Varying results from seemingly repetitive processes afflict many plants. Indeed, as a consultant for mixing processes and equipment, I find that a large proportion of my projects involve tackling such issues.
Companies call upon me because they think that at least part of their problems relate to mixing. In most cases, the difficulties result from not paying enough attention to the process and, sometimes, just from inadequate attention to the details. Some inconsistencies become obvious with careful observation of the entire process from the raw materials to the product and even packaging. However, some mixing problems may be more difficult to assess and correct.
Everyone knows something about mixing. After all, it plays a role in preparing meals in the kitchen and in performing some do-it-yourself household projects like painting. However, this familiarity sometimes hinders rather than helps. For instance, duplicating what happens in a kitchen mixing bowl or a laboratory beaker can be difficult on an industrial scale just because of size. Indeed, some industrial mixing problems develop simply because of the increased batch size. Mixing several thousand gallons or pounds of material is tougher than a similar operation in a kitchen mixing bowl. In other cases, the products being mixed may have different components, some with unusual physical properties. Even food ingredients can cause process problems on the industrial scale.
Most mixing inconsistencies stem from one underlying misconception. Mixing isn’t just one process and can’t always be done successfully in one way or by one type of equipment. Creating an oil-in-water emulsion requires different mixing functions than those for suspending solids. Heat transfer varies considerably depending on the service, e.g., blending versus gas dispersion. Correctly identifying the type of mixing process and the most appropriate equipment is an essential step in creating a consistently successful mixing process.
Different Mixing Processes
To succeed, at a minimum a mixing process must ensure that all the vessel contents are moving. Whether the process is low viscosity blending, high viscosity turnover, solids suspension or gas dispersion, everything must be in motion to achieve a practical degree of uniformity. Increased uniformity is the most common characteristic that defines mixing, regardless of process details or the phases present. Even dry powder blending has greater uniformity as its primary objective. A sufficient degree of uniformity for powders is a random or chaotic distribution of different particles.
Liquid blending often is the simplest and easiest mixing process to define and monitor. Whether combining large quantities of a few materials or adding many ingredients to create a batch, mixing usually is measured both by the degree of uniformity and the time required to achieve that result. When two or more components have similar physical properties achieving a uniform combination generally doesn’t pose great difficulty. However, if one component differs significantly in a physical property, e.g., viscosity, even miscible liquid blending can take a long time and require intense mixing. Adding the more viscous liquid to the less viscous one almost always works better than doing the opposite. The lower viscosity liquid is easier to move and even may be turbulent enough to help disperse the higher viscosity addition. Putting a low viscosity liquid into a high viscosity fluid can be extremely difficult. The flow pattern in a high viscosity liquid often is laminar with stretching flow that only creates streaks or sheets of the low viscosity fluid. It may take a considerable amount of time to divide and stretch the low viscosity liquid well enough to achieve an acceptable blend.
A mixer often is the main piece of equipment that helps transform raw materials into a product. The success of that process step depends on both the raw materials and the equipment. First, to have any hope of making a quality product, sufficiently consistent raw materials are essential. The raw materials most likely to cause problems are natural ones, whether minerals or agricultural products. Minerals taken from the ground can differ in physical or chemical properties depending upon their origin, even within a single deposit or mine. When the minerals are refined before use, differences in properties still may exist; these can change the ability of a mixer to produce the desired product. Agricultural products also may vary in properties because of moisture content, growing conditions or other factors. Manufactured compounds typically are less variable.
Eliminating differences in batches of raw materials obviously is important to avoid product inconsistencies. At the most fundamental level, a plant must purchase components to the same specifications and test them to ensure compliance with those specifications. For some ingredients, achieving consistent processing and product quality requires meeting tight specifications. In other cases, a relatively wide range of physical and chemical parameters may be acceptable. If material specifications can’t be assured, the site must have a mixing process sufficiently robust to handle the variability.
Depending on the type of process, everything from chemical purity to particle size or viscosity may be an important property. One of the more common problems is an inconsistent starting temperature. If a process doesn’t begin at the same temperature for each run, the fluid viscosity or reaction rate may differ. Unfortunately, initial temperature often varies highly depending on time of day, day of the week, operator observation, ambient temperature, etc. In one case I encountered, a plant always heated a batch of polymer before starting the process but gave its operators no instructions as to how high or low the temperature should be. The process began when the operator was ready — so, the temperature differed from batch to batch.