Operation at high S/C ratio

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Operation at high S/C ratio

Post by debjyoti0047 » Tue May 28, 2019 6:42 am

Can anyone please list out the pros and cons of operating a steam methane reformer at higher S/C ratio.
The plant is operating at 30% load and S/C of 8-9.
Thank you in advance.

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Re: Operation at high S/C ratio

Post by bedmonds » Sat Jun 01, 2019 1:41 pm

HI, My only thoughts are:
a. CO2 removal is being driven harder than required -does it suffer if S/C is reduced?
b. dewpoint at the LTS will be rather low-be sure you are not close to condensing-best to have 20 deg of margin to avoid condensation in pores
c. without convection bank details it's hard to know if this very high S/C is necessary for any reason-for sure gas efficiency suffer
An interesting question

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Re: Operation at high S/C ratio

Post by pbaboo » Sat Jul 06, 2019 2:51 pm

Dear Ghosh Jee
Why do you want to have more S/C ratio, this is your compulsion or any other problems. The Higher S/C ratio means more Energy Consumption. A thumb rule lowering the S/C ratio from 4 to 3 the energy saving about 0.2 G.Cal/T of Amm. Primary reformer inlet steam-to-carbon (s/c) ratio is an important factor in reformer design. First, because a high s/c ratio favors the products in the reforming reaction equilibrium, maintained to prevent carbon deposition on the catalyst, shift conversion of carbon mono oxide and reduce carburization damage to the tube material. it lowers the amount of unreacted methane(less methane leak), or methane slip, out of the secondary reformer and increases the production of hydrogen.The design steam/carbon ratio is 2.85-3.0,The optimum S/C ratio has the advantages low pressure drop in the front end of ammonia plant.The S/C ratio depends upon Natural gas composition. However process efficiency declines with increasing S/C ratio for storage applications.Sufficiently above the ratio where carbon formation on an active catalyst is possible and sufficiently high to reduce the methane leakage.
During Start up
Only during reformer start-up or shut down the S/C ratio will be kept higher; during start up, load is gradually increased and the S/C ratio is decreased from an initial value of 6-8 towards normal operating value;
During Shut down.
During shut down, load is reduced and S/C ratio is gradually increased towards even above a value of 10.Lower Steam/Carbon ratios may cause damage to catalyst in case of sudden increase of higher hydrocarbons in feed gas. It may also lead to side reactions like cracking resulting in carbon deposition on catalyst.
Design value for steam/carbon ratio is decided based on various factors, including:
1. Reformer tubes skin temperature;
2. Sudden fluctuations of feed gas composition;
3. Presence of higher hydrocarbons in reformer feed gas;
4. Distribution of duty between primary and secondary reformer;
5. Material of construction of reformer tubes, and;
6. Requirement of steam in CO2 removal section.
Prem Baboo
DGM(Production & Process)
Dangote Fertilizer Limited, Lagos, Nigeria

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Re: Operation at high S/C ratio

Post by teji009 » Thu Sep 05, 2019 4:57 am

At a fixed production rate,thefollowing changes will occur with increasing S/C ratios.
(1) The required amount of process natural gas will decrease. This observation in plants some times has led to the mis- conception that higher S/C ratios result in increased production for a fixed total natural gas rate to the plant.
(2) The fuel requirements will increase.
(3) The loop purge rate will decrease because of the lower methane level in the fresh synthesis gas.
(4) More steam will be generated in the waste heat boilers.
(5) Power requirements for compression of fresh synthesis gas and process air to the secondary reformer will be lower.
(6) Convection section heat requirements will increase.
(7) Pressure drop through the system up to the carbon dioxide removal unit will increase.
(8) Because of the lower process natural gas requirement, the volume of carbon dioxide that must be removed will be reduced.
To have valid conclusions about the optimum steam/carbon ratio, all of these factors must be considered and, preferably, converted to one common unit.

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