I admit, I stole that title from my grandma Bertha. But even in this day, the concept is applicable. Cleaning one thing is one thing, cleaning thousands of things is another. Let’s think about that.
Cleaning, as we have discussed before, really amounts to moving contaminants from the surfaces being cleaned and sequestering them in another location. In some cases, particles for example, the contaminants remain basically intact while in others such as soluble contaminants, they may be emulsified, dissolved or chemically converted to another form or compound. In any case, the addition of contaminants to the cleaning solution has a critical effect. To begin with, let’s consider particles.
For simplicity, assume that each part to be cleaned is contaminated by 1,000 particles. The goal of cleaning is to reduce the number of particles on the part from 1,000 to, let’s say, 50 (no cleaning process is 100% effective). The particles removed from the parts become suspended in the cleaning solution. So, if we clean one part, 50 particles remain on the part while 950 particles are suspended in the surrounding liquid. Now consider cleaning the second part. In the case of the second part, another 950 particles are added to those already suspended in the cleaning solution leaving 50 of the initial particles on the part. However, because of the increased number of particles suspended in solution, there is an increased likelihood that some of the particles in suspension will re-attach to the part with the result that instead of only 50 particles, the second part may leave the cleaning process with an additional number of re-attached particles, let’s say 5. As a result, the second part is not as clean as the first. In addition, the increased concentration of suspended particles often has a deleterious effect on the cleaning mechanism resulting in even less effective removal of the initial particles from the part surface. If you carry this logic out, it is obvious that at some point, the concentration of removed particles in the cleaning solution will become so high that the cleaning process fails. Basically it’s like trying to clean in dirt!
Using filters to remove particles is a practical solution in many instances. The filter must be effective at removing not only a specific size of particles but must also be sized to accommodate the volume of particles that will need to be removed in a practical interval of time which may range from hours to weeks. See this blog for an interesting treatment of just how effective (or ineffective) filtration can be.
Soluble contaminants present a very similar challenge. The reduced capacity of a cleaning chemistry to dissolve contaminants as it either becomes contaminated or weakened due to chemical depletion is usually the path to reduced cleaning effectiveness. In some cases cleaning effectiveness may be maintained for a while by increasing the chemical concentration or replenishing depleted chemical components but there will eventually come a time when a fresh mix of chemistry is required.
The consequence of all of the above is that in the course of specifying process equipment, consideration must be given to what long term effects there may be on the cleaning process as a result of the buildup of contaminants over time. The equipment design must accommodate these factors. As suggested above, the selection of filters with higher retention capability (bag filters vs. canister filters for example) may increase filter life. Series or “duplex” filtration may also be beneficial in some instances. In the case of soluble contaminants, the use of larger tanks or reservoirs to hold and replenish chemistry on a continuous basis may be a viable alternative. Removing dissolved contaminants using ultrafiltration or membrane filtration may also be an option.
As a final note, the first step to preventing capacity problems as a result of contamination buildup is to identify potential issues in the beginning – in the testing stage of process development. Discovering a process that works is important but applying it in a way that makes it efficient and effective in a production situation is the key to a successful process overall.