Coolant Flow, Heat Transfer And Numerical Characteristics In A Multitubular Fixed-Bed Reactor

Multitubular fixed-bed reactors which are the main reactors to carry out highly exothermic heterogeneous catalytic reactions have been widely used in chemical industry. With the expansion of the Multitubular reactor scales and the highly-demand of the operational requirement, how to optimize the reactor design and operation, and how to improve the stability and economy of the reaction process are the important issues in the field of chemical reaction engineering. In this work, the coolant flow, heat transfer and numerical characteristics in a multitubular fixed-bed reactor are studied from the reactor design and operation point of view, so as to provide the reference and basis for large-scale industrial reactor design and operation.In the work, a coolant distribution and parallel flow model for a multitubular fixed-bed reactor and a coolant flow and heat transfer model for a cross flow multitubular reactor equipped with disk and doughnut baffles are respectively established. The main factors which have an influence on hole-distribution on the distributing plate in the parallel flow reactor and the main structural parameters which have an influence on operational performance in the cross flow reactor equipped with disk and doughnut baffles are studied. The numerical characteristics are explored on the basis of research on the cross flow reactor, which could be further applied to reactor monitoring and control. The main achievements in the thesis are as follows:As for parallel flow reactors, in the process of distributing tube holes and additional orifices on the distributing plate, additional orifice adjustment which fixes the size of the tube holes, and distributes the additional orifices on the distributing plate along the radial direction can obtain much wider range of the orifice size distribution than tube-to-plate clearance adjustment does. It is more feasible to adjust and manufacture the distributing plate through the additional orifice adjustment in an engineering sense. With the increase of designed velocity in the parallel flow region, the range of the orifice size distribution will decrease when the tube-to-plate clearances on the distributing plate keeps constant. In order to increase the orifice size distribution range, the perforation pressure drop in the margin of the distributing plate could be designed a little higher than the pressure drop in the distributing chamber in the design stage. In the process of scaling up the parallel flow reactor, the pressure drop in the distributing chamber will increase rapidly. The fluid pressure drop in the chamber will increase by31-33%when the diameter of the reactor increases by10%. The required tube-to-plate clearance accordingly needs to be decreased even though the additional orifices are well distributed on the plate. This small clearance is the bottleneck for manufacturing the distributing plate and assembling the reactor. Removing the tubes in the reactor central region and/or increasing the height of distributing chamber can reduce the fluid pressure drop in distributing chamber, and enlarge the required tube-to-plate clearance. The goal to scale up the reactor could be achieved. When the diameter of the reactor increases by10%, the height of the distributing chamber has to be increased by20%to balance the increased pressure drop in the chamber in process of scaling up the reactor. Setting a large area of non-tube region in the reactor center could also evidently reduce the pressure drop in distributing chamber. When the reactor size increases by10%, the diameter of the central non-tube region has to be set at least30%of the reactor inner diameter to balance the increased pressure drop in the chamber in process of scaling up the reactor.As for cross flow reactors equipped with disk and doughnut baffles, reactor window zones make the coolant flow rate decay rapidly. Low value of the coolant flow rate is unfavorable for heat transfer in the reactor central and wall adjacent region. Removing the tubes in the reactor central region ensures the proper fluid environment around reactor innermost tubes, keeps the hot spot temperature in the central region in a proper level, and thus reduces the difference of the hot spot temperature in the reactor. For a reactor with4.6m in diameter, the optimal size of the central non-tube region is10%of the reactor inner diameter. The coolant flow rate in the zone between the disk and doughnut baffles and that in the central window zone can be adjusted through adjusting the size of the doughnut baffle opening. The larger the doughnut baffle opening, the higher the coolant flow rate in the zone between the disk and doughnut baffles, but the large opening causes the central window zone large, which makes the flow rate in the reactor center low. Decreasing the doughnut baffle opening could increase the flow rate in the central region, but the flow rate in the zone between the disk and doughnut baffles decreases at the same time. The pressure drop of the reactor increases significantly with the doughnut baffle opening decrease. When the size of the central non-tube region is large, it is advisable to make doughnut baffle opening large, and thus the flow rate in the zone between the disk and doughnut baffles could be high. When the size of the central non-tube region is10%of the reactor inner diameter, the optimal size of the doughnut baffle opening is25%of the reactor inner diameter. Similarly, the wall adjacent window zone could be adjusted through adjusting the size of the disk baffles. Tube to baffle clearance has a great influence on coolant flow and temperature distribution. When the tube to baffle clearance is small, the cross flow is the predominant flow pattern in the reactor, and the coolant temperature increases along the coolant flow direction. The catalyst temperature distribution is desirable in this situation. When the tube to baffle clearance is relatively large, there is obvious leakage flow on the baffle, which makes coolant flow and temperature show marked radial distribution and makes the difference of the hot spot temperature large in the reactor. Decreasing the baffle clearance helps to strengthen the coolant capability of heat removal in the zone between the disk and doughnut baffles, but the hot spot position inside the tubes moves to the depth of the wall adjacent window zone, where the hot spot temperature in the tubes turns high, and makes the difference of the hot spot temperature large in the reactor.As for the research on the numerical characteristics of the reactor, the Karhunen-Loeve (K-L) expansion is performed in a two-step mode on the dataset of the catalyst bed temperature distribution in a multitubular fixed-bed reactor equipped with disk and doughnut baffles under different operating conditions. The two-step K-L expansion, which uses the first three eigenvectors in the first step, and the first eigenvectors in the second step, can reconstruct the bed temperature well. The catalyst bed temperature distribution in both axial and radial directions can be represented by three coefficients related to a particular operating condition. Thus the bed temperature distribution can be reduced to a state point in a coefficient space. The deviation of a particular operating variable makes the state point move along a specific trajectory. Two model parameters (θ and г) are established to recognize the deviated operating variable and quantify the deviated extent.