Brewer s Spent Grain (BSG) is the granular byproduct from beer brewing. It mainly consists of barley as this is the major raw material used in the

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1 Evaluation of Pilot Scale Drying of Brewer s Spent Grain in a Keith Rotary Superheated Steam Dryer - a Preliminary Investigation Lise-Lott Ström Department of Chemical Engineering, Lund Institute of Technology, Lund, Sweden Abstract Relative to conventional air drying techniques, superheated steam drying provides a number of advantages, including increased efficiency, reduced fire or explosion risks, faster drying rates and no particulate or odorous emissions. Consequently, superheated steam drying is currently receiving increased focus for drying in a range of industries.brewer s Spent Grain or BSG is the by-product from beer brewing. It is a sticky material which causes problems with sticky deposits on the drying equipment. This work aims to evaluate the feasibility of drying of BSG in a rotary superheated steam dryer at pilot-scale, developed by Keith Engineering of Australia. The evaluation is based on to what extent sticking can be avoided (evaluated by the mass of sticky deposits collected at different run conditions), achievable moisture content, energy consumption, product water quality and a residence time study. The results of an experimental design for three system parameters (namely steam temperature, coal feed rate and steam velocity) tested at two processing levels are presented. It is illustrated that the critical parameters which most significantly affect the amount of sticky deposits are the feed rate and the inlet steam temperature. The moisture contents achieved for the different processing conditions ranged from % wet basis, illustrating that practically any desired moisture content can be achieved. An evaluation of the energy usage of the pilot-scale rig indicated that the usage was significantly higher than would be expected based on literature values. It is proposed that the major reasons for the reduced efficiency are non-optimal flight design and air entrainment. A preliminary analysis of the product water quality showed high concentrations of organic constituents compared to available guideline values, which indicate that treatment of the product water will be required. Finally, two residence time models presented in the literature have been modified for BSG drying in superheated steam. The correlation between the predicted residence times and the experimental data is good. The overall evaluation of the direct rotary superheated steam drying process indicates that the process has potential for the drying of BSG (and other materials) but further research is required to improve the understanding of the process and to optimise the design. Introduction Superheated Steam (SHS) drying is an attractive alternative to conventional air drying due to a range of potential benefits which include increased efficiency, improved safety through the reduced risk of fire and explosion, no odorous or particulate emissions, a combination of drying with material sterilization, pasteurization and/or deodorization, and faster drying rates. In principle, any direct dryer can be converted to SHS operation, so the potential exists to utilize SHS drying in a range of industries and for a variety of materials. Consequently an increased interest for SHS drying is developing. However there are limitations to SHS drying which include increased system complexity, effective utilisation of the steam generated, higher temperature requirements, greater cost of the ancillaries and the limited experience in using SHS drying [1], [2]. Consequently, SHS drying will only be preferable to air drying if the benefits outweigh the limitations. Brewer s Spent Grain (BSG) is the granular byproduct from beer brewing. It mainly consists of barley as this is the major raw material used in the brewing process. Today, it is mostly used as cattle feed. In its wet form BSG contains large amounts of water, 75-80%, making it hard to store and transport which decreases its market value. Therefore drying the grain is required in order to improve its value. Existing drying equipment is energy intensive and experiences problems with BSG sticking to the dryer surfaces. Sticking also causes product degradation from the inclusion of blackened deposits, and can increase the risk for fire and explosion. It is well known that stickiness depends strongly on the moisture content and the temperature of a material. Above certain values, the material behaves in an adhesive or cohesive manner whereas at lower moisture contents and temperatures it is non-sticky [3]. A common way to define food stickiness is by the so called sticky-point temperature, which defines the temperature at which a food product of specific moisture content becomes sticky. Combining the sticky-point temperature at different moisture contents give rise to the sticky-point curve, which provides a

2 boundary between the sticky and the non-sticky regions. Figure 1 below shows a theoretical stickypoint curve for BSG combined with a theoretical drying curve 1. In the figure it can be seen that as the BSG dries, it traverses the sticky-point curve from the sticky to the non-sticky region, illustrating that the BSG is sticky up to a certain point to thereafter become non-sticky. Temperature Non-sticky Region Sticky Region Moisture Content Falling rate period Heat up period Figure 1. A theoretical sticky-point curve defining the sticky and non-sticky regions and a theoretical drying curve for BSG. In the 1990s the concept of the glass transition temperature was introduced in order to provide a fundamental understanding and predictability of the sticky-point temperature [4]. The glass transition is the intermediary stage from a very viscous glassy state to a more liquid rubbery state, and there is a clear indication of the dependence of the sticky point temperature of a product on the glass transition temperature [5]. As a general rule it is stated that the sticky-point temperature lies approximately ºC above the glass transition temperature for a material. All studies presented in the literature regard sticky behaviour of food products while drying in air. To the author s knowledge, no research on sticky behaviour in superheated steam has been published. When drying in superheated steam the material temperature is at the boiling point of the steam. For most sticky materials the glass transition temperature would be less than 100 ºC, and thus below the operating temperature in a superheated steam dryer. Hence it would not be possible to dry sticky materials in SHS. However, the rate of drying is also important and SHS drying may reduce the moisture content of the material faster to the level where it is no longer sticky. It may prove possible to reduce or even avoid sticking by using SHS. Hence, there is an obvious need to conduct research into drying of sticky materials in SHS. 1 Determining the drying and sticky point curves experimentally is a separate study and requires laboratory equipment that unfortunately was not accessible for this work. However, the author clearly sees the need of this in future work. In this work the feasibility of drying of BSG using a SHS rotary dryer at pilot-scale, developed by Keith Engineering of Australia, is investigated. The evaluation is based on mass of sticky deposits collected from the surfaces of the drum, moisture reduction, energy use, water quality and a residence time study Experimental The BSG used in this work was obtained from Mountain Goat Microbrewery (Melbourne, Australia). The grain was stored in a deep freezer at -15ºC and thawed in a refrigerator before the drying experiments. The initial moisture content of the grain used in the runs was 76.5 ± 0.3 % wet basis. The true density of the grain, measured using 3 helium pycnometry, was 1352 kg/m and the average particle size, determined with wet sieving, was 2 mm. A schematic of the Keith pilot-scale SHS rotary dryer used in experiments is presented in Figure 2. The dimensions of the drum are 0.37 m in diameter and 3 m in length while the slope of the drum was close to zero. In a test, a given rate of BSG was fed through a feed hopper and feed screw into the rotating drum. The combined action of the drum rotation and the flow of SHS resulted in the BSG moving through the drum from which it exited through a product screw. The SHS exiting the drum was heated to a higher temperature and returned to the drum through the action of a fan. A proportion of the steam, equal in mass to the amount of water evaporated from the BSG, was removed from the system via a condenser. Temperatures in the system were recorded using thermocouples attached to a data logger. Condenser Condensate Fan Feed (Wet Grain) Heater Rotating Drum Figure 2. Schematic of Keith Engineering s SHS drying process. T and F refer to temperature and flow measurement respectively. The system parameters which were varied during testing were the feed BSG rate, inlet steam temperature and steam velocity. A full factorial series of tests with these parameters at two levels was completed. The levels for each variable were selected based on a combination of the equipment limits and the desired operating conditions. The Cyclone Product (Dry Grain)

3 Product levels selected were 14 and 23 kg/hr for feed rate, 180 and 230 C for steam temperature and 1 and 2.5 m/s for steam velocity. Due to the 20-week time limitation for this work, the fourth parameter of interest for the series, the drum rotation rate, was excluded and all tests were run at 3rpm. After the statistical series was completed two additional tests at 6 and 12rpm, and otherwise the best test conditions were conducted. Results and Discussion Sticky Deposits In order to investigate the effect of the three system parameters on the sticking of BSG to the dryer walls, the drum was divided into three sections, see Figure 3 below. The grain was collected from the different sections with a vacuum cleaner. Outlet Centre Inlet 1m Feed Figure 3. Figure illustrating the three sections the drum was divided in for collection of the sticky deposits The results were analyzed using ANOG (Analysis of Goodness), which is a simple and effective statistical method to evaluate the effect of several parameters at once [6]. Generally for the three sections it was determined that low feed rate was most important for low sticking. Low feed rate is favoured over a high rate for the reason that that with less material in the system there is more energy per grain for heat up and drying. This results in that the grain heats up and dries faster and hence dries past the critical moisture content, at which it traverses the stickypoint curve into the non-sticky region. The second most important parameter was high inlet steam temperature. The hotter steam has a higher degree of superheat. This means that more sensible heat is available and also a larger temperature driving force, which consequently provides faster drying. The third investigated parameter, the steam velocity, was found to have a double sided effect on the system. For two of the sections, the inlet and the centre, there was a trend that a low steam velocity is preferable. The low steam velocity result can be explained by the smaller conveying effect the steam has on the grain, which leads to longer residence time in each section of the drum. This provides the grain with more time for heat up and drying. For this reason the positive effect of the larger heat transfer coefficient for the higher velocity is overpowered by the large conveying effect the steam has on the grain in these sections. However, the steam also aids in faster drying which can be seen in the results for the outlet section 2, in which the higher steam velocity is preferred, see Table 1. Further, the results for sticking in the outlet section were found to be closely related to the product moisture content. Table 1 below, shows the results for sticking in the outlet section combined with the product moisture content and the energy consumption. Certain feed rates and steam velocities have been highlighted, this has been done to illustrate that these parameters are the critical parameters for low sticking and low moisture content. Some of the results have been shaded grey, this is to highlight that no sticking in the outlet section occurred for these conditions. Table 1. Results for sticking in the outlet section, product moisture content and energy consumption. Experimental processing conditions are included. Test No. Feed Rate (kg/h) Inlet Steam Temp. (ºC) Steam Velocity (m/s) Amount of Sticky Deposits - Outlet Section (g/m 2 ) Moisture Content (% wet basis) A comparison between three tests at different rotation rates (3, 6 and 12 rpm) showed significantly better results for 6rpm, regarding sticking. This indicates that for minimized sticking there is an optimum around 6rpm for the rotation rate. However, more extensive testing is required in order to determine the actual optimum rotation rate. Moisture Content A wide range of product moisture contents, ranging from 10.6% to 65.7%, were achieved for the different tests. The best result, 10.6%, corresponds to a 96% moisture removal from the feed, and indicates that practically any moisture content can be achieved by carefully selecting the processing conditions. Energy Usage The energy usage per kilogram of water removed, assuming recovery of the steam generated at 80ºC, is higher ( MJ/kg) than suggested in the literature (1-1.5MJ/kg) [1]. This has been ascribed to air entrainment at the ends of the drum which are hard to seal due to the rotating movement, to heat 2 Results for the other sections can be found in [7] 3 No energy is assumed to be recovered from the condensate below 80ºC Energy per kg of water removedsteam recovery to 80 o C (MJ/kg) (0.7)

4 losses, and to poor internal design of the drum (which consist of only four linear flights at 90º increments) which leads to the system being underloaded. It is thought that these issues could be overcome in a designed system and with improved insulation, leading to energy usage values approaching those reported in the literature. Water Quality A preliminary analysis of the product water quality was made for selected physical properties and common inorganic constituents. The water showed high concentrations of organic constituents compared to available guideline values ( mg/l compared to 40mg/l), which indicate that treatment of the product water would be required. For the other selected properties, the water was able to meet most of the quality requirements for the Latrobe Valley, Victoria, Australia and recommended guidelines for Australian and New Zealand fresh waters, provided by ANZECC 4. Residence Time Study A residence time study for the transport of BSG through the drum has been presented. Two residence time models presented in the literature (Friedman & Marshall and Alvarez & Shene) have been modified for BSG drying in SHS. This was done using an empirical approach with flexible constants and exponents. The modified models and the models proposed in the literature are presented in detail in [7]. The correlation between the predicted residence times and the experimental data is good, see Figure 4 below. It is obvious that there is a need to develop a fundamental model for the residence time, based on drying particles in SHS. run at the low steam velocity (1m/s), while the lower data represent the results from tests run at high steam velocity (2.5m/s). It is evident that the steam velocity plays a major role in the transportation through the drum, and for the achieved level of drying. These aspects require considerable care when selecting the steam velocity. Conclusions Overall, it has been illustrated that BSG can be dried successfully with the proposed technology and that sticking can be minimized by careful selection of the process parameters. However further research on sticky materials dried in SHS is required in order to improve the fundamental understanding and to aid in design and process optimization. References [1] A. S. Mujumdar, Superheated Steam Drying Handbook of Industrial Drying, A. S. Mujumdar, New York: Marcel Dekker, 1995, pp [2] C. Beeby, O. Potter, Steam Drying, in Drying 85 Proceedings from the 4 th International Drying Symposium,9-12 July 1984, Kyoto, Japan, R. Toei, A. Mujumdar, Washington: Hemisphere, 1984, pp [3] C. Hennings, T. K. Kockel, T. A. G. Langrish, New Measurement Techniques of the Sticky Behaviour of Skim Milk Powder, Drying Technology, vol.19 (3-4), pp , 2001 [4] B. Adhikari, T. Howes, D. Lecomte, B. R. Bhandari, A Glass Transition Temperature Approach for the Prediction of the Surface Stickiness of a Drying Droplet during Spray Drying, Powder Technology, vol. 149, pp , 2005 [5] R. J. Lloyd, X. D. Chen, J. B. Hargreaves, Glass Transition and Caking of Spray Dried Lactose, International Journal of Food Science and Technology, vol. 31, pp , 1996 Figure 4. Parity plot between experimental data and predicted residence time results As can be seen in Figure 4 there is a large gap in the data. Data in the upper section of the plot represent residence times obtained from the tests 4 Australia and New Zealand Environment and Conservation Council [6] W. J. Diamond, Analysis of Goodness, Practical Experiment Designs for Engineers and Scientists, 3 rd Edition, New York: John Wiley & Sons, 2001, pp [7] L. Ström, Drying of Brewer s Spent Grain in a Keith Rotary Superheated Steam Dryer-a Preliminary Investigation, Master s Thesis in Chemical Engineering, Lund Institute of Technology, Lund, Sweden, 2007

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