Heat exchanger

Mecánica Computacional Vol. XXII M. B. Rosales, V. H. Cortínez y D. V. Bambill (Editores) Bahía Blanca, Argentina, Noviembre 2003.

DYNAMIC OPTIMIZATION WITH A SIMULTANEOU METHOD: APPLICATION TO A HEAT EXCHANGER
Mariela A. Rodríguez, Maria Soledad Díaz, Alberto J. Bandoni
Planta Piloto de Ingeniería Química, PLAPIQUI (UNS – CONICET) Camino La Carrindanga, Km.7 (8000) Bahía Blanca, ARGENTINATel.: +54 291 486 1700 Fax: +54 291 486 1600 E-mails: {marodriguez, sdiaz, abandoni}@plapiqui.edu.ar

Keywords: Dynamic optimization, OptControlCentre, Heat exchanger. Abstract. In this work we use a numeric method based on a combined discretization and simultaneous dynamic optimization approach to solve a system consisting of partial differential and algebraic equations. The spatialderivatives are discretized by finite differences while the resulting DAE (Differential-Algebraic Equations) optimization problem is transformed into a large-scale NLP (Nonlinear Programming) problem through collocation over finite elements. This method is implemented in a computer package resident in a remote computer located at the Department of Chemical Engineering Carnegie Mellon University, which isaccessed via a high-speed internet connection (Internet 2) from a client computer at the Centro Regional de Investigaciones Basicas y Aplicadas Bahia Blanca (CRIBABB). We have applied this strategy to the resolution of the dynamic optimization model of a gas gas heat exchanger, which is part of a larger model under development. The goal is to minimize the transient between two set points of anoutlet stream temperature. The dynamic model provides profiles of controlled and manipulated variables which are in agreement with available data, and the remote optimization system performed very well.

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M. A. Rodríguez, M. S. Díaz, A. J. Bandoni

1 INTRODUCTION Natural Gas Liquids processing plants provide feedstock, mainly ethane and propane, for production of olefins and otherpetrochemicals. In our country, there are several natural gas processing plants, two of which currently provide raw material for the most important petrochemical complex in the country, located next to Bahía Blanca. The extraction of ethane and heavier hydrocarbons from natural gas can be efficiently performed through turboexpansion processes. The separation is carried out at high pressure and cryogenicconditions. However, cryogenic processes involve intensive material and energy integration, complex process flowsheet, small driving forces for flow and heat exchange, tight operational requirements and very high product purities. These attributes place them on the complex side of the spectrum of potential simulation, optimization and control applications. During the last two decades, much researchand development work has been devoted to the determination of more efficient expansion processes and their optimal operating conditions. Bandoni et al.1 have developed a methodology based on an energy analysis in the cryogenic sector for the selection of natural gas processing plant designs. Diaz et al.2 have solved the debottlenecking problem of an ethane extraction plant as a Mixed IntegerNonlinear Programming (MINLP) model. They have shown that significant improvement in the plant operation and economics could be achieved by simultaneously considering minor structural modifications. Diaz et al.3 have studied the detailed design of a turboexpansion plant for a wide range of natural gas mixtures by means of an MINLP strategy integrated to rigorous process and costs models. They havealso analyzed similar process technologies that can be used to obtain propane as the main product and re-inject ethane to pipeline. More recently, Diaz et al.4 have studied dynamic behavior of main units in the plant, in particular the cryogenic highpressure demethanizing column, through a simultaneous approach to solve the DAE optimization problem. As part of this project we are now modeling...