H - Acid Caustic Fusion Stage

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Process Engineering Guide:
GBHE-PEG-RXT-813
H - Acid Caustic Fusion Stage
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Process Engineering Guide: H - Acid Caustic Fusion Stage
CONTENTS
0 INTRODUCTION
1 DESIGN INFORMATION
1.1 Reactor Type
1.2 Temperature Range
1.3 Pressure Range
1.4 Chemical System
2 BACKGROUND
3 KINETICS AND MECHANISM
4 MAXIMUM YIELD AND IMPLICATIONS FOR REACTOR DESIGN
5 USE OF DESIGN MODEL FOR START-UP AND MANUFACTURING
MONITORING
6 BIBLIOGRAPHY
FIGURES
1 FUSION MODEL OUTLINE MECHANISM AND KINETIC SCHEME
2 TEST RUN OPTIMISATION OF HEATING TIME 3600 kg/h STEAM

0 INTRODUCTION
This document supersedes previous GBHE Report; Process Engineering
Design Guide GBHE-PEG-RXT-818.
1 DESIGN INFORMATION
1.1 Reactor Type
Batch autoclave, optimized heating/cooling cycle.
1.2 Temperature Range
120°C to approximately 220°C.
1.3 Pressure Range
1 to 23 bar.
1.4 Chemical System
Series and parallel complex reaction scheme, homogeneous, non-catalytic
2 BACKGROUND
H-Acid (4 amino 5 hydroxy - 2, 7 naphthalene disulfonic acid) is an
important intermediate in dyestuffs manufacture. It is produced by the
''caustic fusion'' of Koch Acid (4 amino - 2, 5, 7 naphthalene trisulfonic
acid) traditionally in batch autoclaves. Since the desired reaction involves
the replacement of one of three SO3H groups by an OH, not surprisingly
isomer byproducts are obtained. In addition the amino group itself
undergoes substitution. The major reactions are shown schematically in
Figure 1 (the traditional shorthand notation S = SO3H being adopted).

For etymological consistency the kinetic modelers christened the tarry
residues "Ghost Acid" (that portion of the starting materials whose useful
life has departed). Process and laboratory experience showed that yield
was crucially affected by variations in the time/temperature history of the
batch. The emergence of analytical techniques in the 1960s made
analysis of process intermediates possible and as a consequence
elucidation of kinetic data became a distinct probability. This case study
summarizes the work carried out aimed at both optimizing existing
manufacturing facilities and designing new plant. The technology
described has been incorporated into a European Plants.
3 KINETICS AND MECHANISM
Since the reaction is carried out in the presence of excess caustic soda
the scheme of Figure 1 is most simply represented by a set of pseudo first
order mass action kinetic equations. This simple model was subsequently
found to fit the experimental data within the accuracy of that data. From
isothermal laboratory fusions at different temperature levels the activation
energies and pre-exponential factors were determined. In terms of H-Acid
yield, only reactions I, II and III (see Figure 1) are of direct interest for
further modeling. Mass balances on Koch and H-Acids may be written:

4 MAXIMUM YIELD AND IMPLICATIONS FOR REACTOR DESIGN
An unconstrained optimization on the kinetic Equations (1) and (2) showed
that a maximum yield of some 84% was achievable under (unreal)
conditions of rapid heat-up and instantaneous quench. Since this figure
was over 10% higher than current manufacturing attainments in heat
transfer limited autoclaves the margin for improvement was considerable.
Clearly the nearest approach to the desired temperature profile predicted
by the unconstrained optimization would be approximated in a jacketed
plug flow reactor. Pilot plant experiments on such a reactor system were
unsuccessful due to a variety of mechanical details rendering operation
hazardous. A compromise approach was investigated whereby extra heat
transfer surface (in the form of coils) for rapid heating and pressure let
down for flash evaporative rapid cooling in a batch autoclave was
adopted. A mathematical model incorporating the heat transfer
phenomena was constructed around which optimization of temperature
profile (by Rosenbrock) was introduced. With the limitation of jacket
heating/cooling on the existing process plant only small increases in
overall yield proved possible. For new autoclave designs incorporating
coils, yields as high as 81.5% were predicted at cycle times much shorter
than current plant practice. With finite steam supply the optimization
reduced to the single variable of change-over from heating to cooling.
This permitted the Rosenbrock technique to be replaced by a simple
Fibonacci search (Ref. [1]). Laboratory checks on the predicted profiles
gave yield figures within 0.1% of the model's predicted yields. As a design
tool the mathematical model was refined to incorporate corrosion data,
programmed cooling (to avoid stress cracking) and fitted with digital plotter
output (see example in Figure 2).

5 USE OF DESIGN MODEL FOR START-UP AND MANUFACTURING
MONITORING
Under start-up and indeed routine manufacturing operations various
mishaps are likely to make strict adherence to the temperature profiles
impossible. The most likely cause of such problems will be "hiccoughs" in
the steam supply. The complexity of the model lies with reproducing the
reactor environment and optimization routines rather than the simple
kinetic equations. In principle the latter can be incorporated into the
process control computer scheme to predict the H-Acid yield pattern
developing in time as a consequence of the achieved time temperature
sequence for a particular batch. In new manufacturing plant this will be
done and used as a production management tool.
6 BIBLIOGRAPHY
[1] Gill P E, Murray W & Wright M H, Practical Optimization, Academic Press
(1981), London, pp 89 - 90.

FIGURE 2 TEST RUN OPTIMIZATION OF HEATING TIME 3600 kg/h STEAM

H - Acid Caustic Fusion Stage

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