This paper will review how liquid caustic soda interacts with various chelators, scale inhibitors, dispersants, and surfactants during the clean in place process. The paper will explain why the various additive packages work and takes into account new breakthroughs in surfactant technology.
Caustic soda is primarily used for clean in place (CIP) because of its excellent organic dissolving and saponifying powers, its considerable germicidal activity, and its comparative cheapness. Liquid caustic soda and various additives have been used for many years in a wide variety of applications. In general, caustic soda “cleans” because of its strong alkalinity and is widely used due to its low cost.
Over the years, compounders have formulated a wide variety of “cleaning compounds” and the end user is sold a trade name product. When this occurs, many people are unaware as to how the additives are used. For this reason, it is important to fully understand why certain additives are used.
Liquid Caustic Soda
Commercially, there are several methods of producing liquid caustic soda. Sodium chloride (salt) brine is reacted in a cell with a strong direct current. This electrolytic reaction with water and salt yields caustic soda (NaOH), hydrogen (H₂), and chlorine (Cl₂) in accordance with the following formula:
2NaC1 + 2H₂O 2NaOH + H₂ + C1₂
The industry terminology is known as diaphragm (commercial grade), mercury cell (high purity, rayon, etc.), membrane (equivalent to mercury, etc.), and chemical grade. Please note chemical grade is manufactured by a different process and does not produce chlorine as a byproduct. The specifications vary by process with mercury and membrane prices at a premium because they produce low salt, low carbonates, and low iron.
The market is generally sold on a “dry ton basis” and is generally billed on the active NaOH content. Commercial grade liquid caustic soda is manufactured at 50%.
Definitions of Additive Packages in Clean in Place (CIP) Processes
The word “chelant” is from the Latin word “chela” meaning “claw.” Chelants are materials that work in aqueous solutions to tie up metal ions so they are no longer effective. The term “chelating agent” refers to a molecule that contains two or more complexing sites (or claws) capable of coordinating around a metal ion. In technical terms, chelation is a kind of sequestration process whereby the chelating agent forms a non-ionic ring structure with a di- or polyvalent cation by ionic valence bonding, supplemented by coordinate covalent bonding. Sequestration, on the other hand, is a generic term which covers all the ways in which chemical species may be protected from unwanted chemical reactions in its solution environment.
Dispersants attack the undesired species by using a high negative charge. This works much like two magnets with like charges, the magnets repel from one another.
Surface active agents are used in formulations primarily to reduce surface tension. The uses of surfactants give the working solution a reduction in surface tension and, hence, are called wetting agents. The wetting agent increases the ability to wet and aid in rinsing.
The Right Choice for Clean in Place (CIP)
When selecting the best product, the chemical product or system which produces the best results at the lowest cost is declared the winner.
For example, if a particular chelating agent forms a more stable chelate (reaction product of the chelating agent and the cation to be chelated) than does the union which would normally precipitate or otherwise undesirably react with the cation, no precipitation occurs.
In strong caustic soda solutions, the primary chemical competitive influence that a sequestering agent encounters is the hydroxyl ion. Metals tend to form more or less insoluble hydroxides and the sequestering agent must be able to protect the metal ion from the precipitating agent in strong caustic so scale will not form.
In strong caustic, the so-called amphoteric metals may behave at times as cations in solution and at other times, particularly in strong caustic, react with OH ions to form negatively-charged anions.
For example, aluminum in strong caustic exists as aluminate ion (A1O₂) and in related forms associated with water molecules. The implication is that chelating agents for metals in mildly alkaline solutions may fail totally in strong alkaline solutions. This is why EDTA should not be used in caustic formulations.
Thus EDTA is an unsuccessful chelator in strong caustic for metals with highly insoluble hydroxides and fails as a chelator for aluminum in caustic since EDTA can chelate only as long as the chelate is more stable than the metal hydroxide, and EDTA will not chelate anions like aluminate.
To sequester a metal with an insoluble hydroxide or an amphoteric metal with a negatively charged ion like aluminate, a different sequestration chemistry is required.
This different required chemistry is typified by gluconates which have the ability to form complexes with anions of amphoteric metals. In addition ,they will chelate divalent metals with soluble hydroxides (like calcium and magnesium) within certain limitation of caustic strength.
For our purposes we will discuss the sugar acids and their salts. The common ones used especially with caustic soda include:
- Gluconic Acid
- Sodium Gluconate (Dry and Liquid)
Gluconic acid for years was used as the basic chelator with caustic soda. It was first discovered in 1878 and was commercially introduced by Pfizer. The material is made at 50% concentration and is outlined as follows.
In recent years liquid sodium gluconate has been the most popular form to purchase primarily because the product is much more soluble in liquid form. The amount required to clean in place (CIP) for ethanol plant formulations varies with percent concentration of the final solution and temperature.
Scale Inhibitors and Dispersion
Phosphonates have valuable properties other than strictly sequestering power and are sometimes the agents of choice because of these properties. They have the ability to deflocculate and suspend water insoluble materials in colloidal form and prevent re-deposition of suspended dirt particles during rinsing cycles. This ability makes them valuable when formulating clean in place (CIP) cleaners to the ethanol industry. Until water pollution became such a factor, almost every cleaning formulation contained phosphates.
Phosphonates also exhibit a “threshold effect” in that they can prevent the formation of calcium and magnesium carbonate precipitates in hard water even when present in relatively small amounts that are not stoichiometrically enough to fully sequester all the calcium and magnesium ions present.
In summary, the phosphonates act in five ways:
- The ability to chelate.
- The ability to inhibit scale.
- The ability to disperse.
- They do not degrade as temperature increases.
- They do not add to environmental damage sometimes associated with phosphates (inorganic). Commercial products are approved by UL for direct use in potable water for scale and corrosion control.
The right choice of product must be matched with pH range and the evaluation of water hardness.
Other choices, particularly with temperatures exceeding 185 degrees Fahrenheit, are co-polymers. Polyacrylates should be used at 50 ppm in the final working solution. Polymers prevent scale from forming by disrupting the structure of newly-formed crystals. This phenomenon slows crystal growth and reduces the agglomeration of crystallites that have already formed. These polymers are effective at sub-stoichiometric concentrations, unlike chelating agents which must be used in stoichiometric concentrations to yield optimum results. These polymers are anionic molecules which absorb into suspended particles, imparting to them a greater negative charge. When this occurs the particles repel each other, resulting in greater dispersency. This was discussed earlier concerning the definition of dispersants.
There are several companies that produce a wide variety of products. These are generally sold by molecular weights. For clean in place (CIP) formulations, a molecular content between 4,000 and 5,000 is used in combination with high alkaline products. Care must be taken because of caustic’s high pH range and particularly with solubility.
The use of surfactants has been limited over the years due to caustic soda’s high concentration (50% NaOH). Surfactants are known as “surface active agents.” They have the ability to reduce surface tension which will allow better rinsing. For this reason, they are also known as “wetting agents.”
Wetting agents are more generally known as synthetic detergents of which there are four types: anionic, nonionic, cationic, and amphoteric.
The anionic types are more commonly used in detergent formulations, the most important being the sulfated alcohols and alkyl aryl sulfonates.
The principal nonionic wetting agents are polyethylene oxides which are compatible with anionic and cationic types but which, unfortunately, can cause excessive foaming in certain circumstances. Both anionic and nonionic detergents have very good wetting, dispersing, and rinsing agents but are of little use against most deposits such as beerstone.
The cationic wetting agents are essentially quatemary ammonium compounds and are far more widely used as sterilants (sanitizers) than wetting agents. The high concentration of caustic soda limits the surfactant’s ability to couple and the result is the product will have a difficult time going into solution or hydrotroping.
A New Breakthrough for Clean in Place (CIP) Technology
In 1995 a new type of surfactant was developed primarily for the brewing industry. This new technology was able to “hydrotrope” into 50% liquid caustic soda and could reduce the typical surfactants in the 50 dynes/cm range down to about 35 dynes/cm. Each reduction in dynes is a logarithmic value. In other words, this behaves much like earthquake measurements. If we have an earthquake of 7.5 vs 5.0 magnitude, the effect is thousands of times greater. Surface tension of chemicals is similar. In 1998 a second generation of caustic-stable surfactants was developed to lower the surface tension while being soluble in 50% liquid caustic soda down to 24 dynes/cm. In 2002 the surfactant was improved to 22 dyres/cm. In 2005 the surfactant was again improved down to 19 dynes/cm and is patented and licensed to Clear Solution USA, LLC. Because of the extreme low surface tension, the additive package in conjunction with 50% liquid caustic soda eliminates “beerstone” or “milkstone,” as well as provides other benefits depending upon the industry and application.
There has not been much written about “beerstone” in the textbooks or technical papers. It is surprising that so little is said about the subject because of the problems it poses for the ethanol plants in terms of microbiological instability and buildup.
When there is an abundance of calcium in the solution, it may precipitate out of the solution as calcium oxalate (“beerstone”) in tanks, transfer lines, etc.
The color of beerstone changes as the alcohol is processed in the initial stage. If the beerstone is darker, it has been there a long time. If the beerstone is lighter, it has been there for a shorter period of time. Filtering will help remove some of the beerstone.
Removal of Beerstone
While some facilities have the good fortune of being able to cope with beerstone using only one particular method, others have to resort to several different methods of removal, depending on the stage in the production process when the problem is noted.
On stainless steel tanks it is usually mandatory to use an acid beerstone remover, such as a 1.5 to 2.0 per cent sulfamic acid solution. There are other methods that will produce results.
In summary, each plant has a different set of circumstances. The following questions need to be addressed:
- Plant water hardness and does it vary. What is the maximum expressed in grains or ppm (17.1 ppm = 1 Grain).
- Maximum temperature in the cleaning cycle.
- What is the % NaOH in the working solution, otherwise known as the set point.
Once that is determined, the proper additive package can be determined. The additive package is generally 10% mixed with 90% liquid caustic soda (50%). Once the fermenters are micro clean, the low surface tension surfactant will result in a quicker wash cycle. The use of ATP meters will allow the surface to read much lower than 150 RLUs which is accepted as food standard. Our testing using ATP will produce readings far less than 150 RLUs.