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Latin American Applied Research 33:161-165 (2003) IV. DISCUSSION Optimal solvent recovery scheme for the deterpenation of lemon peel oil has been determined for supercritical extraction with carbon dioxide together with optimal operating conditions. The plant processes 20 kg/h feed oil, which is composed of 75% limonene and 25% citral (weight basis), a mixture that closely represents natural lemon oil. A twenty-stage column has been selected. Nonlinear constraints, which correspond to process specifications, are shown in Table 1. The MINLP problem has been solved with an im- plementation of the Outer Approximation algorithm (Duran and Grossmann, 1986) that can interface a proc- ess simulator (Diaz and Bandoni, 1996); NLP subprob- lems have been solved with an SQP algorithm (Biegler and Cuthrell, 1985) and MILP problems, with LINDO (Schrage, 1987). Lemon oil is corrosive so stainless steel process units are required. The extraction column is a structured packed column with Sulzer packing (Mellapack 250). Shell and tube heat exchangers, compressors and recip- rocating pump costs curves have been obtained from Ulrich (1984). Capital cost curves for the extraction column have been obtained from Institut Français du Pétrole (1981). Investment cost has been annualized considering a project life of three years. Raw material cost is 28 $/kg (Chemical Market Reporter, Apr/01) and product prices strongly depend on product purity. Ter- peneless lemon oil (citral, in this work) price is 847 $/kg (TGSC, www.execpc.com/~goodscnt/, Apr/01). Li- monene price has been considered as 1.44 $/kg (Chemi- cal Market Reporter, Apr/01). The MINLP optimum corresponds to a solvent re- covery system in compression mode. Numerical results, which are reported in Table 2, show that a compressor cycle is the best option because this process operates at pressure below 300 bar (Brunner, 1994). Terpeneless lemon oil high-commercial value justifies working with larger solvent-to-feed ratio. Figure 7 shows a compari- son between solvent cycle consumption in compressor mode and in pump mode for the optimal determined conditions, in a T-S diagram. It can be clearly seen that a compressor is the option with lower energy consump- tion at this pressure level. Table 2. Continuous optimization variables and main costs for the countercurrent extraction with reflux at solvent cycle MINLP optimum Table 1. Nonlinear constraints Nonlinear constraint Citral purity (CO2 free) (% mole) Citral recovery (%) Limonene purity (CO2 free) (% mole) Limonene recovery (%) Carbon dioxide in vapor (% mole) Lower Bound 99.00 95.00 98.00 87.00 99.97 A simple countercurrent scheme has been studied as a first step, but none of the nonlinear constraints could be fulfilled with this process scheme. An 89 % product purity (carbon dioxide free) with 66.50% citral recovery could be obtained, together with 97.4% and 89.70% limonene recovery and purity in top product, respec- tively. Extractor operating conditions were 93.22 bar and 345 K. The addition of external reflux to the ex- traction step, as it is shown in Fig. 6, increases both product recovery and purity. The objective function, net profit maximization, has been calculated as : Net profit = (price * production)terpeneless lemon oil + (price * production)limonene – (cost * consump- tion)lemon oil – (Operating Costs + Capital Costs), where operating costs include electrical motor con- sumption, either as pump or compressor driver, cooling water and steam consumption. Capital cost models are ofthegeneralform:Ci =ai yi+bi wiηi ,whereai, biand ηi are cost parameters. These functions are defined over the entire domain of the design variable wi (which stands for heat exchanger area, power consumption in pumps and compressors, etc.) and they have been derived for each unit in the superstructure based on graphical cor- relations from Ulrich (1984) and Institut Français du Pétrole (1981). In the MILP master problem, these functions have been replaced with linear underestima- tors. Also, for a variable wi , associated to unit or path i, the following inequality constraints have been written: wi-Miyi≤ 0, w ≥0, i where Mi is a known upper bound on wi ; therefore, for a non-existing unit i (yi =0), the associated variable wi must be zero. If yi =1, the upper bound holds on wi . Variable Extractor Pressure (bar) Extractor Temperature (K) Separator Pressure (bar) Reflux Ratio Solvent-to feed ratio (mass) Citral in Raffinate, CO2 free (%mole) Citral Recovery (%) Limonene in Top Product (%mole) Limonene Recovery (%) CO2 in Sep. Vapor (% mole) Extractor volume (m3) Compressor consumption (KW) Pumps consumption (KW) Heat exchanger area HE1 (m2) Capital cost ($/Kg product) Operating cost ($/Kg product) Profit ($/Kg product) MINLP Opt. 95.00 333.14 44.60 0.586 125.31 99.98 96.03 98.72 91.54 99.98 0.15 30.00 0.13 4.94 8.09 0.62 721.80 164PDF Image | SOLVENT CYCLE DESIGN IN SUPERCRITICAL FLUID PROCESSES
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