具有预制孔隙的维生素C水溶液微波冷冻干燥

设计和组装了一套实验室规模的多功能微波冷冻干燥装置，探究了具有初始孔隙的非饱和物料微波冷冻干燥过程。以维生素C为溶质，采用“软冰”冷冻技术制备了初始饱和与非饱和的冷冻样品。结果表明，软冰冷冻制备的样品能够避免崩塌。在35℃和20 Pa条件下，初始非饱和物料的干燥时间比饱和物料缩短了30.4%。SEM表征显示，非饱和物料具有疏松的球状孔隙结构、连通性好，有利于水蒸气的迁移。采用吸波材料碳化硅辅助的微波加热能够进一步强化冷冻干燥过程。在相同条件下，非饱和物料的微波冷冻干燥(5 W功率)时间比常规冷冻干燥(0 W功率)缩短了28.1%，比饱和物料的常规冷冻干燥缩短了50.0%。吸波材料辅助的初始非饱和物料微波冷冻干燥实现了传热传质的同时强化。     

Numerical Investigation on Dielectric Material Assisted Microwave Freeze-Drying of Aqueous Mannitol Solution

The dielectric material assisted microwave freeze-drying was investigated theoretically in this study. A coupled heat and mass transfer model was developed considering distributions of the temperature, ice saturation and vapor mass concentration inside the material being dried, as well as the vapor sublimation-desublimation in the frozen region. The effects of temperature and saturation on the effective conductivities were analyzed based on heat and mass flux equations. The model was solved numerically by the variable time-step finite-deference technique with two movable boundaries in an initially unsaturated porous sphere frozen from an aqueous solution of mannitol. The sintered silicon carbide (SiC) was selected as the dielectric material. The results show that dielectric material can significantly enhance microwave freeze-drying process. For case of the dielectric field strength, E = 4000 V/m under typical operating conditions, the drying time is 2081 s, 30.1% shorter and 47.2% longer, respectively, than those for E = 2000V/m and E = 6000 V/m. The heat and mass transfer mechanisms during the drying process were discussed.     

Numerical Investigation on Dielectric Material Assisted Microwave Freeze-Drying of Aqueous Mannitol Solution

Abstract

The dielectric material assisted microwave freeze-drying was investigated theoretically in this study. A coupled heat and mass transfer model was developed considering distributions of the temperature, ice saturation and vapor mass concentration inside the material being dried, as well as the vapor sublimation-desublimation in the frozen region. The effects of temperature and saturation on the effective conductivities were analyzed based on heat and mass flux equations. The model was solved numerically by the variable time-step finite-deference technique with two movable boundaries in an initially unsaturated porous sphere frozen from an aqueous solution of mannitol. The sintered silicon carbide (SiC) was selected as the dielectric material. The results show that dielectric material can significantly enhance microwave freeze-drying process. For case of the dielectric field strength, E = 4000 V/m under typical operating conditions, the drying time is 2081 s, 30.1% shorter and 47.2% longer, respectively, than those for E = 2000V/m and E = 6000 V/m. The heat and mass transfer mechanisms during the drying process were discussed.     

初始非饱和多孔物料对冷冻干燥过程的影响

提出了“初始非饱和多孔物料冷冻干燥”的思想,从实验上验证具有一定初始孔隙的非饱和多孔物料对液体物料冷冻干燥过程的强化作用。设计、加工和组装了一套实验室规模的多功能冷冻干燥装置。采用“液氮制作冰激凌法”,将以甘露醇为主要溶质的液体物料制备成具有不同初始孔隙的冷冻物料。对于相同质量和相同湿含量的非饱和冷冻物料,在相同的操作条件下,进行冷冻干燥实验,并与常规冷冻干燥相比较。结果表明,初始非饱和物料对冷冻干燥过程确实具有显著的强化作用。非饱和冷冻物料(初始饱和度0.28)的干燥时间比常规冷冻物料(初始饱和度1.00)能够节省36.4%。初始饱和度越小,干燥时间越短,干燥产品的含水率越低。     

Porous frozen material approach to freeze-drying of instant coffee

Abstract

Porous frozen material with a certain initial porosity was prepared to explore its influence on freeze-drying experimentally. Soluble coffee was selected as the solute in aqueous solution and liquid nitrogen ice-cream making method was used to prepare the frozen materials. Results showed that freeze-drying can be significantly enhanced using the initially porous frozen material compared with the traditionally solid one. By keeping the same sample mass and moisture content with sole radiation heating, drying time of the porous frozen sample was about one third shorter than that of the solid one under the same tested operating conditions. SEM images of dried products revealed that the porous material had a loose and tenuous structure that was favorable to the transportation of sublimated vapor and the desorption of bound moisture. Appropriately increasing the chamber surface temperature benefited the freeze-drying process and changing the chamber pressure had little effect on the process. Combined radiation and conduction heating can further promote the freeze-drying process and save as much as 36.4% of the drying time. The porous frozen material was found to have a wider range of operating temperature and result in relatively lower residual moisture content.     

Freeze‐drying of aqueous solution frozen with prebuilt pores

To save drying time and increase productivity, a novel idea was proposed for freeze‐drying of liquid materials by creating an initially unsaturated frozen structure. An experimental investigation was carried out aiming at verifying the idea using a multifunctional freeze‐drying apparatus. Mannitol was selected as the primary solute in aqueous solution. Liquid nitrogen ice‐cream making method was used to prepare the frozen materials with different initial porosities. Results show that freeze‐drying can be significantly enhanced with the initially unsaturated frozen material, and substantial drying time can be saved compared with conventional freeze‐drying of the initially saturated one. Drying time was found to decrease with the decrease in the initial saturation. The drying time for the initially unsaturated frozen sample (S₀ = 0.28 or 0.69 of initial porosity) can be at best 32% shorter than that required for the saturated one (S₀ = 1.00 or zero porosity). This unique technique is easy to implement and improves the freeze‐drying performance of liquid materials. © 2015 American Institute of Chemical Engineers AIChE J, 61: 2048–2057, 2015     

Measurement of lyophilisation primary drying rates by freeze-drying microscopy

The mass transfer processes that influence the rate of primary drying during lyophilisation are characterised by freeze-drying microscopy. A simple diffusion model is shown to describe the observed progress of drying fronts through isothermal, one-dimensional systems of slab and cylindrical geometry. Measured drying rates are modelled by an effective diffusion coefficient, D_eff, which describes the diffusion of water vapour from the inner frozen core through the outer dried cake of the lyophilising mass. In this way, the relationship between the instantaneous thickness of the dried cake and the drying time is determined for different drying temperatures. Values of D_eff determined by analysis of freeze-drying microscopy data for two simple model systems are shown to be in satisfactory agreement with those predicted from theory. Furthermore, the lyophilisation behaviour of a citrate and a tris-HCl buffer are well described by the model and values of D_eff are constant at temperatures below the collapse temperatures of the dried cakes. However, values of D_eff increase approximately four- and six-fold, respectively as the collapse temperatures are approached. Microscopic examination of the citrate buffer dried cake shows its structure to be homogeneous at temperatures well below the collapse temperature, as required by the model, but indicates significant cracking of the cake as the collapse temperature for this buffer is approached. As a result, channeling of the water vapour through the dried cake causes enhanced drying rates. Finally, we show that the total time required to sublime water from aqueous slurries of glass beads in a conventional laboratory lyophiliser are in reasonable agreement with those times estimated using the values of D_eff determined by freeze-drying miscroscopy.     

On the upscaling of reaction-transport processes in porous media with fast or finite kinetics

We show that for reaction-transport processes with fast kinetics (in the limit of thermodynamic equilibrium), conventional volume averaging for determining effective kinetic parameters applies only when the macroscopic variable approaches its equilibrium value. Even under such conditions, computing the effective mass transfer coefficient requires solving an eigenvalue problem, which couples the local microstructure with the global. Two examples, one involving a simple advection-dissolution problem and another a drying problem in a pore network, illustrate the theoretical predictions. Similar considerations apply for the case of finite kinetics, when the macroscale concentration approaches an equilibrium value. In that case, the effective kinetic parameter is not equal to the local, as typically assumed, but it becomes a function of the local Thiele modulus.     

Evaluation of assisting methods on fluidization of hydrophilic nanoagglomerates by monitoring moisture in the gas phase

Agglomerates of nanoparticles were fluidized conventionally and under the influence of assisting methods such as vibration and/or moving magnetic particles. The adsorption/desorption rate of moisture of fluidized hydrophilic nanopowders was monitored during humidification/drying of the powder in order to find their adsorption isotherms at room temperature and to evaluate the assisting methods. Adsorption isotherms were verified by a gravimetric method. The nanopowders studied were Degussa Aerosil^® 200 and Aerosil^® 90, which were chosen because of their different fluidization behaviors.The moisture level in the nitrogen gas used to fluidize the powders was monitored on-line by using humidity sensors upstream and downstream of the fluidized bed. Moisture was added to the fluidizing nitrogen by bubbling it through water. The amount of moisture adsorbed/desorbed by the powders was obtained by integration of the time dependant moisture concentration. It was found that when the bed of powder is assisted during fluidization, the mass transfer between the gas and the nanopowder, as measured by the amount of moisture adsorbed/desorbed, is larger than when the powder is conventionally fluidized. Vibration assistance was found to be more effective for Aerosil^® 200, an APF type nanopowder, but magnetic assistance was needed for Aerosil^® 90 in order to break down the very large agglomerates formed in this ABF nanopowder.     

Transmission probabilities and particle-wall contact for Knudsen diffusion in pores of variable diameter

Knudsen dynamics simulations were performed in cylindrical pores with multiple sections of different diameters. The number and location of hits between particles and the pore walls can be related to the known distributions in the uniform diameter cases. Both sweep-gas and pass-through modes of operation were examined. The contact of the particles traveling inside the pores with the pore walls can be regulated by adjusting the aspect ratio (section length/section diameter) of the pore sections and the ratio of their cross-sectional areas. The results were found to be independent of the transition angle between sections except for very smooth transitions. Analytical expressions were developed to relate the transmission probability in pores of multiple sections to the transmission probabilities of the constituent sections.     

Convective drying of porous material containing a partially miscible mixture

Non-hygroscopic capillary porous bodies were dried by convection to investigate the influence of a two-phase moisture on the drying characteristics. The samples were wetted with the ternary liquid mixture 2-propanol/water/1-butanol. This system forms two liquid phases at low propanol concentrations. The drying rate is not significantly influenced by a two-phase moisture. Selectivity is influenced by the drying conditions as well as the characteristics of the capillary porous body. In the two-phase region selectivity can be shifted most effectively by the temperature of the drying air and by the pore size of the porous body. Selectivity can even inverted if the initial moisture composition is close to the boundary line. In the porous body a moisture profile, as well as a concentration profile, was measured.     

Heat and mass transfer model of dielectric-material-assisted microwave freeze-drying of skim milk with hygroscopic effect

A new porous media mathematical model for freeze-drying was developed based on the adsorption-desorption relationship proposed in this paper. A finite difference solution was obtained from a moving boundary problem for the dielectric-material-assisted microwave freeze-drying process. Silicon carbide (SiC) was selected as the dielectric material; and frozen skim milk was used as the aqueous solution to be dried. Simulation results showed that the dielectric material can significantly enhance the microwave freeze-drying process. The drying time was 33.1% shorter than that of ordinary microwave freeze-drying under typical operating conditions. When the solid content of the solution to be freeze-dried was very low, or the solid product had a very small loss factor, microwave heating was less effective without the assistance of dielectric material. The mechanisms of heat and mass transfer during drying were analyzed based on profiles of ice saturation, temperature and vapor concentration. Drying rate-controlling factors were discussed. A comparison was made between the model predictions and the reported experimental data.     

Mathematical modelling of styrene drying by adsorption onto activated alumina

This work is focused on the analysis and modelling of styrene drying, raw material in the manufacture of synthetic rubber, by means of adsorption onto activated alumina. Equilibrium experiments, carried out under isothermal conditions at 10°C, correlated to the equation q (kg/kg)=2.659×10⁻⁴ C (mg/kg). Fixed bed column experiments were performed working with different flow rates and using different bed lengths in order to obtain experimental breakthrough curves. A mathematical model that considers the influence of both film and pore mass transfer resistances described satisfactorily well the experimental results. Finally a value of D_p=6.101×10⁻⁹ m²/s was obtained from correlation of experimental data to simulated curves and using the minimum weighted standard deviation as optimisation criterion.     

吸波材料辅助的液体物料微波冷冻干燥多物理场耦合模型

为了研究吸波材料辅助微波加热对传统冷冻干燥过程的强化作用,建立了多孔介质温度、浓度和电磁场耦合的多相传递模型;以烧结的碳化硅(SiC)为吸波材料、以甘露醇水溶液为待干料液进行了微波冷冻干燥实验,并测定了甘露醇固体的介电特性。模拟和实验结果均表明,吸波材料对初始非饱和多孔物料微波冷冻干燥具有显著的强化作用。初始非饱和样品微波冷冻干燥时间比传统冷冻干燥缩短了18%,比常规饱和样品传统冷冻干燥缩短了30%。模拟结果与实验数据吻合良好。这表明提出的新型干燥方法确实能够实现过程传热传质的同时强化。通过考察样品内部温度、饱和度和电场强度的实时分布,分析了微波冷冻干燥过程的传热传质和电磁波传播与耗散机理。在微波冷冻干燥过程中,初始非饱和样品累计吸收的辐射能和微波能的总和与传统冷冻干燥相当。这说明,该干燥方法只是提高了能量效率,从而大幅缩短了冷冻干燥时间。     

Thin-layer drying of porous materials: Selection of the appropriate mathematical model and relationships between thin-layer models parameters

Experimental investigation of the drying kinetics of various types of materials was carried out in laboratory-scale dryers under different conditions of temperature, microwave heating power and pressure. Leather samples (mechanically and vacuum-dewatered bull napa and wet blue cutting), paperboards (grafopack, testliner), wood (alder, birch, willow) and two pharmaceutical powders (chlorpropamide and hydrochlorotiazide) were dried in a microwave dryer. Thin clay slabs, Al–Ni catalyst and chlorpropamide were dried in a convection dryer, while chlorpropamide and ketoprofen were dried in a vacuum dryer. In order to compare drying kinetics, experimentally obtained data, X = f(t), were correlated with the Lewis “thin-layer” equation, the modified Page equation and Fick's second law. The drying constant, effective diffusion coefficient, mass transfer coefficient and modified Page model parameters were estimated by fitting the selected mathematical models to experimental data. High levels of correlation between measured and calculated data were obtained for all materials and dryers using modified Page model. The application of the Lewis and Fick's equation is justified only for drying of clay, catalyst and leather. Mass transfer coefficient depends linearly on the drying constant. The relation between the modified Page model parameter and the drying constant can be represented by a unique power function.     

SAXS investigation of nanoporous structure of thermal-modified carbon materials

The article investigates the effect of thermal modification of porous carbon material (PCM), obtained from plant feedstock, on its morphology and fractal structure by small-angle X-ray scattering (SAXS) method. The analysis of the scattering intensity curves serve the basis for calculating the parameters of the PCM porous structure: the Porod constant, the Porod invariant, average pore radius, specific surface area, and mass and surface fractal dimensions. It has been found out that the PCMs obtained have fractal structure, formed from mass and surface fractals, the sizes of which increase at the growth of temperature and modification time.

PACS

81.05.Uw; 61.05.cf; 82.47.Aa     

Comparison of the adsorption performance of compound adsorbent in a refrigeration cycle with and without mass recovery

Adsorption and desorption were performed on a compound adsorbent composed of CaCl₂ and activated carbon in cycles both with and without mass recovery and the performances obtained were compared with those of equilibrium adsorption. Experimental results of the cycles without mass recovery carried out on an adsorption refrigeration unit yielded performances that were slightly less than those of equilibrium adsorption. The adsorption performances of the cycle with mass recovery were measured to be much better than those of the cycle without mass recovery. At of evaporating temperature, the cycle adsorption capacity was as high as 0.78 kg/kg for the cycle with mass recovery while it was only 0.55 kg/kg for the cycle without mass recovery. The average adsorption/desorption rate of the cycle with mass recovery, which was 0.031 (kg/kg)/min, has been improved by 47.6% compared with that of the cycle without mass recovery. Research on the cycle adsorption capacity at different evaporating temperatures showed that the improvement of cycle adsorption capacity, with mass recovery, was higher under the condition of lower evaporating temperature. At evaporating temperature, the mass recovery operation had improved the adsorption capacity by 78% compared with the cycle without mass recovery. In addition, refrigeration performances for the cycles with and without mass recovery at an evaporating temperature of were studied. Compared with the results of the cycle without mass recovery, SCP (specific cooling power) and COP (coefficient of refrigeration performance) have been improved by 48.6% and 54.5%, respectively, when mass recovery is performed.     

Adsorption of chlorinated hydrocarbons from aqueous solutions by wetted and non-wetted synthetic sorbents: dynamics

In the present investigation the dynamics of the adsorption of several chlorinated hydrocarbons onto wetted and non-wetted synthetic sorbents was studied. A single particle model was developed to describe the adsorption behavior. The values of the mass transfer coefficient, needed to describe the experimental results, were found to be considerably less than those predicted from theory. The difference between measured and expected mass transfer coefficient was found to increase with non-ideality of the solute and to depend on type of sorbent. Apparently, the rate of mass transfer does not only depend on hydrodynamics but also on possible interaction between solute, solvent and sorbent.     

Experimental investigation of methyl tert-butyl ether dissolution in saturated porous media

This study aims to investigate methyl tert-butyl ether (MTBE) dissolution in saturated porous media. A series of 1D columnexperimentswere conducted in laboratory to obtain MTBE dissolution datawith different groundwater velocity, initial MTBE saturation and grain size of porous medium, and in the presence of other nonaqueous liquids. Results indicate that higher groundwater velocity increases MTBE dissolution rate and higher initial MTBE saturation reduces effective permeability to slow MTBE dissolution rate. Smaller grain size medium gives higher MTBE dissolution rate because of higher permeability. The addition of trichloroethylene enhances MTBE dissolution, with an optimal mass ratio of 10:2, while the presence of p-xylene prolongs complete dissolution of MTBE. Mass transfer correlations are developed for MTBE dissolution rate based on the degree of MTBE saturation Sn. Mass transfer rate is characterized by Re' with a high exponent for 0.3000 b Sn b 0.5482, while it is related to medium grain size and Sn for Sn ≤ 0.3000.     

Kinetics and equilibrium of dissolved oxygen adsorption on activated carbon

The macroscopic adsorption behavior of dissolved oxygen on a coconut shell-derived granular activated carbon has been studied in batch mode at 301 and 313 K for initial dissolved oxygen concentrations of 10-30 mg/l and oxygen/carbon ratios of 2-180 mg/g. BET (Brunauer, Emmett, and Teller) surface area, micropore volume, and pore size distribution were determined from N₂ isotherm data for fresh and used samples of carbon. The surface groups were characterized using Boehm titrations, potentiometric titrations, and FTIR study. The material is characterized by its high specific surface area , microporocity (micropore volume ), its basic character ( total basic groups) and its high iron content (15,480 ppm Fe). BET n-layer isotherm describes adsorption equilibrium suggesting cooperative adsorption and important adsorbate-adsorbate interactions. Kinetic data suggest a process dependent on surface coverage. At low coverage a Fickian, intraparticle diffusion rate model assuming a local equilibrium isotherm (oxygen dissociation reaction) adequately describes the process. The calculated diffusion coefficients (D) vary between and for initial oxygen concentration of 10 and 20 mg/l, respectively. Sensitivity analysis shows that the oxygen dissociation equilibrium constant determines the equilibrium concentration, whereas the diffusion coefficient controls the kinetic rate of the adsorption process having no effect at the final equilibrium concentration. A combined kinetic mass transfer model with concentration-dependent diffusion (parabolic form) has been developed and successfully applied on the dissolved oxygen adsorption system at high surface coverage. For equilibrium uptake of the estimated mean mass transfer coefficient and adsorption rate constant are and , respectively.