Chemical Process Optimization, Most Common Mistakes

Chemical process optimization has some goals. The goal of chemical process optimization is to find the best condition possible to manufacture a material through a process that will give us the best quality of the end material, no problems in the operation, and the best productivity possible.

Based on this definition, the most common mistakes are when we are disconnecting between all the phenomena that are involved in the chemical process. For instance, if we think about a reaction, and we think only about the reactivity of the materials, and we are not taking into consideration how they will interact, the optimization will not be the best optimization possible, because our goal is not the final well. And if we’re working in the industry, the main goal is to be sure that we will be able first to manufacture on a big scale or in the scale required, and second, to avoid safety process problems, low productivity, low quality, and problems in operation.

So, the first common mistake is to separate between all the phenomena that happen in the process.

The second common mistake is to assume that we have well-mixed equipment. This is very critical, because the chemical affinity between the materials for reaction rate, dissolution rate, dissolution by itself, spontaneous crystallization, and mass phase transfer, all of these activities are chemical interactions and chemical affinity. But in order for these kinds of activities to progress, we need to provide the materials to the environment, that is the interaction between the materials to go to the next step. So, if we have a reaction which we’re not able to have an appropriate connection between the reactants that will generate results that maybe will give us an impurity profile that will not be what is expected. So, the second typical and common mistake is to think that we have well-mixed equipment.

The third typical mistake is that we assume that in the laboratory we have a perfect interaction between the materials, so the mixing is ideal. So, it is exactly the opposite. Normally when we are in the lab, because the dimensions of the equipment are small, 0.5 l, 1 l, 250 ml, the dimensions of this equipment generate interaction between the materials that are very close to a laminar regime. So, it’s a full capability to interact between the materials. When we go to the big equipment, the capability to interact between the materials is better because we are in the turbulence regime, and in the turbulence regime, the capability to have a good interaction between the materials is higher. So, when we are coming to very low Reynolds numbers, meaning of this laminar regime, the interaction between materials is not so representative of what we will have after that in the production step. And all these deviations in the interaction between materials will generate maybe different kinds of phenomena that are happening into the process that will not happen in the next step because we are not in the same regime of work. This is the third one.

And the fourth one is more some kind of intuition. For instance, we’re talking about exothermic reactions. The small equipment has a lot of surface per volume, and we are able to transfer heat in a very effective way. So, if we are working in the lab and my equipment is growing in temperature let’s say by 10 degrees during the reaction in the lab, the meaning of this, is that when we go to the production, when we have order of magnitudes less surface per volume, the meaning of this is that the capability to transfer heat to the jacket will be lower, so we are close to adiabatic systems. And when we have adiabatic systems, the temperature will grow fast and will be bigger. And this can confuse us, and maybe we think that our process is not exothermic or is not a problem of exothermic, but when we go to the next step, it will be really a big problem, because we are not able to extract all the heat from the equipment.

The other point that continues to be a phenomenon, is, we assume that when we have homogeneity in our tank, the process will always be good. And this is not true. The process requires some interaction between the materials. And not always when we have a good interaction between the materials, we are generating results that are good for the process. A very simple example. For instance, when we have two liquid phases that are not miscible and we have a process, for instance, emulsification, we will like to generate emulsification, so oil drops that will be small enough that will not separate after we stop the mixing. So, this is emulsification, creams, and ointments. And for this, we need a high shear rate. We need high velocity, high shear rate, we need homogeneity. And if our goal will be different with the same materials and the same equipment, we need only to transfer material from one phase to another phase. And after that, when we stop the mixing, separate phases, it will not happen, because we have a very good mixing and the meaning of this, it will not separate. And for us, we will be very, very proud. Or another example, we have a reaction that is a fast reaction and is controlled by mass transfer. If we are applying a high velocity, maybe we will promote a secondary reaction that is not good for me, it has generated some impurity, and we are losing the productivity and the quality of the end material. So, in this case, when we have homogeneity, maybe it’s not good. So, the assumption that good mixing is equal to good process is a common mistake, and it is not a good approach. And like this, we have other kinds of conclusion that we can think about then, for instance, mixing times that will not be exactly the same mixing times in the lab and the production step or we assume that when we have a very big vortex, the meaning of this is good mixing and it is good for the process. All of this assumption is not good. Opposite normally, when we have a very big vortex, the meaning of this is that we have no capability to have interaction between the materials, and at least more and more.

So, it is very important to take in consideration that we have two main fields; the chemical affinity between the materials, that is the reactivity, solubility, gas consumption, etc., and the physics. So, what are the conditions we’re providing in our materials to progress with the process, and how is the connection between these two main fields that happen in the tank anyway, because normally we’re manufacturing our process into the sterile vessel, and we need to take care of it. So, hydrodynamics separately and chemistry separately is good only to understand, but after that, if you want to go to optimization, the goal is to have good conditions in the productivity state, in the industry stage, we need to connect between them, and we need to understand how is the interaction between these two main phenomena that happen into the sterile vessels.