Pekka Oinas

Hydrodynamics and mass transfer in an airlift reactor

Abstract Hydrodynamics and mass transfer in internal and external flow airlift reactors have been studied in this work. Gas hold-ups and liquid velocities have been determined in a concentric tube airlift reactor, while oxygen mass transfer has been measured in an external loop airlift reactor. First, tracer measurements were used to verify a conventional axial dispersion model. Further, a model derived from multi-phase Navier–Stokes equations was verified with experimental data from the oxygen mass transfer measurements. Both models were in a good agreement with the measurements.

Mass transfer in an external-loop airlift reactor: experiments and modeling

The mass transfer in an airlift reactor is modeled using simple elementary models: the liquid flow in the riser and the downcomer is represented as plug flow with axial dispersion, while the gas-liquid separator and the bottom junction are considered as CSTRs for the liquid. The gas flow in the riser is represented as plug flow. The system of differential equations resulting from the mass balances applied to the different sections of the reactor are solved in the real-time domain using a commercial software (MODEST). The model parameters are evaluated by adjusting the variation with time of experimental and simulated oxygen concentration profiles in the reactor after change from deoxygenation to oxygenation of the recirculating liquid (dynamic method), at six different locations in the riser, gas-liquid separator and downcomer. The model is tested using experimental data obtained with advanced measuring techniques in a pilot airlift reactor. The data agree well with some correlations from literature.

Hydrodynamics of an airlift reactor: Experiments and modeling

Abstract The hydrodynamic behaviour of an external airlift reactor is modelled using simple elementary models: the liquid flow in the riser and the downcomer is represented as plug flow with axial dispersion, while the gas-liquid separator and the bottom junction are considered as CSTRs for the liquid. The system of differential equations resulting from the mass balances applied to the different sections of the reactor are solved in the real time domain using a powerful software (MODEST). The model parameters are evaluated by adjusting the experimental and simulated responses reactor to a tracer impulse injection at six different locations in the riser and the downcomer. A sensitivity analysis shows that the liquid velocities determined this way are much more precisely known than the axial dispersion coefficients. The velocities thus obtained compare well with data measured by a thermal tracer technique.

Application of CFD simulation to suspension crystallization—factors affecting size-dependent classification

Abstract A size-dependent classification function, which is a parameter used to characterise crystallization in imperfectly mixed suspensions, was simulated using CFD. The CFX4.2 program was employed to simulate the local particle-size distribution in a multifluid flow using the k−e turbulence model with respect to interface transfer between the particles and the solution. The problem was studied three dimensionally. A sliding grid technique was used to simulate the transient flow in the mixing tank. The size-dependent classification was calculated based on the local particle-size distribution. The factors that affect the classification function are discussed.

The influence of multicomponent diffusion on crystal growth in electrolyte solutions

Abstract Mass transfer phenomena in crystallizing solutes and the concentrations of impurities in solution and in the crystal have been studied based on Maxwell–Stefans theory of multi-component diffusion. The Maxwell–Stefan theory requires data on the driving force, i.e. the concentration gradient for each species. The effect of the electrolyte mixture on the diffusion coefficient of the crystallizing solute and the impurity solute was studied based on the Pitzer theory, which can be used to determine the activity coefficients of a solute in electrolyte mixtures. The studied crystallization systems were the continuous suspension crystallization of potassium sulfate with sodium sulfate as an impurity, and the crystal growth study using potassium nitrate as a single crystal with calcium nitrate as an impurity in the solution. These two crystallization systems have different charge types of electrolytes. The proposed model was applied in calculating the mass transfer coefficient between the crystallizing solute and the impurity solute. Also, the molar flux of the specified species for the studied cases was estimated.

Closed cycle production of concentrated and dry redispersible colloidal lignin particles with a three solvent polarity exchange method

Lignin, an aromatic biopolymer, is the main by-product of pulp manufacture, and has been under intense study, as it offers great promise as an alternative for petrochemical polymers. However due to its heterogeneity, the applications where lignin can be used have been limited, leading to the vast majority of it being burned for fuel. Colloidal lignin particles (CLPs) offer a means to disperse lignin homogenously into both water and other media, such as polymers. However, no means thus far have been presented that would allow for a large-scale production of CLPs. Herein we show an industrially scalable closed cycle process of CLP production. In the process, a concentrated solution of lignin in tetrahydrofuran (THF) and ethanol (EtOH) is added into the non-solvent water, instantaneously forming CLPs through self-assembly. The organic solvents are recovered and reused in the process. The aqueous CLPs are concentrated by ultrafiltration and the concentrated particles are spray dried, leading to redispersible microclusters. CLPs can be used in multiple applications, such as Pickering emulsions and composite materials. A significant portion of the 50 million tons of lignin produced by the pulp industry could be made into CLPs with this low cost process, which would open a whole new class of materials for industrial applications.

Elemental balance of SRF production process: solid recovered fuel produced from municipal solid waste.

In the production of solid recovered fuel (SRF), certain waste components have excessive influence on the quality of product. The proportion of rubber, plastic (hard) and certain textiles was found to be critical as to the elemental quality of SRF. The mass flow of rubber, plastic (hard) and textiles (to certain extent, especially synthetic textile) components from input waste stream into the output streams of SRF production was found to play the decisive role in defining the elemental quality of SRF. This paper presents the mass flow of polluting and potentially toxic elements (PTEs) in SRF production. The SRF was produced from municipal solid waste (MSW) through mechanical treatment (MT). The results showed that of the total input chlorine content to process, 55% was found in the SRF and 30% in reject material. Of the total input arsenic content, 30% was found in the SRF and 45% in fine fraction. In case of cadmium, lead and mercury, of their total input content to the process, 62%, 38% and 30%, respectively, was found in the SRF. Among the components of MSW, rubber material was identified as potential source of chlorine, containing 8.0 wt.% of chlorine. Plastic (hard) and textile components contained 1.6 and 1.1. wt.% of chlorine, respectively. Plastic (hard) contained higher lead and cadmium content compared with other waste components, i.e. 500 mg kg−1 and 9.0 mg kg−1, respectively.

Experimental design with steady-state and dynamic models of multiphase reactors

Abstract The verification of multiphase reactor model parameters is a complicated practical problem because of the diversity of phenomena involved in the reaction process. Due to this complexity, designing experiments for selecting and estimating the essential parameters affecting the mechanisms of simultaneous mass transfer and reaction is challenging. To our knowledge, there are not much published information on experimental design with these very models. This paper presents some of our recent studies in which experimental design methods were applied for analyzing mass transfer and reaction in stirred gas—liquid and gas—liquid—solid reactors. The first case deals with a moderately fast gas—liquid reaction in a continuous—flow reactor. Transport characteristics were described by a steady-state one-response model based on material balances in the bulk of the liquid and the interfacial film. The effective experimental design proved to be beneficial since only five sequentially designed experiments in addition to ten screening experiments were required. Experimental strategy with dynamic model of a three-phase catalytic slurry system is illustrated secondly. The system was studied in three stages: gas—to—liquid absorption, liquid—to—solid adsorption and the surface reaction.

Perturbation solutions for film models

Abstract The interaction between mass transfer and reaction in gas-liquid systems cannot be described by analytical formulae when second- or higher-order kinetics is considered. the purpose of this paper is to emphasize the usefulness of approximative perturbation solutions with an experimental case where fluxes are needed on both the gas and liquid sides of the interfacial film. For the familiar stagnant film model with second-order chemical reaction, we present approximate solutions in cases which are diffusion controlled for at least one of the reacting components. Some widely used approximations, like the pseudo-first-order assumption, are obtained as special cases. Analytical expressions are derived for both gas- and liquid-side fluxes, valid in the borderline reaction region. The approximate solutions are compared to numerical solutions of the original system.

Transient models for multiphase slurry reactors

Abstract Modelling of catalytic reactions is usually rather a complex task since several mass transfer and reaction effects take place simultaneously in gas, liquid and solid phases, not to mention the problems encountered when heat effects or changes in flow conditions cannot be excluded. Multiphase reactor models retrieved from existing literature are often not applicable as such for practical industrial problems. They might be oversimplified - or computationally too complicated, loaded with unidentifiable parameters. In this paper, a general set of dynamic model equations is presented, valid for a variety of non-catalytic gas-liquid or catalytic gas-liquid-solid reactors with simultaneous reaction and mass transfer. In order to decrease the number of model equations and parameters to be estimated from experimental data, analyses with simplified models are common. The simplified models are obtained as limiting cases, for example with respect to given reaction/diffusion parameters. We show how the validity of some simplifying hypotheses can be analysed in this paper by simulation with existing literature data of α-methylstyrene hydrogenation. The procedure used here may further be used in the investigation of identifiability of parameters or model discrimination, for example to check whether joint assumption of rapid adsorption and intraparticle diffusion can be accepted in the view of experimental data.