ANALYSIS OF DIALYSIS IN CROSS-FLOW PARALLEL-PLATE MEMBRANE MODULES WITH FEED-STREAM RECYCLE FOR IMPROVED PERFORMANCE

The effect of external recycle on the performance of dialysis in cross-flow rectangular membrane modules with feed-stream recycle was investigated theoretically. The analysis of mass transfer in parallel-plate devices with and without recycle is analogous to heat transfer in parallel-plate heat exchangers. In contrast to a device without recycle, improvement in mass transfer is achievable if cross-flow parallel-plate dialysis is operated in a device of the same size with recycle, which provides increased fluid velocity, resulting in reduction of the mass-transfer resistance.     

Recycle effect on membrane extraction through cross-flow rectangular modules

The effects of recycle on membrane extraction through a cross-flow rectangular module have been studied both theoretically and experimentally. Theoretical analysis of mass transfer in cross-flow device with and without recycle is analogous to heat transfer in cross-flow heat exchangers. Experiments were carried out with the use of a membrane sheet made of microporous polypropylene as a permeable barrier to extract acetic acid from aqueous solution by methyl isobutyl ketone. Theoretical predictions are in agreement with the experimental results. In contrast to a device without recycle, improvement in mass transfer is achievable if cross-flow membrane extraction is operated in a device of same size with recycle which provides the increase of fluid velocity, resulting in reduction of mass-transfer resistance. It is shown that recycle can enhance mass transfer, especially for large feed-concentration solution with high distribution coefficient operated under high reflux ratio.     

Application of internal reflux in the raffinate phase for membrane extraction in cross-flow rectangular modules

The effect of internal reflux in the raffinate phase for membrane extraction through a cross-flow rectangular module was investigated. Theoretical analysis of mass transfer in cross-flow devices with or without recycle was analogous to heat transfer in cross-flow heat exchangers. In contrast to a device without reflux, considerable improvement in mass transfer is achievable if cross-flow membrane extractors are operated with reflux which provides the increase of fluid velocity, resulting in reduction of mass-transfer resistance. It is concluded that reflux can enhance mass transfer, especially for large flow rate and feed-concentration solution operated under high reflux ratio.     

Analysis of mass transfer in parallel-flow dialyzer with internal recycle for improved performance

The internal reflux effect on dialysis through the retentate phase of a parallel-flow rectangular module is investigated. Theoretical analysis of mass transfer in the membrane devices with or without recycling is analogous to heat transfer in heat exchangers. In contrast to a device without reflux, considerable mass transfer is achievable if parallel-flow dialyzers are operated with reflux, which provides an increase in fluid velocity, resulting in a reduction in mass-transfer resistance. It is concluded that reflux can enhance mass transfer, especially for large flow rate and feed-concentration operated under high reflux ratio.     

Membrane extraction in rectangular modules with external recycle

The analysis of solvent extraction in the membrane modules of cocurrent, countercurrent and cross-flows with external recycle, has been carried out by mass-transfer theory, analogous to heat transfer in recycled heat exchangers. Considerable improvement is achievable by recycle operation if the increase in mass-transfer coefficient by applying the recycle effect can compensate for the decrease in the driving force of mass transfer due to the dilution of inlet feed solution. Under comparable conditions most solute is extracted in countercurrent-flow arrangement, the least in cocurrent-flow. However, the separation efficiencies of the three flow-type devices approach in unity for lower feed concentration and/or larger flow rate and/or larger reflux ratio.     

MASS TRANSFER ENHANCEMENT OF CONJUGATED GRAETZ PROBLEMS IN MULTI-PASS PARALLEL-PLATE MASS EXCHANGERS WITH EXTERNAL RECYCLE

A new external recycle design at the ends of multi-pass parallel plate mass exchangers with uniform wall concentration is theoretically investigated in this study. The analytical solutions were obtained using the orthogonal expansion technique and the eigenfunction expanding in power series. The results are graphically represented. The influences of the recycle ratio, mass-transfer Graetz number, channel thickness ratio, and characteristic parameter of the permeable barrier on the mass transfer rate are presented in this study. Compared to the single-pass device, considerable mass transfer improvement is obtainable by employing the new external recycle design at both ends.     

EXTERNAL RECYCLE ON UPWARD-TYPE DOUBLE-PASS SOLAR AIR HEATERS

The external recycle effect on collector efficiency in an upward-type double-pass flat-plate solar air heater is investigated theoretically. Application of recycle to a solar air heater has two conflicting effects: the desirable effect of increasing the heat transfer coefficient and the undesirable effect of decreasing the heat transfer driving force (temperature difference). The theoretical predictions show that more than 150% improvement in collector efficiency is obtainable if external recycling is carried out in a way that the advantage effect compensates for the disadvantage effect. The collector performance improvement increases with increasing recycling ratio, especially when operating at a lower airflow rate with higher inlet air temperature. The collector efficiency obtained with a double-pass device with recycling improves the efficiency obtained with a single-pass device of the same size with recycling.     

循环流化床锅炉一体化外置式换热器

大型循环流化床锅炉广泛采用各种形式的外置式换热器。传统的换热器只能实现换热器（EHE）和返料机构（loop seal）之间的物料流量的调节,此时进入EHE的物料将会全部顺次流过各个换热仓室,很难对外置床中的各种受热面的传热量进行单独调节。本文首次提出了一种全新的非机械阀式外置式换热器布置方式,通过控制流入各个换热仓室的固体物料流量,从而达到对各换热仓室中布置的受热面的换热情况进行单独调节。同时把EHE和loop seal结合在一起,保证向炉膛的返料。文中对这种一体化外置式换热器及其返料机构中的物料流动特性进行了冷态试验研究。试验结果表明,这种外置式换热器有很好的物料分流和流量控制特性。可以通过调节运行参数和结构参数来控制两个换热室、EHE和loop seal以及两个返料口之间的物料流量和比例。同时还得到了物料在换热室的流动特性。通过对可见输送分离高度的测量,提出了分配室和换热室相对隔板高度的设计方法。     

Fast gas–liquid–solid reactions in monoliths: A case study of nitro-aromatic hydrogenation

Monolith catalyst supports are attractive as fixed bed reactors that, at the scale of the catalyst dimension, exhibit the mass transfer characteristics of slurry reactors. This paper presents a reactor design study for the single-pass conversion of dinitrotoluene in a loop configuration with an external heat exchanger. The advantage of such a loop system is the elimination of a solvent, which in turn allows more reaction heat to be recovered. The advantages of using a monolith are the low pressure drop at high recycle ratio, while maintaining good mass transfer characteristics. The modelling includes internal diffusion limitation, external mass transfer characteristics, heat effects, maldistribution and flow stability. The optimal design is found at the lowest hydrodynamic stable flow rates, where the mass transfer is fastest and the residence time in the column maximal.     

A study of mass transfer efficiency in a parallel-plate channel with external refluxes

The phenomenon of mass transfer through a parallel-plate channel with uniform wall concentration and external refluxes has been investigated by use of an orthogonal calculation technique. Considerable improvement is achieved when the external refluxes and barrier position are suitably adjusted. Analytical results show that recycle can enhance the mass transfer efficiency for high inlet flow rate compared with that in a single-pass device (without a permeable barrier inserted).     

On the examination of recycle on heat (and mass) transfer in concentric tubes

The effect of recycle on heat (and mass) transfer in concentric circular tubes has been investigated by finite difference methods. Theoretical results show that recycle can enhance the heat transfer rate for large Graetz numbers compared with that in an open tube (without an inner tube inserted and without recycle). Competition between a preheating effect and a residence-time effect can be used to explain the heat transfer behavior. The heat transfer rate can be further augmented, with nearly no increase in total pressure drop, by employing flow pattern B instead of flow pattern A.     

A Theoretical Study of the Recycle Effect on Multi‐Pass Mass Exchangers with Three Idealized Membranes Inserted

A new device of multi‐pass mass exchanger was developed by inserting three idealized membranes to divide an open conduit into four subchannels. The mathematical formulations of the device with external recycle, as referred to a well‐known conjugated Graetz problem, were conducted to improve the mass transfer efficiency. The eigenfunction expansion technique with the eigenfunction expanding in terms of an extended power series was used to solve such a conjugated Graetz problem analytically. The theoretical predictions were presented graphically and compared with those in the single‐pass operations (without any idealized membrane inserted and external recycle) of the same working dimensions.     

Synthesis of tertiary amyl methyl ether (TAME): Influence of internal and external mass transfer resistance

An experimental set-up is presented for the measurement of reaction rates in the liquid phase synthesis of tertiary amyl methyl ether (TAME) from methanol and a mixture of 2-methyl-1-butene and 2-methyl-2-butene using an acidic ion exchange resin (Lewatit SPC 118) as catalyst. The reaction was performed in a continuous-flow recycle reactor at a temperature of 333.15 K and a pressure of 1.6 MPa. Determination of the age distribution (Fcurve) of the reactor showed that mixed flow is approached at a recycle ratio of R = 10. Experiments were performed to reveal the limits of operating conditions where the various resistances do not affect the rate. In agreement with estimation of Reynolds numbers, hindrance by external mass transfer can be excluded at a volumetric flow rate of 10 ml/min and a recycle ratio of R = 10. The maximum temperature increase of the whole particle resulting from limitation of external heat transfer was estimated to be about 3 K. Variation of the size of resin particles revealed the existence of a critical methanol concentration, above which reaction rates can be determined without influence of internal mass transport phenomena. The maximum temperature increase in the particle was estimated to be 0.3 K.     

Recycle Effect on Double‐Pass Concentric Circular Mass Exchangers under Uniform Wall Fluxes

A study on improvement in device performance of a double‐pass concentric circular mass exchanger with uniform wall fluxes and external refluxes was conducted under counterflow operations. The linear superposition of an asymptotic solution and a homogeneous solution with the use of an orthogonal expansion technique for solving such a conjugated Graetz problem is formulated theoretically and the analytical predictions are applied to investigate a substantial masstransfer improvement. A mathematical expression for the average Sherwood numbers and the outlet dimensionless concentration is given. Comparisons of analytical results are made with the single‐pass operations of the same working dimensions (without permeable barriers inserted and without recycle). Considerable device improvement in mass transfer rate is obtained by introducing the recycle‐effect concept in designing such double‐pass devices. The effect of permeable barriers on the mass‐transfer efficiency improvement as well as on the power consumption increment has also been delineated.     

An investigation in the effects of recycles on laminar heat transfer enhancement of parallel-flow heat exchangers

It was demonstrated that fluid recycling could effectively enhance heat transfer rates of heat exchangers, however, related investigations were limited. In the current work, parallel-flow heat exchangers with basic recycles or revised recycles are investigated in the laminar regime. Theoretical models of thermo-hydraulic performances are established. The effects of reflux ratio, capacitance rate ratio, heat transfer area, and recycle length are investigated. The results demonstrate that the dimensionless heat transfer rate rises with the increase of reflux ratio or capacitance rate ratio, or with the decrease of heat transfer area, and the maximum values reach up to 127% and 121% for basic internal and external recycles, respectively. Basic internal recycles generate larger dimensionless heat transfer rates under larger reflux ratios, while basic external recycles perform more reliably over the whole reflux ratio range. Compared with basic recycles, revised recycles (i.e., partial-length recycles) require smaller pumping powers. Thus, partial-length recycles can improve the dimensionless overall performance of full-length recycle heat exchangers, e.g., half-length recycles increase the dimensionless overall performance by 65%. Fluid recycling does not need to change geometrical structures and fluid flow rates, thus it is a competitive approach of thermal augmentation in heat exchangers.     

错流旋转填料床的质、热同传性能及传热机理研究

为了研究错流旋转填料床的质、热同传性能,采用热空气-氨水体系,考察了进气温度T、超重力因子β、液体喷淋密度q和气速u对错流旋转填料床传热性能的影响,在相同实验条件下对比了丝网填料和乱堆填料的传热性能。研究结果表明：气相体积传质系数k_ya_e、体积传热系数(Ua)_s随进气温度、超重力因子、气速、液体喷淋密度的增大而增大;传热效率ε、传热面积A随超重力因子、气速、液体喷淋密度的增大而增大;传热系数K随超重力因子、气速、液体喷淋密度的增大几乎不变,从而揭示了错流旋转填料床强化气液直接传热的机理是通过提高传热面积进而提高体积传热系数,而不是显著提高传热系数。在相同条件下,以丝网为填料时k_ya_e和(Ua)_s分别是乱堆填料的1.09~1.63倍和1.24~3.53倍。     

气固流化床外取热器内流动和换热特性分析

外取热器是维持催化裂化反应-再生系统热平衡和保持装置平稳运行的关键设备之一。外取热器的优化设计和合理调控,要求深入理解外取热器内的流动特性、换热特性及两者之间关系。在一套大型冷模热态实验装置上,分别考察了表观气速、颗粒质量流率对换热管附近的局部固含率和气泡频率、床层与换热管间传热系数的影响。结果表明:增加表观气速可以降低局部固含率、增加局部气泡频率、强化床层与换热管间换热;随着颗粒质量流率增加,局部固含率和局部气泡频率均增加;在较低表观气速下,增加颗粒质量流率不利于换热,而在较高表观气速下,传热系数随颗粒质量流率逐渐增加。不同流型下,气固流动特性对换热特性的影响不同。在鼓泡床流型下,过高的局部固含率不利于颗粒在换热表面的更新,增加换热管附近的局部气泡频率可以明显强化换热;而在湍流床流型下,换热管附近的局部固含率和气泡频率的增加,均使传热系数逐渐增大。建立了针对不同流型的换热经验关联式,预测值与实验值的平均相对偏差分别为6.9%和1.3%。     

溶液再生器传热传质过程的数值模拟

根据湿空气和溶液热质交换的基本理论,建立了叉流溶液再生器的传热传质数学模型,并将该模型简化,得出空气和溶液的质量、能量控制方程。根据数值求解的方法,对方程进行离散简化,利用Matlab语言编程模拟计算。将模拟结果和实验结果进行了比较,结果表明本模型可靠。     

减压膜蒸馏过程纤维膜表面热质传递特征模拟分析

采用CFD数值模拟的方法,建立了错流式单根中空纤维减压膜蒸馏过程的三维计算模型,对纤维膜表面不同位置的局部热量和质量传递特征进行了研究。研究结果表明,纤维膜表面的速度分布、温度分布、对流传热系数分布以及蒸发速率均随着位置的改变而改变,且具有相似的规律,在相位角60°到90°之间有最大值,而在相位角0°和180°附近其值较小;料液流速显著影响纤维膜表面的热质传递参数的分布,蒸发速率和传热系数均随着流速的提高而增大;传热系数随着真空度的提高而迅速降低。本研究结果加深了对中空纤维减压膜蒸馏过程的认识,对膜过程强化具有指导意义。