Métodos discriminatorios 1 Pruebas de diferenciación.

In chapter 1, drying and evaporation were highlighted as the main energy consumers. Therefore, this work focussed on those two processes. Other opportunities for emerging technologies are discussed in this section, i.e. enzymatic cleaning of equipment, and the use of renewable energy sources. These aspects have not yet been extensively mentioned in this work so far, but have potential to reduce the energy consumption in industrial processing.

3.1 Enzymatic cleaning

The cleaning of process equipment is responsible for 10 – 26% of the overall energy consumption in milk production (Krebbekx, Lambregts, Wolf, & Seventer, 2011; Ramirez, Patel, & Blok, 2006), and for this reason alternative cleaning methods deserve attention. In dairy processing, cleaning is applied to guarantee microbial safety of the product and to maintain process performance. Also fouling on heat exchangers, pipes, and membranes by proteins, minerals, and fat can occur (Jeurnink & Brinkman, 1994). As a result of fouling an adherence surface for microorganisms in milk is formed which in turn results in the formation of unwanted biofilms. Furthermore, heat transfer is affected, and pasteurisation temperatures might not be reached, posing another potential safety problem (Flint, Bremer, & Brooks, 1997). For this reason, regular cleaning is of high importance in milk processing. In Chapter 4 the scheduling of cleaning, and effect on process performance was included. However, opportunities for energy reduction in cleaning are not yet addressed in this thesis.

The main sources for energy consumption in cleaning are: 1) heat loss from the storage tank for cleaning agents, 2) heat loss from the processing equipment, and piping towards/from the processing equipment, and 3) energy required for heating fresh cleaning solutions to the required temperature. Potential ways to reduce energy consumption in cleaning are 1) reducing temperature, 2) reducing the number of cleaning steps, 3) reduction of the length of the cleaning steps, and 4) using alternative cleaning agents. Traditionally cleaning consist of a combination of alkaline and acidic cleaning at temperatures ranging from 70 – 90°C (Timmerman, Mogensen, & Graßhoff, 2016). As part of the EU project ENTHALPY, a new enzymatic cleaning method was developed (Guerrero-Navarro et al., 2019). Advantage of using enzymes for cleaning are: the lower working temperatures (around 50°C), the reduced amounts of

153 chemical waste, and a reduction in water consumption. These all contribute to a reduced environmental impact.

Model calculations showed that if the traditional two-step alkaline/acidic cleaning is replaced by a one-step enzymatic cleaning (while keeping the rinsing steps the same), the energy consumption of the cleaning step can be reduced by 70%, and water consumption can be reduced by 50% (assuming lowering process temperature from 70 to 50°C, and reducing the amount cleaning steps from 5 to 3). Although these are basic calculations, these numbers highlight the large potential of enzymatic cleaning.

An additional advantage for the reduction of environmental impact is the potential to reduce the water consumption for cleaning. The reuse of the permeate produced by for example membrane distillation (chapter 4). The retention of membranes used in MD is 99 – 100%, and the permeate is therefore directly suitable for cleaning processes (Hausmann et al., 2013). Further integration of the water cycles has not been investigated in this thesis, but provides additional opportunities for environmental impact reduction.

3.2 Renewable energy sources

Besides the reduction of energy consumption, renewable energy sources can provide part of the solution to reduce the environmental impact of a production chain by reducing the consumption of fossil fuels. In a continuous production processes, it is essential to have a stable and reliable network of renewable energy which does not affect the product or continuity of the processes. 3.2.1 Photovoltaic cells

In chapter 5 radio frequency heating (RF) was included as an innovative technology for milk pasteurisation. RF is electricity driven and if the electricity is generated by a non-renewable energy source, RF cannot compete with traditional pasteurisation based on steam heating. With the use of electricity from photovoltaic cells (PV’s) RF becomes an attractive alternative. It has been observed that each time the total number of PVs manufactured doubles, the cost of PV cell drops 20% (Swanson, 2006). With the current focus on renewable energy sources it is expected that the shipping volume will continue to increase. This trend will lead to a reduction in PV installation cost, and a change in the balance with steam heating.

For a small model milk powder factory (10,000 kg raw milk/h) at least 1 ha of PVs is needed to fulfil the electrical demand on a bright day (assuming: maximum solar intensity of at least 600 W/m2_{, a solar cell efficiency of 20%, and 10% of factory energy demand is electric). With} the increasing number of domestically installed PVs, and their production being out of synchronisation with domestic demand, usage of their production peaks is of interest (Timilsina, Kurdgelashvili, & Narbel, 2011). Industries, like a milk powder plant, could utilize those peaks and adapt their consumption and production for optimal integration. This is an alternative for the grid, instead to invest in storage technologies for the excess energy produced.

3.2.2 Solar heating system

Milk powder production requires mainly thermal energy rather than electrical. In this sense, the use of solar heating is of more interest than PV’s. The working temperatures in milk powder production make the use of solar thermal energy possible (Lauterbach, Schmitt, Jordan, & Vajen, 2012). Just like with PV’s, is the integration with a day round production scheme a challenge, which can be met with hybrid boiler systems. For a large-scale factory (100,000 kg raw milk/h) the contribution of a solar heating field of 1 ha and bright radiation (maximum solar intensity of at least 600 W/m2_{) would lead to a 10 – 20% steam reduction. For smaller factories} this share increases. Depending on the location, and thus solar radiation, of the factory, the applicability of solar heating will be of interest as a hybrid boiler system. The energy demand used in this example is of a state-of-the-art milk powder plant without the previously discussed improvements. By applying the technologies discussed in this thesis, the contribution of solar powered energy will increase.

3.2.3 Opportunities for heat pumps

In food production most of the available waste heat has a relative low temperature (below 100°C) (Hammond & Norman, 2014). These low temperatures make heat recovery often difficult and inefficient. For this reason, heat pumps are of interest, by increasing the temperature of waste heat to a usable temperature level for other processes. Heat pumps can be used in addition to the dehumidification technologies as proposed in chapter 3. Krokida et al. (2004) already showed that the integration of a heat pump to the heat recovery from the dryer exhaust increased the total heat recovery by 15%. Walmsley et al. (2017) modelled a hybrid heat pump for a spray dryer system, which resulted in a total potential energy reduction of 47%. The challenge for heat pump technology is the low efficiency at elevated temperatures. For spray drying, temperatures between 180 and 220°C are needed (chapter 3). This level cannot be reached by the currently available heat pumps with a maximum at 160°C, and additional heating is necessary (Arpagaus, Bless, Uhlmann, Schiffmann, & Bertsch, 2018). High temperature heat pumps are still in development and of growing importance with the increasing availability of renewable electricity.

By means of a next step in the development of an integrated process design, the superstructure optimisation as applied in chapter 5, should be extended by the integration of renewable energy sources, leading to a complete industrial site optimisation. Walmsley et al. (2018) proposed a total site heat integration approach, which should be extended with alternative energy sources and environmental impact evaluation.