Treating Digester Off-Gas from Wet Mill Ethanol Production

By Kenneth D. Jones, Steve Black and Tony Barnette | July 08, 2008
Recently, the Energy Independence and Security Act of 2007 has coupled increasing fuel efficiency standards and renewable fuel use in America. Under this law, ethanol production from dry and wet milling processes is projected to increase. As ethanol production increases, so will waste emissions from fermentation processes. Merichem Gas Technology Product's trademarked LO-CAT technology is a viable alternative for removing hydrogen sulfide from digester gas produced as part of wet milling processes. Use of LO-CAT treated digester gas helps maximize use of valuable natural resources.

The majority of ethanol in the United States is produced during dry grind fermentation of total mash from crops such as corn, wheat, barley, milo/sorghum and potatoes. Distillers dried grain with solubles, a cellulosic coproduct of the dry grind process, is valuable as animal feed or fertilizer.

Figure 1 . Potential energy savings per 1 MMBtu


Wet milling is more elaborate than dry grind because grain must be separated into its components. After corn milling, corn is heated in a water solution to loosen germ and fiber.

In subsequent steps, germ is removed from the kernel, corn oil is extracted, germ meal is added to hulls and fiber to make corn gluten feed, and gluten is separated to create corn gluten meal for use in animal food. Only the starch of the corn is fermented to produce ethanol. The process also produces wastewater that must be treated before disposal.

Although more ethanol is produced by dry versus wet milling, increasing demand for ethanol as a biofuel, has prompted increased use of new wet milling processes to maximize the yield of ethanol from many cellulosic materials such as corn stalks, grain straw, paper pulp, municipal wastes, switchgrass and other sources. New processes for fermenting woody and herbaceous biomass are being developed to yield more ethanol. These processes are projected to yield more sulfurous wastewater and sludge.

Wastewater and sludge from the new wet milling processes can be treated in anaerobic digesters before disposal. During this digestion process, sour gas containing hydrogen sulfide is produced. When the gas stream is treated by LO-CAT, hydrogen sulfide is removed, yielding gas that may be used as an alternative fuel source while meeting environmental standards. Treated digester gas can be used to lower ethanol plant energy costs by lowering natural gas usage.

Potential Energy Savings
Natural gas prices have almost doubled since January 2000, reaching $11.06/MMBtu this January. At press time the New York Mercantile Exchange natural gas futures contract settlement price was $11.60/MMBtu. Typical digester off-gas can yield as much as 600 Btu/standard cubic foot (SCF) versus natural gas at 1,000 Btu/SCF.

Ethanol production energy costs could be reduced by more than half if energy requirements are supplemented through use of LO-CAT cleaned digester off-gas (Figure 1). Every 1,000 SCF of treated digester off-gas could yield 0.6 MMBtu, equivalent to $6.96 in saved natural gas cost. Actual energy savings may vary based on the availability and composition of treated digester gas.

Sour gas is routed to a LO-CAT absorber. The type of absorber used is dependent upon the product gas hydrogen sulfide specification. The conventional liquid full scheme is used when approximately 99.99 percent hydrogen sulfide removal efficiency is required. In fact, as low as 1 parts per million by volume hydrogen sulfide can be achieved in the treated gas. The venturi-mobile bed option is used when moderate hydrogen sulfide removal efficiency is required and low pressure feed gas, less than 15 pounds per square inch, is treated.

In the absorber, sour digester gas is contacted with LO-CAT solution. Hydrogen sulfide is absorbed from the gas into the LO-CAT solution where chelated iron catalyst promotes a reaction to convert hydrogen sulfide to elemental sulfur. Treated gas then exits the top of the absorber and sulfur rich LO-CAT solution flows to the oxidizer.

In the oxidizer, LO-CAT solution is mixed with air as elemental sulfur settles to the bottom of the vessel. Oxygen in the air oxidizes (regenerates) the chelated iron in the LO-CAT solution returning it to its active state. In its active state, iron and LO-CAT solution are recycled to the absorber to remove more hydrogen sulfide.

Elemental sulfur that settles to the bottom of the vessel forms a 15 weight percent slurry which is removed using a filter. Sulfur is then washed and filtered resulting in 60 percent to 65 percent sulfur cake. The balance of the cake is water and trace amounts of LO-CAT solution. Wash water is recycled back to the oxidizer.

LO-CAT Chemistry
The LO-CAT process was developed to provide an isothermal, low operating cost method for carrying out the modified Claus reaction:
H2S + 1/2 O2 4 H2O + S

The overall reaction is explained using the following equations.
Absorption of hydrogen sulfide: H2S (g) + H2O (l) 1 H2S (aq) + H2O (aq) (1)
Ionization of hydrogen sulfide: H2S (aq) 4 H+ + HS- (2)
Sulfide oxidation: HS- + 2Fe+++ 4 S(s) + 2Fe++ + H+ (3)
Absorption of oxygen: 1/2 O2 (g) + H2O (l) 1 1/2 O2 (aq) + H2O (aq) (4)
Iron oxidation: 1/2 O2 (aq) + H2O + 2Fe++42 OH- + 2Fe+++ (5)

Equations 1 and 2 represent the absorption of hydrogen sulfide into the aqueous chelated iron solution and its subsequent ionization, while equation 3 represents the oxidation of hydrogen sulfide ions to elemental sulfur and the accompanying reduction of the ferric (active) iron to the ferrous (inactive) state. Equations 4 and 5 represent the absorption of oxygen from ambient air into the aqueous solution, followed by oxidation of the ferrous iron back to the ferric state. Equations 3 and 5 are very rapid. Consequently, iron-based systems generally produce relatively small amounts of byproduct thiosulfate ions. However, equations 1 and 4 are relatively slow and are the rate controlling steps in all chelated iron processes.

It is interesting to note that the chelating agents do not appear in the process chemistry, and in the overall chemical reaction the iron cancels out. So the obvious question is why is chelated iron required at all, if it doesn't take part in the overall reaction? The iron serves two purposes in the process chemistry. First, it serves as an electron donor and acceptorin other words, a reagent. Second, it serves as a catalyst in accelerating the overall reaction. Because of this dual purpose, the iron is often called a catalytic reagent. Although there are many metals, which can perform these functions, iron was chosen for the LO-CAT process because it is inexpensive and nontoxic.

The chelating agent(s) do not take part in the process reactions. Their role is simply to hold the iron ions in solution. Neither ferrous nor ferric ions are soluble or stable in aqueous solutions. Iron will ordinarily precipitate at low concentrations as either ferric hydroxide or ferrous sulfide. The chelating agents are organic compounds that wrap around the iron in a claw-like fashion, preventing the iron ions from forming precipitates. The LO-CAT process uses a proprietary system of chelating agents to hold the iron in solution over a very wide pH range.

LO-CAT has developed into a versatile processing scheme for treating gas streams containing moderate amounts of hydrogen sulfide. Advantages of these systems include the ability to treat both aerobic and anaerobic gas streams, removal efficiencies in excess of 99.9 percent, essentially 100 percent turndown on hydrogen sulfide concentration and quantity, and the production of innocuous products and byproducts.

Sulfur Product
Elemental sulfur produced by the LO-CAT process has quite profound differences from sulfur produced from other processes. LO-CAT sulfur has a particle size ranging between 8 to 45 microns, which is much smaller than sulfur produced by other processes. The sulfur is produced in a cake form that is 60 percent to 65 percent sulfur, water and trace amounts of LO-CAT solution. These distinct differences give LO-CAT sulfur softer particle texture, hydrophilic nature and water miscibility, and faster soil absorption characteristic and degradation.

These qualities benefit the agricultural industry locally and internationally. Specifically, fertilizing companies recognize the benefits of using LO-CAT sulfur to make sulfur-based fertilizer products. Currently LO-CAT sulfur is used for soil pH adjustments, plant nutrients and foliage fungicides.

LO-CAT sulfur is nontoxic and has been approved for use by the Organic Materials Review Institute. OMRI establishes standards for crops to receive organic designation. OMRI follows the guidelines required for compliant use of sulfur under the USDA National Organic Program Rule 7 CFR Part 205.

Kenneth D. Jones, Steve Black and Tony Barnette are with the Gas Technology Products division of Merichem Chemicals & Refinery Services LLC. All can be reached at (713) 428-5000.