Increasing the Energy Efficiency of Batch-Type Drying with Partial Air Recirculation Thomas Ziegler, Hasan Jubaer*, and Marco Schu ¨ tz DOI: 10.1002/cite.201400150 Partial recirculation of the dryer outlet air is a common practice in many batch-type drying processes. However, energy consumption and total duration of drying are directly affected by the recycled air ratio (RAR). In order to assess the influ- ence of different ambient air conditions on favorable values of RAR, a new quantity is introduced. The so-called critical drying efficiency determines the most favorable starting point for partial air recirculation. It is shown that by increasing RAR successively in the further course of drying, substantial energy savings can be achieved with only a marginal exten- sion of drying duration. It is also pointed out that optimized control of drying plants should be based not only on dryer outlet air conditions but also on ambient air conditions. Keywords: Batch-type drying, Energy efficiency, Medicinal plants, Process control Received: October 28, 2014; revised: October 06, 2015; accepted: November 09, 2015 1 Introduction Drying generally accounts for about 12 – 20 % of total industrial energy consumption [1]. It is the most important and mostly implemented process for a quick and safe pre- servation of food, animal feed and numerous other agricul- tural products. However, it is one of the most energy-inten- sive unit operations in the post-harvest chain of agricultural products [2]. The agricultural drying methods are particu- larly energy-intensive due to several reasons. Especially in case of medicinal and aromatic plants the stipulation that the material be treated gently after harvesting demands rela- tively low drying temperatures, which contributes consider- ably to the high energy consumption. In the production of medicinal and aromatic plants, the drying is the most ener- gy and cost-intensive process step [3]. Mellmann and Fu ¨rll [4] showed in their study that the energy costs of drying account for 30 – 55 % of the total production cost of medici- nal plants. The reduction in energy consumption of drying represents, therefore, a serious challenge for research and development. Among various available processes, the most widely applied and established fixed-bed drying of medicinal and aromatic plants was developed based on the convection dry- ing of hay [5]. The main advantages of this method are sim- plicity, applicability to different types of product, immediate drying without intermediate storage and low labor costs [6]. On the other hand, many of the dryers are confronted with high energy consumption due to uneven distribution of the air flow, poor insulation and inadequate dryer control. Intelligent systems for active heat recovery possess great potential to increase the efficiency [7]. Partial air recirculation as a measure to reduce energy consumption was discussed and recommended in many lit- erature sources, e.g. Maltry [8]. It was shown that due to the air recycle the required energy for heating up the fresh air decreases. This leads to reduced specific energy consump- tion (see Eq. (4)), if no significant reduction in water take up is given. It must be mentioned here that air recycle with- out heat recovery and condensation is implemented in practice also to protect the quality of products that need controlled air humidity during the drying process. However, the same principle can be applied to other products, the quality of which is not affected by higher air humidity. In the case of medicinal plants, air recycle is done in practice primarily in order to save energy. This applies especially for the later stages of drying. Although the operation in partial air recirculation mode is familiar to many drying facilities, the amount of recircu- lated air is often regulated manually by plant operators only based on their past experiences, which can cause a substan- tial loss in energy efficiency. This applies not only to fixed- bed drying of medicinal and aromatic plants but to other batch-type drying processes, as well. Therefore, the objec- tive of this work was to develop a theoretical basis in order to determine the required optimal amount of recirculated air in the course of drying. The theoretical analysis is supposed to function as the foundation for an energy use optimized automated control for batch-type drying processes. Fixed-bed drying of medic- inal plants with conventional air heating at 40 °C drying temperature was taken as an example in order to demon- strate possible energy savings by means of the partial air www.cit-journal.com ª 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Ing. Tech. 2016, 88, No. 1–2, 208–214 – Dr. Thomas Ziegler, Hasan Jubaer (hjubaer@atb-potsdam.de), Marco Schu ¨tz, Leibniz-Institute for Agricultural Engineering Potsdam-Bornim (ATB), Department of Post Harvest Technology, Max-Eyth-Allee 100, 14469 Potsdam, Germany. 208 Research Article Chemie Ingenieur Technik