EVIDENCIA QUE SE ADJUNTA (SOLO NOMBRAR)

1.1. Self-Heating in Spray Dried Detergents

The focus of this thesis is on the problem of self-heating that is observed to occur in the spray drying of laundry detergent powders. Laundry detergents are used across the globe to help with the cleaning of clothing and other fabrics. Detergents come in an array of forms, but detergent powders are still commonly used, particularly in developing countries where the majority of consumers still hand wash their laundry. Detergent powders are typically manufactured in two ways: agglomeration and spray drying. Agglomeration consists of mixing smaller particles with a liquid binder in order to produce larger granules. Spray drying on the other hand is a process by which a slurry of ingredients is atomised into small droplets, which when introduced into hot air are dried and form a powder product.

In the spray drying of these detergent powders, it is common for layers of the newly formed powder product to deposit on the inner walls of the spray drying tower (Francia, et al., 2015) (Hassall, 2011). This is not a problem exclusive to detergent powders and has also been shown to occur in the spray drying of milk powders (Beever, 1985) (Chen, et al., 1993). At the high temperatures at which these towers operate these powder deposits have the propensity to “self-heat”. Self-heating is the process by which some materials can increase in temperature without the application of an external energy source. Exothermic reactions within the material causes an increase in temperature. The stability of these systems is a balance between the internal heat generation and heat loss from the boundaries. If the rate of heat generation is lower than the rate of heat loss from the boundaries, these systems will reach a steady elevated temperature and remain stable. However, if the rate of heat generation exceeds the rate of heat loss from the boundaries, then thermal runaway will occur, whereby a runaway reaction causes a substantial rise in temperature. Many materials exhibit self-heating behaviour including milk powder (Chong, et al., 1996), coal (Sujanti, et al., 1999), and biomass (Caballos, et al., 2015). Self-heating is not only a problem in spray drying, but is also a problem in the storage and transport of these materials.

Consumers have come to expect certain requirements from their laundry detergent. The obvious requirements are that the detergent performs well in cleaning their laundry, while also protecting it from damage. In addition to this, the consumer expects the detergent itself to look and smell clean and appealing. The problem with self-heating is not only the heat generated in the process, but the effect is has on the product. Significant self-heating in the powder deposits on the inner spray dryer walls can cause the powder to char. This produces charred, or burnt, particles which are at risk of falling from the walls and getting collected along with the finished product. Over 20% of the finished product comes from the wall (Francia, et al., 2015) highlighting why self-heating in these build-ups is an issue. This compromises the quality of the finished product, and leaves the consumer with a product that does not meet their requirements of looking clean and appealing.

Limiting the threat of self-heating and charring requires an understanding of the mathematics behind these reactions. The fundamentals of self-heating can be traced back to Frank-Kamenetskii and his Theory of Thermal Explosions (Frank-Kamenetskii, 1969). Frank-Kamenetskii explored the mathematics governing self-heating in an idealised system with a zero-order reaction self-heating reaction. In doing so he derived a dimensionless parameter, 𝛿, often referred to as the Frank-Kamenetskii parameter. This parameter is a ratio of the heat generated to the heat dissipated and encompasses all the quantities required to describe the problems associated with self-heating, inflammation, and ignition. This parameter is a function of the geometry of the problem, the reaction kinetics, and the boundary conditions, such that if these are known, then predictions of self-heating and thermal runaway can easily be made. Much of the subsequent research was based around this important parameter.

Different approaches have previously been applied to address the problem of self- heating. Experimental methods have been applied to measure the self-heating reaction kinetics of similar materials. The long established method is the steady-state method based on Frank-Kamenetskii’s theory of thermal explosions (Frank-Kamenetskii, 1969). This is a basket heating method which was originally used to estimate self-heating kinetics for activated carbons (Bowes & Cameron, 1971). More recent work makes use of the cross-point temperature method, first proposed by Chong et al. (1996), and originally used to estimate the self-heating kinetics of skimmed and whole milk powders.

These two methods are based around oven heated baskets, and have been effectively used to make predictions of self-heating and thermal runaway in a range of materials.

1.2. Objective of Thesis

The overall objective of this thesis is to understand, and address the problem of self- heating in spray drying and spray dryer wall build-up. In doing this, the research seeks to determine the best means of measuring the self-heating reaction kinetics of a typical detergent powder, and seeks to apply these kinetics in order to predict self-heating and charring in oven heated powder baskets and spray dryer wall deposits. This is broken down further into the following objectives:

1. Review the literature that addresses the fundamentals of self-heating and thermal runaway.

2. Evaluate the self-heating behaviour that occurs in detergent powders and the detrimental effects of this under different heating conditions.

3. Apply a range of experimental techniques to measure the self-heating reaction kinetics of a typical detergent powder and determine the best methods for measuring these self-heating reaction kinetics.

4. Develop numerical models capable of predicting self-heating and thermal runaway in oven heated baskets of powder and spray dryer wall build-up. 5. Advance the overall research in this area such that the knowledge gained and

methods develop in this investigation can be applied by industry to address the problem of self-heating in the spray drying of detergent powders.

1.3. Structure of Thesis

This thesis aims to meet these objectives by using a combination of experimental methods and numerical models to explore the self-heating behaviour of detergent powders. The breakdown of the thesis chapters is outlined below.