Farm-scale ethanol fuel production plant
Part I - Introduction & Plant Description
The Gildred/Butterfield Fuel Alcohol Plant
Gildred/Butterfield Limited Partnership
The report that follows, authored by Floyd Butterfield, the designer and operator of the Gildred/Butterfield plant, documents the plant's design, construction, operation and performance.
The Gildred/Butterfield ethanol fuel plant design is offered as one workable approach to ethanol fuel production. Though the design is adaptable to a variety of feedstocks and operating conditions, it may not be the most suitable design for every situation. Careful consideration of site specific factors including feedstock costs and availability, capital requirements, labor inputs, energy inputs and ethanol and co-product markets is critical in determining the technical and economic feasibility of any ethanol production project.
This report provides detailed information concerning the design, construction, operation and performance of the Gildred/Butterfield Fuel Alcohol Plant. In January, 1982 this plant was awarded first place in the California Alcohol Fuel Plant Design Competition. The competition was sponsored jointly by the California Department of Food and Agriculture and the State Solid Waste Management Board.
The Gildred/Butterfield Fuel Alcohol Plant is located on a ranch approximately six miles north of Paso Robles, California. It presently is in operation on a daily basis.
The plant currently processes off-market barley, wheat and screenings, although it is adaptable to a wide range of feedstocks.
For additional information on this plant contact Floyd Butterfield at 805-467-3417.
Front-End Handling Equipment
Front-end handling involves receiving and storing the feedstock and preparing it for processing. The equipment in this portion of the plant may vary substantially for different feedstocks. The plant designer/builder should specify and select equipment suitable for the proposed feedstock.
Grain may be received and stored in any of the commonly accepted ways. The first important step in processing for alcohol recovery is the reduction of the feedstock particle size. Size reduction will render the feedstock more susceptible to rapid enzyme breakdown and efficient mixing. In selecting a specific grinding technique, one must also consider the solid-liquid separating method used later in the process and the end-use of the processed feedstock.
There are two basic types of mills for grinding grains -- the roller mill and the hammer mill. The roller mill is suitable for low-moisture grains that crack easily. On such feedstocks, the roller mill produces uniformly reduced grain with a low percentage of fines. A hammer mill can successfully grind a wider variety of feedstocks.
Feedstocks should be ground with either type of mill to the point where finer grinding provides no further increase in alcohol yield or makes processed feedstock recovery uneconomic.
If the feedstock contains foreign material (such as dirt clods, rocks, trash, etc.) it should be cleaned before being sent to the mill. Any mill should also be protected by a magnetic trap.
For the plant shown on Plate l, feedstock receiving, storage and milling is done at another location. Trucks unload over a bottom-dump hopper and grain is augered into a 100-ton storage bin. As needed, the grain is milled by a 5HP roller mill, which is fed from storage by an auger via a scalper and magnetic trap. The ground grain is augered into a truck and transported to the plant.
Following particle-size reduction, the feedstock is augered into the feedstock storage tank shown on Plate 1 (No. 28). From there it is augered into the scale hopper, weighed and then augered into the cook tank. The scale hopper shown on Plate 1 has only a 500 lb. capacity so it must be loaded several times for a typical 3000 lb. Batch.
A more efficient design would be to have the feedstock storage tank or cooker on a scale itself.
Cooking involves the breakdown of the starch contained within the feedstock to sugar. This breakdown is accomplished by mixing the feedstock with warm water and enzymes and then injecting steam to further heat the mixture.
The batch cooker shown on Plate 1 (No. 20) is a 3000 gallon mild steel tank. It is insulated with a minimum of two inches of urethane foam. The tank is positioned on two steel support pillars and tilted toward the outlet at an approximately 10 degree angle (Plate 3).
The cooker is equipped internally with a 1-1/4-inch diameter, perforated steam line for direct injection of steam. The steam line runs the length of the tank and is positioned slightly off-bottom as shown. It has 1/8-inch diameter holes spaced 2.5 inches apart along its entire length. The steam line from the boiler to the cooker is 1-1/4-inch diameter and fitted with a globe valve for steam flow regulation and a check valve to prevent backflow to the boiler. A back-pressure regulator may also be required in this steam line depending on the other steam demands placed on the boiler during cooking.
The cooker also contains a paddle-type mixer as shown in detail on Plate 3. The mixer shaft is coupled by a chain drive to a motor/gear reducer combination on top of the cooker.
Inlets and outlets to the cooker are via 2-inch diameter, full opening ball valves and hose connections.
External Heat Exchanger
The external heat exchanger (No. 19, Plate 1) is used to cool the contents of the cooker and to cool fermenters when necessary. It consists of a 2-inch diameter copper pipe within a 2-1/2-inch diameter copper pipe. The material to be cooled is pumped through the inside pipe while water flows through the annulus around the inside pipe and in the opposite direction. The smaller tube has baffles welded on its outside each 3 feet along its length to support it inside the larger tube and to create turbulent water flow.
This exchanger should be a minimum of 20 feet long. (An alternative to this external heat exchanger would be to install copper coils inside the cooker and pump cooling water through them. That method has the advantage of lower pumping cost but makes cooker cleanout more difficult.)
Connections to this heat exchanger are made via 2-inch hose connections.
In the cooker, the starch contained within the feedstock is converted to soluble sugar. At this stage, the entire mash could be fermented. An alternative is to first separate the solids from the liquids after the starch has been converted to soluble sugar and then ferment the liquids only. The plant designer and/or operator must make this decision based on feedstock type and cost, by-product value, need for fermenter agitation, separation efficiency, etc. In any case, the solids and liquids are usually separated at some stage in the process.
The plant shown on Plate 1 uses a two-stage separation process. First, the mash is pumped from the cooker to a Sweco separator (No. 13, Plates 1 and 2). This separator is a vibrating screen type. Liquids that contain fermentable sugar pass through the screen and discharge to one of the fermenters via a two-inch hose. The solids move off the screen and fall into the hopper of the screw press (No. 12, Plates 1 and 2).
The screw press is shovn in detail on Plates 4 and 5. As the mash is augered through the press tube, the excess liquids that fall through the perforations are collected in the catch basin and pumped back onto the Sweco. The solids continue along the tube, are pressed in a short, augerless section at the end of the tube and then fall by gravity into the mash storage bin (No. 14, Plates 1 and 2).
The pressed mash, at about 65% moisture, can be stored until enough is accumulated for transport or direct use.
The mash is stored in a fully enclosed wooden box (No. 14, Plates 1 and 2). This box has a metal frame that is faced on the inside with 3/4- inch-thick plywood. The plywood should be treated with a moisture sealant.
Mash falls into the box by gravity through a small door at the top. The bin can be emptied by opening the full-width doors on front and using a front-end loader.
Carbon dioxide from the fermenters is piped to the mash storage bin. This keeps the mash fresh (for up to one month) until it can be used.
Each fermenter (No. 8, Plates 1 and 2) consists of a 1700-gallon plastic tank. Each tank is positioned on a stand consisting of a two-inch box steel floor frame with two-inch diameter steel pipe legs. The stand tilts the fermenter toward the outlet at an approximately 7 degree angle. Each fermenter is equipped with a thermometer and a second, off-bottom outlet for drawing samples and mixing the fermenter contents with a pump. It may be desireable to equip each fermenter with a built-in mechanical agitator to help speed the fermentation and aid in complete tank drainage.
Discharge from each fermenter is through a two-inch diameter hose via a two-inch, full-opening ball valve.
The distillation section of the plant is shown on Plates 1 and 2 and in detail on Plate 6. The distillation column internals are shown on Plate 7.
Following fermentation, each fermenter's contents are pumped to the beer well (No. 2, Plates 1 and 2). The beer well is a 2300 gallon plastic tank on a stand. During distillation, the beer is pumped continuously from the well by the beer feed pump (No. 10, Plate 1) to the tube-side of the condenser/preheater (No. 7, Plate 6). This is a 4-pass, tube-in-shell heat exchanger as specified in Appendix A. The beer then flows to the tube-side inlet of the second beer preheater (No. 1, Plate 6). From the outlet of this preheater, the beer is fed into the downcomer of the second highest plate in the stripping column.
In the stripping column, the feed falls down the downcomer and impinges on the blank portion of the plate below (see illustration of sieve plates, Plate 7). It then travels across the perforated portion of that plate, over the outlet weir, and down the downcomer. After traveling down the entire length of the column and across each sieve plate, the resulting stillage is pumped through the shell side of the second stage beer preheater (No. 1, Plate 6) and to the stillage holding tank (No. 7, Plates 1 and 2).
The purpose of the stripping column is to remove the alcohol from the beer feed stream. Steam is injected into the base of the stripping column. The rate of steam addition can be controlled by a pressure reducing valve.
More accurate steam flow control can be obtained through the use of a temperature-controlled steam valve. The steam rises through the plate perforations and carries with it the alcohol in the feed and some water vapor.
The vapors from the top of the stripping column proceed through the two-inch diameter exhaust tubing shown on Plate 6 and enter the base of the rectifying column. These vapors then rise through the perforations in the plates of this column and exit at the top, into the shell side of the condenser/preheater (No. 7, Plate 6). Since the condenser/preheater is only a partial condenser, the ethanol proceeds next to the air-cooled heat exchanger (No. 8, Plate 6). This exchanger completely condenses and may sub-cool the product slightly. The alcohol then flows by gravity through a single-pass, water-cooled, tube-in-shell heat exchanger (No. 10, Plate 6). By controlling the rate of water flow through this exchanger, the temperature of the alcohol entering the reflux drum can be maintained at a constant. These multiple condensers/subcoolers allow maximum beer preheat and minimize the use of cooling water.
The reflux flow rate is controlled by a temperature-regulated valve whose sensor is placed in the vapor space at the top of the rectifying column (operating experience indicates that a more appropriate sensor position is approximately two-thirds of the way down the rectifying column). The reflux controller shown on Plate 6 (No. 12) contains a flowmeter to monitor the reflux flow.
Ethanol that is not returned to the rectifying column as reflux flows out the overflow of the reflux drum and to the plastic, final product holding tank.
Heat in the form of steam is supplied to the cooker and distillation section by a 10 horsepower, propane-fired boiler (No. 17, Plate 1). This is a fully packaged unit which contains all combustion controls and feedwater reservoir, pump and controls.
Feedwater to the boiler must first pass through a water softener (No. 6, Plate 1) and should be preheated as much as possible. Feedwater preheat can be obtained from the stillage stream during distillation, from boiler exhaust gas, from water discharged from heat exchanger No. 10, Plate 6, or from the external heat exchanger.
The stillage from the stripping column consists of hot water containing dissolved and suspended feedstock solids not removed by the separators and dead yeast cells. The stillage is pumped from the stripping column to the stillage holding tank (No. 7, Plate 1).
Approximately 25% to 40% of the hot stillage can be reused in the next cook batch. This accomplishes a substantial savings in process water and energy consumption since each cook begins with preheated water. As the amount of stillage that is recycled in subsequent batches increases, mixer motor load increases and solid-liquid separation become more difficult. Very high rates of recycling (i.e. greater than about 40%) may impair fermentation.
The stillage that is not recycled can be sold as a liquid feed or fertilizer or discharged to an outside holding pond. If treated as a waste, local regulations must be followed as the stillage is usually high in B.O.D. and total solids, and can create an odor problem if not disposed of properly.
Warm Water Storage
This system consists of a 2300-gallon plastic water storage tank, pump and pneumatic tank as shown on Plate 1 (Nos. 3, 4, & 5). Warm water from the outlet of the water-cooled heat exchanger in the distillation section (No. 10, Plate 6) and from the external heat exchanger (No. 19, Plate 1) is piped into the water holding tank. This water may then be used in the next cook batch, as plant washdown water, or directed to the softener and used as boiler feedwater.
The method of wiring the plant will depend on its location (indoors or out), layout and size. In general, an electrical contractor should be contacted to do this work as it should all meet the National Electrical Code and any other applicable local codes.
Excluding front-end equipment, plant electrical demand can be met by a 20 kVA transformer. A roller or hammer mill may significantly increase this requirement as will a feedstock storage and handling system. Three phase service of at least 220 volts is recommended for motors above 1-2 horsepower to increase motor reliability and useful life.