What is sintering in relation to catalyst deactivation and how can it be avoided? (PTQ Q&A)
Response to a question in the Q3 2020 issue of PTQ
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Simerjeet Singh, Principal Hydroprocessing Specialist, Honeywell UOP - Simerjeet.Singh@Honeywell.com
Irrespective of the technology type, catalyst deactivation can be described as a physical or chemical process that reduces the activity of the given catalysts. One of the most common ways of measuring the deactivation rate is to estimate drop in conversion or reaction rate at a given temperature for the same feed rate and quality. Several mechanisms exist for catalysts deactivation, some of which are reversible while others cause irreversible loss of catalyst activity.
Under normal operating conditions, catalysts deactivate continuously over the cycle due to coke formation and gradual loss of active sites. This type of deactivation begins with adsorption of high molecular weight molecules and proceeds with further loss of hydrogen due to formation of polynuclear aromatic domains and, eventually, coke. Coke deposition is a time-temperature phenomenon. Deactivation increases with time and the temperature of the catalyst. Coke deposition occurs at a relatively slow rate and the catalyst can operate effectively for a couple of years or more before regeneration becomes necessary. The coke can cover active sites and, in extreme cases, prevent access to these sites by physical blockage of the entrance to the pores leading to the active sites. Reactor temperatures must be increased to compensate for the decline in activity caused by the accumulation of coke (and metals) on the catalyst. Unplanned upsets can expose catalyst to operating conditions beyond the design limits of the unit, accelerating the coke formation and reducing the catalyst life drastically at times.
Feedstock type, catalyst composition, reaction temperature, time on stream, and other process variables affect the yield and nature of coke. In most cases, coke on the catalysts can be burnt readily for its reuse by performing either an in situ or ex situ regeneration. Regenerated catalyst performance depends on the quality and frequency of regeneration.
At the front end of a processing unit, metallo-organic compounds in the feed decompose at the reactor operating conditions and deposit on the catalysts, often referred to as metal poisoning. Strong bonds formed as a result of chemisorption may at times make it difficult to remove these impurities, resulting in irreversible damage to catalyst performance. In most cases, these poisons block or hinder the access of reactants to active sites, resulting in a drop in catalyst activity. Other sources of metals are inorganic compounds entrained in the feed with particle sizes too small to be filtered out and which, for example, may be due to incomplete desalting operations. Efficient crude desalting, limiting the total metals in the feed and customising the guard bed in fixed bed units to selectively remove the catalyst poisons, can eliminate or reduce the poisoning of the main catalysts. The amount of poison required to kill the catalysts is usually very small (ppm or ppb levels in feed to the unit) in comparison to total catalyst quantity and varies from one impurity to other.
Impact of each impurity on catalyst performance varies with catalyst support (alumina/zeolite, supported/unsupported), type (bi- or uni-function), metal (base or noble), and reaction chemistry. For bifunctional catalysts having both metal and acidic functions, like hydrocracking or reforming catalysts, each impurity can selectively target one of the two functions. For example, organic nitrogen in hydrocracker feed is a temporary poison for acid sites/function only, but the resulting drop in activity would necessitate an increase in temperature, promoting coke make.
Sintering is broadly termed as a physical and/or thermal phenomenon that leads to agglomeration, a reduction in the surface to volume ratio of catalyst. It normally results in the loss of active sites due to alteration of the catalyst’s structure. Depending on the catalyst’s type, it can either result in loss of active sites due to agglomeration of dispersed metal or crystallites to larger ones or partial to total collapse of the internal pore structure and a corresponding loss of surface area.
Catalyst support and active metal sites can be sintered upon exposure to high temperatures usually during the process of regeneration or a temperature excursion/runaway. High water partial pressures can also lead to sintering, especially in the case of noble metal and high activity catalysts with chelating agent. Too much water increases the mobility of active metals, increasing the probability of agglomeration under these conditions. For some zeolitic catalysts like FCC, feed contaminants like sodium also promote sintering by acting as a flux agent which lowers the catalyst support melting point. Sintering occurs when the catalyst melts just sufficiently close to pores, blocking the access of oxygen for coke burning. All of these scenarios can result in loss of active surface, reduction in catalyst activity, and degradation of catalyst performance. In most cases, alteration to catalyst structure is permanent and the resultant loss in activity is irreversible.
Catalyst sintering can be avoided by controlling the temperature of the burn front during the catalyst regeneration process. If the temperature gets too high, there can be localised sintering of the base, causing a loss of surface area. A high temperature also causes metal crystallites to cluster together (agglomerate) and significantly reduce the catalyst’s metal function. If temperatures get even higher, the support can change and permanently deactivate the catalyst. Many technologies have built-in safety logics in place to shut off the regenerator or stop the reactions to avoid catalyst temperatures from exceeding the recommended limits.
For fixed bed reactors loaded with noble metal catalysts or high activity hydroprocessing catalysts, limiting free water in the feed to the reactor, especially prior to catalyst wetting and activation, is usually an effective way of reducing the risk of sintering. Similarly, proper crude desalting can help limit the sodium carryover to HVGO/FCC feedstocks and minimise the risk of sintering in FCC catalysts.
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