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Direct Freeze Concentration Using Liquid Nitrogen

In the existing progressive freeze concentration systems, a freezer based on a vapor compression cycle is needed acting as a heat exchange surface. In the present study, a novel direct progressive freeze concentration method is proposed where the freezer is replaced by cryogenic liquid nitrogen as the cooling medium spraying above the solution. This process relies on the ability to concentrate the solute by gradual formation of the ice layer resulting from the enormous amount of heat that is absorbed during the liquid nitrogen evaporation process.

Sucrose is selected as a model solute component in the study since it is a major component in various food products. The objective of the present research is to demonstrate the feasibility of spraying liquid nitrogen directly into the solution to achieve progressive freeze concentration and improve the separation effectiveness. The main questions from the industry are : What is the maximum concentration level attainable? What is the heat extraction efficiency in direct LIN cooling?

The project consists of the following parts:

1. Single droplet evaporation study

The understanding of cryogenic droplets evaporating on a liquid surface is essential since it determines the heat extraction efficiency of LIN. However, due to the complexity of the physics involved, the entire process is far from being understood.

A single liquid droplet floating on the surface of a pool acts as a starting point to gain insights into the problem. A crude theoretical model modified from the one proposed by R.M.A Spijkers is developed for the evaporation process of a floating nitrogen droplet to calculate the vaporization rate of the droplet and the vapor layer thickness as a function of the radius of the droplet. This model still needs to be improved by involving the heat flux from the ambient.

2. Proof-of -concept experiments on freeze concentration using liquid nitrogen

The test rig is built to perform the direct freeze concentration using LIN. The efficiency of the system is evaluated according to the following parameters: the cooling rate, the cooling duration, and the initial solution concentration. Partition coefficient K and ice yield are main assessment indexes during the processes. From the experimental results, it is proved that the concept of using liquid nitrogen to achieve direct freeze concentration is feasible.

3. Thermodynamic model & validation - ice growth and solute inclusion during progressive freeze concentration

To allow scaling of progressive concentration towards industrial scale, modeling can be used to predict the processes and to estimate the efficiency and economics.  A crude model is proposed coupling heat and mass transfer problem to predict the solute concentration of the solution as a function of time during progressive freeze concentration process.

4. Liquid nitrogen spray system design and characterization

Contact: Zhuo Zhang