Research On Optical Detection Method Of CO₂ Bubble Characteristic Parameters In Biochemical Reactor

In recent years,with the rapid consumption of global fossil energy and the massive emission of greenhouse gas（CO₂）,serious energy shortage and environmental pollution have been caused.Green energy research needs to be developed urgently.Among them,microalgae biomass energy has the advantages of low production energy consumption,green and recyclable,and wide production range,and is considered to be one of the new energy sources with the most development potential at present.In the process of microalgae biomass energy production,CO₂,as one of the main carbon sources for microalgae growth,rises,dissolves and mass transfers in the microalgae suspension in the form of bubbles.However,the low CO₂conversion efficiency,low biomass yield and low oil content of microalgae in the biochemical reaction of microalgae are difficult to meet the actual requirements of large-scale industrial applications.Therefore,it is necessary to conduct in-depth research on the mechanism of CO₂energy and mass transfer and transformation in microalgae biochemical reactors,and to regulate and strengthen the microalgae biochemical reaction system;at the same time,CO₂fixation and conversion efficiency,microalgae growth rate,microalgae oil content,etc.It is significantly affected by the characteristic parameters of CO₂bubbles in the system.It can be seen that the regulation and strengthening of the biochemical reaction system of microalgae needs to be based on the characteristic parameters of CO₂bubbles.Therefore,the online non-destructive detection of the characteristic parameters of CO₂bubbles in the microalgae biochemical reaction system is very important;however,the existing detection methods are difficult to realize the online detection of the characteristic parameters of CO₂bubbles in the microalgae biochemical reaction system.Based on this,this paper proposes a new method for measuring the characteristic parameters of CO₂bubbles in biochemical reactors based on fiber optic spectroscopy,and builds a fiber optic spectroscopy detection system for the measurement requirements of the characteristic parameters of CO₂bubbles in the biochemical reaction system.The research work is mainly carried out in the following three aspects:（1）In order to detect the characteristic parameters of air bubbles in biochemical reactors,an optical fiber spectral method for on-line quantitative detection of characteristic parameters of air bubbles in biochemical reactors is proposed in this paper.The theoretical models of bubble size,frequency and rate measurement are established.The light transmission path is simulated and analyzed by Zemax software,and biochemical reactors and spatial optical coupling devices are designed.The optical fiber detection system is constructed by using broadband light source,optical fiber spectrometer,precision injection pump,high-speed camera and so on.The system detection band and system detection position suitable for detecting gas-liquid two-phase flow in biochemical reactor were explored,and then three parameters of air bubble size,frequency and rate in air-water gas-liquid two-phase flow in biochemical reactor were tested.The results show that the 1071nm band in the spectral scanning band（900 nm to1700 nm）of the system can effectively avoid the strong absorption of water and can be used as the detection band of the system.The optical detection system constructed in this paper can obtain the response signal of the characteristic parameters of air bubbles in the biochemical reactor in real time,and combined with the calculation model,the quantitative measurement of bubble size,frequency and rate can be realized.The maximum relative errors of bubble size,frequency and rate measurement are 9.8%,8.1%and 8.7%respectively.（2）In order to detect the parameters of the dissolution process of CO₂bubbles in the liquid phase under different p H,the effect of different p H values on the response of the optical detection system was investigated,and the dissolution process of CO₂bubbles with initial radius of 1.13 mm in dilute solutions with different p H values（3-11）was measured.The results show that the influence of the change of p H value in sodium hydroxide and acetic acid diluted solution on the response of optical detection system can be ignored when the temperature is 25±0.5℃.When the detection band is 1071 nm,the quantitative measurement of CO₂bubble size,CO₂bubble dissolution rate and CO₂volume mass transfer coefficient in different p H（3-11）solutions can be obtained in real time by the optical detection system,and the maximum relative error of bubble radius measurement is 8.9%.The initial radius reduction rate,dissolution rate and mass transfer coefficient of CO₂bubbles increase with the increase of liquid p H（3-11）.（3）In order to measure the characteristic parameters of CO₂in microalgae suspensions with different concentrations in a biochemical reactor,the optical detection system responses of microalgae suspensions with different concentrations(OD_900nmvalues 0 to 0.8)were analyzed experimentally,and the dissolution process of static CO₂bubbles with initial radius of 1.13 mm in microalgae suspensions with different concentrations(OD_900nmvalues 0 to 0.8)was measured.The results show that when the temperature is 25±0.5℃,the response signal（900nm to 1140nm）of the optical detection system decreases with the increase of microalgae suspension concentration(OD_900nmvalue 0 to 0.8).When the detection band is 1071nm,the optical detection system can real-time obtain the CO₂bubble size,CO₂bubble dissolution rate and CO₂volume mass transfer coefficient in microalgae suspensions with different concentrations(OD_900nmvalues 0 to 0.8),in which the maximum relative error of bubble radius is 9.5%.The initial radius reduction rate,dissolution rate and mass transfer coefficient of CO₂bubbles increase with the increase of microalgae suspension concentration(OD_900nmvalue 0～0.8).