Coming to grips with arterial plaque

Differences among carbonates, oxylates and MAP must be understood to effectively minimize and treat precipitates. This contribution appears in the November issue of EPM.
By Dennis Bayrock | October 19, 2015

Precipitates are a real problem for all fermentation production facilities as they decrease the heat transfer efficiencies of heat exchangers, reduce volumetric flow within pipes and vessels, reduce mass transfer of chemicals within vessels, and harbor bacterial contamination. Periodic clean-in-place (CIP) between fermentation cycles is required to remove precipitates that build up within process equipment. Annual CIP caustic costs at fuel ethanol plants can range from $490,000 to $890,000 at 100 MMgy plants and hydroblasting costs can run $45,000 for each treatment at a 50 MMgy plant.

There are various precipitates found at industrial fermentation facilities that can be broadly classified into inert, organic and inorganic. Inert materials such as sand, fiber, mash and dead bacteria and yeast are the simplest. Once in the slurry, these inert materials can settle out over time, typically within liquefaction vessels, propagators/fermenters, beerwell, evaporators, and recycle streams. Once built up, physical removal of these materials via a shutdown is the only certain way to remove them as there are no known CIP cleaning agents to effectively remove them. Limiting the amount of foreign material coming in with the feedstock is the easiest form of prevention.

Inorganic precipitates can include sulfides, phosphorus compounds, carbonates and struvite.
Organic precipitates can include proteins, oils, oxalates, phytic acid and succinic acid. Many are present in the feedstock—corn contains proteins, oils, oxalates and phytic acid—but they can also be produced by yeast or bacteria. Bacteria can also synthesize exopolysaccharides, creating an organic adhesive that allows the bacteria to cement to surfaces and form biofilms. More than 40 exopolysaccharides have been characterized from the lactic acid bacteria family, each with unique physical properties.

All precipitates allow bacteria to colonize and adhere to the large surface area created by the precipitate scaffolding. To illustrate the dramatic increase in surface area, consider that a 1-cubic-foot solid sphere of material has a surface area of 0.4492 square meters. If replaced by an equal volume of sand particles (diameter of 0.1 mm), the surface area increases by 376 fold. If replaced by an equal volume of bacteria (diameter of 0.0005 mm), the surface increases by more than a million fold. Thus, smaller diameter particle sizes in precipitates exponentially increase the surface area for colonization by bacteria.

Problematic Participates
One inorganic precipitate at ethanol plants detected more recently is struvite (MgNH4PO4·6H2O), also known as MAP for its 1:1:1 ratio of magnesium, ammonia and phosphorus. The highest rates of MAP formation occur at temperatures above 99 degrees Fahrenheit and a pH greater than 7, but it will form at lower rates at different temperatures and pH. MAP is off-white in color and has a soft, putty-like consistency, unlike most other inorganic precipitates, which are hard. Once formed, MAP is difficult to remove and does not solubilize in water, caustic or quat solutions.

MAP formation is a chemical process that can be found as early as the liquefaction hydroheater. At a typical fuel ethanol plant, ammonia is added to slurry while magnesium is already present in the mash (about 1,500 parts per million are already present in corn, and with any magnesium salts added to methanators). Phosphate ion concentration is generally not elevated at an ethanol plant as phosphate-containing chemicals are not normally used. One potential source of phosphate is in well water. Another source is the corn itself. About 85 percent of the phosphorus in corn is bound as phytic acid that is released when phytases are added. 

With magnesium and phosphate present in the mash, MAP formation can also occur during fermentation when yeast and bacteria produce additional ammonia as they hydrolyze urea, or in the case of bacteria, hydrolyze protein. More worrisome is the real potential for MAP formation during inadequate CIP procedures. If all residual mash is not removed prior to treating with caustic, then ammonia is formed due to the hydrolysis of proteins with caustic. This liberated ammonia, together with the magnesium, phosphate, high pH and temperature during CIP, quickly forms and precipitates MAP.

MAP production can be limited by reducing the concentration of at least one of the three required components—ammonia (or urea), magnesium or phosphate, or by reducing the kinetics by lowering the pH. MAP formation cannot be eliminated, however, and will still continue at less than optimal conditions.

Oxalates—also known as beerstone, CaC2O4—are hard, white organic insoluble precipitates with well-defined crystals visible under the microscope. The acid form, oxalic acid, is soluble in water. Unlike MAP, oxalates are a normal part of metabolism in corn, bacteria and yeasts. Oxalates will solubilize metals in soil to provide a source of soluble metal ions required for metabolism. As oxylates and calcium concentrations cannot practically be limited or prevented at an ethanol plant, other means to prevent their formation must be used. At pH values below 4.27 it transitions from insoluble to the soluble oxalic acid form. This simple fact is employed at most ethanol plants to prevent scaling in the distillation system, either by acidifying the beerwell or ensuring that complete fermentation is achieved prior to distillation (target ferm drop at pH 4.5).

Carbonates form hard, insoluble inorganic precipitates, created when carbon dioxide, produced in copious amounts by yeast, dissolves in water to form carbonic acid which can then participate in multiple reactions leading to the formation of carbonate ions. Significant amounts of carbonate ions are produced at pH 8 or higher, which can combine with the calcium, magnesium or iron in corn mash to form carbonate precipitates. In addition, caustic itself is very effective at solubilizing CO2. As in the case with MAP formation during CIP, carbonate precipitates can quickly form if CO2-containing mash comes in contact with caustic.
Treating the Problem   
Which is the worst precipitate to have? Chemically formed carbonates and biologically formed oxalates are easily created, if there is an overlap of caustic and mash. Prevention mainly relies on maintaining proper pH so that the insoluble forms do not precipitate.  The chemically formed MAP is more complicated. Once formed, MAP is more chemically inert. In addition, due to its softness, MAP can coat and penetrate crevices and surfaces, is more mobile and can more easily be colonized by bacteria.

Complicating the precipitate picture is the fact that most precipitates found within pipes and vessels at ethanol plants are sandwiches of multiple layers of organic and inorganic precipitates. Although caustic is extremely good at removing organic residues, it is not as effective at removing inorganic precipitates. Acidic solutions such as sulfamic acid are good at removing inorganic precipitates, but not as effective at removing organic precipitates such as proteins and oils. Traditionally, CIP procedures that alternate between caustic and acidic solutions that follow the optimal 4 T’s of CIP—time, temperature, titration and turbulence have been used to combat all precipitates.

Knowing these limitations of traditional acidic and caustic CIP chemicals, what can be done to improve the effectiveness of CIP? Scientists at Phibro Ethanol Performance Group have researched and developed two solutions for the fuel ethanol industry. The trademarked PhibroClean is a blend of surfactants that, when added to traditional caustic CIP solutions, significantly improves the penetrating and wetting ability of caustic against organic fouling. The trademarked PhibroAC is a custom blend of acids that can replace sulfamic acid to remove significantly more inorganic precipitates and does not influence the delicate microbial consortia within anaerobic digestors. Various precipitates from multiple ethanol plants have been tested at Phibro's R&D lab. The accompanying charts show the effectiveness of Phibro’s treatments compared to the standard caustic and sulfamic acid treatments.

At one 100 MMgy ethanol plant where PhibroClean was permanently incorporated into the standard operating procedures (SOPs), annual savings in caustic costs of $403,200 were realized. The original caustic CIP concentration at this plant was 2 percent weight/weight. 

For the fuel ethanol plant, proper attention to pH and CIP SOPs can help prevent many organic and inorganic precipitates from forming. By definition, inert precipitates are not affected chemically by caustic and acid wash solutions, but in many cases those inert precipitates are coremoved once the organic and/or inorganic precipitate sandwiches are taken care of.

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

Contributing author: Wayne Mattsfield
Staff Scientist, Phibro Ethanol Performance Group

CONTRIBUTION: The claims and statements made in this article belong exclusively to the author(s) and do not necessarily reflect the views of Ethanol Producer Magazine or its advertisers. All questions pertaining to this article should be directed to the author(s).