Drying in the feed industry
donderdag 8 oktober 2020
Auteur: David Hollestelle, IP&D experts
The world population is growing continuously and hence the amount of food produced needs to increase as well. Soy, cereals, rice, and corn are amongst others the major staple crops for food and feed industry. These materials are partly directly consumed by people, and partly indirectly serving as feedstock for cattle to produce dairy and meat. The scope of this report is the feed industry. It summarizes the major processing steps for raw materials and related energy consumption. The relevance of drying in this process chain is explained for some materials. There are also quite some specialty products, composing of various oils and milk products, like fat filled whey powders. For all processes there are possibilities to save energy or at least use renewable energy sources.
This short report is written for technologists and inspires them hopefully to design an energy efficient process. It is meant for managers too, to inform them on the focus improvement areas in order to make sustainable production systems. It contains energy figures and information on the total process of raw materials for workers in public service for making policy towards a more sustainable environment.
As it is a short report only, it is far from complete. It contains sufficient references for further study.
2.0 Energy requirements for RAW materials productions
2.1 Size of the feed industry
Figure 1: The size of the feed industry in Europe and the world
Figure 1 gives an idea of how big the (compound) feed industry is, and thus how much small changes can have a huge impact on financial and environmental situations. In addition, Figure 2 shows the different kind of materials and their respective amounts that are used in the compound feed production.
Figure 2: The materials used in the compound feed industry
2.2 RAW MATERIALS; energy requirements, its production processes including drying
Figure 2 shows that cereals, cakes & meals are often used materials for feed preparation. Amongst these raw materials, fish meal, soybean (meal), corn, (broken) rice, cassava chip and pulp, oil seed cake and palm kernel meal are prevalent (1, 2, 3). For the base raw materials the process looks like is shown in Figure 3 and Figure 4.This might not make clear why drying is such an important step in the feed production process, as it is mostly used to obtain the raw (dry) materials that are used during further production. The reason that drying is such an important step is that it leads to a product that has a final moisture content that allows for proper conditions for storage and transportation (Teixeira Freire et al., 2013). Drying reduces the water activity to a level at which no fungi nor bacteria can develop and biochemical reactions promoting product degradation can occur. Next to the drying of raw materials, a drying step is involved in the pelleting step, in which often extrusion is involved. Extrusion requires heat and moisture (steam) to structure the end products. It is an energy intensive step. See also Table 2 in Appendix 2. From Figure 6 it becomes clear that indeed most of the energy is used. Figure 5 shows that in the subsequent steps the pelleting is most energy intensive (4).
Figure 3: Schematic of the feed production process
The remainder of this section will look into the amount of energy involved in drying of these ingredients.
Figure 4: Schematic of feed manufacturing process (Beumer, 1986)
|Figure 5: Subdivision of energy usage in the production chain of compound feed (Sevenster and Hueting, 2007).||Figure 6: Share (in % /tonnes compound feed) of the production process in overall energy consumption (4)|
To have an overview of how much energy is used in the production of the raw materials and what is actually causing this, Figure 7 and Figure 8 are presented.
Figure 7: Energy usage for the 10 most used types of raw materials for compound feed for dairy cattle. Transport, processing and farming are displayed in blue, red and green respectively (Kool, 2008)
Figure 8: Origin of energy usage for different raw materials (Kool, 2008)
In addition to the energy use, one could also consider the greenhouse gas equivalents in relation to the total amount of material and/or specific nutrients. Blonk e.a. (2008) discuss this in more detail, Appendix A shows an example of this. While considering all of the above it has to be kept in mind that other types of feed are complementing the compound feed and thus can contribute to the optimum ratio of ingredients with regard to the nutritional value and energy usage. In addition to the drying energy, the specific energy required for pelleting (use by the pellet mill motor) may range from 4 to 40 kWh/t (Israelsen et al., 1981; Stevens, 1987).
2.3 Different drying technologies for RAW materials
Figure 9: Types of driers usually applied to grain drying (Teixeira Freire et al., 2013)
Figure 9 provides an overview of the different types of drying methods that are generally encountered in the drying of grains. First, we can distinguish between natural drying, by using solar radiation, and artificial drying. The latter employs several mechanical processes that increase e.g. the contact between air and grain and also allow for better temperature control. It has been found that this results in faster drying and that it usually gives a better product quality, although it can be useful in developing countries (Sharma, Chen and Vu Lan, 2009). It is important to differentiate between ‘solar drying’ and ‘sun drying’, where the first uses equipment to collect the radiation and use the energy for drying. The first simply lets the sun’s radiation get in direct contact with the grains. This last option however has some disadvantages as there is the risk of spoilage due to unexpected rain. Also, during high temperature days a hard shell might develop on the grains that could lead to problems during further processing. Advantages of solar drying over sun drying are that it is faster, more efficient, more hygienic, healthier (i.e. the grains contain more nutrients. Artificial drying can be subdivided into low and high temperature drying, of which high temperature is usually encountered. The first four methods of the high temperature share are known to result in less mechanical damage to the grains than other types of moving bed dryers. Cocurrent flow has some advantages over cross flow drying, these are mostly related to product homogeneity (Barrozo, Murata and Costa, 1998). However, cross flow dryers require a smaller pressure drop across the bed (Barrozo, Sartori, Freire, 2000). The use of moving bed dryers, although these can be subdivided into many types itself, seems to be the prevalent method (see e.g. (Pfeifer, Murata and Barrozo, 2010 and citations). A reason that this technique is used so often is that it has relatively low investment costs and provides energy savings compared to other types of drying methods. (Barrozo, Murata and Costa, 1998). Also, “to achieve cost reductions and to minimize loss of product quality, several drying technologies have been proposed, where those in fluidized bed dryers were considered superior” (Reyes, Moyano and Paz, 2007). “This technique (sliding bed) requires lower investments, consumes less energy, and causes less mechanical damage to the seeds than other types of moving bed dryers. Sliding beds have advantages over fixed bed dryers because they involve a continuous process and yield a more homogeneous final product” (Barrozo, Mujumdar and Freire, 2014).
In addition to all the different types of bed drying, microwave drying can be used. This technique has been found to be well applicable and results in shorter drying time, higher product quality and lower energy consumption (Reyes, Campos and Vega, 2006). It can be used in addition to bed drying but also in combination with rotary drum drying it has proven to be worthwhile (Wang et al., 2009). In addition, pneumatic drying technology has been successfully employed by Bunyawanichakul et al. (2007). Drying rate was found to increase when dryer diameter and inlet air velocity were increased while the feed rate was decreased. They note however, that for the paddy rice investigated the simple pneumatic dryer is not practical to use but rather as a flash dryer to remove the moisture on free surfaces. To conclude, spouted bed and fluidized bed drying are also used for the drying of grains (Jittanit, Srzednicki and Driscoll, 2010; Perea-Flores et al., 2012).
2.4 Some ideas for energy savings
Figure 10: Energy usage in the dairy chain (Ketenkaart zuivelindustrie, 2006)
Table 1: Energy usage for each raw material (Kool, 2008)
Figure 10 and Table 1 provide information to use as a start for reducing the amount of energy used in producing compound feed. It becomes clear that in the overall dairy chain the production itself contributes most to the overall energy usage. Furthermore, when considering the production of the different ingredients used, it becomes apparent that not only the processing requires a lot of energy, but also the transport. Next to that, the numbers given show a huge variance, pinpointing the materials that would require most attention when reduction of energy usage is key.
If we look into the processing stage with more detail, (again) specifically focussing on the drying stage, research has been performed with regard to the influence of drying conditions on drying performance (Darvishi, H., Khoshtaghaza, M. H., & Minaee, 2014). For example, Figure 11 shows the influence of the drying air velocity and surrounding temperature on the required drying time of soy. Drying was defined as going from the initial moisture content to about 10% (wb).
Figure 11: Drying temperature for soybeans under various temperatures and air velocities. Columns with differnet superscripts mean that the data are significantly different (P<0.05) (Darvishi, H., Khoshtaghaza, M. H., & Minaee, 2014)
Of course not only the drying time is important, the specific energy consumed during the drying is of importance as well, as shown in Figure 12. From this figure it can be seen that the specific energy consumption decreases with increasing temperature at all air velocities (a minimum of 1.8 m/s was necessary to fluidize the soy). At constant temperature, a reduction in specific energy consumption was found for decreased air velocity. In the case of the soybean seeds used here, selecting a temperature of 140⁰C and an air velocity of 1.8 m/s is optimal. These results can be used as a starting point for optimization of other drying situations.
Figure 12: Specific energy consumption at different temperatures and drying air velocities (Darvishi, H., Khoshtaghaza, M. H., & Minaee, 2014)
In addition to temperature and air velocity other factors can play a role in the energy consumption and efficiency of drying. E.g. it was shown by Kulig and Laskowski (2005) that an increase in fat concentration in feed material from 2 to 5.5% reduces energy consumption during pelleting by 30%. In addition, steam conditioning/preheating the feed may require considerable energy, e.g., Skoch et al. (1981) estimated that steam conditioning to increase the temperature from 27 to 80 ⁰C consumed about 26 kWh/t. Adding steam however improves pellet durability and by providing heat and moisture it also helps to reduce energy consumption during pelleting.
In the previous section we briefly touched upon the use of microwave irradiation. It has been shown (for turnip seeds) that this technique can significantly reduce the required drying time (see Figure 13). Also here there seems to be an optimum as an increase from 150 W to 300 W reduces drying time far less than an increase from 0 W to 150 W. The addition of the microwave irradiation increases water evaporation within the particles, leading to an increase in internal pressure that in turn increases water diffusion towards the surface of the particles, where it is removed convectively by the drying air.
Figure 13: Drying curves for tests with pulsating fluidized beds at 0.55 m=s and 30% relative humidity. The SVDM curve represents modelling results (Reyes, Campos and Vega, 2006).
2.5 Determining product quality with regard to drying aspects
“Feed production can have an even closer connection to the quality of the finished animal product than it has ever had before” (FEFAC, 2015-2016). As an example of how the drying aspects can influence the product quality, Figure 11 is presented. This figure shows that an increase in the number of stages in a conventional sliding bed dryer leads to an increase in moisture removal. It was also found that increasing the number of stages increases the seed quality. Low outlet seed temperatures were necessary to ensure good seed quality indices (Pfeifer, Murata and Barrozo, 2010).
Figure 14: Effect of number of stages on moisture removal. *, single-stage configuration; Δ,two-stage configuration; о, three-stage configuration; ▪, four-stage configuration
Another drying method that was mentioned earlier was using microwave drying. Amongst others this technique was used for the drying of rapeseeds, in which the quality in terms of damaged seeds were compared to the used microwave power level (Łupińska et al., 2009). As can be seen from Figure 15, using less than 400 W or the use of higher power levels (800 W) gives the least amount of damaged seeds, whereas moderate power levels gives higher number of damaged seeds. Importantly, it was found that the number of damaged seeds was nearly three times higher in traditional hot air drying than in microwave drying. Moreover, the ability to sprout is totally blocked with microwaves, which can be very useful in the preparation of fresh seed for long-term storage and designated for industrial applications (such as the feed industry). However, microwave-assisted drying of biological material can be very risky in order to obtain the best quality of expected product, as it can change the structure of sensitive and live organisms like various seeds (as it can concentrate in the moistest regions of the material). Also, penetration into a bed of seeds might prove to be difficult due to dissipation and attenuation of microwaves in moist materials.
Figure 15: The number of damaged seeds versus microwave power. The bed was moistened twofold: in the first case, the sample was moistened for 10 min (sample A). Time of moistening was 24 h in the second case (sample B). (Łupińska et al., 2009)
3.0 Example of SPECIALS –fat concentrates and milk replacers
The production of dairy is in general as shown below. Fat concentrates and milk replacers for the feed industry is originating from whole milk and refined palm oil or coconut oils, or hydrolysed wheat proteins. Fats are encapsulated in a protein structure via homogenisation and spray drier process. Spray drier processes are very energy intensive and quite some opportunities for energy savings exist. Examples are
- increasing the solid content of the feed for the spray drier
- energy recovery above dew point (interchange air inlet/outlet) and up to/including dew point, using corrosive resistant heat exchangers.
4.0 Use heat sources from sustainable sources
Incineration of sustainable heating sources like wood pellets and wood chips for use in the production process is promoted by the RVO. Steam can be produced to 30 barg using these heating sources. Wood pellets is an easy and relatively cheap sustainable heating source.
5. NOVEL TECHNIQUES -FOOD for THOUGHTS- Innovations
Novel drying methods are focused on minimizing mechanical damage to the product. For this reason, vibrating beds rather than fluidized beds may be a better option (Barrozo, Mujumdar and Freire, 2014). Also, spouted beds can be designed in such a way that they are suitable for numerous drying applications. The use of ultrasound is also topic of interest. However, although it may increase the speed of drying, the risk of damaging the structure and the cost of applying the field do not seem to outweigh the former (Clemente et al., 2014). Kozanoglu et al. (2012) showed that good-quality product can be obtained using a low-pressure superheated steam dryer as long as the drying takes place under damaged temperature of the seeds. It is more complex and expensive however. Another technique that has also been shown earlier in this report is the use of microwave drying. By selecting the appropriate field intensity, good-quality products can be obtained (Barrozo, Mujumdar and Freire, 2014). The following tables summarize the innovations (waiting to be) implemented in the field of drying. While reading through these tables one has to keep in mind though that these methods are not solely aimed at drying for the feed industry. A division has been made between different types of drying methods amongst the different tables shown. All tables are adapted from Mujumdar (2004), Jangam (2011) provides similar considerations.
Table 2: Fluidized bed dryers: conventional vs. innovative concepts (Mujumdar, 2004)
Table 3: Vibrated bed dryers: conventional vs. innovative concepts
Table 4: Two-stage fluidized bed dryers
Table 5: Spouted bed dryers: conventional vs. innovative concepts
1 http://www.sustainabletable.org/260/animal-feed Accessed 22-10
2 http://www.fao.org/docrep/004/AC797E/ac797e09.htm Accessed 22-10
3 http://www.amis- outlook.org/fileadmin/user_upload/amis/docs/resources/Thailand%20feed%20consumption%20report.pdf Accessed 22-10
4 http://www.bine.info/fileadmin/content/Publikationen/Projekt-Infos/2014/Projekt_07-2014/ProjektInfo_0714_engl_internetx.pdf Accessed 24-10
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