Absorption of steam bubbles in lithium bromide solution Philip Donnellan a,n , Kevin Cronin a , William Lee b , Shane Duggan c , Edmond Byrne a a Department of Process and Chemical Engineering, University College Cork, Ireland b MACSI, Department of Mathematics and Statistics, University of Limerick, Ireland c Department of Physics, University College Cork, Ireland HIGHLIGHTS  An experimental bubble absorption column is constructed and operated.  The absorption of steam bubbles in a hotter lithium bromide solution is tracked.  A simple ordinary differential equation model is developed to describe the collapse.  The model is demonstrated to explain 96% of the observed experimental variance.  Parametric studies are conducted examining factors influencing the rate of absorption. article info Article history: Received 26 March 2014 Received in revised form 21 July 2014 Accepted 29 July 2014 Available online 5 August 2014 Keywords: Bubble Absorption Lithium bromide Mass transfer Heat transfer Absorber abstract Absorption heat transformers are thermodynamic cycles that are capable of recycling waste heat energy by increasing its temperature. One of the most important unit operations in a heat transformer is the exothermic absorption of water vapour into a solution of choice at a higher temperature. Bubble columns are potentially an efficient means of achieving this. An experimental analysis is conducted which examines the absorption of single steam bubbles into a concentrated aqueous lithium bromide solution. The bubbles are tracked using a high speed camera, and their rate of absorption is modelled using a simple ordinary differential equation model. Accurate model predictions are obtained when oscillating bubble Nusselt and the Sherwood number correlations are utilised. The proposed model is capable of describing 96% of the observed experimental variability. Very large mass transfer coefficients of approximately 0.0012 m/s are obtained, which is higher than any previously reported values used in heat transformer absorber design. & 2014 Elsevier Ltd. All rights reserved. 1. Introduction Due to rising energy prices and the increasing regulation of industrial emissions, chemical and processing sectors are under ever increasing pressure to increase energy efficiency and to reduce thermal waste. Heat energy is one of the largest sources of industrial energy wastage, with up to 50% of the energy input to this sector leaving in the form of exhaust gases, cooling water, heated products and from surfaces of hot equipment (Johnson and Choate, 2008). Heat transformers are systems which can recover such low grade heat energy. These are closed cycle thermody- namic units, which are capable of increasing the temperature of waste heat streams so that they may be recycled within a plant (Donnellan et al., 2013). Such systems are generally capable of upgrading up to 50% of the energy supplied to them (Ma et al., 2003). In a heat transformer, it is vital to minimise equipment scale, in order to enhance economic feasibility. To do this the efficiency of all units within the cycle should ideally be examined and optimised. It has been demonstrated that the absorber can contribute up to 50% of the irreversibility within a heat transformer (Rivera, 2000), and therefore it is of primary interest in terms of design optimisation. This unit is also one of the most critical ones to the process. In a heat transformer, the absorption of saturated water vapour into concentrated salt solutions at higher temperatures (solutions may be 450 1C hotter than entering steam/water vapour) in the absorber enables the system to increase the temperature of the waste heat energy (Donnellan et al., 2014). The conventional method of vapour absorption is the falling film method where a lithium bromide solution (LiBr–H 2 O) flows down either in vertical or horizontal tubes as a thin film while absorbing water vapour from the surrounding environment (Guo et al., 2012). The heat of absorption is removed by a cooling fluid flowing on the inside of the tubes. Several studies have been Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/ces Chemical Engineering Science http://dx.doi.org/10.1016/j.ces.2014.07.060 0009-2509/& 2014 Elsevier Ltd. All rights reserved. n Corresponding author. Tel.: þ353 214903096. E-mail address: p.donnellan@umail.ucc.ie (P. Donnellan). Chemical Engineering Science 119 (2014) 10–21