Dynamic modeling of the circulating fluidized bed riser

Abstract

Dynamic modeling is required to understand hydrodynamics of a circulating fluidized bed (CFB) riser as a function of time. The paper develops a dynamic model of the riser from the conservation of solids mass. Solids mass that flows out of the riser requires particle residence time to close the mass balance equation. The mean residence time of solids was derived as the riser length divided by the superficial riser gas velocity normalized by a ratio of solids concentration in the riser outlet to that in the riser itself. For bed materials including high-density polyethylene beads, glass beads, and cork, the percentage of solids concentration in the horizontal crossover pipe connecting the riser exit and the cyclone inlet was found to be 22% of the average concentration of solids in the riser. Upon finding the particle residence time, we calculated the solids flow rate out of the riser and along with the knowledge of solids flowing into the riser column we determined the riser inventory as a function of time. We used the estimated solids inventory to model overall pressure drop across the riser from the conservation of momentum. The momentum equation included the balance of pressure force by the hydrostatic head of solids, and the wall frictions due to gas and solids in a fully developed zone. The prediction of overall riser pressure drop compared well with its measured values at many dynamic conditions. Therefore, our dynamic model is useful for developing advanced control strategy or for improving existing control systems of the CFB plant. The residence time approach is also useful for the design of a transport reactor.

Effects of Operating Parameters on Solids Flux in a High-Density/-Flux Circulating Fluidized Bed Riser Reactor

“…If the wall friction or solids acceleration is neglected compared with the static head of the solids, the pressure balance of the system can be obtained by eq 5 as follows. 40…”

“…For this experimental setup, the driving force derives from the pressure drop of the solids in the downcomer and the storage tank, while the resistance mainly includes the pressure drops of the feed pipe, the valve mounted in the feed pipe, the riser column, the separating system, etc. If the wall friction or solids acceleration is neglected compared with the static head of the solids, the pressure balance of the system can be obtained by eq as follows. − where Δ P dc is the pressure head of the solids in the downcomer and storage tank, Δ P r is the pressure drop of the whole riser, Δ P sep is the pressure drop of the separating system, and Δ P fp is the pressure drop of the feed pipe.…”

A Brief Review on Hydrodynamic Behaviour Analysis of Coal Gasification in a Circulating Fluidized Bed Gasifier

“…In the CFB gasification of coal, solid-holdup and solid circulation rate still being commonly treated as the parameters determining the hydrodynamic of circulating fluidized bed with valve controlled solid circulation in the riser. Solid-holdup decreases when it is accelerated by the high velocity gas stream [35]. As explained above in the gas-solid contact efficiency also helps to justify the hydrodynamic characteristic of CFB system.…”

Crusher to Mill Transportation Time Calculation—The Aitik Case

“…Modeling the system through its mass balance has the advantage that the previously presented problem of time delay is incorporated in the model [12,13]. Mass balance models can be applied on single units, as in the case of circulating fluidized bed reactors in energy engineering [14,15], or a sequence of different units in a process line, e.g., in cement production [16]. When the interest lays in the overall transportation properties of the system, one way to model such a transportation system is through the virtual silo concept introduced by Itävuo et al [12,13].…”