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Modelling and simulation of reactive gas-solid flows in circulating fluidized bed reactors

Sánchez, Rafael Antonio
Doctoral thesis
Åpne
PhD (Låst)
Permanent lenke
http://hdl.handle.net/11250/276655
Utgivelsesdato
2014
Metadata
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Samlinger
  • Institutt for kjemisk prosessteknologi [1425]
Sammendrag
The thesis presents a study on mathematical modelling and numerical simulation of

reactive gas-solid flows operating in circulating fluidized bed (CFB) reactors, in

particular environmentally-friendly methane reforming processes for hydrogen

production. The total mass, momentum, heat and species mass balances, simplified to

their one-dimensional forms, were discretized over the problem domain using the Finite

Volume Method and implemented numerically with Matlab. The transfer of solids

between the reactor units constituting a CFB reactor was modelled via the addition of

source and sink terms in the governing equations.

A Proportional-Integral-Differential (PID) controller model was used in each reactor unit

to regulate the temperature of the solid phase and keep it close to optimal values. A PID

controller was also used to regulate the solid circulation rates between the CFB’s reactor

units through control of the inlet gas velocities.

The model developed represents a step further in complexity from the conventional

Kunii-Levenspiel type of models for CFB reactors. The Kunii-Levenspiel models assume

a stagnant or steady state condition for the solids in the bed, which is not adequate because

the solid circulation rates must be calculated and because the properties of the solids

change over time. Yet the one-dimensional model developed represents a trade-off

between accuracy and computational cost.

The chemical processes studied were the Steam Methane Reforming (SMR) process

operated in a bubbling bed fluidized bed reactor as well as the following processes

operated in a CFB reactor: the Sorption-Enhanced Steam Methane Reforming (SE-SMR)

process, the Chemical Looping Reforming (CLR) process and the CO2 post-combustion

adsorption process. In order to validate the fluid dynamic model and the kinetic models

of the different chemical processes, simulation results were compared with experimental

data from the literature. The performance of each process was investigated, and the solid

circulation rates as well as the heat transfer between the reactor units of the CFB due to

solid circulation were found to be of importance. In the CO2 post-combustion adsorption

process it was found that there is a maximum calciner diameter that can be employed due

to the high heat exchange requirements imposed by CO2 desorption. Simulations of the

SE-SMR process usually indicated a dry hydrogen mole fraction above 95%. In one

publication, the optimal range of operation for the SE-SMR process was found to be

between 830 K and 930 K. For the CLR process, a model was developed and validated in which Ni-based solid particles act both as catalyst for the SMR reactions and as oxygen

carriers.

A study on the energy requirements for hydrogen production via three different chemical

processes was also conducted. In a first stage, the fuel was reformed via either SMR, CLR

or SE-SMR. In a second stage, the product stream information from the first stage was

used as input in a Pressure Swing Adsorption (PSA) unit model for hydrogen purification.

The results indicated that the SMR process required the least energy for hydrogen

production, followed by the CLR and the SE-SMR processes.
Utgiver
NTNU
Serie
Doctoral thesis at NTNU;2014:368

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