| A microbial fermentation process belongs to a cell metabolic process with material conversion. To enhance the conversion ratio to the target product, it's necessary to have a real-time analysis on the dynamic characteristics of a microbial fermentation process, so as to optimize the fermentation process in real time. A lot of fermentation processes discharge a large amount of off-gas. The component, volume and production rate of off-gas reflect in macro-scale the material transformation rule inside the microbes. In this thesis, the acetone-butanol-ethanol (ABE) fermentation process is studied and correspondingly, online off-gas monitoring system is designed for anaerobic fermentation. The established system can analyze the components of off-gas in real time, and display the real-time results with a historical record the memory.By studying on the principle of ABE fermentation, it is recognized that the fermentation process consist of two phases, namely, acidogensis and solventogensis. At the initial phase of acidogensis, acidic compounds like acetate and butyrate are produced and the pH value of the medium drops down rapidly, while off-gas of CO2 and H2 are discharged. When the phase of solventogensis comes up, the chroteridium acetobutylicum consumes acetate and butyrate to turn out acetone, butanol and ethanol, while CO2 and H2 are also produced. This indicateds that the material transformation process in micro-scale via cells is reflected by the variation on the components of the medium and off-gas in macro-scale.Due to that ABE fermentation requires a rigorous anaerobic environment, a primary problem to be solved for monitoring the fermentation process is to ensure the anaerobic environment against external disturbance. An entirely closed monitoring method is proposed in this thesis, based on testing different connection between an anaerobic bioreactor and gas analyzer. That is, a conical flask is used to isolate the bioreactor and the external environment, such that a cyclic measurement circuit is constructed between the conical flask and the gas analyzer. In this way, the fermentation environment can be effectively protected from external disturbance, such that the real-time variation of the off-gas components can be effectively monitored on line.To cope with the measurement error arising from electromagnetic disturbance, sensor aging and environmental change to the multi-component gas analyzer, a self-correction method is proposed based on using a least-squares recursive model. By performing calibration and correction tests with the standard gas tanks, a polynomial calibration model and the optimal correction formula are established. Experimental results demonstrate that the proposed method can effectively reduce the measurement error.By applying the proposed online off-gas monitoring system to fermentation experiments under different conditions, and comparing the results with off-line sampled data and fermentation principle, it is verified that the obtained data of off-gas components, production rate volume are coherent with those of off-line sampled data, and the online analyzed data can be used to distinguish different phases together with the shift points. Therefore, the online analyzed results can offer an important reference for optimizing the fermentation process and implementing automatic control. |
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