Stainless Steels: Cost-effective Materials
Stainless steels are the materials of choice for numerous applications in ethanol plants and have an excellent track record. They offer excellent corrosion resistance, good strength and high ductility and toughness. Stainless steels are easily maintained and give an attractive, hygienic, high-tech appearance. The 100 percent recyclability of stainless steel supports the biofuels industry’s long term sustainability concepts and goals.
An increasing number of ethanol process engineering companies specify stainless steel for equipment. Companies are aware that construction, handling and fabrication costs can frequently be lower when stainless steels are used, compared to much heavier carbon steels.
There is sometimes still a perception that stainless steels are expensive materials, and that they should be confined to more corrosive applications. The many advantages of stainless steels, however, allow them to be used cost effectively. This has been demonstrated extensively and successfully in ethanol plants in all main producing regions. As the industry grows and develops, this trend becomes clearer. This does not mean that stainless steels are economically justified in every application. Nevertheless, ethanol plants will increasingly be looking for reliable operation, low-maintenance cost and long-lasting service, which stainless steels provide.
General Process Conditions
Ethanol production processes are moderately corrosive, with pH ranging from 5.8 to 2 (slightly to moderately acidic) in most parts of the production process. Temperatures are often relatively low, with a few exceptions where they will reach 115 degrees Celsius (240 degrees Fahrenheit). Standard austenitic stainless steels are therefore widely used and recognized as cost-effective and reliable materials solutions.
We estimate that close to 90 percent of stainless steel used in existing ethanol plants is accounted for by Type 304 (UNS S30400) or its low-carbon version, 304L (S30403) stainless steel. The L grade is preferable where welding will be done. These popular stainless steel grades often meet the required corrosion resistance and are readily available in all required product forms all over the world.
Sometimes 316L (S31603) stainless steel will be specified where the operating conditions in the ethanol production process are more corrosive due to higher temperatures and/or the presence of acids, cleaning agents or higher levels of chloride from feedstocks. In addition, there can be selected applications for specialty stainless steel grades and even for nickel alloys.
There are some differences between the stainless steel requirements for dry milling and wet milling. In dry milling, the stainless steel used is primarily Type 304(L). In the milling section of the wet milling process, Type 316(L) stainless will be specified for equipment as the corn will be steeped in a more corrosive solution, containing sulfur dioxide (SO2) and lactic acid.
There is a similar scenario regarding materials used for evaporators and dryers. In a dry milling ethanol plant, this equipment will typically be made from Type 304(L). The same equipment used in wet milling plants preferably will be made of Type 316(L) stainless steel, which offers improved corrosion resistance.
Cellulosic Pretreatment Needs
The pretreatment phase of cellulosic ethanol process designs can be considerably more corrosive than the rest of the production process. This is particularly the case in plants using acid hydrolysis, where the biomass is treated with diluted sulfuric acid at high temperatures up to 240 C and under high pressure. With such a highly corrosive operating environment, nickel alloys such as Alloy C276 (UNS N10276) and other C-type nickel alloys must be used. The concentrated acid hydrolysis process, using higher sulfuric acid concentrations but much lower temperatures, is somewhat less corrosive.
Many pretreatment process technologies designed for enzymatic hydrolysis break down the crystalline structure of the lignocellulose to remove the lignin and liberate the cellulose and hemicellulose molecules. Depending on the biomass feedstock, physical or chemical methods will be used. Some of the most cost-effective technologies may involve a combination of an acid pretreatment stage and enzymatic hydrolysis. Various stainless steel grades and nickel alloys have been tested and used for the pretreatment production equipment in both pilot and commercial plants. These include Types 316(L) and 316Ti (S31653) and specialty stainless grades, such as 904L (N08904), duplex 2205 (S32205) and super duplex grades such as 2507 (S32750). Super-austenitic 6 percent molybdenum alloys have been tested and used. Trademarked names for these grades include 254SMO and AL-6XN.
Thermochemical technologies can also be used for biomass pretreatment processing. Specialized high temperature alloys may be required for equipment used in the related operating environments.
Stainless Steel in Brazil
Stainless steels are also increasingly used in the integrated sugar/ethanol plants in Brazil. Carbon steels are more commonly used in the sugar plants, while the distilleries typically use stainless steel. The use of carbon steel in sugar plants causes frequent wear and corrosion issues, which many plants have resolved by switching to stainless steels.
Where sugarcane is fed on a conveyor to be transported to a crushing mill, carbon steel suffers severe erosion and wear. Donnelly chutes are often made from Type 304(L) or S20400. A 12 percent Cr stainless steel (S41003), called 410D in Brazil, has sometimes been used because it has improved erosion and abrasion resistance over carbon steel and lower cost than the higher alloyed stainless steel grades.
For several other areas in sugar plants where corrosion is more severe, Type 304(L) will be used. In South America, ferritic stainless steel Type S43932 (a modified 439 type) and 444 (S44400) are commonly available, especially in thinner sheet thicknesses, and therefore are often used.
Types 304L, S43932 and/or 444 stainless steels are suitable for most applications in a sugar mill, such as in the cane juice extraction process (ducts, diffuser top), steam generation process, cane juice treatment and evaporation process, and in the sugar milling. Types 444 and S43932 are not recommended for the sulphitation (addition of SO2) stage of the process, where columns require 316(L) stainless. The distillation columns are also typically made of either Type 304(L) or 316(L) stainless steel.
Ethanol plants require substantial amounts of pipe, tubes and fittings—of which 75 percent is estimated to be stainless steel, and Type 304(L) accounts for the vast majority. In the U.S., some 80 to 90 percent of the pipe used is ASTM A 778 as-welded pipe, with ASTM A 774 as-welded fittings, also in Type 304(L). Ethanol plants in Canada have opted for a higher quality of product, using ASTM A 312 (welded and annealed) in Type 304(L) but also Type 316(L) where required. The Types 304(L) and 316(L) fittings used in Canada are specified to ASTM A 403 Class WP, similarly welded and annealed.
Other systems in ethanol plants require stainless steel as well. Heat exchangers require Type 316(L) stainless tubes. Tubing for evaporators can be both type 304(L) and 316(L). Brazil’s sugar cane ethanol industry also uses Types 444 and S43932 for evaporator tubing. In Europe, Type 304(L) is commonly used and at times Type S43932 will also be suitable. The cleaning systems and chemicals used, which may contain strong chemicals, will impact material choices for evaporators.
The power washing, cleaning-in-place (CIP) systems to clean fermentation tanks use caustic soda with an acid rinse and evaporators require regular cleaning due to scale build up. Stainless steels are used within CIP systems, with Type 316(L) used in heat exchangers and in the associated piping systems. Various components within the sulfuric acid skid systems typically use N08020 (Alloy 20).
The numerous mixers in ethanol plants are mostly made from Type 304(L) stainless steel but also Type 316(L) or Alloy 20. It is expected that the usage of the Type 316(L) alloy will increase when commercial scale cellulosic ethanol plants will be built.
Decanter centrifuges are mostly made from Type 316(L), but also from Type 317L (S31703) and 2205 duplex. The numerous processing pumps at ethanol plants are made from cast Type 316 (ACI CF8M) or Type 316L (CF3M) stainless steel, but increasingly also from duplex cast alloys, such as ASTM 890 Grade 1B (CD4MCuN), 3A (CD6MN) or 4A (CD-3MN).
Leading pump manufacturers indicate that some 60 to 70 percent of pumps supplied to ethanol plants are made of Type 316(L) and 30 to 40 percent of one of the duplex alloys. As duplex alloys are more erosion-corrosion resistant, performing well where solid particles are present, there has been a clear increase in their use in the past five years.
Ethanol plants also contain stainless steel valves, filters, regulators, quills, thermo-compressors, steam ejectors, screens and various other parts and components, mainly from Types 304(L) and 316(L). Alloy 20 is also used in sulfuric acid environments. Applications for Type 304(L) stainless can also be found in equipment used to control air pollutants and odor emissions and for food-grade ethanol applications. Some of the new high-strength, lean-duplex grades such as S32101 will be suitable for the very large tanks that use heavier plate.
As new processes are commercialized, stainless steel usage will only increase. Properly chosen and fabricated, stainless steels will be the material of choice for generations to come.
Author: Kristina Osterman
Consultant to the Nickel Institute
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).