SPECIAL ISSUE ORIGINAL ARTICLE Evaluation of immersion-contact type heat transfer for continuous pharmaceutical spin freeze-drying process Ganeshmurthy Srinivasan | Balakrishnan Raja Department of Mechanical Engineering, Indian Institute of Information Technology, Design and Manufacturing (IIITD&M), Chennai, Tamil Nadu, India Correspondence Dr. Balakrishnan Raja, Department of Mechanical Engineering, Indian Institute of Information Technology, Design and Manufacturing (IIITD&M) Kancheepuram, Chennai 600 127, Tamil Nadu, India. Email: rajab@iiitdm.ac.in Funding information Science and Engineering Research Board, Grant/Award Number: SR/S3/ MMER/0005/2014; Department of Science and Technology (DST-SERB), India, Grant/ Award Number: SR/S3/MMER/0005/2014 Abstract Spin freeze-drying process is a recently proposed method to achieve continuous processing of unit doses in freeze-drying application. An experimental study is carried out to investigate the heat transfer and drying characteristics of a spin-frozen liquid product. To enhance heat transfer, unlike freeze drying performed in shelves, the shell is immersed into a temperature-controlled bath. The influence of different ramping condi- tion of bath temperature on drying rate, time and variation of moisture content in spin- frozen products (deionized water and skimmed milk) are analyzed and reported in this paper. Among different ramping conditions, ramping of bath temperature to 10  C resulted in higher drying rate (1.8 kg hr -1 m -2 ) and lesser drying time (44%). The overall heat transfer coefficient (K v ) between the bath and product is evaluated for the above condition in both the test liquids. This direct contact type heat transfer helps in reducing air gap resistance and radiation effect, which primarily helps in minimizing thermal resis- tances. The effect of dry layer resistance is minimal in the spin-frozen product due to lower product thickness and larger ice-vapor surface area. Practical Applications The spin-frozen sample is dried either by using an infrared (IR) heater or by placing on a shelf. Limitations using IR heater include positioning of the vials (the distance between the vials and IR source) and selecting suitable IR window material and thick- ness. In the case of the shelf, conduction resistance between shelf and vial, radiation resistance inside the chamber and air gap resistance present between vial and shelf plays a major role. The thermal resistance is minimal in the immersion method due to the absence of air gap resistance and radiation effect. Hence, an alternative approach is discussed using immersion drying method to achieve continuous vial processing and uniformity among vials. Hence, freeze-drying process can also be shifted to con- tinuous production mode like prefilling and packaging. 1 | INTRODUCTION Freeze-drying is a dehydration process carried out in three consecutive stages. In freezing stage, the water content in the product is frozen below the eutectic temperature under atmospheric pressure. The frozen content is sublimated and desorbed under vacuum during primary and secondary drying stages, respectively (Cohen & Yang, 1995; Jennings, 1999; Mujumdar, 2014; Oetjen & Haseley, 2004). The pressure–temperature diagram of freeze-drying process is sche- matically shown in Figure 1. Batch wise freeze-drying process has certain limitations in achieving inter-vial uniformity while handling Abbreviations: DW, deionized water; IR, infrared; RH (10  C) , ramp and hold at 10  C; RH (-5  C) , ramp and hold at -5  C; RH (s) , ramping and holding in sequence; SM, skimmed milk; X, fraction of moisture content. Received: 16 January 2019 Revised: 11 April 2019 Accepted: 17 May 2019 DOI: 10.1111/jfpe.13153 J Food Process Eng. 2019;e13153. wileyonlinelibrary.com/journal/jfpe © 2019 Wiley Periodicals, Inc. 1 of 7 https://doi.org/10.1111/jfpe.13153