Use of Acid-Blend Detergents and the Avoidance of Foam

by Jim Watson
October 2004

CIP is a much-discussed concept in this day and age. There are many issues surrounding it, including flow rates, flooding velocities, sprayball types, TOC sampling techniques, etc. This article discusses issues regarding the use of chemicals in CIP operations. Specifically, the use of high- or low-pH surfactant blends in lieu of pure acids or bases and the additional issues with using these blends. In the example described here, a CIP OQ was being performed, however, the vessels were to use surfactant blends, whereas their entire previous service lives had been CIP’d with pure acids and bases.

There are several factors that must be taken into account when using high- and low-pH detergent blends. Care must be taken when determining the appropriate concentration to be used else ‘foam out’ could occur. For this article, ‘foam out’ is defined as an event where instead of recirculating an aqueous solution, air becomes entrained in the bulk solution, foam is generated and eventually cavitation of recirculation pumps occurs. The study referenced here focused on an acid detergent blend and the analysis of its foam generation.

In many cases, acid rinses following caustic washes are simply to ensure that any caustic that was not flushed out with a water rinse has been neutralized. Manufacturer-recommended concentrations in these scenarios are usually very low, typically less than 1% and usually closer to 0.5%. Additionally, some manufacturers make the claim that their acid detergents can perform de-rouging (such as those which contain glycolic acid), which can probably lead some users to decide that using a higher concentration ‘can’t hurt.’ Making this assumption without benchtop or other studies can lead to the serious issues discussed here.

When issues were discovered during validation execution, testing was undertaken to determine the reasons for the foam-out and cavitation. Testing performed on multiple vessels with static sprayballs revealed that vessels with a dual-sprayball arrangement circulating between 1.5% and 1.85% of an acid-based detergent solution would cause large amounts of air to be dissolved into the bulk solution (as the bulk solution’s surface tension was significantly reduced), creating a strong emulsion and causing  the flow and pressure regime to collapse. Single-sprayball CIP systems did not show this flow/pressure degradation.

After extensive testing of three vessels with pure water and with varying concentrations of a surfactant-based acid cleaner, the following general conclusions were made:

1. Each vessel tested with the acid cleaner had foam in the vessel at various depths (2"-5"). The foam would eventually break up, but it was persistent beyond 5-6 minutes. It is probable that the foam layer generated on the surface of the solution inside the vessel was itself not the cause of the loss of flow and pressure.

2. The solution was forming a strong emulsion after being run through a high-shear pump (~3500 rpm) and sprayball(s). It is assumed that the surfactant reduces the surface tension of the water enough to allow for this. Air was being forced into solution beyond what was expected with the use of an amphoteric surfactant that is present is the acid cleaner. It is hypothesized that using this surfactant caused foaming through the use of the sprayball(s), and then the solution would get pulled into the CIP pump suction. The high shear would break up any bubbles and force air into solution. With the high pressures in the line from the discharge of the pump to the sprayballs, the fluid being discharged into the vessel at atmospheric pressure generated thick, small-bubble foam. After just a few minutes, this scenario allowed for enough air to be put into solution that it would start to effervesce at the bottom of the vessel, right before being drawn back into the suction of the pump. Samples from all three CIP: Use of Acid-Blend Detergents and the Avoidance of Foam Jim Watson, Senior Process Engineer, Integrated Process Technologies, Inc. vessels tested showed this effervescent effect. There was probably little to no phase boundary between bulk solution and top foam layer during pumping, rather a gradient between high foam concentration to low foam concentration.

3. Use of two sprayballs in a vessel appeared to lead to the loss of pressure and flowrate. Each run performed with a single sprayball, regardless of vessel, showed no loss in flow and pressure. This leads to the conclusion that the flow/pressure regime must be questioned. Simply stated, at high concentrations of the acid cleaner (in this case, 2- 3x higher than the manufacturer’s recommended concentrations) a high flow, lower-pressure regime led to cavitation, where a lower flow and a higher-pressure regime did not.

4. It is believed that a correlation with residence time (τ ) can be found. This was not examined in this study.

 In summary, the first thing to ensure is that testing with cleaning chemicals is done during cycle development or commissioning to determine the parameters for high and low concentrations of blended chemical cleaners to determine the maximum tolerable levels. If your new chemistry is only being tested during validation, it’s too late. Cycle development or commissioning is vitally important for automated CIP since indirect measurement methods and system variances could potentially bring you close to foam out. Other, more specific benchtop studies that can be conducted include:

1. Chemistry Studies:
a. study the reduction in surface tension at various cleaner
concentrations
b. determine the contact angles and wetting on 316L SS, using pure
water and various concentrations of acid cleaner
c. study whether glycolic acid alone can cause foaming issues
d. if the generation of phosphates (or other residuals) is an issue, study
the use of other pure acids (citric, glycolic, etc.) in full-scale testing
e. study the use of an acidic cleaner that does not contain surfactants

2. Fluid Dynamics Studies:
a. study the flow dynamics of various flowrate/pressure regimes using
multiple CIP setups (varying pump size, vessel height/diameter
ratios, number of sprayballs, line sizes, chemical concentrations,
temperature, etc.) and attempt to correlate with residence time (τ )
b. study the sheeting action of cleaning solution on vessels of varying
sizes