Gerda M. van Rosmalen

Precipitation of ultrafine organic crystals from the rapid expansion of supercritical solutions over a capillary and a frit nozzle

Benzoic acid, salicylic acid, aspirin or phenanthrene was dissolved in supercritical CO2 and precipitated as a fine powder by the rapid expansion of the supercritical solution over a nozzle (RESS process). A new kind of expansion device is presented (the frit nozzle) which can be manufactured more easily than the capillary nozzle, especially if small holes are required. The performances of the two expansion devices are compared. The process operating conditions (extraction pressure and temperature, pre- and post-expansion temperature, mass flow rate and nozzle design) were varied systematically to analyze their impact on the crystal habit and size of the precipitated material. Except for phenanthrene, the crystals obtained with the frit nozzle were smaller than those precipitated with the capillary nozzle, as can be explained from expansion paths and solubility diagrams.

Carboxymethyl inulin: A new inhibitor for calcium carbonate precipitation

A new polysaccharide-based polycarboxylate, carboxymethyl inulin (CMI), was synthesized recently. The influence of small amounts (0.1–200 ppm) of this material on the crystallization of calcium carbonate, an important scale-forming salt, is studied. The effects of CMI are compared to those of a commercial inhibitor (a copolymer of acrylate and maleate) and of other carboxymethylated saccharides. It is shown that CMI is a good calcium carbonate precipitation inhibitor. CMI influences the spontaneous precipitation of calcium carbonate, the morphology of the formed crystals (vaterite and calcite), and the growth rate of calcium carbonate seed crystals. The effect is related to the carboxylate content, the chainlength, and the concentration of the additive. For the application of CMI as crystallization inhibitor, products with a high degree of substitution (degree of substitution>1) and a high degree of polymerization (average degree of polymerization = 30) are the most effective. Also, other carboxymethylated polysaccharides (dextrins, cellulose) show good crystallization-inhibition properties, although the performance of the copolymer of acrylate and maleate is not met. A great advantage of CMI, as compared to carboxymethyl cellulose (CMC), is that aqueous solutions of CMI display, contrary to those of CMC, a very low viscosity. A carboxymethylated disaccharide (carboxymethyl sucrose) has no influence on the calcium carbonate crystallization which shows that the long-chain character is essential for a polycarboxylate inhibitor.

Design of industrial crystallisers for a given product quality

Abstract The main challenge in the design of industrial crystallisers is to predict the influence of crystalliser geometry, scale, operating conditions and process actuators on the process behaviour and product quality. The quality characteristics, such as the crystal size distribution, inclusion content and morphology determine to a large extent the product performance and are therefore of importance. The quality of the product crystals is basically determined by the rates at which crystals are born and attrited, grow or dissolve and agglomerate in the different regions of the crystalliser. An analysis technique is therefore introduced to describe the various crystallisation phenomena as a function of local process conditions such as supersaturation and energy dissipation. This technique is based upon: • the derivation of pure kinetic parameters from an MSMPR experiment. • setting up compartmental models for design alternatives in order to separate kinetic and hydrodynamic phenomena. • analysis of the process behaviour of the design alternatives by applying the same kinetic model and parameters for each compartments. • optimisation of the design alternatives with respect to product quality using crystalliser geometry, operating conditions and the appropriate process actuators, like a fines removal system, as degrees of freedom. The advantage of this technique over conventional techniques is illustrated for an evaporative DTB crystalliser.

Towards on-scale crystalliser design using compartmental models

Abstract In this paper the importance of on-scale crystalliser design is outlined. An on-scale approach is specifically required for the analysis and optimisation tasks in design. The need for this approach is a direct consequence of the nonlinear dependency of most physical processes in crystallisation on the degree of saturation, the energy dissipation, the crystal size, and its distribution. The hydrodynamics in a crystalliser vessel are typically such, that these process variables are distributed non-uniformly throughout the vessel. The conventional, geometrically lumped description of the physical process inside a crystalliser vessel, i.e nucleation, growth, dissolution, attrition, breakage agglomeration and particle segregation, has therefore never proven to be reliable for scale-up purposes. Furthermore, as the interactions between these processes lead to an intricate dynamic behaviour, models describing the effect of changes in time of process variables on the product quality are essential. Compartmental modelling, a well known technique in reactor engineering and applied within crystallisation since a number of years, facilitates on-scale design since it allows a natural separation of kinetic and hydrodynamic mechanisms. The resulting dynamic models (order of 10 4 equations) can be easily tackled with standard DAE solvers. Here we will focus upon the need for a proper physical description of the aforementioned crystallisation mechanisms. First of all, a brief description of the dependencies of these mechanisms upon local supersaturation or undersaturation, local energy dissipation and crystal size is given. Depending on the type of crystallisation process, suspension crystallisation or precipitation, the dependencies necessary to be included in the compartmental model, in order to describe their overall effect are discussed. The next step is deriving the geometric structure of a compartmental model for a certain scale crystalliser and material, for which two methodologies will be presented. Finally, the approach will be illustrated for evaporative crystallisation of ammonium sulphate from water in 0.15 and 18.5 m 3 FC (Forced Circulation) and 0.022 and 1.1 m 3 DTB (Draft Tube Baffle) crystallisers, using size dependent nucleation, growth, dissolution, attrition and segregation models.

The effect of aluminium fluoride on the formation of calcium sulfate hydrates

Abstract Different phases, hydrates or polymorphs are often developed in mineral ore conversion processes where sparingly soluble salts are formed. Their development not only depends on the operating conditions but also on the specific composition of the ore. In this study the effect of aluminium fluoride, a constituent encountered in fluorapatite rock in varying concentrations, on the formation of calcium sulfate hydrates during phosphoric acid processing is presented. Solubility experiments and continuous crystallization experiments of calcium sulfate hemihydrate ( CaSO 4 · 1 2 H 2 O , HH) and calcium sulfate, dihydrate (CaSO 4 · 2H 2 O, DH) were performed in concentrated phosphoric/sulfuric acid solutions at 75°C and 90°C in the presence of small amounts of aluminium fluoride or hydrogen fluoride. Both AlF 3 and HF increase the solubilities of HH and DH via their effect on the activity coefficients of calcium and sulfate. Aluminium fluoride retards the HH and DH growth rate and an AlF 3 concentration of 95 mmol (kg solution) −1 increases the calcium sulfate concentration far above the DH solubility line. This leads to DH formation under conditions where normally HH is formed. The presence of aluminium fluoride also results in crystal habit modification and agglomeration of the HH and DH crystals formed.

The effect of the impeller speed on the product crystal size distribution (CSD) a 22 liter draft tube (DT) crystallizer

Abstract In draft tube (DT) crystallizers secondary nuclei originate predominantly from collisions between crystals and the impeller. The formation step of these nuclei is a complex process depending on the attrition behavior of the crystalline material and the two-phase flow pattern in the vicinity of the impeller. In order to develop crystallizer models with a predictive value for different operational conditions or for different scales of operation a mechanistic description of the secondary nucleation rate is a prerequisite. In this paper the predictive capability of a mechanistic model framework was studied by comparing the dynamic trend of the predicted median crystal size L 50 with the experimentally obtained one. The model is in good agreement with the experimental data regarding the predicted steady-state L 50 and the amplitude of the start-up oscillation. The dynamic behavior predicted by the model, however, differs from the measured one regarding the oscillation frequency and phase for reasons that will be addressed in the future.

SELECTIVE DISSOLUTION OF COPPER OXALATE USING SUPPORTED LIQUID MEMBRANES

ABSTRACT A supported liquid membrane has been used to dissolve selectively copper oxalate from a suspension of copper, calcium and cadmium oxalate, which have low, similar solubilities. 2-Hydroxy-5-nonyl-acetophenone oxime (HX) dissolved in kerosene was used as a carrier for copper transport from the suspension to the stripping solution. A mathematical model of the copper permeation is presented. The model takes into account the dissolution kinetics of CuC204-1/2H20, the diffusion of copper ions through an aqueous stagnant layer, the chemical reaction at the aqueous/membrane interface, and the diffusion of the CuX2 complex in the membrane. The model fits the experimental data well with a unique parameter set, except for the transport from an acetate buffered system, for which a lower rate constant for the reaction at the membrane interface had to be assumed. In a separate set of experiments the dissolution of copper oxalate hemihydrate in water was found to be surface reaction controlled.

The influence of aluminium fluoride in hemi-dihydrate phosphoric acid processes

Abstract Bench-scale continuous crystallization experiments were performed with three different types of phosphate ores in a cascade of bench-scale crystallizers, thus simulating the calcium sulfate hemihydrate ( CaSO 4 · 1 2 H 2 O ) crystallization section in a two-filter hemi-dihydrate phosphoric acid process. The effect of aluminium fluoride in the system was investigated by the addition of an aluminium salt and the use of ores with different aluminium contents. Aluminium fluoride, which might affect the crystallization by its growth-retarding AlF 5 2− complex, promotes the formation of DH (CaSO 4 · 2H 2 O) modification and the formation of agglomerates of short crystals, and therefore influences the permeability of the crystal product formed. The aluminium distribution coefficient ([Al] in solid/[Al] in liquid) in calcium sulfate hemihydrate decreases strongly with increasing aluminium concentration in the liquid. Consequently, low aluminium distribution coefficients are encountered when an aluminium-rich phosphate ore is used and vice versa. Finally, from mass balance calculations, the maximum allowable aluminium concentrations in the phosphate ores are predicted for hemi(dihydrate) phosphoric acid processes. Above these concentrations, unwanted calcium sulfate dihydrate DH formation can take place but this does not always occur.

Modeling the Attrition Process in a 22 Liter Draft Tube Crystallizer

In Draft Tube (DT) crystallizers secondary nuclei are predominantly produced due to crystals colliding with the impeller. The formation step of these nuclei is a complex process depending on the local two-phase flow pattern in the vicinity of the impeller and on the attrition behavior of the crystalline material. In order to develop crystallizer models with a predictive value for different operational conditions or for different scales of operation a mechanistic description of the secondary nucleation rate is a prerequisite.

Industrial crystallization in practice: from process to product

Scope of the book Crystallization refers to the phase transformation of a compound from a fluid or an amorphous solid state to a crystalline solid state. However, a crystallization process is not just a separation process; it is also a production process and a purification technique, as well as a branch of particle technology. It thus encompasses key areas of chemical and process engineering (Davey and Garside, 2000). Crystallization is an extremely old unit operation, but is still used to produce highly specified speciality chemicals, and pharmaceuticals. In fact, there are few branches of the chemical and process industries that do not, at some stage, employ crystallization or precipitation for production or separation purposes (Mullin, 2003). Crystalline products include bulk chemicals such as sodium chloride and sucrose, fertilizer chemicals such as ammonium nitrate, potassium chloride, ammonium phosphates and urea; valuable products such as pharmaceuticals, platinum group metal salts and organic fine chemicals; products from the new and rapidly expanding field of engineered nanoparticles and crystals for the electronics industry, as well as biotechnology products such as protein crystals. Although crystallization is an increasingly important industrial process, one that is governed by thermodynamics of phase separation, mass and heat transfer, fluid flow and reaction kinetics, it is not usually explicitly covered in any of the existing core chemical engineering material. A large percentage of final or intermediate industrial products consist of a product of a crystallization process, i.e. tiny crystals or particles that have to conform to product specifications with respect to crystal size and shape, crystal size distribution, degree of agglomeration and uptake of either liquid or solid impurities. These product properties relate to the selected type of crystallization process as well as to the specific crystallization mode and type of hardware used for production. Crystallization is therefore much more than just a simple separation process. Unfortunately, the technology to design, operate and optimize crystallization processes is usually covered only very briefly as part of a broader overview on separations or particle technology, such as the chapter on Crystallization and Precipitation by Mullin in Ullmanns Encyclopaedia of Industrial Chemistry (Mullin, 2003) or other, similar, volumes (Richardson et al ., 2002 and Ruthven, 1997).