Retrofit of Distillation Columns in Biodiesel Production Plants

Column grand composite curves and the exergy loss profiles produced by the Column-Targeting Tool of the Aspen Plus simulator are used to assess the performance of the existing distillation columns, and reduce the costs of operation by appropriate retrofits in a biodiesel production plant. Effectiveness of the retrofits is assessed by means of thermodynamics and economic improvements. We have considered a biodiesel plant utilizing three distillation columns to purify biodiesel (fatty acid methyl ester) and byproduct glycerol as well as reduce the waste. The assessments of the base case simulation have indicated the need for modifications for the distillation columns. For column T202, the retrofits consisting of a feed preheating and reflux ratio modification have reduced the total exergy loss by 47%, while T301 and T302 columns exergy losses decreased by 61% and 52%, respectively. After the retrofits, the overall exergy loss for the three columns has decreased from 7491.86 kW to 3627.97 kW. The retrofits required a fixed capital cost of approximately $239,900 and saved approximately $1,900,000/year worth of electricity. The retrofits have reduced the consumption of energy considerably, and leaded to a more environmentally-friendly operation for the biodiesel plant considered. 1626 Demirel & NguyeN iN EnErgy (2010) 35 2.1. Column Grand Composite Curve (CGCC) The CGCC displays the net enthalpies for the actual and ideal operations at each stage, and theoretical minimum cooling and heating requirements in the temperature range of separation. The area between the actual and the ideal operations in a CGCC should be small. The CGCCs help in identifying the following retrofits: (i) feed location (appropriate placement), (ii) reflux ratio (reflux ratio vs. number of stages), (iii) feed conditioning (heating or cooling), and (iv) side condensing and reboiling. A sharp enthalpy change occurring on the Stage-HCGCC on the condenser side indicates that a feed has been introduced too high up in the column and should be moved down. Similarly, a feed introduced too low in the column will cause a sharp enthalpy change on the Stage-HCGCC on the reboiler side and should be moved up. Appropriate feed placement removes the distortions in the Stage-HCGCC but also reduces the condenser or reboiler duty. Reflux ratio reduction lowers the condenser and reformed theoretically by bringing a resource into equilibrium with its surrounding through a reversible process. Molar exergy ex is defined by ex = Δh T0Δs = (h-h0) T0 (s-s0) (1) where h is the molar enthalpy, s is the molar entropy, and T0 is the reference temperature, which is usually assumed as the environmental temperature of 25.0°C. Exergy is a function of both the physical properties of a resource and its environment. Exergy Loss profiles are available as stage-exergy loss and temperature-exergy loss profiles, and measure their reversibility in the column due to momentum loss (pressuredriving force), thermal loss (temperature-driving force/ mixing), and chemical-potential loss (mass transfer driving force/mixing). These profiles can be used as a tool to examine the degradation of accessible work for all the internal trays of the column. Figure 1. Flow diagram for biodiesel plant: (a) base case design; (b) retrofitted design. retrofit of DistillatioN ColumNs iN BioDiesel ProDuCtioN PlaNts 1627 obtained from the molar flow ratios Here * and * are the molar flow rates at equilibrium, and and are enthalpies of vapor and liquid streams leaving the same stage at equilibrium, respectively. From the enthalpy balances at each stage, the net enthalpy deficits are obtained by [3,13] Hdef = HLmin – HVmin + HD (before the feed stage) (6) Hdef = HLmin – HVmin + HD – Hfeed (after the feed stage) (7) After adding the individual stage-enthalpy deficits to the condenser duty, the enthalpy values are cascaded, and plotted in the CGCC. This is called the top-down calculation procedure, which will be the same with the bottom-up calculations for a stage without any feed. At the feed stage, mass and energy balances differ from a stage without feed. For the two procedures to yield similar results, the rate of enthalpy deficit at the feed stage becomes The values of and maybe obtained from an adiabatic flash for a single phase feed, or from the constant relative volatility estimated with the converged compositions at the feed stage and feed quality. This procedure can be reformulated for multiple feeds and side products as well as for different choices of the key components. In a CGCC, a pinch point near the feed stage occurs for nearly binary ideal mixtures. However, for nonideal multicomponent systems pinch exists in rectifying and stripping sections. boiler duties, decreases operating costs, however, it will increase the number of stages, increase capital costs, to preserve the separation. User must carefully analyze to determine whether saving in operating costs compensate higher capital costs. Feed preheating or cooling can reduce thermal loss on the feed stage. Using existing heat sources available in the plant are desirable and side condensing or side reboiling provides the column with a cheaper cold or hot utlity [3-7]. Using the equilibrium compositions of light (L) and heavy (H) key components minimum vapor and liquid flow rates leaving the same stage with the same temperatures can be estimated from the following mass balances The enthalpies for the minimum vapor and liquid flows are Table 1 Streams properties of the base case design of biodiesel production plant. Fame Glycerol Methanol Methout Oil Water Waterout Temperature °C 4.40E+01 2.55E+02 2.50E+01 6.68E+01 2.50E+01 2.50E+01 1.02E+02 Pressurebar 1.00E-01 5.00E-01 1.00E+00 1.10E+00 1.00E+00 1.10E+00 1.10E+00 Vapor Frac 0.00E+00 0.00E+00 0.00E+00 0.00E+00 0.00E+00 0.00E+00 0.00E+00 Mass Flow kg/h 2.44E+04 2.51E+03 2.73E+03 2.91E+03 2.41E+04 5.50E+02 2.40E+03 Enthalpy MMBtu/h 5.66E+01 1.60E+01 1.94E+01 2.02E+01 5.66E+01 8.27E+00 3.53E+01 Mass Flow kg/h METHANOL 5.42E+01 7.52E-01 2.73E+03 2.91E+03 0.00E+00 0.00E+00 8.45E+00 OIL 3.96E+01 0.00E+00 0.00E+00 0.00E+00 2.41E+04 0.00E+00 0.00E+00 FAME 2.42E+04 5.73E-04 0.00E+00 1.11E-66 0.00E+00 0.00E+00 8.36E-01 GLYCEROL 1.43E-08 2.51E+03 0.00E+00 5.91E-57 0.00E+00 0.00E+00 4.96E-04 NAOH 7.45E-04 0.00E+00 0.00E+00 0.00E+00 0.00E+00 0.00E+00 0.00E+00 WATER 4.78E+01 1.96E+00 0.00E+00 3.37E+00 0.00E+00 5.50E+02 2.39E+03 H3PO4 0.00E+00 0.0E+00 0.00E+00 0.00E+00 0.00E+00 0.00E+00 0.00E+00 NA3PO4 0.00E+00 0.00E+00 0.00E+00 0.00E+00 0.00E+00 0.00E+00 0.00E+00 Mass Frac METHANOL 2.22E-03 3.00E-04 1.00E+00 9.99E-01 0.00E+00 0.00E+00 3.51E-03 OIL 1.62E-03 0.00E+00 0.00E+00 0.00E+00 1.00E+00 0.00E+00 0.00E+00 FAME 9.94E-01 2.28E-07 0.00E+00 3.81E-70 0.00E+00 0.00E+00 3.48E-04 GLYCEROL 5.86E-13 9.99E-01 0.00E+00 2.03E-60 0.00E+00 0.00E+00 2.06E-07 NAOH 3.06E-08 0.00E+00 0.00E+00 0.00E+00 0.00E+00 0.00E+00 0.00E+00 WATER 1.96E-03 7.82E-04 0.00E+00 1.16E-03 0.00E+00 1.00E+00 9.96E-01 H3PO4 0.00E+00 0.00E+00 0.00E+00 0.00E+00 0.00E+00 0.00E+00 0.00E+00 NA3PO4 0.00E+00 0.00E+00 0.00E+00 0.00E+00 0.00E+00 0.00E+00 0.00E+00 Table 2 Comparison of operating conditions and configurations of designs 1 and 2 for distillation column T202. Conditions & Configurations Design 1 (base case) Design 2 (retrofitted) No. of stages 6 8 Feed stage 3 3 Feed temperature,°C 60.00 86.50 Reflux ratio 1.10 0.19 Condenser duty, kW -10897.10 -6129.29 Distillate rate, kmol/h 90.47 90.45 Condenser temperature,° C 44.01 44.08 Condenser pressure, bar 0.10 0.10 Reboiler duty, kW 10985.16 5840.57 Boil up rate, kmol/h 217.59 108.32 Bottoms rate, kmol/h 1.75 1.71 Reboiler temperature,°C 340.34 347.12 Reboiler pressure, bar 0.20 0.2