Applying surfactants to improve the absorption capacity of mixtures of lithium bromide and formates in absorption refrigeration coolers

Abstract

The addition of a surfactant like n-octanol improves the absorption capacity of the absorbent solutions employed in absorption refrigeration coolers.

The effect of the addition of n-octanol on the water vapour absorption properties of LiBr + CHO₂Na (sodium formate) + water (LiBr/CHO₂Na = 2 by mass) and LiBr + CHO₂K (potassium formate) + water (LiBr/CHO₂K = 2 by mass) solutions has been investigated.

The enthalpies of dilution, vapour pressures, densities and viscosities of the mixtures were measured in the range of temperature from 293.15 to 333.15 K and concentrations between 20.0 and 45.0% by mass.

The mixture LiBr + CHO₂Na (sodium formate) + 1000 ppm of n-octanol solution at 45% by weight exhibits a greater vapour absorption capacity than the LiBr solution conventionally employed in this type of refrigerant machines.

Introduction

Today the technology developments have made the absorption refrigeration an economic and effective alternative to the vapour compression cooling cycle. The increase of electricity cost and environmental problems has made this heat-operated cycle more attractive for both residential and industrial applications. Absorption chillers are widely used in the air-conditioning industry, in part because they can be activated by hot water, steam and direct-fired natural gas, among others, instead of electricity (Herold et al., 1996, Kim et al., 1996, Uemura and Hasaba, 1964).

For absorption refrigeration applications, a refrigerant–absorbent combination should satisfy the following conditions (Ameel et al., 1995).

• The solute should be highly soluble in the solvent. Utilizing a high-solubility mixture could permit a higher difference between the concentrations leaving the generator and absorber. This could in turn reduce the solution circulation factor, increase the temperature lift and reduce the heat load of the generator.

• The strong solution leaving the generator should be coolable to nearer the absorber temperature without salt crystallization. This could improve the performance of the heat exchanger and can in effect reduce the heat load of the generator.

• The refrigerant and absorbent should be non-toxic, non-inflammable, non-corrosive, cheap and readily available.

The most used refrigerant–absorbent combination is water–lithium bromide that complies with all the previous requirements. These absorption units are based on two factors to produce refrigeration effect.

• Water that boils and gets cold by vaporization at low temperatures when it is kept under high vacuum could be used as coolant fluid.

• Certain substances, like salts at high concentrations, can absorb water vapour and are able to cause important decreases in the vapour pressure of water.

The absorption refrigeration machines are big industrial devices. It has been generally employed to get cheap refrigeration power in chemical or manufacturing factories using the excess of steam or hot water. A large volume of absorbent solution has to be moving inside the machine if a large refrigeration capacity has to be provided. It means that a large contact surface is required and consequently the volume of these engines is very large. An increase of the absorbent efficiency could make possible to reduce the size of the required machine for a given refrigeration capacity or increase the duty of a working machine.

For that reason one of the most important considerations for enhancing the performance of an absorption chiller using water as a refrigerant is to improve the performance of the absorber.

A variety of mechanical and physicochemical methods have been adopted for the improvement of absorber performance. One of the possible mechanical ways is to bring up the wavy motions in the vertical falling film absorber (Morioka and Kiyota, 1991).

It is well known that the addition of a certain amount of proper surfactants to the working fluids greatly enhance the mass transfer producing a vigorous surface instability (Elkassabgi and Perez-Blanco, 1991, Hihara and Saito, 1993, Hozawa et al., 1991, Jung et al., 1993, Kim et al., 1999, Kim et al., 1993, Kim and Berman, 1995, Kashiwagi, 1988, Park and Lee, 2002). Eight-carbon alcohols such as n-octanol are commonly used as the effective additives for enhancing the mass transfer of the water + lithium bromide system which is the typical refrigerant and absorbent pair.

When the water vapour is absorbed into the lithium bromide solution containing small amount of alcohol additive, a vigorous surface turbulence known as Marangoni convection results from the presence of a surface tension gradient which plays a key role in enhancing mass transfer. Probably the convection is induced by a change of surface tension around the droplets of surfactant floating on the solution. Although there are some experimental and theoretical works reported in the literature on the subject most of them are limited to the LiBr + water solutions.

In previous papers of this group, mixtures of lithium bromide and organic salts of sodium and potassium (formate, acetate and lactate) have been evaluated as alternative absorbents for absorption refrigeration machines (De Lucas et al., 2003, De Lucas et al., 2004, De Lucas et al., 2006, Donate et al., 2006, Kulankara and Herold, 2000). Among these five working fluids, the LiBr + CHO₂Na (sodium formate) + water solution was confirmed to have a better absorption capacity and greater thermodynamic efficiency than lithium bromide alone.

On the other hand as claimed in previous works (De Lucas et al., 2004, De Lucas et al., 2006, Donate et al., 2006), the heating requirements in the generator section can be dramatically decreased using this kind of mixtures, being enough using a waste stream with only a ΔT ≈ 10 K over 334.15 K to reach the boiling point of the diluted absorbent mixture. This fact allows the use of condensate water or any other residual stream of the industrial process, even under 383.15 K, to get the evaporation of the absorbent mixture, not necessarily overheated water or steam.

Both facts together provide an important economic advantage and improve the efficiency of conventional absorption refrigeration engines, so that the LiBr + CHO₂Na + water solution can be a promising candidate as a new working fluid from the viewpoints of both thermophysical properties and mass transfer characteristics.

In the present work, the main goal is to improve even more the absorption efficiency of the aforementioned mixtures. The effect of a surfactant on the mass transfer process is experimentally investigated for the case of water vapour absorption into aqueous solutions of the systems selected in a previous work, LiBr + CHO₂Na (sodium formate) + water (LiBr/CHO₂Na = 2 by mass) and LiBr + CHO₂K (potassium formate) + water (LiBr/CHO₂K = 2 by mass) solutions.