CFD. André França de Almeida, Mechanical Engineer

Transcription

1 SCOT Heater Analysis using CFD André França de Almeida, Mechanical Engineer Fábio Sousa do Fundo, Chemical Engineer Sylvio Lopes Junior, Mechanical Engineer Daniel dos Santos Lustosa, Mechanical Engineer

2 Summary Mixing Mixing improvements

3 Mixing Sulfur Removal Units are present in refineries processing high sulfur crude oils. The SCOT heater is an equipment used in such units; it heats the gas with high content of hydrogen sulfide (acid gas) by mixing it with hot combustion gases. This equipment poses a complex flow and combustion problem, where a heater has to be designed to promote adequate combustion between air and fuel while heating a significant flow of tail gas and avoid burning the hydrogen sulfide present in this stream. The purpose of this paper is to analyze the performance of a typical configuration of an inline SCOT heater, with a burner with separate inlets of air and fuel and a side entrance of tail gas. The analysis was made focusing on the flow simulation, in light of preferable flow paths, high gas velocities and occurrence of hotspots. A simple model was used for the natural gas combustion SCRS (simple chemical reaction system) where oxidant and fuel react in a single step whereas the tail gas was considered inert. Possible problems regarding the tail gas entrance such as hotspots or preferable paths were identified in this configuration and two alternative geometries were proposed and simulated.

4 The figure below shows the proposed geometry for the SCOT heater. The Heater simulation was divided into two parts: burner simulation (only flow) and mixing chamber simulation (combustion, heat transfer and flow). Mixing BURNER SIMULATION Flow of air and gas AIR INLET MIXING CHAMBER SIMULATION Combustion reaction, flow and heat transfer MIXED GAS FUEL GAS TAIL GAS INTERFACE Pressure, velocity and concentration fields The results obtained in the burner simulation were used as boundary conditions for the mixing chamber simulation.

5 The figure below shows the proposed geometry for the SCOT burner. AIR INLET Interface between Burner and Simulations Mixing FUEL INLET OUTLET

6 Mixing The following premises were taken into consideration: Steady flow No chemical reaction Non-slip walls. Traditional k-e model was used. Table 1 presents the boundary conditions for the air and fuel streams used in this simulation Design Case Air Fuel Flow (kg/s) Temperature (C) Density (kg/m 3 )

7 The figure below presents the velocity field obtained for the burner simulation at the cross section. Mixing Even though the analysis of the burner was not the primary purpose of this work, it was possible to make some observations regarding the poor distribution of air around the impact plate and guide vanes, which could lead to an asymmetric flow into the mixing chamber.

8 Mixing Cross section location

9 The velocity field at the end of the burner (before the fuel and air streams mix) shown on the figure below will be used as a boundary condition for the mixing chamber simulation. Mixing Cross Section Location

10 The figure below shows the geometry for the mixing chamber. Mixing Table 2 presents the boundary conditions for the air, fuel and tail gas streams used in the simulation. Design Case Air + Fuel Tail gas Flow (kg/s) 21.8 Burner Temperature (ºC) 159 Simulation Density (kg/m 3 ) 0.87

11 Mixing The following premises were taken into consideration for the mixing chamber simulation: Steady flow Ideal gases Gravity effects No radiation effects Adiabatic walls Single chemical reaction of the type: FUEL + AIR = PRODUCTS Properties of the PRODUCTS flow were calculated considering the slightly substoichiometric combustion of natural gas Rate of the reaction was diffusion controlled. Tail gas as inert stream

12 Mixing The figure below shows the velocity field thoughout the mixing chamber. Gas Outlet Mixing SIDE VIEW Tail Gas Entrance TOP VIEW

13 Mixing The figure below shows the streamlines throughout the mixing chamber. Gas Outlet Mixing Fuel gas streamlines Tail gas streamlines SIDE VIEW Tail Gas Entrance TOP VIEW

14 Mixing The figure below shows the temperature fields for the mixing chamber. Gas Outlet Mixing SIDE VIEW Tail Gas Entrance TOP VIEW

15 Mixing Mixing It was possible to observe from these results that: The tail gas flow appears to block the flame in between the burner and the tail gas side entrance. The hot flue gas is forced to pass in the small spaces between the tail gas flow and refractory walls, where it will develop the highest velocities. The above mentioned phenomena concentrates the hot flue gas in the space between the burner and conic section. This will lead to high wall temperatures, which can be the cause of hotspots formation and refractory failure. In light of these observations, CHEMTECH proposed alterations in the tail gas entrance, in a way to obtain overall lower wall temperatures.

16 Similar cases were found in specialized literature with results that corroborate the validity of the results obtained in this simulation. The John Zink combustion handbook presented the simulation results for a burner with separate inlets of air and fuel that presented similar patterns. Henneke et al simulated an inline SCOT heater which also presented similar concentration fields. Mixing

17 CHEMTECH simulated two alternate cases to improve the temperature distribution in the mixing chamber. CASE 1 Mixing CASE 2

18 The figures below show the velocity fields for the alternate cases: CASE 1 Mixing CASE 2 TOP VIEW SIDE VIEW

19 The figures below show the temperature fields for the alternate cases: CASE 1 Mixing CASE 2 TOP VIEW SIDE VIEW

20 The results show that Case 1 did not improve the temperature distribution near the wall. The opposite entrances for the tail gas created a barrier which caused the hot flue gas concentration near the burner. Case 2 showed improvement in relation to the base case. The flue gas developed more naturally, creating a longer flame and moving it away from the wall. The figures below indicate clearly that the temperature in the conic section in case 2 diminished in comparison to the base case. Mixing BASE CASE CASE 2

21 Mixing The present work presented a study of the flow phenomena involved in the design of a SCOT inline heater. The results indicate the importance of an adequate design for the tail gas entrance into the mixing chamber. The base case a typical design of inline heaters, used in the current industry showed a tendency for hotspots occurrence and high wall temperatures. Two alterations to the heater geometry were suggested and simulated. The best results were obtained from the alteration that could successfully lead the flame away from the walls and towards the center of the chamber.

22 Mixing E McKenty, L. Gravel and R. Camarero. Numerical simulation of industrial boilers ; Korean Journal of Chemical Engineering, Nº 16, P , Ruprecht, A., Heitele, M., Helmrich, T., Moser, W., Aschenbrenner, T. Numerical Simulation of a Complete Francis Turbine including unsteady rotor/stator interactions. IAHR Symposium, Charlotte, Baukal, Charles. The John Zink combustion handbook. Florida, CRC Press, Henneke, M. Et al. Scot Inline Heater combustion and mixing. Available at: < Access on: 3 agust

23 A little bit about CHEMTECH

24 Global Solutions control and optimization information engineering

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