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Methanol Plant Capacity Enhancement By: Mr. C D ... - Fibre2fashion

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<strong>Methanol</strong> <strong>Plant</strong> <strong>Capacity</strong><br />

<strong>Enhancement</strong><br />

<strong>By</strong>:<br />

<strong>Mr</strong>. C D Bhakta, <strong>Mr</strong>. A I Shaikh,<br />

<strong>Mr</strong>. Y N Patel & <strong>Mr</strong>. S J Darjee


<strong>Methanol</strong> <strong>Plant</strong> <strong>Capacity</strong> <strong>Enhancement</strong><br />

<strong>By</strong>: <strong>Mr</strong>. C D Bhakta, <strong>Mr</strong>. A I Shaikh, <strong>Mr</strong>. Y N Patel & <strong>Mr</strong>. S J Darjee<br />

The authors share their experience in debottlenecking a methanol plant at GNFC Ltd. The project<br />

involved the commissioning of a state of the art Isothermal reactor from Linde.<br />

GNFC is located at Bharuch, Gujarat, India and is engaged in manufacturing of fertilizers. The key<br />

products are urea, ANP, CAN, formic acid, methanol, acetic acid, and nitric acid (weak/strong).<br />

GNFC owns two (2) methanol plants. A small, old plant called <strong>Methanol</strong>-I with a capacity of 60 MTD<br />

and another <strong>Methanol</strong>-II plant with a capacity of 300 MTD. <strong>Methanol</strong>-I was commissioned in 1985. It<br />

was designed to operate on the feed gas from the rectisol wash unit of an ammonia plant. With the<br />

methanol market improvement in the late 90’s, this plant became an attractive option for a capacity<br />

increase. It is now producing more than 180 MTD (three times the design capacity) due to<br />

implemented, stepwise modifications in the plant.<br />

Brief History - Enhancing the <strong>Methanol</strong>-I <strong>Plant</strong> <strong>Capacity</strong><br />

Originally, this plant was designed to operate on feed gas from an ammonia plant consisting of a gas<br />

mixture of 75% hydrogen, 22% carbon dioxide, 1% carbon monoxide and some inerts. The reaction of the<br />

methanol in gas rich in CO2 is milder as it produces water along with methanol. The crude methanol<br />

concentration is also lower. Water further retards the rate of reaction. The two reactions involved here are:<br />

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

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

Normally, a gas mixture of H2 + CO + CO2 is used in a proportion measured in terms of a “R” value (H2-<br />

CO2)/(CO+CO2) equal to 2.0 to get an optimum methanol conversion per pass.<br />

Changes to the process included:<br />

1. A methanol chiller was introduced in the gas cooling circuit at the reactor outlet to reduce the methanol<br />

concentration and temperature in the recycle gas which helped to increase the methanol production from<br />

4~5 MTD up to 75 MTD level.<br />

2. Setting up a synthesis gas generation unit (SGGU) to supply CO rich gas from natural gas reformer in<br />

February 1998. This gas composition is better for methanol production compared to the rectisol wash gas<br />

which is rich in CO2. The synthesis gas and distillation loops were debottlenecked by replacing of some<br />

control valves, installation of exchangers, and other modifications. The capacity was boosted to 100 to 120<br />

MTD.<br />

3. Replacement of the refining column trays with high capacity Superfrac trays from Kostch Glistch India<br />

Ltd in October 2002. This, along with other peripheral modifications, was made to increase distillation<br />

capacity to 145 MTD.<br />

4. Replacement of the quench adiabatic methanol convertor to Linde’s Isothermal Reactor and<br />

debottlenecking of the distillation loops for higher capacity. The capacity of the plant was increased to 160<br />

MTD in September 2003.<br />

Major advantages of Isothermal Reactor include:<br />

Lower pressure drop in reactor<br />

Less temperature variation<br />

Increased life of catalyst<br />

Narrow band of temperature differences in the reactor catalyst bed<br />

Sustained Production Level throughout the catalyst life due to better conversion<br />

Less by-product formation<br />

Effective heat recovery


Figure 1 below shows the temperature profile in the isothermal reactor.<br />

Figure 1: Comparing Reactor Temperature Profiles Before and After<br />

the Changes<br />

Compared to the expected 160 MTD production capacity, the unit has achieved a stable production level of<br />

185~190 MTD.<br />

A flow diagram of the new loop is shown in Figure 2 below. In this article, we’ll focus on this latest<br />

dimension added to the plant, highlighting the re-commissioning experiences.


Figure 2: Changes in the <strong>Methanol</strong> Synthesis and Distillation Loops<br />

Figure 3: <strong>Methanol</strong> Synthesis and Distillation Loops After Changes


<strong>Plant</strong> Re-commissioning with the Isothermal Reactor<br />

Following the replacement of the quench reactor with the Isothermal reactor from Linde, the plant was ready<br />

for start up. The following details the activities associated with start up after the changes were made.<br />

Basic and Detail Engineering - Design Fundamentals<br />

The original plant was designed by Linde with process licensing from ICI. Linde performed the basic<br />

engineering for the loop modification and the detailed engineering for the new Isothermal reactor. Based on<br />

the data for the new design conditions, a debottlenecking study on the distillation section was carried out inhouse<br />

by our Technical Services department. Major pre-fabrication work and in-plant erection of the loops<br />

which were to be replaced was completed before the final shutdown of the plant. A shutdown schedule of<br />

11 days was planned.<br />

Outline of the Pre commissioning activities<br />

The piping loops were identified and broken down into various process loops per the P & IDs. The plant was<br />

broadly classified into three independent sections: synthesis loop, makeup gas loop, and distillation loop.<br />

This helped prioritize tasks such that the synthesis and related loops were made ready first. The loops,<br />

which were erected before shutdown, were prepared for commissioning by flushing / blowing. Based on the<br />

service, the plans for flushing / blowing were prepared and discussed with the mechanical and instrument<br />

groups to streamline the activities. All instruments in the circuit were removed from the lines.<br />

The following procedures were used:<br />

For gas lines: Gasket blowing with plant air was carried out starting from 1.0 barg up to 3.5 barg repeatedly,<br />

until there was no rust / dust in the line. This was followed by nitrogen passivation / drying.<br />

For liquid lines: Air blowing followed by water flushing was carried out. This was followed by nitrogen<br />

passivation / drying.<br />

For steam lines: Gradual warming of the header before insulation was applied for grease removal and rust<br />

flushing through the trap bypass. Then steam blowing at full capacity was carried out for half an hour by<br />

diverting the open end at a safe location. The header was allowed to cool. This cycle was repeated again till<br />

clear condensate was discharged in the trap bypass.<br />

For Running Machines: There was a pair of process pumps in each service. One pump online and one<br />

spare. With the higher capacity, some pumps were replaced for higher capacity. The main crude feed<br />

pumps and refining column reflux pumps were replaced. With spare pumps, the plant operation was not<br />

interrupted during the pump changes. Each replacement took 12 days and included the modification of the<br />

base, pipeline, motor, and other ancillary pieces.<br />

Likewise, four control valves were replaced via proper coordination between the operations and project<br />

teams. The prefabricated loops were also washed or blown and then dried with nitrogen. These were kept<br />

inert and sealed at their ends until they were to hooked up during the shutdown. This also helped reduce<br />

the pre-commissioning time for the plant. The start-up boiler feed water circulation pump was commissioned<br />

and stabilized prior to shutdown as soon as the errection of the reactor steam drum system was completed.<br />

Both <strong>Methanol</strong>-I and SGGU operate independently. It was not necessary to shutdown SGGU for the<br />

commissioning of Isothermal reactor in the <strong>Methanol</strong>-I synthesis loop. The natural gas compressors in the<br />

SGGU plant get cooling water from the <strong>Methanol</strong>-I plant header. Since cooling tower was to be taken<br />

offline, temporary arrangements to supply an alternate water source was planned to keep the natural gas<br />

compressors in the SGGU running. This was implemented prior to shutdown, avoiding a stoppage of the<br />

SGGU plant.<br />

Reactor Catalyst Charging<br />

This was the first reactor of its kind at GNFC with spiral wound coils within the shell. The catalyst is to be<br />

charged on the shell side, while the cooling medium (boiler feed water) flows in the tubes via thermosiphon.<br />

During the startup, the boiler feed water circulation pump maintains the water circulation.


As shown in Figures 4 and 5, the reactor coils are supported at both the ends by six support strips moving<br />

radially outward from the central mandrill. This divides the cross section of the reactor into six equal parts.<br />

These were taken as the basis for charging the catalyst and numbered from 1 to 6 inside the reactor.<br />

Figure 4: Internal View of the Isothermal Reactor<br />

Figure 5: Top Internal View of the Isothermal Reactor


Two catalyst charging nozzles were used with hoppers and 2 ½” dia flexible hoses for charging the catalyst -<br />

SACK WISE under the supervision of Linde. A table was prepared to log the number of bags charged per<br />

round and the subsequent dip achieved, which showed the packing uniformity. This proved to be a very<br />

successful method of charging with good packing density with less than 20 mm of variation in the final height<br />

adjustment. A flat and heavy plumb with strong cotton thread was used for taking the dip.<br />

Approximately equal quantities of 20 kg balls/catalyst were filled in HDPE sacks before the start of loading.<br />

About 5.2 m 3 alumina balls were filled first in four rounds of sack charging. The catalyst bed was leveled so<br />

that the balls were just inside the tube coiled bundle. The first dip of catalyst was taken after charging<br />

almost half of the catalyst. Thereafter, while monitoring the height, charging continued to completion over<br />

approximately two (2) days.<br />

Commissioning activities<br />

The synthesis loop was made available earlier than the distillation loop (6 ½ days) while the total shutdown<br />

period was compressed to 8 days by effective identification of the priority of each job. The effectiveness of<br />

the pre-commissioning activities was evident during post-commissioning. There were no plugged strainers,<br />

control valves by-passing, nor false signals during or after the startup of the plant. Re-commissioning of the<br />

plant was completed in less than 4 days time. While, the distillation section modifications were being<br />

completed, the synthesis loop pre-commissioning activities were completed. While the catalyst heat up and<br />

reduction was proceeding, crude methanol production was coming online.<br />

Peak production levels for the plant were achieved while testing the plant at different feed gas mixtures. The<br />

plant has met all process guarantees. Of particular interest has been an improved yield of methanol due to a<br />

higher conversion rate and stable reaction conditions. Less by-product formation has led to a reduction of<br />

loading in the distillation section.<br />

Conclusion<br />

From this experience, we see that the plant capacity can be increased by understanding the basic principles<br />

of reaction kinetics and unit operations. Through integration of technology and the use of improved catalyst,<br />

this little plant had been transformed into a giant producer. Proper planning of critical activities like catalyst<br />

charging, pre-commissioning of loops, commissioning and guarantee test runs can ensure success.<br />

About the Authors:<br />

The authors are associated with GNFC Ltd.<br />

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