Chemical En@eeriy Science, Vol. 48, No. 4, PP. 661-679, 1993. Printi in Orcat Britain. lIoo%?509/93 55.00 + 0.00 Q 1992 Pcr&anme Press Ltd MODELLING, SIMULATION AND CONTROL OIL PREHEATING FURNACE OF A CRUDE zyxwvutsrqponm AVRAHAM FUCHS and DANIEL R. LEWIN’ zyxwvutsrqponmlkjihgfedcbaZY Department of Chemical Engineering, Technion, Haifa, Israel and SAMUEL J. WAJC IM I-Institute for Research and Development, P.O. Box 313, Haifa, Israel zyxwvutsrqponmlkjihgfe (First received 16 November 1991; accepted in revised form 14 May 1992) Abstract-The energy necessary for the first fractionation of crude oil is supplied by a furnace which preheats and partially vaporizes the atmospheric column feed. This paper describes a dynamic nonlinear mathematical model for a typical preheating furnace. As a first approximation, all the crude volatile fractions arc lumped into one phase, represented by n-decane. Analysis of the time scales associated with the furnace components leads to the conclusion that the long-term system dynamics are dominated by the thermal accumulation in the furnace walls, while the significant faster modes are contributed by thermal, mass and momentum accumulation in the flowing crude oil. The model is used principally to account for the effect of variations in thefurnace fuelflow rateand theinfluence of changes in the feed temperature and feed flow rate on the furnace effluent temperature. Using the model, dynamic open-loop simulations are performed, and approximations based on parallel linear transfer function models are fitted to the responses. A robust feedback controller is designed to ensure that the exit temperature tracks its set point, on the basis of the variations in the parameters of the linear model relating fuel flow to the furnace exit temperature. Feedforward controllers are computed in order to gauge the level of disturbance rejection improvement possible. It is shown that both disturbances investigated can be eliminated within a reasonable time by manipulating the fuel fed to the burners. Comparing the combination feedforward-feedback control to feedback alone reveals that adding feedlorward action is advantageous only in rejecting the temperature disturbance. This is because the significant dead timewhichprecedes the effect of thisdisturbance on the effluent temperature gives the feedforward controller ample time to act. 1. 1NTRODUmION Crude oil consists of a mixture of hundreds of organic components with a broad range of boiling points, and whose specific composition is essentially dependent on its source. The first step in separating the mixture is atmospheric distillation. As shown schematically in Fig. 1, the energy for the distillation is supplied by a furnace which preheats and partially vaporizes the atmospheric column feed. The control of such a sys- tem is complex for three main reasons: (a) the control of the atmospheric column is difficult in itself; (b) the overall process is heat-integrated, leading to addi- tional dynamic interaction in the system; and (c) the actual composition of the raw crude oil, which has significant impact on the operation of the column, cannot usually be accurately assessed on-line. Since the capacity of the crude column to reject disturbances in the crude composition is limited, it is natural to consider the furnace as a means to adsorb these disturbances. This is the motivation for generat- ing a dynamic model of the furnace. Crude distillation is a high-volume, high-energy-consuming operation, and several papers have been published dealing with the control of the crude column. On the other hand, +Author to whom correspondence should be addressed. surprisingly little has been presented on the modelling and control of the furnace. Indeed, the only work found was that of Nigmatulin et al. (1977), who pres- ented a steady-state model of a crude oil preheating furnace. A cross-sectional view of a typical furnace is given in Fig. 2. The data used for the furnace modelled in this study are somewhat similar to those of a furnace currently operated by the Haifa Oil Refinery, used to preheat crude oil at a nominal processing rate of 360 m3/h. The physical dimensions of the furnace are: length 18.3 m, width 8 m, height 12 m. The crude oil enters the furnace in the overhead convection section and from there descends to the radiation section, splitting into four parallel paths, two on each side of the furnace. The oil is pumped through 6” piping, arranged horizontally back and forth along the length of the furnace, and connected externally by 180” bends. In each of the four parallel convection and radiation paths, there are 20 bends, with a total of 732 m of pipe for each path. The furnace control system must deal with changes in the composition, flow rate and temperature of the crude feed and be as insensitive as possible to changes in the fuel quality. The fuel quality changes result from the use of both fuel oil and fuel gas to fire the 661