STRATEGIES FOR DRYING TOMATOES IN A TUNNEL DEHYDRATOR

ABSTRACT

A tunnel dehydrator can be operated using batch or semi-batch modes and while a batch mode is the simplest, it has the inherent characteristic of producing non-uniform moisture content. Although semi-batch mode does not have this problem, because all product experiences the same drying conditions, it is not used when drying tomatoes in Australia. The operation of most dehydrators is based on operator experience and rules of thumb, methods that are not likely to give optimum performance. A model that is suitable for simulating the operation of a tunnel dehydrator with any fruit has been developed. This model has been applied to the development of strategies for drying tomatoes with the objective of achieving optimum throughput, colour and drying efficiency. The results of these simulations are presented in this paper.     

Intensification of enzymatic conversion of glucose to lactic acid by reactive extraction

Lactic acid is an important commercial product and extracting this out of aqueous solution is a growing requirement in fermentation-based industries and recovery from waste streams. The design of an amine extraction process requires (i) equilibrium and (ii) kinetic data for the acid-amine (solvent) system used. Equilibria and kinetics for lactic acid extraction by Alamine 336 in octanol as a diluent have been determined and compared with other diluents studied earlier. An approach for extracting the lactic acid by a long-chain tertiary amine, which is in the dispersed phase as a liquid ion exchanger (LIX), is presented. A mathematical model for slurry phase reactor with glucose in the continuous aqueous phase, the amine with a diluent in the dispersed phase and the immobilized enzyme as the solid catalyst, has been developed using equilibrium and kinetic data for reactive extraction. Effects of various parameters affecting the conversion of glucose have been discussed. The model has been solved for batch and semi-batch modes. It has been shown that the semi-batch mode yields approximately five times higher productivity than batch mode.     

Open-loop optimization for batch reaction systems: a case study

A comparison of the results obtained from deterministic and stochastic model-based optimization approaches for the determination of the optimal open-loop operating policy for a semi-batch reaction system are presented. The commercial synthesis of an intermediate in the production of the hypertension agent Eprosartan is considered in this case study. Identification of the reaction mechanism and estimation of the kinetic model parameters from experimental data are carried out as part of the development of a detailed semi-batch reaction system model that is used with the optimization approaches. The results of this study indicate that conversion optimization of the operating conditions can provide a good approximation to the economic optimum in the case of high conversion operation. This study also indicates that the optimal operating policy for the catalyst and reactant are batch, with the catalyst addition occurring prior to the reactant, and that a reduction in the amount of reactant from current practice is possible.     

Modeling and Optimization of a Tunnel Crape Dryer

A mathematical model of a tunnel dryer for the dehydration of grapes is presented and applied to the determination of optimal operating conditions of the dryer. The dryer is of semi-batch structure, operating with trucks and trays. The cycle period is determined by meeting appropriate quality specifications for the final product. The nominal conditions were evaluated bv suitably minimizing. the total fuel demand, expressed as fuel consumption to production capacity, under some constraints regarding the production rate of the dryer and the maximum permissible air temperature. An nominal air humidity value was evaluated suggesting a minimum cycle period value for the production capacity and fuel demand. The nominal conditions required operation of the dryer on the maximum permissible air temperature. The optimum operation was evaluated by maximizing the total profit resulting from the operation of the dryer. The optimization variables were temperature and humidity of the drying air stream. A charteristic case study of industrial grape was included to illustrate the effectiveness of the proposed approach.     

Modeling and Optimization of a Tunnel Crape Dryer

ABSTRACT

A mathematical model of a tunnel dryer for the dehydration of grapes is presented and applied to the determination of optimal operating conditions of the dryer. The dryer is of semi-batch structure, operating with trucks and trays. The cycle period is determined by meeting appropriate quality specifications for the final product. The nominal conditions were evaluated bv suitably minimizing. the total fuel demand, expressed as fuel consumption to production capacity, under some constraints regarding the production rate of the dryer and the maximum permissible air temperature. An nominal air humidity value was evaluated suggesting a minimum cycle period value for the production capacity and fuel demand. The nominal conditions required operation of the dryer on the maximum permissible air temperature. The optimum operation was evaluated by maximizing the total profit resulting from the operation of the dryer. The optimization variables were temperature and humidity of the drying air stream. A charteristic case study of industrial grape was included to illustrate the effectiveness of the proposed approach.     

DRYING '80 VOLUME 1: DEVELOPMENTS IN DRYING

ABSTRACT

The purpose of this research was to investigate strategies for papaya glacé drying in tunnel. To evaluate the optimum conditions of drying, corresponding mathematical models were also considered. The criteria set for this study included low drying time, low specific energy consumption and acceptable qualities of papaya glace. The results obtained from the model of batch tunnel drying were in good agreement with the experimental ones. From the mathematical models, it was found that the optimum conditions of the first stage of drying of papaya glace (3.1×7.8×1.4 cm) were drying temperature of 70°C, specific air flow rate of 12 kg/h-kg dry papaya glace (velocity of 1.25 m/s) and air recycle ratio of 70%. In the second stage of drying papaya glace (0.98×0.98×0.98 cm), it was found that the optimum drying conditions were: drying air temperature of 55°C, specific air flow rate of 10 kg/h-kg dry papaya glace (velocity of 0.6 m/s) and air recycle ratio of 80%. Ambient air temperature and relative humidity were 30°C and 70% respectively.     

Intensification of enzymatic hydrolysis of penicillin G: Part 2. model for enzymatic reaction with reactive extraction

During the enzymatic hydrolysis of the potassium salt of Penicillin-G (PenGK) into Phenylacetic acid (PAA) and potassium salt of 6-Aminopenicillanic acid (6-APA), the pH of the reaction mixture falls on account of accumulation of PAA. This lowers the stability and activity of the enzyme used, viz., Penicillin-G acylase (PGA). A new approach of extracting the PAA by a long-chain tertiary amine, which is in the dispersed phase, as a liquid ion exchanger (LIX), is presented. A mathematical model has been developed for this slurry phase reactor with PenGK in the continuous aqueous phase, the amine alongwith a diluent in the dispersed organic phase and the immobilized PGA enzyme as the solid catalyst. Effects of various parameters affecting the conversion of PenG have been discussed. The model has been solved for batch and semi-batch modes. It has been shown that the semi-batch mode yields a higher productivity. This approach can also be advantageously used for other intermediates like 7-ADCA for cephalosporins.     

Conjugate heat and mass transfer model for predicting thin-layer drying uniformity in a compact,crossflow dehydrator

Abstract

A conjugate heat and mass transfer model was implemented into a commercial CFD code to analyze the convective drying of corn. The Navier–Stokes equations for drying air flow were coupled to diffusion equations for heat and moisture transport in a corn kernel during drying. Model formulation and implementation in the commercial software is discussed. Validation simulations were conducted to compare numerical results to experimental, thin-layer drying data. The model was then used to analyze drying performance for a compact, crossflow dehydrator. At low inlet air temperatures, the drying rate in the compact dehydrator matched the thin-layer drying rate. At higher temperatures, heat losses through the external walls resulted in temperature and moisture variations across the dehydrator.     

ON THE BEHAVIOR OF CONSECUTIVE ENZYMATIC REACTIONS IN CYCLICALLY OPERATED REACTORS

The dynamics of a process involving consecutive enzymatic reactions were investigated for a case in which the reactions are carried out in a cyclically operated reactor. Each cycle of the operation protocol involves three phases. During phase I the reactor operates in a semi-batch mode involving input of the reactants. During the second phase, the vessel operates in a batch mode. During phase III the reactor has an output only and a fraction of its contents are emptied before another identical cycle begins.

It was found that there are regimes in the operating parameter space where the system can reach more than one limit cycle (multistability). Using computer software based on the bifurcation theory for forced systems, as well as one- and two-parameter continuation algorithms, the impact of various parameters on the dynamics of the system was investigated. The results are presented in the form of diagrams. Conditions under which formation of the intermediate product in the reaction sequence is maximized were also investigated.

Production of the intermediate product in a limit cycle was compared to that obtained in batch and semi-batch operation. Implications of the proposed operation protocol for process optimization and pollution prevention are discussed.     

The Efficacy of Chlorinated Water Treatments in Minimizing Yeast and Mold Growth in Fresh and Semi-dried Tomatoes

Abstract

A key problem with dried tomatoes for the industries is that the products tend to have limited shelf-life due to yeast and mold growth. This study was undertaken to evaluate the efficacy of chlorine treatments on minimizing yeast and mold populations on fresh and semi-dried tomatoes. Tomatoes were inoculated with molds (4.2 log CFU/g) from contaminated pack of semi-dried tomatoes. These inoculated tomatoes were then treated by washing with water (control) and concentrations of 50, 100, and 200 ppm of chlorinated water for specific length of time (1, 5, and 10 min), and finally analyzed for yeast and mold populations. In this study, the results showed that the maximum log reduction of yeast and mold cells on tomato surface was found to be chlorinated water with a concentration of 200 ppm. At this concentration, washing for 5 and 10 min resulted in 1.7 log reduction and 3.1 log reduction in yeast and mold growth respectively. This was significant (P<0.01) as compared with washing with tap water (control). Drying at 60°C further reduce the microbial load of these pre-treated tomatoes. Results showed that there were reductions in three of the twelve runs while seven of the runs demonstrated an increase in microbial load. This finding is important as it demonstrated that when using a typical drying temperature of 60°C, if the initial microbial load is high, there is a risk that the dehydrator can act as an incubator for more microbial growth. This finding also demonstrated the importance of pretreatment to reduce the initial microbial load before drying commences.     

A general simulation model of batch chemical reactors for thermal control investigations

This paper describes the development of a dynamic simulation model for stirred tank batch or semi-batch chemical reactors fitted with an alternative heating-cooling system. Heat and mass balances are established for the reactor and its jacket. Since the general purpose of our research is the thermal control of these reactors, special attention is devoted to the behaviour of the heating-cooling system. In this article, we are particularly concerned with an alternative system, i.e. different fluids at a constant temperature can be alternatively delivered to the jacket. The computer simulation programme is flexible, enabling simulation of a batch or semi-batch reaction vessel, ranging from a laboratory pilot plant to a full-scale production plant. A control algorithm is included which allows reactor operation with open or closed-loop temperature control. To demonstrate the good performance of the simulation model, experimental results are presented for both a pilot plant and an industrial reactor.     

Atmospheric Freeze Drying—Modeling and Simulation of a Tunnel Dryer

Drying is an important unit operation in processing of foods and other biotechnological products. Vacuum freeze drying is said to be the best drying technology regarding product quality of the end product, but the disadvantages are, among others, expensive operational costs and batch drying. Atmospheric freeze drying was introduced to lower the production costs of high-quality dried foods, and the need of simulation tools became important in estimations of the industrial drying processes.

A simplified mathematical model (AFDsim) is developed based on uniformly retreating ice front (URIF) considerations. The model is used to calculate theoretical drying curves of atmospheric freeze dried foods in a tunnel dryer. Studies of thermal and mass transfer properties during drying are essential for understanding the changes in product quality and for designing and dimensioning the drying process. The model can be used to simulate industrial atmospheric freeze drying of different foodstuff in a tunnel. The results from AFDsim modeling are in good accordance with the experimental results.     

Novel strategies for optimization of batch, semi-batch and heating/cooling evaporative crystallization

A novel optimization framework based on a stochastic optimization algorithm is proposed for optimizing batch, semi-batch and non-isothermal evaporative crystallizations. The mathematical models for these dynamic processes are highly non-linear and hence involve prominent non-convexities in the solution space of optimization. The framework not only significantly reduces the likelihood of being trapped into local optima and convergence problems in optimization and simulations, but also employs a novel and efficient algorithm for optimization of control profiles. A new concept in combining heating/cooling and evaporation in a batch crystallization operation is also proposed in this work. Optimization results of the citric acid-water system using this concept indicate that the maximum average crystal size obtainable is 19% and 27% larger than those attainable in the optimized unseeded evaporative and cooling operations respectively. The crystal size distribution is significantly narrower than that under seeded operations. Yield increase of more than 42% is also observed in this operating methodology. Optimization results for the batch and semi-batch evaporative crystallizations also demonstrate improvements that otherwise cannot be attained by using conventional approaches and heuristic rules.     

Atmospheric Freeze Drying—Modeling and Simulation of a Tunnel Dryer

Drying is an important unit operation in processing of foods and other biotechnological products. Vacuum freeze drying is said to be the best drying technology regarding product quality of the end product, but the disadvantages are, among others, expensive operational costs and batch drying. Atmospheric freeze drying was introduced to lower the production costs of high-quality dried foods, and the need of simulation tools became important in estimations of the industrial drying processes.

A simplified mathematical model (AFDsim) is developed based on uniformly retreating ice front (URIF) considerations. The model is used to calculate theoretical drying curves of atmospheric freeze dried foods in a tunnel dryer. Studies of thermal and mass transfer properties during drying are essential for understanding the changes in product quality and for designing and dimensioning the drying process. The model can be used to simulate industrial atmospheric freeze drying of different foodstuff in a tunnel. The results from AFDsim modeling are in good accordance with the experimental results.     

半间歇釜式反应器安全高效操作的数值优化策略

半间歇釜式反应器内物料积累过多且反应放热过快时,容易引发热失控风险,造成化工安全事故。设计较优的加料操作,既可以避免热失控风险,又可以缩短操作周期、提高生产效率。针对半间歇釜式反应器,以乙酸酐水解为模型反应,基于反应器数学模型,提出了一种安全高效加料操作的数值优化策略。在该策略中,首先确定不同加料方式（如一段、二段和三段加料）下的安全操作温度区间,然后在该温度区间内寻找最短操作周期对应的操作温度,该温度即为最佳的加料操作温度。优化结果表明三段加料的安全操作温度区间和最佳操作温度分别比一段加料宽60.9%和低1.9 K,六段加料已经基本可以实现操作周期最短,增加操作压力也有利于缩短操作周期。     

Design and optimisation of batch and semi-batch reactors

This paper addresses a systematic methodology for batch and semi-batch reactor design and optimisation for both ideal and non-ideal mixing. It can be applied to non-isothermal and multiphase systems. The method starts from a general representation in the form of a temporal superstructure based on the similarity of between plug flow reactors and ideal batch reactors. The temporal superstructure of a batch reactor exists in both the space and time dimensions. For non-ideal mixing, this paper addresses a mixing compartment network model to represent mixing inside reactors. The mixing compartment network is then included into the temporal superstructure to model non-ideally mixed batch reactors and the mixing pattern optimised with the other variables. Besides the operation variables for batch reactors, this method can also suggest the optimum mixing pattern and promising reactor configurations for mechanical design. A profile-based approach is proposed to make a search of the profiles for temperature, pressure and feed addition. This approach starts from a set of initial profiles of temperature, pressure and feed addition. Then the performance of the batch reactor is evaluated against the objective function under different profiles. An optimal set of profiles is then found by this profile searching process. A stochastic optimisation technique based on simulated annealing is employed to obtain optimal solutions. This method is also extended to multiphase reaction systems based on the concept of shadow reactor compartments. A number of case studies are presented to illustrate the use of the proposed methodology.     

DESIGN OF BATCH GRAPE DRYERS

The mathematical model describing the batch operation of industrial dryers with trucks and trays is presented and analysed for the case of grape dehydrators. The optimum flowsheet configuration and operation conditions for the specific mode of operation and type of dryer employed, are sought and verified by appropriate formulation of design and optimization strategies. The optimization objective is the total annual cost of the plant, subject to constraints imposed by the operation of the dryer, thermodynamics, and construction reasoning. The decision variables were the number of trucks and the drying air stream conditions involving temperature and humidity. The MINLP nature of the design problem required mathematical programming techniques for its solution. The optimization was carried out for a wide range of production capacities, and the optimal points were evaluated in each case. A characteristic design study was presented in order to demonstrate the effectiveness of the proposed approach.     

Multi-Scale Analysis of Vacuum Contact Drying

The modeling of the unit operation of vacuum contact drying is approached as a multi-scale problem. At the particle assembly length scale, effective transport properties (thermal conductivity, relative gas- and liquid-phase permeability) have been determined computationally by simulations on reconstructed porous media and verified by direct measurements. A distributed-parameter model of vacuum contact drying including liquid and vapor flow and differential energy balance has been formulated and used for the calculation of drying time as function of vacuum level, temperature, vessel diameter, and batch size at the unit operation length scale. Drying curves for a model system of sodium carbonate-isopropanol have been measured experimentally and compared with the model predictions. A very good agreement has been found.     

Multi-Scale Analysis of Vacuum Contact Drying

The modeling of the unit operation of vacuum contact drying is approached as a multi-scale problem. At the particle assembly length scale, effective transport properties (thermal conductivity, relative gas- and liquid-phase permeability) have been determined computationally by simulations on reconstructed porous media and verified by direct measurements. A distributed-parameter model of vacuum contact drying including liquid and vapor flow and differential energy balance has been formulated and used for the calculation of drying time as function of vacuum level, temperature, vessel diameter, and batch size at the unit operation length scale. Drying curves for a model system of sodium carbonate–isopropanol have been measured experimentally and compared with the model predictions. A very good agreement has been found.     

异丙苯液相氧化反应动力学研究

本文研究了在半间歇操作条件下,在100°—130℃的温度范围内,采用异丙苯氢过氧化物为引发剂的异丙苯液相空气氧化反应动力学.根据烃类氧化的链式自由基反应机理以及对在氧化反应中异丙苯氢过氧化物分解方式的假设,导出该氧化反应在链稳定增长阶段的反应动力学模型.根据所获得的实验结果,确定了异丙苯液相氧化反应及氢过氧化物分解副反应的速率常数、表观活化能及反应速率常数的Arrhenius关系式.由反应动力学模型计算出的异丙苯和它的氢过氧化物浓度与实验结果吻合良好.文中还讨论了在半间歇操作中存在的诱导期以及反应温度对异丙苯转化率和异丙苯氢过氧化物选择率的影响.