Additives for real-time FCC catalyst optimisation
The ability to optimise the circulating inventory composition remains an elusive independent variable. A solution to this often overlooked variable is discussed
Ray Fletcher and Martin Evans
Intercat (Johnson Matthey)
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The FCCU has long been considered the heart of a high conversion modern refinery due to its unique capability of processing a wide range of feedstocks under an equally wide range of process conditions.1, 2 The unit has matured in terms of control systems over the past six decades, with most units now being operated under advanced distributed control systems. Additionally, many units are equipped with multiple on-line analysers and kinetic-based simulators, all designed to integrate with the control systems to maximise unit profitability. One striking exception to this trend is found in an independent variable, which has remained elusive in terms of on-line, real-time optimisation: the composition of the FCC catalyst circulating inventory.
For very good reasons, many refiners generally limit catalyst evaluations to once every two to three years. Thorough testing and selection of catalyst technology is typically a medium- to long-term process, requiring dedicated laboratory equipment and six months or more before decisions can be finalised. The FCCU, however, responds to feedstock variation and operating changes in a matter of hours. The ease of catalyst additions combined with the equilibrium nature of the circulating inventory make the unit unique among all refining processes in terms of real-time optimisation. However, the ability to actually optimise the circulating inventory composition remains an elusive independent variable potentially providing an additional degree of freedom in the maximisation of FCC profitability. Figure 1 demonstrates the actual variability of feedstock composition in a typical North American refinery.
In addition to feedstock variation, today’s FCC operator faces an increasingly dynamic market some-times subject to significant volatility. Many fuel markets are observing a transition in which diesel is being favoured over gasoline. Propylene markets continue to evolve seemingly from week to week. The refiner who blends ZSM-5 additive into their base catalyst will often be at a competitive disadvantage to the competitive refiner who injects on demand as the market demands. The ability to alter product slates with dexterity may determine, in the long run, which refineries are profitable vs marginal. The ability to control the circulating catalyst inventory in real-time will be an additional enabling factor, allowing rapid response to market demands.
Two factors have eliminated the possibility for most refiners to optimise their circulating catalyst inventory composition in real-time: the first limitation being hardware related (multi-component addition systems and catalyst hoppers) and the second limitation being the catalytic components themselves. Most operating units today are equipped with one fresh catalyst hopper and possibly one additive addition system added to the unit subsequent to startup. Significant advances in catalyst and additive addition technology, plus catalytic technology have been achieved, which now permit the real-time, on-line optimisation of the FCC circulating inventory.
Overcoming existing technology barriers
The two barriers preventing the implementation of real-time FCC catalyst circulating inventory control are related to hardware and the long lead times necessary for catalyst selection. These two barriers are to be described in further detail within the context of:
• Catalyst and additive loader technology
• Catalyst evaluation lag time.
Catalyst and additive loader technology
Intercat has developed a full line of automated, catalyst and additive addition systems. More than 250 of these additive systems are currently in use around the world, designed to ensure the exact amount of catalyst or additive that is targeted is actually added to the units in small, consistent shots throughout the day. These units are equipped with feedback control systems to ensure the targeted addition levels are actually achieved. The hallmark of these systems has been their high accuracy, reliability and low maintenance (Table 1). The standard precision observed with these loaders is typically a greater than 99% approach to target. This is observed for both fresh catalyst and additive addition systems.
Multiple compartment catalyst and additive hoppers have been developed that are capable of accurately injecting three separate materials into an FCCU simultaneously (Figure 2). Hopper capacities range from 1–120 tonnes. These systems deliver to the refiner the ultimate level of flexibility and control with respect to catalyst additions.
These loaders present multiple advantages for the innovative refiner choosing to eliminate bottlenecks limiting maximum operating flexibility. The most obvious of these include the capability to begin optimising FCC circulating inventory in real-time. These loaders provide the capability to add an additional degree of freedom to the operation of their units. Specifically, the circulating inventory can be manipulated with the same degree of precision as preheat or riser outlet temperatures. Furthermore, refiners are capable of achieving precision addition rates from a wide range of containers, such as tote bins, super sacks and drums, enabling multi-component catalytic system trials without the necessity to first invest in hopper capacity.
The end result will be a system designed to deliver both the capacity and capability to begin manipulating circulating inventory compositions. This can be achieved via a combination of a wide range of selective additive technologies designed to maximise refinery profitability together with state-of-the-art catalyst and additive addition system technology.
Catalyst evaluation lag time
The second significant barrier to the implementation of on-line, real-time FCC catalyst circulating inventory control is the long evaluation time required for the minimum risk catalyst selection procedures currently in place among most refiners. An example of a typical catalyst selection protocol in terms of timing is demonstrated in Table 2. This table demonstrates that for most refineries the catalyst selection procedure can require up to 50 weeks from the time of issuing bid requests until the new catalyst formulation selectivities have been achieved. The time impact of each element of the standard procedure is described as follows:
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