Synthesis of Rice Processing Plants. II. MINLP Optimization

A systematic design approach was applied to develop the optimal process flowsheet for a rice processing plant. The optimization problem was formulated as a Mixed-Integer NonLinear Programme, MINLP, consisting of vectors of binary and continuous variables. A superstructure flowsheet comprising all serial structures of drying, cooling, and tempering units in the process was postulated. The set of optimum decision variables including the number of drying, cooling, and tempering units, temperature and relative humidity of drying air, drying time, cooling time, and tempering time were determine as the solution of the corresponding MINLP. Six objective functions were investigated as possible performance criteria: production time, number of the operating units, energy consumption, total operating cost, head rice yield, and the profit. The choice of objective function was found to have a significant effect on the optimal solution. Comparison with typical design and operating conditions, the MINLP results showed that a 22% reduction in energy consumption was possible along with a 2.4% increase in head rice yield. These savings, if applied to the world-wide rice industry, translate into more than $3 billion dollars/year increase in profit.     

Synthesis of Rice Processing Plants. III. Sensitivity Analysis

Abstract

An optimal process flowsheet for a rice processing plant has been developed. The optimization problem was formulated as a Mixed-Integer Nonlinear Programme, MINLP, consisting of a vector of binary and continuous variables. The set of optimum decision variables including the number of drying, cooling, and tempering units, temperature and relative humidity of drying air, drying time, cooling time, and tempering time were determined as the solution of the MINLP problem. Six objective functions were investigated as performance criteria. A sensitivity analysis of each model parameter was conducted to determine its influence on the optimal flowsheet. The MINLP approach is an efficient tool for optimization and the simplified models were adequate for use at the synthesis stage. The solution is insensitive to uncertainty in the models. However, due to the nonconvex nature of MINLPs, the solution was found to depend on the initial starting values, especially for the maximum profit flowsheet.     

Synthesis of Rice Processing Plants. III. Sensitivity Analysis

An optimal process flowsheet for a rice processing plant has been developed. The optimization problem was formulated as a Mixed-Integer Nonlinear Programme, MINLP, consisting of a vector of binary and continuous variables. The set of optimum decision variables including the number of drying, cooling, and tempering units, temperature and relative humidity of drying air, drying time, cooling time, and tempering time were determined as the solution of the MINLP problem. Six objective functions were investigated as performance criteria. A sensitivity analysis of each model parameter was conducted to determine its influence on the optimal flowsheet. The MINLP approach is an efficient tool for optimization and the simplified models were adequate for use at the synthesis stage. The solution is insensitive to uncertainty in the models. However, due to the nonconvex nature of MINLPs, the solution was found to depend on the initial starting values, especially for the maximum profit flowsheet.     

Synthesis of Rice Processing Plants. I. Development of Simplified Models

Abstract

This article, the first of three articles on the synthesis of rice processing plants, focuses on the development of simplified mathematical models necessary for use in optimizing rice processing plants. The second concentrates on the optimal synthesis of a rice plant and the third on the sensitivity of the optimization to uncertainty in model parameters. Existing models for rice processing unit operations are not suitable for flowsheet optimization and new models need to be developed to overcome numerical difficulties that occur in optimization applications, specifically in mixed integer nonlinear programming (MINLP) applications. Simplified models of the drying, cooling, and tempering units are developed. In addition head rice yield models, used as a quality indicator, energy consumption, and economic models were also developed. Naturally, the new models exhibit some mismatch with respect to the existing models from which they were developed. However, a sensitivity analysis, presented in Part III, has shown that the optimal flowsheet structure was not sensitive to a lack of fit between the simplified and complex models. The simplified models were found adequate to be appropriate for use at the synthesis stage.     

Synthesis of Rice Processing Plants. I. Development of Simplified Models

This article, the first of three articles on the synthesis of rice processing plants, focuses on the development of simplified mathematical models necessary for use in optimizing rice processing plants. The second concentrates on the optimal synthesis of a rice plant and the third on the sensitivity of the optimization to uncertainty in model parameters. Existing models for rice processing unit operations are not suitable for flowsheet optimization and new models need to be developed to overcome numerical difficulties that occur in optimization applications, specifically in mixed integer nonlinear programming (MINLP) applications. Simplified models of the drying, cooling, and tempering units are developed. In addition head rice yield models, used as a quality indicator, energy consumption, and economic models were also developed. Naturally, the new models exhibit some mismatch with respect to the existing models from which they were developed. However, a sensitivity analysis, presented in Part III, has shown that the optimal flowsheet structure was not sensitive to a lack of fit between the simplified and complex models. The simplified models were found adequate to be appropriate for use at the synthesis stage.     

Drying Characteristics of Malaysian Paddy: Kinetics & Grain Cracking Quality

Abstract

Drying kinetics of Malaysian paddy dried in a laboratory scale batch rapid bin dryer was studied. Quality of paddy after drying at temperatures of 60 and 80°C, bed heights of 2 and 4 cm, and at different drying stages was presented in terms of cracking percentage. Moisture content of paddy reduced exponentially with time and became stable at equilibrium moisture content. Characteristic drying rate curves exhibited mainly falling rate period, with an induction period in deep beds, but without the constant rate period. A custom-made light box was built to visually analyze the cracking in paddy kernel. The extent of breakage is directly related to the crack percentage. The percentage of cracks increased with drying and tempering temperatures and was higher after 2 weeks of storage before milling. Bed heights did not significantly affect the head rice yield as much as tempering between two drying stages. Temperatures above 80°C caused severe cracking in paddy kernel and produced miserable yield of head rice after milling, which could be slightly improved by tempering. The use of a proper miller is also important in ensuring good head yield results and whiteness quality.     

Effects of operating parameters of impinging stream dryer on parboiled rice quality and energy consumption

Abstract

It is very important to know how operating condition of a coaxial impinging stream dryer affects the drying time, parboiled rice quality and energy consumption. The drying temperature, parboiled paddy feed rate, drying air velocity and impinging distance were experimentally investigated. Increases in drying temperature and air velocity and a decrease in paddy feed rate provided higher evaporation rate whilst impinging distance, ranging from 5 to 13?cm, did not affect the evaporation rate. Collision between kernels within the impinging zone caused the hull’s lemma splitting from the kernel and the percentage of the split kernel strongly depended on the air velocity and feed rate. The acceleration in the rate of drying by changing the above operating parameters did not affect the head rice yield. In addition, the collision of kernels also did not influence the head rice yield since the mechanical properties of rice are strengthened during steaming step in the parboiling process. However, the change of head rice quality was only governed by the moisture content after drying. The total energy consumption including electricity and heat was strongly depended on the air velocity and feed rate whilst it was slightly changed with the drying temperature. From the present study, it was recommended that the parboiled paddy should not be dried below 25% d.b. and the highest temperature that could possibly be used was 190?°C and the inlet air velocity should not be below 15?m/s.     

Rapid Drying of Parboiled Paddy Using Hot Air Impinging Stream Dryer

A method for rapid drying of parboiled paddy via the use of an impinging stream dryer was proposed and assessed. The effects of the drying air temperature, number of drying cycles, as well as time of tempering between each drying cycle on the moisture reduction, head rice yield, and whiteness index of the dried parboiled paddy were studied. The drying experiments were carried out at drying air temperatures of 130, 150, and 170°C; inlet air velocity of 20 m/s; impinging distance of 5 cm; and paddy feed rate of 40 kg_{dry_paddy}/h. Parboiled paddy was dried for up to seven cycles. Between each drying cycle the parboiled paddy was tempered for a period of either 0 (no tempering), 15, 30, 60, or 120 min. After impinging stream drying, paddy was ventilated by ambient air flow until its moisture content reached 16% (db). Moisture reduction of the paddy was noted to depend on both the impinging stream drying temperature and tempering time. Drying at a high temperature along with tempering for a suitable period of time could maintain the head rice yield of the paddy at a level similar to that of the reference parboiled paddy. To avoid discoloration and low head rice yield, parboiled paddy should not be dried at a temperature higher than 150°C and should be tempered for at least 30 min.     

EFFECTS OF DRYING ON HEAD RICE YIELD USING FLUIDIZATION TECHNIQUE

ABSTRACT

Experimental results of fluidized bed paddy drying using high inlet air temperatures (140 and 150 °C) showed that head rice yield could be increased to a maximum value at a range of paddy final moisture contents of 19 to 22 % wet-basis. In case of reducing moisture content of paddy to lower than 19 % wet-basis, head rice yield of tempered paddy was higher than that of no-tempered one. Initial moisture contents of paddy that could increase head rice yield were in a range of 23 to 31 % wet-basis. As initial moisture content increased head rice yield increased. Whiteness of dried paddy was mostly accepted. However, if tempering temperature was higher than 60 °C, it may cause the problems for trade.     

INTERMITTENT DRYING OF ROUGH RICE

Abstract

This work obtains thin-layer drying data for rough rice from 108 treatments. A thin-layer drying equation is also derived using these data with drying air absolute humidity, drying air temperature, tempering time interval and drying time interval as the independent variables. In addition, an intermittent drying equation is developed to predict the drying behavior of rough rice in a re-circulating type rice dryer.     

Drying Characteristics of Malaysian Paddy: Kinetics & Grain Cracking Quality

Drying kinetics of Malaysian paddy dried in a laboratory scale batch rapid bin dryer was studied. Quality of paddy after drying at temperatures of 60 and 80°C, bed heights of 2 and 4 cm, and at different drying stages was presented in terms of cracking percentage. Moisture content of paddy reduced exponentially with time and became stable at equilibrium moisture content. Characteristic drying rate curves exhibited mainly falling rate period, with an induction period in deep beds, but without the constant rate period. A custom-made light box was built to visually analyze the cracking in paddy kernel. The extent of breakage is directly related to the crack percentage. The percentage of cracks increased with drying and tempering temperatures and was higher after 2 weeks of storage before milling. Bed heights did not significantly affect the head rice yield as much as tempering between two drying stages. Temperatures above 80°C caused severe cracking in paddy kernel and produced miserable yield of head rice after milling, which could be slightly improved by tempering. The use of a proper miller is also important in ensuring good head yield results and whiteness quality.     

Effect of tempering approach following cross-flow drying on rice milling yields

Abstract

Three tempering approaches were followed after drying rough rice at 16.3% and 20.5% initial moisture contents (IMCs) using 57?°C/13% RH air at an airflow of 0.56 (m³/s)/m² for 30, 60, and 90?min in an experimentally simulated cross-flow drying column. For the longer drying durations, post-tempering head rice yields were consistently less when the interstitial air from rice from different cross sections of the drying column was allowed to “interact” during tempering than when the rice from these different cross sections was tempered separately; this effect was more prominent at the greater rice IMC. RH of the interstitial air during tempering was measured and used to estimate the minimum tempering durations required for the different tempering approaches.     

化工过程综合问题MINLP算法中整型变量的连续化

化工过程系统综合问题需要同时考虑设备结构参数和工艺操作参数,一般用整型变量表示设备的取舍,用连续变量表示操作参数,这就构成一个流程的超结构,在数学形式表现为一个混合整型非线性规划（MINLP）问题。混合整型非线性规划问题的求解成为化工过程综合优化的关键。今根据超结构中整型变量的特征,提出整型变量连续化处理的思路,将MINLP问题简化为NLP问题,然后采用罚函数法求解。最后将该算法运用于加氢脱烷基化（HDA）过程综合的实例研究,结果表明该算法克服了传统方法在处理整型变量时出现的麻烦,为有效快速地进行化工过程综合优化问题提供了一种新的途径。     

Evolution of mechanical properties of parboiled brown rice kernels during impinging stream drying

Information on mechanical properties of parboiled brown rice kernels upon impinging stream drying, which is important for effective control of kernel fissure and head rice yield, is reported. Experiments were performed at the drying temperatures of 130, 150, and 170°C; inlet air velocity of 20?m/s; impinging distance of 5?cm and paddy feed rate of 40?kg_{dry_paddy}/h. Parboiled paddy was dried for up to seven cycles. Between each drying cycle, the paddy was tempered for a period of either 0 (without tempering) or 30?min. The moisture evaporation rate was noted to be very high during the first two drying cycles and rapidly dropped in the later drying cycles. When tempering was included after a particular drying cycle, the drying rate in a subsequent cycle was higher than without tempering. At the kernel moisture contents immediately after drying of 25.3–47.5% (d.b.), the drying temperature and existence of tempering did not affect the mechanical properties although microcracks were formed in the kernels. However, both factors played a more important role on the mechanical properties when the kernels were evaluated at 16% (d.b.). The head rice yield correlated well with the tensile strength of the kernels.     

DESIGN OF BATCH GRAPE DRYERS

ABSTRACT

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.     

MANAGING MOIST PADDY BY DRYING,TEMPERING AND AMBIENT AIR VENTILATION

ABSTRACT

This paper describes a strategy for reducing moisture in paddy by fluidized bed drying, tempering and ambient air cooling. Experimental results showed that after the three processes, moisture content was reduced from 33 % to 16.5 % dry-basis within approximately 53 minutes. During the first process, a fluidized-bed dryer was used to reduce the moisture content of paddy down to 19.5 % dry-basis within 3 minutes. Then the paddy was tempered for 30 minutes. Finally, it was cooled by ambient air (temperature and relative humidity of 30 °C and 55-60% respectively) with air velocity of 0.15 m/s for 20 minutes. Quality of paddy in terms of head rice yield and whiteness was acceptable.     

STUDY ON ROUGH RICE FISSURING DURING INTERMITTENT DRYING

Abstract

The Assuring of rough rice during intermittent drying was studied through experimental and numerical methods. The moisture distribution inside the rice kernel during drying and tempering was obtained by diffusion model. The moisture gradients were used to analyze the hydro stresses in the rice kernel during intermittent drying.

Discontinuing the drying process with tempering can decrease the hydro stresses in the rice kernel. Decreased unit drying time or increased intermittent ratio caused decrease of the stresses in the rice kernel during intermittent drying. Less fissured rice was also observed by discontinuing the drying process in the experiments. Higher intermittent ratio or lower unit drying time caused lower percentage of fissured rice in the experiments.     

EFFECT OF A TEMPERING PERIOD ON DRYING OF CARROT IN A VffiRO-FLUlDIZED BED

ABSTRACT

Drying of diced carrot in a vibrofluidized bed was studied experimentally for various air temperatures, bed heights and size of the cubes. Effect of a tempering period that mav be implemented into ¦ drying cycle was examined with respect to drying kinetics and energy consumption. Although two stage drying with a tempering period increases the overall drying time, it shortens the drying time in a VFB dryer thus reduces energy consumption     

Energy optimization of bioethanol production via hydrolysis of switchgrass

The optimal flowsheet for the production of bioethanol from switchgrass via hydrolysis is proposed in this work. A superstructure embedding a number of alternatives is proposed. Two technologies are considered for switchgrass pretreatment, dilute acid and ammonia fiber explosion so that the structure of the grass is broken down. Next, enzymatic hydrolysis follows any of the pretreatments to obtain fermentable sugars, mainly xylose and glucose. Ethanol is obtained by fermentation of the sugars. To obtain fuel quality, ethanol and water must be removed from the water–ethanol mixture. A number of dehydration technologies is considered including rectification, adsorption in corn grits, molecular sieves, and pervaporation. The problem is formulated as a mixed‐integer nonlinear programming (MINLP). The superstructure is optimized by decomposing the MINLP for each of the pretreatments. Then, multieffect columns and heat integration are used to reduce the energy consumption and cooling needs. Finally, an economic evaluation is performed. The optimal flowsheet consists of using dilute acid hydrolysis followed by molecular sieves as dehydration technology, which requires less energy and cooling and yielding a promising production price of 0.8 $/gal. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

Multi-Pass Sorption Drying of Paddy Using Rice Husk Adsorbent: Experiments and Simulation

A single-stage sorption drying of paddy with rice husk adsorbent cannot reduce its moisture content to a safe level since the relative humidity of air inside the mixing tank is very high. To obtain a safe moisture content, multi-pass sorption drying was used and the effect of operating parameters—i.e., rice husk density, rice husk initial moisture content, and volumetric mixing ratio of paddy to rice husk—on the drying time and paddy quality were investigated for two paddy varieties, Suphanburi 1 and Khao Dawk Mali 105. A mathematical model associated with the transfers of heat and mass between the grain and adsorbent was also formulated to explore the sensitivity of the operating parameters to the drying time. Both the experiment and the simulation showed that the drying time was shorter by using a lower initial moisture content of rice husk, a higher rice husk bulk density, and a higher volumetric mixing ratio, the last factor being less important. For quality assessment, the color of white rice for both paddy varieties changed insignificantly under the influence of all the operating parameters, but the head rice quality was rather sensitive to the operating conditions especially for the Khao Dawk Mali 105, which had a 6–10% lower in head rice yield compared to that of the reference samples.