Revamp of Ammonia Plant at
RCF Trombay
C M T Britto and H S Karangle
Rashtriya Chemicals
& Fertilizers Limited
Priyadarshini
Sion
Mumbai – 400 022
ABSTRACT
Rashtriya Chemicals
& Fertilizers (RCF) operates two ammonia plants at Trombay- one with a original
capacity of 900 MTPD (Trombay V) and the other with a capacity of 350
MTPD(Trombay I). The Trombay V ammonia plant employed technology of late
seventies based on associated gas as feedstock and fuel. Due to aging and
obsolescence of technology energy consumption level was much higher than
desirable.
Ammonia production
process is energy intensive. Against the backdrop of increasing energy costs
generally energy saving measures quickly pay back the investment. Thus to bring down energy consumption and also
to enhance production capacity RCF undertook a revamp scheme at a cost of Rs.
250 crore with the involvement of process licensor M/s Haldor Topsoe. The
revamp scheme is expected to improve reliability besides reducing energy
consumption by approximately 20%.
Introduction
Rashtriya
Chemicals & Fertilizers Limited (RCF) is an established manufacturer of
fertilizers and industrial chemicals with a track record of operating success
over the last 35 years. RCF has two manufacturing facilities, one at Trombay,
in the Mumbai City, and the other at Thal, in Alibag Taluka
which is 100 kilometers from Mumbai.
Trombay
unit of RCF manufactures complex fertilizers and industrial chemicals. It has
two ammonia plants- one with a capacity of 350 MTPD(Trombay I) and the other
with a capacity of 900 MTPD (Trombay V). The Trombay V ammonia plant employed
technology of late seventies based on associated gas as feedstock and fuel.
This ammonia plant could not achieve the expected level of performance from the
beginning and the same deteriorated with passing of time due to poor
performance of machinery and obsolescence of technology. In the increasingly
competitive world of ammonia production and ever increasing energy costs, it is
imperative to look for ways to improve energy efficiency, enhance plant
throughput and lower costs.
RCF
focused its revamp objectives for the plant on the following:
·
Reduction
in the specific energy consumption in order to meet the continued shortage of
allocation of gas
·
Improvement
in reliability
·
Utilizing
design margin in capacity
·
Minimizing
the downtime to incorporate the changes
Past Performance of the
Plant
The
production performance of the plant was below the desirable level since
inception. This was further impacted in recent years due to severe feedstock
gas limitation. The average annual capacity utilization of the plant for last
seven years has been about 77% of the rated capacity. Even though the design
specific energy consumption was 10.93 Gcal/MT of ammonia the actual energy
consumption was much above this level. Some of the contributing factors for
increased energy consumption was excess fuel gas in primary reformer, high
import of steam due to inefficient turbines especially synthesis compressor
turbine as well as increased numbers of startup and shut down of the plant due
to poor reliability.
Methodology for
Revamping
A
detailed study of the plant through M/s Haldor Topsoe, original process licensor,
was undertaken. The study went through following stages:
1.
Base
case data collection at site and validation
2.
Base
case process evaluation
3.
Revamp
options
4.
Economic
evaluation of the options
5.
Finalization
of scheme for Revamp
After
conducting a detailed performance audit of the plant and equipment, various
options of potential plant modifications were developed, analyzed and
documented as Option Screening Report. Each of the options was evaluated
against the following criteria:
·
Making
maximum use of capacity margin in a equipment
·
Keeping
the number of additional equipment to the minimum by retaining the existing equipment by simple modification
·
Minimizing
the downtime to incorporate the changes
Based
on the above criteria the revamp scheme for implementation was finalized.
Revamp Measures
The
main objective was to reduce energy consumption with utilization of capacity margin
for which following modifications were selected and implemented:
·
Up-gradation
of primary reformer
·
Modification
of steam superheater
·
Modification
of process air compressor
·
Modification
in Carbon Dioxide removal system
·
Medium
pressure condensate stripping
·
Installation
of S-50 converter and a loop boiler
·
Replacement
of synthesis gas compressor
The
total time required for implementation was 24 months. This could be achieved by
adopting maximum possible supply to prefabricated items and erection of the
same during normal operation. The actual
duration of shutdown of the plant for hook up was limited to 45 days. The
overall installed cost of the revamp was budgeted at Rs. 250 crores.
Up-gradation of Primary
Reformer
The
Primary reformer was a typical side fired furnace consisting of two staggered
rows of tube containing 120 tubes in each row located inside the radiant
chamber. There were six numbers of heat recovery coil banks in the convection
section of primary reformer. The staggered configuration of the reformer tube
layout caused shadow effect resulting into poor heat flux and inadequate
reforming. There was no scope to increase heat flux further. This difficulty
was circumvented by operating the reformer with high steam carbon ratio leading
to poor energy efficiency. Further flue gas exiting the reformer stack at 250
deg C was also a colossal loss of energy.
The
revamp measures in reformer area were aimed at addressing the above problems. The
following modifications were carried out to reduce steam carbon ratio from 4.04
to 3.2. and to bring down reformer stack temperature to 150 deg C:
·
Single
row of catalyst tubes instead of staggered row for better distribution of heat.
The furnace was extended on one side to add two more collectors to accommodate
required number of tubes. With improved heat flux and effective utilization of
catalyst it was possible to reduce the number of reformer tubes to 168 from
240.
·
Increase
in tube size OD/ID from 143/117 to 152/129 mm
·
Installation
of triple decker catalyst
·
Replacement
of reformer burners by force draught type
·
Replacement
of inlet distributors and pigtails
·
Replacement
of outlet hot collectors and pigtails
·
Modification
of roof, floor and its refractory
·
Installation
of combustion air pre-heater in reformer convection in place of boiler feed
water pre-heater
With
incorporation of above changes, the radiant section of the reformer is now operating
under much less severe conditions as shown in the table below:
|
Particulars
|
Before
revamp
|
After
revamp
|
|
Inlet Gas temperature, deg C
|
545
|
520
|
|
Outlet gas temperature, deg C
|
827
|
792
|
|
Maximum tube wall temperature, deg C
|
892
|
877
|
|
Gas Inlet pressure, Kg/cm2g
|
38
|
33
|
A
considerable drop in tube wall temperature is also expected to give a big boost
to reformer tube life thereby bringing down unscheduled shutdown of the
reformer.
The
old primary reformer was provided with natural draft type burners. These have
been replaced with forced draft radiant wall burners. The use of these burners with
preheated combustion air will increase the energy efficiency of the primary
reformer system. The combustion air preheat coil is located in such a way that
it absorbs low grade heat from flue gases exiting reformer stack. Prior to
revamp this low grade heat was absorbed by boiler feed water heater which has
been shifted to carbon monoxide conversion section for better heat integration
after revamp. Total number of burners provided after revamp is 504.
The
revamp changes have moderated the severity of operation in the convection
section, whereby various coils are now operated much below their design
temperature. It is expected to resolve the problems of repeated maintenance and
downtime. Most of the coils in the convection section were not modified except
for the boiler feed water heater.
Fig – 1 : Schematic of Modified Primary
Reformer
Modification in Auxiliary Steam Superheater
The
auxiliary steam superheater was thermally inefficient as there was no
combustion air pre-heat. Moreover the heat loss through stack temperature was
high as stack temperature was 465 degree C.
In order to integrate the waste heat recovery and to reduce energy
consumption following modifications were carried out to auxiliary steam
superheater:
·
Installation
of coils for preheating of feed gas and combustion air in preheat coil banks of convection section
·
Installation
of 2 combustion air blowers in order to pre-heat combustion air up to 400 deg C
and its distribution to new forced draught burners.
·
Replacement
of natural draught burners with forced draught type
·
Repair
of stack damper to avoid hot flue gas bypassing.
Figure – 2 : Schematic
Diagram of Steam Superheater
|
|||
 |
|||
Process Air Compressor
To
cater to increased requirement of air for secondary reforming as well as to
improve the compressor efficiency, the process air compressor internals were
changed. This job was entrusted to the original equipment manufacturer.
Compressor inter-stage moisture separators were replaced with improved types.
Carbon Dioxide Removal
In
order to improve the energy
efficiency of carbon dioxide removal system single stage flash vessel system in
regenerator section was replaced with 5-stage flash vessel with ejectors
including a mechanical steam compressor. The tower packing was replaced with
IMTP. A hydraulic turbine with generator was also installed on rich solution
line to recover energy. Additionally a DM water preheater was installed at
regenerator over head.
Medium Pressure
Condensate Stripper
The
low pressure process condensate stripper was converted to medium pressure
condensate stripper whereby the steam used for stripping is recycled back to
reformer as process steam. To this extent the steam addition to primary
reformer has been brought down. Further, the condensate quality has also
improved and fed directly to polishing unit.
Modification in
Synthesis Loop
Major
modifications were also carried out in synthesis section that included
installation of S-50 Converter and a loop boiler. The old synthesis gas
compressor was inefficient and prone to frequent downtime. This was completely
replaced with a new one.
Energy Reduction
Benefits
The
various revamp measures have been implemented to bring down specific energy
consumption in the following manner:
|
Sr.
No.
|
Scheme
|
Estimated
Savings
Gcal/MT
|
|
1
|
Primary
Reformer
|
0.63
|
|
2
|
Aux.
Steam Superheater
|
0.08
|
|
3
|
MP
condensate stripper
|
0.25
|
|
4
|
Carbon
Dioxide removal system
|
0.54
|
|
5
|
Other
schemes (Synthesis, turbines, compressors etc.)
|
0.76
|
|
|
Total
|
2.26
|
The
energy consumption prior to revamp was of the order of 11.0 -11.2 Gcal/Mt of
ammonia on sustained load operation. After revamp the energy level of 8.7 - 8.8
Gcal/MT of ammonia is expected on annualized basis. The other consequent benefits
such as improved operability and reliability and reduced downtime will further augment
the benefits of revamp on year to year basis.
Conclusion
Ammonia
production process is highly energy intensive. Given the high cost of energy
any reduction in energy consumption will have impact on lowering down the cost
of ammonia. This ammonia, being an important building block of other products
of RCF Trombay complex, will in turn have significant impact on operations of
the company. Besides being a cost effective investment, the revamp project of
Trombay V ammonia has become a significant technical improvement for RCF
Trombay complex in terms of giving fresh lease of life for aging plants.
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