Drying of yam with solar adsorption system

Abstract

Yam is an important food crop in Ghana. It is a delicacy that is eaten after cooking or frying the fresh sample when cut into slices. The powders of the dried yam can be incorporated into soups, baby food and bread as composite flours. However, the high moisture content of yam when harvested (about 70%wb) makes it susceptible to spoilage leading to reduced shelf life and onward rejection by the consumer. This adds to the waste basket. Drying is a worldwide postharvest method for extending the shelf life of agricultural food products such as yam. An energy source that is cheap and clean has always been from the sun. The open sun method has been utilized severally and for many years, but this is subject to various contaminants and thus reducing its quality. A confined solar drying method is a better option but the energy source is limited to the day’s solar energy. Solar adsorption drying is an opportunity to dry day and night in an efficient and product conserving manner with a reduced drying cycle. Prior to this thesis work, several aspects were insufficiently known to really judge the potential. The thesis tackles some of the bottlenecks and problems: lack of knowledge about the drying properties of yam; how to design appropriate solar collectors for drying and regeneration and to share experiences with the actual implementation of the system to see its possibilities an opportunities. In this thesis the sorption properties of yam which gives an indication of the equilibrium moisture content in relation to water activity at constant temperature, but has been given less attention in literature, is studied. The analytical Crank solution to the Fick’s second law has been utilized in the prediction of drying curves of many food products. However the Crank solution puts limitations on the extent to which the equation could be utilized. The limitation is that the particle of food sample to be dried must be infinite, shrinkage should not exist, while moisture diffusion coefficient must be constant. In practice, these limitations are not realistic since most biological food products go through shrinkage together with variable moisture diffusion. Meanwhile the source of energy for drying has largely come from the sun and has led the use of the so-called open sun drying (OSD) method especially in tropical countries, for so many years. This method exposes the food to weather conditions and contaminants reducing the quality of the dried product. Recent research has shown that a confined drying system is most appropriate in terms of reduced drying time and quality of the final product. The shortcoming of the solar drying (SD) method is that, drying is limited to a time interval of 6-9 hours of the day after which the sun’s energy can no more be explored. Solar adsorption drying (SAD), as an alternative drying to solar drying, is an integrated drying method that helps in drying day and night in a continuous manner. Research in solar adsorption drying systems is limited in literature, therefore its performance is yet to be explored. Solar collectors are an integral part of solar and solar adsorption dryers. However, most researchers construct collectors to see what happens or use heuristic rules, whereas others do not link to dryers. The output air temperature has an effect on the quality of the dried food product, therefore much attention must be given to the output temperature of the air that enters the dryer. On the other hand after adsorption drying, the adsorbent needs to be regenerated. The easiest way to regenerate the adsorbent (silica gel), which requires temperatures over 70°C is by conventional ovens, which is costly. In this thesis, the condition and requirements for solar adsorption drying of yam (Dioscorea rotundata) is investigated. The approach is first to investigate the sorption properties of yam, then the drying characteristics of yam by modifying the Crank solution of the Fick’s diffusion equation, subsequently the model based design and construction of solar collectors for drying and regeneration purposes, and then finally, the proof of principle of solar - adsorption for both drying and regeneration. The first part of this thesis concerns investigating the equilibrium moisture isotherms for yam at 25°C and 50°C, both for sorption and desorption, experimentally using the gravimetric method. The strength of this work is that a Dynamic Vapour Sorption (DVS) analyzer, which is very sensitive to micro changes in weight loss during dehydration or rehydration, was used. A graphical presentation of equilibrium moisture content, as a function of water activity from the outcome of nonlinear regression analyses of desorption and adsorption isotherms respectively, is shown. The findings contradict most of the findings in literature. The parameters and their uncertainty range for each of the models (GAB, Henderson, Halsey, Oswin, Smith, BET and Peleg) are estimated. The strength of the statistical analysis is that an objective criteria is used to select the model that best describes the experimental desorption and adsorption isotherms over the relevant range of moisture contents. This has been presented in a table form where statiscally, the standard error (SE), the percent average relative deviation (PRD), Akaike Information Criterion (AIC), coefficient of performance (R2), and most importantly, the 2-σ bound confidence interval of the parameters, in particular those that bear the same unit as the equilibrium moisture content (Xe), have been taken into consideration. The GAB model was selected to describe both the desorption and adsorption isotherms, because it allows a physical interpretation. Next, Crank’s analytical approximation to Fick’s diffusion equation is used to investigate the effect of moisture dependent sample thickness and diffusivity on the drying behavior of yam (Dioscoreaceae rotundata) cubicles. Separate experiments of drying and shrinkage at constant temperatures of 30, 40 and 50oC were conducted in a cabinet dryer. The comparative study shows an interdependence between diffusivity and shrinkage due to water loss during drying. The analytical expression for the diffusion in a slab results in non-Fickian behavior for smaller cubicles and consequently, results in a higher effective diffusion coefficient. The drying rate trajectory shows two stages as a function of moisture ratio. The drying behavior of yam is better described by observing both drying and drying rate curves concurrently. This gives good agreement between observed and model data by a combination of fractal moisture dependent shrinkage and moisture dependent diffusion. The advantage of this modified Crank’s approximation is that the moisture trajectory of finite food cubicles can be predicted appropriately. Coupled partial differential equations are developed to investigate which collector lengths are appropriate for drying and for adsorbent regeneration. The output air temperature of the model was validated with an experiment. The spatially distributed model is a powerful tool to aid in multi-purpose collector design leading to solar dryer and regeneration system construction. The advantage of this model is that, the spatial mean temperature of the absorber plate, a requirement for the determination of the overall heat loss coefficient, which hitherto is absent in the approach of Duffie and Beckmann, is no more a problem. The result shows a spatially distribution of temperature in air, absorber plate, and glass cover as functions of collector length and time (day) at constant air velocity and presented in a graphical from. While the temperature of absorber plate, glass cover and air are functions of time (day), the air temperature additionally varies directly with collector length. The study shows the operational overall heat loss coefficient, radiative and convective heat gain and efficiency as functions of the operational air velocity. There was a good agreement between the observed and the model air output temperature. The proof of principle of solar adsorption drying (SAD) and its benefits, compared to solar drying (SD) and open sun drying (OSD) and the effects of these drying methods on the quality of different yam cultivars and regeneration of silica gel is considered. The drying methods SAD, SD and OSD are compared in terms of drying cycles. The SAD had, the shortest drying cycle while maintaining the whitish colour of yam, followed by SD and OSD. However, due to the night drying, the SAD dried product is longer exposed to the drying medium (air), leading to slightly lower vitamin C content of dried sample compared to SD dried samples. OSD had the worst effect on vitamin C. The study shows that the different cultivars of yam dried at different rates. Drying has no effect on composition. Regeneration was possible with solar energy but more work must be done to obtain the information needed for improved designs and operation strategies for the desorption. Finally the perspective, impact and the potential application of the study has also been discussed.