Ideal Continuous Stirred Tank Reactor Research Articles

Optimizing cellulase usage for improved mixing and rheological properties of acid-pretreated sugarcane bagasse

Consolidation of bioprocessing steps with lignocellulose is limited by hydrolysate toxicity, the fibrous nature of suspensions, and low activity of cellulase enzymes. Combinations of enzyme dose and treatment conditions improved the flow properties and pumping of acid-pretreated sugarcane bagasse slurries (10% dry weight). Low levels of cellulase enzyme (0.1 and 0.5 FPU/g dry weight acid-pretreated bagasse) were found to reduce viscosities by 77-95% after 6 h, solubilizing 3.5% of the bagasse dry weight. Flow of slurries through small funnels was a useful predictor of success with centrifugal and diaphragm pumps. Equations were derived that describe viscosity and solubilized carbohydrates as a function of time and cellulase dosage. Blending of acid-pretreated bagasse (10% dry weight) with suspensions of acid-pretreated bagasse (10% dry weight) that had been previously digested with cellulase enzymes (low viscosity) did not increase viscosity in a linear fashion. Viscosity of these mixtures remained relatively constant until a threshold level of new fiber was reached, followed by a rapid increase with further additions. Up to 35% fresh acid-pretreated bagasse could be blended with enzyme-digested fiber (5.0 FPU/g dry weight acid-pretreated fiber; 6 h) with only a modest increase in viscosity. The smooth surfaces of enzyme-treated fiber are proposed to hinder the frequency and extent of interactions between fibrils of fresh fiber particles (acid-pretreated) until a threshold concentration is achieved, after which fiber interactions and viscosity increase dramatically. These results were used to model the viscosity in an ideal continuous stirred tank reactor (liquefaction) as a function of residence time and enzyme dosage.

Simulation of Slurry Polymerization of Ethylene

A model of slurry polymerization of ethylene in a multistage continuous stirred-tank reactor (CSTR) was developed in order to find the effect of stagewise variation of parameters on polymer polydispersity and rate of polymerization. Higbie's penetration theory has been used to calculate monomer absorption rate in the presence of micron-sized single site type catalyst in slurry phase. The residence time distribution of the growing polymer particles in an ideal CSTR was modelled by a relaxation-type unsteady state approach. The effect of gas-liquid mass transfer limitations on polymer properties is predicted for various reactor configurations. An analysis of various possible configurations of two-stage CSTR with stagewise variation of the partial pressures of ethylene and/or hydrogen has been made.

Removal of Arsenic from Wastewaters by Airlift Electrocoagulation. Part 2: Continuous Reactor Experiments

Arsenic removal from wastewater is a key problem for copper smelters. Conventional methods either prove to be complicated, expensive, or not sufficiently effective. This work shows the results of electrocoagulation (EC) in aqueous solutions containing arsenic in a newly designed and constructed cylindrical continuous airlift reactor. The residence time distribution measurements showed that the reactor behaved as an ideal continuous stirred tank reactor (CSTR) with perfect mixing. Ten EC experiments were carried out in the continuous airlift reactor with sacrificial iron electrodes. The variables were: initial As(V) concentration, liquid flow rate, and electric current density. The results showed that the airlift EC process could reduce an initial As concentration from 1000 mg L−1 to 220 mg L−1 – corresponding to a reduction of 78%. In addition, a 100 mg L−1 solution was reduced by 88%. The Fe-to-As (mol/mol) ratio, when EC was working properly, was in the range of 1.3–1.5, which is very promising for the future development of the reactor. The arsenic removal is proportional with the electric current, the electric charge and the CSTR residence time. On the other hand, when the flow rate is increased, the arsenic removal decreases.

Multiparameter models for performance analysis of UASB reactors

AbstractBACKGROUND: UASB (upflow anaerobic sludge blanket) bioreactors have the distinct advantage that they do not demand support particles and provide a high rate of bioconversion even with high strength feedstocks. Although apparently simple in construction, the performance analysis of these reactors involves a high degree of mathematical complexity. Most simulation models reported in the literature are rudimentary in nature as they involve gross approximations. In the present paper, two multiparameter simulation packages are presented that make no simplifying assumptions and hence are more rigorous in nature.RESULTS: The first package assumes the sludge bed to be a plug‐flow reactor (PFR) and the sludge blanket to be an ideal continuous stirred tank reactor (CSTR). The second package equates the reactor to a plug flow dispersion reactor (PFDR), the axial dispersion coefficient however being a function of axial distance. The three phase nature of the sludge blanket has been considered and the variation of gas velocity in the axial direction has been taken into account. Three different kinetic equations have been considered. Resistance to diffusion of substrate into sludge granules has been accounted for by incorporating appropriately defined effectiveness factors. The applicability of simulation packages developed has been ascertained by comparing with real‐life data collected from industrial/pilot plant/laboratory UASB reactors. The maximum deviation observed is ± 15%.CONCLUSIONS: Although the software packages developed have high computational load, their applicability has been successfully ascertained and they may be recommended for design and installation of industrial UASB reactors and also for the rating of existing installations. Copyright © 2008 Society of Chemical Industry

Particle mixing and residence time when drying sawdust in a continuous spouted bed

When unscreened sawdust is dried in a spouted bed dryer using superheated steam with a temperature of 240 °C, there is no active discharge of the dried material. Instead the particles lose density as they dry, while being pneumatically transported out of the dryer. The residence time distribution was measured by injecting LiCl tracer added sawdust into the dryer. The measured residence time distributions show that particle mixing in the spouted bed dryer is very similar to the complete mixing occurring in an ideal continuous stirred tank reactor. A comparison between the residence time distribution and the results of stop-and-empty trials indicates that the experiments were made with a small accumulation of inert material in the dryer. Under identical conditions regarding in- and outgoing steam temperatures, steam flow, material flow and moisture contents, the spouted bed dryer can operate under different sets of operating conditions as regards the mass of the material in the dryer and the residence time.

Selective Oxidation of Methacrolein towards Methacrylic Acid on Mixed Oxide (Mo, V, W) Catalysts. Part 1. Studies on Kinetics

Comprehensive kinetic studies of the oxidation of methacrolein on mixed oxide (Mo, V, W) catalysts have been undertaken in order to find ways of catalyst improvement to achieve a high selectivity toward methacrylic acid. Steady-state kinetic experiments were carried out in a differential recycle reactor that behaves like an ideal continuous stirred tank reactor (CSTR). A kinetic model based on the scheme of Mars−van Krevelen was developed for a fair representation of the reaction rates of methacrolein conversion, methacrylic acid formation, byproduct formation, and selectivity toward methacrylic acid. For a better understanding of the kinetics, transient experiments were carried out in an apparatus to render possible sorption studies as well as transient kinetic experiments, which are powerful tools to study independently both the oxidation of the aldehydes on the catalyst in the absence of oxygen and the reoxidation of the catalyst. Steady-state kinetic data and transient experiments agree well. It could be clearly shown that the conversion of methacrolein is mainly determined by the reoxidation of the catalyst and that the selective oxidation of the aldehydes and the consecutive oxidation of the acids occur on different domains of the catalyst.

Optimizing panose production by modeling and simulation using factorial design and surface response analysis

Dextransucrase is the biocatalyst for dextran synthesis and other products of commercial interest such as fructose and panose from sucrose and maltose. When an acceptor is used (besides sucrose as substrate), some of the glycosyl units are deviated from dextran synthesis and acceptor products are formed. When maltose is the acceptor the prime product is a trisaccharide, panose, composed by one maltose unit and one glucose unit linked by an α-1,6-glycosidic bond. In this work, a mechanism accounting for maltose in the reactor as well as its prime acceptor product (panose) is considered to model an ideal continuous stirred tank reactor (ICSTR). The concepts of factorial design were used to simulate the model and surface response graphs were used to analyze the effect of feed flow rate, enzyme, sucrose and maltose concentration on panose and higher oligosaccharide formation.

Mixing Time, Homogenization Energy and Residence Time Distribution in a Gas-Induced Contactor

Mixing time, homogenization energy and residence time distribution (RTD) experiments have been carried out in a gas-induced contactor in order to determine mixing characteristics of the vessel. Effects of sensor location, impeller speed, gas induction, and gassing on mixing time have been studied. Experiments on a semi-technical scale vessel have identified optimal mixing time conditions and optimal impeller speeds for most efficient homogenization energy. The effects of liquid flow rate and impeller speed on liquid phase RTD have also been investigated. The contactor mixing behaviour was modelled by a continuous stirred tank reactor (CSTR) with dead zones and bypassing. Good agreement between experimental and model results was observed. Analysis shows that contactor performance is close to that of an ideal CSTR; however, dead zones (as large as 16.7%) at low liquid flow rates and bypassing (as large as 10%) at high liquid rates need to be considered.

Molecular Weight Distribution Design with Living Polymerization Reactions

It has been previously shown that it is feasible to control the molecular weight distribution (MWD) of polymers produced from living polymerization processes in flow reactors through the control of reactant feeds. Here, attention is given to the problem of establishing an inverse process model as a step toward a fully automatic control strategy for the synthesis of polymers with predefined MWD. Particular attention is given to the prediction of instantaneous reactor feed conditions for a specified product MWD. The reactor is modeled as an ideal continuous stirred tank reactor with constant monomer concentration in the reactor. The way in which a real laboratory-scale polymerization system can be developed from this approach is outlined.

Response surface analysis and simulation as a tool for bioprocess design and optimization

In the present work, factorial design and response surface techniques were used in combination with modeling and simulation to design and optimize an industrial bioprocess. Alcoholic fermentation process with multiple stages was considered. The fermentation system is composed of four ideal continuous-stirred tank reactors (ICSTR), linked in series, with cell recycling. Operational conditions for maximal yield and productivity were determined with ten parameters under consideration: temperature (four stages), residence time for each stage, cell recycling concentration, and the fraction of fresh medium fed into the second fermentation stage. Initially, screening design methodology was used to evaluate the process variables which were relevant in relation to yield and productivity. Five statistically significant parameters for each response were selected and utilized in factorial design in order to optimize the process. With the models obtained from the factorial design, response surfaces were generated, and the productivity improved to 12 g/l·h (an increase of 52% in relation to the control version of the bioprocess), while maintaining the high yield of 86.28% (99.1% conversion).

Dynamic modelling of pH controlled precipitation and dissolution in an aqueous solution

This paper presents a dynamic model for a chemical precipitation and dissolution process in an aqueous solution. The precipitation process is a part of the electrolyte purification section at the Falconbridge Nikkelverk in Kristiansand, Norway. The model is partly mechanistic, and partly based on first principles. The state variables in the model are chosen as a combination of linear reaction invariants and non-linear variants. The effect of dispersion and radial mixing in the non-ideal plug flow reactor (PFR), where the precipitation takes place, is modelled as a combination of ideal continuous stirred tank reactors (CSTR). The CSTR-elements can be viewed as general building blocks or objects, which may also form a larger and more complex reactor system. Easier maintenance and better chance for reuse are some of the benefits of using only one central building block when modelling reactor systems. The chemical precipitation and dissolution rates in the model are controlled by the pH value and the reduction potential. The simulation results shows a high degree of correspondence with actual measurements at the refinery, and therefore form a promising basis for design of control and operator support systems.

MODELLING OF MIXING CHARACTERISTIC IN A REACTOR USING A MICROCOMPUTER

Abstract This article presents a computer procedure for modelling the mixing characteristic in a reactor. The basic approach employed in this study is to model the mixing pattern of a real reactor by using an equivalent reactor system consisting of several ideal continuous stirred tank reactors (CSTRs) placed in series. The concentration profile yielded by this CSTRs system is compared with the actual profile of the real reactor obtained experimentally. The concentration profile of the CSTRs system that is closest to the actual profile is then taken as the system that will adequately represent the mixing characteristic of the actual reactor. The proposed procedure is tested on an IBM personal computer

The response of linear cascades to variations of holding times

An arbitrary system of isothermal first order reactions with no volume change on reaction is considered. A comparison is made between the response of the system in a cascade of ideal continuous stirred tank reactors and an ideal tubular reactor to variations of holding times. The performance criterion chosen is a linear combination of the exit concentrations and, contrary to what has been published previously, examples are given to show that for an m tank cascade non-equal tank cascades can be locally stationary and even localy extreme. In spite of this (except for certain trivial and degenerate cases) it can be shown that the global extrema occur for equal tank cascades. Furthermore, these extrema are dominated by those for an m + 1 tank cascade and these are all dominated by those of the turbular reactor. Cases where the performance criterion is augmented by a cost associated with the total holding time or where the total holding time is constrained (equality or inequality) are also discussed. In the first case the above results are essentially unchanged, while in the constrained cases examples are given to show that non-equal tank cascades can be optimal.

Prediction of molecular weight distribution of high‐density polyethylene made in a continuous reactor from fixed ethylene concentration batch reactor data

AbstractThe polydispersity index, Mw/Mn, of the product of an ideal continuous stirred tank reactor can be predicted from batch reactor data. A method for the prediction, in the case of high‐density polyethylene synthesis, comes from finding the effect of catalyst age upon yield, Mw, and Mn in a batch reactor operated at constant pressure. These catalyst age effects are combined with the catalyst age distribution in the continuous reactor, and integration over all ages gives the yield and the polydispersity index for the continuous reactor product. This scheme for prediction was applied to a particular catalyst system, and the calculated values have been found to agree with observations.

H1. Systematische ausbeuteberechnung für isotherme reaktoren im falle zusammengesetzter reaktionen

This paper constitutes an attempt to treat the most important aspects of isothermal reactor systems in the steady state from a unified point of view. Starting from the stoichiometric structure of complex reactions, differential and integral yields for the ideal tubular reactor, the ideal continuous stirred tank reactor and the C.S.T.R. chain are given. Using the yield relationships, the improvement of the selectivity of a chemical process by suitable choice of reactor is discussed. The main problem in this respect, namely the optimization of an N-stage C.S.T.R. chain with regard to the distribution of residence times is dealt with by using the theory of dynamic programming. The stirred tank reactor and the tubular reactor appear as limiting cases. These considerations are illustrated by means of examples.