(173c) Overcoming Heat Transfer Bottlenecks in Non-Equilibrium Chemical Processes through Autothermal Operation

Overcoming Heat Transfer Bottlenecks in Non-Equilibrium
Chemical Processes through Autothermal Operation

Robert C. Brown^1,
2 and Chad Peterson²

¹Bioeconomy
Institute

² Department
of Mechanical Engineering

Iowa State
University

Innovations in Process
Intensification Session

Topical C: Process
Intensification

AIChE Spring Meeting, Orlando, FL

April 22-26, 2018

Abstract

Heat transfer is the bottleneck in many chemical
processes, limiting throughput for a reactor even when chemical reaction and mass
transfer are very fast. Efforts to enhance heat transfer can add significantly
to equipment costs. Furthermore, with heat transfer rate limiting, reactor
capacity only scales as the square of reactor diameter, constraining throughput
as reactors are built larger and increasing capital costs faster than expected
from economies of scale principles.

Autothermal operation,
which balances the energy demand of endothermic chemical reactions with energy
supply from exothermic chemical reactions, can overcome heat transfer
bottlenecks. Although autothermal operation has been previously demonstrated
for equilibrium chemical processes
(prominent examples include methane steam reformers and biomass/coal
gasifiers), application to non-equilibrium
processes has been little explored. 
Identifying complementary endothermic and exothermic reactions that can
occur simultaneously while preserving the same products of an externally heated
(or cooled) chemical process is not trivial. However, the gains in process intensification would be
substantial, increasing throughput and reducing capital and operating costs of
a chemical process.

We have experimentally demonstrated the principle of
non-equilibrium autothermal operation for the fast pyrolysis of biomass.  Autothermal pyrolysis was achieved by
admitting a small amount of air into a fluidized bed pyrolyzer, which partially
oxidizes biomass reactant or pyrolysis products to provide the enthalpy of
pyrolysis without substantially affecting the desired pyrolysis products.  Decreases in non-condensable gases such as
carbon monoxide and methane during autothermal pyrolysis would suggest their
oxidation is responsible for a significant fraction of the exothermic energy
released during this process.  However,
autoignition temperatures for the non-condensable gases in pyrolysis products is
higher than typically occurs in fast pyrolysis, suggesting other exothermic
chemical reactions provide the energy to support autothermal pyrolysis.

We are developing a chemical kinetic model of autothermal
pyrolysis to better understand the source of energy for the process.  The model indicates that non-condensable
gases are not substantially oxidized during autothermal pyrolysis. Instead,
precursors of these gases along with other light oxygenated compounds are
oxidizing and providing the enthalpy for pyrolysis. The model is also
investigating solid-gas reactions to investigate the contribution of lignin and
char to the energy required for autothermal pyrolysis.