Herman J. M. Kramer

Extracellular Polymeric Substances Govern the Surface Charge of Biogenic Elemental Selenium Nanoparticles

The origin of the organic layer covering colloidal biogenic elemental selenium nanoparticles (BioSeNPs) is not known, particularly in the case when they are synthesized by complex microbial communities. This study investigated the presence of extracellular polymeric substances (EPS) on BioSeNPs. The role of EPS in capping the extracellularly available BioSeNPs was also examined. Fourier transform infrared (FT-IR) spectroscopy and colorimetric measurements confirmed the presence of functional groups characteristic of proteins and carbohydrates on the BioSeNPs, suggesting the presence of EPS. Chemical synthesis of elemental selenium nanoparticles in the presence of EPS, extracted from selenite fed anaerobic granular sludge, yielded stable colloidal spherical selenium nanoparticles. Furthermore, extracted EPS, BioSeNPs, and chemically synthesized EPS-capped selenium nanoparticles had similar surface properties, as shown by ζ-potential versus pH profiles and isoelectric point measurements. This study shows that the EPS of anaerobic granular sludge form the organic layer present on the BioSeNPs synthesized by these granules. The EPS also govern the surface charge of these BioSeNPs, thereby contributing to their colloidal properties, hence affecting their fate in the environment and the efficiency of bioremediation technologies.

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.

Nonlinear Model-Based Control of a Semi-Industrial Batch Crystallizer Using a Population Balance Modeling Framework

This paper presents an output feedback nonlinear model-based control approach for optimal operation of industrial batch crystallizers. A full population balance model is utilized as the cornerstone of the control approach. The modeling framework allows us to describe the dynamics of a wide range of industrial batch crystallizers. In addition, it facilitates the use of performance objectives expressed in terms of crystal size distribution. The core component of the control approach is an optimal control problem, which is solved by the direct multiple shooting strategy. To ensure the effectiveness of the optimal operating policies in the presence of model imperfections and process uncertainties, the model predictions are adapted on the basis of online measurements using a moving horizon state estimator. The nonlinear model-based control approach is applied to a semi-industrial crystallizer. The simulation results suggest that the feasibility of real-time control of the crystallizer is largely dependent on the discretization coarseness of the population balance model. The control performance can be greatly deteriorated due to inadequate discretization of the population balance equation. This results from structural model imperfection, which is effectively compensated for by using the online measurements to confer an integrating action to the dynamic optimizer. The real-time feasibility of the output feedback control approach is experimentally corroborated for fed-batch evaporative crystallization of ammonium sulphate. It is observed that the use of the control approach leads to a substantial increase, i.e., up to 15%, in the batch crystal content as the product quality is sustained.

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.

Influence of sulfide concentration and macronutrients on the characteristics of metal precipitates relevant to metal recovery in bioreactors.

Purity and settling properties determine metal sulfide recovery from bioreactors. The influence of macronutrients commonly present in mineral media and wastewaters on Cu, Pb, Cd and Zn depletion kinetics and characteristics was evaluated in batch experiments with chemically produced sulfide at different concentrations. The metal depletion kinetics showed that metals with slower depletion rates (Zn and Cd) are susceptible to other removal mechanisms such as biosorption onto the sulfate reducing biofilm and precipitation with macronutrients when sulfide is below the stoichiometric metal to sulfide ratio. For Zn, the main mechanism of removal is its sorption onto apatite (Ca(5)(PO(4)))(3)(+)(OH(-)), a compound formed due to the presence of CaCl(2)·2H(2)O and KH(2)PO(4) in the mineral medium. All precipitates were 8.1-10.0μm regardless the sulfide concentration demonstrating that this parameter is less relevant for particle growth and settling, compared to the agglomeration of the precipitates.

Modeling of industrial crystallizers for control and design purposes

Abstract The design of forced circulation (FC) crystallizers, which are widely used for the bulk crystallization of inorganic salts, is hindered by the lack of rules for scale-up, caused by the lack of reliable process models. The absence of a stirrer in the crystallizer and the relatively high temperature increase in the circulation line result, as expected, in a strong deviation from the well-mixed vessel approach often used in modeling. To improve process models, a compartmental approach is presented here to describe the crystallization process of evaporative and cooling suspension crystallizers. The model has been implemented in the dynamic flowsheeting program, speedup , and applied to simulate a 200-l evaporative FC crystallizer, with five compartments. Preliminary simulation results indicate that large supersaturation profiles are present in the crystallizer and that only a part of the crystallizer volume is effectively used for the growth of crystals.

A Comparative Study of Various Size Distribution

One of the major product specifications of a crystalline material is the crystal size distribution (CSD). In order to monitor and control the CSD in an industrial crystallization process, on-line sensors are required. Over the years, a number of techniques to measure the CSD have been established. In this paper, three instruments operated in an on-line fashion and an off-line method are compared. The instruments were the OPUS, a HELOS/VARIO (both manufactured by Sympatec) and a Malvern 2600c (manufactured by Malvern). They were implemented on an 1100-l evaporative draft tube baffle (DTB) crystallizer producing ammonium sulfate crystals from aqueous solution. Samples from this crystallizer were also analyzed offline by wet sieving. The results show reasonably good agreement between the different on-line techniques and the wet sieving technique concerning the shape of the distribution. However, there is a discrepancy regarding the absolute values, which can be explained by the fact that the techniques used are based on different measuring principles.

The microscopic modelling of hydrodynamics in industrial crystallisers

Abstract In this contribution a method for the calculation of crystal–crystal collisions in the flow field of an industrial crystalliser is proposed. The method consists of two steps. The first step is to simulate the internal flow of the crystalliser as a whole. For this purpose, the simulation of the internal flow of an 1100 l draft tube baffled crystalliser at a Reynolds number of 240,000 is presented. This simulation was done with a lattice-Boltzmann scheme with a Smagorinsky sub-grid-scale turbulence model ( c s was 0.11) on approximately 35.5×10 6 grid nodes. The second step of the method consists of simulating individual crystals in a fully periodic box with turbulent conditions that represent the conditions in a point of the crystalliser. Thus collision frequencies and intensities of the crystals under the local hydrodynamic regime can be obtained. In this contribution a feasibility study of this second step is described. A theoretical framework is established to identify the key parameters that determine the relationship between the crystalliser flow and the box simulations. Based on this framework, conditions for box simulations representing three monitor points in the simulated crystalliser are calculated. Finally, to demonstrate the method of predicting the motion of individual particles, sedimentation and consecutive collision of a single sphere with a solid wall is simulated.

Modelling of industrial crystallizers, a compartmental approach using a dynamic flow-sheeting tool

A compartmental approach is presented to describe the crystallization process of evaporative and cooling suspension crystallizers. The model has been implemented in the dynamic flow-sheeting program Speedup and applied to simulate a 970 litre evaporative DTB crystallizer using four compartments. Preliminary simulation results indicate that even in DTB crystallizers, profiles of relevant process variables can be expected.

Control of industrial crystallizers

The particle size distribution produced in particulate processes generally has a large impact on process economics, and the crystal size distribution produced in industrial crystallizers is no exception. Crystal Size Distribution (CSD) control is therefore desirable. Its establishment is the main objective of a joint research program between three research groups within the Delft University of Technology and several industrial participants. This paper presents a review of this project and intends (1) to illustrate the development of an on-line crystal size measuring technique, (2) to illustrate how information on CSD can be used to derive a dynamic process model and (3) to emphasize the need for effective process inputs and to make suitable suggestions in that direction.