Initial Condensation During Superheated Steam Drying and the Effect of Orientation of DSG Pellets During the Drying Process

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1 The Canadian Society for Bioengineering The Canadian society for engineering in agricultural, food, environmental, and biological systems. La Société Canadienne de Génie Agroalimentaire et de Bioingénierie La société canadienne de génie agroalimentaire, de la bioingénierie et de l environnement Paper No. CSBE Initial Condensation During Superheated Steam Drying and the Effect of Orientation of DSG Pellets During the Drying Process Justin Bourassa Department of Biosystems Engineering, University of Manitoba, Canada. umbouras@myumanitoba.ca Rani Puthukulangara Ramachandran Department of Biosystems Engineering, University of Manitoba, Canada. puthuku3@myumanitoba.ca Jitendra Paliwal Associate Professor, Department of Biosystems Engineering, University of Manitoba, Canada. J.Paliwal@umanitoba.ca Stefan Cenkowski Professor, Department of Biosystems Engineering, University of Manitoba, Canada. Stefan.Cenkowski@umanitoba.ca Written for presentation at the CSBE/SCGAB 2015 Annual Conference Delta Edmonton South Hotel, Edmonton, Alberta 5-8 July 2015 ABSTRACT Distiller s spent grain (DSG), which is a major by-product of ethanol production, contains high amounts of protein and fiber making it an excellent supplement for cattle feed. However, the high initial moisture content of DSG makes it unsuitable for long term storage and transportation due to spoilage. Superheated steam (SS) drying can be used as an alternative to air drying for biological materials such as DSG. One characteristic unique to SS drying is the presence of initial condensation on the material at the start of drying. The objective of this research was: (i) to measure the maximum amount of condensation during SS drying for the temperature range between 120 and 180C and SS velocity from m/s, and (ii) to determine the effect of the orientation of pellets with respect to SS flow on moisture diffusivity. Drying characteristic curves were modeled based on Page s equation. Moisture diffusivity was calculated using a finite cylinder Papers presented before CSBE/SCGAB meetings are considered the property of the Society. In general, the Society reserves the right of first publication of such papers, in complete form; however, CSBE/SCGAB has no objections to publication, in condensed form, with credit to the Society and the author, in other publications prior to use in Society publications. Permission to publish a paper in full may be requested from the CSBE/SCGAB Secretary, Department of Biosystems Engineering, E2-376 EITC Bldg, 75A, Chancellor Circle, University of Manitoba, Winnipeg, Manitoba, Canada R3T 5V6, bioeng@csbe-scgab.ca. The Society is not responsible for statements or opinions advanced in papers or discussions at its meetings.

2 model accounting for any change in dimensions that the DSG pellets experience while drying is SS. Results have shown that orientation has a significant effect on drying characteristic when comparing specific drying periods (constant vs falling drying-rate period). The overall mass diffusivity was higher for the vertical orientation only in the first drying rate period. The results of this study could help to develop a computer model to simulate SS drying of biological materials such as DSG. Keywords: distillers spent grains, superheated steam, moisture diffusivity, drying. INTRODUCTION Distillers spent grain (DSG) is a major by-product of ethanol production. DSG is also a by-product from producing alcoholic beverages. It has been reported by Bonnardeaux, J. (2007) that the fermentation of one ton of wheat or corn produces approximately 375 L of ethanol. The DSG that remains after fermentation per ton of wheat or corn is approximately 470 kg (Graham, et al., 2013). The high initial moisture content of the DSG slurry (also known as whole stillage) which leaves the distillation column makes it impractical for transportation and long-term storage. In general, the whole stillage is screened and pressed or centrifuged to obtain thin stillage and coarse grains. The thin stillage is later evaporated to produce thick syrup with high nutritional value for livestock or added back to the spent grains during drying to produce DSG with solubles (Stroem, et al., 2009). The remaining coarse grain fraction is usually dried to about 10-12% wb (wet basis) in rotary driers using hot air (Bonnardeaux, J., 2007; Johnson, et al., 2014). The coarse grain fraction of DSG has been found to be an excellent source of protein and energy (Nyachoti, et al., 2005). DSG makes a good supplement for swine and cattle feed since it contains high amounts of digestible fibre, protein, and fatty acids, as well as a low amount of starch (Cromwell, et al., 1993; Ayoade, et al., 2014). Superheated steam (SS) drying can be used as an alternative to the traditional hot air drying methods of DSG. In general, the benefits of SS drying in comparison to hot air drying have been reported to include lower energy consumption, elimination of risk for fire, the elimination of harmful emissions, and preservation of nutritional quality (Pronyk, et al., 2010; Cenkowski et al., 2012). The elimination for risk of fire exists because the replacement of hot air with SS eliminates oxygen from the drying medium, which also reduces the potential for oxidation (Head, et al., 2011). Furthermore, it has been suggested by Pronyk et al. (2008) that the future of utilizing SS may not always be in the form of strictly drying, but also decontamination and product processing. Deoxynivalenol (DON) is a contaminant that is commonly found in wheat and corn crops, and consequently is present in DSG. DON is problematic for DSG consumption by swine in particular because higher levels can result in oxidative stress, increasing intestinal permeability, and inhibiting protein synthesis (Wu, et al., 2014). In a study by Pronyk, et al. (2006) it was found that DON levels were reduced by up to 52% in naturally contaminated wheat kernels when processed in SS at 185 C. Current hot air drying techniques are unable to reduce mycotoxins to such a level (Cenkowski, et al., 2007). The initial condensation period, which is unique to SS drying, occurs when a material at a temperature lower than the saturation temperature is exposed to SS. This stage is very important for the modeling of SS drying characteristics as it affects drying kinetics (Taechapairoj, et al., 2003). The SS coming in contact with the material then becomes cooled and condenses on the material surface as sensible heat is transferred to the material. This results in an overall increase in mass of the sample resulting in increased moisture content. This phenomenon can have a significant effect on the material quality after drying due to quicker heating of the interior of the material (Iyota, et al., 2001). Depending on the porosity of the material the condensed moisture has the ability to permeate into the interior of the material. A study by Iyota et al. (2001) reported that the moisture content slightly inside the surface of potato samples had higher moisture content than the original value shortly after the start of SS drying. Once the material surface temperature increases to the 2

3 saturation temperature the condensation period is over and the restoration period begins. During the restoration period the initial condensation evaporates and the moisture content of the material returns to its original moisture content. The determination of moisture diffusivity within a food material over the course of drying can be a complex task since it can exist in many forms such as molecular diffusion, liquid diffusion, vapour diffusion, hydrodynamic flow, capillary flow, Knudsen flow, and surface diffusion (Karathanos et al., 1990; Hashemi, et al. 2009; Kim & Bhowmik, 1995). Specifically for granular material it has been widely accepted that the majority of moisture loss is comprised of liquid and vapour diffusion. Therefore, the term effective diffusivity is used to describe all possible mechanisms of moisture movement within a biological material. For drying applications, replacing the diffusion coefficient with effective diffusivity helps to alleviate the problem of unsteady moisture diffusion (Maroulis, et al., 2001). Knowing the effective moisture diffusivity of a material is necessary for designing and modeling mass transfer processes such as dehydration, adsorption, and desorption of moisture during storage (Erdogdo & Turhan, 2006). Fick s law of diffusion incorporated into drying experiments has been widely used to determine effective moisture diffusivity for many biological materials (Da Silva, et al., 2008). The criteria for using either finite or infinite geometry assumptions have been discussed previously in detail (Ruiz-Lopez, et al., 2013; Erdogdu and Turhan, 2006) with the conclusion that inadequate assumptions can lead to significant errors with respect to analysis of mass transfer properties. In the study done by Da Silva et al. (2008), an infinite cylinder model is suggested for use when the radius of the product is much smaller than its length. It has been suggested by Incropera & DeWitt (1996) that an infinite cylinder model could be used accurately if the length of the sample is at least ten times larger than its radius. However, in a study done by Rovedo & Viollaz (2000) it has been suggested that even with the length being twenty times larger than its radius the model can be inaccurate and the finite models should be used. Furthermore, it has been shown that neglecting volumetric changes of biological materials over the course of drying can lead to inaccurate predictions of moisture content, which directly affect the calculated moisture diffusivity values (Thuwapanichayanan, et al., 2011). The effect of SS temperature on the effective moisture diffusivity of DSG was reported by Zielinska and Cenkowski (2012) who used a Teflon sphere coated with a layer of DSG and accounted for volumetric shrinkage in their mathematical simulation. They concluded that the overall moisture diffusivity increased with a decrease in moisture content and increase in temperature of SS. The objective of this research was: (i) to measure the maximum amount of condensation during SS drying for the temperature range between 120 and 180 C and SS velocity from m/s, and (ii) to determine the effect of the orientation of pellets with respect to SS flow on moisture diffusivity. MATERIALS AND METHODS Raw Material The DSG used in this study was a mixture of 90% corn and 10% wheat stillage obtained from a local distillery (Mohawk Canada Limited, a division of Husky Oil Limited, Minnedosa, MB). The raw material was kept in a freezer at -15 C in a plastic pail sealed with a lid. Before sample preparation, the sealed plastic pail was taken out of the freezer and allowed to thaw for 2 days. Initial Sample Preparation The DSG was centrifuged in a Sorvall General Purpose, RC-3 centrifuge (Thermo Scientific Co., Asheville, NC) to separate the raw material into different fractions. The centrifuge was operated at a relative centrifugal force of 790 g, with four 1000 ml sample containers, rotating at a speed of 2200 rpm on a radius of m for 10 min. After centrifugation, the liquid fraction (thin stillage) was discarded. The remaining semi-solid solubles fraction and coarse grain fractions were placed in separate airtight bags and stored in a freezer. Before each set of experiments, the required amount of sample was thawed at room temperature 3

4 for 3 h. The initial moisture content of the coarse grain fraction was 78.9% wb (wet basis). Moisture content was calculated using the air-oven drying method (AACC standards, 2000). Pelletizing the Sample Thawed coarse grain was placed in an air-oven (Thermo Electron Corporation, Waltham, MA) at 50 C and allowed to dry to a moisture content of 0.33 db (dry basis). Either 2.1 g of sample (short pellet) or 4.2 g (long pellet) was placed inside a cylindrical mold with a diameter of 12.1 mm depending on the experiment to be conducted, initial condensation or diffusivity, respectively. A cylindrical die attached to a 10 kn load cell on a universal testing machine (Model 3366 Universal Testing Systems, Instron Corp., Norwood, MA) applied a pressure of 60.3 MPa to the sample for 5 minutes. The sample was then removed from the mold and a vernier caliper with an accuracy of 0.01 mm was used to measure the pellets length and diameter. The mass of the pellet was measured using an electronic balance with an accuracy of g (Adventurer Pro AV313, Ohaus Corporation, Pine Brook, NJ). Superheated Steam Dryer The SS drying equipment was designed and fabricated by the Department of Biosystems Engineering at the University of Manitoba. The system is comprised of a steam generator (boiler), a pressure reducing valve, a superheater, a drying chamber, as well as conveying pipelines, valves, and a data acquisition system. The system has previously been described in detail (Pronyk, et al., 2010). Initial Condensation Experiments A total of 4 short pellets were made using the procedure outlined above for each trial. The pellets were placed on a holding tray and kept in place with 4 thin steel pins. The holding tray was inserted into the SS drying chamber and attached to a thin still rod which was attached to the underfloor weighing hook of an electronic balance (PGW 453e, Adam Equipment Inc., Danbury, CT) located above the drying chamber. The pellets were dried for 5 minutes with three different SS temperatures (120, 150 and 180 C) and three SS velocities (1.0, 1.2, and 1.4 m/s). All trials were conducted in triplicate. A schematic showing the drying chamber, holding tray and position of the electronic balance can be seen in Fig. 1. Figure 1. Schematic diagram of the SS drying chamber showing the position of the holding tray and electronic balance used for mass recording. 4

5 Diffusivity Experiments Two long pellets were made using the procedure outlined above for each trial. The pellets were stored in a saturated NaCl desiccator overnight which allowed them to relax to uniform dimensions and moisture content. The pellets were placed on the holding tray oriented either horizontally or vertically with respect to the direction of SS flow, and then placed in the SS drying chamber for 90 min with a SS temperature of 120 C and a SS velocity of 1.0 m/s. Both trials were conducted in triplicate. The same electronic balance recorded the mass of the samples over the course of drying. A finite cylinder model, which has been published by Pabis et al. (1998), was used to calculate effective moisture diffusivity: MR exp μ μ (1) Where and were calculated using the following equations: (2) (3) μ 2 1 (4) μ 0 (5) Where J 0 represents the Bessel function of the first kind of order zero. The value calculated for K at any time was found using the following equation: Where and refer to the average radius and length of the two pellets at given time respectively. MR( ) is defined as the moisture ratio and is a function of time: MR (7) Where M( ) is the average moisture content of the two pellets at given time after accounting for the lifting force, M 0 is the average initial moisture content of the two pellets, and M e is the equilibrium moisture content determined experimentally. Using Eqn. 1 with 100 terms (with both n and m ranging from 1 to 10) to calculate the Fourier number (Fo mass ), the effective moisture diffusivity value (D m ) can be calculated: (8) Using the average MR( ) values calculated from Eqn. 7, the coefficients k and m for the Page equation (Eqn. 9) were found for each orientation using nonlinear regression in order to model the drying characteristic curve: MR exp (9) RESULTS AND DISCUSSION Initial Condensation Experiments The average values for maximum condensation, condensation time, and restoration time that were calculated from the data obtained from the electronic balance are shown in Table 1. (6) 5

6 Table 1. Average maximum condensation, condensation time, and restoration for DSG pellets dried under varying SS temperatures and velocities Specifications of four experimental drying runs used for validation SS Temperature ( C) SS Velocity (m/s) Maximum Condensation (g) Condensation Time (s) Restoration Time (s) Increasing the SS temperature from 120 to 180 C reduced the maximum amount of condensation present on DSG pellets by 97%. Furthermore, the condensation time was reduced 90% over the same temperature range. The 1.2 and 1.4 m/s velocities were not included for the SS temperature of 180 C since only a very small amount of condensation was found for the 1.0 m/s trials. Diffusivity Experiments The drying characteristic modeled by the Page equation (Eqn. 9) for both orientations of DSG pellets is shown in Fig. 2. The coefficients for the Page for the horizontal drying experiments were calculated to be k = and m = 1.35 and for the vertical drying experiments were calculated to be k = and m = 1.05 when θ is in s. Figure 2. Drying characteristic based on the Page equation for different orientations of DSG pellets dried in SS at 120 C. Using the above data sets in the finite cylinder equation (Eqn. 1) the effective moisture diffusivity characteristics for both orientations was obtained and is shown in Fig. 3. 6

7 Figure 3. Effective moisture diffusivity characteristic for horizontal and vertically oriented DSG pellets dried in SS at 120 C. The effective moisture diffusivity values for horizontally oriented pellets ranged from to m 2 /s. The effective moisture diffusivity values for vertically oriented pellets ranged from to m 2 /s. The diffusivity values for both orientations were within the general range for food materials (Rafiee, et al., 2010). Effective moisture diffusivity began to decrease once the DSG pellets approached their equilibrium moisture content value of 0.04 db. Statistical analysis showed that there was no significant effect (P 0.84) of orientation on the overall effective diffusivity characteristic. Furthermore, the maximum values of effective diffusivity that occur at approximately 0.05 db are not significantly different due to the standard deviation being m 2 /s. Therefore, it can be concluded that the diffusivity coefficient of DSG pellets dried in SS is independent of pellet orientation. However, when only the constant drying-rate period is considered (when the moisture content ranged from approximately db), the effective moisture diffusivity characteristic was found to be significantly different (P<0.0015) with respect to change in orientation of the DSG pellets. CONCLUSION The maximum amount of condensation, total condensation time, and restoration time was determined for DSG pellets dried in SS at 120, 150, and 180 C with SS velocities of 1.0, 1.2, and 1.4 m/s. It was found that increasing the SS temperature from 120 to 180 C resulted in a 97% reduction in maximum condensation. The effective moisture diffusivity characteristic for DSG pellets dried in SS at 120 C oriented horizontally and vertically was determined using a finite cylinder model. Taking the entire characteristic into account there was no significant effect of orientation. However, when only the constant drying-rate period was considered there was a significant effect on the effective moisture diffusivity of DSG pellets dried in SS based on orientation. Acknowledgements. The authors thank the Natural Sciences and Engineering Research Council of Canada for their financial support 7

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