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.