Fired Heater Simulation, Modelling and Optimization

In this study a dynamic model Simulation was carried out using ASPEN HYSY for industrial refinery fired heater, PID controller was applied to control the flue gas exit temperature. the simulation shows perfect agreement with the datasheet of the furnace. It was found that Increasing the number of the tube rows in convection bank from 2 to 3 allows us to recover approximately 5% of the overall efficiency, hence the duty furnace has increased from 65.9MW to 68.1MW and the fuel flow in like manner has increased from 5597kg/h to 5807kg/h moreover Adding more rows has a reverse return as we start to notice increase on the flue gas temperature. Furthermore, sensitivity analysis was conducted with HYSYS to determine one of the most important parameters that affect the performance of the heater based on the data generated from the simulation. MATLAB code was generated for efficiency calculation and for parameter manipulation, the code was designed to be flexible as possible, user will just need to replace the nominal fuel and air characteristics. for gaining optimum furnace operation the heat transfer inside the furnace was studied, and the results were compared with previous research involving furnace analysis to validate models, the heat transfer coefficient is determined by analysis of conductive heat transfer through the boundary layer. well-stirred model was used for mathematical model calculations and for optimization of furnace operation, the models were validated with ASPEN Exchanger Design and Rating(EDR) alongside ASPEN HYSYS


Introduction Furnace Heat Transfer:
The problem of transferring heat to crude oil in the primary furnace before it enters the crude fractionator will be undertaken.The heat from the flames passes by radiation and convection to the pipes in the furnace, by conduction through the pipe walls, and by forced convection from the inside of the pipe to the oil.Here all three modes of transfer are involved.After prolonged usage, solid deposits may form on both the inner and outer walls of the pipes, and these will then contribute additional resistance to the transfer of heat. 1

Evaluating convective heat transfer coefficients
There are three approaches to evaluating convective heat transfer coefficients.1.By analysis of conductive heat transfer through the boundary layer.2. By analogy between heat, mass and momentum transfer processes; 3.By direct measurement under limited conditions and extrapolation using dynamic similarity.
For the problem of heat transfer between a fluid and a tube wall, the boundary layers are limited in thickness to the radius of the pipe and, furthermore, the effective area for heat flow decreases with distance from the surface.The problem can conveniently be divided into two parts.Firstly, heat transfer in the entry length in which the boundary layers are developing, and, secondly, heat transfer under conditions of fully developed flow. 2   For the region of fully developed flow in a pipe of length L, diameter d and radius r, the rate of flow of heat Q through a cylindrical surface in the fluid at a distance y from the wall is given by: For the region of fully developed flow in a pipe of length L, diameter d and radius r, the rate of flow of heat Q through a cylindrical surface in the fluid at a distance y from the wall is given by: (7) Close to the wall, the fluid velocity is low and a negligible amount of heat is carried along the pipe by the flowing fluid in this region and Q is independent of y.And thus: ( ) and and And then: Assuming that the temperature of the walls remains constant at the datum temperature and that the temperature at any distance y from the walls is given by a polynomial, then: (9) Thus: and and Thus : from eq(8) And If the temperature of the fluid at the axis of the pipe is and the temperature gradient at the axis, from symmetry, is zero, then: Giving: And then: Or: and thus: consequently : and or: (10) Thus the rate of heat transfer per unit area at the wall: q q q q --+ = In general, the temperature at the axis is not known, and the heat transfer coefficient is related to the temperature difference between the walls and the bulk fluid.The bulk temperature of the fluid is defined as the ratio of the heat content to the heat capacity of the fluid flowing at any section.Thus the bulk temperature is given by: ( From Poiseuille's law:

=
The heat transfer coefficient h is then given by: (17 where q is the rate of heat transfer per unit area of tube.

Furnace Efficiency: -
Running furnaces efficiently is a major operating concern because two thirds of a plant's fuel budget is needed for furnace fuel cost.Furnace efficiency is linked to environmental regulations that stipulate a clean operation.Most furnaces use fuel gas or fuel oil.Natural gas burns cleaner and more efficiently than oil. 4  Furnace efficiency or total furnace efficiency is the ratio of heat usefully absorbed and total heat supplied.
−Actual oxygen required = Stoichiometric oxygen + (excess air * Stoichiometric oxygen) − Actual air required = actual oxygen required * g 100 21 i j ( 9) −Amount of CO n in flue gas = (total formed + CO n reported as fuel)

sensible heat correction for combustion of air (H > ):
- T ?= combustion air temperature.

Combustion and heat transfer modelling
Designing an industrial furnace generally requires the simultaneous resolution of transient heat, mass and momentum transfer phenomena in a complex chemically reacting system modelling is indisputably the most reliable technique, relying on the representation of the significant variables of the full size system in a physical and/or mathematical model that is based on sound, proven and validated theory.Modelling is termed as partial because it is not possible to satisfy all the scaling criteria or algorithms required for a complete model.and requires the modeler to be skilled in both an understanding of the design objectives, as well as the mechanics of the modelling process. 2   Mathematical modelling: According to Lobo and Evans (1939) and Hottel (1954) two models methods which are significantly important when considering modelling of fired heaters, they will be discussed in the following section

well-stirred furnace model
The most important development of fired heater model was made by Hottel  and is based on the assumption that many industrial furnaces works with sufficient momentum in the air and/or fuel streams to create a reasonably well-stirred furnace chamber.
This assumption allows most of the complex geometric problems associated with radiative heat transfer calculations to be reduced to a numerically simple solution.For this type of model, the following simplifying assumptions are made: (1) Mixing is intense so that there are no concentration or temperature gradients in the furnace.
(2) Reactants-products can be considered as a single zone.
The heat release from gas can be obtained thus: (1) Calculate the radiative heat transfer to the furnace surfaces from gases at gas temperature,  z .This requires: (a) Furnace geometry and surface properties ; (b) Gas composition (usually known for a specific fuel and combustion air); (c) gas flame (2) Calculate the convective heat transfer from gases at gas temperature,  z .This requires the convective heat transfer coefficients which can be found from: (a) Published data for flows over surfaces, tube banks, etc.; (b) Published data for jets expanding in a duct; (c) Mass transfer measurements on small scale models of furnace burner system. 5  the net heat exchange, Q, If the flux geometry is calculated from: and energy balance could be written as: For convective heat transfer, and for radiation losses from furnace openings, thus equation ( 19) can be rewritten as (28) This can be calculated from flux geometry as ( ) ( ) ) To eliminate T g between equations as before, and define some extra dimensionless groups as follows: then thus a new design equation driven as:

Simulation:
In this section refinery fired heater is simulated and the Long furnace models and Simple wellstirred furnace models are validated.

Conclusion:
we can notice that the comparison of the effect of oxygen percent on the inlet air of the furnace to the above results in figure (13) and in figure (14) which were conducted by MATLAB and ASPEN HYSYS environment simultaneously its almost has the same effect, increasing oxygen percent improve the efficiency of the furnace Increasing the fraction of the oxygen in the inlet air will lead to increase in thermal efficiency based on the fact that nitrogen absorb heat hence decease thermal efficiency.Fired heater

− 1 -
Amount of H n O in flue gas = (total formed + H n O reported as fuel) (11) −Amount of SO n in flue gas = (total formed + SO n reported as fuel) (12) −Amount of O n in flue gas = (actual O n supplied − actual O n used during combustion) (13) −Amount of N n in flue gas = (79%of moles of air + N n reported as fuel) (amount of stack component * molecular weight) (16) 2-(result from (1) / amount of fuel enter the furnace).3-result from (2) * enthalpy for each stack component).4-summation of all result in (3).

From
a constant of proportionality, thus we have(27) furnace efficiency, , is then calculated from (31) This equation indicates that the efficiency reaches a maximum value at a particular D' for any set of  ,  } ~ and  } • conditions, as shown in Figure above , thus we can potentially use equation (31) to optimize furnace designs.

Table 1 .
Combustion work sheet