Forced Circulation Crystallizer Research Articles

Long-term simulation of crystallization fouling in a forced circulation crystallizer

Most heat exchangers in wastewater treatment systems and the thermal desalination industry experience the deposition of unwanted materials on their surfaces. One of the primary causes is the supersaturation of salts with inverse solubility near the heat exchanger's heated surfaces. In this paper, a long-term simulation of the crystallization fouling of calcium carbonate by developing an efficient solver in the OpenFOAM software is proposed. The k-ω SST model was used to simulate turbulence, and the level-set model was adopted for high-precision and high-quality capturing of the fouling front inside the heat transfer tubes of a forced circulation crystallizer. The results demonstrated that the developed solver is a promising tool for accurately predicting moving boundary conjugate heat transfer. Upon validation of the results, the dynamic evolution of the fouling front and its sophisticated interactions with hydrodynamics were examined. The crystallization fouling was found to be under reaction control. Furthermore, the relatively slight supersaturation caused delayed front growth, which prolonged the fouling time to several months. Besides, findings demonstrated that the intensive competition between removal and deposition mechanisms at higher inlet velocities resulted in decreased asymptotic thermal resistance. Additionally, the velocity increase had a smaller impact on reducing the deposit's asymptotic resistance.

Implementing a one-dimensional quick model for dynamic simulation and economic analysis of a forced circulation crystallizer

Forced circulation crystallizer is one of the main contributors in Zero Liquid Discharge wastewater treatment processes. This technology enables the extraction of pure water from wastewater and separates impurities in the form of crystals. In the present study, a fast one-dimensional (nodal) model with high accuracy is introduced to predict the dynamic performance of the crystallization process in an FC crystallizer. Besides, to design and optimize a crystallizer, the parameters affecting the system performance should be evaluated from thermodynamic and economic perspectives. For this purpose, the developed model is used to investigate the crystallizer thermo-economic performance. Results show that the performance of the crystallizer system has the most significant sensitivity to varying the inlet feed concentration, suspension concentration, and rotational speed of the circulation pump. Increasing the suspension concentration reduces the mean diameter of output crystals. Also, increasing the pump rotational speed by about two times decreases the mean size of output crystals by approximately 41.39 %. From an economic point of view, increasing the heat input has a significant effect on the reduction of the dynamic payback period. Also, increasing the inlet feed concentration by 36 % leads to a 7.84 % increase in the internal rate of return.

A hybrid compartmental-CFD model to investigate forced circulation crystallizer performance

Forced Circulation (FC) crystallizer plays a key role in a Zero Liquid Discharge desalination process. The hydrodynamics of an FC crystallizer involves a complex turbulent flow field. Computational Fluid Dynamics as a powerful method for simulating flow hydrodynamics suffers from high computational cost. In contrast, the compartmental method has widely been used in practical applications. This method does not have the challenge of high computational cost, but it may give unrealistic results since the hydrodynamics of the flow is not taken into account. In the present work, a hybrid compartmental-CFD model, as a powerful method, is utilized. In this model, a simple CFD simulation is made at a relatively low cost. The CFD results are then translated into several proper parameters to modify the compartmental model. Results show that taking 3D hydrodynamic effects into account in the compartmental model, after 15E4 seconds confrontation between growth and attrition mechanisms, finally narrower distribution with a 37% decrease in Sauter diameter is obtained. Furthermore, boiling zone is intensely susceptible to particle segregation, preventing the presence of particles above 583 μm. In addition, mixing zone with moderate volume fraction and supersaturation plays a significant role in crystallization, contributing 54% of the total mass transfer.

Numerical investigation of hydrodynamics and crystal growth in a forced circulation crystallizer

Industrial waste and brine discharge into nature, has had destructive effects on the environment; solution to which is using Zero Liquid Discharge (ZLD) waste water treatment. Crystallizer as a heart of Zero Liquid Discharge process undertakes creating and growing crystals from the inlet feed solution. Studying the hydrodynamics of the flow assisted by Computational Fluid Dynamics (CFD) is a powerful method of identifying microscopic phenomena in crystallizers. The present paper simulated a common industrial Forced Circulation (FC) crystallizer using the two phase Euler-Euler model, assisted by kinetic theory of granular flow. Turbulence is accounted for by Reynolds Stress Model (RSM), while inter-particle interactions are considered using the Population Balance Model (PBM). Results showed that approximately 65% of the increase in size of crystals happens in the boiling and mixing zone of the crystallizer. Applying the growth rate of 5e-9 [m/s], breakage and aggregation in the crystallizer dominates, contributing 84% of total size increase; while increasing the growth rate and reaching the value of 5e-7 [m/s] results in the growth mechanism surpassing the other mechanisms, contributing 55% of total size increase. Additionally, by 10% increase in particles concentration at the inlet, particles mean diameter at the outlet increases about 6.2%.

Experimental evaluation of forced circulation crystallizer performance in production of sugar crystals

AbstractOperating conditions of forced circulation crystallizer, which is equipped with a condenser, are investigated in this research. The results can be used as tally rules for design and process optimization. The effect of operating pressure, operating temperature, and cooling water flow rate on the performance of forced circulation crystallizer as an industrial type of evaporators is evaluated in this research. Moreover, the relationship between the amounts of produced sugar crystal, acidity, distilled water, sugar crystal size, and energy consumption in the heat exchanger has been investigated, experimentally. Finally, the relationship between the concentration of sugar in liquor and related density is measured in this article. The experimental results show, with increasing operating temperature from 87.1 to 91.6 °C, the density of outlet liquor from the crystallizer decreases from 1,122.37 to 1,121.48 kg/m3. In addition, 6.24 kWh as the minimum amount of energy consumption is obtained at a temperature of 27.3 °C. Results show that the difference between feed temperature and heat exchanger temperature is between 5.9 and 9.8% for different cooling water flow rates.Practical applicationsA forced‐circulation crystallizer operates under reduced pressure. A high recirculation rate through the external heat exchanger is used to provide good heat transfer and minimize encrustation. The crystal magma is circulated from the lower conical section of the evaporator body through the vertical tubular heat exchanger and reintroduced tangentially into the evaporator below the liquor level to create a swirling action and prevent flashing (sudden evaporation). Feedstock enters on the pump inlet side of the circulation system. Product crystal magma is removed below the conical section. Its purpose is to concentrate nonvolatile materials such as organic compounds, mineral sugars, acids, and bases. Totally, a forced circulation evaporator utilizes a heavily recirculated heat exchanger with suppressed boiling. Moreover, it is typically used for fouling or crystallizing products. A high liquid rate combined with induced turbulence deters scaling on the heat transfer surfaces. Totally, the forced circulation crystallizers apply in food industries. Some of the industries, which are related to the forced circulation crystallizer, are mineral ingredients companies, chemical companies, and feed industries. The main applications of forced circulation crystallizer are as follows: (a) suitable for material with bigger concentration ratio, higher viscosity, and easily crystallization; (b) widely used in the evaporation and concentration of water phase solution or organic phase solution, in food, pharmaceutical, chemical, biological engineering, and other industries; (c) widely used for the evaporation and concentration of tomato sauce, malt dextrin, starch, sugar, glucose, lysine, inositol, fructose, oligosaccharides, peptides, and so on; (d) compact structure and convenient operation, ideal evaporator for treating corn syrup, tomato paste, and industrial sewage; and (e) for the production of chemical and petrochemical compositions such as phosphoric acid, ammonium sulfate, soda, sodium sulfate, and gelatin, urea.

Experimental Study of Solar Pond Coupled With Forced Circulation Crystallizer as Major Stages of Proposed Zero Discharge Desalination Process

Pretreatment, solar pond (SP), and forced circulation crystallizer (FCC) are the basic stages of one option to provide the goals of zero discharge desalination (ZDD) process. This work represents the performance of a solar pond that is coupled with forced circulation crystallizer as second and third stages of proposed zero discharge desalination process. The purpose of ZDD in this paper is gathering fresh water and saline crystals from effluent wastewater of the desalination unit of Mobin petrochemical complex. So, the SP unit is constructed after the pretreatment unit to concentrate the treated wastewater to about 20 wt. % as a suited feed for the FCC unit. Effects of solar insolation rate are investigated experimentally, during a year. In addition, the effect of cooling water flow rate of FCC on quality of effluent stream from SP as feed crystallizer is studied in this paper. The experimental results show the maximum evaporation rate from SP is obtained 5 l/(m2·d) when the insolation rate was about 2.5 × 104 kJ/(m2·d). Experiments show the suitable range of crystals growth (710 μm to 830 μm) in FCC is occurred when the cooling water flow rate in condenser is 9 kg/min. The size and the color of produced salt crystals will be optimized in this flow rate and energy consumption is measured as 6.98 kW·h.

Experimental study of a solar desalination pond as second stage in proposed zero discharge desalination process

This work represents the efficiency of a solar desalination pond as a second stage of proposed zero discharge desalination processes to reach fresh water and also concentrated brine from the effluent wastewater of the desalination unit of Mobin petrochemical complex. So a solar desalination pond is constructed after a pretreatment unit to concentrate the softened wastewater to about 20wt%. The concentrated wastewater is as a suited feed for a forced circulation crystallizer. During one year, the effects of major parameters such as ambient temperature and solar insolation rate are investigated, experimentally. specific gravity in each layer of concentrated brine wastewater is evaluated. Also, evaporation rates are calculated theoretically and are verified by experimental data. Theoretical values predict evaporation rate accurately. Results show good agreement with experimental data. According to results, maximum evaporation rate is 5l/m2day when the insolation rate is about 24,602kJ/m2day Solar energy absorption factor on June is max. Also, experimental results show the best proposed time to gain highest thermal energy is on spring therefore performance efficiency of solar desalination pond promote on spring comparing with the other months. Extracted data for specific gravity prove the bottom of solar desalination pond, layer 1, is best zone for energy saving and energy utilization.Also, theoretical values of evaporation rate are calculated according to measured temperatures and related mass conservation equation. Comparison between theoretical and experimental values shows dusty weather, humidity and wind velocity affects on heat transfer coefficients approximately. So, deviations between theoretical data and measured values can be explained. Results show good agreements with experimental data.

Experimental study of forced circulation evaporator in zero discharge desalination process

Zero discharge desalination process is the most promising technology to prevent salinity and thermal shocks to ecosystem by effluent streams of desalination unit drained into the sea. Pretreatment, solar pond and forced circulation crystallizer are the major steps of one option to provide the purposes of zero discharge desalination process. Reduction of total hardness of wastewater occurs in pretreatment unit and the solar pond is proposed for effluent concentrated brine wastewater of pretreatment unit which seeks production of potable water and concentrated brine. The effluent stream from solar pond is conveyed to one forced circulation evaporator in order to produce salt and distilled water. This work presents the effective optimum parameters of one forced circulation evaporator such as occupied capacity of crystallizer main body, residence time, optimum temperature of heat exchanger, flow rate of cooling water, color and size distribution of salt crystals, amount of distilled water and energy consumption. Results show that the suitable range of crystal growth (700–830 μm) occurs when the occupied capacity of crystallizer is 70%. Also, the flow rate of cooling water in condenser is 10 kg/min and the efficient residence time is 4 h.

Experimental study and mathematical simulation of the crystallization of ammonium sulfate in a forced circulation crystallizer

The growth mechanism of ammonium sulfate crystals is studied. A mathematical model for the process of continuous crystallization in an experimental FC-crystallizer based on the previously reported model of crystal attrition is developed. A mathematical model for the mass crystallization of ammonium sulfate in an industrial crystallizer with forced circulation by an axial three-blade propeller pump, which takes into account the impact of crystals onto its blades, is developed as well. Optimal operating conditions for an industrial crystallizer, the parameters of an axial propeller pump, and its operating conditions for producing crystals of a given size at minimal power consumption are chosen.

Modeling of industrial crystallizers for control and design purposes

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.

Modelling the Settling, Dissolution and Non-Uniform Nucleation Kinetics in a 150-Litre Forced Circulation Crystallizer

Experiments on a pilot-scale 150-litre forced circulation crystallizer with external circulation flow rates of 2 and 3 l/s revealed a high sensitivity of the product crystal size distribution for this operational variable. It is believed that a change in the circulation flow rate influences (1) the nucleation rate, which is mainly caused by the mechanical action of the pump, (2) the degree of internal dissolution in the heat exchanger and (3) the settling of the larger crystals in the crystallizer body. This paper presents a modelling framework which allows for a mechanistic description of growth and dissolution. attrition and secondary nucleation and settling. Such a description is seen as a prerequisite for the development of crystallization models with a predictive value for different operational conditions or for another scale of operation. Growth, attrition and nucleation kinetics are modelled using relationships from both Eek and O Meadhra, whereas a diffusion-controlled model is applied to describe size-dependent dissolution kinetics. The settling behaviour of crystals is described using Stokes law, combined with Richardson-Zaki's equation for hindered settling. From the simulation results it can be concluded that this modelling framework is capable of qualitatively predicting the effects of a change in the circulation flow rate. The quantitative predictions of the median crystal size are within 15% of the experimental values, whereas the oscillations predicted during start-up show a far larger deviation from the experimental results.

Design criteria for the crystallization of an organic salt from an aqueous solution

Results from laboratory experiments and a 500 tonnes per month pilot plant have been used to draw up criteria for designing the crystallization of an organic salt from its aqueous solution. The crystallization kinetics were determined in a continuous forced circulation crystallizer. The experiments in the forced circulation evaporative crystallizer were carried out and evaluated in accordance with the MSMPR (mixed suspension mixed product removal) concept. The residence time was varied between 0.8 and 10 hours, and the density of the suspension between 180 and 870 kg m −3. It was found that, within the residence time and suspension density ranges studied, growth and nucleation rates have the same dependence on the supersaturation. On the other hand, the nucleation rate decreases with increasing suspension density. If the crystallization kinetics and other design criteria, such as vapour velocity, i.e. supersaturation ratio in the vicinity of the evaporative surface, or height of liquid above the heat exchangers, are taken into account, it is possible to obtain a space—time yield which is five times higher than in the pilot plant, so that the plant to be built can be designed to be correspondingly smaller.