3.1. Determination of the Bubble Diameter of H2S
The bubble size cannot be determined by mathematical models for this type of reactors, because the system is a complex mixture, that is why we have proceeded to measure directly by photographic captures using the microscope (UNIMAKE, USB Digital Microscope 1000X) to determine the size of the bubbles closest to the surface of the reactor.
The photographic capture (Digital Camera, Nikon Coolpix S6500) was made stopping untimely agitation and gas entering the reactor. The measurement must be fast because the bubbles reduce their size since they are absorbed in the aqueous solution or they abandon the reaction medium. A uniform distribution of the bubbles in the liquid is observed and it can be inferred that the bubbles have diameters of less than 300 μm measured at a reaction temperature of 53°C, stirring speed of 700 rpm and a flow of H2S 0.4 vvmin. The bubbles were (Fig. 7a) (A= 300µm, B= 80μm, C= 60µm). The bubbles are of H2S and it occurs quite frequently when the reaction solution is almost saturated with H2S and when NaHS is already formed in its entirety (the pH of the reaction mixture is around 8). On the other hand, when air enters the system, O2 becomes thiosulphate and collapses, and if there are bubbles, it is due to Nitrogen (N2). The vacuum pressure prevents the entry of air, the gas flow meter does not show the passage of any gas, and if CO2 has passed must be negligible because the content of thiosulphate is minimal and almost always maintained at values of 100 mg /L, and has no impact on the specification of the final product. Fig. (7b) shows the multiphase reactor in operation, the foam that is formed when the flow rate of H2S (g) is much greater than 0.8 vvmin is observed. This foam mattress arrives until the empty space of the reactor is reached. In other research work [52], the photographic method was used for this purpose, where the bubble diameters were evaluated for selected bubbles in different locations at different times. The foam formed from sodium sulfide crystals Na2S5H2O, when cooled, crystallizes and adheres to the walls of the reactor generating various problems such as the impediment of the flow of H2S. The hydration temperature of Na2S is 73°C [28], and the dehydration temperature is 80 °C (Eq. 11) [24, 53], which represents a process of hysterisis. Mechanically, stirred tank reactors in which the gas phase is dispersed by means of a dispenser and/or a stirrer are used in many chemical processes (gas absorption etc) [54]. Stirred vessels are very commonly used for gas-liquid reactions on account of their flexibility and good performance for mass and heat transfer [55]. When a single phase is present, the fluid flow induced by the impellers is relatively easy to predict. The addition of a second phase to the reactor increases the hydrodynamic system complexity [54, 56]. Gas flow pattern is important. It controls the degree of recirculation and backmixing of the gas phase, which in turn determines the mean concentration driving force for mass transfer. In the homogeneous regime in an agitated vessel, the superficial gas velocity, vS < 0.02 to 0.03 m/s, and the bubbles have a monomodal size distribution with a small mean size, generally between 0.5 and 4 mm. Here, the impeller controls the flow pattern and bubble size [55]. Good mass transfer performance requires large interface area between gas and liquid (resulting directly from small bubble size and high gas fraction, given the fixed gas rate), and a high mass transfer coefficient (associated with local levels of turbulence) [55].