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A conceptual breakthrough in chemical process design – “A reactor system capable of eliminating dynamic coupling among multiple feed streams” Author: Zhong Lintian Source: PROCESS «Process Industry» The patented process developed by Mobil for the conversion of benzene hydrocarbons into ethylbenzene has been widely used in actual production. This patent features many technical advantages, but it has also given rise to new problems. The patent discussed in this article serves as a complement and improvement to Mobil’s patented process for converting benzene hydrocarbons into ethylbenzene. As reactors become more differential in design and molecular sieves continue to advance, this patent will become increasingly widespread. Concept for the process of producing ethylbenzene via the catalytic reforming of benzene in mobile units: In ethylene production projects, there are usually styrene production facilities. Styrene is produced by the dehydrogenation of ethylbenzene, and currently, ethylbenzene is manufactured through a benzene alkylation reaction between ethylene and benzene (in vapor phase) in the presence of a catalyst. In the production process, if a common approach of feeding materials at one location, using monomers, and large integral reactors is adopted, the massive accumulation of heat flow will inevitably destroy the catalyst in the lower layer of the fixed bed, preventing normal production from continuing. Obviously, the problem of the massive heat released during the benzene alkylation reaction leading to catalyst degradation must be addressed by focusing on the design of the benzene alkylation reactor. The basic approach to dealing with the large amount of heat generated (exothermic reaction heat) in benzene alkylation reactions is this: since an integrated reactor cannot withstand the accumulation of such large amounts of reaction heat, it is necessary to divide the reaction into several stages. But how to divide it? How to handle the massive amount of reaction heat after separation? The process flow for producing ethylbenzene via vapor-phase benzene alkylation developed by the American company Mobil, which is widely used in ethylene production projects, effectively solves these problems. As shown in Figure 1, R-101 is a 6-layer differential reactor. The total ethylene feed coming from an ethylene main pipe is divided into 6 streams (pipes 104, 108, 110, 112, 114, and 116). The vapor-phase benzene feed coming from the vapor-phase benzene main pipe of heat exchanger E-101 is also divided into 6 streams (pipes 105, 107, 109, 111, 113, and 115). These vapor-phase benzene streams combine and then enter layers 1 to 6 of R-101, thereby forming a differential reactor. In each layer, the benzene undergoes alkylation to produce ethylbenzene, and the large amount of reaction heat generated is divided into 6 equal portions. However, the clever aspect of the Mobil patent lies in knowing how to handle the heat generated at each layer so that it does not accumulate in the lower layers, causing high temperatures that could damage the catalyst. As shown in Figure 1, the (vapor-phase) benzene feed to tube 105 in the first layer comes from the main (vapor-phase) benzene pipeline; it is slightly superheated – as it is formed by the saturated benzene vapor coming from the liquid benzene evaporator E-102, which is mixed with the high-temperature exhaust gas from R-101 and then superheated in heat exchanger E-101. This (vapor-phase) benzene is further heated in heater F-101 to reach an appropriate temperature for the benzene alkylation reaction. The alkylation reaction takes place in the first layer, and the resulting heat is carried down to the lower layers. But how can the lower layers dispose of the heat of reaction from the upper layers without it accumulating layer by layer? Another highlight of Mobil’s patents! As can be seen from the above, the temperature of the slightly superheated (vapor phase) benzene coming from E-101 does not reach the appropriate temperature for the reaction ； Therefore, to enter the first stage (vapor phase), benzene must be heated in a combustion furnace, and the heat supply to furnace F-101 is controlled by the temperature resulting from the mixing with ethylene from tube 104. Therefore, the feed streams from layers 107, 109, 111, 113, and 115 – which consist of (vapor-phase) benzene coming from main pipe 103 – along with the corresponding ethylene streams from layers 108 to 116, are all “cold” feeds. This “cold” feed mixes with the gaseous mixture from the upper layer, which contains a large amount of reaction heat and reactants as well as products, thereby absorbing all the reaction heat from the upper layer and achieving an appropriate reaction temperature. In other words, the heat of reaction generated at the upper layers is equivalent to the amount of heat (flow) provided by the F-101 heating furnace; this approach not only prevents the gradual accumulation of heat of reaction but also reduces fuel consumption. The two-in-one concept of using 2–6 layers of stratified “cold” feed is the biggest highlight of Mobil’s patent. Defects in the Mobil benzene alkylation process for producing ethylbenzene. As summarized above, in the Mobil (vapor-phase) benzene alkylation process for producing ethylbenzene shown in Figure 1, the 6-stage differential reactor in R-101 not only disperses the large amount of total reaction heat into 6 portions of varying sizes to prevent catalyst destruction, but also utilizes the \"cold\" feed from stages 2 to 6 to mix directly with the reactants and products from the upper stages, thereby absorbing all of the corresponding reaction heat. This approach not only prevents catalyst damage caused by the accumulation of reaction heat but also makes full use of that heat. These clever concepts are all based on considerations of steady-state material and heat balances, but they do not take into account how to achieve and maintain this steady state (equilibrium). However, as shown in Figure 1, the dynamic coupling between the ethylene feed divided into 6 parallel streams from a single main pipe 102, and the (vapor-phase) benzene feed divided into 6 parallel streams from the vapor-phase benzene main pipe at the outlet of heat exchanger E-101, makes it difficult to achieve and maintain a steady-state balance. As a result, even with careful operation, it is hard for the actual production plant to reach a stable state, causing the reaction temperatures in each stage to fluctuate constantly. This not only poses difficulties for actual production operations but also reduces the service life of the catalysts due to these frequent temperature changes. As shown in Figure 1, the ethylene coming from the main pipe 102, being a gas and some of it originating from low-pressure turbines (compressors), results in relatively weak mutual dynamic coupling among the various parallel feed streams. By making careful, gradual adjustments to the feed rates of each branch stream, it is possible to gradually reach a steady-state equilibrium value. But the (vapor-phase) benzene side is different. On the benzene side (in the vapor phase), various layered feed streams experience more frequent and severe dynamic coupling due to the following reasons, which may even lead to temperature runaway. Due to the dynamic coupling issues arising from the fact that the reaction temperatures in layers 2 to 6 in Figure 1, along with the \"cold\" feed flow rates of their respective feed lines (107–115), are all connected to a single main pipe (the ethylene main pipe 102 and the benzene (vapor phase) main pipe 103), these problems can be barely resolved using an automated matrix decoupling method. It takes several cycles of adjusting the opening degrees of the various feed valves before a certain stability is achieved, but this stability cannot be maintained. The 5×5 matrix is asymmetric due to the varying sizes of the individual feed streams, which not only makes it difficult to decouple and invert the matrix but also increases the time and space overhead of the control system (DCS) used. Clearly, the conventional approach to designing (implementing) production processes in the petrochemical industry has so far considered only steady-state material flows and heat balances; it has never taken into account the dynamic processes involved in achieving and maintaining that steady state. This inherent mindset is also reflected in the design of Mobil’s process for producing ethylbenzene through benzene hydroformylation. The massive total reaction heat is dispersed and fully utilized to create 6 layers of differential reactions and feed streams; however, the dynamic coupling resulting from dividing a single main pipe feed into 6 parallel streams makes it difficult to achieve and maintain a steady state, thereby posing the challenge of operating the production facility to the control systems and operators. To address this issue, the author conceived of a “reactor system capable of eliminating the dynamic coupling of multiple feed streams.” “The concept of the patent \"A reactor system capable of eliminating dynamic coupling among multiple feed streams\" involves a detailed analysis of the issues arising from when one main pipe is divided into six parallel feed streams, with dynamic coupling between them. The root cause of the instantaneous shortage in the amount of \"cold\" feed used as a temperature regulator is the lack of storage and insufficient immediate supply. As shown in Figure 2, by adding two parallel empty tanks, A1 and A2, between the ethylene feed main pipe 102 and the various feed branches 104, 108–116, and by adding two parallel empty tanks between the slightly overheated (vapor phase) benzene feed main pipe and the respective (vapor phase) benzene feed lines 105, 107–115, a certain amount of (vapor phase) benzene can be stored for use during periods of high demand. This approach addresses, at the source, the problems related to dynamic coupling among the multiple parallel feed streams originating from one main pipe, as well as the issue of instantaneous high demand as a regulatory mechanism. This is the concept of this patent. In the process design, two parallel empty tanks each, A1-A2 and B1-B2, are installed as buffers to serve as flow stabilizers between the main ethylene feed pipe and its 6 branch feed pipes, as well as between the main (vapor-phase) benzene pipe 103 and its 6 parallel (vapor-phase) benzene feed pipes. This arrangement not only eliminates the dynamic coupling caused by multiple feed streams at the source, enabling rapid attainment and maintenance of a steady state for liquid benzene, but also provides more space for the vaporization of liquid benzene, which facilitates this process. Moreover, the (vapor-phase) benzene stored in the buffers B1-B2 is sufficient to handle occasional increases in feed volume used as a reactant temperature regulator, thereby fully compensating for the supply delay that results from the large heat capacity associated with the latent heat of vaporization when converting liquid benzene into saturated vapor benzene. Clearly, in terms of addressing the issue of multiple (parallel) feed streams into a single main pipe, as well as dynamic disturbances caused by sudden increases in supply volume as a form of regulation, the original design from Mobil’s patent relies on using a 5×5 dynamic decoupling matrix method on-site – a solution that is merely temporary. In contrast, the approach of adding two empty tanks between the main pipe and the various parallel branches to serve as buffers and thus create a flow stabilizer is a simple, effective solution that achieves better results with less effort. Application prospects: The Mobil 6-layer differential feed-reactor is highly suitable for zero-order (independent of reactant concentration) and irreversible chemical reactions such as those involving ethylene and (vapor-phase) benzene, but it may not be suitable for non-zero-order and reversible reactions. Furthermore, in the case of fixed-bed reactors, based on the principles of the Arrhenius equation, the commonly used integrated, large-scale vertical reactors are integral reactors; as the depth within the catalyst bed increases, the product concentration builds up, whereas the driving force for the reaction decreases exponentially, resulting in a decline in conversion rate. Clearly, the differential reactor is the most ideal but difficult to implement. As shown in Figure 3, a reactor consisting of multiple small benzene catalyst beds connected in parallel (n beds) near a differential reactor can be easily implemented with advances in manufacturing technology (to make use of vertical space and save land, the n reaction beds shown in Figure 3 can be stacked; when arranged in layers in parallel, the outlets of each layer are connected to a single main outlet). To address the accumulation of large amounts of reaction heat, as shown in Figure 3, it is divided into equal n portions and fed in parallel; these parallel feed streams originate from the flow stabilizers A1-A2 and B1-B2 ； To utilize the reaction heat, the feed is first externally heated to the reaction temperature; after the reaction begins, the reaction heat from each outlet is used to exchange heat with the incoming feed through a heat exchanger, thereby gradually removing the external heating. This can replace the benzene hydrocarbonation differential internal reactor R-101 patented by Mobil, but the reaction heat is not fully utilized, resulting in lower thermal efficiency compared to the former. Conversely, as shown in Figure 3 with multiple parallel arrangements: for the small reactors containing benzene beds, if the outlets are combined and a molecular sieve is used to separate the products from the unreacted reactants in the outlet gas, with the latter being returned to the feed point, this approach can be applied to reactions of any order, including zero-order and irreversible reactions, as well as reversible reactions. In this way, it not only reduces the energy consumption required for separation methods based on vapor-liquid phase changes but also protects the environment from pollution. With the development of molecular sieves, the author expects that reactors with multiple parallel thin catalyst beds as shown in Figure 3 will become widely used, and accordingly, this patent will also become widespread. As mentioned above, the Mobil patents solved the problems at hand but simultaneously introduced new issues for those working on site (operators and control engineers). Typically, designers of petrochemical processes do not consider how to address these issues by improving the design of the processes themselves, and I believe this traditional approach is unsound. The concept and implementation method of this patent are very simple; minor improvements to the Mobil process design can resolve the problems associated with that process. This also enables the production facility to quickly reach and maintain a stable state over the long term. It not only protects the catalyst from damage caused by frequent temperature fluctuations, thereby extending its lifespan, but also reduces the startup time and frees operators from the need for prolonged, careful, and meticulous operations. The author hopes that in the petrochemical industry, engineering designers will be able to break free from traditional concepts and dare to make bold improvements to established process designs, as a response to China’s call for \"independent innovation\". Finally, it should be noted to the readers that for multi-stream feedings with a main pipe pressure of ≥0.5 MPa, a flow stabilizer composed of buffer tanks is not necessary.