Performance of a recirculating dryer equipped with a

Drying Technology An International Journal

ISSN: 0737-3937 (Print) 1532-2300 (Online) Journal homepage: http://www.tandfonline.com/loi/ldrt20

Performance of a recirculating dryer equipped with a desiccant wheel Sadjad Abasi, Saeid Minaei & Mohammad Hadi Khoshtaghaza To cite this article: Sadjad Abasi, Saeid Minaei & Mohammad Hadi Khoshtaghaza (2016) Performance of a recirculating dryer equipped with a desiccant wheel, Drying Technology, 34:8, 863-870, DOI: 10.1080/07373937.2015.1021421 To link to this article: http://dx.doi.org/10.1080/07373937.2015.1021421

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Date: 23 June 2016, At: 00:25

DRYING TECHNOLOGY 2016, VOL. 34, NO. 8, 863–870 http://dx.doi.org/10.1080/07373937.2015.1021421

Performance of a recirculating dryer equipped with a desiccant wheel Sadjad Abasi, Saeid Minaei, and Mohammad Hadi Khoshtaghaza

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Biosystems Engineering Department, Tarbiat Modares University, Tehran, Iran ABSTRACT

KEYWORDS

This article reports the incorporation of a rotary desiccant wheel unit into an air recirculated convective dryer and testing it by drying corn kernels. Experiments were conducted with and without the desiccant wheel at air temperatures of 50, 60, and 70°C and flow rates of 1, 1.4, and 1.8 kg/min. The effect of drying temperature, air flow rate, and desiccant wheel on drying time, drying rate, energy consumption, and specific moisture extraction rate were investigated. Statistical analysis of data showed that air drying temperature and air flow rate had significant effects on drying time and drying rate and the effect of desiccant wheel on drying time was significant. Results indicated that a desiccant wheel is an economical and useful system to utilize in dryers because it decreases drying time while increasing the drying rate and has a positive influence on energy consumption.

Desiccant wheel; drying performance; energy consumption

Introduction Drying is a method for preservation of fruits and vegetables in which moisture content and water activity of the crop are decreased to minimize chemical, biochemical, and microbiological deterioration. The main objective of drying is to lower the moisture content of agricultural products to a certain level, suitable for long-term storage.[1] Drying is an energy-intensive process, accounting for about 12% of the total energy consumption in the world. The most common energy sources in industrial dryers include natural gas, propane, and other fossil fuels. The low-term rise in fuel prices has increased drying costs. Thus, economic reasons and shortage of these energy sources have led drying industries and researchers to look for ways to reduce energy consumption during drying.[2,3] In convective dryers, a large amount of energy (70–174 kcal/kg of removed water) is released to the environment because the exhaust air has high temperature and humidity (not saturated with water vapor).[3] Therefore, efforts have been focused on recycling part of the exhaust air into the drying chamber. The recycled air is mixed with the ambient air and reused for drying of the material. According to previous studies, this technique improves energy efficiency for drying but increases drying time, which can have detrimental effects on the quality of the dried material.[3,4] Thus, using dehumidification systems for removal of air humidity to be recycled has

been the focus of recent research efforts. Dehumidification of the drying air decreases drying time, which improves the quality of the final product.[5] The methods utilized for removal of moisture from air in dryers include dehumidification in a heat pump drying system, a refrigeration dehumidification system for decreasing air temperature below the dew point, and moisture adsorption by means of a desiccant system.[6] In recent years, rotary desiccant wheels (DWs) have been used for dehumidification due to benefits such as low energy requirement so that solar energy and waste heat energy can be used for its operation, no generation of pollutant gasses, low development and operation costs, and suitability for a wide range of climates.[7–9] An overview of adsorbents in the rotary desiccant dehumidifier for air dehumidification was presented by Wang et al.[10] They concluded that heat of adsorption should be taken into consideration because it increases the temperature of the desiccant material and results in decreased adsorption capacity. According to their studies, the effectiveness of the dehumidification process depends on the adsorbent and adsorbate as well as the substrates, environmental conditions, and variables.[10] Numerous investigations have been conducted to determine the energy efficiency of DWs. A numerical simulation for the amount of energy and cost savings was obtained with a desiccant system combined with other cooling alternatives in the air conditioning of a hot and humid climate. According to

CONTACT Saeid Minaei [email protected] Biosystems Engineering Department, Tarbiat Modares University, P.O. Box 14115-336, Tehran 14117 13116, Iran. Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/ldrt. © 2016 Taylor & Francis

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the results, when using solar energy to regenerate the DW, the use of a DW will be an arrangement with lower operational cost compared to that without a DW.[11] A comparison between air regeneration and superheated steam regeneration of a DW with zeolite as the adsorbent was done by Goldsworthy et al.[12] According to the experimental results, the regeneration temperature for superheated steam is higher than that for air.[12] Yahya et al.[13] used a DW for drying of guava at low temperatures. Studies show that drying at low air temperatures decreases the rate of Maillard reactions, especially for sensitive products such as mushrooms.[14] Hodali and Bougard[15] developed a solar dryer equipped with a DW unit of silica gel for drying apricots. Results showed that use of the DW decreased drying time from 52 to 44 h while improving the quality of the final product. They concluded that DW is useful when air drying temperatures are near ambient temperature and when drying in the falling rate period.[15] In another study, a DW with silica gel as the adsorbing material was integrated into a dryer for drying corn and milo. Solar energy was used for regeneration of the adsorber. Results showed that use of the DW decreased drying time.[16] A solar dryer with and without desiccant bed material of CaCl2 was tested for drying of green peas. Results indicated that the drying process can be continued in off-sunshine hours and that the quality of the product improved with the desiccant unit. Additionally, moisture pickup efficiency increased up to 63% and more uniform drying of the product was observed.[17] Sahnmungam and Natarajan[18] evaluated an indirect convection dryer with silica gel desiccant and a reflective mirror by drying of green peas and pineapple slices. Based on the results, integration of the desiccant bed decreased drying time and improved product quality. The desiccant material removed 40% of the product moisture.[18] A solar dryer equipped with a silica gel desiccant unit was made to retain the quality of dates in storage. The drying efficiency of this system was maximum at noontime when sunshine is intense.[19] Most of the previous studies focused on the integration of a desiccant unit into solar dryers at the inlet of the drying chamber. The effect of a desiccant unit on energy consumption and energy efficiency has not been investigated at different air temperatures and flow rates. Therefore, in this study, a DW unit was integrated in the path of the recirculated air in a recirculating convective dryer. The effect of incorporation of the DW on drying time, drying rate, energy consumption, and specific moisture extraction rate of corn kernels at various air temperatures and flow rates was investigated in this study.

Materials and methods The corn samples (KSC 704 variety) were stored at 3°C and 2% relative humidity. Experiments were conducted in a hot air convective recirculating dryer at the Biosystems Engineering Department of Tarbiat Modares University, Tehran, Iran. An AC blower (Iran) and an electric heater were used to supply the required hot air to conduct the experiments. During the process, the drying temperature and flow rate of the inlet air were measured using an LM35 Thermal IC (China) with an accuracy of �0.5°C and a Lutron-YK 80 AM anemometer (Taiwan) with an accuracy of �0.1 m/s, respectively. The corn samples (KSC 704 variety) were weighed during drying using a Zemic L6D load sensor (Netherlands) with an accuracy of �1 g connected to a TIKA TD1000 indicator (Iran). Ambient air temperature was 21–24°C and relative humidity was in the range of 19–28% during the experiment. Before drying, the dryer worked for half an hour to attain steady-state condition. The initial moisture content of the samples was measured as approximately 34.5% (db) using a PM-600 Seed and Grain Moisture Tester (Japan) with an accuracy of �0.1%.Then, 10 kg of corn kernels were dried to 18% (db) at air temperatures of 50, 60, and 70°C and flow rates of 1, 1.4, and 1.8 kg/min. Experiments were conducted with and without a DW where 60% of the exhaust air was mixed with ambient air and recirculated into the chamber inlet. Figure 1 shows the recirculating dryer equipped with the measurement system and DW. The rotary DW consists of adsorption and regeneration sections that are filled with a solid desiccant material. It rotates between the process humid air and a heated regeneration air stream. The humid air flows

Figure 1. Dryer and desiccant wheel used in the study: (a) computer, (b) wattmeter, (c) load weight indicator, (d) weighting device, (e) dryer chamber, (f) counterweight holder, (g) AC centrifugal blower, (h) heater, (i) desiccant wheel assembly.

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through the adsorption section and its moisture is removed by the adsorber. The adsorbing gel is saturated with moisture in the adsorption section and thus needs to be reactivated. Desiccant regeneration is performed in the regeneration section by the heated air stream. Previous work has shown that at high air flow rates, silica gel performs better than activated alumina and activated charcoal in regeneration and adsorption processes.[20] Due to its low regeneration temperature, silica gel was used as the adsorbing material.[21] The optimum values for DW parameters have been reported as follows: regeneration temperature of 60–90°C, air flow rate of 1–5 kg/min, wheel thickness of 18–26 cm, wheel rotation speed of 15–60 rph, and regeneration ratio of 1/4–1/2.[22,23] These parameters were selected as 60°C, 1 kg/min, 30 cm, 30 rph, and 1/4, respectively, for this study. The DW was rotated using a 14-W AC electro-motor, and a 0.373-W centrifugal blower and 300-W heater were used to supply the air for regeneration of the silica gel.

Drying rate The drying rate is proportional to the difference in moisture content between two consecutive measurements and is defined as Eq. (1)[24]: Drying rate :

DM ¼ Dt

MtþDt Mt : Dt

ð1Þ

Energy consumption During drying, energy consumption for the recirculating dryer was measured using an AC wattmeter shown in Fig. 1. Another AC wattmeter (Lutron DW-6060) was utilized for measurement of DW energy consumption.

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Table 1. Results of ANOVA for drying time and drying rate of maize kernels. Mean square Source of variation

df

Drying time

Drying rate

T FL DW T � FL T � DW FL � DW T � FL � DW Error Total

2 2 1 4 2 2 4 36 53

74,574.130* 92,590.037* 66,360.167* 534.074* 28.389 n.s. 69.500 n.s. 49.556 n.s. 74.481

8.271 � 10 5* 9.804 � 10 5* 1.202 � 10 5 n.s. 7.184 � 10 7 n.s. 3.031 � 10 8 n.s. 1.689 � 10 5 n.s. 1.058 � 10 7 n.s. 3.595 � 10 6

*P < 0.01, n.s. ¼ not significant.

Results and discussion Table 1 shows the results of ANOVA for drying time and drying rate. The effects of drying temperature, air flow rate, DW and the interaction effect of drying temperature and air flow rate on drying time are significant at P < 0.01. In addition, results show that only drying temperature and air flow rate significantly affect the drying rate, whereas other parameters, in particular, DW, had no significant effect on drying rate. The effects of the other independent variables on drying time and drying rate were not significant. Figures 2–4 show the drying curves of corn at various air drying temperatures and flow rates. The maximum drying time was observed at 50°C and air flow rate of 1 kg/min (458 min) without DW, whereas its minimum value was 190 min at 70°C and air flow rate of 1.8 kg/ min with the DW. Increasing the drying temperature from 50 to 60°C and 60 to 70°C decreased the drying time by an average of 17.7 and 16.9% with the DW, respectively, and increased the drying rate by 19.7 and 19.9% without the DW, respectively. As a result of the temperature increase from 50 to 60°C and 60 to 70°C with the DW, drying time was cut by an average of 16.4 and 16.9% and the drying rate increased by 23.5

Specific moisture extraction rate Specific moisture extraction rate (SMER) is another indicator of the dryer performance and is expressed as follows[17]: SMER :

Mi Mf DM � ¼ : Dttot � E Eheater þ Efan Dttot

ð2Þ

Statistical analysis Data were statistically analyzed using SPSS 16.0 software. One-way analysis of variance (ANOVA) in the form of a completely randomized design was applied to investigate the effect of drying temperature, air flow rate, and DW on drying time and drying rate of maize kernels.

Figure 2. Corn drying curves for various air temperatures, with or without use of the DW, at air mass flow rate of 1 kg/min.

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Figure 3. Corn drying curves for various air temperatures, with or without use of the DW, at air mass flow rate of 1.4 kg/min.

and 18.5%, respectively. The temperature difference between the drying air and corn kernel increases and causes more heat transfer from the drying air to the product, which increases its temperature. Thus, moisture removal continues at steeper rates so that drying time decreases and the drying rate increases. This is in agreement with the results of previous studies on fruits and grains.[1,25,26] An increase in the air flow rate from 1 to 1.4 kg/min and 1.4 to 1.8 kg/min reduced drying time by an average of 16.2 and 12.8% and increased the drying rate by 16.4 and 23.7% without the DW, respectively. Moreover, comparison of the means showed that drying time and drying rate at a flow rate of 1.4 kg/min were 13.4% lower and 15.8% higher than those at 1 kg/min. These were respectively 15.2% higher and 25.7% lower than those at 1.8 kg/min with the DW. As a result of the increased air flow rate, the quantity of drying air builds up and leads to an increased capacity to remove moisture from the corn kernels. Similar results have been reported by researchers for other crops.[27–29]

Figure 4. Corn drying curves for various air temperatures, with or without use of the DW, at air mass flow rate of 1.8 kg/min.

Based on Figs. 2 to 4, integration of the DW decreased drying time while increasing the drying rate in all treatments. Humidity of the exhaust air is adsorbed by the silica gel when passing through the process section, and its humidity is reduced. Thus, humidity of the air entering the drying chamber decreases and moisture removal from corn kernels improves, which leads to decreased drying time and increased drying rate.[14] In addition, based on Figs. 2 to 4, it is clear that the DW shortens the drying time more effectively in the final period of drying process and there is no considerable difference between the drying curves with and without DW in the initial period of drying. This is why the DW has no significant effect on drying rate. The effects of the integration of the DW on drying time and drying rate change are shown in Table 2. With the DW in operation, as the air drying temperature and flow rate are increased, the decrease in drying time and increase in drying rate diminish. In other words, drying performance is improved at low values of air temperature and flow rate, because silica gel can better adsorb the humidity of the process air at low air temperatures and flow rates.[30] The maximum decrease in drying time (15.9%) and increase in drying rate (13.6%) were observed at 50°C and 1 kg/min with the DW. In addition, the minimum decrease in drying time (5.9%) and increase in drying rate (6.9%) were observed at 70°C and 1.8 kg/min. Use of the DW, on average, decreased the drying time by 9.75% and increased the drying rate by 7.85%. Madhiyanon et al. integrated a DW unit of silica gel into a recirculating convective dryer. This reduced the drying time of chopped coconut pieces by 25% compared to a conventional hot air–drying system.[5] Values of energy consumption for drying of corn in the dryer with and without the DW are presented in Table 3. The energy consumed for reducing the moisture content of corn kernels from approximately 34.5 to 18% (db) increased with drying temperature and air flow rate with and without the DW. Enthalpy of air increases with drying temperature and flow rate, which results in higher energy consumption for drying. Similar results have been obtained in previous studies with other agricultural products.[31,32] The minimum value for energy consumption was measured as 36.1 kWh at 50°C and 1 kg/min without the DW, whereas the maximum value was 61.8 kWh at 70°C and 1.8 kg/min with the DW. The energy consumption without the DW increased with drying temperature from 50 to 60°C and 60 to 70°C by an average of 18.6 and 8.6%, respectively. Increasing the

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Table 2.

Effect of integration of the desiccant wheel on percentage change in drying time and drying rate. Drying time 50

1 1.4 1.8 Average

15.9 13.7 12.5 14.03

þ ¼ Increase;

Drying rate

Temperature (°C)

Air flow rate (kg/min)

Table 3.

60

70

13.4 11.2 10.3 11.63

12.1 10.4 8.9 10.47

Average 13.8 11.77 10.57 9.75

Temperature (°C)

Air flow rate (kg/min)

50

60

70

Average

1 1.4 1.8 Average

þ13.6 þ10.4 þ9.2 þ8.6

þ11.2 þ9.9 þ8.3 þ8.2

þ9.9 þ5.6 þ6.9 þ7.5

þ11.8 þ8.7 þ2.3 þ7.85

¼ decrease.

Values of energy consumption (kWh) for drying of corn under different conditions. Without desiccant wheel

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Air flow rate (kg/min) 1 1.4 1.8 Average

With desiccant wheel

Temperature (°C) 50

60

70

Average

Air flow rate (kg/min)

50

60

70

Average

36.1 38.3 40.1 38.2

42 45.2 48.7 45.3

45.4 48.9 53.4 49.2

41.2 44.1 47.4 44.2

1 1.4 1.8 Average

46.6 47.9 50.2 48.2

51.5 53 56.7 53.7

54.3 57.8 61.8 58.0

50.8 52.9 56.2 53.3

air flow rate from 1 to 1.4 kg/min and 1.4 to 1.8 kg/min increased the energy consumption by 7 and 7.5%, respectively. Energy consumption with the DW at 60° C was on average 11.4% higher than that at 50°C and was 8% less than that at 70°C. In addition, as a result of an increase in the air flow rate from 1 to 1.4 kg/ min and 1.4 to 1.8 kg/min, energy consumption increased on average by 4 and 6.2%, respectively. It is clear that the rate of increased energy consumption diminishes when operating the DW. Processing the exhaust air from the drying chamber through the DW increases its temperature due to heat transfer from the heated silica gel (in the regeneration section) to the air.[33] Thus, the temperature of the mixed air entering the drying chamber increases, resulting in less power consumption by the heater. Moreover, the shorter drying time with the DW improves this outcome. Table 4 shows the effect of DW on the increase in energy consumption for corn drying. Based on the results, integration of the DW has, on average, increased the total energy consumption by 20.7% due to the energy required for regeneration of air being supplied for regeneration of the silica gel. ΔE decreases with air temperature and flow rate because the regeneration energy changes with drying

Table 4. Effect of desiccant wheel on percentage increase in energy consumption (%ΔE) for drying of corn. Temperature (°C) Air flow rate (kg/min) 1 1.4 1.8 Average

Temperature (°C)

50 29.1 22.6 19.6 23.8

60 25.1 17.3 18.2 20.2

70 25.2 16.4 15.7 19.1

Average 24.5 18.8 17.8 20.7

conditions, namely, lower drying times. The lowest increase in energy consumption occurred at an air temperature of 70°C and flow rate of 1.8 kg/min (15.7%), and the highest value was at 50°C and 1 kg/ min (29.1%). The energy required for supplying regeneration air is displayed in Fig. 5. Regeneration energy decreases with increasing temperature and flow rate of the drying air because regeneration air is supplied at stable conditions (temperature and flow rate), but drying time varies with drying temperature and flow rate, thus affecting the regeneration energy requirement. Drying time is shorter at high air temperatures and flow rates, which results in lower energy consumption for regeneration of silica gel. Regeneration energy was measured as 13.9– 17.3 kWh. In a previous study, this energy at a regeneration temperature of 100°C and air velocity of 0.14 m/s was reported to vary between 13 and 16 MJ/kg of water, [5] which is close to the range measured here. It should be noted that although incorporation of the DW increases the total energy requirement for drying of corn, regeneration energy can be supplied using sunshine or other low-cost energy sources such as biomass, which can make it effective and economical for drying processes. Table 5 shows the effect of DW on the reduction in energy consumption (%) of the air recirculating dryer (energy consumed for regeneration of silica gel not included). It is clear that if the regeneration energy of the silica gel is supplied by alternative energy sources such as sunshine, drying energy consumption decreases for all drying conditions, indicating the usefulness of DW utilization. Moreover, it can be seen that the DW is more effective at lower air temperatures and flow rates

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Figure 5. Relationship of regeneration energy requirement with use of the DW versus air temperature for various mass air flow rates.

because the reduction in energy consumption is lower at high air temperatures and flow rates because energy saving is lower at high temperatures and flow rates. Desiccant materials tend to release moisture at high temperatures so that hot air is required to reactivate them. In addition, at high air flow rates there is not enough time for the desiccant material to extract moisture from the air passing from desiccant bed.[34] Thus, excluding regeneration energy, integration of the DW has, on average, decreased the dryer energy consumption by 15%. Table 6 shows the effect of the DW on SMER under various drying conditions. Comparison of dryers with

and without the DW shows that integration of the DW decreased the SMER due to the extra energy required for regeneration of silica gel. However, if the regeneration energy is not included in energy consumption calculations, the SMER is improved with the use of the DW. Values of SMER varied from 0.026 up to 0.039 kg/kWh without the DW and 0.029 to 0.048 kg/ kWh with the DW, showing an increase of 11.5–25%. These values for total energy consumption (including regeneration energy) were between 0.023 and 0.03 kg/ kWh. Shanmugam and Natarajan[17] reported the SMER of a solar dryer equipped with a DW to be in the range of 0.55 to 0.82 kg/kWh.[17]

Conclusions Table 5. Effect of desiccant wheel on percentage energy savings in drying of corn without including silica gel’s reactivation energy. Temperature (°C) Air flow rate (kg/min) 1 1.4 1.8 Average

50

60

70

Average

18.8 16.7 14.3 16.6

18 16.8 12.7 15.8

14.2 13.7 10.3 12.7

17 15.7 12.4 15

Table 6. Specific moisture extraction rate with and without the desiccant wheel. Treatment

Without DW

With DW

Total

FL1 FL2 FL3 FL1 FL2 FL3 FL1 FL2 FL3

0.039 0.036 0.035 0.033 0.031 0.029 0.031 0.029 0.026

0.048 0.045 0.041 0.040 0.037 0.033 0.036 0.033 0.029

0.03 0.029 0.028 0.027 0.026 0.025 0.026 0.024 0.023

T1 T1 T1 T2 T2 T2 T3 T3 T3

The effect of a desiccant unit on the drying characteristics of maize kernels was studied in an air recirculating dryer. Without the DW, increasing the air temperature from 50 to 60°C and 60 to 70°C, on average, decreased the drying time by 17.7 and 16.9% and increased the drying rate by 19.7 and 19.9%, respectively. As a result of increasing the drying temperature from 50 to 60°C and 60 to 70°C with the DW operating time, drying time was cut by an average of 16.4 and 16.9% and the drying rate increased 23.5 and 18.5%, respectively. Utilization of the DW, on average, decreased drying time by 9.75% and increased the drying rate by 7.85%. Energy requirement for regeneration of the silica gel in various treatments was measured as 13.9–17.3 kWh. Desiccant regeneration energy notwithstanding, use of the DW decreased drying energy by an average of 15% and increased the SMER from 11.5 to 25%. If the regeneration energy requirement is included, total

DRYING TECHNOLOGY

energy consumption increases by an average of 20.7% and SMER decreases from 23.1 to 11.5%. Therefore, if desiccant regeneration energy is supplied from sunshine or other low-cost energy sources, use of a DW may be recommended for economical reasons as well as due to its positive effect on drying performance.

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Nomenclature E ΔE Efan Eheater FL Ṁ ΔM Mf Mi Mt Mtþdt T Δt Δttot

Energy consumed by the dryer (kW) Percentage increase in energy consumption (%) Energy consumed by the fan (kW) Energy consumed by the heater (kW) Air flow rate (kg/min) Total moisture removal rate for each treatment (kg/h) Mass difference during a period of time (kg water/kg dry matter) Final mass of the corn (kg) Initial mass of the corn (kg) Moisture content of material at time t (kg water/kg dry matter) Moisture content of material at time t þ Δt (kg water/kg dry matter) Temperature (°C) Time difference (min) Total drying time of each treatment (h)

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