CFD Simulation of Mass Transfer Efficiency and Pressure Drop in a Structured Packed Distillation Column

The pressure drop and mass transfer efficiency for two‐phase flow in a structured packed column were simulated using a commercial CFD package, CFX version 10. The distillation of the methanol/isopropanol system was carried out in a 0.073 m diameter column, with an element composed of a ceramic structured packing and 0.053 m in height. The Height Equivalent to Theoretical Plate (HETP) value varied from 0.106–0.146 m. Pressure drop experiments were measured with an air/water system. The pressure drops at the flooding and loading points were ca. 173 and 580 Pa/m of packing, respectively. HETPs and pressure drops calculated from the Computational Fluid Dynamics (CFD) model were compared to their experimental counterparts. The average relative error between CFD predictions and the experimental data for the prediction of dry pressure drop, irrigated pressure drop and mass transfer efficiency are 20.3 %, 23 % and 9.15 %, respectively. In all cases, the CFD predictions show a good agreement with the experimental data, indicating that CFD is a reliable, cost saving and suitable technique for the design and optimization of separation processes.     

Hydrodynamics of Distillation Tray Column with Structured Catalyst Containing Envelopes: Experiments versus CFD Simulations

We report studies on the hydrodynamics of a distillation sieve tray column in which structured catalyst containing wire‐gauze envelopes are dispersed along the liquid flow direction. The gas and liquid phases are in cross‐current contact on the tray and were studied using Computational Fluid Dynamics (CFD). Experiments were carried out to determine the clear liquid height on a rectangular tray as a function of geometry and operating conditions. The agreement between the experiments and CFD simulations was found to be very good, suggesting that CFD simulations can be used for design and scale‐up purposes.     

Electrohydrodynamic Spraying (EHDS) of Conductive Liquids and Mass Transfer in EHDS Distillation Columns

A physicomathematical model for electrohydrodynamic spraying (EHDS) of a conductive liquid and the mass transfer in a parallel plate electrodes system of an EHDS distillation column is presented. The influence of the electric field on the formation and general characteristics of the finely dispersed aerosol flow of the conductive liquid in EHDS distillation columns has been determined from the model. It allows to explain in detail the mechanism of the influence of the electric field on the mass transfer in the gas‐liquid system. In addition, the maximum possible liquid flow rate in the mass transfer section of the EHDS distillation column has been estimated. The role of process parameters is discussed in detail.     

CFD Simulation of the Heat and Mass Transfer Process during Centrifugal Short‐Path Distillation

Numerical simulation was carried out for a centrifugal short‐path distillation arrangement using the computational fluid dynamics technology. Both two phases and interfacial transport were taken into account with an attempt to explore the heat and mass transport in the film body and the interface. The model tried to provide a general method to study the short‐path distillation in various situations. The effects of feed flow rates, feed and heating wall temperature controlling the development of the temperature and concentration along the axial and radial direction were investigated. On the basis of the simulation results, detailed characteristics such as distribution of temperature and concentration, heat and mass transfer coefficient throughout the liquid film can be described.     

规整填料内气液两相流动与传质的计算传质学模型

Detailed investigation of flow behavior in structured packing distillation columns is of great importance in accurate prediction of process efficiency and development of more efficient and optimal equipment internals. In this study, a three-dimensional two-phase flow model based on VOF method for simulating the hydrodynamics and mass-transfer behavior in a typical representative unit of the structured packing is developed. In the proposed model, the model is used for the closure of turbulent mass transfer equation. By solving the proposed model, the velocity distribution, phase fraction profile and concentration field are obtained. Using these data, the total liquid holdup, the wetted area and the separation efficiency [height equivalent to a theoretical plate (HETP)] are estimated. For testing the model validation, the simulated HETPs are compared with our previous experimental data obtained in a 150 mm-diameter column containing Mellapak 350Y operating at the pressures of 0.6-1.8 MPa. The compari-son shows that they are in satisfactory agreement, with an average absolute deviation (AAD) of 25.4%.     

利用计算流体力学及计算传质的方法研究精馏塔传质效率

It has long been found that the flow pattern of the liquid phase on distillation tray is of great importance on distillation process performance. But until now, there was very few published work on quantitative investigation of this subject. By combining the computational fluid dynamics (CFD) with the mass transfer equation, a theoretical model is proposed for predicting the details of velocity and concentration distributions as well as the tray efficiency of distillation tray column. Using the proposed model, four different cases corresponding to different assumptions of liquid and vapor flowing condition for a distillation tray column were investigated. In Case I, the distributions of velocity and concentration of the incoming liquid from the downcomer and the uprising vapor from the underneath tray spacing are uniform. In Case Ⅱ, the distribution of the incoming liquid is non-uniform but the uprising vapor is uniform. In Case Ⅲ, the distribution of the incoming liquid is uniform but the uprising vapor is non-uniform.In Case IV, the distributions of both the incoming liquid and the uprising vapor are non-uniform. The details of velocity and concentration distributions on a multiple sieve tray distillation column in four different cases were simulated using the proposed model. It is found that the shape of the simulated concentration profiles of vapor and the liquid is quite different from case to case. The computed results also show that the tray efficiency is highly reduced by the maldistribution of velocity and concentration of the incoming liquid and uprising vapor. The tray efficiency for Case Ⅰ is higher than Case Ⅱ or Case Ⅲ, and that for Case Ⅳis the lowest. It also reveals that the accumulated effect of maldistribution becomes more pronounced when the number of column trays increased. The present study demonstrates that the use of computational method to predict the mass transfer efficiency for the tray column, especially for the large one, is feasible.     

A Novel SiC Foam Valve Tray for Distillation Columns

The novel SiC foam valve tray was made of thin slices of SiC foam material with a high specific surface area. Hydrodynamic performances of the novel SiC foam valve tray were studied with air-water system at atmospheric pressure. These performance parameters included pressure drop, entrainment, weeping and clear liquid height. The mass transfer efficiency of the SiC foam valve tray was measured in laboratory plate column. Compared with the F1 float valve tray, the dry pressure drop was decreased about 25%, the entrainment rate was about 70% lower at high gas load, the weeping was much better, and the mass transfer efficiency was far higher. Thus, the overall performance of the novel SiC foam valve tray was better than that of F1 float valve tray.     

Dynamic Study of Distillation Column Operated with Tray Heat Source Combined with Reboiler

This work reports experimental tests using the distribution of energy in distillation by applying internal heat sources combined with a reboiler in order to reduce the transition time when the process is disturbed. In this way, a dynamic study was undertaken in a pilot column with ethanol and water, comparing the effects of a step disturbance in the reboiler and the same heat quantity in the intermediate tray of the column. The results showed that the use of the combined heat supply reduced the hydraulic delay in the distillation column and, consequently, the transition time. In this way, this study suggests that it is possible to use the heat distribution to control a distillation column using classical strategies and simple controllers, such as proportional-integral-derivatives (PID), and with this obtain faster responses.     

工业甲基萘及苊馏分双塔精馏系统的模拟和优化

苊是一种重要的化工原料,工业苊主要通过精馏和结晶相结合的办法来提取.通常以中质洗油为原料,通过双炉双塔精馏,同时提取70％以上的工业甲基萘和50％以上的苊馏分,然后再通过结品得到90％以上的工业苊.利用Aspen Plus软件工具,按照回流比最小,即消耗最低的原则,对甲基萘精馏塔和苊精馏塔的进行了详细优化,确定了甲基萘馏份的最佳采出量、各物料最佳进料位置、最佳采出位置等,最终达到了设计指标.模拟结果与工业化实施结果基本一致.     

精馏塔的故障诊断方案设计

本文简单地介绍了精馏塔故障诊断的必要性。主要讨论了精馏塔的故障诊断专家系统和故障诊断的具体步骤(其中主要包括建立设备档案、确定检测方法、检测周期及维修时间和性能预测及档案管理)。只有将维修技术、故障诊断技术和计算机远程监测技术有机的结合，才能有效地预防和避免事故、优化操作条件和延长操作周期，为企业取得更大的经济效益和社会效益。     

催化精馏塔装填方式的研究进展

综述了催化精馏塔催化剂的 4种装填方式 ,即板式塔装填方式、填充式装填方式、悬浮式装填方式和催化剂散装填料的技术进展 ,介绍了这些方式的优缺点及适用范围 ,提出了装填方式的发展趋势。     

Efficiency of Distillation Columns Containing Packed Sieve Trays

Abstract

The Murphree efficiency of the fifth tray in a six-sieve tray distillation column, 7.8 cm in diameter and 85 cm high, was increased from 37.5 to 90% when three disks of a wire mesh pad, 9 cm high, were placed on the tray. An acetone-methanol test system was used in this investigation. The pressure drop across the tower corresponding to this packing increased by 33%. Nine disks of Raschig rings of similar height increased the tray efficiency to 75% and the pressure drop by 16%. A model was used for the azeotropic system, and the overall tower efficiency was found to increase by about 20% due to the presence of packing on a single tray.     

雾沫夹带对蒸馏塔板效率的影响

在塔板上液体完全混合的条件下,雾沫夹带对蒸馏塔板效率的影响,导出了一个新的关系式.著名的Colburn关系式是本文关系式的一种特殊情况(λ_o=1).计算结果表明,用于校正雾沫夹带对塔板效率的影响,本文关系式与Colburn关系式相比,具有较小的误差.