Published January 8, 2008

The authors share their experience in debottlenecking a methanol plant at GNFC Ltd.  The project involved the commissioning of a state of the art Isothermal reactor from Linde.

GNFC is located at Bharuch, Gujarat  India and is engaged in manufacturing of fertilizers. The key products are urea, ANP, CAN, formic acid, methanol, acetic acid, and nitric acid (weak/strong).  GNFC owns two (2) methanol plants.  A small, old plant called Methanol-I with a capacity of 60 MTD and another Methanol-II plant with a capacity of 300 MTD. Methanol-I was commissioned in 1985. It was designed to operate on the feed gas from the rectisol wash unit of an ammonia plant. With the methanol market improvement in the late 90’s, this plant became an attractive option for a capacity increase. It is now producing more than 180 MTD (three times the design capacity) due to implemented, stepwise modifications in the plant.

Brief History - Enhancing the Methanol-I  Plant Capacity

Originally, this plant was designed to operate on feed gas from an ammonia plant consisting of a gas mixture of 75% hydrogen, 22% carbon dioxide, 1% carbon monoxide and some inerts.  The reaction of the methanol in gas rich in CO₂ is milder as it produces water along with methanol.  The crude methanol concentration is also lower. Water further retards the rate of reaction. The two reactions involved here are:

H₂  +  CO₂  ---->    CH₃OH  +  H₂O          +            9.8 kcal/kgmol            

H₂  +  CO    ---->   CH₃OH                     +          21.6 kcal/kgmol                       

Normally, a gas mixture of H₂ + CO + CO₂ is used in a proportion measured in terms of a “R” value (H₂-CO₂)/(CO+CO₂) equal to 2.0 to get an optimum methanol conversion per pass.

Changes to the process included:

1.      A methanol chiller was introduced in the gas cooling circuit at the reactor outlet to reduce the methanol concentration and temperature in the recycle gas which helped to increase the methanol production from 4~5 MTD up to 75 MTD level.

2.      Setting up a synthesis gas generation unit (SGGU) to supply CO rich gas from natural gas reformer in February 1998.  This gas composition is better for methanol production compared to the rectisol wash gas which is rich in CO₂. The synthesis gas and distillation loops were debottlenecked by replacing of some control valves, installation of exchangers, and other modifications.  The capacity was boosted to 100 to 120 MTD.

3.      Replacement of the refining column trays with high capacity Superfrac™ trays from Kostch Glistch India Ltd in October 2002.  This, along with other peripheral modifications, were made to increase distillation capacity to 145 MTD.

4.      Replacement of the quench adiabatic methanol convertor to Linde’s Isothermal Reactor and debottlenecking of the distillation loops for higher capacity.  The capacity of the plant was increased to 160 MTD in September 2003. 

Major advantages of Isothermal Reactor include: 

Lower pressure drop in reactor
Less temperature variation
Increased life of catalyst
Narrow band of temperature differences in the reactor catalyst bed
Sustained Production Level throughout the catalyst life due to better conversion
Less by-product formation
Effective heat recovery

Figure 1 below shows the temperature profile in the isothermal reactor.

Figure 1: Comparing Reactor Temperature Profiles Before and After the Changes

Compared to the expected 160 MTD production capacity, the unit has achieved a stable production level of 185~190 MTD.

A flow diagram of the new loop is shown in Figure 2 below.  In this article, we’ll focus on this latest dimension added to the plant, highlighting the re-commissioning experiences.

Figure 2: Changes in the Methanol Synthesis and Distillation Loops

Figure 3: Methanol Synthesis and Distillation Loops After Changes

Plant Re-commissioning with the Isothermal Reactor

Following the replacement of the quench reactor with the Isothermal reactor from Linde, the plant was ready for start up.  The following details the activities associated with start up after the changes were made.

Basic and Detail Engineering - Design Fundamentals

The original plant was designed by Linde with process licensing from ICI. Linde performed the basic engineering for the loop modification and the detailed engineering for the new Isothermal reactor.  Based on the data for the new design conditions, a debottlenecking study on the distillation section was carried out in-house by our Technical Services department.  Major pre-fabrication work and in-plant erection of the loops which were to be replaced was completed before the final shutdown of the plant.  A shutdown schedule of 11 days was planned.

Outline of the Pre commissioning activities

The piping loops were identified and broken down into various process loops per the P & IDs. The plant was broadly classified into three independent sections: synthesis loop, makeup gas loop, and distillation loop.  This helped prioritize tasks such that the synthesis and related loops were made ready first.  The loops, which were erected before shutdown, were prepared for commissioning by flushing / blowing. Based on the service, the plans for flushing / blowing were prepared and discussed with the mechanical and instrument groups to streamline the activities. All instruments in the circuit were removed from the lines.

The following procedures were used:

For gas lines: Gasket blowing with plant air was carried out starting from 1.0 barg up to 3.5 barg repeatedly, until there was no rust / dust in the line. This was followed by nitrogen passivation / drying.

For  liquid lines: Air blowing followed by water flushing was carried out. This was followed by nitrogen passivation / drying.

For steam lines: Gradual warming of the header before insulation was applied for grease removal and rust flushing through the trap bypass.  Then steam blowing at full capacity was carried out for half an hour by diverting the open end at a safe location. The header was allowed to cool. This cycle was repeated again till clear condensate was discharged in the trap bypass.

For Running Machines: There was a pair of process pumps in each service. One pump online and one spare.  With the higher capacity, some pumps were replaced for higher capacity. The main crude feed pumps and refining column reflux pumps were replaced.  With spare pumps, the plant operation was not interrupted during the pump changes. Each replacement took 12 days and included the modification of the base, pipeline, motor, and other ancillary pieces.

Likewise, four control valves were replaced via proper coordination between the operations and project teams.  The prefabricated loops were also washed or blown and then dried with nitrogen. These were kept inert and sealed at their ends until they were to hooked up during the shutdown.  This also helped reduce the pre-commissioning time for the plant.  The start-up boiler feed water circulation pump was commissioned and stabilized prior to shutdown as soon as the errection of the reactor steam drum system was completed.

Both Methanol-I and SGGU operate independently.  It was not necessary to shutdown SGGU for the commissioning of Isothermal reactor in the Methanol-I synthesis loop.  The natural gas compressors in the SGGU plant get cooling water from the Methanol-I plant header.  Since cooling tower was to be taken offline, temporary arrangements to supply an alternate water source was planned to keep the natural gas compressors in the SGGU running.  This was implemented prior to shutdown, avoiding a stoppage of the SGGU plant.

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