Capacity Increase and Efficiency Improvement in Naphtha Stabilizer Reboiler
A Refinery in East Texas has a natural draft Naphtha Stabilizer Reboiler (077H-002) in Ultra low Sulphur Gasoline Unit. 077H-002 is a vertical cylindrical heater with a horizontal convection section and a top mounted stack. The heater was designed for a heat duty of 8.68 MMBtu/hr and an efficiency of 82.7%.
Client requested Furnace Improvements (FIS) to rerate the Naphtha Stabilizer Reboiler for new process conditions. FIS observed that the heater design was very tight. The firebox was very small and volumetric heat release was high. Thus, the proposed heat duty of 10.8 MMBtu/hr and increase in process throughput were not feasible with the existing radiant section. The flue gas temperature leaving convection section was very high, and the stack height was limiting with the proposed firing conditions.
FIS revamp scheme consisted of following:
Modified Radiant Section:
The radiant section height was increased from 16.82 ft to 22 ft. New radiant tubes were installed. The radiant heat transfer area was increased by 35.4%. The increase in height facilitated in maintaining the radiant heat flux below 10,000 Btu/hr/ft2 and volumetric heat flux within API set limit of 16,000 Btu/hr/ft3.
New Convection Section:
Wider convection section with 6 tubes/row was installed. The convection section had 48 tubes and a design heat duty of 3.80 MMBtu/hr.
The new stack had an inside insulation diameter of 2’10” and its length was 40 ft.
The following advantages were realized due to the modifications proposed by FIS:
The capacity of the heater was increased by 13.5% from the design.
The bridge wall temperature was reduced by 76°F.
The flue gas temperature leaving the convection section was reduced by 190°F.
The fin tip temperature was alleviated by 107°F.
The heat duty increased by 24.4% due to availability of more heat transfer surface.
The efficiency of the heater increased by 5.1%.
Increased efficiency also led the client to save $269,100 on fuel per year.