Caustic CIP Optimization

FROM THE FEBRUARY ISSUE: CIP brings unique challenges to ethanol, but they can be overcome.
By Dennis Bayrock | January 17, 2020

Fuel ethanol plants utilize cleaning-in-place (CIP) procedures to prevent bacterial contamination and optimize plant performance. Most unit operations at a plant use CIP as part of a standard operating procedures (SOP) program. Typically, plants depend on using 2.5 to 5 percent weight by volume (w/v) caustic solutions and the CIP guidelines related to time, temperature, titer (solution concentration) and solution turbidity to properly clean multiple locations within the plant.

Caustic not only has excellent cleaning capabilities, but also has unique disinfection properties not found in other cleaners. Saponification, for example, hydrolyzes fats and oils to free fatty acids and glycerol. Caustic also causes saponification in the lipid membranes of yeasts and bacteria. This destroys the cell’s integrity and is fatal. The higher pH of caustic solutions also directly impacts the cell’s ability to uptake nutrients such as amino acids and sugars. These transport mechanisms in the cell membrane rely on a pH gradient across the cell membrane in order to properly operate. By raising the pH, this pH gradient is removed and can cause nutrient starvation.

In most CIP SOPs, an acid wash is also implemented to remove mineral deposits that caustic solutions typically do not remove.

A few measures can optimize pre-rinse and caustic CIP practices.

CIP Challenges 
Significant challenges exist for fuel ethanol plants to effectively use caustic as part of their CIP programs. Unlike CIP procedures in other industries, the corn used to create mash in dry-grind fuel ethanol plants contains significant amounts of carbohydrates, proteins, oils and insoluble fiber. CIP challenges specific to fuel ethanol plants include: thorough and effective primary rinsing; limiting exposure of caustic solutions to carbon dioxide; accurately analyzing caustic concentration of CIP; and monitoring solids contamination in CIP solutions.

The primary rinse of pipes and vessels with process water must be thorough and extensive to be effective. The addition of a surfactant detergent cleaner, such as PhibroClean, to the rinse cycle will improve the overall effectiveness of the CIP process by removing more material before the caustic CIP cycle.

Without effective rinsing, the remaining deposits can form an excellent scaffold for bacteria to colonize as a result of the increased surface area created. Research on fouling and bacterial contamination has shown that, depending on the particle size of deposits, the potential surface area for colonization can increase by over 400 percent compared to a clean, deposit-free surface. Insufficient rinsing also allows organic material in mash to chemically neutralize caustic, reducing and compromising its strength and effectiveness. In addition, multiple studies show chemical byproducts can be generated from Maillard reactions that form furfural, n-substituted glycosylamines, acetaldehyde, and dioxins, which are known to be inhibitory when recycled back to yeast.

Improper rinsing will also lead to insoluble particle accumulation in the recycled caustic solution. Not only will these particles displace the volume of caustic (which requires more frequent additions of fresh caustic to the system), but they will also be recycled in subsequent CIP cycles and increase the risk of bacterial contamination. In addition, the Maillard reaction byproducts from the cleaning process and unreacted components of the mash (a result of carryover to the caustic tank) will also recycle, providing nutrients for contaminating bacteria at other locations in the plant.

Carbonates—insoluble hard crystals—form when caustic is exposed to carbon dioxide. In distillation, carbonates can be dissolved and removed with the addition of an acid-based cleaning system, but it is advantageous to reduce or eliminate their formation through better caustic CIP management. Typically, the strength of this caustic solution is periodically refortified with fresh caustic, but any carbonates within the tank are not usually removed until the plant decides to purge the caustic tank.
Formation of carbonates also neutralizes the strength of caustic solutions. Therefore, the reuse of caustic solutions containing carbonates will ultimately reduce the actual strength of the caustic solution, provide increased colonization power of bacteria when crystals are deposited in pipes and vessels, and decrease the overall effectiveness of CIP within the plant.   

To prevent these effects from occurring and to enhance CIP effectiveness, periodic purging and full turnover of the content of the caustic tank with fresh caustic solution should be implemented as a regular part of any CIP program. The frequency of these purges varies between fuel ethanol plants depending on their overall CIP program.

It is also critical to accurately monitor caustic strength to ensure it is maintained at its target concentration. Most ethanol plants check the concentration of their caustic solution with pH and with a titration using a single indicator (usually phenolpthalein in a one-point titration procedure). The advantages of a one-point titration procedure are that it is inexpensive, convenient, rapid and provides reasonable accuracy when determining the concentration of pure uncontaminated caustic solutions.

However, a one-point titration procedure will overestimate the concentration of a caustic solution if there are any carbonates or any organic carryover from mash in the CIP solution. To overcome this limitation, the Warder titration method can be used.

The Warder procedure expands the one-point phenolphtalein titration to a two-point titration using methyl orange. The phenolpthalein end-point is titrated first (determines caustic concentration) followed by titration in the same flask to the methyl orange end-point (determines carbonate concentration). Simple calculations can then be performed to determine the concentrations of each and thus the true concentration of caustic in CIP. The Warder procedure is simple to perform, and only introduces one additional titration in the same flask to the base one-point titration procedure.

Monitoring the amount of solids (carbonates and insoluble materials) in CIP caustic solutions is important to indicate how quickly the sediment level increases over time. Close monitoring of solids buildup can give an indication of whether sufficient flushing was performed in the initial rinse cycle of the CIP process and will also give the plant another metric that can be used to gauge when the recycled caustic needs to be purged and refreshed. This monitoring can be as simple as drawing samples from the spent caustic tank, allowing them to cool in a graduated cylinder, and recording the amount of settled bulk precipitate.

The addition of a detergent cleaner such as PhibroClean to the pre-rinse and caustic CIP solution improves CIP effectiveness while providing potential savings in caustic usage. When applied during the pre-rinse cycle, PhibroClean removes organic material that would otherwise remain in place, reducing the potential for caustic solution degradation. PhibroClean addition to the caustic CIP solution will lead to better cleaning results through its detergent action, facilitating the use of lower-strength caustic CIP solutions, resulting in potential net savings in overall CIP costs.

Proper and effective CIP practices can reduce the severity and frequency of bacterial contamination at ethanol plants, improve ethanol yields, optimize heat transfer and energy inputs and reduce the need for hydroblasting. Addressing the challenges inherent in using CIP to clean and sanitize mash-based fermentations reduces issues with bacterial contamination and improves plant operational efficiency.

Author: Dennis Bayrock
Global Director, Fermentation Research
Phibro Ethanol Performance Group