(153c) (Micro-)Mixing Effect in Alkylbenzene Sulfonation Using Spinning Disc Technology

Since the introduction
of process intensification numerous chemical processes have been successfully
optimized and have obtained an excellent control on conversion and selectivity,
without making any concessions on process safety and production rate^[1]–[3]. The idea
of working at the intrinsic kinetic rate rather than having mass or heat
transfer limitations gave rise to intensified compact reactors which have the
potential to achieve the same throughput as conventional large batch reactors.
Such compact reactors include micro reactors, rotating packed beds, and
spinning disc reactors (SDR). For example, a significant decrease in heat^{[4], [5]} and mass^[6]–[8] transfer
limitations were reported for the SDR. Furthermore, the local mixing time was
measured to be as low as 1.13×10^-4 seconds^[9].

Technologies such as
the SDR are especially useful when working with reaction systems which are
highly exothermic, are very rapid in terms of reaction rates, consist of
multiple phases, and are very sensitive towards selectivity. An example is the
extremely fast sulfonation of alkylbenzenes using fuming sulfuric acid to form
the para-sulfonic acid. The formation of byproducts, such as unwanted ortho-
and meta-sulfonic acid isomers as well as di sulfonic acids and sulfones,
decreases the selectivity significantly.

In this work, a
Synthetron Spinning Disc Reactor (S-SDR) was used to investigate the effects of
mixing on the selectivity of the sulfonation of toluene and ethylbenzene
reactions using fuming sulfuric acid. The reactor consists of a disc (a disc
radius of 7.62 cm) rotating at a speed range of 50 to 9000 rpm inside a narrow
cylindrical encasing with a gap width of 0.2 millimeter. Reactants were fed
below the disc and the reaction mixture flowed upwards exiting the reactor
around the disc shaft.

The experimental study
was carried out by varying the rotational speed of the disc, while maintaining
a constant residence time. This was tested for different molar flow ratios of the
reactants. Several analysis techniques (C13-NMR, H-NMR, LC-MS and titration)
were used to quantitatively determine the reaction products and their relative
abundance. Additional analyses were conducted to determine the viscosity of the
outflow as function of the composition. Figure 1 depicts the toluene and
ethylbenzene results for different molar flow ratios. The graphs clarify the discussion
below. 

Figure 1 (a) Selectivity towards the para isomer (S_pSA),
(b) conversion (X) and (c) viscosity ratio (𝜂_product/𝜂_AB) versus the rotational speed (Ω) for
constant residence time (τ). All measurements were done for different
molar flow ratios (Ψ) which is defined as the inlet molar flowrate of the
considered alkylbenzene over the inlet molar flowrate of fuming sulfuric
acid. Toluene (red): + Ψ = 0.9, 𝜏 = 6.5 s   ○ Ψ = 2.0, 𝜏 = 6.5 s. Ethylbenzene (blue): + Ψ = 0.8, 𝜏 = 6.4 s   ○ Ψ = 1.6, 𝜏 = 4.3 s   □ Ψ = 2.4, 𝜏 = 4.3 s   × Ψ = 3.2, 𝜏 = 4.3 s

The
results in figure 1 (a) show an increase in selectivity with increasing
rotational speed (mixing intensity) whilst maintaining a constant conversion
for both toluene and ethylbenzene (figure (b)). The selectivity of toluene
sulfonation was predominantly determined by the formation of disulfonic acids
and sulfones. For ethylbenzene sulfonation only the formation of disulfonic
acids was found.

At constant conversion
(i.e. 1000 rpm to 9000 rpm), the increase in selectivity was found to be 10%
for the toluene experiments, while for the ethylbenzene experiments this was 3
to 5% depending on the different flow ratios. The latter showed that
ethylbenzene sulfonation is the least sensitive towards mixing in line with the
lower reaction rate as measured in earlier work^[10]. Given the constant
conversion, the observed shift in selectivity can only be explained in terms of
local mixing phenomena rather than bulk effects such as heat or mass transfer.
This is because bulk effects influence the overall reaction rate so that the
conversion should change subsequently for a constant residence time. As was
discussed above, this was not observed for both toluene and ethylbenzene
sulfonation.

At low rotational speed
the measured viscosity ratio (𝜂_product/𝜂_AB) peaked at
circa 200 for the toluene case, while for the ethylbenzene this was 135 (figure
1 (c)). In both cases, the viscosity ratio decreased for increasing rotational
speed. The lowest ratios at maximum rotational speed (9000 rpm) were 110 and 70
respectively. At this condition, maximum selectivity was obtained for all flow
ratios. The strong increase in viscosity probably worsens the local turbulent
mixing, resulting in locally formed hot-spots and large concentration
gradients. This stresses the importance of local mixing in these systems.

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