Innovative Revamping of Ammonia plants for Capacity up and Energy Efficiency

Ammonia and methanol  manufacturing have been one of the most energy intensive chemical  production.  High consumption and   cost of energy, limited availability and need for minimizing  emission  of GHG gas and other pollutants has led to major technological changes  in last 30-40 years. Ammonia and methanol are produced largely from Natural Gas  with Steam reforming and resultant raw  synthesis gas is treated differently to produce pure synthesis gas as per need of both processes. Broad comparision of synthesis gas quality required for  ammonia and methanol is presented     This study paper  highlights  benefits and ways of revamping vintage ammonia and methanol   plants for capacity increase and efficiency improvement. The paper  brings innovative revamp for substantial capacity increase and energy reduction in a gas based ammonia plant  through use of proven process modules like oxygen fired reformer, inert free ammonia loop, ASU for pure nitrogen and oxygen, cryogenic purification/nitrogen wash etc   which have been  successfully used in different flow sheet of ammonia  plants  based on different feed stock  and with higher capacities .

 Ammonia has remained important building block for production of Fertilizers and chemicals. About 80% of ammonia produced is used as nitrogen source in fertilizers like urea, DAP,  MAP, NPK ,Ammonium sulfate/phosphates, ammonium nitrate ,etc , while other 20 % is used in several industrial application such as plastics,  fibre, explosives, hydrazine, amines, nitriles, dyes and intermediates .Ammonia is also used  for environment protection measures in removal of NOX from flue gases. Liquid ammonia is an important solvent and refrigerant .

Ammonia plants and world capacity

Global Annual ammonia production capacity was more than 160 million tons  in year 2015.Major  of the ammonia production facility based on production data of 2015 are located in China, Russia, India, USA ,Indonesia,  Trinidad,  Ukarine, Canada .,Saudi Arabia and middle East. There are several other plants in different countries. There are more than 400 ammonia plants of size ranging from 100  mtpd to 3300 mtpd .Ammonia plants built in eighties and after are in the capacity range of 1000 to 2200 mtpd. Modern plants are being designed in the range of 2200 to 3300 mtpd.

Ammonia production and Energy requirement

Ammonia production plants are based on energy feed stock such as Natural Gas, Naphtha, fuel oil, coal  and other hydrocarbons. More than 80 % capacity is based on natural gas while several small ammonia plants based on coal as a feed stock are working in China. Many naphtha based ammonia plants are either switched over or in the process of switch over  to Natural Gas. Few ammonia plants based on fuel oil are switched over to natural gas. Flow sheet of  conventional NG based ammonia plant is shown on diagram  I

                                                                Diagram I:   Flow sheet of conventional Ammonia plant     

  

Source :Industrial Efficiency Technology Database Ammonia    ietd.iipnetwork.org

Ammonia production technology  has been one of the most energy intensive technology and it has matured over the years due to development of newer catalyst and CO2 absorption solvents, improved metallurgy for high temperature and pressure requirement, improved separation systems mainly cryogenic purifications, intensive process integration , development of efficient compression and drive systems etc .  Modern Ammonia technology available from reputed  technology suppliers  are quite competitive in terms of reliability, operating cost, energy efficiency. Excellent review of these technology is presented by R K Aggarwal, S Banerjee, R M Maliya

(reference 1,2)

As per IFA  2008-2009 summary report on energy efficiency and CO2 emissions  in ammonia production, energy use for best available technology (BAT ) for natural gas based plant is 28GJ per t ammonia and energy use by best practice technology (BPT) is 32 GJ per t ammonia. Actual  average energy consumption  in various countries  across the globe  is in the range of 34 to 43.6 GJ per t ammonia. Such variation is due to  capacity, plant age, technology and feed stock. This brings out immense energy saving opportunity by way of plant revamp/technology modernization. Use of BAT would result into 25 % energy saving and reduction in green house gases by 30 %. For plants already in operation  energy efficiency optimization can be achieved through integrated infusion of newer technology concepts in existing plant  (Reference 1, 3 )

Efforts to improve energy efficiency:  As mentioned earlier several improvements in the technology has resulted into incremental improvement in energy consumption per unit ton of ammonia produced .These are  :

• Development of catalyst, improved material/metallurgy, improved absorption solvents, equipment design, efficient rotating equipment
• Reduction in Primary Reformer Duty, Part of reforming at lower temperature in Pre-Reformer
• Utilizing Secondary Reformer outlet gas heat for reforming.
• Increasing Conversion share of Secondary Reformer by excess air and recovery of excess Nitrogen downstream.
• More efficient Burner tips to reduce excess air, Insulation / refractory for Furnaces, equipment and piping. More Heat recovery from flue gases.
• Superior Primary Reformer Tube Material with lower thickness and higher catalyst volume
• Steam superheating by Process gas in place of using other furnace.
• Supply Compression Energy directly by Gas Turbine and exhaust to HRSG or Reformer Furnace.
• Use of Vapor Absorption for cooling GT / Compressor suction. Reducing Loop Pressure Drop.
• Power Recovery Unit for High pressure gases let down, Hydraulic Turbine  for liquid   let down.
• More efficient machinery and Advance Instrumentation and control
• Lower Process Steam (requires improved catalyst and lower heat requirement for CO2 Removal System).
• Use of Energy for Process Condensate treatment for generating Process Steam.
• Feed Gas Saturation with Process Condensate to reduce treatment and process steam requirement.
• Lower Energy for CO2 removal. Improved solution activators and efficient packing
• More steam export makes available additional DM water and BFW water more better Heat recovery.
• 2 stage Ammonia synthesis
• Improved ammonia refrigeration system
• Isothermal single stage CO shift reactor
• CO shift and ammonia synthesis reactor with axial-radial design to reduce pressure drop
• Additional and improved Catalyst at various steps.
• Reduce steam to carbon ration in PR . Such reduction in S/C ratio and oxygen firing will  reduce pressure drop   between primary reformer and CO2 removal
• Modify present CO2 removal to MDEA for energy efficiency, low CO2 leakage and any environmental issues

It is pertinent to note the positive effect of these changes on  specific energy consumption for ammonia during last 40 years as shown diagram II.

 Diagram II: Energy Learning Curve for Ammonia

           Source: Chaudhary 2001,PSI 2004

Several of above changes are proprietary  in nature and require careful integration with existing process flow sheet . Implementation of any of above specific improvements for capacity up/energy reduction  can be thought of as per site study , present performance, cost of natural gas, alternative supply etc Revamp invest is detailed out in   FEMA 2005 by Rafiqual  et al(3 ) ,which is summarized as follows

Retrofit measure	Average improvement GJ/T	Range GJ/T	Uncertainty parameter  %	Cost  €per t/year
Reformer large improvement	4.0	±1.0	17	24
Reformer moderate improvement	1.4	±0.4	20	5
Improvement CO2removal	0.9	±0.5	33	15
Low pressure synthesis	0.5	±0.5	67	6
Hydrogen recovery	0.8	±0.5	50	2
Improved process control	0.72	±0.5	50	6
Process integration	3.0	±1.0	23	3

Drivers for Ammonia plant Revamp for capacity and energy efficiency 

1. Need for improving energy efficiency :Vintage plants built in years1970- 1990 were in capacity range 400 to 1350 MTPD. However they    are energy in efficient and consumes  8 to 9.5 Gcal per MT ammonia as compared to modern plants in capacity range of 1350 to 3300 MTPD  with energy consumption 6.5 to 7.5 Gcal/mt
2. High energy cost in India, SEA Countries and other Asian AND European countries provides opportunity to replace/modernize inefficient old compressors for reliability

4   Strict pollution norms and need for reduction in GHG emission

5   Additional ammonia capacity at incremental cost

6  Lower time for implementation  and cost effective revival of old/idle plants

7   Lot many successful examples of revamp across the globe ( more than 150)

This review paper will examine few important  concepts for increasing capacity and energy efficiency in vintage  ammonia plant

Case study  1 :Use of Chilling to improve ammonia production

For incremental improvement and energy efficiency, chilling provide option that avoids expensive upgrades of compressor system which are in many cases limiting both capacity and energy efficiency. This concept has been applied in several ammonia plants for multiple benefits of higher production, reduced power consumption and in some cases saving of costly steam which otherwise is required for vaporizing ammonia  by a consumer downstream fertilizer production unit. Few examples are

1. Addition of synthesis gas chillier at suction of synthesis gas compressor of 450 & 500 MTPD ammonia plants of GSFC Vadodara originally  built in years 1967 &1969 respectively. It helped in increasing synthesis gas compressor capacity by  @8 %.These chillers were generating about 4 mt/hr of vapour ammonia which was sent to  DAP/AS plants about 1 km away by a pipe line thus avoiding steam consumption of about 2 mt/hr otherwise required . Later on, this vapour ammonia network was connected with other ammonia vapour generator units , thus avoiding compression of vapour ammonia generated  in the ammonia and other plants . This is one of the earliest  chilling measure in fertilizer industry  to increase ammonia plant compressor capacity (1,5 )
2. Indo Gulf Fertilizers   debottlenecked the process air compressor of their 1520 mtpd ammonia plant by proving suction chilling. IGFC utilized   Vapour Absorption Machine  VAM to  cool atmospheric air to 15©.( 6 )
3. Concept of Multistage Integrated Chilling MIC to increase ammonia production is presented by Kinetics Process Improvements MIC process modification is  a staged thermal coupling of ammonia compression system with the process air compressor. It is reported that the MIC scheme along with other measures provides the potential to achieve incremental increase in ammonia capacity and energy efficiency improvements up to 15 % with no modifications in process air compressor, synthesis gas compressor and ammonia compressor(7)

To  summarize , following chilling sources can be explored  within ammonia complex

1. Use of generated vapour ammonia in the complex itself in any ammonia consuming plant/section
2. Use of VAM using heat energy from flue gases (in temperature range of   125 to 180 ). It could be hot flue gas in convection section of primary reformer or in HRSG if GT is installed in the complex
3. Low pressure steam otherwise getting vented in the complex or low     level heat up stream of CO2 removal unit
4. Use of existing ammonia refrigeration capacity

Case study  2 :Use of Gas Turbine for process air compressor/power generation;

Several old ammonia plants have been designed with either electric driven  process air ,ammonia refrigeration compressors and in few cases  the synthesis gas compressor also electric driven. Such plants are designed with import of power and  excess steam  is exported .The plants which are designed with steam driven process air and synthesis gas compressors usually  import power and steam from typical off site boiler/power house or state electricity grid . Such plants using steam for power production or import power ,  do  have very low energy efficiency  as more than 50 % of energy contained in steam is transferred to cooling water through condensation and losses. Such cases where both steam and power is required , use of Gas Turbine  provide an energy efficient solution. This concept is practiced in new plants in several ways  such as  (a)  provide a gas turbine to run process air compressor and utilize exhaust gases (at around 500 ©)as combustion air in primary reformer. With this arrangement it is  possible to achieve  high efficiency of the order of 90 %. However this  concept can be considered at design stage itself. In revamp situation this would call for several major modifications . (b) Alternatively exhaust gases can be sent to Heat Recovery Steam Boiler (HRSG ) to generate high/medium pressure steam to meet steam requirement of ammonia-urea plant and excess steam can be used to produce power . This can be applied to new design as well as existing plants

As ammonia plant is considered to be power house producing large steam/power, it is essential to optimize the steam and power balance and driver selection for optimum energy consumption. For a typical 2500 MTPD Ammonia and 3850 MTPD urea  project , developmental work was carried out  to estimate energy reduction with different drive system ,which is summarized on diagram IV

                                                    Diagram IV

                             Effect of Energy consumption with different Drives                 

Serial Number	Driver concept	Energy reduction Gcal/mt urea
1	Gas fired boiler and all machines steam turbine driven	Base case
2	Air compressor by gas turbine and rest all steam turbine driven	-0.26
3	All power and steam from GT+HRSG+Steam Turbine	-0.45

For 500 MTPD Ammonia and 800 MTPD Urea plant based on conventional technology, potential for  energy saving with GTR+HRSG was evaluated as shown on diagram V

Diagram V

Power Generation :Conventional  vs GT+HRSG

Particulars	Present Design	With GT +HRSG
Type of drive and source of power	conventional power plant	power and steam from GT+HRSG
Net power MW	23	23
Net steam for urea mt/hr	50	50
Fuel consumption for steam and power NM3/HR	13190	10567
Reduction in energy use  mmkcal per mt urea	base	-0.65
Savings  $ per year @NG cost 5 $ per mmbtu	base	3.9
Estimated cost of GT+HRSG  million $	base	18

 Many ammonia-urea plants  operating with import of power are  in the process of implementing this concept. RCF-Trombay is implementing GT+ HRSG concept .At present they are importing 40.7 MW power from state distribution and 184 mt/hr of steam is generated in gas fired boiler. Implementation of this concept of GT +HRSG is expected to result into energy saving of 86.5 Gcal/hr for Trombay complex .(8 ).IFFCO Phulpur is also planning to implement GT +HRSG for energy efficiency .

Case study 4  :Application of Pinch Analysis

Pinch analysis initially  developed to optimize energy usage in a new design  can equally be applied to analyse and improve the process in a retrofit situation. It is focused on improving the manner in which way hot and cold utilities (flue gas, steam, process/steam condensate, cooling water , refrigeration, etc )  are utilized to serve needs of the process .In retrofit situations, constraints by existing equipment to achieve optimum use of energy can be overcome through improvement in process –utility interface. Major steps of pinch study are  obtain relevant data on existing process configuration, generate targets for each relevant utility, identify in efficiency in existing heat exchanger network, identify possible process modifications to reduce energy use and decide viability of modification for implementation. To illustrate ,detail pinch analysis of an ammonia plant of about  1000   tpd  has been presented and documented by Natural Resources  Canada (9). This study brought out  several process improvements  with primary benefits of   energy use reduction by 2 GJ/T Total steam  demand reduction of 30 t/hr, shut down of package boilers  and about 11 % reduction in NOx and CO2 emissions. Simple pay- back period of  required modification was 1.5 years at a gas price of 6 can$ per GJ.  The study also brought out other retrofit opportunity  for cost and energy savings .

Case study  5 : Innovative revamp using proven process modules

While increasing production capacity  in existing ammonia plant, major hurdles/bottlenecks faced vary from plant to plant and it is  also dependent on type of technology put to use, design  margins applied during installation, maturity of technology, site specific changes  such as raw material specification ,etc. Major limitations encountered by operating group are

• Limitation of primary reformer viz highest operating temperature, catalyst volume and limiting pressure drop
• Limitation of process air/synthesis gas compressor
• High pressure drop between reformer and synthesis
• Limitation of ammonia synthesis unit

Several successful capacity revamp have been reported with application of technological improvement such as  pre-reformer, use of heat exchanger type reformer, cryogenic purification ,up-gradation of ammonia synthesis . This case study describes the concept of revamping a vintage ammonia plant  through use of modular technology applied in several ammonia and methanol plants. This concept for revamping of conventional ammonia plant  is based on following changes in the flow sheet through add on “ process modules “

1. Convert existing secondary reformer to oxygen firing. This requires new oxygen burner and refractory rework of SR vessel
2. Optimize oxygen addition to modified SR to match required additional capacity and minimize   loading of primary reformer
3. Add dedicated ASU for  pure oxygen and nitrogen.
4. Modify present CO2 removal to MDEA for energy efficiency, low CO2 leakage and eliminate   environmental issues
5. By pass existing  Methanation section to avoid hydrogen loss and extra methane generation
6. Add  cryogenic  purification /Liquid Nitrogen wash   for drying and purifying  synthesis gas and hydrogen/nitrogen ratio control .

With above modification/addition the revamp flow sheet will have features as shown in the diagram  VI and Table  VII

                                                               Diagram  VI

                            Ammonia Revamp using proven process modules

Table   VII

Major impact of Process  changes

Section	Changes	Benefits	Remark
Primary reformer	Lower steam /carbon ratio	Lower pressure drop, higher throughput	Can be implemented independently after process simulation and steam balance
Secondary reformer	Oxygen firing in place of air  firing	Increase in throughput, flexibility to add NG directly to Secondary Reformer  for extra capacity	Existing SR Vessel can be used with new refractory and oxygen burner
HT shift reactor  & LT shift reactor	No changes		check adequacy of catalyst volume/life
CO2 Removal	Revamping for lower CO2 Leakage and energy consumption	Less consumption of steam	Utilize excess low level heat for chilling, DMW heating
Methanation	Bypassing	Saving of hydrogen and reduced methane generation	Reduces pressure drop
Cryogenic purification/nitrogen wash	New section to remove all impurities from synthesis gas  and ratio control	Generation of inert free synthesis gas	Purge gas from this section can be used as fuel
Ammonia Synthesis	As per capacity requirement	Loop working with pure hydrogen: nitrogen will result higher capacity thus avoiding major modifications	Inert free ammonia loop

Introduction of above modular changes in the flow sheet along with all feasible changes like reduction in   steam to carbon ration in  primary  reformer, heat recovery measures  will help in creating positive impact for capacity rise of the order of 15 to 20 % with significant reduction in energy consumption .It may be mentioned here that so far no such revamp with all the  above features has been implemented. However each of the modules such as oxygen fired reformer(or Auto Thermal Reactor ), cryogenic purification, improved CO2removal , Air Separation Unit for pure oxygen and nitrogen, inert free ammonia loop are operating successfully in different ammonia plants based on natural gas, fuel oil, coal  and   Natural Gas  based methanol  plants  of  higher capacity .With good design and process integration of newer process modules, these benefits can be obtained from a revamped ammonia plant .

Concluding Observations

High cost of natural gas, need of additional production requirement and environmental issues are prompting   ammonia   operators to continuously strive for newer improvements in the process. Various case study such as chilling for increasing compressor capacity, use of gas turbine , process simulation for heat integration and use of innovative proven process modules  such as conversion from air to oxygen firing, ASU for pure nitrogen and oxygen, cryogenic purification/nitrogen wash , inert free ammonia loop will help in substantial improvement in capacity and energy efficiency of old ammonia plant.  This can be achieved  by  better understanding of the process by owners themselves and  process intervention with suppliers of such process modules to develop integrated revamp  .

References and  literature cited

1. A Modern Intervention for Ammonia Plants : Harshad P Pandya,” World Fertilizer Magazine” May-June 2017 ,p 34-38(edited version on innovative improvements in Ammonia plants)
2. Ammonia Plant Technology options and KRIBHCO’s operating experience on  KBR  Technology :  R K Aggarwal, S Banerjee, R M Maliya   Indian Journal of Fertilizers  72-87 ,December 2015
3. Industrial Efficiency database, Ammonia: Technology Resource and Benchmark ;  ietd.iipnetwork.org
4. Rafiqul , I. , C. Weber , B. Lehmann , and A. Voss , 2005 : Energy efficiency improvements in ammonia production – perspectives and uncertainties. Energy ,30 : 2487 – 2504.
5. TERI Report on GSFC Energy conservation Efforts
6. Debottlenecking of process air compressor of Ammonia plant by suction chilling :  CK Datta , Anand K Gupta ; Indian Journal of Fertilizers 23-24 ,July 2007
7. Use Multistage integrated chilling to increase ammonia production V K Arora .Hydrocarbon Processing 39-46, April 2015
8. Reports  from www.rcfltd.com and  public domain on energy conservation measures in RCF Thal plants