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There are two basic types of acid processes: dilute acid and concentrated acid, each with variations. Dilute acid processes are conducted under high temperature and pressure, and have reaction times in the range of seconds or minutes, which facilitates continuous processing.

As an example, using a dilute acid process with 1% sulfuric acid in a continuous flow reactor at a residence time of 0.22 minutes and a temperature of 237°C (458°F) with pure cellulose provided a yield over 50% sugars. In this case, 0.9 t (1 ton) of dry wood would yield about 189 L (50 gallons) of pure ethanol. The combination of acid and high temperature and pressure dictate special reactor materials, which can make the reactor expensive.

Most dilute acid processes are limited to a sugar recovery efficiency of around 50%. The reason for this is that at least two reactions are part of this process. The first reaction converts the cellulosic materials to sugar and the second reaction converts the sugars to other chemicals. Unfortunately, the conditions that cause the first reaction to occur also are the right conditions for the second to occur. Thus, once the cellulosic molecules are broken apart, the reaction proceeds rapidly to break down the sugars into other products—most notably furfural, a chemical used in the plastics industry. Not only does sugar degradation reduce sugar yield, but the furfural and other degradation products can be poisonous to the fermentation microorganisms.

The biggest advantage of dilute acid processes is their fast rate of reaction, which facilitates continuous processing. Their biggest disadvantage is their low sugar yield. For rapid continuous processes, in order to allow adequate acid penetration, feedstocks must also be reduced in size so that the maximum particle dimension is in the range of a few millimeters.

Since 5-carbon sugars degrade more rapidly than 6-carbon sugars, one way to decrease sugar degradation is to have a two-stage process. The first stage is conducted under mild process conditions to recover the 5-carbon sugars while the second stage is conducted under harsher conditions to recover the 6-carbon sugars. Unfortunately, sugar degradation is still a problem and yields are limited to around 272 L/t (80 gallons of ethanol/ton) of dry wood.

The concentrated acid process uses relatively mild temperatures and the only pressures involved are usually only those created by pumping materials from vessel to vessel. One concentrated acid process was first developed by USDA and further refined by Purdue University and the Tennessee Valley Authority.

In the TVA concentrated acid process, corn stover is mixed with dilute (10%) sulfuric acid, and heated to 100ºC for 2 to 6 hours in the first (or hemicellulose) hydrolysis reactor. The low temperatures and pressures minimize the degradation of sugars. To recover the sugars, the hydrolyzed material in the first reactor is soaked in water and drained several times.

The solid residue from the first stage is then dewatered and soaked in a 30% to 40% concentration of sulfuric acid for 1 to 4 hr as a pre-cellulose hydrolysis step. This material is then dewatered and dried with the effect that the acid concentration in the material is increased to about 70%. After reacting in another vessel for 1 to 4 hr at 100ºC, the reactor contents are filtered to remove solids and recover the sugar and acid. The sugar/acid solution from the second stage is recycled to the first stage to provide the acid for the first stage hydrolysis. The sugars from the second stage hydrolysis are thus recovered in the liquid from the first stage hydrolysis.

The primary advantage of the concentrated process is the high sugar recovery efficiency, which can be on the order of over 90% of both hemicellulose and cellulose sugars. The low temperatures and pressures employed also allow the use of relatively low cost materials such as fiberglass tanks and piping. Unfortunately, it is a relatively slow process and cost effective acid recovery systems have been difficult to develop. Without acid recovery, large quantities of lime must be used to neutralize the acid in the sugar solution. This neutralization forms large quantities of calcium sulfate, which requires disposal and creates additional expense.

Using some assumed cellulose conversion and fermentation efficiencies, ethanol yields from glucose can be calculated for corn stover (the above-ground part of the corn plant less the ears) as shown in Table 1 showing ethanol yield from glucose. Similarly, ethanol yields from the xylose can be calculated as shown in Table 2.

Table 1. Ethanol yield from glucose.

Dry stover 1 tonne (1000 kg)
Cellulose content × 0.45
Cellulose conversion and recovery efficiency × 0.76
Ethanol stoichiometric yield × 0.51
Glucose fermentation efficiency × 0.75
Yield from glucose 131 kg ethanol = 151 L (40 gallons)

Table 2. Ethanol yield from xylose.

Dry stover 1 tonne (1000 kg)
Hemicellulose content × 0.29
Hemicellulose conversion and recovery efficiency × 0.90
Ethanol stoichiometric yield × 0.51
Xylose fermentation efficiency × 0.50
Yield from xylose 66 kg ethanol = 76 L (20 gallons)

Thus, in this example, the total yield/t of dry stover is about 227 L (60 gallons) of ethanol. These numbers also show how critical sugar conversion and recovery efficiencies and fermentation efficiencies are. If one could attain 95% for both efficiencies, then the yield would be approximately 350 L/t (103 gallons of ethanol/ton).

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Written by Casey McConnell

March 8, 2008 at 6:07 pm

Posted in Bioenergy

Tagged with , , ,

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