Study On The Gas-solids Flow Characteristics In Adjustable Series Combined Riser

Circulating turbulent fluidized bed is a better form of reactor in the development of fluid catalytic cracking for heavy oil processing with high solids concentration,less gas-solid backmixing and homogenous axial-radial distribution.Compared with others,circulating turbulent fluidized bed has particular structure and operation as follows.The riser was made of a small diameter bottom section and another big upper one series where the secondary air supply device was installed.During the operation,a lower superficial velocity was adopted in the small diameter bottom section in order to establish a dense region of circulating turbulent fluidization flow for efficient gas-solid contact and reaction while a higher superficial velocity was adopted in the big diameter upper section in order to establish a dilute region of fast fluidization flow(pneumatic transport)for higher solids circulation rates operation condition and rapid materials export.Many researches have been conducted on the gas-solids flow characteristics in the dense region of circulating turbulent fluidization.But it has been rarely reported in detail about whether there exist other flow regions,how the circulating turbulent fluidization regions transmute into each other and whether there exists an operating regions lower limit,the gas-solids flow characteristics in the dilute region,the effect of the secondary air supply on both the dense and dilute regions and so on.Aimed at providing reference and guidance for industrial design,structure optimization and operation regulation of adjustable series combined riser,particular researches and analysis were conducted on the problems mentioned above in this thesis.The results showed that there was obvious partition between the dense and dilute phases in the adjustable series combined riser.It could be realized for high density flow state in the dense region through adjustment for the adjustable series combined riser structure and operating process when the solids circulation rate was less than200kg/(m~2·s).And the transforming point between lower density and higher density appeared ahead compared with that in the same diameter high density circulating fluidization bed.When the solid/gas feed ratio was not less than 27,the dense region kept for low density as fast fluidization flow.And then it came into high density flow state with increase of the solid/gas feed ratio.At the same time,the particles local time-mean velocity and the net particles mass flow rate performed upward.When the solid/gas feed ratio varied from 27 to 50,the dense region was in the transition stage between high density circulating fluidization and circulating turbulent fluidization.The local flow characteristics were similar to that in circulating turbulent fluidized bed while the cross-sectional averaged and the overall flow characteristics were similar to than in high density circulating fluidized bed.And the dense region came into circulating turbulent fluidization state when the solid/gas feed ratio exceeded 50.The secondary air supply had a restraint effect on the gas-solids flow characteristics in the dense region to cause the solids holdup increase and the particles velocity decrease in the central area(h=0.95 m)of the dense region along the axial direction.It also had a jet reduction effect on the gas-solids flow characteristics in the dilute region to cause the solids holdup increase and the particles velocity decrease in the bottom area(h=3.10 m)of the dilute region.