Revamp of Ammonia Plant at RCF Trombay

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

	AG Preheater

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.