Chemical engineering is the science of solutions (no pun intended), assessing available input, desired output, and using a specialized set of tools to connect the dots at minimal cost. This process entails myriad considerations, from size and baffling configuration of heat exchangers to temperatures and stages of distillation columns. With larger considerations in mind, such as flow rates and resulting efficiencies of separations, it is easy to miss the more minute aspects of process design.
Mixing apparatuses, for example, present these kinds of considerations. Despite HYSYS models suggesting that the appropriate mixing magically occurs with little additional thought, the appropriate impeller for the job can make a big difference in terms of energy savings and process speed; both important when presenting budget projections to decision-makers. By asking the right questions when selecting an impeller, you can save time, money, and maximize process efficiency, all with the help of a spinning hunk of metal. Here are five questions to get you started:
What are we trying to accomplish?
It may seem elementary, but without understanding what the end goal of your mixing process, it is unlikely that you'll know where to go next. Consider your process and determine what the end goal is. Homogeneity? Creation of an emulsion? Consider the chemicals involved and how they react under high-shear or high-agitation conditions. Knowing these facts can help make the choice much easier.
In addition, knowing your process goals can help you decide if an appropriate impeller can improve the totality of the process. High-energy costs on distillation reboil? Utilize a high-efficiency impeller to save on power expenditure. Accurate product consistency a concern? Utilize a high-motion impeller with high-turbulence mixing to ensure homogeneity down the line. In either instance, a design consideration in one part of the process can greatly alleviate bottlenecks in other areas.
What efficiency can it accomplish?
Each impeller is designed with two things in mind: shear and motion. The way that these specific considerations combine to determine efficiency makes an enormous difference, especially when power expenditures span the life of a plant. Even if the particular efficiency of two models in the same design family differ by only a modicum, remember that those energy savings will last over the life of the mixer and each hour of efficiency means more savings for your organization.
When selecting an impeller, consider examining case studies of previous design iterations of similar geometry. Observe plant performance for mixers that used the same impeller configuration and see the efficiency for yourself. Particularly with newer designs, sufficient time may not have elapsed to permit accurate efficiency numbers so see for yourself and save in the process.
What is the motion-to-shear ratio?
Remembering what it is you are trying to accomplish with your mixer will come in handy when assessing this question. Each impeller possesses a certain quantity of mechanical energy as it rotates in a solution. The ratio of motion to shear that occurs from that action will play a large part in determining which impeller to use.
Assess the requirements and characteristics of chemical components and make an educated decision. If you are mixing a gas, consider high-motion mixers, as high-shear impellers are unlikely to achieve sufficient mixing for homogeneity. For creating emulsions or solutions, the components of which would normally be immiscible, utilize a high-shear impeller, since that kinetic action is what achieves the desired results.
Will it fit existing equipment?
Even if you find the appropriate impeller on paper, mechanical considerations can nullify the choice. For example, mixing apparatuses with small apertures may not fit blades of a particular diameter. Fortunately, accommodations can be made. Folding impellers exist that can achieve the same results while bending to the whim of your current configuration.
With these special pieces, make sure to scrutinize reliability. In high-viscosity mixtures, folding impellers may experience excessive strain on structural joints. In the case of more strenuous processes, be sure to request a model of appropriate hardiness to withstand the physical rigors.
What is the expected lifespan?
Obviously, all good pieces of equipment will eventually fail with time. Particularly caustic mixtures may hasten this process, adding to cost and maintenance. For this reason, inquire regarding the lifespan of equipment, since even high-efficiency impellers can have their savings offset by frequent replacement. Doing so will also minimize the amount of maintenance and downtime required of personnel, which translates into more revenue.
Calculating process specifications can mean the difference between product and refuse. Choosing the right mechanical parts for a process can mean the difference between good and great. Look at your specific process and know your goal. Base your decision on efficiency and the design specifications of the impeller in question. Ensure that your new equipment fits your design and ask about lifespan in order to prevent unwanted downtime. By assessing the details and making the right decision, you can impress your board, your colleagues, and your customers alike.