3.1 - Process and Technologies For Grass-Root Ammonia Plants - EnG

 

 

Paper No. 3.1 

Processes and Technologies for Grass-Roots Ammonia Plants 

by F. Baratto, M. Rizzi, P. Nibioli Ammonia Casale S.A. Lugano, Switzerland

Prepared for presentation at the Casale 3rd Customer Symposium Lugano, Switzerland June 6th – 10th, 2011

 

Processes and Technologies for Grass-Roots Ammonia Plants

FOREWORD AMMONIA CASALE S.A. S.A.   is one of the oldest companies active in the field of synthetic ammonia production. It was established in Lugano (Switzerland) (Switzerland) in 1921 for the industrial industrial development and commercialization commercializa tion of Dr Luigi Casale’s inventions relating to the catalytic synthesis of ammonia. From the very start and throughout its history, AMMONIA C ASALE has been engaged in the construction of new plants: to date over 200 Casale-designed plants have been built worldwide. The company has also acquired an unrivalled track record in revamping (retrofitting) existing plants of any type to improve their performance. More recently, AMMONIA C ASALE’s activities have expanded into the fields of urea and methanol production, and the company company structure has evolved evolved accordingly. accordingly. Today C ASALE is a group of four companies, each with a discrete focus on a principal area of the development of ammonia, urea and methanol production technologies. The distinctive strength of C ASALE  lies in the licensing of its technologies; most which are developed in house by teams of people with highly specialized knowledge and experience. Thanks to the tradition of innovation established by the company’s founder, Dr Luigi Casale and maintained and nurtured by subsequent management teams, C ASALE  has always invested significantly in technology technology development development and still does today. Nowadays, though, though, what was formerly a more or less empirical art married to an intuitive sense for good design has a far more systematic basis. Process design is now supported by sound insight into the chemistry of the processes, behaviour of catalysts, kinetic data, heat and mass transfer phenomena, fluid mechanics, the science of construction materials and cost analysis. The present paper will describe the standard C ASALE ammonia process for grass-roots plants from natural gas and coal gasification, which is the result of a wide experience gained both with revamping and grass-roots grass-roots projects. The description will will go into energy saving technologies technologies and glance at other aspects that are of primary importance for an operating plant such as safety, reliability and environmental impact. Between 1980 and 2000 Casale technology was almost always supplied for plants fed with natural gas. But in 2000 the feedstock choice began began to change dramatically dramatically in favour of coal, and of the 44 synthesis loops that AMMONIA C ASALE has supplied in the last 10 years, with capacities ranging from 300 MTD to 2,050 MTD, 30 were were for coal-based plants. plants. This paper describes describes how the design and behaviour of these plants are influenced by the peculiarity of the gas composition and other site-specific factors, and reports on the experience acquired with these synthesis loops and converters.

NATURAL GAS-BASED AMMONIA PROCESS FOR GRASS-ROOTS PLANTS C ASALE has developed, as a natural evolution of its revamping activity, new technologies for grassroots ammonia, urea and methanol plants.

F. Baratto, Baratto, M. Rizzi, P. Nibioli

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Processes and Technologies for Grass-Roots Ammonia Plants

The Standard ammonia process The C ASALE Standard process for natural gas-based ammonia plants is founded on the classical steam reforming route. The main process steps are, as shown in Fig.1:                   

Desulphurization Primary and secondary reforming HT and LT shift conversion CO2 removal Methanation Syngas drying Compression Ammonia synthesis Hydrogen recovery

Fig. 1 – C ASALE Standard ammonia process

Process de dess cript cription ion The feed gas is desulphurized in two steps: hydrogenation and sulphur adsorption on zinc oxide. The desulphurized gas leaving the zinc oxide beds contains less than 0.1 ppm of sulphur. The gas is then mixed with process steam coming from the process condensate stripper and is heated in the mixed feed heater coil before entering the reformer catalyst tubes, where the gas and steam react in the presence presence of a nickel-based nickel-based catalyst to form hydrogen hydrogen and carbon carbon oxides. The steam/carbon (S/C) ratio is 2.9. The fuel for the main burners is mainly natural natural gas, supplemented with tail gas from the hydrogen recovery unit, flash gas from the CO2 removal section and, in some cases, surplus syngas (for balancing CO2/NH3 production).

F. Baratto, Baratto, M. Rizzi, P. Nibioli

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Processes and Technologies for Grass-Roots Ammonia Plants

The air supply for the reformer burners burners is usually usually heated in a combustion combustion air preheater. preheater. The primary reformer can be supplied by most qualified licensors, based on C ASALE’s process specification and preliminary design. Process gas from the primary reformer is further reformed in the secondary (autothermal) reformer using C ASALE  patented burners. burners. The amount of air is is controlled to produce a synthesis gas mixture at the converter inlet with an H2:N2  molar ratio of exactly 3:1, allowing for additional hydrogen recycled recycled from the hydrogen hydrogen recovery unit (HRU). The effluent from the secondary reformer is cooled in the main waste heat boiler and the recovered heat is used for steam production. The gas leaving the waste waste heat boiler is further further shifted to produce carbo carbon n dioxide and hydrogen. This conversion conversion is accomplished accomplished in two steps, the high-temperature high-temperature shift shift (HTS) and low-temperature shift (LTS), which are both equipped with C ASALE  axial-radial axial-radial internals. The reacted gas leaving the LTS is cooled before entering the purification section. Process air for the secondary reformer is filtered and compressed in a two-case, four-stage machine. Where the ammonia ammonia plant plant is associated with a urea plant, this also also provides the passivation air introduced into the urea plant in the carbon dioxide feed. There are three water-cooled intercoolers with with condensate separators. separators. All of the condensate condensate coming from inter-stage separators is recovered as cooling water make-up. The process air from the final stage of the compressor is heated in the convection zone of the primary reformer and then fed to the secondary reformer. Carbon dioxide is is removed from the process process gas using high-efficiency high-efficiency third-party technology. technology. The CO2 content of the gas at the outlet of the absorber is less than 300 ppm if amine-based solutions are used. The gas then flows to the the methanator feed-effluent feed-effluent exchanger. Rich solution leaving the base of the CO2  absorber is regenerated in a stripper column with a “lean/semi lean” arrangement in which only a minority of the solution is regenerated to the highest standard. The gas leaving the absorber is heated heated to the reaction temperature temperature for methanation methanation in the feed/effluent exchanger. exchanger. The gas exiting the methanator, methanator, now containing containing less than 5 p ppmv pmv of carbon oxides (as CO+CO2), is then cooled and sent to the synthesis gas compression section. The centrifugal synthesis gas compressor has two casings, low-pressure (LP) and high-pressure (HP), and four stages in all, including the synthesis loop recycle compression impeller, which is in the HP HP case. case. All stages are provided with intercoolers. The compressor compressor is driven by an an extraction/condensing extraction/conden sing steam turbine. turbine. All of the hydroge hydrogen n recovered by the HRU is added added to the syngas, either in the second stage of the syngas compressor or at compressor suction. The gas leaving leaving the second stage stage is dried by by washing with with liquid ammonia. ammonia. This washing is performed in the make-up gas ammonia scrubber, which uses a special AMMONIA C ASALE design. The expected water and CO2 content after ammonia ammonia washing are less than 0.1 ppmv. The dried synthesis gas flows directly to the suction of the HP case and is compressed to the synthesis loop pressure. The combined make-up and recycle gas stream from the circulator is fed to the hot gas-gas exchangers. The preheated gas gas then enters the ammonia converter, converter, in which it reacts over an iron-based ammonia synthesis catalyst.

F. Baratto, Baratto, M. Rizzi, P. Nibioli

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Processes and Technologies for Grass-Roots Ammonia Plants

The ammonia converter is typically of a well proven AMMONIA C ASALE  design, containing three adiabatic axial-radial beds with intermediate cooling by heat exchange in two inter-bed exchangers. At the converter outlet, outlet, the product gas is cooled and the heat removed is is used for steam production. The gas next passes through the hot side of the hot gas-gas exchangers and then enters the condensation section, where ammonia is condensed in a water cooler and then in ammonia chillers. A considerable amount amount of ammonia is condensed in these coo coolers lers on account of the high ammonia concentration achieved in the highly efficient AMMONIA C ASALE converter design. Purge gas from the synthesis loop and inert gas from the refrigerant receiver are washed with water in two separate packed columns to absorb their ammonia content. The outlet gas leaving the ammonia absorption section has a low ammonia content (less than 10 ppm) and is fed to the HRU unit, which is normally membrane-based. membrane-based. The ammonia refrigeration section usually comprises a four-stage compressor. Gaseous ammonia from the fourth-stage discharge discharge is condensed condensed and colle collected cted in a refrigerant refrigerant receiver. The cold ammonia product product is exported to the urea plant by a pump. pump. In normal operation it passes passes through a heat recovery section inside the refrigeration section and is then delivered to the urea plant at 20°C. When the urea plant plant is down all the ammonia ammonia produced is DELIVERED to storage at -33°C.

C hara haracterizing cterizing eleme elements nts The main characterizing elements of the C ASALE process are the advanced technologies placed in the key section of the plant:          

• • •

•

•

C ASALE high-efficiency design for the secondary reformer. C ASALE axial-radial technology for the shift conversion. C ASALE ejector ammonia wash system. C ASALE axial-radial technology for the ammonia converter. C ASALE advanced waste heat boiler design in the synthesis loop.

The CASALE high-efficiency secondary reformer design is design is based on the most advanced C ASALE  secondary reformer burner technology, which has been developed, using C ASALE’s deep knowledge of combustion and fluid dynamic phenomena, to achieve very high combustion efficiency with low energy consumption. The C ASALE advanced secondary reformer burner has the t he following features:      

• • •

 

•

   

•

•

Superior mixing in the flame; Low pressure drop in both air and process streams; Homogeneo Homogeneous us gas composition and temperature distribution at the catalyst bed entrance; Reduced flame length, avoiding catalyst impingement impingement even at high operating loads; Low temperature of the burner surfaces exposed to the flames; Protection of the refractory lining from the hot core of the flame itself.

Thanks to the above features of the burner, the C ASALE high-efficiency secondary reformer design minimizes the size of the item, guaranteeing also a high reliability with a long duration of the burner itself.

F. Baratto, Baratto, M. Rizzi, P. Nibioli

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Processes and Technologies for Grass-Roots Ammonia Plants

CASALE  Axial-Radial Technology Technology   is used for the design of both the shift converters and the ammonia converter. In an axial-radial catalyst bed (see Fig. 2) most (about 90%) of the gas passes through the catalyst bed in a radial direction, resulting in very low pressure drop. The balance passes down through a top layer of catalyst in an axial direction, thus eliminating the need for a top cover on the catalyst bed. Mechanically the bed is very simple, being made only of the two vertical perforated walls and of one bottom closure plate. The absence of a top cover greatly simplifies and facilitates the construction of the converter internals. The essential advantages of the axial-radial catalyst bed concept are the same wherever in the ammonia plant it is applied, namely: •

   

•

Low pressure drop; High efficiency thanks to the use of small-size catalyst.

Fig. 2 – 2 – Axial-radial bed

Fig. 2 – Axial-radial catalyst catalyst bed

High efficiency and low pressure drop are important features to minimize equipment size and energy consumption.

Fig. 3 – HT and LT shift converters at Iranian plant  The use of C ASALE  axial-radial technology in both HT and LT shift converters  converters  guarantees, in addition to the above mentioned features, the following advantages:    

• •

 

•

Low average CO outlet concentration. High reliability and longer catalyst life thanks to higher higher resistance to poison poison and water carry over. Longer catalyst life thanks to the fact th that at the pressure drop across the unit remains stable all along the catalyst life.

F. Baratto, Baratto, M. Rizzi, P. Nibioli

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Processes and Technologies for Grass-Roots Ammonia Plants

Before entering the synthesis loop, the syngas is dried using the CASALE Ejector ammonia wash system,, which is uses liquid ammonia to dry the syngas. system C ASALE  ejector ammonia wash system consists of a specially designed ejector, which guarantees a perfect contact between the liquid ammonia and the syngas, followed by a separator, and is, therefore, very simple and efficient, completely removing water and CO 2  from the syngas. The main advantages of this system are the following:  

•

 

•

The en energy ergy consumption consumption of the refrigeration compressor is minimized, as the syngas can be sent straight to the ammonia converter The energy consumption of the circulator is also Fig. 4 – Ejector ammonia wash minimized because of the very low suction system temperature. The ammonia synthesis converter is also based on the C ASALE  axial-radial technology. This technology is combined, in the CASALE  axial-radial ammonia converter,   with a three-bed configuration, with two interchangers, to attain converter, very high thermodynamic efficiency and catalyst volume utilization. The conversion per pass is, therefore, maximized, minimizing the energy consumption of the loop and the size of the loop equipment.  Another important feature in the design of the ammonia synthesis loop is the C ASALE advanced design for the waste heat boiler/HP BFW preheater  that   that is downstream the converter. The C ASALE  waste heat boiler is a U-tube exchanger with the boiler feed water on the tube side side and the process gas gas shell side. The pressure shell shell is kept cooled by the outlet colder gas flow. The only ferritic parts in contact with the hot gas are the tubes, which are cooled by the BFW. With this special design design it is possible to avoid avoid any risk of nitriding.  All the above mentioned characterizing elements used in the C ASALE  Standard ammonia process have been used in many applications proving their efficiency, reliability and long operating life.

Performance Thanks to the characterizing elements explained in the previous sections, the C ASALE Standard ammonia process has very outstanding performance.    

•

•

Fig. 5 – C ASALE synthesis converter F. Baratto, Baratto, M. Rizzi, P. Nibioli

•

 

Steam : carbon ratio (referred to NG stream only): 2.9 : 1 CO slip from LTS: less than 0.3% vol. (dry base) CO2 from absorber: less than 300 ppm vol.

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Processes and Technologies for Grass-Roots Ammonia Plants

   

•

•

Ammonia loop pressure: 140-160 bar Ammonia conversion per pass: ~20%

In addition, total energy consumption (evaluated as feed + fuel + steam import from package boiler  – steam export to urea) is is very low:  

•

~ 6.7 Gcal/MT of produced ammonia

On account of its very high efficiency, the size of equipment needed for a given capacity, and therefore the investment cost, is lower for the C ASALE Standard ammonia process than for other processes.

Recent developments Some additional elements can be added to the standard ammonia process to further reduce the energy consumption of the plant:    

The use of gas turbine exhaust as reformer combustion air (Fig. 6); Low-temperatur Low-temperature e heat recovery (Fig. 7);

 

Cryogenic hydrogen recovery.

•

•

•

Fig. 6 – Turbine exhaust as process air

F. Baratto, Baratto, M. Rizzi, P. Nibioli

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Processes and Technologies for Grass-Roots Ammonia Plants

The gas turbine is is used to drive the process air compressor. compressor. It is thermally more more efficient than a steam turbine, while using its exhaust as preheated combustion air makes use of heat which would otherwiseitsbe dispersed dispersed into the pressure atmosphere atmosphere or used used the in aneed less for profitable profita ble way. Furthermore, delivers exhaust at sufficient to obviate a forced-draught fan for theit combustion air. Using the gas turbine option renders the combustion air preheater at the end of the convection section unnecessary, providing the opportunity for additional heat recovery from flue gas at relatively high high temperature. temperature. Therefore, this system is perfectly coupled coupled with Casale lowlowtemperature heat recovery, the object being to recover as much heat as possible from the flue gas without compromising compromising the safe operation of of the flue gas stack. That is achieved by recovering recovering heat at very low temperature with the use of liquid ammonia.

Fig. 7 – Low-temperature heat recovery  A stream of HP liquid ammonia from the loop separator is heated to about 200°C in the last coil of the primary flueant gas duct. The duct. ammoniar or is then expand expanded ed to about 18 bar a in a machine coupled withreformer the refrigerant refriger ammonia compresso compressor an electricity generator. The ammonia is then condensed at the expander expander outlet with cooling water. This results in a stack temperature of 135°C 135°C,, which is far enough above the dewpoint to allow for safe plant operation. The low-temperature heat recovery system can produce about 75 KWh/MT of NH3. Cryogenic purge recovery is aimed at further enhancing the recovery of hydrogen and nitrogen from the standard hydrogen hydrogen recovery units. units. It should be noted that that cryogenic separation separation is more selectivity towards towards hydrogen but has virtually virtually no impact impact on system pressure. That allows the recovery of about 95% (mol/mol) of hydrogen and 60% (mol/mol) of nitrogen in the purge stream without a significant significant pressure drop. The recovered hyd hydrogen rogen can be reintrod reintroduced uced to the synthesis loop at the circulator suction. Given the fact that synthesis gas is the key element in the ammonia loop energy cost, it is evident that recovering hydrogen as efficiently as possible leads to the greatest benefits in terms of overall loop performance, higher conversions per pass, lower circulating flow rates and thus lower overall consumptions.

F. Baratto, Baratto, M. Rizzi, P. Nibioli

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Processes and Technologies for Grass-Roots Ammonia Plants

Through the introduction of these three improvements, the energy consumption of the plant can be lowered to below 6.5 Gcal/MT or about 6.6 Gcal/MT in case it is desired to balance NH 3 and CO2  for urea production.

Projects and industrial experience C ASALE  has completed and is also currently working on several projects that incorporate the technologies described in the previous sections. The C ASALE  Standard ammonia process is currently operating in a 2,050-MTD plant in Iran operated by Razi Petrochemical Petrochemical Company. The new plant, which has been been designed by C ASALE  and built by the Iranian Iranian contractor Pid Pidec, ec, has been in operation since th the e beginning o off 2008. A second 2,050-MTD ammonia plant in Iran, for Shiraz Petrochemical Company, is at an advanced stage of construction. And recently, C ASALE  has been awarded three more contracts for the design of three new 2,050-MTD ammonia plants in various locations in Iran.

Fig. 8 – Pictures from C ASALE Razi III plant C ASALE’s typical scope of supply for grass-roots ammonia projects from natural gas consists of the basic engineering design of the entire plant; detail design and material supply is limited to the proprietary items. But C ASALE reviews the entire detail engineering work (performed by others) to check for compliance with the basic engineering package and  assists with commissioning and start-up activities, including catalyst reduction, through to parameter optimization and test runs. During this period C ASALE personnel also provide training to the client’s staff operators.

F. Baratto, Baratto, M. Rizzi, P. Nibioli

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Processes and Technologies for Grass-Roots Ammonia Plants

COAL-BASED AMMONIA PROCESS FOR GRASS-ROOTS PLANTS Nowadays most of the world world output of ammonia is produced produced from natural gas gas feedstock. But in recent years a new trend has been established towards the exploitation of different types of feedstock, like coal and petroleum coke, where they are abundant and less expensive than natural gas. China led this revolution, constructing and commissioning a number of ammonia plants based on coal gasification which account for almost 33% of world ammonia production. Chinese ammonia production is, in fact, mainly based on coal: the output of coal based ammonia accounts for 75% of total domestic production. The utilization of these solid feedstocks requires different technological approaches, not only in the preparation of the synthesis gas but also in its use to produce ammonia, mainly because of the composition of the synthesis gas obtained from gasification, which is practically free of components that are inert under the conditions in the ammonia synthesis loop. Thanks to its advanced technologies technologies in ammonia plants, C ASALE was able successfully to enter the Chinese market and has become a major player.

CASALE technical involvements in Chinese projects C ASALE is generally involved in Chinese projects as licensor and technology provider, supplying its proprietary equipment for ammonia and methanol projects in the front end section or in the back end section or both.  As a world leader in the design of converters, C ASALE  has designed and supplied the perfect technologies for the sour CO shift sections in seven coal-based ammonia and methanol plants.  As a consequence of the success of its well-known three-bed, two-interchanger two-interchanger ammonia synthesis converter, in 30 Chinese ammonia projects C ASALE has provided license, basic design engineering package (BDEP) or process design package (PDP) of the synthesis section, refrigeration section and, in some cases, ammonia cryogenic storage section, the proprietary internals of reactor and site services. CO  2 & H  2  S

COA L

GA S I F I E R

 

O

CO S HI F T 

 2

 A S U 

 

 

A GR

N 

N2 WASH 

 S YN LOOP

 2

RE F RI GE RA TI ON   A MMON I A

 A MMON I A

 S ECT I ON 

 S TO RA GE 

 

Fig. 9 – 9 – Coal-based ammonia plant block flow f low diagram

F. Baratto, Baratto, M. Rizzi, P. Nibioli

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Processes and Technologies for Grass-Roots Ammonia Plants

Technical aspects of coal-based ammonia projects  A S A LE 

 S our C O-s hift hi ft ttech echnolog nolog y: C 

 axial-radial converters

The ammonia process requires maximum conversion of carbon monoxide and steam to carbon dioxide and hydrogen, for maximum maximum ammonia ammonia production at at a given syngas syngas capacity. That is generally achieved by carrying out the shift conversion reaction in multiple reactors, (two or three), with intermediate cooling. cooling. Staging is also required because of the high CO content, which entails entails a large temperature temperature rise across the converter, especi especially ally the first bed. bed. Addition of nitrogen nitrogen in the stoichiometric amount occurs in the downstream nitrogen wash unit.

It is clear that the best design of the CO-shift section (and converters) is of great importance for the efficiency and reliability of the ammonia plant, bearing in mind that it features more converters in series. Especially in large plants, it is desirable to be able to guarantee a low pressure drop in the shift section over the entire life of the catalyst so as to minimize the power consumption of the downstream syngas compressor. compressor. Also, the shift converter design must be mechan mechanically ically robust and adequate to the challenging reaction environment. The C ASALE  design for shift converters successfully provides these and other advantageous features, through the application of the renowned axial-radial concept (see Fig. 2). In the axial-radial concept, gas is distributed through the perforated walls of the catalyst canister, which ensures a low pressure drop that remains stable with time as the catalyst ages or deteriorates, as it is unaffected by any caking of the catalyst top layer that might result from entrained water droplets. droplets. Hence, by ensuring a higher higher suction pressure to the compressor through the catalyst life, it achieves a higher production rate. Since the pressure drop is unaffected by the catalyst, there is never any need to change of the catalyst prematurely: the catalyst working life can easily reach and exceed four years, even in the case of the first sour shift converter, which is the one operating in the most severe conditions. Low pressure drop is, in fact, the major feature of the C ASALE axial-radial concept, allowing the converter to be designed with a much slimmer pressure vessel than an axial design would require and, consequently, consequently, a thinner pressure ve vessel ssel wall. This results in significant significant capital cost cost saving, especially with respect respect to the first shift converter converter.. On account of the high op operating erating temperature, temperature, an axial bed must be designed either with a SS cartridge and an annulus for flushing the pressure vessel, or with a refractory-lined vessel. vessel. Instead, the axial-radial axial-radial bed with inward flow, keeps the hot gas at the centre of the converter, away from the vessel wall, so that only the outlet nozzle is exposed to high temperatures, allowing much easier, cheaper and robust construction. The advanced design of the C ASALE  sour shift started to be successful in Chinese coal-based plants with the Tianji project in 2005; these sour shift reactors were inspected during catalyst replacement in June 2009 and no cleaning and/or repairing works were necessary on the internals. The proven and reliable C ASALE  design with its attractive cost-saving and process performance has been persuading further clients to select C ASALE sour shift technology.

F. Baratto, Baratto, M. Rizzi, P. Nibioli

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Processes and Technologies for Grass-Roots Ammonia Plants

Client

C ASA LE Scope of Work

Capacity Capacity [MTD]

Status

RONGXIN CHEMICAL CO., LTD. Erdos City, Inner Mongolia P.R. of China

One shift converter for new methanol plant,sour C AS  ASALE axial-radial

3,000

Start-up 2013

LUAN XINJIANG YILI COAL CHEMICAL INDUSTRYCO., LTD. P.R. of China

Two sour shift converters for new ammonia plant, C AS  ASALE axial-radial

1,000

Start-up 2011

JIUTAI ENERGY CO. LTD. P.R. of China

One sour shift converter for new methanol plant, C AS  ASALE axial-radial

3,000

In operation since 2010

TIANJI COAL INDUSTRY GROUP CHEMICAL CO. Lucheng City, Shanxi Province, P.R. of China

Two revamped sour shift converters and one new sour shift converter for ammonia plant, C AS  ASALE axial-radial

1,500

In operation since 2005

Table 1 - C ASALE Chinese sour shift converter reference list

 A mmonia ssyn ynthes thes is loop des desig ig n The synthesis gas composition from coal gasification is characterized by the low inert content. Typically the argon and methane concentration in the make-up gas to the synthesis loop does not exceed 50 ppm by volume, while the hydrogen and nitrogen concentrations are about 75% and 25% respectively. The synthesis loop therefor therefore e has to be designed to take maximum a advantage dvantage of this favourable gas composition.

Fig. 10 – Ammonia synthesis loop process flow diagram

F. Baratto, Baratto, M. Rizzi, P. Nibioli

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Processes and Technologies for Grass-Roots Ammonia Plants

Because the make-up gas normally has a minimum inert content, there is no loop purge and the flash gas from the medium-pressure separator is recycled back to the suction of the syngas compressor so asinlet to minimize mak e-up to gas consumption. consum ption. The overall lowith w inert the ammonia reactor makes it make-up possible achieve high production rateslow low content recycle at ratios and low catalyst volumes. The synthesis loop is quite simple and not significantly different from the loop designed for natural gas feedstock. It comprises the ammonia ammonia synthesis converter, the hot gas-gas exchanger preheating the reacting gas entering the converter, the heat recovery section, the water cooler, the cold gas-gas heat exchanger, the two chillers, the high-pressure separator (needed to separate the liquid ammonia from the unreacted gas), the medium-pressure separator (useful for recovering dissolved gas in the liquid ammonia), and the syngas and circulating compressor.  According to the overall plant steam system specific optimization, the heat recovery section can comprehend a steam superheater, a waste heat boiler and a boiler feed water preheater or, as alternative, only the waste heat boiler and the BFW preheater, or in unusual cases, only the BFW pre-heater. The amount of heat recovered recovered is generally generally greater than 0.6 Gcal / MT NH3. C ASALE designs normally incorporate a direct connection between the ammonia converter and the downstream exchanger, which is is shown in Fig. 11. (C ASALE  has references for each of the alternatives described described above.) above.) This feature of C ASALE’s layout avoids the need for stainless steel piping which would be exposed to a combination of high pressure and high operating temperature (above 440ºC) 440ºC) that is conducive conducive to nitriding attack. This makes a major contribution contribution to the reliability and safety of the system.

Fig. 11 – Direct connection between converter and waste heat boiler in a Chinese plant

F. Baratto, Baratto, M. Rizzi, P. Nibioli

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Processes and Technologies for Grass-Roots Ammonia Plants

C ASALE usually supplies, as proprietary equipment, the heat exchanger directly connected to the converter, which can be a steam superheater, a waste heat boiler or a boiler feed water preheater, according the particular the disposition disposition of the plant. provide However, C ASALEdesign  is open to meet need to minimizetoinvestment costs and can alternatively a reliable which canclients’ be procured directly by the client and manufactured in Chinese workshops. The steam superheater superheater is typically a horizontal exchanger exchanger (TEMA type DFU). DFU). Exchanger tubes tubes are fabricated in Inconel alloy, and the syngas inlet chamber should be overlaid in the same material to avoid nitriding due to the high metal temperature. Generally the waste heat boiler is an NKU special type, horizontally arranged, with process gas flowing tube side. side. The exchanger can be coupled with the converter outlet if no steam supersuperheater is present. Insulated ferrules in nickel alloy material protect inlet tubes, keeping the hot-end tube wall temperature inside inside the tube sheet below 370°C to avoid nitriding. nitriding. All the parts in contact contact with process gas are in low-alloy low-alloy steel to ensure ensure a higher reliability reliability and a lon longer ger lifetime. Parts in contact with medium-pressure steam are in carbon steel as minimum requirement. In case no superheater and waste heat boiler are installed, the BFW preheater is directly connected to the converter converter outlet. The exchanger is NEU NEU type with hot gas shell shell side and BFW tube side. The HP shell will be in low-alloy low-alloy steel and are therefore operated at lower lower temperature. The tubes are “U” type in low-alloy steel, too.

Fig. 12 – Refrigeration section process flow diagram Generally the ammonia plant refrigeration section is designed by C ASALE  as a "closed-loop" scheme (as shown in Fig. 12), strictly dedicated to the ammonia chillers. chillers. However, C ASALE is fully prepared to customize the refrigeration section in order to meet the specific client requirements about cold duties, export or inert specification specification in the liquid ammon ammonia. ia. This could extend, in cases where there is a stringent specification about inerts content in the ammonia product, to adopting an “open loop” scheme.

F. Baratto, Baratto, M. Rizzi, P. Nibioli

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Processes and Technologies for Grass-Roots Ammonia Plants

 A mmonia conv converter erter des ig n The C ASALE technology for this section of the plant relies on the ammonia synthesis converter with three adiabatic beds beds and two interchangers. interchangers. The three catalytic beds beds have axial-radial axial-radial flow, to minimize the pressure drop. The C ASALE design for the ammonia synthesis converter has already proven its performance and reliability, on the grounds of the following key concepts: ü  ü  ü  ü  ü  ü 

the axia axial-radial l-radial flow in the catalyst catalyst beds ensures optimal distribution distribution of the gas in the catalyst, avoiding any hot spots, and full utilization of the catalyst; the temp temperatures eratures in in each of the beds iis s controlled controlled independently, independently, to keep the converter converter optimized under any operating conditions; the mate materials rials of construction used for the internals, internals, i.e. SS 321 and Inconel Inconel 600, 600, are chosen for their resistance to nitriding and hydrogen attack; all the internal parts are connected with sliding sliding joints, joints, allowing allowing unrestricted unrestricted differential differential thermal expansions, and are free from leakages due to material aging; the pres pressure sure vessel vessel is flushed with cool cool gas, ensuring that it is not subject to embrittlement due to nitriding, which may lead, in the long run, to cracking; the converte converterr effluent is is directly conveyed to the downstream downstream exchanger, exchanger, which which is liplipsealed to the converter, thus eliminating the hot converter outlet pipe, and allowing the use of safer horizontal exchangers.

 Altogether, the ammonia synthesis loop for coal-based plants is not significantly different from that used in reforming plants, except for the better overall performance (lower recycle rate, no purge, higher specific heat recovery). Syngas Quality Specific production Converterr out let pressure Converte Converter P Converter NH3  Converter Tout  

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SR

CG

MTD/m3cat

~ 15

~ 20

bar g

160

140

bar