What is a Solid-Liquid Mixture?

What is a solid-liquid mixture and the mechanism of solid-liquid mixing?When we talk about a solid-liquid mixture, we are talking about solids that are present in the liquid media. So, when we are processing this mixture between solid and liquid, the main environment that is moving the solids to advance the process—or even the reactions or a formulation or blending with other solids—is the liquid physics. So, we’re talking about fluid mechanics that are controlling the interaction between the solids and the liquid media. These fluid mechanics are governed by an equation, and the meaning of the equation is to calculate the velocity of the flow at every part of the fluid media, based on mass balance and momentum balance. The solid-liquid mixture is very important in the process industry because most of the materials we use in everyday life are solids. And we use these solids to be dissolved in the liquid media or to remove all the kinds of materials that will protect the solids. But, in principle, the solids we use in everyday life are in this state, and in order to progress with them, we need them to interact; we need to put them in a liquid phase and generate interaction between the solid and liquid.

So, what we normally are looking for when we have solid-liquid systems is also liquid into a liquid media in our sterile vessel. We are looking to see if the solids are moving well and in a convenient distribution to our process. So, the first parameter associated with this activity is solid suspension velocity. What is the minimum velocity the solid should have so it does not precipitate or float into the liquid media? This velocity, this minimum velocity, is controlled by the fluctuation that we have into the flow, because normally we’re talking about fluctuations into the flow that take the material from the bottom. These fluctuations give us the capability to keep the solids moving at a minimum velocity in the tank before the precipitate or float rate.

The other parameter that we normally need to take precautions with is high velocities that will send the material to the wall of the tank, or high velocities that will generate some shear stress resulting in attrition of solids and reduction of the particle size. Those are the main parameters we need to control; it is possible to calculate, modulate, and measure these parameters to ensure they will be appropriate for the process. Not all processes will require the same distribution of solids, and there is no standard good solid distribution by a number. For instance, we cannot say that a 20% deviation in the solid distribution from the bottom to the top of the tank is good or bad because good or bad depends on the process. It could be “good” if we generate our results, quality operation, and productivity in a good way. It could be “bad” if the 20% generates a secondary reaction that is no good or blending that is not homogeneous enough, or if we get some deposition of the solids in the wall or bottom of the tank—even if the number continued to be the same, of which the 20% is an example.

So, when we talk about solid-liquid mixtures and the mechanism of a solid-liquid mixture, we must remember that this mixture is only the tool for the process. And once we know the process well, based on our experience and our calculations, we will be able to understand the good distribution of solids into the liquid phase in our sterile vessel.