2.2 Experimental setup
To eliminate the influence of interphase mass transfer on the reaction
and reduce measurement error, the kinetics of CB nitration were studied
under homogeneous conditions. A known amount of CB was added to a sealed
glass vessel containing sulfuric acid. The mixture was stirred for 1 h
with a magnetic stirrer (RCT basic, IKA, Germany) at room temperature to
form a homogeneous solution (solution 1). According to the
literature,5 a sulfonation reaction between CB and
sulfuric acid is not expected to occur at the temperature. Sulfuric acid
and fuming nitric acid were mixed to form the acid phase (solution 2).
The molar ratio of nitric acid to CB was adjusted by changing the amount
of nitric acid added to solution 2. The mass fraction of sulfuric acid
used in the experiment was between 85% and 95%.
As shown in Figure 1 , solution 1 and solution 2 were loaded
into two airtight syringes (50 mL, CHEMYX, USA) and delivered by two
syringe pumps (Fusion 6000, CHEMYX, USA). The two solutions were allowed
to pass through a sufficiently long, precooled capillary loop so that
they could reach the preset reaction temperature. After being mixed in
the first micromixer (316 stainless steel, 250 μm bore, VICI, USA), the
solutions entered the microchannel reactor for reaction, and the
residence time was controlled by adjusting the length of the reaction
capillary. In the second micromixer (316 stainless steel, 250 μm bore,
VICI, USA), the reaction was quenched in situ by a large amount of
water, which was delivered by a metering pump (2PB-3020, Szweico,
China). According to the literature,22,23 when the
concentration of mixed acid is substantially diluted by water, the
concentration of will decrease to the extent that the reaction
immediately stops. Due to the large amount of water, the contact between
water and sulfuric acid during quenching does not significantly increase
the temperature of the solution, and no side reactions such as
sulfonation occur. The outer and inner diameters of all stainless steel
capillaries were 1/16 inch and 0.5 mm, respectively. The entire
microreactor system, consisting of precooling, mixing, reaction and
quenching stages, was submerged in a circulating thermostat (CORIO
CD-200F, Julabo, Germany) to obtain a uniform reaction temperature.