Article pubs.acs.org/IECR
Optimal Simultaneous Production of Biodiesel (FAEE) and Bioethanol from Switchgrass Mariano Martín* Departamento de Ingeniería Química, Universidad de Salamanca. Plz. Caídos 1-5, Salamanca 37008, Spain
Ignacio E. Grossmann Department of Chemical Engineering. Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States ABSTRACT: In this work, we optimize the simultaneous production of biodiesel (fatty acid ethyl esters, FAEE) and ethanol from switchgrass. Two technologies are considered for switchgrass pretreatment: dilute acid and ammonia fiber explosion (AFEX). Next, enzymatic hydrolysis follows any of the pretreaments to obtain fermentable sugars, mainly xylose and glucose. We separate the lignin, and, with the sugars, we consider the production of bioethanol and/or FAEE. Based on a superstructure of alternatives, the problem is formulated as an mixed-integer nonlinear program (MINLP) with simultaneous optimization and heat integration. An economic evaluation is performed and water consumption is calculated. The simultaneous production of bioethanol and FAEE from switchgrass is shown not to be competitive at the current development of the conversion of sugars to FAEE, but it can become promising if the conversion from sugar exceeds 0.5. The optimal solution indicates bioethanol to be the preferred product because of its higher yield.
1. INTRODUCTION Biofuels are a short- and medium-term alternative to reduce our dependency on fossil fuels. Typically, biorefineries have been presented as the production of a biofuel and some byproducts that improve the economics such as the case of distilled dried grain and solubles (DDGS) for first-generation bioethanol,1 glycerol, and fertilizers in biodiesel production2 and lignin or hydrogen in the production of second-generation bioethanol.3,4 So far, the most flexible biorefinery, in terms of the range of products, has been based on the Fischer−Tropsch technology5,6 that can use biomass, coal, and/or natural gas as raw materials for the production of light hydrocarbons, synthetic gasoline, and diesel, as well as heavier products.7−9 Biomass is a rich source of chemicals comprised of carbohydrates, proteins, lipids, and lignin. In a recent paper, Martı ́n and Grossmann10 presented the possibility of obtaining ethanol and biodiesel (fatty acid ethyl esters, FAEE) from algae simultaneously, because of its composition. We can use the lipids accumulated within to obtain oil that is used for biodiesel production, while the starch is a source of glucose, and, thus, ethanol can also be produced from it, via fermentation. Finally, the proteins are an asset to the process. Because of the lower cost associated with the production of biodiesel, the plant was self-sufficient in the sense that there is no need to buy alcohol to transesterify the oil, but the main product was FAEE with only 10% of the total biofuel production going to bioethanol. Recently, some experimental studies have shown the possibility of obtaining FAEE from lignocellulosic raw materials by aerobic fermentation of glucose.11−14 Therefore, it is a straightforward comparison to evaluate the competitiveness of the simultaneous production of bioethanol and biodiesel from lignocelulosic switchgrass, compared to the one that uses algae as raw material, and both to the stand-alone processes. © XXXX American Chemical Society
In this paper, we study the production process of both FAEE and bioethanol from lignocellulosic raw materials comparing two pretreatments of the lignocellulosic biomass. The goal is to simultaneously optimize and heat integrate the production process of FAEE and bioethanol to assess its competitiveness with current crude based fuels production and other integrated processes. The optimization of the system is formulated as a mixed-integer nonlinear programming (MINLP) problem, where the model involves a set of constraints representing mass and energy balances, experimentally based models and rules of thumb for all the units in the system. Ahmetovic and Grossmann’s model15 is used to design the optimal water network that minimized the freshwater consumption. A sensitivity analysis on the conversion to FAEE is also performed. Finally, a detailed economic evaluation is performed to determine the production cost and the investment.
2. OVERALL PROCESS DESCRIPTION We consider the use of switchgrass as biomass raw material since it is the energy crop of choice in the United States, because of its wide availability.3,4 Grinding is the first stage to reduce the size of the raw material and to increase the contact area before pretreatment. There are several alternative pretreatments, and a few comprehensive reviews have been published on the topic.16−19 Among them, the two most promising ones, because of their feasibility for scaleup, are (1) dilute acid Special Issue: Scott Fogler Festschrift Received: September 30, 2014 Revised: January 24, 2015 Accepted: January 26, 2015
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DOI: 10.1021/ie5038648 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
Figure 1. Superstructure of for the simultaneous production of ligno-based biodiesel (FAEE) and bioethanol.
(H2SO4) pretreatment20−23 and (2) ammonia fiber explosion (AFEX).17,24,25 So far, both have been used to release the cellulose and hemicelullose for their hydrolysis. Once the physical structure of the switchgrass has been broken to allow the contact between the polymers and the enzymes, they are hydrolyzed. This process is carried out in stirred tank reactors at 45−50 °C for 3 days, where the accessible cellulose and hemicellulose are broken into fermentable sugars.20,21,26−28 Next, the sugars, mainly glucose and xylose, are fermented. An aerobic fermentation yields biodiesel (FAEE), with ethanol and glycerol as byproducts.11,12,29−34 On the other hand, anaerobic fermentation yields bioethanol, as reported in previous papers.4,13,20 We propose these two options to evaluate the simultaneous production of bioethanol and biodiesel. From the aerobic fermentation, biodiesel can be easily separated from the rest of the products by centrifugation, and dehydrated using a vacuum dryer,13 while the water− ethanol−glycerol−biomass mixture can be further processed to recover ethanol. This ethanol can be processed with the one produced from the anaerobic fermentation, mainly bioethanol and water. To obtain fuel degree ethanol,we use a multieffect distillation column to separate the water−ethanol mixture and next a molecular sieves system to dehydrate the ethanol as in the 2012 work of Martı ́n and Grossmann4 (see Figure 1).
3.1. Pretreatment. In order for the fermentation to be effective, the bacteria must be able to reach the cellulose and hemicellulose. Any lignocellulosic raw material is created with a matrix of lignocellulose that protects the plant and maintains the structure. Inside the structure of lignin, the hemicelluloses and the cellulose are the major constituents of the plant. This structure must be broken so that the polymers of sugars (cellulose and hemicellulose) can be attacked to release the sugar monomers. First, the feed is washed and the size of the switchgrass is reduced by grinding so that further pretreatments are more effective.16,36 Both stages, washing and grinding, are considered only in terms of energy consumption (45 kWh/ ton36) and their cost, since they do not alter the chemical composition of the feedstock. Next, the two alternatives indicated abovedilute acid pretreatment and AFEXare analyzed, because of their high capability of degrading this structure.17,37−40 3.1.1. Ammonia Fiber Explosion (AFEX). This method consists of treating the lignocellulosic material at mild temperature and high pressure with ammonia to break the physical structure of the crop. In order to reduce the cost, the ammonia remaining in the slurry after the expansion should be recovered, and the slurry of biomass and water is sent to enzymatic treatment to break the polymers.17,24,25,41 The pretreatment is modeled using experimental data on the yield versus the operating variables. Garlock et al.41 developed a DOE-based model, given by eq 1, to predict the yield of sugars liberated from different switchgrass raw materials as a function of the ammonia (kg/kg of biomass) and the water load, the operating temperature (°C) and the contact time (min), at 20 atm.
3. MATHEMATICAL MODELING All the operations in the bioethanol and FAEE production process are modeled using mass and energy balances, design of experiments (DOE) based on experimental data in the literature, rules of thumb, and design correlations. The model is written in terms of total mass flows, component mass flows, component mass fractions, and temperatures of the streams in the network, so that they are the main variables to be determined from the optimization. The components in the system include those present in the switchgrass, as well as those involved in the different stages presented in the previous section. We define the component set J = {water, FAEE, H2SO4, CaO, ammonia, protein, cellulose, hemicellulose, glucose, xylose, lignin, ash, CO2, O2, CelluM, glycerol, succinic acid, acetic acid, lactic acid, gypsum, ethanol}. We describe the main modeling features below. For the case of the pretreatment and the ethanol production, we refer the reader to previous papers.35
yield = 0.01 × ⎡⎣−88.7919 + (26.5272 × ammonia_ratio) − (13.6733 × water_pret) + (1.6561 × T_AFEX) + (3.6793 × time_pret) − (4.4631 × ammonia_ratio2) − (0.0057 × T_AFEX2) − (0.0279 × time_pret2) − (0.4064 × ammonia_ratio × time_pret) + (0.1239 × water_pret × T_AFEX) − (0.0132 × T_AFEX × time_pret)⎤⎦
(1)
Here, T_AFEX is the operating temperature of the pretreatment (°C), ammonia_ratio is the ratio of ammonia to dry biomass in mass, and water_pret is the mass ratio between the water added and the dry biomass. Finally, time_pret is the B
DOI: 10.1021/ie5038648 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
provided in Shi’s paper,46 we have developed DOE-based models for the yield of the glucose and xylose released (see eqs 2 and 3):
residence time in the reactor (min). The upper and lower bounds in Table 1 are based on the range of the experimental data on which the model is based.
yield of glucose: Table 1. Range of Operating Variables for AFEX Pretreatmenta pretreatment temperature, T ammonia water residence time
lower bound
upper bound
90 °C 0.5 g/g dry matter 0.5 g/g dry matter 5 min
180 °C 2 g/g dry matter 2 g/g dry matter 30 min
yield_cellu = −0.00055171 + (0.00355819 × T_acid) + (0.00067402 × conc_acid_mix) + (time_pret × 0.00100531) − (enzyme_add × 0.0394809) − (0.0186704 × T_acid × conc_acid_mix) + (0.00043556 × T_acid × time_pret)
a
Reprinted with permission from ref 35. Copyright Elsevier, Amsterdam, 2014.
+ (0.0002265 × T_acid × enzyme_add) − (0.0013224 × conc_acid_mix × time_pret) − (0.00083728 × time_pret × enzyme_add)
Next, the pressure is released and the content of the reactor discharged to a blowdown tank. Since the reactor operates in batch mode, at least two reactors in parallel are fed into an intermediate storage tank to ensure continuous operation.17,42 Next, the ammonia remaining in the slurry, ∼10% of the initial amount, is recovered by distillation at high pressure (15 atm).23,43 The distillate temperature is 40 °C and the bottoms temperature is 200 °C23,43 and we assume a reflux ratio of 2. The evaporated ammonia is compressed, condensed, and mixed with the ammonia recovered in the distillation column and reused again. Following these stages, we assume that all of the ammonia is recovered. However, traces may be left, typically
