Ind. Eng. Chem. Res. 2010, 49, 1197–1213
1197
Integrated Process Modeling and Product Design of Biodiesel Manufacturing Ai-Fu Chang and Y. A. Liu* SINOPEC/AspenTech Center of Excellence in Process System Engineering, Department of Chemical Engineering, Virginia Polytechnic Institute and State UniVersity, Blacksburg, Virginia 24061
Biodiesel, i.e., a mixture of fatty acid methyl esters (FAMEs), produced from reacting triglyceride with methanol by alkali-catalyzed transesterification, has attracted much attention as an important renewable energy source. To aid in the optimization of biodiesel manufacturing, a number of published studies have applied commercial process simulators to quantify the effects of operating conditions on the process performance. Significantly, all of the reported simulation models are design models for new processes by fixing some level of equipment performance such as the conversion of transesterification reaction. Most models assume the feed oil as pure triolein and the biodiesel fuel as pure methyl oleate, and pay insufficient attention to the feed oil characterization, thermophysical property estimation, rigorous reaction kinetics, phase equilibrium for separation and purification units, and prediction of essential biodiesel fuel qualities. This paper presents first a comprehensive review of published literature pertaining to developing an integrated process modeling and product design of biodiesel manufacturing, and identifies those deficient areas for further development. This paper then presents new modeling tools and a methodology for the integrated process modeling and product design of an entire biodiesel manufacturing train (including transesterification reactor, methanol recovery and recycle, water wash, biodiesel recovery, glycerol separation, etc.). We demonstrate the methodology by simulating an integrated process to predict reactor and separator performance, stream conditions, and product qualities with different feedstocks. The results show that the methodology is effective not only for the rating and optimization of an existing biodiesel manufacturing, but also for the design of a new process to produce biodiesel with specified fuel properties. 1. Biodiesel Production by Transesterification Process The American Society for Testing and Materials (ASTM) defines biodiesel as a fuel comprised exclusively of monoalkyl esters of long-chain fatty acids derived from vegetable oils or animal fats, designated B100 (100% pure biodiesel), and meeting the requirements of ASTM designation D6751.1 The transesterification process is the most well-known biodiesel production technology that essentially involves the reaction of alcohol with triglyceride (from vegetable oils or animal fats) to produce fatty acid methyl esters (FAMEs), the biodiesel fuel. Alkali-catalyzed transesterification is the most common manufacturing method because of its faster rate and it is less corrosive to industrial equipment.2 Methanol is the most popular alcohol used because of its low cost and suitable properties,3 and the corresponding product is a mixture of FAMEs. This article focuses on alkali-catalyzed transesterification of methanol with triglyceride. Figure 1 shows a simplified flow sheet of an alkali-catalyzed transesterification process. There are many options for the separation and purification processes following the transesterification step.4 2. Integrated Process Modeling and Product Design Many of the alkali-catalyzed transesterification process models4–12 are developed by flow-sheet simulators such as Aspen Plus and Aspen HYSYS. Table 1 summarizes the key features of the process models in the literature. We distinguish the reported simulation models into design mode and rating mode. In the design mode, we develop simulation models by assuming some level of equipment performance * To whom correspondence should be addressed. Tel.: (540) 2317800. Fax: (540) 231-5022. E-mail:
[email protected].
(e.g., reactors with known component conversion, or with product yields calculated according to Gibbs thermodynamic equilibrium), and then design the equipment to meet this level of performance. In the rating mode, however, the equipment already exists and we develop simulation models with quantitative rate equations (e.g., reactors with known reaction equations and kinetic parameters) to predict how the equipment will perform under a variety of process conditions. Significantly, all of the reported simulation models are design mode, fixing the conversion of transesterification reaction. Most models assume the feed oil as pure triolein and the biodiesel fuel as pure methyl oleate, and ignore the saponification reaction which is important only when the reactor feed is crude or used oil containing much water and free fatty acids initially. The reported models pay insufficient attention to the characterization and thermophysical properties of feed oil, and to the prediction of essential product fuel qualities. This paper focuses on those issues that are not well covered or are essentially ignored in reported studies. 3. Reaction Kinetics The raw materials of transesterification reaction, vegetable oil and animal fat, are mixtures of several oils and fats, and the compositions vary with oil sources and growth conditions. The major feed component is triglyceride (TG) in which glycerol (GL) is esterified with fatty acids. Figure 2 shows the structure of TG, where R1, R2, and R3 are long fatty acid chains. Vegetable oil and animal fat usually contain small amounts of water and free fatty acids.14 Table 2 lists the chemical structures of common fatty acid chains.14,15 In the common acronym column in Table 2, the first number denotes the number of carbon atoms in the chain and the second number indicates the number of double bonds. The number of
10.1021/ie9010047 2010 American Chemical Society Published on Web 12/22/2009
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Ind. Eng. Chem. Res., Vol. 49, No. 3, 2010
Figure 1. Simplified flow sheet of an alkali-catalyzed transesterification process. Table 1. Key Features of Reported Simulation Models feed/product characterization
reference
feed/product definition
description of thermophysical property models
reactor model
kinetics
calculation of fuel properties saponification of biodiesel
thermodynamic model of biodiesel purification unit
Myint and El-Halwagi4
feed: pure triolein + oleic acid; product: methyl oleate
none
yield model
included
none
West et al.5
feed: pure triolein + oleic acid; product: methyl oleate feed: pure triolein; product: methyl oleate
none
yield model
not included
none
none
yield model
not included
none
none
yield model
not included
none
NRTL (nonrandom two-liquid) activity coefficient model with RKS (Redlich-Kwong-Soave) equation of state; UNIFAC for estimating missing parameters NRTL; UNIFAC for estimating missing parameters and fixed efficiency for centrifuge NRTL; UNIFAC for estimating missing parameters and fixed efficiency for centrifuge no description
none
yield model
not included
none
none
yield model
included
none
rigorous model
none Tang et al.13
Zhang et al.6 Harding et al.7
feed: pure triolein; product: methyl oleate feed: pure triolein Haas et al. product: methyl oleate Tapasvi et al..9 feed: mixture of simple triglycerides; product: mixture of FAMEs Kapilakarn and feed: no description; 10 product: no description Peutong feed: pure triolein; Stiefel and product: methyl oleate + Dassori11 diolein + monoolein feed: pure triolein; Apostolakou product: methyl oleate et al.12 8
mode
Aspen Plus
design
Aspen HYSYS design Aspen HYSYS design Aspen Plus
design
fixed efficiency for centrifuge
Aspen Plus
design
none
fixed efficiency for centrifuge
MS Excel
design
not included
none
fixed efficiency for centrifuge
Aspen HYSYS design
rigorous model
not included
none
fixed efficiency
Aspen Plus
yield model
not included
none
NRTL; UNIFAC for estimating missing parameters and fixed efficiency for centrifuge
Aspen HYSYS design
carbon atoms includes the carboxylic carbon. Table 3 shows the typical compositions of various oil sources. Freedman et al.2 show the overall reaction scheme of transesterification of methanol with triglyceride (TG) to give the fatty acid methyl ester (FAME), i.e., the biodiesel, and byproduct, glycerol (GL). See Figure 3. Figure 4 represents the stepwise reaction scheme which includes reaction intermediates, monoglyceride (MG) and diglyceride (DG),2 and fatty acid methyl esters (FAMEs). The reaction takes place through three stages: (1) slow masstransfer stage, (2) fast kinetic stage, and (3) slow chemical equilibrium stage.16–21 The slow mass-transfer stage at the beginning results from the immiscibility of oil and methanol. The reaction
design
rate is limited by the mass transfer of TG to methanol and oil phases. The fast kinetic stage occurs when the reaction intermediates, MG and DG, act as surfactants and stabilize the methanol drops in the continuous oil phase.18 The increasing interfacial area Table 2. Chemical Structure of Common Fatty Acid Chainsa fatty acid chain lauric acid myristic acid palmitic acid stearic acid oleic acid linoleic acid linolenic acid arachidic acid bechnic acid lignoceric acid
Figure 2. Chemical structure of triglyceride (TG).
program
structure HOOC-(CH2)10-CH3 HOOC-(CH2)12-CH3 HOOC-(CH2)14-CH3 HOOC-(CH2)16-CH3 HOOC-(CH2)7-CHd CH-(CH2)7-CH3 HOOC-(CH2)7-CHdCH-CH2-CHd CH-(CH2)4-CH3 HOOC-(CH2)7-(CHdCH-CH2)3-CH3 HOOC-(CH2)18-CH3 HOOC-(CH2)20-CH3 HOOC-(CH2)22-CH3
common acronym C12:0 C14:0 C16:0 C18:0 C18:1 C18:2 C18:3 C20:0 C22:0 C24:0
a By replacing the H atom in the HOOC- group with CH3 group, we have the structure of the fatty acid methyl ester (FAME). For example, the structure of lauric acid methyl ester is CH3-OOC-(CH2)10-CH3.
Ind. Eng. Chem. Res., Vol. 49, No. 3, 2010 Table 3. Compositions (wt %) of Various Oil Sources
1199
15
fatty acid chain
common acronym
palm
olive
peanut
rape
soybean
sunflower
grape
almond
corn
lauric acid chain myristic acid chain palmitic acid chain palmitoleic acid chain stearic acid chain oleic acid chain linoleic acid chain linolenic acid chain arachidic acid chain eicosenoic acid chain behenic acid chain erucic acid chain liqnoceric acid chain nervonic acid chain
C12:0 C14:0 C16:0 C16:1 C18:0 C18:1 C18:2 C18:3 C20:0 C20:1 C22:0 C22:1 C24:0 C24:1
0.10 0.70 36.70 0.10 6.60 46.10 8.60 0.30 0.40 0.20 0.10 0.00 0.10 0.00
0.00 0.00 11.60 1.00 3.10 75.00 7.80 0.60 0.30 0.00 0.10 0.00 0.50 0.00
0.00 0.10 8.00 0.00 1.80 53.30 28.40 0.30 0.90 2.40 3.00 0.00 1.80 0.00
0.00 0.00 4.90 0.00 1.60 33.00 20.40 7.90 0.00 9.30 0.00 23.00 0.00 0.00
0.00 0.00 11.30 0.10 3.60 24.90 53.00 6.10 0.30 0.30 0.00 0.30 0.10 0.00
0.00 0.00 6.20 0.10 3.70 25.20 63.10 0.20 0.30 0.20 0.70 0.10 0.20 0.00
0.00 0.10 6.90 0.10 4.00 19.00 69.10 0.30 0.30 0.00 0.00 0.00 0.00 0.00
0.00 0.00 10.40 0.50 2.90 77.10 7.60 0.80 0.30 0.00 0.10 0.00 0.20 0.40
0.00 0.00 6.50 0.60 1.40 65.60 25.20 0.10 0.10 0.10 0.00 0.10 0.10 0.00
between oil and methanol phases makes the mass transfer of TG to methanol and oil phases no longer limiting the reaction rate. In the last stage, the reaction approaches chemical equilibrium and the reaction rate slows down. In addition, saponification of free fatty acid and TG with hydroxide ion (OH-) may also occur in the reaction system (see Figures 5 and 6).4 Saponification leads to lower production of biodiesel and higher cost of the purification process.4 For the alkali-catalyzed transesterification of methanol with oils, Freedman et al.20 demonstrate that, at 60 °C or higher, 1 wt % NaOH, methanol/oil molar ratio at least 6:1 and fully refined oil (free fatty acid 101 51 96.5
