Problem Statement

Is it possible to model an ethanol from corn process using Aspen Plus?

Solution

Attached is an example of this process. This example will run in Aspen Plus 2006 and higher.

1. Introduction

This file is a model of a dry-grind corn-to-ethanol plant process and it is intended for the following uses:

• Understanding the relationships between key variables for producing bioethanol from corn.
• Provides an example of how to model the different areas of this process.
• Supply a starting set of components and physical property parameters for modeling processes of this type

 The model is not intended for equipment design or specifying other engineering documents without further review by a process engineer with experience of corn-to-ethanol processes.

 The bioethanol from corn model includes the following features:

• A nominal set of chemical species and property parameters for this process.
• Typical process areas including: liquefaction, saccharification, fermentation, distillation, evaporation, dehydration, and the main streams connecting these units.
• Key process control specifications such as backset flow, fermenter ethanol percent, solids concentration, specifications for distillation columns and near-zero-net water balance.
• Usability features such as an Excel SCALE calculator which allows the user to scale plant production rate and apply corns of varying starch content. 

2. Components

The following components represent the chemical species present in the process:

 SOLID component types represent non-library chemicals with user specified property parameters. CONV components such as NFDS, XYLOSE, and PROTSOL originate as ?clones? of glucose and are later modified with their own property parameters. For example, the molecular weight of XYLOSE is modified to that of xylose (C5) in a Pure Component Paragraph.

3. Process Description

The process includes the following stages: 

4. Physical Properties

This category includes the models and methods used to calculate the chemical and thermodynamic equilibrium, and the physical properties of all streams. The models and methods used in Aspen Plus are grouped into Option-Sets named after the central model, e.g., Ideal, Redlich-Kwong-Soave, NRTL (Non-Random Two Liquid). The property Option-Set used in this model is NRTL.

Physical Properties are usually the most important and often the most difficult part of a simulation. The accuracy of physical property calculations strongly influences the reliability of the results and ultimately affects the estimated cost of process equipment.

5. Chemical Reactions

Dozens if not hundreds of chemical reactions occur in this process. These have been simplified in this model to the following:

1.       Saccharification

STARCH + WATER = GLUCOSE

99% conversion of STARCH

2.       Fermentation

GLUCOSE = 1.9 ETOH + 1.9 CO2 + .06 NFDS 100% conversion of GLUCOSE

NFDS = PROTSOL molar extent 3.31 lbmol/hr at 25 mmgal/yr

Saccharification and fermentation reactors are simplified to continuous operations. Conversions and molar extents are adjustable parameters in the model. No attempt has been made to model the action of enzymes and yeast in the reactors.

6. Simulation Approach

Unit Operations - Major unit operations have been represented by Aspen Plus models as in the table below.

Aspen Plus Unit Operation Models Used in the Bioethanol from Corn Model

Streams - Streams represent the material and energy flows in, out and around the process. Streams can be of three types: Material, Heat, and Work. Feeds to the process are corn, energy, water, acid, enzyme and yeast; the later three are represented by NFDS, WATER and WATER components respectively for simplicity. There are several internal streams that represent the crossover of material and heat between blocks. A key internal stream is 59BS representing backset.

Design-Specs, Calculator Blocks and Convergence - The simulation is augmented with a combination of flowsheeting capabilities such as Convergence, Design Specs and Calculator Blocks.

Sequencing and Convergence paragraphs are included that produce a relatively stable model at varied rates. The model has been tested at production capacities as low as 15 mmgal/yr and as high as 180 mmgal/yr and has run successfully aided by these convergence elements.

The following tables outline key flowsheeting capabilities of this model:

Design Specs Used in the Corn to Ethanol Model

Flowsheet Calculators Used in the Corn to Ethanol Model

Calculator blocks SCALE and EVALUATE are Spreadsheets in Excel, and are both embedded in the file with extension .apmbd. Use SCALE to change the characteristics of the corn feed as follows:

Calculator block EVALUATE may be easily modified to add more comparison variables that are of interest.

7. Simulation Results

The Aspen Plus simulation flowsheet and key results are shown below:

Key Simulation Results

8. Conclusions

The bioethanol from corn model provides a useful description of the process. The simulation has been developed using many of the capabilities of Aspen Plus including unit operation models, physical property methods, models and data, and flowsheeting capabilities like convergence design specs.

The model may be used as a guide for understanding the process and the economics, and also as a starting point for more sophisticated models for plant design and specifying process equipment. 

9. References