3) OBJECTIVE FUNCTION & DISTURBANCES Objective function: Assuming product prices are the same, p D = p S = p B and (p-p F ) = p’, with F given and Q =  H·V, gives: Disturbances: 6 Feedrate disturbances, F± 20 % 6 Composition disturbances, z B,F ± 0.1 (mole fraction) Selecting Appropriate Control Variables for a Heat Integrated Distillation System with Prefractionator INTRODUCTION Classical separation schemes H.K. Engelien and S. Skogestad Norwegian University of Science and Technology, Department of Chemical Engineering, Trondheim, Norway ENERGY SAVINGS Shortcut calculations for minimum vapour flowrate indicates that integrated prefractionator arrangements can have high energy savings- up to 70 %. CASE STUDIED Separation process: forward integrated prefractionator (PF)Separation:propane/butane/pentane Feed data:z F = [ ] Column Stages:N HP = 20 F = 300 mol/s N LP = 40 q= 1 Aim: Identify good control variables SELF-OPTIMIZING CONTROL: The method of self-optimizing control involves a search for the variables that, when kept constant, indirectly lead to near-optimal operation with acceptable loss. 1) DOF ANALYSIS & CONSTRAINTS 11 DOF’s: HP column LP column boilup (Q B,HP )boilup (Q B,LP ) condensation rate (Q C,HP ) condensation rate (Q C,LP ) reflux (LT HP )reflux (LT LP ) distillate (D HP )distillate (D LP ) bottom flowrate (B HP )bottom flowrate (B LP ) sidestream flowrate (S LP ) Process Constraints: 6 The pressure in the LP column should be  1 bar. 6 The pressure in the HP column should be  15 bar. 6 The reboiler duty in the LP column (Q B,LP ) = condenser duty in the HP column (Q C,HP ) 6The product purities (x A,D ), (x B,S ) and (x C,B ) should be  99 mol%. 6 The area in the combined reboiler/condenser should be  A max. 2) SIMULATIONS Main assumptions behind model: 6Simple equilibrium relationship: K-values 6Partial pressure calculated from Antoine equation 6A max calculated from optimal steady state solution when (T cond,HP - T reb,LP ) = 5 o C 6Constant pressure in each column Optimization is done in Matlab/Tomlab, using SOL optimization routines. CONCLUSIONS 6The integrated prefractionator arrangement can give high energy savings compared with non- integrated arrangements. 6Good control systems are important in order to achieve the expected energy savings. 6The self-optimization method has been used as a method for selecting the control variables. 6Control variables were identifies that will give low energy losses during operation CONTACT INFORMATION Prof.. Sigurd SkogestadHilde K. Engelien Department of Chemical EngineeringDepartment of Chemical EngineeringNTNU REFERENCES Bildea, C.S., Dimian, A.C., 1999, Interaction between design and control of a heat-integrated distillation system with prefractionator, Tans IChemE, Vol. 77, Part A, Cheng, H. C., Luyben, W., 1985, Heat-integrated distillation columns for ternary separations, Ind. Eng. Chem. Process Des. Dev., 24, Ding, S.S., Luyben, W., 1990, Control of a heat-integrated complex distillation configuration, Ind. Eng. Chem. Res.¸ 29, Halvorsen, I.J., 1999, Optimal operation of Petlyuk distillation: steady-state behavior, J. Process Control, 9, King, C.J., 1980, Separation Processes, McGraw-Hill Book Co. Rev, E., Emtir, M., Szitkai, Z., Mizsey, P., Fonyo, Z, 'Energy savings of integrated and coupled distillation systems', Computers and Chemical Engineering, 2001, 25, pp Skogestad, S., 2000, Plantwide control: the search for the self-optimizing control structure, J. Proc. Control, Vol.10, ) LOSS CALCULATIONS Calculate loss: L = (J opt - J) for a number of variables at the selected disturbances and identify the best variable(s) for control, where the loss is small. 4) OPTIMIZATION RESULTS 4 active constraints:P LP = 1 barx B,S = 0.99x C,B = 0.99A = A max 4 levels with no steady state effect 1 match heat duty in integrated reboiler/condenser  Implement active constraint control for these variables  Leaves a system with (11-9) = 2 DOF for which the choice of control variable is not clear Fix concentration in top of LP column (x A,D = 0.99)  leaves 1 DOF for self-optimizing control 6) PROPOSED CONTROL STRUCTURE N T N U Examples of integrated separation schemes Direct split (DS) Indirect split (IS) Prefractionator (ABC) (A) (B) (C) (BC) (AB) (B) (C) (BC) (A) (ABC) HP LP (B) (C) (BC) (A) (ABC) (A) (B) (ABC) HP LP (C) (AB) (ABC) (A) (B) (C) (AB) Comparison of energy savings (V min ) of different systems (compared to the best of the non-integrated direct or indirect sequence), sharp split: propane-butane-pentane,  = [ ]. Z F DS (%) IS (%) Petlyuk (%) DSF (%) DSB (%) ISF (%) ISB (%) PF (%) PB (%) [1/3 1/3 1/3] [ ] [ ] [ ] [ ] [ ] [ ] Suggested control structure (for illustration): Control: Distillate composition (x D,LP ) Sidestream composition (x S,LP ) Bottom stream composition (x B,LP ) Pressure in LP column (P LP ) Self-optimizing variable: Ratio of distillate to feed flow D HP /F (A) HP LP (B) (C) (BC) (AB) (ABC) DSF ISF PF Keeping different control variables constant at the nominal value will increase the duty required when there are disturbances. The table shows the extra duty as a percentage of the optimal duty at the various disturbances. 1 % extra duty cost corresponds to about $ per year Best control variable: D HP /F (or B HP /F) Implementation error for D HP /F is 2.9 % CAN THESE SAVINGS BE ACHIEVED IN PRACTICE ?