Ethanol production by Fermentation of Grain to be employed as Bio-Fuel
Liquid Bio-based fuels are mainly used as vehicle fuels, the major ones being Biodiesel and Bioethanol. Bioethanol (also known as fuel ethanol) is anhydrous ethanol, which is at least 99% pure. The history of ethanol as a fuel dates back to the early days of the automobile. However, cheap Gasoline quickly replaced ethanol as the fuel of choice and it was not until the early 1980’s, when the Brazilian government launched the Protocol program, that ethanol made a come back to the market place. Pure ethanol is rarely used as a fuel for transportation (with few exceptions in case of modified engines). Usually it is mixed with Gasoline. Thus, ethanol as an automotive fuel can be used in two ways: Firstly, it can replace gasoline outright in dedicated internal combustion engines and secondly it can be mixed with gasoline in blends of 5 to 30%. In this case no engine modifications are required. These blends achieve the same octane boosting or anti-knock effect as petroleum derived aromatics like benzene or metallic additives like lead. The most popular blend for light-duty vehicles is known as E85, which is 85 percent ethanol and 15 percent Gasoline. Heavy-duty trucks typically use E95 (ethanol blended with five percent unleaded Gasoline) and E93 (ethanol blended with five percent methanol and two percent kerosene). In some European countries, ethanol has also been used as a 10 percent mixture with Gasoline in a blend called “gasohol” or E10. Ethyl alcohol can be produced from a variety of sources, including fermentation of carbohydrates derived from starch crops such as grain and potatoes, from sugar crops such as cane and beets, from cellulosic agricultural residues such as bagasse and corn stalks, and from wood and wood by-products. It can also be produced by hydration of ethylene or from synthesis gas containing hydrogen and carbon monoxide.

Your company is considering a bid to purchase an ethanol production plant owned by a smaller company, and you have been given the assignment of reviewing the process. This review is to include a quantitative study of the operation and should de¬velop detailed information regarding key process variables. Although the study need not be concerned with economics, consideration should be given to the energy re¬quirements of the process. Suggestions for alternative processing schemes should be included in the final report.

In the process to be described, a portion of the starch in corn is converted to ethyl alcohol by two biological processes: saccharification and fermentation. In saccharification, the polymeric structure of starch (a polysaccharide) is hydrolyzed in the presence of the enzymes (biological catalysts) a-amylase and amyloglucosidase. The primary products of hydrolysis are maltose (a disaccharide consisting of two glucose units) and oligomers consisting of several glucose units as seen below.
-(C6H10O5)x – C12H22O11 + H20 + -(C6H10O5)y-
Starch Maltose Oligomers
The fermentation process is based on the growth of a yeast culture that converts maltose to ethyl alcohol and carbon dioxide:

C12H22O11 + H20 + (yeast) 4 C2H5OH + 4 C02 + (yeast + H20)

Properties of 199-Proof Ethyl Alcohol
Specific Gravity 0.7962
Composition
Vol % ethyl alcohol 99.50
Wt % ethyl alcohol 99.19
Mol % ethyl alcohol 97.95
Composition of Corn (Grain)

Component Wt%
Starch 72
Protein 9
Oil 3
Fiber and ash 16
Moisture 15.5
As the yeast culture grows, 0.0794 lbm of yeast is produced for every lbm ethyl alcohol formed, and 0.291 lbm water is produced for every lbm of yeast formed. The residue from the alcohol recovery and dehydration operation can be further processed to recover distillers’ dried grains and solubles (DDGS). The yeast solids become a part of the by-product grains, and the sale of DDGS as animal feed improves process economics and minimizes waste disposal problems.
The alcohol plant under consideration is designed to produce 50 million gallons of 199-proof (99.5 vol%) ethyl alcohol per year. Existing technology provides a yield of 2.57 gal of alcohol from a bushel of corn. (See Tables for properties of 199-proof ethyl alcohol and corn.) The process is to be operated continuously, with the exception of the fermentation and fungal amylase sections of the plant, which must be operated in a batch wise fashion to allow for frequent sterilization of equip¬ment. The nature of the plant dictates that it operates at least 330 days per year.

PROCESS DESCRIPTION

Corn is milled and the resulting meal is conveyed to a mixing tank where it is blended with recycled condensates and water to produce a mash. The total water input to this rank is controlled to produce 22 gal of mash per bushel of corn input. The specific gravity of the mash is approximately 1.1. From the mixing tank, the slurry flows to a precook vessel, which is maintained at 145° F. The condensates, which have been added hot to the mixing tank, yield a mash at 100°F. Live saturated steam at 15 psig is added to the precook vessel to increase the mash temperature to 145°F. (Live steam is steam injected directly into a process vessel.)

Mash from the precooking vessel is heated to 230°F by condensing saturated steam at 15 psig, and then to 320°F with saturated 150-psig live steam. The mash is then sent to a cooking tank and held there long enough for the starch structure to be broken down in preparation for the saccharification reactions. The cooked mash is flashed to 15 psig to produce saturated steam and a concentrated mash, which is cooled to 145°F by flashing to a vacuum. The flash-tank vacuum is main¬tained by drawing the vapor into a condenser where most of the steam is condensed and passing the remaining uncondensed vapor to a steam ejector. (Steam ejectors are described in Perry’s Chemical Engineers’ Handbook.) Steam at 150 psig is fed to the ejector along with the uncondensed vapor, and the resulting mixture is sent to a condenser operating at 15 psig. The ratio of vapor drawn from the flash condenser to 150-psig steam fed to the ejector is 0.04 lbm vapor/lbm steam. Approximately 30 lbm/h of vapor is drawn into the ejector from the flash condenser. All condensates from these units are pumped back to the mash mixing tank.

Mash from the vacuum flash is mixed with a small amount of fungal amylase, and the mixture is sent to the saccharification reactors where the starch is converted at 140°F to fermentable sugars. Thin stillage (s.g. = 1.1) recycled from a down¬stream unit is added to the liquor from the saccharification reactors to lower the pH, provide yeast nutrients, and obtain a final mash volume of 25 gal per bushel of corn fed to the process. The volume of the thin stillage added is 16% of the final mash volume going to the fermenters. The mixture of converted mash and thin stillage is cooled in heat exchangers to 100°F by cooling-tower water at 85°F, and then to 85°F by well water at 60°F. The temperature increases of both cooling water streams are limited to 25°F.
The cooled mash is fed to the fermentation reactors, which are operated in a batch mode. The reactor network consists of eight vessels, each having a capacity of 550 000 gal. The vessels are arranged in sets of two with one heat exchanger and circulation pump for each set of fermenters. Liquid loading is 90% of vessel capacity. Cooling is needed for only about 24 h out of the 48-h fermentation cycle, which makes it possible for one exchanger to service two fermenters. The fermenters are filled on an 8-h cycle; in other words, the flow rate of mash is sufficient to fill a single reactor every eight hours.

About 300 lbm of yeast is added to each fermentation batch. During fermentation, heat is released by the exothermic conversion of sugars to ethanol and carbon dioxide, and the batch temperature is allowed to increase from 85°F to 95°F. Removal of heat from the mash as it is recirculated through the heat exchangers is used to prevent the temperature from increasing above 95°F. Well water at 60°F is the cooling fluid used in these exchangers, and each fermenter requires a flow of about 2400 gal/min during the peak period. The temperature increase of the well water is limited to 25°F.
The fermentation products are sent to a storage vessel from which dilute alcohol is pumped at 90°F and 1 atm and heated to 280°F by condensing 150-psig saturated steam. The dilute alcohol is then flashed to separate essentially all of the carbon dioxide. Water and alcohol vaporized with the carbon dioxide are condensed and returned to the flash drum. The liquid leaving the flash drum is at 250°F and con¬sists of 7.1 wt% alcohol, 6.9 wt% soluble and suspended solids, and water. This liquid is fed to a recovery system consisting of a distillation followed by an adsorption and desorption (regeneration) column.

The dilute liquid from the flash tank is fed to the first distillation column, which is operated at 50 psig and is known as the ethanol concentrator. Heat is supplied to the ethanol concentrator by condensing approximately 110 000 lbm/h of 150-psig saturated steam in the reboiler. Condensate from the reboiler is saturated at 150 psig. The distillate and reflux are saturated liquids at 250°F and contain 95 vol% (190-proof) ethyl alcohol. The bottoms from the column are at a temperature of about 305°F, and contain all of the solids fed to the column and approximately 0.02 wt% alcohols on a solids-free basis.
“General information about Distillation column. Feed enters each distillation column at a location between the top and bottom, and two product streams are withdrawn. A vapor stream is removed from the top of the column and condensed. A portion of the condensate (reflux) is returned to the top of the column, and the remainder is taken as overhead product or distillate. A liquid stream leaves the bottom of the column and goes to a reboiler that vaporizes a portion of the liquid. The generated vapor is returned to the column as boilup, and the remaining liquid is taken as product and is referred to as bottoms.”

The distillate obtained from the distillation column is an azeotropic mixture of Ethanol and water. An azeotrope containing two or more components is a mixture whose bubble point is greater or less than all pure-component bubble points (Find the description of azeotrope from Perry’s Chemical Engineering Handbook). If an azeotrope is encoun¬tered in distillation, its composition represents a limit to the separation or concentra¬tion that can be achieved. However, In-order to utilize ethanol as a bio-fuel, it needs to be dehydrated to achieve almost 99.5 vol% concentration. The dehydration of ethanol is obtained using an adsorption system, which consists of Adsorption-Desorption column. While the adsorption column is used for purifying the ethanol, the other column is being thermally regenerated. One the first column gets saturated the 2nd column is employed for the adsorption and the 1st column is then meanwhile regenerated. Adsorption is a phenomenon, wherein one component of the mixture is preferentially retained on the surface of the solid (adsorbent) and thus making purifying the liquid phase. In the given case, the azeotropic mixture of the ethanol/water is passed through the bottom of the 1st adsorption column consisting of 3A molecular sieves which has high capacity to adsorb water. Prior to sending the mixture through the adsorption column it is cooled to 25 0C. The adsorbent capacity of 3A molecular sieves is 0.16wt/wt (wt of water/ wt of Zeolite) and 0.001 wt/wt (wt of ethanol/wt of Zeolite). The product coming out of the top of the column contains 99.5 vol% ethanol and 0.5% water is sent to the storage tank. The properties of 3A molecular Sieves is given in table below. The 2nd column which was previously saturated is thermally regenerated by hot flue gases entering the bottom of the column at 600 0F. The heat capacity of molecular sieves can be assumed to be similar to silica.

Properties of 3A Molecular Sieves beads
Properties Values

Molecular formula 3A Molecular sieve (assuming pure Zeolite) [K+12 (H2O)27]8 [Al12Si12 O48]8
Heat of Adsorption 40.00 kJ/mol
Density 2.75 gm/ml

After the bottoms from the ethanol concentrator has been flashed to atmospheric pressure, the resulting liquor is divided by centrifugation into two fractions: a thin stillage containing 4 wt% total solids and a wet cake containing about 30 wt% total solids. As described earlier, part of the thin stillage is recycled to join the mash leaving the saccharification reactor. The remaining thin stillage is heated from 165°F to 208°F and is then sent to an atmospheric evaporator to produce a syrup with a solids con¬tent of 40 wt%.

The wet cake is mixed with the syrup and recycled dried grains, which have a moisture content of 10 wt%. The recycle of dried grains is adjusted so that the re¬sulting mixture of wet cake, syrup, and recycled dry grains has a moisture content of about 30 wt%. The wet grains mixture is fed at 100°F to a dryer where it is contacted with hot flue gas from a boiler. The hot gas, whose composition is given in Table below, enters the dryer at about 600°F, and gas and dried grains leave the dryer at 190°F. The dried grains, which contain 10 wt% moisture, are split into two portions, with one portion being recycled to regulate the moisture content of the feed to the dryer and the remainder being sent to storage and shipping.

Composition of Flue Gas Fed to Dryer

Component Vol%
CarbonDioxide 12.6
Water 9.3
Nitrogen 73.3
Oxygen 4.5
Sulphur Dioxide 0.3

 

Problems:

The heat capacity of mash may be assumed to be the same as that of water. The specific gravity of the mash prior to fermentation is 1.1; after fermentation it is 1.05. The heat capacity of grains is 0.25 Btu/Ibm°F. In your solutions to the following problems, cite all sources of data not given in the process description, and clearly state all assumptions.

1. Construct a flowchart of the process and a separate diagram for one of the fermentation batteries containing two reaction vessels, one pump, and one heat exchanger. Calcu¬late and fill in the component flow rates (lb,„/h) and temperatures of each stream on the chart to the extent possible from the given information.

2. Calculate the feed rate of corn in Ibm/h. Estimate the acreage required to supply this plant with corn. (Mote: According to the U.S. Department of Agriculture, Crop Production, 11/9/78, p. A-3, 101.2 bushels of corn are harvested from an acre.)

3. Calculate the molar flow rate (lb-moles/h) of ethanol product.

4. How much water (combined fresh water and condensates) must be added to the mixing tank to obtain the desired mash volume?

5. The C02 produced as a by-product from the fermentation reactor may be useful if it is recovered in sufficient quantity. Estimate the production rate of C02 in pound-moles per hour and standard cubic feet per minute.

6. What fraction of the dried grains must be recycled to control the moisture content of the grains entering the dryer? What production rate of dried grains can be expected from this process?

7. What is the minimum pressure at which the cooking vessel must operate? What is the pressure to which the mash must be flashed to provide a resultant liquor temperature of 145°F? At what rate (lbm/h) is water vaporized in this step?

8. Determine the rate (lbm’h) at which 15-psig steam must be supplied to heat the mash from the precooking vessel to 230°F. How much live steam must be injected to raise the tem¬perature of this stream to 320°F? It has been suggested that a portion of these steam requirements could be met by the vapor from the flash occurring at 15 psig. Is this feasible? If so, what fraction of the required 15-psig steam can be supplied by this means?

9. At what rates must heat be removed to condense the vapors from the distillation column? Suppose cooling water at 85°F is available and the tempera¬ture increase of this water is limited to 25°F. At what rates must water be supplied to the condenser?

10. Estimate the quantity of 3A molecular sieves required in one adsorption bed?

11. At what rates must heat be removed in-order to cool the distillate prior to sending it to the adsorption column? Cooling water from well at 60°F is available and the tempera¬ture increase of this water is limited to 25°F. At what rates must water be supplied to the heat exchanger?

12. At what rate must the hot gas be fed to the drier to provide the required drying of grains? If this gas is to be obtained from the combustion of Illinois No. 6 coal with 30% excess air, how much coal is required?

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