Equation oriented modelling of UOP FCC units with high-efficiency regenerators

Transcription

1 Equation oriented modelling of UOP FCC units with high-efficiency regenerators Alexandre J.S. Chambel a,b, Jorge F.P. Rocha a, Carla I.C. Pinheiro b and Nuno M.C. Oliveira a a CIEPQPF - Centre for Chemical Processes Engineering and Forest Products, Department of Chemical Engineering, University of Coimbra, R. Sílvio Lima Pólo II, Coimbra, Portugal. b IBB/CRERG - Institute for Biotechnology and Bioengineering, Department of Chemical Engineering, Instituto Superior Técnico/Technical University of Lisbon, Av. Rovisco Pais, 1, Lisboa, Portugal Presented at ChemPor2011, Univ. Nova de Lisboa Outline Work Motivation and Objectives Fluid Catalytic Cracking (FCC) Process FCC Modelling and Assumptions FCC Model Validation Sensitivity Analysis Preliminary Optimization studies Conclusions Future Work 2/22 1

2 Work Motivation Nowadays, modelling and simulation are essential tools in the improvement and optimization of industrial processes. Why consider the FCC Process? Incorporates several areas of Chemical Engineering such as catalysis, fluidization, heat and mass transfer, etc.. Is a complex Nonlinear System, with strong interactions between the riser reactor and regenerator blocks; Due to its Economical Relevance, often small improvements lead to significant profits. Optimal performance is achieved near various Physical and Operating Constraints. 3/22 Main Objective To develop a rigorous model for the FCC process based on first principles, suitable for RT supervision and optimization. This model should Accurately predict the steady state and the dynamic characteristics of the system; Have low computational requirements, suitable for RT applications; Use only input data routinely available in a typical refinery. Be flexible enough to simulate a wide range of operating conditions without noticeable loss of precision or lack of convergence; 4/22 2

3 FCC Unit Where is the FCC unit in the refinery processes? LPG VGO GLN LCO HCO LCO Typical Refinery Flowchart adapted from Scherzer and Gruia, /22 FCC Unit FCC unit design change over the last 75 years: FCC Model IV FCC R2R Unit FCC UOP High -Efficiency Adapted from Montgomery (1993). 6/22 3

4 FCC Unit FCC unit overview: Primary conversion process in petroleum refining; Converts high-boiling petroleum fractions (gas oil) to high-value, high-octane gasoline and heating oil; Largest contributor to the gasoline pool (~35%vol); Extremely flexible process processes a range of feeds and produces a variety of valuable products; Low cost operation operates near P atm and is autothermic. 7/22 FCC Unit The FCC unit considered in the model developed is a FCC UOP High-Efficiency (HE) Unit. Main differences relative to traditional FCC units: Most of the combustion reactions occur in the combustion riser. Catalyst recirculation is promoted to maintain constant density in the combustor and control the dense bed temperature. Residence times are much lower than in traditional bubbling bed regenerators. 8/22 4

5 FCC Unit Modelling A FCC HE unit model was built in FORTRAN (Fernandes et al., 2007). Sequential solution (+ iter.); Single purpose code; High computational effort. A 2nd generation model was built in GAMS*. * General Algebraic Modeling System: Simultaneous solution; More flexible unit models; Computationally more efficient; Full algebraic model (OCFE + ). + Orthogonal Collocation with Finite Elements 9/22 FCC Unit Modelling Main model assumptions in Riser Catalytic Cracking Kinetic Model w/ 6-Lumps Deactivation model: dφ/dy ck =-αφ d García-Dopico et al. (2006) Hydrodynamic Model: Steady State Plug-flow VGO LCO GLN LPG COKE FG Takatsuka et al. (1987) Stripper Hydrodynamic Model: Dynamic State CST Accounts for the hydrocarbons that remain adsorbed after stripping: Cat-to-Oil Coke Correlation 10/22 5

6 FCC Unit Modelling Main model assumptions in HE Regenerator Combustor and Lift : Steady State Plug-flow Regenerator Dense Bed: Dynamic State CSTR Regenerator Freeboard: Steady State Plug-flow Overall coke combustion reactions scheme: 2 x 1 x CH x ( s) O2 ( g) CO( g) CO2 ( g) H 2O( g) CO( g) O2 ( g) 2CO2 ( g) Sadeghbeigi (2000) 11/22 FCC Unit Modelling How is the GAMS model solved? 1. Set the unit inputs with SS values. 2. Sequential initialization of each model. 3. Simultaneous solution of all models. Gams FCC Unit model flowchart. 12/22 6

7 Model Validation E.g. of comparison between GAMS and Fortran results: VGO FORTRAN VGO GAMS FVGO (kg/s) ,2 0,4 0,6 0,8 1 Riser Normalized z Temperature (K) T FORTRAN TempT GAMS ,2 0,4 0,6 0,8 1 Combustor Normalized z The results between both models are in agreement. 13/22 Sensitivity Analysis Effect of COR* variation: * COR = F cat /F feed TRiser (K) TrRegenerator(K) Yield (%) 0,6 0,5 0,4 0,3 0,2 0,1 YieldLCO YieldGLN YieldVGO YieldLPG YieldFG YieldCK ,00 5,00 6,00 7,00 8,00 9,00 10,00 COR (kg/kg) 0 4,00 6,00 8,00 10,00 COR (kg/kg) T riser increases w/ COR. T regen decreases w/ COR until 7,5. Higher LPG yield w/ COR increase. 14/22 7

8 Sensitivity Analysis Effect of AOR* variation: * AOR = F air CB/F feed ,6 TRiser (K) TrRegenerator(K) Yield (%) 0,5 0,4 0,3 0,2 0,1 YieldLCO YieldGLN YieldVGO YieldLPG YieldFG YieldCK 792 0,5 0,7 0,9 1,1 AOR (kg/kg) ,50 0,70 0,90 1,10 AOR (kg/kg) Both T riser and T regen decrease w/ higher AOR. Yields remain constant. 15/22 Effect of F feed variation: Sensitivity Analysis TRiser (K) TrRegenerator(K) Yield (%) 0,6 0,5 0,4 0,3 0,2 0,1 YieldLCO YieldGLN YieldVGO YieldLPG YieldFG YieldCK Feed (kg/s) Feed (kg/s) T riser increases w/ higher F feed. T regen decreases w/ higher F feed until F feed =80 kg/s. Yields decrease with higher F feed. 16/22 8

9 Preliminary Optimization Studies Opt. #1 - Maximization of LPG production Tregen 1003,82 977,64 2,61% F air DP 1,15 0,15 86,52% F air CB 57,59 68,65 19,20% * COR = F cat /F feed ** AOR = F air CB/F feed Triser 801,9 809,8 0,99% F H2O RS 3,1615 0, ,00% F cat reg 517,33 721,09 39,39% F LPG 13,83 21,39 54,68% F H2O ST 1,72 8,62 400% COR* 7,1208 9, ,4% AOR** 0,7928 0, ,20% Steady-State Scenario Optimized Scenario Objective: Maximum F LPG DOFs: valves + gas streams 17/22 Preliminary Optimization Studies Opt. #2 - Maximization of products Tregen 1003,82 988,39-1,54% F air DP 1,15 0, ,52 F air CB 57,59 70,57 22,53% * COR = F cat /F feed * AOR = F air CB/F feed Triser 801,9 805,77 0,49% F H2O RS 3,1615 0, ,00% F cat reg 517,33 627,50 21,30% F LPG 13,83 19,60 41,72% F H2O ST 1,72 10,34 500% F GLN 33,56 33,27-0,85% F LCO 10,10 7,82-22,59% AOR** 0,7928 0, ,53% Steady-State Scenario Optimized Scenario COR* 7,1208 8, ,30% Objective: Maximize Sum of Products DOFs: valves + gas streams 18/22 9

10 Conclusions AFCC HE unit model was successfully built in GAMS. The FCC model was validated against the FORTRAN model, which was previously validated with industrial data. The FCC model allows simultaneous solution of all the individual blocks and unit optimization. The model is suitable for RT applications, sinceafterss initialization it only needs 1-10s CPU to solve (~ vars, ~ 10 DOFs). Suitable for sensitivity analysis and optimization studies, as seen in opt. #1 and #2 results. 19/22 Future Work Further comparison with the Fortran model, such as steady state multiplicity, cluster formation, catalyst deactivation function, etc Further studies, such as hydrodynamics studies, kinetic studies, etc Extend to dynamic model. Integrate the FCC model with advanced RT control schemes and compare performance w/ typical linear MPC algorithms. Optimization studies w/ additional economics objectives, including additional specific constraints (equipment). Extend to include ancillary units (e.g., feed preheater, blower) and main fractionator. 20/22 10

11 References T.Takatsuka,S.Sato,Y.Morimoto,H.Hashimoto; A Reaction Model for Fluidizied-Bed Catalytic Cracking of Residual Oil, Int. Chem. Eng., 1987,27, 107 Joana L. Fernandes, Carla I.C. Pinheiro, Nuno M.C. Oliveira, José Inverno, and Fernando Ramôa Ribeiro; Model Development and Validation of an Industrial UOP Fluid Catalytic Cracking Unit with a High-Efficiency Regenerator, Ind. Eng. Chem. Res., 47 (2008) J. Montgomery; The Grace Davidson Guide to Fluid Cracking Catalytic Part One, W.R. Grace & Co.-Conn., 1993 J. Scherzer, A. J. Gruia; Hydrocracking Science and Technology. Chemical Industries, 66, Marcel Dekker, 1996 M. García-Dopico, A. García, A. Santos García; Modelling coke formation and deactivation in a FCCU, Appl. Catal. A., 2006, 303, /22 Acknowledgements The authors would like to thank for the financial support of Fundação Ciência e Tecnologia (FCT) through the project DynOpt (PTDC/EQU-ESI/74358/2006). 22/22 11