Nonlinear Dynamic Analysis For Suspension Catalytic Distillation

A novel kind of heterogeneously catalytic distillation technology,suspension catalytic distillation(SCD),has been proposed and developing by Research Institute of Petroleum Processing,China.Basing on the conceive that allows on-stream removal and regeneration of catalyst,the very fine catalyst particles are suspended in the liquid and recycled or regenerated partly via a solid/liquid separator equipped outside the column.SCD process not only inherits the advantages of the conventional catalytic distillation(CD),but also overcomes the limitation of intraparticle diffusion involved in the traditional CD.Therefore,the catalytic efficiency is significantly improved and the amount of used catalyst is largely reduced.At present,the SCD process is mainly used for the alkylation reaction with a new-developed supported PW heteropoly acid catalyst.Exploratory experiment of this new process has already been carried out with promising results showing it is a vitally important innovation and is worthwhile for further R&D efforts.In addition,it is needed to consider if this kind of CD technology has potential advantages for other reaction systems.However,the introductions of solid/liquid separator and liquid external recycling,along with the strong nonlinear interaction introduced by the coupling between dispersion and chemical kinetics in counter-current contacting,not only lead SCD process more difficult to modeling and simulation,but also can trigger the phenomenon of complex nonlinear dynamics.Therefore,successful industrialization of SCD technology requires careful attention to the modeling and nonlinear dynamics aspects.Firstly,based on some reasonable simplifications,we develop generic steady-state and dynamic equilibrium stage models for SCD process,proposed corresponding steady-state calculation method and dynamic simulation arithmetic, and introduced the method of nonlinear dynamic analysis detailedly.Secondly,steady-state and dynamic simulations are presented for linear alkylbenzene synthesis by SCD.The results show that the predicted values of the state variables agree with the experimental data well.The models developed can be used for in-depth steady-state and dynamic simulations,and nonlinear dynamic study for SCD process.Only one steady state can exist in this system for a given interested range of inputs.The operation pressure is the most important manipulated parameter,and the product purity(the liquid mole fraction of major product in bottom)has its maximum at about 0.2 MPa.The last stage in reactive section is the optimal location of benzene feed stream.The system is stable for the imposed disturbances,and its anti-disturb ability is very strong.Although the ratio of benzene to olefins in the feed is very small,about 1:1,this ratio in the reactive section is very high owing to distillation effect.This not only helps to prevent the olefins from further transforming to waste byproducts,but also prolongs the life of catalyst because the excessive benzene washes the catalyst particles continuously.Then,steady-state simulation and rigorous bifurcation analysis are presented for methyl tert-butyl ether(MTBE)synthesis by SCD.The results show that,to obtain excellent performance,the process has to be operated in a region with multiple attractors with the resulting difficulties for process operation and control. An unsuitable start-up strategy can yield an inappropriate steady state.It is suggested that the SCD process can be started up at sufficiently low reflux ratio where only one steady state is present.Once the column reaches the corresponding steady state,the reflux ratio must be increased slowly up to its design value.Thus, this column can track the high-conversion solution branch to attain the desire steady state with high conversion of isobutene and high product purity.The solid/liquid separator ratioφis an important manipulated parameter for this SCD process,and the superior column performance can be reached under practical operating conditions because the upper bound of the multiplicity range for the parameterφis very small,about 0.025.The parameter plane of input variablesφand bottom flowrate is divided into three subfields by limitation point curves,and the top right one is the acceptable operation region with high conversion of isobutene and high product purity.The solution branch diagram displays very intricate and highly nonlinear dynamic behavior with respect to the operation pressure,and the SCD column shows four types of multiplicities:two large-scale three-branch multiplicities,a large-scale five-branch multiplicity and a small-scale "switchbacking" multiplicity,which can result in as many as seven steady-state solutions with identical column specifications.Finally,steady-state simulation and rigorous nonlinear dynamic analysis are presented for ethylene glycol synthesis using SCD technology from ethylene oxide and water.The results show that,for the high-selectivity steady state,although the water ratio(the molar fraction ratio of water to ethylene oxide)in the feed is very small,about 0.95:1,this ratio in the reactive section is very high owing to distillation effect.This can prevent the major product,ethylene glycol,from further reacting with the excessive ethylene oxide to yield waste byproducts.To obtain satisfactory performance,the process has to be operated in a region with three distinct attractors and second-order input multiplicities with the resulting difficulties for process start-up and control.Maybe,certain perturbations in manipulated variables cause the system suffer an undesired transition from a high-selectivity steady state to a low one.The SCD process can be started up at sufficiently low boilup ratio where only one steady state is present.Once the column reaches the corresponding steady state,the ratio must be increased slowly up to its design value.Thus,this SCD column can track the high-selectivity solution branch to enter the domain of attraction of the desire attractor.The solid/liquid separator ratioφ,does not result in qualitative change in the number and form of steady-state solutions.Compared with the external recycling,the internal recycling(i.e.,reciprocal of reflux ratio)has a more significant and complex effect upon this system.The operation pressure is the crucial manipulated parameter for this SCD process,and the superior column performance can be reached under practical operating conditions because the multiplicity range is narrower and the upper bound of this range for the pressure is very small,about 0.2 MPa.The solution branch diagram displays very intricate and highly nonlinear dynamic behavior with respect to the holdup volume.The column has two types of multiplicities:a large-scale three-branch multiplicity and a complex small-scale "switchbacking" multiplicity,which can result in as many as nine steady-state solutions with identical column specifications.