Investigation of the Fluidization Quality in Fluidized Bed Methanation Reactors

Author:Zhang Yu Li

Supervisor:xiao zuo ye mao

Database:Doctor

Degree Year:2016

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Pages:134

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The demand for natural gas grows rapidly in China’s market.And the coal to methane technology is playing a critical role in transforming the abundant coal into clean natural gas in China.Several SNG processes are commercially available today,and in these SNG processes fixed bed is adopted as methanation reactors.The inherent temperature gradient in fixed bed requires high product gas recycle ratios and serial connection of multiple gas coolers/reactors to remove the reaction heat and control temperature distribution in the reactors,which certainly add extra capital expenditure and increase the energy consumption.Compared to fixed bed reactors,fluidized bed reactors have been shown more suitable for heterogeneously catalytic reactions with strong exothermicity,owing to the great heat transfer performance and operational flexibility.Yet there is a challenge to be addressed in developing industrial fluidized bed methanation reactors:the fluidization of catalyst with a reduced gas volumetric flow will worsen the fluidization quality in the reactor.In the carbon monoxide methanation reaction,the number of gas moles is reduced from 4 to 2,which will lead to a significant reduction of gas volumetric flow in the emulsion phase since the methanation reaction mainly takes place in the emulsion phase.If the gas volumetric flow decrease in the emulsion phase cannot be compensated by the gas flow from the bubble phase,it will cause contraction of emulsion phase and reduction in fluidization quality.Good fluidization quality is highly desired for the optimal and safe operation of fluidized bed methanation reactors.In order to understand the fluidization quality in the methanation reactors,this thesis is organized by four parts:(1)Establish a measurement technique for in situ characterization of fluidization quality;(2)Validation of the in situ method in a cold fluidized bed;(3)Experimental research on fluidization quality in a methanation bubbling fluidized bed reactor;(4)A discrete element simulation study of the fluidization quality in bubbling fluidized bed methanation reactors.The following conclusions can be obtained via the study in this thesis:1.Good fluidization quality usually means small bubble size and high bubble frequency,and thereby accurate estimation of bubble parameters can well characterize fluidization quality.First the reasons of the relatively large error of the existing pressure fluctuations measurement methods are analyzed by CFD simulations.Then a modification on the existing measurement methods is proposed accordingly:The distribution of the lateral distance between the pressure sensor and bubble centers will be interpreted by the theory of probability.And the bubble passage induced local pressure fluctuation is realized by a modification on the traditional Davidson and Harrison model.Finally a relation between the real bubble size and the measuring bubble size can be established.2.A cold model fluidized bed setup with inner diameter of 60 mm and height of 1.6 m,is built for validation of above modified methods.The high frequency pressure probes and an electrical capacitance tomography system(ECT)are used to measure the bubble size simultaneously.A reliable reference for contrast experiment measurements can be obtained with the capacitance tomography(ECT)method.The ratios between the ECT and pressure results are 3-6 before correction,while 0.77-0.92 after correlation,indicating the modified method can well describe the bubble size evolution in the fluidized bed in this study.3.A high pressure methanation reactor is built up which has the same dimension as that of the cold setup.The NiLaMn/Al2O3 methanation catalyst is then prepared.The bubble size and frequency are calculated according to the modified method for characterizing the fluidization quality in the fluidized methanation reactor.The effect of superficial velocity,pressure,initial bed height on fluidization is investigated.It is found that higher superficial velocity or initial bed height will worsen the fluidization quality,and higher pressure will improve the fluidization quality.On the other hand,the in bed temperature gradient is minimized by addition of inert particle into the catalyst,and in the meanwhile,the fluidization quality is also improved with the presence of inert particles.4.The expansion ratio of the emulsion phase in a fluidized methanation reactor is predicted by the DEM-CFD model.The results of DEM-CFD indicated that the fluidization quality with the operating condition has the same trend with the experiments.By investigating the fluidization quality,reaction conversion and meso-scale parameters by DEM-CFD results under different operating conditions,it can be concluded that the interphase mass transfer rate and in particular back mixing has a close relationship with the fluidization quality.Higher superficial velocity/initial bed height correspond to higher back mixing rate and poor fluidization quality.The fluidization quality is improved at high pressure because of the increased interphase mass transfer rate and the reduced the back mixing rate.