Exergetic Analysis of Bioethanol Production from Tunisian Waste Dates

The results obtained show that approximately 60% of the exergy fed to the process is recovered in the useful products (bioethanol and exhausted pulp used as feedstuff). The overall exergy destroyed in the process considered is about 377 kW which represents 7% of the exergy reaching the process. The distillation section, the most energy-intensive stage, constitutes the main contributor of exergy destruction, followed by the fermentation reactor with contributions of 47% and 33%, respectively.


INTRODUCTION
Anthropogenic activities exert distressing pressure on natural resources.In fact, the high fossil fuel consumption is confronted with several problems, including the limitation of natural resources and greenhouse gas GHG emissions (CO 2 , CH 4 and N 2 O).This alarming situation is marked by global warming, loss of biodiversity and depletion of resources.Thus, the energy transition towards renewable energy seems an emergency in the last decades.Several sources of renewable energies have already been topics of research works such as wind, solar, hydraulic, geothermal, biomass, etc.Currently, biomass is becoming increasingly important in research of renewable energies field.It can be converted into high-energy products, mainly through thermo-chemical routes with or without chemical catalysts such as combustion, gasification and pyrolysis [1], and biochemical processes using enzymes and / or microorganisms such as anaerobic digestion for the production of biomethane [2], alcoholic fermentation for the production of bioethanol [3], etc. Bioethanol is considered as an important contributor to possible solutions to our sustainable fuel needs [4].It burns cleaner than gasoline, producing less CO, CO 2 and NO x emissions [5].However, bioethanol derived from maize and sugar cane, classified as first-generation bioethanol, is facing an intense debate "food versus fuel" [6].Therefore, in order to avoid competition with agro-food chains, it is necessary to use non-food material such as agricultural wastes in view of their abundance and their renewable characteristics.
Tunisia is considered to be one of the major dates producing countries in the world [7].The production of dates is estimated to be about 195,000 tones in 2013 [8].Approximately, 30% of the total production is generally discarded due to inadequate texture (too soft or too hard), fungal contamination, or insect infestation [9].Date wastes are generally used as feed for livestock or rejected posing environmental problems.The use of this waste for the production of bioethanol could be a strategic solution for its abundance and high-sugar content.The date is composed of a flesh and a hard seed representing approximately (10-15%) of the total weight of the date [10].The flesh is composed of sugar-rich food fiber about 72.8% (sucrose, glucose, and fructose), mineral salts (potassium, phosphorus, magnesium, calcium, etc.) and proteins [11].Waste dates are considered a better non-edible alternative source for alcoholic fermentation [12].For this reason, several studies have been conducted to produce bioethanol from waste dates.B. Louhichi et al. [13] carried out a synthesis study of bioethanol from three secondary varieties of dates (Kunta, Eguoua and Bouhatem) in the Gabes-Tunisia region.The alcoholic fermentation was investigated under sugar concentration near to 200 g L -1 at 30°C and natural pH.The results obtained showed that all the varieties tested allow producing ethanol with a mass yield (mass of ethanol / initial mass of sugar) equal to 11%. S. Chniti et al. [14] evaluated the bioethanol production capacity of the three yeasts, Saccharomyces cerevisiae, Zygosaccharomyces rouxii and Candida pelliculosa, from waste dates.The best result is retained with the strain S. cerevisiae with an initial concentration of 174 g L -1 and with 38% mass yield of ethanol.
Currently, exergy analysis is used to evaluate the performance of different systems and to improve their efficiencies [15].The link between the exergy analysis and the sustainability shows that reduction of the loss of energy quality (exergy) leads to ensure sustainability of energy use [15].Exergy analysis is based on both the first and second laws of thermodynamics able to locate the imperfections of a system and identifying their causes [16].Exergy is a relevant tool in terms of energy efficiency, thus contributing significantly to improving energy efficiency and thus increasing environmental efficiency (evaluating our consumption and production lead to less pollution).From a thermodynamic point of view, exergy is defined as the maximum amount of work that can be produced by a flow of material, heat or work in interaction with a reference environment (characterized by temperature T and pressure P and a composition of reference state) [17].Many recent bioethanol studies have focused on exergy analysis to assess the performance and sustainability of the bioethanol production process [16, 18 -20].
The main objective of this study is to evaluate the energy performance of the bioethanol production process from waste dates via an exergy analysis.This analysis serves as a guide to improving efficiency by focusing on exergy destroyed during key stages of bioethanol production in order to identify areas of imperfection and to understand the sources of thermodynamics imperfections

Characterization of Waste Dates
Waste dates, considered in this work, comes mainly from the two varieties of dates: " Allig" and " Deglet Nour" produced in the desert regions of southern Tunisia.These two Tunisian varieties constitute the large part of the production (about 195,000 tons in 2013 [8]).A significant fraction of production is lost during picking, packaging and storage [21].The date is composed of soft, edible flesh and a hard seed, representing approximately (10-15%) of the total date weight [10].The flesh consists of sugar-rich food fiber (sucrose, glucose, and fructose), mineral salts (potassium, phosphorus, magnesium, calcium, etc.) and proteins.Table 1 shows the average chemical composition of the dates considered [11].These data are used subsequently to study bioethanol production process by alcoholic fermentation of waste dates.3.02±0.13 (a) In % dry matter (b) In mg/100 g dry matter.

Simulation of the Process
The bioethanol production process from waste dates includes three main sections: (1) pretreatment, (2) fermentation and (3) purification.The purification section was adopted from a published article [22] dealing with the production of bioethanol from sugarcanes and blue agaves.To describe the process and calculate the material and energy exchanges, the bioethanol plant was simulated by SuperPro Designer ® 8.5 (Intelligen, Inc.).The flowsheet is given in Fig. (1).The simulation results are subsequently used to compute all exergy forms exchanged in the process.

Pretreatment
The pretreatment section is devoted for sugar extraction, using various equipments.A conveyor belt (BC) is used as a mechanical supplier to feed the process (F1).Dates are then washed (F5) and grinded (F6) to make them suitable for the sugar extraction stage respecting optimums conditions described in B.louhichi et al. [13].To separate sugar-rich juice (F11) from the solid phase (F10), a following centrifugation operation is carried out.The exhausted pulp (F10) is considered a byproduct which it could be used as feed for livestock.

Fermentation
The operating conditions in the fermentor are fixed on the basis of results published by B. Loufichi et al. [13].The reactions extents are closely related to the yeast strain retained for this study (S.cerevisiae FRCH strain [23,24]).The main reactions involved in the fermentor are summarized in Table 2.An adjustable flow mixer (MX-1) is used to adjust the stream (F14) to the desirable concentration of sugars.

Purification
The distillation section is dedicated to the recovery of bioethanol.For this purpose, three distillation columns (Dist-1, Dist-2 and Dist-3) followed by a molecular sieve (P-20, /MS) are used.

Different forms of Exergy
In this paper, three forms of exergy are considered: exergy associated with heat transfer, exergy associated with mechanical work and exergy associated with a steady stream of matter.The environmental temperature and pressure used in this work are respectively 298.15 K and 1 atm.

a. Exergy associated with heat and work transfer
The exergy associated with a heat flow is expressed as follows (Eq.1) [25].

(1)
Where T is the reference temperature (298.15K), T is the temperature at which the heattransfer occurs, Q is amount of heat transferred.
-The exergy associated with the transfer of mechanical work (Eq.2).
(2) b.Exergy associated to material stream This step consists in determining the exergy associated to material stream (Ex M ) that includes five terms, namely physical exergy (Ex phy ), chemical exergy (Ex chem ), the exergy of mixing (Ex mix ), the kinetic exergy (Ex kin ) and the potential exergy (Ex pot ).Only the chemical, physical and mixtures exergy are calculated in the present study.The variations of the kinetic and potential exergies are often neglected in the expression of matter [16,18].

(3)
The exergy associated with the material flow is computed accordingly to the methods described by A.P. Hinderink et al. [26] frequently adopted in research topics dealing with exergetic analysis [27,28].
The physical exergy (Ex phy ) is the maximum amount of work that can be retrieved when a material flow is brought from its initial state (T, P) to the state of the defined environment by (T, P) by a process involving only a thermal interaction with the environment [25].This component is given by A. P. Hinderink et al. [26] (Eq.4).

(4)
where n is the number of chemical species in a flow of material, D is the mass flow, xl and xv are the mass fraction of liquid and vapor, respectively, in the flow of material, xi and yi are respectively the mass fraction of species i in the liquid and vapor phases steam, Hi and Si are respectively mass enthalpy and entropy of the pure component i. Indices l 0 ( 1) and v refer respectively to the liquid and vapor phases.
-The chemical exergy (Ex chem ) is the maximum work that can be recovered when the substance under consideration is brought from the environment, defined by the parameters T and P, to the dead state by a process involving only heat transfer and material exchange with the environment (Kotas, 1995) [25].The chemical exergy is given by [26] A.P. Hinderink and et al. (Eq.5).
Where ε chem,i 0l , ε chem,i 0v are respectively the standard chemical exergy of the species (i) in liquid and vapor phase, determined from Kotas [25].Chemical exergies process components are evaluated using the composition of the reference environment defined by Kotas [25].The mixing exergy (Ex mix ) always has a negative value (Eq.6) [26]. Where

Exergetic Evaluation
To establish an exergy balance, we consider an open system operating in steady state.This balance consists on coupling an enthalpic and entropic balance that highlighting the term of destroyed exergy (Ex destroyed ).The arrangement of material, enthalpy and entropy balances leads to Eq.( 9) [25].This expression, known as the exergy balance, includes four terms: 1: Specific exergy associated with matter (Ex M = Ex phy + Ex chem + Ex mix ).Q k : The quantity of heat exchanged by the system at a temperature T k .
W k : Non-thermal energy (mechanical, electrical ...) received by the system.
To compute the destroyed exergy, the detailed method is considered which provides the contribution of each equipment (e.g.distillation column) to the total exergy destroyed.More details about the calculations are provided in supplementary materials.

Process Design and Simulation
The simulation results show the thermal and mechanical powers exchanged at the various equipments (Table 3) and the characteristics of the different material flows (flow, temperature, pressure, composition, enthalpy) (Table 4).The main operating conditions needed in the process simulation are retrieved from experimental study [13] to match as possible real operating unit and to get valuable results.The novelty of this work is the relevant application of waste dates in the field of bioethanol.However, conduct a rational comparative analysis between the studied system and real ones which involve the same transformations steps is a big challenge.

Exergy Analysis Results
The different forms of exergy exchanged in the process (matter, work, heat) are gathered in Tables 5 and 6.

Aspen Plus
TM is employed in order to estimate the thermodynamic properties (enthalpy and entropy) of each stream.The hypothesis considered in this step is to assimilate all organic compounds that constitute date fleshes to dextrose (sugar predefined in AspenPlus database), thus dates are modulated as (95% dextrose and 5% water).To calculate the exergy associated to the stream material F1 (waste dates entering the system) we proceed as describe in the text.
Ex pky = 0 Because the flux is in environmental conditions (T=100 kPa, and T= 298.15 K).Where ε 0 chem, i is the standard chemical exergy of the species (i) determined from Kotas [25].
Exergy associated to different streams are listed in Table 6.We display in Table 7 the outcomes of exergy balance carried on the main components of the process.More details about the calculations of exergy balance are provided in supplementary materials.The distillation section has the most exergy destruction (Fig. 2) of 177.8 kW about 47% of the total exergy destruction in the bioethanol production process.According to the fractionating principles, reducing the reflux ratio, ie increasing the number of stages, could lead to a substantial reduction in the irreversibility of the column [29].Heat exchangers (condenser and reboiler) are also a source of irreversibility in the distillation column due to the considerable temperature differences between the hot and cold streams.To see in details, the distribution of the exergy destroyed through the various distillation columns, the contribution of each column to the exergy destroyed is shown in Fig. (3).It is clear that the first distillation column "Dist-1" "constitutes the most zone of thermodynamic imperfection of the process.It contributes to more than half of the destroyed exergy of the distillation columns and thus accounts for 26.71% of the total exergy destruction from the process.The fermentation process contributes about 33% of the exergy destruction of the system.Its inefficiency is mainly caused by the creation of entropies during biochemical reactions and the dissipation of heat due to the exothermic nature of the reactions [30].The heat exchanger (HX-4) is the third contributor to the exergy destruction with about 11.5% of the exergy destroyed this is due to the temperature difference between the hot and cold flow which is around 35° C. Finally, the extraction tank also contributes to the exergy destroyed about (4.43% of the total exergy destroyed) which can be at the origin of concentration gradient during perpetual diffusion of sugars from the concentrated zones to the diluted zones.The method conducted to compute the exergy destroyed is a detailed one i.e., we focus on the exergy destroyed in each equipment to figure out its contribution to the thermodynamic imperfections.While the total exergy that entering the system is: where Ex F1 , Ex F7 , Ex F12 , Ex F15 , Ex F24 are exergy associated to material streams F1, F7, F12, F15, F24 respectively.ΣEx Q , ΣEx w are exergy associated with heat flow and mechanical work provided to the main steps of the system.Thus the overall exergy destruction (377.54 kW) represents 7.21% of the exergy entering the system.Furthermore to calculate the exergetic efficiency of the process we adopt the rational exergy efficiency which is the ratio of exergy content of useful products to the total exergy input [27].The useful products leaving the system are bioethanol (F36) and exhausted pulp used as feed for animal (F10), exergetic efficiency is computed as below To the best of our knowledge exergetic analysis of bioethanol production from waste dates has not yet been carried out to measure the thermodynamic performance of such system which constitutes the novelty of this work.Exergy analysis can deal with any systems of different features and is powerful in qualitative analysis for any elements of a system.Furthermore, the exergy analysis can indicate the major cause of imperfections of a system.However, the lack of a formal procedure in using the results from an exergy analysis is one of its limitations.Therefore, the combination of two relevant methods the pinch and exergy analysis constitutes a rational basis for process modifications.Pinch or exergy analysis alone cannot fulfill the task for effective process modification, since they have their own intrinsic advantages and disadvantages.The power of pinch analysis (which is not the purpose of this work) is in showing the main features of a system on simple diagrams.Therefore, the proper combination of these two methods can brought significant benefits for process modifications, since it is essential to display the system information graphically to visualize its performance and quantitatively identify major directions for improvement [31].

CONCLUSION
The main objective of this study is to provide an exergetic investigation of bioethanol production system from waste dates.Several chemical engineering assessment tools have been simultaneously applied.The main outcomes of this research can be summarized in these points: -Approximately 60% of the exergy fed to the process is recovered in the useful product (bioethanol and feedstuff).About 7% of the exergy reaching the process is destroyed due to the imperfections of the thermodynamic transformations.
The overall exergy destruction in the process is about 377.54 kW.
The greatest loss of exergy is localized in the distillation section; this is due to the mixture of the streams with unbalanced compositions and the differences in temperatures in the stages of the column.
The fermentation reactor occupies the second place in term of exergy destruction with a contribution of 33%

2 :
Exergy associated with heat transfer.3: Exergy associated with mechanical work exchange.4: Term corresponding to the creation of entropy due to the different irreversibility of the system, it is exergy destroyed.With: D in : Mass flow of material at system inlet.D out : Mass flow of material at the system outlet.

Fig. ( 3 )
Fig. (3).Contribution of each column distillation to the total exergy destroyed associated to the the distillation section [% to the total exergy destruction].