Included in this chapter is a discussion about all the work and researches that has been done relating to the drying of maize

Included in this chapter is a discussion about all the work and researches that has been done relating to the drying of maize. Since this project is mainly about the drying technique of maize, various types techniques, important parameters in the drying process, and all other components’ literature that will help in the design of the maize dryer will all be discussed in this chapter. A discussion of the previous researches relating to this project that was done before is given in this chapter. The literature reviews in this project are important to assist in the design and simulation of this project. The following literature review was extracted from websites, journals, books, and articles.
2.1 Types of Drying Method
There exist various types of grain drying mechanisms which are mainly distinguished by the method of drying. The two methods are the powering mechanisms of drying the grain. Drying of maize is done either via forced mechanisms or natural drying without any external force to force warm air through the grain to dry it in the process. The two, unforced air-drying and forced air-drying mechanisms are discussed in the coming sections 2.1.1 and 2.1.2. The Figure 2.1 shows the structure of drying mechanisms classifications.

Figure 2.1 Drying Mechanisms Classifications
2.1.1 Unforced Air-Drying
Unforced air-drying techniques consist of drying maize using no external forces to enhance drying. The main advantage of unforced air-drying techniques is that they are cheap and require little to know investment in terms of equipment. They are depended to the natural air circulation and the drying rate is very low taking weeks and months to dry maize. The natural methods of drying are vulnerable to weather conditions like high humidity, rain and excess temperatures which affect quality of dried maize and results in huge losses in quality and quantity of maize during the drying process. Unforced drying techniques are also characterised with much manual labour and the techniques are labour intensive. In addition to these general characteristics of unforced air-drying methods, particular types of unforced air-drying techniques possess other characteristics specific to them only. The discussion of these techniques is illustrated as under the following sub-headings. In-field Drying
The in-field maize drying systems is the one mainly used by the majority of farmers in Zimbabwe and other geographic locations as well. In this drying technique which is usually done when grain is first harvested, maize is made to remain in the field in stalks. In-field maize has no limit in terms of the quantity of maize it can dry and also it uses direct sunlight which is readily available to many geographic locations. However, this drying technique takes more than 30days to drying maize and also it delays land preparation for the upcoming farming season. The drying rate of in-field drying is 0.8% to 1% moisture reduction per day on a day having an average of 24? and the drying can be as low as 0.4% moisture reduction per day on a day having an average of 13?. Figure 2.2 below shows diagram of In-field drying technique.

Figure 2.2 In-Field Drying Natural Drying Direct Sun
The natural drying of direct on direct sunlight is similar to in-field drying techniques in terms of source of energy of energy advantages and disadvantages. This technique consists of maize being laid on a tarp and dried in the sun. In this drying technique, harvested maize is spread on hard ground, roof tops and/or designed platforms to lay down the maize which will be mixed manually. This drying can stretch up to one week to dry maize from 27% to around 14% moisture content. Many users of this type of drying employ the use of a black drying surface that absorbs sun energy effectively. Figure 2.3 below shows a diagram of natural drying in direct sunlight.

Figure 2.3 Natural Drying Direct Sun Grain Crib Drying
The most common type of maize drying in local communities is grain crib drying. This method consists of the construction of a platform to contain maize cobs. The grain cribs (Dura in Shona) are designed in such a way to allow air reach the maize due to openings they possess. Construction material of the grains is usually wood. This method is small in that it cannot accommodate tonnage more than 10 tonnes of grain and still be able to effectively dry the maize. Termites and insects easily find their way to the exposed maize in the grain crib. Drying rate in Grain Crib Drying is usually weeks. The diagram in Figure 2.4 shows the grain cribs usually employed by small communities locally.

Figure 2.4 Grain Crib
2.1.2 Forced Air Drying
Forced Air-drying techniques consists of an external air flow directed to maize to cause fast effective drying of grain. Various sources of energy are employed to cause air (usually heated) to pass through grain and cause drying in the process. The most common sources of energy being used are electricity, hydrocarbon, biogas and solar. Forced air-drying techniques are effective in achieving high drying rate, better hygienic drying, quality dried maize, reduced dependence to natural drying and reduced labour in processing. However, the disadvantages of these techniques include need of investment which in most cases is beyond the reach of many farmers, depended on the availability of the source of energy accessibility and the need of skilled labour to operate the machines employed in drying. The following subheadings gives a discussion of the various forms of forced air drying with specific characteristics other than the generics characteristics discussed in this section. Electric Powered Air Flow Dryers
Electric powered air flow dryers mainly consist of air blowing systems being powered by electricity. This system is fastest drying to as many as 100 tonnes of maize per week. Electricity powered drying is usually on a commercial level and is very effective in assuring quality product after drying. However, electricity is expensive and not accessible in many geographic locations. There are also various types of electricity powered air flow dryers depending on the orientation of the drying chamber and materials handling systems. Examples of electricity powered dryers are thin layer dryer, shallow layer dryer, radially vented bin, deep layer dryer, sack drying systems to mention a few. Figure 2.5 below shows In-storage Natural Air-Drying System.

Figure 2.5 Natural Air-Drying System Hydrocarbon Powered Air Flow Dryers
Hydrocarbon powered air flow dryers are those being powered by hydrocarbon. This method is highly used in areas where there is need of high output rate of drying and the presence of a hydrocarbon fuel to power air flow to maize. Mainly used hydrocarbons are petrol, diesel, coal and charcoal. This mechanism of using hydrocarbons is not clean as it depletes natural resources and they cause environmental pollution during combustion to produce energy to power air flow to maize. There are also various types of hydrocarbon powered air flow dryers depending on the orientation of the drying chamber and materials handling systems. Examples include floor drying systems, rotary drying systems and fluid bed drying systems. Figure