The document provides an engineering design guide from GBH Enterprises on preventing carbon formation in mixed feed preheat coils, emphasizing the importance of calculating maximum allowable pre-heat temperatures based on feed composition. It details the calculation method using component indices and s/c ratios, illustrating two examples with natural gas and LPG compositions. The publication includes a disclaimer regarding the accuracy of the information and liability.

1. GBH Enterprises, Ltd.
Engineering Design Guide
Carbon Formation in Mixed Feed Preheat
Coils
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2. Carbon Formation in Mixed Feed Preheat Coils
Maximum Mixed Feed Pre-heat Temperature
What follows is a crude but effective routine, which evaluates the maximum
possible temperature allowable to prevent excessive carbon laydown in the
mixed feed pre-heat coil.
The method assumes that 100% CH4 mixed with steam (S/C =3) could in theory
be heated-up with steam to 645oC. As heavier hydrocarbons are added, this
temperature will fall. As hydrogen is added, or S/C increased, the temperature
can increase. A table of indices for the calculation is as follows:
Component
C1
C2
C3
C4
C5
C6
C7
C8
H2
N2, CO, CO2 etc
S/C (molar)
Index
[X]
0.05
1.2
1.5
1.7
1.55
1.65
1.75
1.8
-0.5
0
10
Each component composition in the feed (mol %) is then multiplied by the
respective index number [X] and totalled. The S/C ratio correction is then
calculated by subtracting the S/C ratio from 3 and multiplied by the index number
[X = 10]. The total summation of these numbers is then subtracted from 650oC to
yield the maximum allowable pre-heat temperature.
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3. Example 1 (NG)
NG composition......
C1
86.0 mol %
C2
6.6 mol %
C3
3.6 mol %
C4
1.5 mol %
C5
0.4 mol %
H2
2.0 mol %
N2
1.0 mol %
S/C ratio
3.3
So maximum pre-heat temperature
= 650 - [(86 x 0.05) + (5.5 x 1.2) + (3.6 x 1.5) + (1.5 x 1.7) + (0.4 x 1.55) +
(2 x -0.5) + (3 - 3.3)x10]
= 632oC
Example 2 (LPG)
LPG composition......
C3
0.47 mol %
C4
83.52 mol %
C5
0.35 mol %
H2
15.66 mol %
S/C ratio
3.3
So maximum pre-heat temperature
= 650 - [(0.47 x 1.5) + (83.52 x 1.7) + (0.35 x 1.55) + (15.66 x -0.5) +
(3 - 3.3)x10]
= 518oC
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Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals
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4. Note: This approach takes no account of pre-heat coil positioning, size,
counter/co-current etc. notwithstanding the other approximations.
However, even a crude approach may help in recommending maximum pre-heat
temperatures.
Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown
Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass
Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance
Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts /
Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals
Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries
Web Site: www.GBHEnterprises.com