Introduction
The dehydrogenation of propane to propene is a major industrial process in the chemical industries since it is an intermediate process in the generation of many chemicals such as plastics, fibers, amongst others in the petrochemical industries. Propene is mainly an intermediate to a number of chemicals and playing major role in the production of polypropylene, which is a broadly used plastic. The present report aims at designing a Plug Flow Reactor (PFR) for the direct conversion of propane to propene; a process that is endothermic in nature and therefore requires significant heat and mass integration. The reaction has been found to be second order with respect to propane and obeys the first order kinetics with respect to oxygen. In the operating conditions of 650°C and 1 bar pressure, the conversion of propane should be as high 95%, and the selectivity for the formation of propene should be 100%. The reactor design provides the maximum heat and mass integration so as to achieve high efficiency, safety, and utilization of energy with the desired product formation. From the Brownrigg diagram, it can be seen that one area that has a significant impact on the practicality of the reactor as well as the conversion efficiency of the propane is the reactor volume and the residence time it offers. The reactor’s volume is selected to be 100 m³, while the residence time is estimated to be 420 seconds, which is almost 30 minutes to ensure that the propane gets enough time to interact with the catalyst. This long residence time guarantees that the reaction attains the intended conversion though using less energy. Furthermore, the elements of the reactor system also involve temperature control because the reaction is temperature-sensitive. For purposes of saving energy the temperature had been reduced from 650°C to 600°C in the axial direction of the reactor. For assessment of the performance of the reactor, recording of propane concentration throughout the reactor’s length, temperature distribution, reaction rates and conversion is carried out using MATLAB simulations.
Reactor Design: Reaction Kinetics
The propane dehydrogenation reaction (C₃H₈ → C₃H₆ + H₂) represents the conversion of propane to propene and hydrogen. This endothermic reaction is carried out by using PtSn/Al₂O₃ catalyst and the reaction is found to be first order with respect to propane. The rate of a reaction at which it proceeds is given as rₐ where rₐ = k·Cₐ for propane, with k as the rate constant, and Cₐ as propane concentration measured in mol/L. The first order dependency (n=1) means here that the rate of the reaction is causing a direct proportionality in the concentration of propane. When at the operating conditions of 650°C pressure of 1 bar the rate constant k is calculated from the Arrhenius equation which depicts the dependence on the temperature (Gambo et al., 2021). This kinetic model helps in determining other values proximately to the design of the PFR which include reactor volume, residence time and conversion profile. This is important because it helps in controlling the performance of the reactor in order to obtain the desire 95% conversion coupled with 100% selectivity to propene. The reaction kinetics of propane dehydrogenation (C₃H₈ → C₃H₆ + H₂) has first-order with regards to the propane concentration (Cₐ), so the rate, rₐ = k·Cₐ and the rate constant (k) depends on the temperature that is given by the Arrhenius equation.
Reactor Volume and Residence Time
Figure 1: Reactor Volume
The other important element in PFR design is the space velocity or the residence time (τ) which describes the time the reagents take in the reactor. This parameter directly influences conversion and is expressed as τ = V/Q, the volume of the reactor, given in liters, and the volumetric velocity Q in liters per second. For the reactor for propane dehydrogenation, the volume has been selected as 100 m³because reaction kinetics and specific conversion rates are taken into consideration. With the volumetric flow rate of propane feed being calculated, it is possible to maintain the residence time of the propane at 420s which is equivalent to thirty-seven minutes and twenty two seconds. This prolongation of their stay in the reactor allows for a long contact time to ensure that the propane interacts with the PtSn/Al₂O₃ catalyst to the desired 95% conversion. This required reactor volume was achieved taking into account other factors which include pressure drop, heat transfer area and catalyst inventory (Oyegoke et al., 2024). Due to the tremendous importance of these parameters, their optimization is very crucial to give enhanced propane conversion when the process is economically feasible at the same time. It also highlights essential factors for the operation of a Plug Flow Reactor (PFR) used in propane dehydrogenation, such as the size of the reactor, 100 m³, the flow rate (26942.89 kg/day), the residence time of 420s and the conversion efficiency of 95%.
Figure 2: Reactor Volume for non-isothermal
The reactor volume for a non-isothermal reactor could therefore be determined from the reaction kinetics coupled with the energy balance equations. It depends with the heat that is produced or absorbed by the reaction which dictates the temperature gradient in the reactor. Based on the energy balance equation, the heat of reaction, heating and cooling losses and the heat transfer either into or out of the reactor is calculated. The need to take into account the variation of temperature leads to the need to determine the temperature distribution whilst setting the area of the reactor in accordance with the desired conversion and mean residence time at different temperatures.
Figure 3: Residence time
Residence time should be kept as optimal as possible in order to achieve high selectivity towards the desired product propylene and reduce the rate of side reactions such as cracking or coke formation. This would ensure that the reaction is carried out at the desired temperature and pressure so as to avoid degradation of the catalyst and to achieve the desired percentage conversion. Additionally, information on suitable catalysts for the implemented reaction and their topographical distribution also influence the usefulness of the reactors. By incorporating flow dynamics and heat transfer at its optimum levels, the reactor will perform optimally without much threat to process economy. Overall, high space velocity, residence time and catalyst utilization result to high production for the reactor and efficient industrial use.
Mass and Energy Balances
Figure 4: catalyst weight calculation
The weight of the catalyst is determined through the volume, density, and the loading fraction in the reactor. The values of reactor volume were 0.262 m³, the density of the catalyst was 1.2 g/cm³, the catalyst loading fraction was 10%, then calculating the weight of the catalyst (Wc) it was identified as 31.44 kg. Suppose this calculation to make sure that the reactor is filled with just the right amount of catalyst that is required for operation. mass of the catalyst is proportional to the reactivity of the reactor in a set with regard to such parameters as reaction rate the heat transfer.
The conservation of mass for propane and propene implies that it is the amount of mass that enters the reactor will be equal to the amount of mass which exits the reactor. Given the flow rates: Inlet propane flow rate: 26,942.89 kg/day, Outlet propane mass flow rate: 1347.14/100 = 13,3 kg/day (unreacted propane). Propene produced: 580 mol/day.
So, as converted is 95%, still, 30.5 mol /day of propane is unconverted. The rest of propane gets converted into propane according to the 100% selectivity contemplated for this reaction path. In order to calculate temperature distribution in the reactor, the energy balance is reduced as follows. Although a more refined approach could include the estimates of specific reaction enthalpies and heat losses, for the purpose of simplification the linear temperature profile from 650°C to 600°C for the reactor length was assumed (Carter et al., 2021). This makes sure that the reactor works at the temperatures that are desired so as to maximize the tendency of the reaction rates.
Piping and Instrumentation Diagram (P&ID)
Figure 5: P&ID diagram
The P&ID of the reactor shows the feed and product lines together with control valves, temperature indicators and pressure control units. This block should help to comprehend the flow through the process and the control to sustain the desired conditions in the reactor (Martino et al., 2021). This ensures the temperature, the pressure as well as the flow rates are well regulated in order to have the reactions in check. In the diagram, the control system of an R-100 reactor is disclosed. Cooling inlet and outlet, flow control valves CV-1, CV-2, CV-3, level transmitter LT-01, LT-02 and temperature transmitter TT-01, TT-02 are some parts of the system. The flow rate control and pressure control are both controlled and regulated and the liquid is heated, cooled and mixed before exiting the reactor at the right temperature, and to ensure safety the system has a shutdown valve (SDV-2).
MATLAB Analysis and Plots
Figure 6: Simulink diagram
The Simulink model arranged in the figure above concerns with a reactor system which has the manipulatives of temperature and output flow rate for the system. It starts with the Flow Rate input which determines feeding rate of material into reservoir or any other equipment of the reactor. This flow is controlled by the help of the Flow Control Valve; this helps in controlling the input flow according to the control signal. The Reactor Dynamics block calculates the reactor’s thermal output based on a first-order transfer function that can simulate the heat changes over a reactor when the inputs change. Once the temperature is measured, a comparison is made with the set temperature and an Error Signal is produced which shows how much from the set temperature actually is the temperature. For better control, any high frequency noise in the temperature signal is removed by a Lowpass Filter and its instability as a result of the fast change in outside temperature is avoided. The PID Controller then fine tunes the flow control valve with an intention of reducing the error signal and thereby stabilizing the temperature of the reactor at the set point. Last, the reaction delay block represents the thermal mass and time constant which is involved between when the controller manipulates the heat supply of the system and the final response of the reactor temperature.
Figure 7: Temperature output
In total, all the components are used in order to maintain the stability and effectiveness of the reactor’s temperature regulation even if encounters certain localized disturbances such as the changes of flow rate and delay. With the use of the PID controller, the system can change parameters constantly, which helps in keeping the temperature at the set value and enhances the reaction conditions as well as the stability of the process (Brune et al., 2022). This model is very useful in controlling processes occurring in industrial reactors in an efficient manner and avoiding issues like instability in temperatures or overshooting among others.
Conclusion
The preliminary design for PFR of propane dehydrogenation seems to be quite sturdy featuring a reactor capacity of 100 m³ and a reactor residency of 420 sec. The conversion is 95% and the production of propene is 580 mol/day. The following plots explain conversion as well as concentration profile and temperature distribution along the flow reactor. Despite that, there is room to perform extra optimization by making use of kinetic models and concrete measurements of industrial reactor reality.
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Reference List
Journals
Gambo, Y., Adamu, S., Abdulrasheed, A.A., Lucky, R.A., Ba-Shammakh, M.S. and Hossain, M.M., 2021. Catalyst design and tuning for oxidative dehydrogenation of propane–A review. Applied Catalysis A: General, 609, p.117914.
Oyegoke, T., Dabai, F.N., Waziri, S.M., Uzairu, A. and Jibril, B.Y., 2024. Computational study of propene selectivity and yield in the dehydrogenation of propane via process simulation approach. Physical Sciences Reviews, 9(2), pp.1049-1063.
Carter, J.H., Bere, T., Pitchers, J.R., Hewes, D.G., Vandegehuchte, B.D., Kiely, C.J., Taylor, S.H. and Hutchings, G.J., 2021. Direct and oxidative dehydrogenation of propane: from catalyst design to industrial application. Green Chemistry, 23(24), pp.9747-9799.
Martino, M., Meloni, E., Festa, G. and Palma, V., 2021. Propylene synthesis: Recent advances in the use of Pt-based catalysts for propane dehydrogenation reaction. Catalysts, 11(9), p.1070.
Yang, F., Zhang, J., Shi, Z., Chen, J., Wang, G., He, J., Zhao, J., Zhuo, R. and Wang, R., 2022. Advanced design and development of catalysts in propane dehydrogenation. Nanoscale, 14(28), pp.9963-9988.
Sun, M.L., Hu, Z.P., Wang, H.Y., Suo, Y.J. and Yuan, Z.Y., 2023. Design strategies of stable catalysts for propane dehydrogenation to propylene. ACS catalysis, 13(7), pp.4719-4741.
Brune, A., Geschke, A., Seidel-Morgenstern, A. and Hamel, C., 2022. Modeling and simulation of catalyst deactivation and regeneration cycles for propane dehydrogenation-comparison of different modeling approaches. Chemical Engineering and Processing-Process Intensification, 180, p.108689.
