Ebook: BIWIC 2007

The field of crystallization holds many challenges, with the physical and chemical complexity of the crystallization process being core to the dynamic nature of the field. Exciting advances are currently being achieved in the areas of nanoparticle formation, product and particle design and methods of particle characterisation. There is also significant progress and innovation in the design, scale-up and control of crystallizers.

These key developments are reflected in the session themes of the 14th BIWIC (Bremen International Workshop on Industrial Crystallization) with the technical programme incorporating a wide range of topics, such as:

• The formation and stabilisation of nano particles,

• Polymorphs and co-crystals in pharmaceutical preparation

• Product and particle design

• Kinetics of crystallization and measurement of crystal properties

• Freeze, Antisolvent, Reactive and Melt crystallization

• Design, scale-up and control of crystallization processes at the industrial scale.

Although the BIWIC workshop is still a relaxed and informal gathering, it has developed an international flavour, having been held in recent years in Germany, France, the Netherlands, and South Korea. It is a real pleasure and honour to be hosting this 14th BIWIC in Cape Town, adding our beautiful city to the illustrious host cities that have gone before us.

We would like to express our heartfelt thanks to the members of the scientific and organising committees for their helpful contributions, and their ready and willing participation in the process of producing this BIWIC. The financial support from our sponsors is also gratefully acknowledged. Lastly, we would like to extend our special appreciation to Professor Ulrich, both for his enthusiasm and support for holding the workshop in Cape Town, as well as for his guidance and helpful assistance during the organising process.

Wishing you a productive and enjoyable workshop

Alison Lewis, Dolf Bruinsma, Christine Olsen and Meg Winter

Crystallization and Precipitation Unit, Chemical Engineering Department, University of Cape Town

While from the thermodynamic point of view chiral resolutions for systems showing miscibility in the solid state are often not feasible, the kinetics of both solid-solid transformation and crystallization must not be neglected. It has been aimed to determine the course of the solvus line over temperature. Different crystallization conditions have been investigated with regard to their capability to alter the formation of a partial solid solution. Tie lines inside the 2-phase region at thermodynamic equilibrium have been measured. The solid phase composition in equilibrium differs from the phase obtained primarily from cooling crystallization. It has been focussed on the change of the solid phase composition during crystal growth.

In this work, a model has been developed allowing for a rapid calculation of chord length distributions (as obtained from Focused Beam Reflectance Measurements (FBRM)) as well as the prediction of measurements acquired by in-situ microscopy. The data from the FBRM and in-situ microscopy are interpreted in terms of bidimensional particle size distributions (PSD). Using the model, an algorithm was developed to monitor bidimensional growth rates from FBRM and in situ microscopy measurements. The algorithm assumes knowledge on the initial PSD, as for example in a seeded crystallization, and will provide insight in how temperature and supersaturation affect growth rates. Knowledge of multidimensional growth rates and their dependence on operating parameters is a key requirement for the control of crystal size and shape in industrial crystallization. The models and techniques developed in this work provide a better understanding of multidimensional crystal growth and can form a basis for on-line crystal shape control.

Present study aims at investigation of crystallization kinetics of bisphenol A (BPA) adducts under the influence of impurities present in the industrial liquor. Experiments were carried out in a seeded batch cooling crystallizer. A population balance equation coupled with a mass balance equation was used to model the adductive crystallization processes. These model equations were solved simultaneously utilizing the finite difference method of Crank Nicolson. Kinetic parameters of the rate equations of growth, nucleation and agglomeration were estimated by minimizing the difference between the model predictions of product size distributions and experimental data.

(R)-Chloro Acid was an intermediate in the synthesis of a candidate drug in development. This paper describes the approach taken to develop a reverse classical resolution to give (R)-Chloro Acid. Knowledge of the eutectic position and eutectic solubility were key to the development of this process.

Experiments using Quinine as the resolving agent gave a eutectic position of 0.02 (resolvability = 0.98, max. = 1[1][2]) and a potential yield of 49 % at 100 % diastereomeric excess (d.e), however, it was found that the undesired salt, (S)-Chloro Acid Quinine, crystallised from solution. We chose to exploit the excellent eutectic position of the Quinine salts by developing a reverse classical resolution process, i.e. keeping the desired salt, (R)-Chloro Acid Quinine, in solution. The eutectic solubility was fundamental to achieving a process with practical working volumes and finding a suitable solvent proved to be a challenge. Batch mobility issues complicated the determination of the eutectic solubility. Roche Resolution software [3] was used as a guide to the optimum concentration and thus process volumes.

Elimination of an undesirable compound in water treatment may be performed by precipitation of an insoluble salt. Nevertheless, because of the low concentration of the undesirable compound, the solid concentration in the reactor is low, and the precipitation reactor volume is consequently very large.

The paper shows some principal methods for the drastic decrease of the precipitator volume, based on the recycling and/or in situ separation of the solid phase. The detailed mass and population balances of the installation prove that the efficiency depends on two main parameters, recycling ratio and solid-liquid separation factor in the thickener. Some quantitative relations are obtained to estimate the reactor volume saving. These methods can be successfully used to improve the functioning of existing plants and/or to design new plants having reduced dimension.

The perfectly mixed precipitation zone has a relatively low elimination rate due to the low supersaturation corresponding to the outlet limit concentration of the undesirable compound. To avoid this disadvantage, a fluidized bed column can be used. The fluidized bed precipitator combined with a classifier can be largely used for the treatment of waste water containing undesirable compounds in low or very low concentrations. It represents the most efficient device for the process intensification in the environmental engineering or other purification industrial activities.

The influence of mono ethylene glycol (MEG) on nucleation and the resulting polymorphism of calcium carbonate has been studied at supersaturation ratios in the range of 3-10 and temperatures from 25-80 °C in mixed solvents of ethylene glycol and water. The results show that increased concentration of MEG extends the induction time for precipitation of calcium carbonate significantly, even at the same supersaturation level. The interfacial tension with respect to calcite is lowered as the MEG content increases, but this change is not sufficient to explain the wide variation in the observed induction times. MEG also affects the polymorphic abundance in the precipitates. High concentrations of MEG favour the precipitation of vaterite and higher temperatures promote the formation of aragonite. The particle size is remarkably reduced in solvents with high MEG concentration.

This work presents a general approach to employ different complementary spectroscopic in-situ monitoring techniques in combination with multivariate analysis to estimate liquid and solid compositions in suspensions. The polymorph transformation of L-glutamic acid was selected as a representative process to apply the developed methods. ATR-FTIR and Raman spectroscopy in combination with multivariate analysis were employed to measure solute concentrations and solid composition, respectively. Additionally it was shown Raman spectroscopy can be employed to estimate solute concentrations. The kinetics of the transformation process was determined by combination of a population balance model and a parameter estimation technique. This model can be used to describe the polymorph transformation in a wide range of operating conditions.

The option to improve the shelf life of fertilizers by upgrading the morphology is examined. The researched fertilizer consists of a ternary melt suspension with two fertilizer compounds and an additive. The first part of the study shows that the crystal size of the non-molten fertilizer compound (the remaining solids) has a significant effect on the morphology of granules. Due to the fact, an assumption concerning the roughness and sphericity of granules manufactured with a small crystal size of the suspended fertilizer compound in fluid bed granulator is introduced and proven. The roughness of the granules decreases with smaller crystal size fractions of the suspended solids in the suspension.

Conclusions concerning coverage of the water-repellent finishing and shelf life of the granules are introduced. A reduced roughness of the granules will decrease the dust products in transport processes and hence increase the shelf life. Since dust as attrition products of the roughnesses is one of the key factors to boost the caking of granules. Furthermore, the reduced roughness of the granules will also improve the coverage of the water-repellent finishing. The roughnesses are responsible for failure in the coverage of the coating of granules. Therefore, the coverage of the water-repellent finishing is another key factor for the reduction of caking of the granules and hence thereafter an improved shelf life. An improvement in shelf life could be achieved in the same order of magnitude as the roughness is reduced of the granules.

This paper deals with the design and on-line implementation of a real-time dynamic optimizer for fed-batch evaporative crystallization of an ammonium sulphate-water system. An optimal heat input profile to a 75 liter draft tube crystallizer is sought such that an objective function pertinent to the batch crystal yield is maximized while the reproducibility of batches and the fulfillment of desired product specifications are sustained. The seeded fed-batch crystallizer is represented by a non-linear moment model that is experimentally verified to be an adequate description of the process at hand. The optimal control problem is solved by the sequential approach. Due to lack of supersaturation measurements, the evolution of supersaturation during the process is estimated using the extended Luenberger observation technique. A closed-loop implementation of the dynamic optimizer reveals that a substantial increase in the crystal volume fraction at the batch end can be attained by an optimal heat input profile, whereas the impact of the heat input on the median crystal size is insignificant.

Transformation of polymorph during the crystallization was monitored by using in-situ measurement of ultrasonic velocity in the crystallizer. From the in-situ measurement of the ultrasonic velocity, the degree of transformation and the mechanisms of nucleation and crystal growth were found during the transformation of polymorph.

Environmental considerations as well as the economical benefits of water re-use have raised the interest of the power industry to improve zero-liquid-discharge technologies for the treatment of effluents from power plants.

Typical streams to be treated are the effluents from the water demineralization system, first rain water and various blow-down. Such streams are diluted saline waters containing chlorides, sulphates, nitrates and carbonates, usually saturated in CaSO4. Due to their nature, evaporative crystallization of such brines leads to heavy encrustation problems.

Scope of such ZLD units is to recover high quality process condensate and separated solids of disposable quality, satisfying at the same time the stringent requirements of a power plant in terms of system reliability and availability.

GEA has developed a concept for modular systems with a special approach in order to obtain very long operating cycles between consequent washouts, acting both on the process design (seen as a real crystallization system) and on the mechanical design of the equipment itself.

This paper describes the main design criteria, comparing the expectations with the operating behaviour of a plant designed accordingly, that is on stream successfully since two years.

The solvent mediated anhydrous to monohydrate phase transition of citric acid was monitored using in situ Raman spectroscopy and image analysis. The solid phase composition, solute concentration and CSD (Crystal Size Distribution) were measured during specific experiments designed to study the process: dissolution of the metastable anhydrous form and nucleation and growth of the monohydrate stable form. A Population Balance Model was developed to depict the time evolutions of the populations of particles involved. Nucleation and growth kinetic parameters were estimated through the non-linear least squares minimization of model-experiments prediction errors. The overall model is shown to satisfactorily describe the solvent mediated phase transitions process. Secondary nucleation of stable monohydrate particles was found to depend on both the supersaturation and the overall solid content. As far as the nucleation of stable particles in the presence of metastable solid is concerned, the rate of particle generation exhibits identical dependency on the metastable and stable solid content; a feature which remains to be further investigated and explained.

The use of different types of seeding material to cause the layer deposition of desired iron phases is an attractive technique for the treatment of industrial waste waters, synthesis of iron products and the study of adsorption and dissolution reactions involving iron oxides. In this study, recycled solids (consisting of mainly ferrihydrite), goethite and haematite seeds were used during batch oxidation and precipitation of iron from ferrous sulphate solutions. A 4L standard stirred tank reactor was used. Air was used as the oxidising agent. High ferrous iron concentrations typical of acid mine drainage associated with South African coal and gold mining were used. All the three oxides of iron catalysed the oxidation reaction rate in the order, recycled solids < goethite < haematite. The products formed were mainly the same as the seed material with varying amounts of ferrihydrite in lesser proportions. The product formation process involved two mechanisms; the direct formation of precipitate on the seed material and formation of small particles in the bulk which later adhered onto the seed material. The latter mechanism appeared to be the dominant one due to the high supersaturation levels induced by the catalytic effect of the seeds on the oxidation rate.

The article describes a typical procedure for the industrial implementation of a melt crystallization project starting with pilot testing to determine feasibility and the major design parameters followed by the engineering, construction and start-up of the unit. The presentation is attempting to provide a summary overview about how crystallization theory is included in the design of an industrial melt crystallization plant. A time accelerated sequence of photos will be used during the presentation to illustrate the progress of the construction works during the completion phase. The melt crystallization plant consists of a single stage suspension crystallization unit with external heat exchangers and a separate mixing vessel to provide the necessary residence time for crystal growth. The crystal separation is efficiently accomplished in a wash column where the crystals are counter-currently washed in order to achieve the required high purities. This process is referred to as SCWS (Suspension Crystallization Wash-column Separation) process.

A case study is presented on the specific layer growth history of an α-Al₂O₃ supported NaA zeolite membrane synthesised from a clear solution. Using a defined set of synthesis parameters, the layer development over time (1.0-4.0 h) was described in terms of morphology, growth rate and elemental composition. It was shown that membrane growth proceeds along two distinct morphological pathways over the duration of synthesis. A two-step growth rate trend was observed and could be correlated to the respective growth phases within the two underlying morphology types. The development of the hemisphere-shaped grains was associated with a period of accelerated growth during the first 2.5 h of synthesis (3.3 × 10^-10 m.s^-1), followed by a period of slower growth for the formation of the cubic morphology (1.9 × 10^-10 m.s^-1). Localised changes in supersaturation, combined with the possible effects of grain crowding, were offered as feasible explanations for the observed morphology and growth rate tendencies. Following the elemental make-up of the developing membrane showed a gradual decrease in the Na/Si ratio with increasing crystallisation times, which was explained by the consumption of the amorphous content in the membrane as growth proceeds.

In hard water, dodecyl sulfate (DS) tends to precipitate with the counterions, Ca²⁺ and Mg²⁺ forming soap scum, and is thus no longer available to participate in the cleaning action. The scum formed from the precipitation is also a problem in that it causes graying of the fabric. Solubility data of the precipitated calcium dodecyl sulfate (CDS) crystals remains quite limited. Consequently, this work has investigated the activity-based solubility product of CDS in aqueous solutions as a function of temperature, with modeling between 10 and 55°C. Two techniques of measuring the solubility were used. Below the Krafft temperature, approximately 50°C for CDS, the solutions were saturated at fixed temperatures and then analyzed using atomic-absorption spectroscopy. Above the Krafft temperature a conductivity technique was used. Plotting the conductivity of a fixed concentration solution against temperature allows the solubility to be found from a discontinuity in the derivative. The activity-based solubility product, K_SP, was calculated from the experimental data based on modeling of the micellar phase in solutions for the samples which were above the critical micelle concentration. The two techniques for measuring the solubility of CDS yield consistent values of the solubility product, which can be used to model the phase boundary of CDS precipitation in solutions of sodium dodecyl sulfate (SDS) and calcium chloride as a function of temperature.

Co-crystals of Carbamazepine and Isonicotinamide form through a solvent mediated transformation process when a dry mixture of the pure crystalline components is suspended in ethanol. First, needle-like crystals grow while the pure crystalline components dissolve. Eventually plate-like crystals grow at the expense of the needle-like crystals. We are confident that both the needle-like and the plate-like crystals are co-crystal forms of Carbamazepine and Isonicotinamide.

Bisphenol-A (BPA), an organic compound used as a primary monomer in epoxy resins and polycarbonate plastics, is produced by a reaction of two phenols with one acetone catalyzed by a cation exchange resin. BPA is separated from the reaction products through the crystallization of BPA-phenol adduct crystals. Adduct crystal consists of an equimolar solid mixture of BPA and phenol. Products of side reactions and the unreacted compounds may affect the crystallization yield and product quality. This study aims at investigating the influence of different concentrations of water and acetone on the yield and product size distribution in adductive crystallization of BPA. Experiments were performed in a comparative method from phenolic solutions in the absence of impurity and in the presence of different concentrations of acetone and water as impurities. Crystallization experiments were carried out in 250-ml-plastic-bottles installed in a rotating water bath. Experimental results showed that water and acetone will decrease the crystallization yield and the average size of product particles. However, water in reduction of yield was more influential than acetone and in reduction of average size of product was less influential. These effects may be attributed to the increase in solubility of BPA in phenolic solution and the blockage of active sites on seed crystals by water and acetone molecules.

The most important influences on the width of the metastable zone are the kind and concentration of the impurities existing in the soultion. The presence of impurities in solution leads to different disturbances on the yield of a product. Only an experimental determination for the effects of impurities exists in the relevant literature without any interpretation of these results. Most explanations of crystal growth under additive effects start at the solid phase but do not take into account the change in the structure of the solution when additives are added to a solution.

Here it was investigated if the properties of intermolecular interactions cations and anions each for additives, solvent and salt in the solution are the key of the interpretation of influences of additives on the metastable zone width of inorganic compounds.

The polythermal method was used to measure the effect of the additives, e.g. Al³⁺, Fe²⁺, Ba²⁺, Li¹⁺ and K¹⁺ on the MSZ width of ZnSO₄. These additive ions were selected according to a suggested rule, which takes the amount of hydration enthalpy values into account. The results show that the additives affect the MSZ width not always as expected. Also it is explained that the effect of Al³⁺, Fe²⁺ and Li¹⁺ on the MSZ width of zinc sulfate can be given in terms of hydration enthalpy values and ion-ion interaction forces. This approach is, however, still not sufficient to interpret the effect of all additives e.g. not for BaCl₂ and K₂SO₄ on the MSZ width of ZnSO₄.

Knowledge of metastable zone width (MZW) can help to control the quality of the produced crystals. In this study, the maximum MZW in adductive crystallization of Bisphenol-A (BPA) from a phenolic solution was determined using the concepts of thermodynamic phase equilibria. Using these concepts, it was possible to determine the maximum MZW in the absence of the influence of the kinetic factors such as the rate of cooling, and the rotational speed of the mixer. The method included calculating the necessary parameters in various excess Gibbs energy models, and determining their temperature dependencies. The excess Gibbs energy models studied were the Symmetrical, Margules, Wilson, and NRTL models. Experimental transparency (solubility) and cloud point (instability) data were obtained for five different binary solutions of phenol containing 9.5, 13, 18, 23, and 25 wt% BPA. Calculations showed that the Margules and Symmetrical models were able to predict the maximum MZW.

The effect of supersaturation and temperature on the particle morphology of L-Glutamic acid and an aromatic amine derivative has been compared by pH-shift precipitation in water at temperatures of 5-80 °C. Both systems show switching from single crystals to polycrystalline particles as the initial supersaturation is increased. The polycrystalline particles have been identified to grow by a spherulitic growth mechanism. The aromatic amine derivative shows switching from monocrystalline plate-like crystals at a constant supersaturation value, whereas L-Glutamic acid displays a transition from needles to spherulites, at a supersaturation level that becomes lower as the temperature increases. The spherulite morphology changes with supersaturation, becoming more compact at higher supersaturation ratios.