(98c) Inherently Safer Reactor Design: Challenges in the Changeover from Semi-Batch to Continuous

Abstract for 2017 AIChE Spring Meeting and 13th Global Congress on
Process Safety March 26-30, 2017 San Antonio, TX

Session Selection: 19th Process Plant Safety Symposium (PPSS)
(T1A): Process Safety Challenges in Batch Operations - Oral Session

Inherently Safer Reactor Design: Challenges in the Changeover from
Semi-batch to Continuous

Jingyao Wang¹, Maria Papadaki², M. Sam
Mannan¹

1      
Mary Kay O'Connor Process Safety Center, Artie McFerrin Department
of Chemical Engineering, Texas A&M University, College Station, Texas
77843-3122, USA,

+1(732)910-0929, jingyao@tamu.edu; +1(979) 862-3985, mannan@tamu.edu

2      
Department of Environmental & Natural Resources Management,
School of Engineering, University of Patras, Seferi 2, Agrinio GR30100,
Greece, marpapadaki@upatras.gr

Abstract: A significant number
of chemical incidents are related to thermal runaways in batch reactor operation.
For example, 189 industrial batch reactor incidents were reported from 1967 to
1989 (Barton & Nolan,
1989). Those
incidents involved multipurpose batch or semi-batch reactors, which are
intensively utilized in agrochemical, fine-chemicals and pharmaceutical production.
The reactions performed often employ thermally unstable and/or toxic reactants,
products or intermediates. Typical reactions of this type are oxidation,
epoxidation and halogenation reactions of organics. Most highly exothermic
reactions are performed in semi-batch mode with one of the reactants being
added over the remaining ones, thus controlling the reaction rate and the
amount of the generated thermal power. Although inherently safer than the batch
mode, still this option has significant draw-backs in terms of both the safety
and the efficiency of the process.

In the present work
the changeover from semi-batch to continuous of a highly exothermic catalytic organic
oxidation is researched. The oxidant of choice is hydrogen peroxide, the
thermal and catalytic decomposition of which is always a concern to the safety
of such processes:

There are several
hazards related to those oxidations. However, it is envisaged that the reaction
can be performed more efficiently and in an inherently safer manner, with
higher selectivity towards the final product, if the semi-batch reactor is
replaced by a continuous one.

In view of that,
different reactor geometries have been considered and their pros and cons are
discussed. The goal is to design a reactor in which reactant A concentration is
always maximal (plug-flow) while hydrogen peroxide concentration is minimal
(CSTR flow). Therefore, it is critical to design the reactor so as to achieve
the optimum mixing conditions for hydrogen peroxide, which can be potentially
achieved if it is distributed uniformly along the reactor through rows of
radial jet flows. After developing the mass, energy and momentum balances, different
reaction geometries and hydrogen peroxide distribution patterns were simulated
using Computational Fluid Dynamic in ANSYS Fluent. Figure 1 shows the effect
which hydrogen peroxide injection-speed has in a tubular reactor of 0.1 m diameter
and 1 m length, which can produce up to 1000 ton/year of fine chemicals per
year. As can be seen in Figure 1, the optimum hydrogen peroxide injection speed
was 7.5 m/s; under these conditions uniform distribution of the temperature
profile, to prevent hot spots can be achieved. The heat generation will be also
integrated in the final design to preheat the feed stream, thus reducing
utility costs. The study shows that such a design is economically sounder, more
efficient and safer compared to traditional semi-batch processes.

Figure 1. Temperature contours
within reactor at different jet velocity

Keywords

Process Intensification, Reactor design and
simulations, Computational Fluid Dynamics, Inherent Safer Design

References

Barton, J., & Nolan, P. (1989). Incidents in the chemical industry due to thermal
runaway chemical reactions. Hazards X: Process Safety in Fine and Speciality
Chemical Plants(115), 3-18.