19-08-2014, 10:21 AM
Gas liquid dispersions are widely encountered in industrial reactors, because many important synthesis reactions involve reaction of gas with liquid. Proper distribution of bubbles inside the reactor serves duel purpose: it improves the contacting of gas with liquid being reacted, and it also generates flow currents inside the reactor. However, in order to achieve the desired result with reliable design, it is important to understand the characteristics of bubbles, particularly when they are in large groups forming a dispersion. Efforts have also been made for getting the quantitative estimates of forces (such as drag, lift, virtual mass, etc.) on a single bubble or in a swarm. The local bubble size distribution is important for determining the spread in bubble sizes encountered, and it signifies the amount of breakup/coalescence present in dispersion.
Gas liquid dispersions are widely encountered in industrial reactors, because many important synthesis reactions involve reaction of gas with liquid. Proper distribution of bubbles inside the reactor serves duel purpose: it improves the contacting of gas with liquid being reacted, and it also generates flow currents inside the reactor. However, in order to achieve the desired result with reliable design, it is important to understand the characteristics of bubbles, particularly when they are in large groups forming a dispersion. Efforts have also been made for getting the quantitative estimates of forces (such as drag, lift, virtual mass, etc.) on a single bubble or in a swarm. The local bubble size distribution is important for determining the spread in bubble sizes encountered, and it signifies the amount of breakup/coalescence present in dispersion.
The current study focuses on measurement of bubble size and liquid circulation velocity. Velocimetry techniques are mainly of two types: Non intrusive techniques such as LDA and PIV, and Intrusive velocimetry techniques like hot film anemometry. The conductivity probe utilizes the large difference in electrical conductivity of gas and liquid to measure the gas hold-up, gas velocity and bubble size. The onductivity probe to be used in the present work has been developed in-house. Pressure fluctuation measurement carried out with miniature piezoresistive diaphragm type sensors facilitates the estimation of some of the quantities like averaged liquid velocity, frequency of velocity fluctuations and the bubble ize information in operating equipment at industrial scale, and offers an alternative for process monitoring of pilot and plant scale reactors for their performance evaluation.
The velocity data obtained from LDA was analyzed with wavelet based multiresolution analysis to separate gas-and liquid velocities and to infer gas hold-up and bubble sizes. PIV images were analyzed for liquid velocity and bubble size and shape measurement. The data from pressure fluctuation signals was analyzed to obtain bubble size and liquid circulation velocity. The data from conductivity probe is analyzed to obtain gas holdup, gas velocity and bubble size. High speed camera was used for measuring bubble sizes for calibration of conductivity and pressure probe.
Measurements have been made in a rectangular (200x15 mm cross section) column and in a cylindrical ubble column (385 mm diameter). In the rectangular column, pressure time series were recorded with one sensor near the sparger region and the other at the measurement location i.e. at 400 and 600 mm height (H/D=2 and 3). Data were collected at two different sampling frequencies of 200 Hz and 20 kHz n order to facilitate different ways of signal analysis. The bubble sizes at the measurement location were also captured from the images by a high speed camera (Photron Fastcam super 10kc). Images were processed with a program written in MATLAB. Two spargers were used: a single point sparger and a five point sparger with equal free area of 0.16 % to see the effect of the difference in sparger geometry n the robustness of measurement technique.
In the experiments planned for cylindrical column, the pressure sensors will be fixed at H/D 1, 2, 3, 4 and 5. The pressure fluctuation signals thus recorded will be processed for bubble size and liquid velocity variation in radial direction at each axial location. The bubble diameters obtained with PFS analysis will be cross checked with sizes obtained from the conductivity probe.