Experimental Study and Modeling of Freeze-Drying in Syringe Configuration. Part I: Freezing Step

This article concerns the experimental study and the modeling of the freezing step during freeze-drying of a model lyoprotectant formulation in syringe configuration. First, ice crystal morphology observations were carried out in a cold chamber equipped with stereomicroscopy to determine the ice crystal size distribution. We observed that the ice crystal morphology was homogeneous as a function of the radial position and that, contrary to our previous observations in glass vial configuration, no significant ice morphology differences existed between the bottom and the top of syringes. Thus, it seems that the gas cooling by free convection at the walls of the syringe during the freezing step led to higher homogeneity in the ice crystal morphology. These data allowed calculation of the mean ice crystal diameter (which was supposed to represent the mean porous diameter) and then the permeability of the freeze-dried cake. Moreover, a two-dimensional axisymmetric (2D) mathematical model in cylindrical geometry of the freezing step for aqueous sucrose solution was elaborated. It has been proved that this model was able to simulate quite precisely the whole temperature profiles during the freezing step, except the exothermic peak of nucleation. In the last part of this work on syringe geometry, the impact of annealing treatments on the freeze-dried matrix morphology was investigated. Some heterogeneity of sublimation rates was observed, with significant variation of residual humidity. Additionally, the water vapor mass transfer resistance values decreased if a convenient annealing treatment (shelf temperature at ?10°C) was applied.     

Ice crystal formation in the carbon nanotube suspension: A modelling approach

Freeze-dried porous solid foams were prepared from carbon nanotube (CNT) suspensions via either contact freezing with a heat exchanger or immersion freezing into a cryo-bath. Microstructures in the freeze-dried foam cast by ice crystals formed during the freezing step. It was found that domains of the carbon nanotubes in the freeze-dried solid foams were fairly well interconnected, and the microstructures were largely influenced by the freezing condition and freezing method. A mathematical model was proposed to simulate the observed thermal history during the freezing step; then theoretical predictions of the microstructural features were attempted. The simulated thermal history was in good agreement with experimental result. The resulting mean ice crystal sizes L* could be estimated from the calculated freezing front velocity (R) and the temperature gradient in the frozen zone (G). Interestingly, it was found that a correlation based on the power law (L*∝R^?0.2G^?0.2) was applicable to the present system for both the contact and immersion freezing methods. Though the fundamental ice crystallization phenomena were essentially the same for both freezing methods, actual temperature distribution and movement of the freezing front through the whole freezing bulk could control the morphology of ice crystal in the frozen matrix. In other words, good control of thermal flow in the freezing system would greatly contribute to rational design of microstructures of the CNT foam.     

Freezing of aqueous solutions in a porous medium Part I. Freezing of air-entraining agent solutions

In this investigation, a pre-formed, autoclaved porous material, of stable pore characteristic, was used to study the freezing characteristics of wet porous materials. Prior to freezing, prisms made from this material were filled with solutions of air-entraining agents. The results show that (i) a major part of a solution could be frozen without any dilation of the matrix, (ii) the dilation, when it occurs, is connected with the later stages of the freezing process when only a limited amount of ice forms, (iii) initially, ice forms on the surfaces of large air-filled pores, (iv) ice formation strengthens the matrix.It is proposed that the initially formed ice-layers on the pore surfaces seal in the remainder of the solutions. Continued ice formation generates a bursting pressure within the unfrozen solution. When this bursting pressure exceeds the tensile strength of the ice-strengthened matrix, it fails.Attention has been drawn to the relevance of the above observations and the hypothesis to freezing of cement paste.     

Separation of diethylbenzene isomers by distillative freezing

Due to the close boiling points of the mixed diethylbenzene (DEB) isomers, it is rather complicated to separate them by distillation. A new separation technique, distillative freezing (DF), is successfully applied to separate p-DEB from the diethylbenzene isomers in this study. Basically, the DF process operates at a triple point condition, in which the liquid mixture is simultaneously vaporized and solidified due to the three-phase equilibrium. It results in the formation of pure crystals along with the liquid phase and vapor phase of the mixtures. The process can be continued until the liquid phase is completely eliminated and only the pure solid crystals remain in the feed. A model, whereby the DF process is simulated in a series of equilibrium stage operations, is proposed to direct the DF operation. In the model, each stage is operated under an adiabatic condition at a three-phase equilibrium. The experiments show that, in the p-DEB/m-DEB mixtures, p-DEB can be purified from 80% to 99% through several DF operations with the experimental recovery rate of p-DEB in one DF operation ranging from 58% to 77%. The unique feature of this new separation technique is that no filtration and crystal washing is required after the p-DEB crystals are obtained by DF.     

单向冷冻过程溶液中冰晶界面的竞争现象

通过对溶液冷冻过程冰晶界面发展变化的实验观察,定义了界面发展过程的3个阶段,分析各个阶段的现象特征,并从界面区浓度分布探析界面规则形态的物理本质。界面的发展过程中,会因为来自内部或者外界的扰动,使界面局部区域产生不均匀性,导致晶枝间的竞争现象,而决定这些竞争现象及冰晶生长的驱动因素是界面附近的过冷度分布。竞争现象机理的分析,有助于更清晰地了解生物材料在冷冻过程中的结冰特性。     

Photoconductivity observations on phosphorous-doped n-type CVD diamond at low temperature and varying electric field

Photocurrent measurements on phosphorous-doped n-type diamond have been performed at liquid helium temperature and varying electric field. For increasing fields the onset of the experimental photocurrent versus photon energy shifts towards smaller photon energy. At relative small hν its slope is nearly constant, although smooth structures can be observed in the spectrum. These and other observations at low photon energy are interpreted using an impurity-assisted tunnelling model. The idea behind this model is that, at finite electric fields, the impurity electron tunnels a few times towards neighbouring neutral impurity atoms, until it arrives in a non-localized conduction band state and contributes to the photocurrent. The experimental results could be explained in a semi-quantitative way. The consequences for various experimental circumstances (varying field, temperature, photon energy, impurity concentration) are discussed.     

A generalized correlation of solute inclusion in ice formed from aqueous solutions and food liquids on sub‐cooled surface

Solute inclusion in an ice layer formed on a smooth stainless steel surface under sub‐cooled flow conditions has been studied experimentally. The effects of ice growth rate, solution velocity and bulk concentration of solute on the solute inclusion in the ice layer have been investigated using a controlled flow surface crystallizer (CFSC). For several aqueous solutions (sucrose, NaCI, glycol, fructose), food liquids (skim milk, whole milk, orange juice) and particulate suspension (potato starch particles), the effective molecular weights were calculated and their effects on solid inclusion determined. This effect was then conveniently related to the freezing point depression (FPD) of each liquid tested. A generalized empirical correlation has been established for all the liquids tested.     

海水雾化冷冻脱盐技术研究

首先利用雾化冷冻方法来处理渤海湾海水,使得海水部分冷冻为海冰,更多的盐分被包含在未发生冷冻的海水中而脱离冰体.在海冰形成过程中,由于冰晶形成速度要大于海水脱离冰晶的速度,所以最初形成的冰晶、冰针和冰片会发生合并,合并过程中包裹海水形成了"盐胞",因此海冰具有一定的含盐量.然后依次利用重力法和离心法对雾化冷冻得到的海冰进行深层脱盐.依次通过上述3种方法,能够将海水的盐度从25.5‰降低到2.9‰,脱盐率为88.63%.     

Experiments and insights of desalination by a freezing/thawing method at low subcooling

Desalination by freezing/thawing method was a very important method to obtain fresh water from sea water. In this work, desalination by freezing/thawing method was conducted with initial sodium chloride of 3.5 wt% in consideration of stirring speed, freezing time and subcooling. The subcooling ranged from 1.2 K to 4 K. The optimum conditions for desalination in this work were stirring speed of 200 r·min⁻¹, freezing time of 120 min, and subcooling of 3 K. The results also showed that sodium chloride cannot be effectively removed by once freezing/thawing process. The maximum removal efficiency of sodium chloride was 64.3%. Two major reasons resulting in the impurity of obtained melted water by freezing/thawing method were proposed. The first reason was the inevitable adhesion of salt solution to the surface of ice, which could be removed easily by distilled water flushing. The second reason was that salt solution was heterogeneously wrapped in the accumulated ice, which was difficult to be removed by distilled water flushing. The liquid flushing method was proposed to verify the conjecture, and the results were in accordance with the two reasons mentioned above. Additional method, such as multiple flushing liquid method, was suggested to be used during the freezing/thawing process for effectively removing sodium chloride, and obtaining fresh water.     

Ice-mould freeze casting of porous ceramic components

Porous, hollow ceramic components were produced by freeze casting technique. For this purpose aqueous slurries with high solid contents were prepared which were stable against freezing down to at least −5 °C. Ice cores were made by coating steel components with freezing water which were subsequently dip-coated with the ceramic suspensions. After freeze drying which removes both, the ice core and the frozen suspension liquid, and sintering, ceramic components with a high amount of open porosity including steel parts could be achieved. As an example hydroxyapatite was used for showing the opportunities of the freeze casting technology among others for applications in the field of bone replacement. The influence of the solid content of the hydroxyapatite slurries on the ice crystal growth has been investigated by means of compact hydroxyapatite bodies which were prepared by freeze casting using ice moulds with cylindrical cavities.     

含水量对粗对苯二甲酸熟化的影响

研究了高温下含水量对粗对苯二甲酸晶体粒度和固液两相中杂质浓度的影响,结果表明:熟化能增加对苯二甲酸晶体的平均粒径,但含水量变化对熟化过程影响不明显。含水量的减小有利于固相和液相中4-CBA浓度的降低,但此含水量存在一个阈值,低于此含水量对杂质的减少速率影响微小。     

The Solid–Liquid Phase Diagram of Binary Mixtures Dissolved in an Inert Oil: Application to Ternary Blends that Can Form Organogels

A simple generalization of the Hildebrand equation is presented for the prediction of the solid–liquid phase diagram of a binary mixture of structuring agents dissolved in an inert liquid. The model is a thermodynamic interpolation between three well-known limits: (1) the freezing depression curve of liquid A under influence of the addition of the solvent, (2) the freezing depression curve of liquid B under influence of the addition of the solvent, and (3) the binary solid–liquid phase diagram of substances A and B. The theory is shown to be valid as long as the freezing temperature of the liquid is well below the freezing temperatures of the structurants. The theory is compared to literature data for three different mixtures: (1) stearyl alcohol and stearic acid dissolved in sunflower oil, (2) hentriacontane+melissic acid in safflower oil, and (3) lauric acid + behenic acid in canola oil. The agreement between the thermodynamic calculation and experimental data is good. The solid–liquid phase behavior of glyceryl tristearate + stearic acid in edible oil is also studied. The location of the eutectic point of the latter system is predicted to shift toward an axis with zero tristearate concentration as the structurant concentration decreases from 50% to 5% (w/w).     

Experiments and insights of desalination by a freezing/thawing method at low subcooling

Desalination by freezing/thawing method was a very important method to obtain fresh water from sea water. In this work, desalination by freezing/thawing method was conducted with initial sodium chloride of 3.5 wt% in consideration of stirring speed, freezing time and subcooling. The subcooling ranged from 1.2 K to 4 K. The optimum conditions for desalination in this work were stirring speed of 200 r·min^-1, freezing time of 120 min, and subcooling of 3 K. The results also showed that sodium chloride cannot be effectively removed by once freezing/thawing process. The maximum removal efficiency of sodium chloride was 64.3%. Two major reasons resulting in the impurity of obtained melted water by freezing/thawing method were proposed. The first reason was the inevitable adhesion of salt solution to the surface of ice, which could be removed easily by distilled water flushing. The second reason was that salt solution was heterogeneously wrapped in the accumulated ice, which was difficult to be removed by distilled water flushing. The liquid flushing method was proposed to verify the conjecture, and the results were in accordance with the two reasons mentioned above. Additional method, such as multiple flushing liquid method, was suggested to be used during the freezing/thawing process for effectively removing sodium chloride, and obtaining fresh water.     

Formation of locust bean gum hydrogel by freezing–thawing

The gelation process of locust bean gum (LBG) aqueous solution has been studied by rheological and calorimetric measurements. The storage modulus of the gel was found to be dependent on repeating cycles of the freezing–thawing process and freezing speed. When freezing–thawing was repeated more than three times, the dynamic rigidity of the hydrogel was about 50% more than that of the original polymer solution. Furthermore, the storage modulus was greater for the hydrogel prepared by 12°C min^-1 freezing than that by 1°C min^-1 freezing. These observations are explained in terms of the formation of ice crystals during the freezing process, observed by differential scanning calorimetry (DSC). © 1998 SCI.     

Pore size and shape in mortar by thermoporometry

The pore structure of mortar (w/c = 0.55) was examined using thermoporometry (TPM), nitrogen adsorption/desorption (NAD), and mercury intrusion porosimetry (MIP). The TPM measurements were calibrated by comparison to NAD and MIP measurements on porous glass; similar comparisons were made on dried and resaturated mortars. For undried mortars, TPM provides the size of pore entries (from the freezing cycle) and interiors (from the melting cycle). In keeping with previous studies, we find that there is an unfrozen layer of water between the ice and the pore wall in porous glass that is about 0.8 nm thick; when lime-saturated water is used, the thickness of that layer increases by about 10%. In mortar, the unfrozen layer is about 1.0-1.2 nm thick, so no freezing occurs in pores with diameters ≤ 4.5 nm, at least down to − 40 °C (where the radius of the crystal/liquid interface is ∼ 1.5 nm). Based on the hysteresis in the freezing and melting curves, the larger mesopores in mortar were found to be rather spheroidal, while the smaller ones were more cylindrical.     

临界条件(0℃)下溶液蒸发冷冻过程中的传质规律

通过对相际间力学特性的分析,结合湍流的流动特征,考虑了冰体存在孔隙和毛细作用,给出了临界条件(0℃)湿度差驱动下溶液蒸发冷冻过程中的质扩散分析模型,并对溶液在不同状态下(液态、液固共存和固态)气流界面间的质扩散变化规律进行了研究,结果表明,随着溶液从液态向固态的转变,其表面质扩散系数逐渐减小,即表面质扩散能力逐渐减弱,因此,对于溶液自身蒸发冻结过程而言,为了强化溶液表面的质传递和冻结,应控制冻结过程中溶液表面的结冰状况。这一研究结果为蒸发冷冻传质的研究以及强化蒸发冻结的工程应用提供了理论指导和参考。     

Dye removal in steady-state continuous countercurrent liquid–solid adsorber

Mass transfer studies have been carried out in a continuous countercurrent liquid–solid adsorber to remove methylene blue from wastewater using activated carbon under steady-state conditions. Experimental data on axial concentration profile and overall removal efficiency has been obtained using a pilot plant set-up. The overall colour removal efficiency of the adsorber increases with increase in solid velocity, and a decrease in liquid velocity and initial dye concentration. A mathematical model has been developed to predict the axial concentration profiles, which compare with the experimental data satisfactorily. The overall mass transfer coefficient has also been evaluated using this model.     

A mechanism of breakdown of limestone nodules in a freeze-thaw environment

When exposed to a freeze-thaw environment, water soaked limestone nodules show two types of disintegration. In the first type the nodule fractures into a few, comparatively large pieces and in the second the surface of the nodules becomes pitted. The causes of these breakdowns have been investigated. It appears that the most probable cause of the fracture is the development of a $\text{hydrostatic}$ pressure in the unfrozen part of the original water content as it becomes encased by initial ice formation. For breakdown to occur the hydrostatic pressure has to overcome the tensile strength of the $\text{ice-limestone composite}$. The required pressure can only be generated if the nodule contained a critical volume of water at the beginning of freezing. This mechanism suggests, that for a fully saturated limestone nodule, there is a critical size, determined by its porosity, below which the nodule will not be broken. This mechanism also suggests that for bulk water there is a critical volume below which no fracturing will occur when frozen. The existence of this critical volume of water has been experimentally demonstrated.     

Temperature Dependence of Excited-State Proton Transfer and Geminate Recombination Processes in Water and in Glycerol-Doped Ice

The reversible proton dissociation and geminate recombination of photoacids was studied as a function of temperature in neat water, binary water mixture containing 0.6 mol% glycerol, and doped ice containing 0.6 mol% glycerol. The deuterium isotope effect on both condensed phases was also studied. 8-hydroxypyrene-1,3,6 trisulfonate trisodium salt was used as the electronically-excited-state proton emitter. The experimental data are analyzed by the Debye–Smoluchowski equation solved numerically with boundary conditions to account for the reversibility of the reaction. We propose a qualitative model to describe the unusual temperature dependence of the proton transfer rate in the liquid phase. We also propose a model for proton transfer in solid ice based on L-defects transport as proton acceptors. While in the liquid phase at t > 10°C the proton dissociation rate constant is almost temperature independent, in glycerol-doped ice we find a large temperature dependence.     

The effect of variation in pore structure on the frost resistance of porous materials

Processes of freezing of water and melting of ice were studied using samples of hardened suspensions of plaster-of-paris with different mean pore sizes and constant porosities. Changes in the structure of the material were analyzed by measurement of the thermal and volume changes during cooling and heating cycles, and by the measurement of pore-size distribution and the changes of strength after the freezing cycles. It was shown that changes of volume within a temperature limit T=±20°C depend on the mean pore size and on the degree of water saturation. The disruption of the hardened plaster-of-paris structure was evaluated by determining the decrease in strength, if the nucleation of the ice crystals occurs during supercooling and the rate of their growth is greater than the rate of the amount of water forced out of the pores. The experimental observations support Power's theory concerning the frost resistance of porous material.