A study of mass transfer behaviour in a catalytic distillation column

Catalytic distillation (CD) is a process where reaction and separation take place simultaneously in a distillation column. Recently, a new CD process for the aldol condensation of acetone to diacetone alcohol and mesityl oxide was developed in our laboratory. Owing to the unique structure used to support catalysts in the CD column, mass transfer data are needed to model the CD process. The mass transfer behaviour in the CD column has been studied both for the reactive and non-reactive sections of the column in our laboratory. Based on the experimental results, general correlations for the mass transfer coefficients have been established and the 95% confidence intervals for the associated parameters were obtained. The experimental results indicated that the catalyst bags did not play a significant role for the separation between the vapour and liquid phases. The mass transfer correlations developed in this study have been used successfully to model the CD process for the aldol condensation of acetone carried out in our laboratory.     

Simultaneous heat and mass transfer in a packed binary distillation column

A simultaneous heat and mass transfer model was applied to a binary system in a 3-in. distillation column packed with $\text{1}\text{4}$in. Raschig Rings. MaThe experiments carried out with the toluene-trichloroethylene system show that the liquid phase is saturated, indicating no resistance to mass transfe     

Simultaneous heat and mass transfer for multicomponent distillation in a wetted-wall column

A simultaneous heat and mass transfer model was applied to multicomponent distillation in a wetted-wall column. Experiments were carried out, using the benzene-toluene-ethylbenzene ternary system, in a wetted-wall column of 2.2 cm i.d. and 100 cm long equipped with special probes designed for simultaneous liquid sampling and temperature measurements. The experimental results show that the bulk liquid phase was saturated, indicating no resistance to mass transfer in the liquid film. Confirmation of the liquid phase saturation was made through a comparison of the experimentally measured liquid temperature with the calculated bubble point temperature. The average deviation between the measured and calculated temperatures was found to be 0.26°C.Individual mass transfer rates were evaluated locally by measuring compositions and temperatures as functions of column height and were compared to theoretical predictions using the exact film model solution of the Maxwell-Stefan multicomponent equations developed by Krishna and Standart. The comparison shows good agreement with average deviations of 15.76% for benzene, 23.09% for toluene and 23.23% for ethylbenzene.     

Liquid phase mass transfer resistance in a small scale packed distillation column

Binary distillation experiments using the toluene-trichloroethylene system were conducted in a 3-in. diameter column packed with 0.25 in. Raschig rings. In order to determine the liquid phase mass transfer characteristics, gas absorption studies with CO₂-water system were also performed on the same column. It was shown in both cases that end-effects can be ettectively removed by using a “differential height” technique. It was observed in the distillation experiments that the liquid phase was saturated, indicating a negligible resistance to mass transfer in the liquid. Results from the absorption experiments indicate that this resistance is small but not negligible.     

A modified model of computational mass transfer for distillation column

The computational mass transfer (CMT) model is composed of the basic differential mass transfer equation, closing with auxiliary equations, and the appropriate accompanying CFD formulation. In the present modified CMT model, the closing auxiliary equations [Liu, B.T., 2003. Study of a new mass transfer model of CFD and its application on distillation tray. Ph.D. Dissertation, Tianjin University, Tianjin, China; Sun, Z.M., Liu, B.T., Yuan, X.G., Liu, C.J., Yu, K.T., 2005. New turbulent model for computational mass transfer and its application to a commercial-scale distillation column. Industrial and Engineering Chemistry Research 44, 4427-4434] are further simplified for reducing the complication of computation. At the same time, the CFD formulation is also improved for better velocity field prediction. By this complex model, the turbulent mass transfer diffusivity, the three-dimensional velocity/concentration profiles and the efficiency of mass transfer equipment can be predicted simultaneously. To demonstrate the feasibility of the proposed simplified CMT model, simulation was made for distillation column, and the simulated results are compared with the experimental data taken from literatures. The predicted distribution of liquid velocity on a tray and the average mass transfer diffusivity are in reasonable agreement with the reported experimental measurement [Solari, R.B., Bell, R.L., 1986. Fluid flow patterns and velocity distribution on commercial-scale sieve trays. AI.Ch.E. Journal 32, 640-649; Cai, T.J., Chen, G.X., 2004. Liquid back-mixing on distillation trays. Industrial and Engineering Chemistry Research 43, 2590-2597]. In applying the modified model to a commercial scale distillation tray column, the predictions of the concentration at the outlet of each tray and the tray efficiency are satisfactorily confirmed by the published experimental data [Sakata, M., Yanagi, T., 1979. Performance of a commercial scale sieve tray. Institution of Chemical Engineers Symposium Series, vol. 56, pp. 3.2/21-3.2/34]. Furthermore, the validity of the present model is also shown by checking the computed results with a reported pilot-scale tray column [Garcia, J.A., Fair, J.R., 2000. A fundamental model for the prediction of distillation sieve tray efficiency. 1. Database development. Industrial and Engineering Chemistry Research 39, 1809-1817] in the bottom concentration and the overall tray efficiency under different operating conditions. The modified CMT model is expected to be useful in the design and analysis of distillation column.     

计算传质法预测精馏塔效率(英文)

A computational mass transfer model is proposed for predicting the concentration profile and Murphree efficiency of sieve tray distillation column. The proposed model is based on using modified two equations formulation for closing the differential turbulent mass transfer equation with improvement by considering the vapor injected from the sieve hole to be three dimensional. The predicted concentration distributions by using proposed model were checked by experimental work conducted on a sieve tray simulator of 1.2 meters in diameter for de-sorbing the dissolved oxygen in the feed water by blowing air. The model predictions were confirmed by the ex-perimental measurement. The validation of the proposed model was further tested by comparing the simulated re-sult with the performance of an industrial scale sieve tray distillation column reported by Kunesh et al. for the strip-ping of toluene from its water solution. The predicted outlet concentration of each tray and the Murphree tray effi-ciencies under different operating conditions were in agreement with the published data. The simulated turbulent mass transfer diffusivity on each tray was within the range of the experimental result in the same sieve column re-ported by Cai et al. In addition, the prediction of the influence of sieve tray structure on the tray efficiency by using the proposed model was demonstrated.     

Efficiency of mass transfer trays with allowance made for nonequimolar counterdiffusion

An analysis of experimental data on the distillation of methanol-water, ethanol-water, and water-acetic acid mixtures is performed. A considerable difference in the efficiency of separating a water-acetic acid mixture and aqueous alcohol solutions is noted. This phenomenon is explained by the difference in the heats of evaporation of components of the studied mixtures and the associated nonequimolar counter-diffusion. Taking into account nonequimolar counter-diffusion made it possible to describe the dependence of mass transfer coefficients by a unified equation for all of the studied mixtures.     

炼油减压精馏塔汽提工艺传质过程模拟与分析

为了揭示减压塔汽提工艺的传质过程特性,进而改进汽提操作,采用改进的减压精馏过程模拟方法和传质计算方法,分析了炼油常减压装置的汽提工艺的传质特点。在深化对于汽提工艺的理解基础上论证了采用填料汽提塔(段)的必要性以及设计、分析的基本方法。计算分析结果表明侧线汽提塔内填料分离性能保持恒定,汽提塔的分离效率与减压塔对应分离段接近,所以在设计和操作中应给予汽提塔(段)充分重视。减压塔底汽提段的工艺和水力学特点与侧线汽提塔相同。塔底汽提蒸汽质量流率应小于减压塔进料质量流率1%,否则将导致减压塔整体分离效率降低。     

Mass transfer in a packed extractive distillation column

The kinetics of mass transfer in the extractive distillation of the benzene–heptane mixture (which imitates the catalytic reforming product mixture) in the presence of N-methylpyrrolidone as the separating agent has been investigated. The separating agent exerts an effect on the overall efficiency of the process. The partial heights of transfer units in the vapor and liquid have been determined, and equations for calculating these parameters have been set up.     

乙烯生产流程中联塔模拟与优化

在建立精馏塔单塔严格机理形式的开放式模型方程, 并对单塔进行模拟与优化的基础上, 对大规模乙烯生产过程中脱丙烷塔和脱丁烷塔组成的联塔系统进行模拟与操作优化研究.在对整个系统分析和处理后, 成功对其进行了模拟与优化, 结果对脱丙烷和脱丁烷塔精馏过程和操作优化具有指导意义.在优化过程中, 改进过的简约空间序列二次规划(reduced-space sequential quadratic programming, RSQP)算法具有比SNOPT和FMINCON更高的求解效率也得到验证.本文对联塔系统的研究和在模拟与优化过程中的处理、优化方法为将来整个乙烯生产流程系统的模拟与优化奠定了良好的基础.     

Effectiveness of mass transfer in a packed distillation column in relation to surface tension gradients

In packed columns large differences occur in the wetting of the particles and especially in the refreshing of the liquid on the wetted particles due to gradients in surface tension of the liquid/gas interface. Mass transfer rates may differ with a factor 2. In a column packed with Berl saddles distillation experiments were performed with a mixture n-heptane/cyclohexane. Ceramic Berl saddles of 4, 6 and 10 mm were used as well as aluminum Berl saddles of 4 mm. In some of the experiments the saddles were coated with PTFE (teflon). The driving force for mass transfer was varied over a wide range. Both negative and positive driving forces were realized. The influence of the surface tension driven refreshment of the interface is most pronounced for small particles; for larger liquid flow rates, that may be applied in beds with larger particles, the effect is obscured by the inertia of the downcoming liquid.     

Impact of mass transfer coefficient correlations on prediction of reactive distillation column behaviour

A significant part of the safety analysis of a reactive distillation column is the identification of multiple steady states and their stability. A reliable prediction of multiple steady states in a reactive distillation column is influenced by the selection of an adequate mathematical model.For modelling reactive distillation columns, equilibrium (EQ) and nonequilibrium (NEQ) models are available in the literature. The accuracy of the nonequilibrium stage model seems to be limited mainly by the accuracy of the correlations used to estimate the mass transfer coefficient and interfacial area.The binary mass transfer coefficients obtained from empirical correlations are functions of the tray design and layout, or of the packing type and size, as well as of the operational conditions and physical properties of the vapour and liquid mixtures.In this contribution, the nonequilibrium model was used for the simulation of a reactive distillation column. For prediction of the binary mass transfer coefficient for a sieve tray, four correlations were chosen to show their impact on the prediction of the reactive distillation column behaviour. As a model reactive distillation system, the synthesis of methyl tertiary-butyl ether (MTBE) was chosen. The steady-state analysis and the dynamic simulation of the model system were done. Qualitative differences between the steady states were predicted using the chosen correlations.     

PET生产中蒸馏塔的清洗操作

在PET生产中根据需要停止和开启蒸馏塔,期间出现塔循环易堵塞等问题,通过制定清洗方法较好地解决了该问题。     

Simulation of hydrodynamic behaviour and mass transfer efficiency of reduced pressure distillation

A random packing hydrodynamic simulator is designed specially to carry out experiments under reduced pressures with a counter-current flow air/water system. The simulator results (air/water system) compared to those obtained in a real packed distillation column (benzylchloride/ ethylbenzene system) show a good agreement between hydrodynamic parameters like pressure drop, flooding flow rate, and total liquid hold-up. Empirical relationships derived from the simulator experimental results are proposed allowing the calculation of the pressure drop in a packed distillation column, operating in the pressure range between 10 kPa and atmospheric pressure. The influence of reduced pressure on the HETP in the distillation column and interfacial area in the simulator was also investigated.     

Gas-liquid mass transfer in a vibrating disk column

Gas—liquid mass transfer has been studied in a vibrating disk column with CO₂ gas—water concurrent flow. The overall volumetric mass transfer c     

On the modelling of mass transfer in extractive distillation

Mass transfer in extractive distillation in plate columns on a laboratory and pilot-plant scale was investigated experimentally by the separation of an azeotropic mixture, acetone—methanol, with water and 1,2-propylene glycol as solvents. The decrease in mass transfer efficiency with increasing solvent concentration was explained by a decrease and partial blocking of the interfacial area caused by the solvent. A mathematical model was established for mass transfer in extractive distillation on valve plates of diameter 0.4 m. This model was applied successfully to an industrial column.     

考虑不同产品选择性目标的乙醇胺反应精馏塔模拟与优化

提出以环氧乙烷和液氨为原料,水为催化剂,反应精馏塔合成乙醇胺的工艺。分别以一乙醇胺（MEA）和二乙醇胺（DEA）的选择性最大为目标,探讨了在同一反应精馏塔中实现不同产物选择性目标的可行性和可达到的最大选择性。研究在Aspen Plus模拟软件上进行,通过考察操作压力、水进料量、进料氨烷比、再沸比、环氧乙烷进料位置等参数对反应精馏的影响规律,取得实现不同产物选择性目标的参数优化调节方法。研究表明,大的氨烷比、水进料量和再沸比有利于提高MEA选择性,小的氨烷比、水进料量和再沸比则有利于提高DEA选择性,在优化条件下,MEA选择性可达70.30%,DEA选择性可达41.89%。与文献方法比较,采用反应精馏合成乙醇胺具有明显的优越性和操作柔性。     

吸收膜蒸馏的传热传质过程

由于吸收膜蒸馏系统的进料液和吸收液的温度非常接近甚至相同,其传热过程无相变热损失,有可能实现能耗较低的膜蒸馏过程,本文尝试开展吸收膜蒸馏法海水淡化研究。首先分别以葡萄糖水溶液和氯化钙水溶液为吸收液进行吸收膜蒸馏海水淡化实验,结果表明氯化钙吸收液时的膜通量明显高于葡萄糖吸收液时的膜通量。其次建立了吸收膜蒸馏过程的质量和热量传递模型,并对模型进行了计算,模型计算值与实验值非常接近。最后对葡萄糖水溶液和氯化钙水溶液分别作吸收液时对传质系数和极化现象的影响进行了计算分析,结果显示,氯化钙水溶液作吸收液时传质系数约为葡萄糖水溶液的1.7倍;且极化现象造成的蒸气压力差减小的值为后者的1/4左右。     

Binary and ternary distillation in a stirred mass transfer cell

A continuous flow stirred cell operating at point conditions has been developed for the study of mass transfer in distillation. Results for the methanol-isopropanol and methanol-ethanol binary systems showed that 85–100% of the resistance to mass transfer resided in the vapour phase, and that the vapour side coefficients correlated well with those obtained in the same cell for gas phase controlled absorption. Flux rates for the ternary system methanol-ethanol-isopropanol were in fair agreement with those predicted from the binary data, using a modified form of the Krishna-Standart theory. It is concluded that the experimental technique shows considerable promise, but requires further development to overcome discrepancies in the interfacial heat balance.     

动力学不确定的甲苯二异氰酸酯装置光化反应精馏塔的模拟分析

以甲苯二异氰酸酯的实际工业生产为基础,建立胺盐光化反应的数学模型,并计算出反应活化能。针对动力学参数的不确定性,提出一种模拟分析方法。该方法应用Aspen Plus软件,假设胺盐反应完全,对反应精馏塔内不同工况进行指前因子估算及模拟分析。结果表明,胺盐消耗量在第N-1块塔板处出现最小值;不同工况沿塔板温度变化不大;沿塔板液相胺盐摩尔分数变化较大,光气摩尔分数变化较为缓和。