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Abstract
Odours from the drying process at rubber processing plants have been identified as a major malodour contributor. The increasing number of complaints from these operations has resulted in the suspension and/or shutdown due to environmental impacts by local communities. Previous studies have indicated that packed-bed wet scrubber typically adopted as an odour abatement technology is ineffective at treating odours from these plants. Literature review reveals there is various chemical groups of volatile organic compounds emitted from the raw rubber processing. To date, no comprehensive analytical study was conducted on the effectiveness of wet scrubber in removing the VOCs. This research aims to understand the composition of VOCs emitted at full-scale drying processes in Malaysian rubber processing plants in terms of their chemical composition and sensorial profile. This led to the identification of key odorants responsible for the malodour issue. The wet scrubber performance to remove the VOCs was investigated as well as the operation optimisation to improve its performance. The adsorption by activated carbon was explored as a potential secondary treatment to improve the overall VOCs removal for emissions from rubber drying processes. Lastly, the sustainability of all options to upgrade the wet scrubber system was studied.
The VOCs emissions samplings were performed at two typical commercial rubber processing plants in Malaysia. VOCs samplings were collected at both inlet and outlet of the wet scrubbers using a nalpohan bag attached to a vacuum drum and subsequently transferred to sorbent tubes at dry and wet seasons as well as at different operational times. The VOCs quantification was performed using a gas chromatography-mass spectrometry/olfactometry (GC-MS/O). Additionally, the presence of hydrogen sulfide (H2S) was determined using a H2S analyzer (Jerome 631-X) in one of the sampling periods. For the optimisation, the design and operating conditions of one of the plants were used in simulation and experimental studies. Meanwhile, the adsorption behaviour of selected VOCs was investigated on two types of virgin activated carbon (AC) namely coconut-based AC (CSAC) and palm kernel-based AC (PKSAC) manufactured in Malaysia. The ACs were characterised accordingly to determine their surface characteristics. The environmental impact of the proposed improved odour abatement technologies was compared by Life cycle Assessment (LCA).
A total of 80 VOCs from various chemical functional groups was frequently detected by GC-MS/O with 11 new compounds not previously reported. 50 % of the chemical concentration of the emissions was dominated by the volatile fatty acids (VFAs) and 16 critical VOCs were identified including key odorants. H2S was detected in the emissions and potentially contributes to the odour impact. The composition of the emissions was observed to be influenced by the seasonal variation in terms of the number of VOCs detected for each season. Odour wheels were developed for the first time based on the sensory analysis of full-scale rubber emission before and after treatment by the wet scrubber as a management tool for on-site plant operators and regulatory authorities to assess the malodour impact on surrounding communities. The performance analysis of the existing wet scrubber technology revealed that it was ineffective at removing VOCs, indicated by the high chemical concentration and odour activity value (OAV) detected at the outlet emissions of the wet scrubber except for single-chain VFAs. The comparable odour categories and the number of sensory-related VOCs in both inlet and outlet emissions further revealed the wet scrubber’s poor sensory removal. The wet season observed a higher wet scrubber performance because of the greater VFAs concentration detected in the emissions. However, the wet scrubber is not suitable to remove the H2S due to its poor and inconsistent removal. The study demonstrated that the combination of sensory and quantitative analysis improved the accuracy to identify the odorants from rubber emissions and investigated wet scrubber poor performance.
The simulation study revealed that the performance of the wet scrubber can be optimised (> 80 % chemical concentration removal) by modification of some of the operating conditions, namely the application of a higher liquid/gas ratio and greater interfacial area of packing to remove water-soluble VOCs and subsequently, reduce the odour impact (> 90 % OAV reduction). Laboratory-scale optimisation experiments demonstrated that the VOCs solubility is highly correlated with their absorption efficiency. Furthermore, the absorption of the VOCs is best at higher gas temperature (> 45 C) and low liquid temperature (< 10 C). The experimental results show the condition in the gas phase has a greater influence on the removal efficiency compared to the liquid condition. Optimal removal of the key odorants and other critical VOCs at this stage is necessary to minimise the odour impact and organic loading before subsequent treatment.
The adsorption by activated carbon has the potential to be incorporated as a secondary treatment to remove the remaining low water-soluble VOCs that are inefficient to be removed by the wet scrubber. The boiling point of the VOCs was found to be the primary factor that influences AC adsorption behaviour whereas polarity and molecular structure were secondary factors. The emissions consist of multi-component VOCs, an adsorption competition has been observed where higher boiling point VOCs have a stronger affinity with the AC and displace the weaker adsorbed lower boiling point VOCs. The presence of high relative humidity (RH >70 %) was found to shorten the breakthrough times greatly and adversely affect the AC adsorption performance. The breakthrough of VOCs categorised as odorant is more critical because of the greater odour impact contribution than its chemical concentration. CSAC demonstrated greater adsorption capacity (average 38 %) than PKSAC (average 11 %) due to the different surface characteristics. However, both ACs showed comparable adsorption behaviour. Therefore, AC adsorption could be employed as a polishing (or secondary) stage after full-scale wet scrubber abatement to improve the overall odour mitigation from rubber drying processes.
Life cycle assessment (LCA) was performed on the existing wet scrubber (WS), optimised wet scrubber (OWS) and a hybrid of the optimised wet scrubber with activated carbon (OWS+AC) at full-scale operation. It was found that the impact of malodorous emissions was the greatest for the direct emissions (> 99 %) of WS due to its lower efficiency in removing the malodour. 3-methylbutanal and 3-methylbutanoic acid have the greatest contribution from the direct emission of WS whereas H2S has a greater contribution from additional components used in OWS and OWS+AC. Electricity was found to be a major contributor to global warming and either ozone formation or human health potential impact categories with the increasing trend in WS, OWS and hybrid OWS+AC, accordingly. Meanwhile, the upgrading of the wet scrubber has also impacted the operational cost. In general, the environmental impact contributed by WS primarily came from direct emissions whereas the whole supply chain of OWS and hybrid OWS+AC has a greater environmental impact than their direct emissions. Nevertheless, the application of renewable energy is a good option to reduce the environmental impact.
