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Cracking Furnace


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#1 Chem32

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Posted 17 April 2014 - 09:46 AM

Sir,

 In our plant we have a EDC cracking furnace of capacity 5 tonnes/hr. the furnace is radiant wall cabin type. There are 24 burnerns in three rows using superior kerosene as a fuel.

The design kerosene flow in each row is given below

Top row - 1.5 gallons/hr

Middle row - 3 gallons/hr

Bottom row - 4.5 gallons/hr

 

In top row instead of 1.5 gallons/hr of kerosene the flow was increased. Because of this the coke formation was so frequent so we isolated the top row burners. Now the stack temperature has increased to 400oC .

 

Do the isolation of the top row burners have any impact in the stack temperature?



#2 Pilesar

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Posted 17 April 2014 - 12:11 PM

Stack temperature will increase if you have extra air leaving the radiant section. Perhaps the top burners used some of the excess oxygen from the lower burners for combustion. Or perhaps when you isolated the fuel to the top burners, you still have air entering there. Higher stack temperature means your furnace as a whole is not as efficient as previously.



#3 Chem32

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Posted 18 April 2014 - 02:48 AM

Dear pilesar,

 

We isolated both the fuel and air to the top row burners. the excess air is 25%.

 

Do chamber vacuum have any impact in the stack temperature?



#4 Pilesar

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Posted 18 April 2014 - 05:50 AM   Best Answer

   Stack temperature reflects the leftover energy after all the heat transfer in the radiant and convection section. The higher stack temperature means that the furnace is not as efficient at using energy as it was before. Efficiency changes with firing rate, heat transfer coking, excess air. If you are pushing production very far from design conditions, then you may have equipment limitations such as too few convection section coils for your rate. Do you have flue gas temperature measurements in the convection section? Do you have convection coil outlet temperature measurements? If you have enough instrumentation, you can do an overall heat and material balance showing each individual section of your furnace. The hot side duties and cold side duties have to match overall and also over each exchanger service in the convection section. The mass flow rate of the flue gas in the convection section is constant so you use that information in your balance. You will have to make adjustments to your temperatures as your measurements may not be representative of the bulk stream. 

   If you contact the cracking furnace designer, they might give you insight over the phone. Engineers like to show off their knowledge and will often give free advice if you bypass the sales person. They put the top burners in the box for reasons that may not all be apparent. Changing the operating mode of highly integrated equipment can have consequences. By turning off the top burners, you are trying to compensate for a problem elsewhere in the furnace. Unless the top burners are impinging directly on the coils, then the coking rate may be better controlled by another method. I have not worked with EDC -- my experience is with olefins and syngas furnaces. Your 400 C stack temperature seems way too high to be reasonable. What temperature do you normally see at the stack?



#5 Chem32

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Posted 18 April 2014 - 09:30 AM

We are operating below the design condition. The furnace was erected at 1967.

our normal stack temperature will be 260 C.

 

The design conditions are given below:

 

Heat absorbed  (MM BTU/hr)

Convection         2.285

Shock                 1.005

Radiant               3.84

Total                 7.13

 

 

No of Coils

Convection      18

Shock              8  

Radiant            32

Total               58

 

Outside surface area (Sq.ft)

Convection        1170

Shock               70

Radiant              281

Total                 1521

 

Heat flux (BTU/hr.ft2)

Convection       1960

Shock              14400

Radiant             13650

Total                4180

 

% Heat distribution

Convection        32

Shock               14

Radiant              54

 

% Area Distribution

Convection        77

Shock                5

Radiant              18


Edited by sarathkumar, 18 April 2014 - 10:07 AM.





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