TECHNO-ECONOMIC EVALUATION OF CITRUS WASTE VALORIZATION FOR THE PRODUCTION OF BIOFUELS AND HIGH ADDED-VALUE PRODUCTS

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1 TECHNO-ECONOMIC EVALUATION OF CITRUS WASTE VALORIZATION FOR THE PRODUCTION OF BIOFUELS AND HIGH ADDED-VALUE PRODUCTS Anestis Vlysides 1, Nikolaos Kopsahelis 1, Seraphim Papanikolaou 1, Apostolis A. Koutinas 1, Ioannis K. Kookos 2 * 1. Department of Food Science and Human Nutrition, Agricultural University of Athens, Iera Odos 75, Athens , Greece 2. Department of Chemical Engineering, University of Patras, Rio, 26504, Greece * Corresponding author: i.kookos@chemeng.upatras.gr ABSTRACT Annual world production of citrus fruits is approximately 90 Mt and almost half is used for juice production. The by-products of the juice production including peel, segment membranes and other by-products are considered as citrus wastes (CWs). These CWs can be dried and used as raw material for pectin extraction or pelletized for animal feed but a significant fraction of CWs is still deposited every year. As CWs contain different carbohydrate polymers they are a potential source for production of biogas, bioethanol and many high valueadded materials. In this work a comprehensive techno-economic analysis is presented to estimate the economic potential for CWs valorization. The process for the production of biofuels and high value-added products is synthesized and simulated in commercial software. The relevant equipment is designed in detail. Finally, indices of economic performance are calculated and the economic viability of the venture is evaluated. INTRODUCTION World production of citrus fruits is around 90 Mt/y and it is estimated that half of these fruits is squeezed to juice, while the remainder including peel, segment membranes and other by-products is considered as CWs (Wilkins et al., 7). CWs contain different carbohydrates, limonene and pectin and can be used as raw material for producing of fuels and high value-added products. Limonene is toxic for microorganisms and as it can, for instance, decrease the biogas yield or the production of chemicals (such as ethanol) it should be separated from the CWs prior to bio-processing steps. Two alternative processes for production of ethanol from CWs can be used. In the first process enzymatic hydrolysis can be used to produce a hydrolyzate rich is fermentable sugars and limonene is removed by filtration prior to fermentation. In the second alternative limonene is removed by steam explosion following a dilute-acid hydrolysis step that takes place under high pressure and temperature. The limonene-free hydrolyzate if then fed to a fermentation process where most of the sugars are transformed to ethanol. The second process has been studied recently by Pourbafrani et al., In this process, the CW is mixed with sulfuric acid and then steamexploded to hydrolyze the CW and also remove limonene. The slurry produced is first centrifuged and the liquid part is fermented to ethanol and distilled. The stillage from the distillation and solids from the centrifuge were mixed and digested to biogas (methane) than can partly be utilized to produce steam and electricity. The aim of this work is to investigate the economic potential of a skeleton industrial process of ethanol and limonene production from CW as a base case for further investigation of the CW valorization. The process is simulated using UniSim Design from Honeywell and then standard sizing and costing procedures are applied in order to estimate the fixed capital cost. Raw material consumption, utilities consumption and operating labor are also estimated to calculate the cost of manufacture. Finally, cash flow analysis is performed and the net present value of the investment is estimated. PROCESS DESCRIPTION The proposed process is broken down into two areas. Area, which is shown in Figure 1, is the pretreatment and limonene recovery area. Area, which is shown in Figure 2, is the fermentation and ethanol recovery area. All solids produced from the process are fed to a solids combustion area (also part of Area ) for steam generation. The CW contains 20% dry matter with the composition shown in Table 1. The plant operates for 4000 h/y. The raw CW (50,000 t/y) is fed through a conveyor belt and a mill to the hydrolysis vessel R-101 together with superheated steam and hydrochloric acid so as to achieve 15 bar, 150 o C and 0.5 v/v HCl. The hydrolysis vessel R-101 is a train of four vessels operating in batch mode with a cycle time of 20 min. After the completion of hydrolysis the hydrolyzate is flashed into atmospheric pressure in the flash vessel V-101. The vapor produced contains almost all limonene present in the feed. The vapor stream is condensed in the heat

2 exchanger E-101 and almost pure limonene is recovered in the decanter V-102 and stored (V-103). The liquid stream is first cooled in heat exchangers E-102 and E-103 and then fed to vessel V-104 where neutralization is taking place using a strong basis. The cooled and neutralized hydrolyzate is stored in V-104 and then fed to the fermentation section that consists of a 0 m 3 fermentor R-201 (supported by a train of five seed fermentors m 3, 10 m 3, 1 m 3, 0.1 m 3 and 0.01 m 3 ) with a cycle time of 60 h. The fermentor is cooled using an external heat exchanger (E-204). 95 % of C6 sugars and 85 % of C5 sugars are transformed into ethanol and 2 % of C6 or C5 fermentable sugars are transformed into biomass (CH 1.8 O 0.5 N 0.2 ). An absorption column is used to recover the ethanol lost in the gas stream that contains mostly CO 2. Unconverted solids (including pectin) and cell mass are separated from the liquid content of the fermentor in the filter F-201 and are fed to the solids combustor BH-201. BH-201 is used to produce high pressure steam (HPS) used in heat exchanger E-104 to preheat the water fed to the hydrolysis vessels. The liquid stream from the filter is mixed with the liquid stream that comes out from the absorber and after they are preheated they are fed to the distillation column T-202. To bottoms product from the column is almost pure water while the top product stream is an ethanol rich stream (95% w/w). Ethanol and water form an azeotropic mixture at 95% w/w ethanol that prevents further purification of ethanol in a single distillation column. Ethanol is then stored in V-202 for further purification. RESULTS AND DISCUSSION The process described in the previous section is simulated in UniSim Design from Honeywell in order to solve consistently all material and energy balances. Components not available in the database (carbohydrate polymers, limonene, hexozes, pentozes, biomass, pectin and insoluble solids) are introduced as Hypothetical components. Following the simulation of the process, standardized sizing and costing procedures (Peters, Timmerhaus and West, 2) are applied in order to determine the fixed capital investment (FCI) the cost of utilities (C UT ) the cost of operating labor (C OL ) the cost of raw materials (C RM ) The summary of the FCI estimation is presented in Table 2 while in Table 3 a summary of the utilities cost estimation is presented. In a similar way it is found that the cost of raw materials is C RM =0.096 M$/y and the cost of operating labor is C OL =0.720 M$/y. The cost of manufacture (COM) is then calculated using the following equation taken from Turton et al., (2012) COM=0.18 FCI+2.73 C OL +1.23(C RM +C UT +C WT ) (1) After substituting the estimation of the FCI, C OL, C RM and C UT it is estimated that the cost of manufacture is approximately 5.26 M$/y. Following the estimation of the cost of manufacture a net present value analysis is performed to determine the potential profitability of the process. To this end the price of limonene is obtained from well known sources ( and then the minimum selling price of ethanol (MSEP) is determined. The minimum selling price of ethanol is determined so as the net present value of the investment to be equal to zero for 20 years of operation of the plant. The results are presented in Figure 3. From this figure we can observe that the selling price of limonene is detrimental to the potential profitability of the process. The price of limonene varies in the range 10 to $/kg. For the minimum value of 10 $/kg of limonene the profitability of the process is not ensured but at the price of 15 $/kg of limonene the profitability is increased dramatically. For the usual range of interest rate considered in preliminary studies (10-15 %) the MSEP is competitive with the current market price of ethanol. CONCLUSIONS In this work a potential process for citrus waste valorization is presented where limonene and ethanol are the products. This is a base case process for determining the economic potential of the process as pectin, unconverted carbohydrates and cell mass are combusted to partially satisfy the energy requirements of the process. Future research will be focused in the production of other chemicals (instead of ethanol) and the recovery of pectin and microbial mass. As the profitability of the base case process is marginal it is expected that process improvements will make economics more favorable and support industrial application of the proposed method for valorizing CWs.

3 C-101 M-101 R-101 V-101 E-101 V-102 E-102 V-104 V-103 Feed Rotary Hydrolysis Vapour Condenser Decanter Cooler Neutralization Limonene conveyor cutter vessel liquid vessel storage separator CW 6 C E-101 V-102 V-103 M Acid 98% w/w 5 2 R-101 Base A-101 V-101 CW 3 E V-104 Process water E-103 HPS E-104 Sup. steam 22 E-105 Condensate Figure 1. Process flow diagram of the pre-treatment and limonene recovery area.

4 C-201 Air compressor R-201 Ethanol fermentor T-201 Absorption column F-201 Filter E-201 Feed preheater E-202 Column reboiler E-203 Column condenser T-202 Ethanol recovery column V-201 Column reflux drum BH-201 Boiler V-202 Ethanol storage Process water E-203 CW 5 V-201 T V-202 E-201 T Condensate 16 E-202 Vent Supplements 13 A To waste treatment R F-201 Flue gas Air C CW 17 Solids Sup. steam Sat. steam 22 E-204 Combust air BH-201 Figure 2. Process flow diagram of the fermentation area, ethanol recovery and solids combustion area.

5 Table 1. Composition of dry matter contained in CW. Component Amount in kg/ of dry CW Glucose 8.10 Fructose Sucrose 2.80 Pectin Protein 6.07 Cellulose Hemicellulose lignin 2.19 Limonene 3.78 Ash 3.73 Table 2. Fixed capital estimation summary. UNIT Description f.o.b (M$) Source CEPCI FM CBM C101 CS, 0.7 m width, m length PTW, $@ M101 CS, 12.5 t/h JBEI, $@ R101 SS316, 1.67 m 3, 4 units JBEI, $@ V101 SS316, 1.15 m diameter, 3.45 m height PTW, $@ V102 SS316, m diameter, 3.85 m height PTW, $@ V103 SS304, 25.2 m NREL, $@ V104 SS304, 0 m 3, includes agitator NREL, $@ E101 SS304/CS, 1.53 m PTW, $@ E102 SS316/CS, 35.8 m PTW, $@ E103 SS316/CS, 50.6 m PTW, $@ E104 CS/CS, 13.2 m PTW, $@ E105 CS/CS, 74.8 m PTW, $@ TOTAL INSTALLED EQUIPMENT COST OF AREA R201 SS304, 0 m 3, includes agitator & coil - NREL, $@ th seed fermenter m 3, SS304 - NREL, $@ th seed ferm. 10 m 3, SS304, skid complete NREL, $@ rd seed ferm. 1 m 3, SS304, skid complete NREL, $@ nd seed ferm. 0.1 m 3, SS304, skid complete NREL, $@ st seed ferm m 3, SS304, skid complete NREL, $@ T201 SS316, D=0.4 m, H=7 m PTW, $@ sieve trays PTW, $@ T202 SS316, D=0.84 m, H=38.3 m PTW, $@ sieve trays PTW, $@ V201 SS316, D=0.7 m, H=2.1 m, horizontal PTW, $@ V202 CS gr. C, V=144 m 3, floating roof NREL, $@ E201 SS304/CS, 130 m PTW, $@ E202 SS304/CS, 8 m PTW, $@ E203 SS304/CS, 43 m PTW, $@ F201 Centrifuge, 2 kg/s solids, SS PTW, $@ BH201 Boiler, 5 t/h of HPS JBEI, $@ TOTAL INSTALLED EQUIPMENT COST OF AREA TOTAL INSTALLED EQUIPMENT COST 13.8 FIXED CAPITAL INVESTMENT 16.6 Table 3. Summary of utilities cost estimation. UNIT Electricity (kw) HPS (t/h) CW (t/h) C101 1 M A F E E E E TOTAL COST (M$/y) TOTAL UTILITIES COST 0.150

6 Limonene sel. pr. 15 $/kg MESP ($/t) Limonene sel. pr. 10 $/kg IR (%) Figure 3. Minimum ethanol selling price as a function of interest rate and limonene selling price. REFERENCES [1] Peters, M., Timmerhaus, K., West, R. 2. Plant Design and Economics for Chemical Engineers, 5 th ed., McGraw Hill. [2] Pourbafrani, M., Forgacs, G., Horvath, I.S., Niklasson, C., Taherzadeh, M.J., Production of biofuels, limonene and pectin from citrus wastes. Bioresour. Technol. 101 (11), [3] Turton, R., Bailie, R.C., Whiting, W.B., Shaeiwitz, J.A., Bhattacharyya, D., Analysis, Synthesis and Design of Chemical Processes, 4 th ed., Prentice Hall. [4] Wilkins, M.R., Widmer, W.W., Grohmann, K., Cameron, R.G., 7. Hydrolysis of grapefruit peel waste with cellulase and pectinase enzymes. Bioresour. Technol. 98,