Tham, Thing Chai
(2018)
Improving drying efficiency and energy saving for crumb natural rubber drying with combined drying technologies.
PhD thesis, University of Nottingham.
Abstract
Natural rubber (NR) or cis-1, 4-polyisoprene is obtained from the latex that is tapped from a tree known as Hevea Brasiliensis. It can be in the form of preserved latex concentrates or further processed into sheets, crepes, block rubber that associated with its unique characteristics such as great elongation strength, excellent building tack, low rolling resistance, low heat build-up and good low-temperature performance. These are essential characteristics in dynamic working environment and application especially for vehicle tyre and rubber thread. To produce good quality of technically specified rubber or block rubber from raw natural rubber, drying process plays a vital role in the raw rubber processing. To date, the level of technology involves rubber drying is still inefficient and primitive. Non-uniform drying and the occurrence of wet rubber in the final product are the major drawbacks when using the conventional type of trolley dryer. Also, fuel and electricity usage for a single drying operation is noticeably intense and costly. Indeed, the main limitation of conventional drying method is the drying time that is mainly dependent on temperature gradient inside the product and cannot be sustainably reduced because the product is heat sensitive; whereas drying at high temperature for prolong hours could result in quality deterioration too.
The prime objective of this research was to investigate combined drying as a mean of crumb rubber processing to reduce the drying time, energy consumption and to produce the desirable dried product. In the present study, crumb rubbers were dried by alternative drying strategies whereby additional drying technologies such as hot air, vacuum, and microwave was integrated into the drying system. Both engineering properties (drying kinetics, effective moisture diffusivity and specific energy consumption) and physicochemical properties (colour, thermal oxidative level, rheological, textural and surface topography) of all dried products were measured and evaluated. The comparisons were made against samples from hot air drying.
The finding shows the drying time of microwave-convective drying (MWHA) was found 89% and 50% shorter than convective hot air drying (HA) and microwave (MW) drying, respectively. In terms of drying rate performance, the maximum drying rate of MWHA was found 455% and 62.5% higher than HA and MW drying, respectively. On the other hand, effective diffusion coefficient (Deff) values of both MW drying and MWHA were ranging from 1.48 x 10-8 m2/s to 5.59 x 10-8 m2/s that is six times higher than HA which has lowest Deff recorded at 0.22 x 10-8 m2/s. It is noteworthy to mention that the energy consumption of MWHA is much lower than other drying strategies due to its excellent time saving and energy efficient features in the drying process. The average specific energy consumption (SEC) values of MWHA’s drying strategies were scored below 0.22 MJ/ gH2O whereby HA drying yields SEC values of 0.36-0.44 MJ/gH2O and highest was recorded in vacuum assisted drying which scored at 0.75 MJ/ gH2O. MWHA dried rubber has exhibits lightest colours, minimum total colour change and consistent colour appearance over the range of drying methods. The lesser colour change can be an indication of reduced oxidative reaction and hence better ageing properties of dried rubber. This is corresponding to the high value of activation energy obtained by MWHA’s dried rubber in the thermal degradation study by using the Coats-Redfern methods. Similarly, the rheological properties of MWHA dried rubber including PRI and Po were found in par and the volatile matter was 28.6% lower in comparison with SMR 20 block rubber. In terms of springiness indices, MWHA dried rubbers gave the highest at 72.23% showing marked performance in mechanical properties. In a nutshell, the potential of combined drying techniques in crumb rubber has been proven through this research and able to improve both engineering and physicochemical properties while the drying time is significantly reduced. Furthermore, these findings will supplement the knowledge on crumb rubber drying process which is beneficial to raw rubber processing industries in having economically feasible drying option to be applied to the existing plants.
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