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Energy consumption and rice milling quality upon drying paddy with a newly-designed horizontal rotary dryer
Energy xxx (2016) 1e8
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Energy consumption and rice milling quality upon drying paddy with a newly-designed horizontal rotary dryer Saeed Firouzi a, *, Mohammad Reza Alizadeh b, Didar Haghtalab b a b
Department of Agronomy, Rasht Branch, Islamic Azad University, Rasht, Iran Rice Research Institute of Iran, Agricultural Research, Education and Extension Organization (AREEO), Rasht, Iran
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 October 2015 Received in revised form 22 October 2016 Accepted 6 November 2016 Available online xxx
Paddy drying is an energy-intensive process which affects final product quality. Energy consumption and rice milling quality upon drying paddy with an industrial horizontal rotary dryer (IHRD) developed in northern Iran were investigated and compared with those of the conventional industrial batch-type bed dryer (IBBD) at five paddy final moisture levels (8.0, 9.0, 10.0, 11.0, and 12.0%, w.b.). The results indicated that the IHRD consumed specific electrical energy between 5.5 and 17.41 MJ kg1 water evaporated compared to 2.64e7.48 MJ kg-1 water evaporated in IBBD and specific thermal energy between 11.5 and 36.44 MJ kg1 water evaporated in IHRD compared to 7.78e22.09 MJ kg1 water evaporated in IBBD with a decrease in paddy final moisture content in the range of 12.0%e8.0% (w.b.). Thermal energy use efficiency was estimated to be as 38.8% and 26.3% for IBBD and IHRD, respectively, at moisture drop range of 14.5%e12%, w. b. Major milling quality attributes of the final product of paddy dried with IHRD were significantly superior to those of IBBD. It was concluded that the drying time of IHRD can be cut down about 26% without any significant changes in final milled rice quality; resulting to reduce the IBBD's total specific energy use around 10.43 MJ kg1 water evaporated. Completely discharging the drying air and kick turning the drying drum of IHRD with a suitable time interval were proposed as a subject of investigation to improve the energy efficiency of the tested dryer. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Paddy Drying Industrial paddy dryer Energy consumption Milling recovery
1. Introduction Paddy drying is a major concern in all rice-producing countries [1]. It is an energy-intensive process which significantly affects the quality of milled rice particularly in terms of head rice yield [2,3]. Paddy drying process requires the most energy, accounting for 55% of the whole energy needed to produce milled rice [4]. The energy required to dry food materials mainly involves the thermal energy of fuel used to remove undesirable moisture from the foodstuffs [5]. The amount of fuel consumed to dry grain will vary widely with the type of method used [6]. There are various methods to dry paddy rice. They involve different drying technologies of different scale and complexity. There is no ideal dryer for drying paddy since each drying technique has its own inherent advantages and
* Corresponding author. Department of Agronomy, Rasht Branch, Islamic Azad University, Rasht, Iran. Tel.: þ98 911 3362546 (mobile). E-mail addresses: fi[email protected], saeedfi[email protected] (S. Firouzi). URL: http://firouzi.iaurasht.ac.ir/faculty/fa/index.html, https://scholar.google. com/citations?user¼DFP4f0cAAAAJ&hl¼en
disadvantages [7]. The most common dryer for paddy drying in Asia is the batchtype bed dryer that includes rectangular and circular bin dryers [8]. In Iran, more than of 90% of paddy is dried by the rectangular batch-type bed dryer. Due to the high thickness of paddy grain placed in the drying box of batch-type dryers, the bottom layers are dried more compared to the top layers, which leads to excessive milled-rice breakage. Furthermore, to achieve the favorable moisture level for paddy in top layers, a high amount of energy is consumed for paddy drying in industrial batch-type bed dryer (IBBD). Extensive labor requirement is another problem in the use of IBBDs [7]. Review of reports showed that some efforts were made to find energy-efficient drying techniques for improving the milling quality of rice [1,2,9]. The aims of the majority of the studies were to develop and check the performances of the newly-designed dryers and methods for drying the freshly harvested paddy in tropical countries [10]. Most of them were based on laboratory scale experiments and results on industrial scale are seldom reported in scientific publications [11]. Along with these attempts, some new
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designs of paddy dryers were presented to resolve the shortcomings of IBBD in northern Iran. Industrial rotary horizontal dryer is a recently developed paddy dryer which attracted the attention of many researchers and mill owners in northern Iran (Fig. 1). It is a recirculating batch type grain dryer where the grain is loaded as a batch and is constantly mixed during the drying process. Agitation of the paddy bulk during drying operation resulting in higher moisture uniformity of paddy grains, as well as the mechanized unloading of the dryer are the noteworthy benefits of industrial horizontal batch-type bed dryer (IHRD). However, energy consumption and milling quality of the final product of any industrial paddy dryer must be checked to verify its present status and to suggest for further efficient operation [1]. To compare the energy consumption among industrial drying options, the data of drying paddy with the industrial scale dryers must be obtained [12]. All proceedings to minimize the milled rice breakage in paddy drying process may be a solution to mitigate the fuel cost for thermal energy supplied to the paddy dryers [13]. It seems that agitating paddy in drying with newly developed paddy dryer IHRD results in more uniformity in moisture content of individual paddy grains and then minimizing the milled rice breakage compared to that of the traditional IBBD [7]. Moreover, due to recycling a part of the exhaust moist drying air in IHRD, paddy drying process in this dryer is expected to be gentler than that of traditional IBBD; leading to further decrease in milled rice breakage; but the key questions are: How much improvement happens in milling quality of paddy dried by using this dryer? How much is the energy use of drying process with IHRD compared to that of traditional IBBD? Therefore,
Fig. 1. Industrial horizontal rotary paddy dryer (IHRD).
in this study, the technical specifications of the new dryer IHRD are described in brief at first and then, the key parameters of its drying performance including drying residence time, energy consumption, and milling quality of the final product will be checked and discussed compared to the traditional IBBD. 2. Materials and methods Rice is the second most important food crop after wheat in Iran. In 2013, approximately 564,000 ha were under rice production in this country, around 273,000 ha of which are located in Guilan Province, making it the second most important rice-producing region in Iran, in terms of cultivated area while, the first in terms of total production [14,15]. This research was conducted in a paddy mill in Sangar district of Rasht, center of Guilan Province, northern Iran. Long grain paddy variety of Hashemi, the most frequently cultivated paddy variety in the Guilan Province, was considered as the test material. To keep the uniformity of paddy mass used for the experiments, the experimental paddy was prepared from a unit farm with the same agronomic practices. Because of semimechanized and manual harvesting of paddy in northern Iran, and due to the relative stability of climatic conditions at harvest time, the initial paddy moisture content is around 14e15% w. b.; hence, most of the studies on paddy drying in Iran are based on 14e15% w. b. initial moisture level. The experiment was conducted as a completely randomized design in a factorial arrangement with four replicates. Factors were the type of dryer at two levels (IBBD and IHRD), and paddy final moisture content at five levels (8.0%, 9.0%, 10.0%, 11.0%, and 12.0%, w. b.). Different levels of paddy final moisture contents were chosen in order to study the behavior of the tested dryers in terms of energy consumption and milling quality of the final product at various final moisture levels. The experiment location consisted of two tested industrial dryers of 3 tons loading capacity (Table 1). According to Fig. 2, in the industrial batch-type dryer (IBBD), paddy is placed in a rectangular container with a floor fabricated from a perforated screen. Drying air heated by a natural gas burner using a fan powered by an electrical motor is passed through the paddy mass in the drying box. Paddy loading and unlading are performed manually. During the unloading of the drying box, the pressure of labors' feet easily breaks some paddy grains which leads to a decrease in head rice yield. Non-uniform drying of different layers of paddy mass and high energy demand for the process are the main problems in the use of batch-type dryers. Fig. 3 also shows various elements of IHRD. The inlet air is heated by the natural gas burner (No. 7) and is transferred from the warming box by the vacuum generated by a strong fan (No. 10) powered by a 7.5 hp electrical motor and is then pushed into the drying drum. The heated air is passed through the inlet, which is the perforated curved screen of the drying drum, into the grain mass. The discharged air exits towards the duster cylinder through the curved perforated screen of the outlet; about 50% of the filtered humid discharging air is mixed with some fresh air vacuumed at the warming box and are fed into the burner inlet and re-circulates in the next drying cycle. This action softens the moisture removal of the paddy grains in the drying drum and prevents excessive internal stress in the paddy kernels; which is expected to lessen the broken milled rice. However, due to the increasing the relative humidity of the drying air, the paddy drying rate is expected to be reduced or the paddy residence time in the dryer is expected to be increased. A gear and chain mechanism drives the drying drum at a speed of 0.25 rpm (4 rotations per minute); therefore, paddy is moved and agitated by the inner helix. Agitation of paddy during the drying process, de-awning of paddy grains by small holes of
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Table 1 Details of operating parameters used in industrial bed dryer (IBBD) and industrial rotary dryer (IHRD) for drying paddy in northern Iran. Type of paddy dryer
Dimensional parameters
Drying operating conditions
IBBD
Drying bed area: 6 m 2 m Paddy depth: 0.7 m Approximate capacity: 3 tonnes Length of drying drum: 4 m Diam. of drying drum: 1.90 m Approximate capacity: 3 tonnes
Drying air temperature 38e40 C Relative humidity: 55e75%; Air flow: 0.015 m3 s1 m2 Drying air temperature: 38e40 C Relative humidity: 55e90%; Air flow: 0.171 m3 s1 m2
IHRD
and IBBD. Moisture content of paddy samples were determined using the digital grain moisture meter (GMK 303R5, Korea). In order to carry out paddy sampling for milling tests at various paddy final moisture levels, 20 perforated boxes made from metal mesh were filled with experimental paddy grain and placed into paddy bed in four equal sections of IBBD length as four replicates for five final moisture levels of paddy (8.0%, 9.0%, 10.0%, 11.0%, and 12.0% w. b.). In the case of IHRD, the milling samples were obtained at discharge gates. The temperature and relative humidity of drying air were measured by Jenway-digital Psychrometer (model 5100, England). Fig. 2. Industrial batch-type bed dryer (IBBD).
Fig. 3. Elements of industrial horizontal rotary paddy dryer (IHRD) 1:Drying drum 2:Perforated sheets 3:Moving screw 4:Air inlet 5:Air outlet 6:Duster 7:Oven 8:Warming box 9:Ventilation gate 10:Fan 11:Drum driver electrical motor.
perforated screens, and mechanized unloading of the dryer are the remarkable benefits of paddy drying with IHRD. Details of the operating parameters of industrial bed dryer (IBBD) and industrial rotary dryer (IHRD) are shown in Table 1. Necessary data such as the amount of paddy, moisture content, drying air temperature, drying time, air flow, and motor power were recorded during the experiment. In order to avoid over-drying of paddy for milling tests, except for the beginning of drying process, all samples were collected at 1 h time intervals. In order to determine the sample moisture content, the whole bed of IBBD was divided into four equal sections along the length and three depths (as the top, middle, and bottom sections). The average moisture content of all twelve paddy samples was used to study the behavior of IBBD against the drying time. To study the paddy moisture gradient in IHRD, the paddy samples were collected at four peripheral discharge gates and at three depths toward the center of drying drum (superficial, medium, and central). A long hand auger with short pitch was fabricated to collect the moisture samples at different points of IHBD
The inlet air velocity of dryers was measured by Testo-405 velocity stick hot wire Anemometer, Italy with ±0.3 m/s accuracy. The total volume of air passing through each dryer was calculated by continuity equation (eq. (1)). Bed air velocity of each dryer also was calculated using the same equation [1].
Q ¼AV
(1)
where Q is the drying air flow (m3 s1), A the cross-sectional area at the air inlet (m2), V the average velocity of the drying air across the air inlet (m s1).
2.1. The energy consumption of dryers The specific energy consumption is defined as the energy used to evaporate the unit mass of water from the bulk grain in a dryer. It is a key indicator for evaluating the performance of industrial dryers. Given the preservation or improvement of the quality of final product, the less specific energy consumed by a dryer denotes
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its better drying performance [1]. In the case of IBBD, the energy consumption includes thermal energy for warming up the drying air and electrical energy for driving blower fan at air inlet; while in the case of IHRD, the drying energy includes the electrical energy for driving blower and burner fans, electrical energy required to rotate the drying drum, and thermal energy for warming up the drying air. Turbine gas meter LUXI 2000/TZ, Actaris Gasz€ ahlerbau GmbH, Germany with an accuracy of ±1% was used to measure the amount of natural gas consumed for heating the drying air. The specific thermal energy consumption (STEC) was computed in MJ kg1 water removed using the following equation [16]:
STEC ¼
V eq mw
(2)
where STEC is the thermal energy consumed to dry paddy (kJ kg1 water removed),V the volume of natural gas consumed (m3), eq the energy equivalent of natural gas (49.5 MJ m3) [10], mw mass of water removed. Based on the efficiency of power plants for conversion of the thermal to the electrical energy, the following equation was used to calculate the initial thermal energy consumed to produce the electrical energy [1,17,18]:
Eel ¼ 2:6 P t
(3)
where Eel is the electrical energy in terms of primary energy (MJ), P the power of electrical motors used in different parts of paddy dryers (kW), t the drying process duration (h). The specific electrical energy consumption (SEEC) as the electrical energy used per unit mass of water removed was computed by the following equation:
SEEC ¼
Eel mw
(4)
where SEEC is the specific electrical energy consumption in kJ kg1 water removed, Eel the electrical energy use (mJ), mw the mass of water removed. The total specific energy consumption was determined as the ratio of summation of electrical and thermal energy divided by the amount of water removed from initial paddy at the end of drying process [1]. The amount of water removed from the paddy bulk at each final moisture level was calculated based on the following equation [16]:
mi mci mcf mw ¼ 100 mcf
moisture levels. It was determined through milling the paddy samples using the laboratory rubber roll paddy huller (SATAKE Co. Ltd, Japan) and laboratory friction type rice whitener (McGill Miller, USA) with 45 s milling time, and then separating the broken and unbroken kernels by the laboratory rotary sieve SATAKE Coe Type TRG 058, Japan. The HRY index was calculated as the mass ratio of unbroken rice kernels after milling to the paddy before milling process. Kernels having a length longer than three-fourths of intact milled rice are defined as unbroken or head rice [20,21]. (2) Milling recovery (MR) as an indicator of the quantity of total milled rice was measured for different samples derived from two dryers at different paddy final moisture levels. It was determined as the mass of total milled rice including head and broken milled rice divided by mass of dried paddy sample multiplied by 100 [22]. (3) In order to determine the degree of milling, milled rice whiteness (WT) was measured by the rice whiteness tester model “KET-C300, Tokyo, Japan” [19]. Besides HRY and MR, degree of milling is also an important issue for judging milled rice quality [19,23,24]. All laboratory measurements were performed in Iran rice research institute (IRRI). The statistical software package SAS 9.1 was used for the analysis of variance (ANOVA) and a least significant difference (LSD) test to compare the mean values of measured attributes.
3. Results and discussion 3.1. Drying behavior of dryers Fig. 4 illustrates the drying behaviors of the IBBD and IHRD by plotting the percentage of the paddy moisture content against the drying time. Drying time or grain residence time in the dryer is an important parameter to evaluate the performance of grain dryers. It shows the rate of moisture removal in individual grains [16]. It has been suggested that the drying rate, more so than the drying air temperature, affects the milled rice quality [25]. As shown in Fig. 4, the total time required for paddy moisture drop from initial moisture content (14.5%, w. b.) to the final moisture content (8.0%, w. b.) in IBBD and IHRD were 30 and 46 h, respectively. Shorter drying time in drying with IBBD can be explained by the continuous fresh air being fed to its inlet and free discharge of its exhausted
(5)
where, mci and mcf are the initial and final moisture levels of paddy dried (w.b., %), respectively. mw and mi are the mass of water removed (kg) and initial mass of paddy bulk (kg), respectively.
2.2. Laboratory measurements The following milling quality attributes were evaluated for dried paddy samples. Head rice yield (HRY), total rice yield (TRY) or milling recovery (MR), and degree of milling are the most important indices to assess the milling quality of rice [19]: (1) Head rice yield (HRY) as the universal indicator for the quantity of head rice kernels was measured for different paddy samples derived from two dryers at different final
Fig. 4. Drying curves during with Industrial Batch-type bed paddy dryer [IBBD; drying temp., 38e40 C; air flow, 0.015 m3 m2 s1 and bed depth, 0.7 m] and industrial horizontal rotary paddy dryer [IHRD; drying temp., 38e40 C; air flow, 0.171 m3 m2 s1].
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humid drying air. However, in IHRD, a part of the exhausted drying air participates in the next drying cycle, resulting in delay in the drying time. Longer drying time in IBBD (30 h) compared to the study of inclined bed dryer (13 and 20 h) can also be related to differences in the moisture drop range between the first (about 20%, w. b.) and last (about 10%, w. b.) paddy moisture content in the study of inclined bed dryer [1]. It is obvious that the evaporation of the superficial moisture of paddy grains is much faster than their inner moisture leading to higher drying rate.
3.2. The energy consumption of dryers Fig. 5 shows the amounts of specific electrical energy consumption (SEEC) for two tested dryers at various levels of paddy final moisture level. The SEEC was found to vary between 2.64 and 7.48 MJ kg1 water removed and 5.50e17.41 MJ kg1 water removed for IBBD and IHRD, respectively. Amount of SEEC increased with a decrease in paddy final moisture content for both dryers. This can be explained by the fact that the surface moisture will evaporate readily when the paddy grain is exposed to hot air; therefore it takes less time than the evaporation of internal moisture. The internal moisture has to move from the inside of the paddy kernel to the outside surface. As a result, evaporation of surface and internal moistures take different durations of time [7]. The increasing trend of the time intervals between lower paddy moisture levels confirms this result (Fig. 4). Therefore, due to the direct relationship of the electrical energy with the time of drying (Eq. (2)), the increasing trend of electrical energy consumption with a decrease in the final paddy moisture level of paddy can be justified. Fig. 5 also shows that the SEEC of IHRD was more than that of IBBD at all final desired moisture levels of paddy. This can be explained by the participation of some of the exhaust humid air in next drying cycles in this dryer. Hence, evaporation of paddy moisture is delayed in drying drum of IHRD. Fully discharging the exhaust humid air and feeding fresh air to the burner can reduce the overall drying time and then reduce electrical energy consumed. However, the milling quality of final product is the other main factor which determines the ultimate result. The SEECs of inclined bed dryer were also determined to be in the ranges of 1.86e1.95 and 1.44e1.83 MJ kg1 water removed for Bukit Besar and Simpang Empat complexes in Malaysia, respectively [1]. Differences between SEECs in inclined bed dryer and the dryers studied in this research mostly contributed to the differences between moisture drop ranges and then, different drying time in tested dryers.
Fig. 5. Electrical energy consumption during drying of paddy with Industrial batchtype bed dryer (IBBD) & Industrial horizontal rotary dryer (IHRD) in northern Iran.
5
Fig. 6 shows the specific thermal energy consumption (STEC) for drying paddy in IBBD and IHRD. According to this Fig. 6, depending on the paddy final moisture level, the amount of STEC varied from 7.78 to 22.09 MJ kg1 and 11.5e36.44 MJ kg1 water removed while drying with IBBD and IHRD, respectively. Similar to the case of SEEC, the increasing trend of STEC with a decrease in final desired moisture level of paddy is related to the increased time intervals at lower moisture contents and higher values of STEC in IHRD compared to the IBBD are related to the shortcoming of the IHRD in fully discharging the exhaust drying air and feeding fresh air to the dryer inlet. The value of STEC of IBBD (7.78 MJ kg1 water removed) at the moisture drop range of 14.5 to 12%, w. b. is within the range of 3.48e10.45 MJ kg1 water removed stated for cross-flow grain dryer reported by Maier and Bakker-Arkema [16]. In addition, the value for IHRD (11.5 MJ kg1 water removed) is somewhat more than the topmost level reported for cross-flow grain dryer (10.45 MJ kg1 water removed) [16]. Thermal energy needed to drying paddy using commercial, cross flow paddy dryers in Arkansas were also reported to be within the range of 6.90e9.67 MJ kg1 water removed in 2011 and 8.81 to 9.62 in 2012 with paddy moisture drop from 19.0e21.7% to 12.2e13%, and from 15.4e18.3% to 11.7e12.2%, w. b., respectively [26]. The value of specific thermal energy consumption for IBBD (7.78 MJ kg1 water removed) at the moisture drop range of 14.5 to 12%,w.b. is within the ranges measured for commercial, cross flow paddy dryers in Arkansas, but the value for IHRD at the same conditions for IBBD (11.5 MJ kg1 water removed) is somewhat more than that reported for commercial, cross flow paddy dryers in Arkansas. It was also reported that the STEC of industrial-scale spouted bed paddy dryer prototype to be in the range of 3.5e7.00 MJ kg1 water removed to reduce paddy moisture level from 28% to 21.5%, d.b., and those of inclined bed dryers of Bukit Besar and Simpang Empat complexes in Malaysia was reported to be in the ranges of 2.77e3.27 and 3.20e3.47 MJ kg1 water removed, respectively, for paddy moisture drop from ~22.5 to 12.5%, w. b [1,27]. It is noticed that the differences between the results of these studies can be mainly due to unequal drying time for the complete drying of paddy because of different initial moisture levels, and differences in moisture drop in the studies. Moreover, the differences in drying conditions in the study of inclined bed dryer [1], such as the depth of paddy bulk in industrial inclined dryer compared to IBBD and the different airflows in dryers can explain the different results of energy efficiencies in the studies. Higher values for STEC in IBBD and IHRD in lower moisture levels compared to the similar studies can be contributed to the differences between initial moisture contents of paddy grains in the studies, the different drying way in IBBD and
Fig. 6. Thermal energy consumption during drying of paddy with Industrial batchtype bed dryer (IBBD) & Industrial horizontal rotary dryer (IHRD) in northern Iran.
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Fig. 7. Total energy consumption during drying of paddy with Industrial batch-type bed dryer (IBBD) & Industrial horizontal rotary dryer (IHRD) in northern Iran.
IHRD, and the fact of hardening the moisture removal from grain's inner layers in grain drying process [16]. The theoretical thermal energy required to dry a single grain under ideal conditions was reported to be in the range of 2.55e3.02 MJ kg1 water removed [16]. Regarding the uppermost level of foregoing STEC ranges, the energy efficiencies of IBBD and IHRD (theoretical thermal energy divided by the thermal energy supplied to the dryer) would be as 38.8 and 26.3, respectively, to dry paddy from moisture level of 14.5%e12%, w. b. However, the ideal conditions to evaporate extra water from grains are very different from true conditions of drying in industrial paddy dryers. The values of thermal energy efficiency for commercial, cross flow paddy dryers in Arkansas were also determined to be within the ranges of 26%e36% and 27%e29% in 2011 and 2012, respectively [26]. Finally, according to Fig. 7, the total specific energy consumption (SEC) as the sum of SEEC and STEC had trends similar to the trends of SEEC and STEC with a decrease in paddy final moisture level. The amount of SEC increased from 10.41 to 29.58 and 17.00e53.86 MJ kg1 water removed in drying with IBBD and IHRD, respectively. The values of SECs for IHRD were 84 and 216.7% more than those of IBBD at the first (12.0%) and the last (8.0%) paddy final moisture level (w.b.), respectively. The ratio of SEEC to SEC in all final moisture levels was around 0.25 and 0.30 for drying paddy in IBBD and IHRD, respectively. This result shows the important role of electrical energy of the total energy consumed in IHRD, compared to drying with IBBD.
Fig. 8. Comparison of head rice yield (HRY) at different final moisture content of paddy dried in industrial batch-type bed dryer (IBBD) and industrial horizontal rotary dryer (IHRD). Values with the same letters are not significantly different (p < 0.01).
significantly more than those of the IBBD samples at all corresponding paddy final moisture levels (Figs. 8 and 9). It seems that the continuous agitation of the paddy during the drying process and lower drying rate or lesser drying capacity of drying air in IHRD are the main factors which lead to the significant improvement of HRY and MRC in drying with IHRD. It has been shown that the milled rice quality can be adversely affected by drying air with high evaporative capacity [28]. The amount of HRY increased significantly from 43.11 to 54.92% and 35.03e52.77% with moisture drop in drying with IBBD and IHRD, respectively (Fig. 8). The increasing trend in HRY with a decrease in paddy moisture level can be attributed to the higher mechanical strength of rice kernels in terms of their bending strength and fracture energy at lower moisture contents [29]. There were no significant differences between HRY and MRC at paddy final moisture levels of 11.0% and 9.0%, w. b. in drying with IHRD and IBBD, respectively. A similar state is seen for HRY at paddy final moisture contents of 12.0% and 10.0%, w. b. in drying upon IHRD and IBBD, respectively (Figs. 8 and 9). As stated previously,
3.3. Product quality assessment Main effects of the dryer type (DT) and final moisture content of paddy (FMC) and DT FMC interaction effect on HRY and MRC were significant at 0.01 probability level (Table 2). Results show that the HRY and MRC of the samples obtained from IHRD were
Table 2 Mean squares of ANOVA for head rice yield (HRY), milling recovery (MRC), and degree of whiteness (WT) of dried paddy samples as affected by the dryer type and paddy final moisture content.
*
Source of variance
df
HRY
MRC
WT
Dryer type (DT) Final milling moisture level of paddy (FMC) DT FMC Error CV (%)
1 4
414.672** 271.096**
143.717** 29.271**
6.241** 9.847**
4 30 e
12.440** 0.659 1.72
2.351** 0.508 1.09
2.806* 0.789 1.53
and
**
represent significant at 0.05 and 0.01 probability levels, respectively.
Fig. 9. Comparison of milling recovery (MRC) at different final moisture content of paddy dried in industrial batch-type bed dryer (IBBD) and industrial horizontal rotary dryer (IHRD). Values with the same letters are not significantly different (p < 0.01).
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Table 3 Degree of whiteness for different final moisture content of paddy dried in industrial batch-type bed dryer (IBBD) and industrial horizontal rotary dryer (IHRD). Name of dryer
Final moisture content (w.b.) 12 ± 0.2%
IBBD IHRD
11 ± 0.2% abc
58.89 ± 0.15 59.61 ± 0.37ab
10 ± 0.2% cd
58.21 ± 0.17 60.13 ± 0.50a
9 ± 0.2% bcd
58.35 ± 0.75 57.98 ± 0.70cd
8 ± 0.2% cd
57.84 ± 0.18 57.46 ± 0.22d
55.56 ± 0.60e 57.65 ± 0.17cd
Values with the same letters are not significantly different (p < 0.05).
gentle drying with more humid drying air in IHRD associated with longer paddy residence time or lower drying rate leads to less thermal and moisture stresses in rice kernels. This can preserve end use grain quality more efficiently [16]. Therefore, the same results for main quality attributes of milled rice were achieved at higher moisture contents in drying paddy upon IHRD compared to the IBBD. Interestingly, the values of HRY, MRC, and WT for paddy dried in IHRD at 9.0% moisture level are significantly more than those of IBBD at 8.0% moisture level. Thus, all milling quality attributes for the final product of paddy dried with IHRD at 9.0% paddy moisture level are superior to those of IBBD (Figs. 8 and 9, and Table 3). Considering the non-significant differences between all quality attributes of final milled rice at 8.0% and 9.0% final paddy moisture levels in drying with IHRD, (Figs. 8 and 9 and Table 3), in order to save energy, the final paddy moisture of 9.0% is suggested as the appropriate final moisture level when drying paddy with IHRD (the SEC decreases by 10.43 MJ kg1 water removed). Therefore, the required drying time of IHRD can be shortened to about 12 h or 26% (the paddy drying residence time at 8.0% final moisture level, 46 h, can be replaced by that of 9.0% paddy final moisture level or 34 h in IHRD).
4. Conclusion The actual drying performance attributes including the drying time, specific energy consumption and final product quality of IBBD and IHRD paddy dryers were presented and discussed in this article. The HRY and MRC of the samples obtained from IHRD were significantly more than those from IBBD at all corresponding paddy final moisture levels. In order to save energy when drying with IHRD, paddy final moisture level of 9.0% was suggested as the appropriate final moisture level in drying paddy with IHRD in northern Iran. Due to the non-significant differences between final product quality attributes at 8.0% and 9.0% paddy final moisture levels, the paddy residence time in IHRD may be reduced about 12 h (~26%); therefore, specific energy consummation may be saved around 10.43 MJ kg1 water removed. It seems that the economic benefits resulting from improvement in milled rice quality, especially for HRY and MRC can compensate the cost of higher thermal energy demand in IHRD. Further studies are needed to improve the drying rate and then save energy, along with the preservation or even improvement of the final product milling quality in IHRD dryers. Among them, kick turning of the drying drum of IHRD with an appropriate rotation time interval, along with complete discharge the drying air should be considered in future studies.
Acknowledgements The authors would like to thank Rasht branch, Islamic Azad University for their financial support, rice mill of Zare in Sangar district of Rasht, center of Guilan Province, Iran, for allowing to conduct the experiments, and Rice Research Institute of Iran (RRII) for facilitating the milling tests.
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Energy journal homepage: www.elsevier.com/locate/energy
Energy consumption and rice milling quality upon drying paddy with a newly-designed horizontal rotary dryer Saeed Firouzi a, *, Mohammad Reza Alizadeh b, Didar Haghtalab b a b
Department of Agronomy, Rasht Branch, Islamic Azad University, Rasht, Iran Rice Research Institute of Iran, Agricultural Research, Education and Extension Organization (AREEO), Rasht, Iran
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 October 2015 Received in revised form 22 October 2016 Accepted 6 November 2016 Available online xxx
Paddy drying is an energy-intensive process which affects final product quality. Energy consumption and rice milling quality upon drying paddy with an industrial horizontal rotary dryer (IHRD) developed in northern Iran were investigated and compared with those of the conventional industrial batch-type bed dryer (IBBD) at five paddy final moisture levels (8.0, 9.0, 10.0, 11.0, and 12.0%, w.b.). The results indicated that the IHRD consumed specific electrical energy between 5.5 and 17.41 MJ kg1 water evaporated compared to 2.64e7.48 MJ kg-1 water evaporated in IBBD and specific thermal energy between 11.5 and 36.44 MJ kg1 water evaporated in IHRD compared to 7.78e22.09 MJ kg1 water evaporated in IBBD with a decrease in paddy final moisture content in the range of 12.0%e8.0% (w.b.). Thermal energy use efficiency was estimated to be as 38.8% and 26.3% for IBBD and IHRD, respectively, at moisture drop range of 14.5%e12%, w. b. Major milling quality attributes of the final product of paddy dried with IHRD were significantly superior to those of IBBD. It was concluded that the drying time of IHRD can be cut down about 26% without any significant changes in final milled rice quality; resulting to reduce the IBBD's total specific energy use around 10.43 MJ kg1 water evaporated. Completely discharging the drying air and kick turning the drying drum of IHRD with a suitable time interval were proposed as a subject of investigation to improve the energy efficiency of the tested dryer. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Paddy Drying Industrial paddy dryer Energy consumption Milling recovery
1. Introduction Paddy drying is a major concern in all rice-producing countries [1]. It is an energy-intensive process which significantly affects the quality of milled rice particularly in terms of head rice yield [2,3]. Paddy drying process requires the most energy, accounting for 55% of the whole energy needed to produce milled rice [4]. The energy required to dry food materials mainly involves the thermal energy of fuel used to remove undesirable moisture from the foodstuffs [5]. The amount of fuel consumed to dry grain will vary widely with the type of method used [6]. There are various methods to dry paddy rice. They involve different drying technologies of different scale and complexity. There is no ideal dryer for drying paddy since each drying technique has its own inherent advantages and
* Corresponding author. Department of Agronomy, Rasht Branch, Islamic Azad University, Rasht, Iran. Tel.: þ98 911 3362546 (mobile). E-mail addresses: fi[email protected], saeedfi[email protected] (S. Firouzi). URL: http://firouzi.iaurasht.ac.ir/faculty/fa/index.html, https://scholar.google. com/citations?user¼DFP4f0cAAAAJ&hl¼en
disadvantages [7]. The most common dryer for paddy drying in Asia is the batchtype bed dryer that includes rectangular and circular bin dryers [8]. In Iran, more than of 90% of paddy is dried by the rectangular batch-type bed dryer. Due to the high thickness of paddy grain placed in the drying box of batch-type dryers, the bottom layers are dried more compared to the top layers, which leads to excessive milled-rice breakage. Furthermore, to achieve the favorable moisture level for paddy in top layers, a high amount of energy is consumed for paddy drying in industrial batch-type bed dryer (IBBD). Extensive labor requirement is another problem in the use of IBBDs [7]. Review of reports showed that some efforts were made to find energy-efficient drying techniques for improving the milling quality of rice [1,2,9]. The aims of the majority of the studies were to develop and check the performances of the newly-designed dryers and methods for drying the freshly harvested paddy in tropical countries [10]. Most of them were based on laboratory scale experiments and results on industrial scale are seldom reported in scientific publications [11]. Along with these attempts, some new
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designs of paddy dryers were presented to resolve the shortcomings of IBBD in northern Iran. Industrial rotary horizontal dryer is a recently developed paddy dryer which attracted the attention of many researchers and mill owners in northern Iran (Fig. 1). It is a recirculating batch type grain dryer where the grain is loaded as a batch and is constantly mixed during the drying process. Agitation of the paddy bulk during drying operation resulting in higher moisture uniformity of paddy grains, as well as the mechanized unloading of the dryer are the noteworthy benefits of industrial horizontal batch-type bed dryer (IHRD). However, energy consumption and milling quality of the final product of any industrial paddy dryer must be checked to verify its present status and to suggest for further efficient operation [1]. To compare the energy consumption among industrial drying options, the data of drying paddy with the industrial scale dryers must be obtained [12]. All proceedings to minimize the milled rice breakage in paddy drying process may be a solution to mitigate the fuel cost for thermal energy supplied to the paddy dryers [13]. It seems that agitating paddy in drying with newly developed paddy dryer IHRD results in more uniformity in moisture content of individual paddy grains and then minimizing the milled rice breakage compared to that of the traditional IBBD [7]. Moreover, due to recycling a part of the exhaust moist drying air in IHRD, paddy drying process in this dryer is expected to be gentler than that of traditional IBBD; leading to further decrease in milled rice breakage; but the key questions are: How much improvement happens in milling quality of paddy dried by using this dryer? How much is the energy use of drying process with IHRD compared to that of traditional IBBD? Therefore,
Fig. 1. Industrial horizontal rotary paddy dryer (IHRD).
in this study, the technical specifications of the new dryer IHRD are described in brief at first and then, the key parameters of its drying performance including drying residence time, energy consumption, and milling quality of the final product will be checked and discussed compared to the traditional IBBD. 2. Materials and methods Rice is the second most important food crop after wheat in Iran. In 2013, approximately 564,000 ha were under rice production in this country, around 273,000 ha of which are located in Guilan Province, making it the second most important rice-producing region in Iran, in terms of cultivated area while, the first in terms of total production [14,15]. This research was conducted in a paddy mill in Sangar district of Rasht, center of Guilan Province, northern Iran. Long grain paddy variety of Hashemi, the most frequently cultivated paddy variety in the Guilan Province, was considered as the test material. To keep the uniformity of paddy mass used for the experiments, the experimental paddy was prepared from a unit farm with the same agronomic practices. Because of semimechanized and manual harvesting of paddy in northern Iran, and due to the relative stability of climatic conditions at harvest time, the initial paddy moisture content is around 14e15% w. b.; hence, most of the studies on paddy drying in Iran are based on 14e15% w. b. initial moisture level. The experiment was conducted as a completely randomized design in a factorial arrangement with four replicates. Factors were the type of dryer at two levels (IBBD and IHRD), and paddy final moisture content at five levels (8.0%, 9.0%, 10.0%, 11.0%, and 12.0%, w. b.). Different levels of paddy final moisture contents were chosen in order to study the behavior of the tested dryers in terms of energy consumption and milling quality of the final product at various final moisture levels. The experiment location consisted of two tested industrial dryers of 3 tons loading capacity (Table 1). According to Fig. 2, in the industrial batch-type dryer (IBBD), paddy is placed in a rectangular container with a floor fabricated from a perforated screen. Drying air heated by a natural gas burner using a fan powered by an electrical motor is passed through the paddy mass in the drying box. Paddy loading and unlading are performed manually. During the unloading of the drying box, the pressure of labors' feet easily breaks some paddy grains which leads to a decrease in head rice yield. Non-uniform drying of different layers of paddy mass and high energy demand for the process are the main problems in the use of batch-type dryers. Fig. 3 also shows various elements of IHRD. The inlet air is heated by the natural gas burner (No. 7) and is transferred from the warming box by the vacuum generated by a strong fan (No. 10) powered by a 7.5 hp electrical motor and is then pushed into the drying drum. The heated air is passed through the inlet, which is the perforated curved screen of the drying drum, into the grain mass. The discharged air exits towards the duster cylinder through the curved perforated screen of the outlet; about 50% of the filtered humid discharging air is mixed with some fresh air vacuumed at the warming box and are fed into the burner inlet and re-circulates in the next drying cycle. This action softens the moisture removal of the paddy grains in the drying drum and prevents excessive internal stress in the paddy kernels; which is expected to lessen the broken milled rice. However, due to the increasing the relative humidity of the drying air, the paddy drying rate is expected to be reduced or the paddy residence time in the dryer is expected to be increased. A gear and chain mechanism drives the drying drum at a speed of 0.25 rpm (4 rotations per minute); therefore, paddy is moved and agitated by the inner helix. Agitation of paddy during the drying process, de-awning of paddy grains by small holes of
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Table 1 Details of operating parameters used in industrial bed dryer (IBBD) and industrial rotary dryer (IHRD) for drying paddy in northern Iran. Type of paddy dryer
Dimensional parameters
Drying operating conditions
IBBD
Drying bed area: 6 m 2 m Paddy depth: 0.7 m Approximate capacity: 3 tonnes Length of drying drum: 4 m Diam. of drying drum: 1.90 m Approximate capacity: 3 tonnes
Drying air temperature 38e40 C Relative humidity: 55e75%; Air flow: 0.015 m3 s1 m2 Drying air temperature: 38e40 C Relative humidity: 55e90%; Air flow: 0.171 m3 s1 m2
IHRD
and IBBD. Moisture content of paddy samples were determined using the digital grain moisture meter (GMK 303R5, Korea). In order to carry out paddy sampling for milling tests at various paddy final moisture levels, 20 perforated boxes made from metal mesh were filled with experimental paddy grain and placed into paddy bed in four equal sections of IBBD length as four replicates for five final moisture levels of paddy (8.0%, 9.0%, 10.0%, 11.0%, and 12.0% w. b.). In the case of IHRD, the milling samples were obtained at discharge gates. The temperature and relative humidity of drying air were measured by Jenway-digital Psychrometer (model 5100, England). Fig. 2. Industrial batch-type bed dryer (IBBD).
Fig. 3. Elements of industrial horizontal rotary paddy dryer (IHRD) 1:Drying drum 2:Perforated sheets 3:Moving screw 4:Air inlet 5:Air outlet 6:Duster 7:Oven 8:Warming box 9:Ventilation gate 10:Fan 11:Drum driver electrical motor.
perforated screens, and mechanized unloading of the dryer are the remarkable benefits of paddy drying with IHRD. Details of the operating parameters of industrial bed dryer (IBBD) and industrial rotary dryer (IHRD) are shown in Table 1. Necessary data such as the amount of paddy, moisture content, drying air temperature, drying time, air flow, and motor power were recorded during the experiment. In order to avoid over-drying of paddy for milling tests, except for the beginning of drying process, all samples were collected at 1 h time intervals. In order to determine the sample moisture content, the whole bed of IBBD was divided into four equal sections along the length and three depths (as the top, middle, and bottom sections). The average moisture content of all twelve paddy samples was used to study the behavior of IBBD against the drying time. To study the paddy moisture gradient in IHRD, the paddy samples were collected at four peripheral discharge gates and at three depths toward the center of drying drum (superficial, medium, and central). A long hand auger with short pitch was fabricated to collect the moisture samples at different points of IHBD
The inlet air velocity of dryers was measured by Testo-405 velocity stick hot wire Anemometer, Italy with ±0.3 m/s accuracy. The total volume of air passing through each dryer was calculated by continuity equation (eq. (1)). Bed air velocity of each dryer also was calculated using the same equation [1].
Q ¼AV
(1)
where Q is the drying air flow (m3 s1), A the cross-sectional area at the air inlet (m2), V the average velocity of the drying air across the air inlet (m s1).
2.1. The energy consumption of dryers The specific energy consumption is defined as the energy used to evaporate the unit mass of water from the bulk grain in a dryer. It is a key indicator for evaluating the performance of industrial dryers. Given the preservation or improvement of the quality of final product, the less specific energy consumed by a dryer denotes
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its better drying performance [1]. In the case of IBBD, the energy consumption includes thermal energy for warming up the drying air and electrical energy for driving blower fan at air inlet; while in the case of IHRD, the drying energy includes the electrical energy for driving blower and burner fans, electrical energy required to rotate the drying drum, and thermal energy for warming up the drying air. Turbine gas meter LUXI 2000/TZ, Actaris Gasz€ ahlerbau GmbH, Germany with an accuracy of ±1% was used to measure the amount of natural gas consumed for heating the drying air. The specific thermal energy consumption (STEC) was computed in MJ kg1 water removed using the following equation [16]:
STEC ¼
V eq mw
(2)
where STEC is the thermal energy consumed to dry paddy (kJ kg1 water removed),V the volume of natural gas consumed (m3), eq the energy equivalent of natural gas (49.5 MJ m3) [10], mw mass of water removed. Based on the efficiency of power plants for conversion of the thermal to the electrical energy, the following equation was used to calculate the initial thermal energy consumed to produce the electrical energy [1,17,18]:
Eel ¼ 2:6 P t
(3)
where Eel is the electrical energy in terms of primary energy (MJ), P the power of electrical motors used in different parts of paddy dryers (kW), t the drying process duration (h). The specific electrical energy consumption (SEEC) as the electrical energy used per unit mass of water removed was computed by the following equation:
SEEC ¼
Eel mw
(4)
where SEEC is the specific electrical energy consumption in kJ kg1 water removed, Eel the electrical energy use (mJ), mw the mass of water removed. The total specific energy consumption was determined as the ratio of summation of electrical and thermal energy divided by the amount of water removed from initial paddy at the end of drying process [1]. The amount of water removed from the paddy bulk at each final moisture level was calculated based on the following equation [16]:
mi mci mcf mw ¼ 100 mcf
moisture levels. It was determined through milling the paddy samples using the laboratory rubber roll paddy huller (SATAKE Co. Ltd, Japan) and laboratory friction type rice whitener (McGill Miller, USA) with 45 s milling time, and then separating the broken and unbroken kernels by the laboratory rotary sieve SATAKE Coe Type TRG 058, Japan. The HRY index was calculated as the mass ratio of unbroken rice kernels after milling to the paddy before milling process. Kernels having a length longer than three-fourths of intact milled rice are defined as unbroken or head rice [20,21]. (2) Milling recovery (MR) as an indicator of the quantity of total milled rice was measured for different samples derived from two dryers at different paddy final moisture levels. It was determined as the mass of total milled rice including head and broken milled rice divided by mass of dried paddy sample multiplied by 100 [22]. (3) In order to determine the degree of milling, milled rice whiteness (WT) was measured by the rice whiteness tester model “KET-C300, Tokyo, Japan” [19]. Besides HRY and MR, degree of milling is also an important issue for judging milled rice quality [19,23,24]. All laboratory measurements were performed in Iran rice research institute (IRRI). The statistical software package SAS 9.1 was used for the analysis of variance (ANOVA) and a least significant difference (LSD) test to compare the mean values of measured attributes.
3. Results and discussion 3.1. Drying behavior of dryers Fig. 4 illustrates the drying behaviors of the IBBD and IHRD by plotting the percentage of the paddy moisture content against the drying time. Drying time or grain residence time in the dryer is an important parameter to evaluate the performance of grain dryers. It shows the rate of moisture removal in individual grains [16]. It has been suggested that the drying rate, more so than the drying air temperature, affects the milled rice quality [25]. As shown in Fig. 4, the total time required for paddy moisture drop from initial moisture content (14.5%, w. b.) to the final moisture content (8.0%, w. b.) in IBBD and IHRD were 30 and 46 h, respectively. Shorter drying time in drying with IBBD can be explained by the continuous fresh air being fed to its inlet and free discharge of its exhausted
(5)
where, mci and mcf are the initial and final moisture levels of paddy dried (w.b., %), respectively. mw and mi are the mass of water removed (kg) and initial mass of paddy bulk (kg), respectively.
2.2. Laboratory measurements The following milling quality attributes were evaluated for dried paddy samples. Head rice yield (HRY), total rice yield (TRY) or milling recovery (MR), and degree of milling are the most important indices to assess the milling quality of rice [19]: (1) Head rice yield (HRY) as the universal indicator for the quantity of head rice kernels was measured for different paddy samples derived from two dryers at different final
Fig. 4. Drying curves during with Industrial Batch-type bed paddy dryer [IBBD; drying temp., 38e40 C; air flow, 0.015 m3 m2 s1 and bed depth, 0.7 m] and industrial horizontal rotary paddy dryer [IHRD; drying temp., 38e40 C; air flow, 0.171 m3 m2 s1].
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humid drying air. However, in IHRD, a part of the exhausted drying air participates in the next drying cycle, resulting in delay in the drying time. Longer drying time in IBBD (30 h) compared to the study of inclined bed dryer (13 and 20 h) can also be related to differences in the moisture drop range between the first (about 20%, w. b.) and last (about 10%, w. b.) paddy moisture content in the study of inclined bed dryer [1]. It is obvious that the evaporation of the superficial moisture of paddy grains is much faster than their inner moisture leading to higher drying rate.
3.2. The energy consumption of dryers Fig. 5 shows the amounts of specific electrical energy consumption (SEEC) for two tested dryers at various levels of paddy final moisture level. The SEEC was found to vary between 2.64 and 7.48 MJ kg1 water removed and 5.50e17.41 MJ kg1 water removed for IBBD and IHRD, respectively. Amount of SEEC increased with a decrease in paddy final moisture content for both dryers. This can be explained by the fact that the surface moisture will evaporate readily when the paddy grain is exposed to hot air; therefore it takes less time than the evaporation of internal moisture. The internal moisture has to move from the inside of the paddy kernel to the outside surface. As a result, evaporation of surface and internal moistures take different durations of time [7]. The increasing trend of the time intervals between lower paddy moisture levels confirms this result (Fig. 4). Therefore, due to the direct relationship of the electrical energy with the time of drying (Eq. (2)), the increasing trend of electrical energy consumption with a decrease in the final paddy moisture level of paddy can be justified. Fig. 5 also shows that the SEEC of IHRD was more than that of IBBD at all final desired moisture levels of paddy. This can be explained by the participation of some of the exhaust humid air in next drying cycles in this dryer. Hence, evaporation of paddy moisture is delayed in drying drum of IHRD. Fully discharging the exhaust humid air and feeding fresh air to the burner can reduce the overall drying time and then reduce electrical energy consumed. However, the milling quality of final product is the other main factor which determines the ultimate result. The SEECs of inclined bed dryer were also determined to be in the ranges of 1.86e1.95 and 1.44e1.83 MJ kg1 water removed for Bukit Besar and Simpang Empat complexes in Malaysia, respectively [1]. Differences between SEECs in inclined bed dryer and the dryers studied in this research mostly contributed to the differences between moisture drop ranges and then, different drying time in tested dryers.
Fig. 5. Electrical energy consumption during drying of paddy with Industrial batchtype bed dryer (IBBD) & Industrial horizontal rotary dryer (IHRD) in northern Iran.
5
Fig. 6 shows the specific thermal energy consumption (STEC) for drying paddy in IBBD and IHRD. According to this Fig. 6, depending on the paddy final moisture level, the amount of STEC varied from 7.78 to 22.09 MJ kg1 and 11.5e36.44 MJ kg1 water removed while drying with IBBD and IHRD, respectively. Similar to the case of SEEC, the increasing trend of STEC with a decrease in final desired moisture level of paddy is related to the increased time intervals at lower moisture contents and higher values of STEC in IHRD compared to the IBBD are related to the shortcoming of the IHRD in fully discharging the exhaust drying air and feeding fresh air to the dryer inlet. The value of STEC of IBBD (7.78 MJ kg1 water removed) at the moisture drop range of 14.5 to 12%, w. b. is within the range of 3.48e10.45 MJ kg1 water removed stated for cross-flow grain dryer reported by Maier and Bakker-Arkema [16]. In addition, the value for IHRD (11.5 MJ kg1 water removed) is somewhat more than the topmost level reported for cross-flow grain dryer (10.45 MJ kg1 water removed) [16]. Thermal energy needed to drying paddy using commercial, cross flow paddy dryers in Arkansas were also reported to be within the range of 6.90e9.67 MJ kg1 water removed in 2011 and 8.81 to 9.62 in 2012 with paddy moisture drop from 19.0e21.7% to 12.2e13%, and from 15.4e18.3% to 11.7e12.2%, w. b., respectively [26]. The value of specific thermal energy consumption for IBBD (7.78 MJ kg1 water removed) at the moisture drop range of 14.5 to 12%,w.b. is within the ranges measured for commercial, cross flow paddy dryers in Arkansas, but the value for IHRD at the same conditions for IBBD (11.5 MJ kg1 water removed) is somewhat more than that reported for commercial, cross flow paddy dryers in Arkansas. It was also reported that the STEC of industrial-scale spouted bed paddy dryer prototype to be in the range of 3.5e7.00 MJ kg1 water removed to reduce paddy moisture level from 28% to 21.5%, d.b., and those of inclined bed dryers of Bukit Besar and Simpang Empat complexes in Malaysia was reported to be in the ranges of 2.77e3.27 and 3.20e3.47 MJ kg1 water removed, respectively, for paddy moisture drop from ~22.5 to 12.5%, w. b [1,27]. It is noticed that the differences between the results of these studies can be mainly due to unequal drying time for the complete drying of paddy because of different initial moisture levels, and differences in moisture drop in the studies. Moreover, the differences in drying conditions in the study of inclined bed dryer [1], such as the depth of paddy bulk in industrial inclined dryer compared to IBBD and the different airflows in dryers can explain the different results of energy efficiencies in the studies. Higher values for STEC in IBBD and IHRD in lower moisture levels compared to the similar studies can be contributed to the differences between initial moisture contents of paddy grains in the studies, the different drying way in IBBD and
Fig. 6. Thermal energy consumption during drying of paddy with Industrial batchtype bed dryer (IBBD) & Industrial horizontal rotary dryer (IHRD) in northern Iran.
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Fig. 7. Total energy consumption during drying of paddy with Industrial batch-type bed dryer (IBBD) & Industrial horizontal rotary dryer (IHRD) in northern Iran.
IHRD, and the fact of hardening the moisture removal from grain's inner layers in grain drying process [16]. The theoretical thermal energy required to dry a single grain under ideal conditions was reported to be in the range of 2.55e3.02 MJ kg1 water removed [16]. Regarding the uppermost level of foregoing STEC ranges, the energy efficiencies of IBBD and IHRD (theoretical thermal energy divided by the thermal energy supplied to the dryer) would be as 38.8 and 26.3, respectively, to dry paddy from moisture level of 14.5%e12%, w. b. However, the ideal conditions to evaporate extra water from grains are very different from true conditions of drying in industrial paddy dryers. The values of thermal energy efficiency for commercial, cross flow paddy dryers in Arkansas were also determined to be within the ranges of 26%e36% and 27%e29% in 2011 and 2012, respectively [26]. Finally, according to Fig. 7, the total specific energy consumption (SEC) as the sum of SEEC and STEC had trends similar to the trends of SEEC and STEC with a decrease in paddy final moisture level. The amount of SEC increased from 10.41 to 29.58 and 17.00e53.86 MJ kg1 water removed in drying with IBBD and IHRD, respectively. The values of SECs for IHRD were 84 and 216.7% more than those of IBBD at the first (12.0%) and the last (8.0%) paddy final moisture level (w.b.), respectively. The ratio of SEEC to SEC in all final moisture levels was around 0.25 and 0.30 for drying paddy in IBBD and IHRD, respectively. This result shows the important role of electrical energy of the total energy consumed in IHRD, compared to drying with IBBD.
Fig. 8. Comparison of head rice yield (HRY) at different final moisture content of paddy dried in industrial batch-type bed dryer (IBBD) and industrial horizontal rotary dryer (IHRD). Values with the same letters are not significantly different (p < 0.01).
significantly more than those of the IBBD samples at all corresponding paddy final moisture levels (Figs. 8 and 9). It seems that the continuous agitation of the paddy during the drying process and lower drying rate or lesser drying capacity of drying air in IHRD are the main factors which lead to the significant improvement of HRY and MRC in drying with IHRD. It has been shown that the milled rice quality can be adversely affected by drying air with high evaporative capacity [28]. The amount of HRY increased significantly from 43.11 to 54.92% and 35.03e52.77% with moisture drop in drying with IBBD and IHRD, respectively (Fig. 8). The increasing trend in HRY with a decrease in paddy moisture level can be attributed to the higher mechanical strength of rice kernels in terms of their bending strength and fracture energy at lower moisture contents [29]. There were no significant differences between HRY and MRC at paddy final moisture levels of 11.0% and 9.0%, w. b. in drying with IHRD and IBBD, respectively. A similar state is seen for HRY at paddy final moisture contents of 12.0% and 10.0%, w. b. in drying upon IHRD and IBBD, respectively (Figs. 8 and 9). As stated previously,
3.3. Product quality assessment Main effects of the dryer type (DT) and final moisture content of paddy (FMC) and DT FMC interaction effect on HRY and MRC were significant at 0.01 probability level (Table 2). Results show that the HRY and MRC of the samples obtained from IHRD were
Table 2 Mean squares of ANOVA for head rice yield (HRY), milling recovery (MRC), and degree of whiteness (WT) of dried paddy samples as affected by the dryer type and paddy final moisture content.
*
Source of variance
df
HRY
MRC
WT
Dryer type (DT) Final milling moisture level of paddy (FMC) DT FMC Error CV (%)
1 4
414.672** 271.096**
143.717** 29.271**
6.241** 9.847**
4 30 e
12.440** 0.659 1.72
2.351** 0.508 1.09
2.806* 0.789 1.53
and
**
represent significant at 0.05 and 0.01 probability levels, respectively.
Fig. 9. Comparison of milling recovery (MRC) at different final moisture content of paddy dried in industrial batch-type bed dryer (IBBD) and industrial horizontal rotary dryer (IHRD). Values with the same letters are not significantly different (p < 0.01).
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Table 3 Degree of whiteness for different final moisture content of paddy dried in industrial batch-type bed dryer (IBBD) and industrial horizontal rotary dryer (IHRD). Name of dryer
Final moisture content (w.b.) 12 ± 0.2%
IBBD IHRD
11 ± 0.2% abc
58.89 ± 0.15 59.61 ± 0.37ab
10 ± 0.2% cd
58.21 ± 0.17 60.13 ± 0.50a
9 ± 0.2% bcd
58.35 ± 0.75 57.98 ± 0.70cd
8 ± 0.2% cd
57.84 ± 0.18 57.46 ± 0.22d
55.56 ± 0.60e 57.65 ± 0.17cd
Values with the same letters are not significantly different (p < 0.05).
gentle drying with more humid drying air in IHRD associated with longer paddy residence time or lower drying rate leads to less thermal and moisture stresses in rice kernels. This can preserve end use grain quality more efficiently [16]. Therefore, the same results for main quality attributes of milled rice were achieved at higher moisture contents in drying paddy upon IHRD compared to the IBBD. Interestingly, the values of HRY, MRC, and WT for paddy dried in IHRD at 9.0% moisture level are significantly more than those of IBBD at 8.0% moisture level. Thus, all milling quality attributes for the final product of paddy dried with IHRD at 9.0% paddy moisture level are superior to those of IBBD (Figs. 8 and 9, and Table 3). Considering the non-significant differences between all quality attributes of final milled rice at 8.0% and 9.0% final paddy moisture levels in drying with IHRD, (Figs. 8 and 9 and Table 3), in order to save energy, the final paddy moisture of 9.0% is suggested as the appropriate final moisture level when drying paddy with IHRD (the SEC decreases by 10.43 MJ kg1 water removed). Therefore, the required drying time of IHRD can be shortened to about 12 h or 26% (the paddy drying residence time at 8.0% final moisture level, 46 h, can be replaced by that of 9.0% paddy final moisture level or 34 h in IHRD).
4. Conclusion The actual drying performance attributes including the drying time, specific energy consumption and final product quality of IBBD and IHRD paddy dryers were presented and discussed in this article. The HRY and MRC of the samples obtained from IHRD were significantly more than those from IBBD at all corresponding paddy final moisture levels. In order to save energy when drying with IHRD, paddy final moisture level of 9.0% was suggested as the appropriate final moisture level in drying paddy with IHRD in northern Iran. Due to the non-significant differences between final product quality attributes at 8.0% and 9.0% paddy final moisture levels, the paddy residence time in IHRD may be reduced about 12 h (~26%); therefore, specific energy consummation may be saved around 10.43 MJ kg1 water removed. It seems that the economic benefits resulting from improvement in milled rice quality, especially for HRY and MRC can compensate the cost of higher thermal energy demand in IHRD. Further studies are needed to improve the drying rate and then save energy, along with the preservation or even improvement of the final product milling quality in IHRD dryers. Among them, kick turning of the drying drum of IHRD with an appropriate rotation time interval, along with complete discharge the drying air should be considered in future studies.
Acknowledgements The authors would like to thank Rasht branch, Islamic Azad University for their financial support, rice mill of Zare in Sangar district of Rasht, center of Guilan Province, Iran, for allowing to conduct the experiments, and Rice Research Institute of Iran (RRII) for facilitating the milling tests.
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Please cite this article in press as: Firouzi S, et al., Energy consumption and rice milling quality upon drying paddy with a newly-designed horizontal rotary dryer, Energy (2016), http://dx.doi.org/10.1016/j.energy.2016.11.026