Using Thermally Coupled Reactive Distillation Columns in Biodiesel Production

Production of ethyl dodecanoate (biodiesel) using lauric acid and methanol with a solid acid catalyst of sulfated zirconia is studied by using two distillation sequences. In the first sequence, the methanol recovery column follows the reactive distillation column. In the second sequence, the reactive distillation and methanol recovery columns are thermally coupled. Thermally coupled distillation sequences may consume less energy by allowing interconnecting vapor and liquid streams between the two columns to elminate reboiler or condenser or both. Here we study the thermally coupled side-stripper reactive distillation and eliminate the condenser of the reactive distillation column. Both the sequences are optimized by using the thermal and hydraulic analyses of the Column Targeting Tools of Aspen Plus simulator. Comparisons of the optimized sequences show that in the thermally coupled sequence, the energy consumption is reduced by 13.1% in the reactive distillation column and 50.0% in the methanol recovery column. The total exergy losses for the columns are reduced by 281.35 kW corresponding to 21.7% available energy saving in the thermally coupled sequence. In addition, the composition profiles indicate that the thermally coupled reactive distillation column operates with the lower concentration of water in the reaction zone which reduces catalytic deactivation. Using Thermally CoUpled reaCTive disTillaTion ColUmns in Biodiesel prodUCTion 4839 2. TA (Thermodynamic Analysis) TA determines the net enthalpy deficits and exergy losses due to irreversibility at each stage of a column to identify the scope and extent of retrofits or improvements by reducing the irreversibility and/or distributing them evenly [30-33]. The Column Targeting Tools of Aspen Plus performs thermal analysis and hydraulic analyses. Thermal analysis produces the CGCC (column grand composite curves) and the exergy loss profiles for rigorous column calculations based on and methanol with a solid acid catalyst of sulfated zirconia (SO4/ZrO2) [27-29]. In this proposed biodiesel plant, TCSRD (thermally-coupled side-stripper reactive distillation) configuration has been considered, since using the reactive distillation column as a side-rectifier may cause lower reaction rate because the reaction zone temperature must be within the separation temperature of methanol and water. Also, using the methanol recovery column as a rectifier may disrupt the reaction zone with the side feed and withdraw [10-12,15-19]. Figure 1. Process flow diagrams for biodiesel plant: (a) base case design; (b) thermally-coupled design. 4840 ngUyen & demirel in EnErgy (2011) 36 the PNMTC (Practical Near-Minimum Thermodynamic Condition). These profiles represent the cumulative heating and cooling requirements for the column to operate at PNMTC. This approximation takes into account the inefficiencies introduced through pressure drops, mixing, and heat and mass transfer. 2.1. CGCC (Column Grand Composite Curve) 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 CGCCs are available as stage-enthalpy (Stage-H) and T-H (temperature eenthalpy) profiles and help in identifying the following scopes: (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. The area between the actual and the ideal operations in a CGCC should be small. A sharp enthalpy change occurring 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 reboiler side and should be moved up. Appropriate feed placement removes the distortions on the CGCC and may reduce the condenser or reboiler duty. Reflux ratio reduction lowers the condenser and reboiler duties and decreases operating costs. However, it will increase the number of stages and increase capital costs to preserve the separation. User must carefully analyze to determine whether savings in operating costs compensate higher capital costs. Feed preheating or cooling can reduce thermal loss on the feed stage. Using heat sources available in the plant are desirable and side condensing and side reboiling provide the column with a cheaper cold or hot utility [34,35]. 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 [4,5] The enthalpies for the minimum vapor and liquid flows are obtained from the molar flow ratios where * and * are the molar flows of equilibrium, and and are enthalpies of equilibrium vapor and liquid streams leaving the same stage, respectively. From the enthalpy balances at each stage, the net enthalpy deficits are calculated by Hdef = HLmin – HVmin + HD (before the feed stage) (5) Hdef = HLmin – HVmin + HD – Hfeed (after the feed stage) (6) After adding the individual stage-enthalpy deficits to the condenser duty, the enthalpy values are cascaded, and plotUsing Thermally CoUpled reaCTive disTillaTion ColUmns in Biodiesel prodUCTion 4841 ted 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 enthalpy deficit at the feed stage becomes [4,5] The values of and may be 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. For nonideal multicomponent systems pinch may exist in rectifying and stripping sections. A horizontal distance between the CGCC pinch point and the vertical axis represents the excessive use of heat, and therefore the scope for reduction in reflux ratio [36]. For smaller reflux ratios, the CGCC will move toward the vertical axis, and hence reduce the reboiler and condenser duties, which may be estimated by where λ is the heat of vaporization and R is the reflux ratio [4,5]. The horizontal distance of the CGCC from the temperature axis, however, determines the targets for installing a side reboiler or side condenser at suitable temperatures [30]. For example, a sharp change on the reboiler side may be due to subcooled feed, and a feed preheater with a heat duty depending on the change can be installed [37]. On the other hand, a sharp change in the enthalpy represents inappropriate feed conditioning, such as feed quality or temperature. Feed conditioning is usually preferred to side condensing or reboiling, since the side heat exchangers are effective at suitable temperature levels or stages only. 2.2. Exergy Loss Profiles Exergy (Ex) is the accessible work potential and defines the maximum amount of work that may be performed theoretically by bringing a resource into equilibrium with its surrounding through a reversible process. Specific exergy is defined by ex = Δh ToΔs = (h ho) To(s so) (9) where h is the molar enthalpy, s is the molar entropy, and To is the reference state temperature. The reference temperature and pressure states are 298.15 K and 1 atm, respectively. Aspen Plus uses ideal gas heat of formation and ideal gas Gibbs free energy of formation to compute enthalpies, entropies, and Gibbs free energies; these formation energies may be set equal to zero if there is no reaction occurred [38]. Exergy is a function of both the physical properties of a system 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 (pressure driving force), thermal loss (temperature driving force/mixing), and chemical potential loss (mass transfer driving force/mixing). These profiles can be used to exam4842 ngUyen & demirel in EnErgy (2011) 36 where and are the shaft work and exergy losses, respectively. For distillation columns, the minimum exergy flow rate required for separation is directly proportional to the differences between the exergy of the product and feed streams ine the degradation of accessible work for the column. Higher value of exergy loss means more thermodynamic imperfection, consequently, higher energy consumption to achieve a desirable separation. For a steady state system, the exergy balance is not conserved [39] Table 3 Comparison of operating conditions and configurations of reactive distillation column RD101 and distillation column T101 of base case and thermally-coupled designs. Base Case Design Thermally Coupled Design Condition & Configurations RD101 T101 Total RD101 T101 Total Number of stages 30 12 30 12 S1 feed stage 3 – 3 – S2B feed stage 29 – 29 – S4B feed stage – – 1 – WATMET feed stage – 9 – 10 S1 temperature, ° C 100 – 100 – S2B temperature, °C 110 – 110 – Molar reflux ratio 0.1 1.55 – 2.02 S4B flow rate, kmol/hr – – 7 – Reaction stages 3-29 – 3-29 – Distillate rate, kmol/hr 196 95 203 95 Column diameter, m 1.04 1.10 0.86 1.25 Liquid hold up, cm3 0.043 – 0.043 – Condenser duty, kW -2168.38 -2375.23 -4543.61 0.00 -2809.62 -2809.62 Condenser temperature,°C 124.12 64.36 155.50 64.36 Condenser pressure, bar 5.5 1.0 9.0 1.0 Side condenser stage – – – 6 Side condenser duty, kW – – – -600.00 -600.00 Reboiler duty, kW 5634.95 2136.23 7771.18 4898.97 1067.38 5966.35 Boil up rate, kmol/hr 177.80 186.19 217.53 89.63 Bottoms rate, kmol/hr 104 101 104 101 Reboiler temperature,°C 311.29 97.59 274.96 96.34 Reboiler pressure, bar 6.0 1.0 9.5 1.0 Total Conversion, mol% 99.99 – 99.76 – Exergy loss, kW 1189.04 108.36 1297.40 907.31 108.743217 1016.05 Figure 2. Comparison of operating conditions for reactive distillation column RD101: (a) temperature profiles; (b) composition profiles; (c) reaction profiles; (d) exergy loss profiles. Using Thermally CoUpled reaCTive disTillaTion ColUmns in Biodiesel prodUCTion 4843 Exergy loss is always greater than zero in every irreversible processes, hence, η must lie between 0 and 1. A conventional distillation column receives heat in the reboiler, performs the separation work, and releases the rest of the amount in the condenser. Thus, it resembles a thermal engine and the efficiency is typically much less than 1. 3. Reactive Distillation Reactive distillation column RD101 shown in Figure 1a and b has 30 stages, rectifying section located at the top, reaction zone located at the middle, and stripping section located at the bottom of the column. The column operates with a total condenser and a kettle reboiler. The activity coefficient model The sign of depends on the energy stored in the inlet and outlet streams. Thermodynamic efficiency (η) is the ratio between the total useful work output and the total exergy input. When is greater than zero, the η is calculated by References [4,5] If the value of is negative, the η is given by Figure 3. Hydraulic analysis and enthalpy deficit profiles for column RD101: (a) stage-liquid flow rate profiles of base case design; (b) stage-liquid flow rate profiles of thermally-coupled design; (c) stage-vapor flow rate profiles of base case design; (d) stage-vapor flow rate profiles of the thermally-coupled design; (e) stage-enthalpy deficit curves of base case design; (f) stage-enthalpy deficit curves of the thermally-coupled design. 4844 ngUyen & demirel in EnErgy (2011) 36 S3B in mixer M101 is preheated to 110°C in heat exchanger HX102 before it is fed to the bottom of the reaction zone at stage 29 of RD101. The feeds enter at the both ends of the reaction zone to maximize conversion [7,10-14]. Although the conversion nearly reaches completion in 23 reactive stages although, 27 stages are set for ensuring flexibility in the operation. Both feed streams are preheated to minimize the loss of exergy caused by the temperature gradient. The plant produces around 99.2 wt% of 21,527 kg/h of methyl dodecanoate and dilute concentration of methanol in water as summarized in Tables 1 and 2. The inlet streams conditions are identical while there is a slight difference of the product concentrations in the outlet streams between the two sequences of distillation columns. The distillate, stream WATMET, of column RD101 is fed to stage 9 and 10 of column T101 of the BCRD and TCSRD sequences, respectively. Aspen Plus N Q curves (plots of heat load Q versus total number of stages N) are used to determine the number of stages and optimum feed locations based on an objective function [5,38]. Column T101 recovers methanol from water and recycles. The column operates with 12 stages, with a kettle reboiler and a total condenser. The activity coefficient model of NRTL is used for predicting the equilibrium and liquid properties in column T101. The top product, stream S3A, containing mostly methanol is pressurized before it is recycled. The bottom product, stream WATA, is treated as a waste. Stream ESTERA and S3A are tear streams for both sequences, while stream S4A and WATMET are tear streams only for the TCSRD sequence. of UNIQUAC containing size and shape parameters is used to estimate the phase equilibrium because of the presence of the components with wide range of molecular weights within the column. The esterification reaction takes place in the reaction zone from stages 3-29 with solid acid catalyst of sulfated zirconia (SO4/ZrO2) [27-29]. Operation temperature of the catalyst ranging from 130 to 200°C and high activity even at high methanol/lauric acid ratio make it a suitable candidate for the RD application. The reverse hydrolysis reaction is negligible and the kinetic rate constant s is given by k(T) = 1.2 × 105 exp(-55,000/RT) [12]. The units for reaction rate, concentration, and activation energy expressed as kmol/(m3h), kmol/ m3, and kJ/kmol, respectively. For each reaction stage, an estimated volume trichold up of 0.043 m3 is used [12]. The bottom separation zones, stage 30, separates the desired product, methyl dodecanoate, while the top separation zone, stages 1 and 2, removes the water and unreacted methanol. 4. Biodiesel Plant A biodiesel production plant using reactive distillation sequence requires at least two columns for biodiesel separation and methanol recovery as seen in Figure 1a [13]. Figure 1b proposes a thermally-coupled reactive distillation column sequence utilizing methanol and lauric acid (oil) to produce product methyl dodecanoate (biodiesel). Tables 1 and 2 summarize the stream properties for both the sequences. Stream LAURIC is preheated by stream ESTERB in heat exchanger HX101 to 100°C and enters to the top of the reaction zone at stage 3 of RD101. Stream METH combined with stream Figure 4. Sensitivity analysis of stream S4A flow rate on: (a) ester mass fraction in the bottom product stream; (b) column RD101 reboiler duty. Figure 5. Comparison of operating conditions for distillation column T101: (a) temperature profiles; (b) exergy loss profiles. Using Thermally CoUpled reaCTive disTillaTion ColUmns in Biodiesel prodUCTion 4845