Energy and Mass Balance Model Description

8/7/2019 Energy and Mass Balance Model Description

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Figure 2. Inputs and Outputs at the System Boundary

2. Mass Balance Model

To resolve the material balance for the corn‐to‐ethanol plant, the unit operations of

the block flow diagram (BFD) must be defined3. Figure 3 shows the BFD of a typical

ethanol plant with all the basic steps included.

Figure 3. Mass Block Flow Diagram of Ethanol Production Process



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For each of the block, the material balance is written as:

Material in = Material out (2‐1)

or oi M M = (i = input; o = output) (2‐2)

or ∑∑==

=n

k

k o

m

j

ji M M 1

,

1

, (2‐3)

where j represents the type of material inputs with a total of m inputs, and k represents

the type of material outputs with a total of n outputs.

The mass balance calculation starts with corn inputs. Table 1 lists the composition of

corn3.

Table 1. Corn CompositionComponent Mass Content % Water 15.0%

Starch 59.5%

Protein 7.7%

Oil 3.4%

Other 14.5%

Total 100%

The starch (the actual material that makes ethanol) is then mixed with water and

turns into glucose followed by fermentation and production of ethanol. The reaction

stoichiometry on a weight basis can be written as:

Starch + 0.1111 Water 1.1111 Glucose (2‐4)

Glucose 0.4589 Ethanol + 0.4641 CO2 + 0.05 other (2‐5)

Apparently, the yield of ethanol is a function of starch composition in corn,

conversion efficiency of starch to glucose, and conversion efficiency of glucose to

ethanol. Assuming a 100% efficiency for both conversion processes, the typical yield is

2.5 to 2.85 gallons ethanol per bushel corn3. In our model, we take the average value,

2.7 gal ethanol per bushel corn, as our corn‐to‐ethanol conversion rate. The CO2

emission profile can also be calculated from equations 2‐4 and 2‐5 with a given mass of

corn (or starch).

Based on the equations 2‐1 to 2‐5 and the process data and assumptions derived

from Mei et al.3, we have established an Excel‐based mass balance model for the corn‐

to‐ethanol production process. Figure 4 shows an example worksheet of the mass



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balance model. It demonstrates that with 1 kg input of corn, 2.68 kg water is needed,

and 0.32 kg ethanol and 0.33 kg DDGS can be produced with 0.31 kg CO2 emission and

2.72 kg wastewater discharge. It should be noted that this model assumes that no water

recycling technology is applied.

Figure 4. Excel worksheet for the mass balance model



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3. Energy Balance Model

The energy needs for ethanol production is of great concern, and the availability of

economical and reliable energy sources is essential for stable operation of the facility.

We have performed a literature review on the total energy consumption of the corn‐to‐

ethanol process, as listed in Table 2. The reported energy consumption varies

significantly from 40,850 to 75,118 Btu/gal, with an average of 53,750 Btu/gal.

Pimentel’s estimates is over 30,000 Btu/gal higher than Wang et al.’s estimate, and over

20,000 Btu/gal higher than the average value of the all the studies. This is because of

Pimentel’s inclusion of energy expended on capital equipment and energy for steel,

cement, and other materials used to construct the ethanol plant, components not

included in most of other studies. In this study, we used the average value from

literature, 53,750 Btu/gal, as the basis of our energy balance calculation.

Table 2. Total Energy Consumption for Corn‐to‐Ethanol Process

Literature Ethanol Conversion Process (Btu/gal) Pimentel and Patzek (2005) 54684

Pimentel (2001) 75118

Lorenz and Morris (1995) 53956

Wang et al. (1999) 40850

Shapouri et al. (2002) 51779

Mei et al. (2006) 46114

Average Total Energy Demand 53750 (Btu/gal) or 15.0 MJ/L Generally, energy demand for an ethanol plant consists of thermal energy and

electricity.

Thermal

energy

as

steam

is

used

for

cooking,

liquefaction,

ethanol

recovery

and dehydration. Natural gas thermal energy is used for dying and stillage processing.

Electricity is used for grinding and running electric motors. Figure 5 shows the diagram

of energy flow through the corn‐to‐ethanol plant.

Figure 5. Energy block flow diagram of ethanol production process

(Et – thermal energy, Ee – electricity energy)



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A general energy balance equation for each individual block can be written as3:

Energy Input = Energy Output (2‐6)

Or ∑∑ =++

streamsoutput

j

streamsinput

j E W QE (2‐7)

where E j represents the total rate of energy transported by the j th input or output

stream of a process, and Q and W are defined as the rate of flow of heat and work into

the process.

The energy balance calculation procedure is adapted from Mei et al3, and the

results are summarized in Table 3. For a ethanol conversion process, the majority of the

energy is used as thermal energy for cooking, liquefaction, distillation, and drying.

Electricity is mainly used for milling, distillation and drying processes.

Table 3. Energy Flow in Corn‐to‐Ethanol Process

Energy Flow Thermal Energy (MJ/L) Electricity Energy (MJ/L) A ‐Milling 0.21 0.10

C‐ Cooking/Liquefaction 2.81 0.06

D ‐ Fermentation ‐ 0.06

F ‐ Distillation 4.76 0.37

J ‐ Drying 6.22 0.41

Total 14.0 1.0

4. Flash Based Interactive Model

In this study, we have developed an interactive model integrating mass and energy

balance at the ethanol plant system boundary. Flash is a popular multimedia software

that can create animation and add interactivity to web pages. As shown in Figure 6, the

users of this Flash based model have two options to start the mass and energy balance

calculation by inputting either corn feed or ethanol plant capacity. For example, as

shown in Figure 6a, if the user chooses “Corn Feed” as the input method, a input text

box will show up and allow the user to type in the amount of corn that will be fed to the

plant. Then by clicking the “Run” button, the model will calculate and display the

amount of water and energy needed for the process, and the amount of ethanol and

DDGS will be produced, as well as the amount of wastewater and CO2 will be generated

and emitted, if no controls are installed. Similarly, as shown in Figure 6b, if the user

chooses “Plant Capacity” as the input method, after typing in the amount of ethanol

that a plant is expected to produce, the model will calculate and display the amount of

corn, water, and energy that are needed, as well as the amount of co‐products and

emissions. The user friendly interface and the interactive feature make this model a



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handy tool for researchers, plant managers, policy makers, and the public to understand

the overall energy and environmental impact of ethanol production process.

Figure 6. Flash‐Based Interactive Model on Energy and Mass Balance – Calculation

Based on (a) Corn Feed and (b) Ethanol Plant Capacity

(a)

(b)



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References

1. ICM, Inc. http://www.icminc.com/ethanol/production_process

2. National Corn-to-Ethanol Research Center. http://www.siue.edu/ETHANOL

3. Mei, F.; Dudukovic, M.; Evans, M.; Carpenter, N. Mass and Energy Balance for a

Corn-To-Ethanol Plant. Washington University in St. Louis, 2005.

Documents

Energy and Mass Balance Model Description