial and full burn regenerator FCC designs. Although NH4Cl deposition will be made worse with lower fractionator temperatures (for example, if an FCC unit has an HCN side-stream draw) or increased partial pressure in the fractionator, deposits have been observed even for overhead fractionator temperatures above the water dew point since localised cold spots (for instance, the top pumparound return section) generate the same mechanism.
Such NH4Cl deposits will reduce both the capacity and efficiency of main fractionator operation. These will manifest either as pressure or temperature instabilities, an increased pressure drop over the trays, flooding of the top section of the fractionator, or some combination of these. Another manifestation might be poor performance overall, leading to, for example, poor fractionation between LCO and naphtha product streams. In addition, there could be long term detrimental effects like under-deposit enhanced corrosion rates in tower internals (trays, supports) and external walls. Each of these negative effects has its own penalties in terms of downtime and/or negative economic impacts.
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