ENERGY SAVING IN AMMONIA-UREA COMPLEXES

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ENERGY SAVING IN AMMONIA-UREA COMPLEXES Plant revamping is an effective way of raising the performance and reliability of an old plant to keep it competitive with new builds, entailing limited investment, relatively quick project execution and minimal plant downtime. Most of the production cost of fertilizers is attributable to the feedstock, the price of which varies according to its type and local availability. Producers in areas such as Russia, North America and the Middle East, which have domestic supplies of feedstocks like natural gas, have the advantage of lower production cost in comparison with countries that depend on imports, as is the case in India or European countries. Many plants depend for their survival on improving efficiency to make them competitive with more recent plants built according to the most up-to-date principles. Casale has developed technologies and know-how to increase the efficiency of the most energy-consuming sections of the plant. MODIFICATIONS IN THE AMMONIA PLANT Different modifications can be done to improve the energy-efficiency of an ammonia plant, concentrating on reducing specific NG consumption, steam import from the utilities and electric power consumption. In the following paragraphs there is a description of the main options for reducing the energy consumption of ammonia plants.

Reformer convection section

Being the most expensive and most energy-intensive single item of an ammonia plant, the primary reformer has a strong impact on the overall plant performance and it is important that is operated and maintained under the best possible conditions to achieve peak performance. Casale has been engaged in revamping these furnaces for many years; to date it has modified 14 units. When the stack temperature is high, a very effective way to improve reformer efficiency is to install a combustion air pre-heating system. However, this modification is often discarded on account of the structural impact that it could have on the existing primary reformer and its inherent cost. A possible alternative means of recovering the heat available in the flue gas is a saturation tower, provided with a new coil installed in the last part of the primary reformer convection section.

Fig. 1. Simplified PFD of the new saturation section



The process condensate stream fed to the saturation coil is directly taken by a new pump from the knock-out drum located upstream of the CO2 absorber and routed, after pre-heating, to the new coil. The desulphurized natural gas is first cooled in a saturator feed/effluent exchanger, then is fed to the new saturator tower, where it is counter-currently washed by the pre-heated process condensate. The steam-to-carbon ratio downstream of the saturator is between 0.5 and 1.0 mol/mol, providing a corresponding saving in MP steam. The new scheme reduces the stack temperature to 190-195°C and the associated energy saving is typically about 0.15-0.2 Gcal/MT. This modification can be typically done in a normal turnaround (less than 30 days), since most of the units can be installed while the plant is running.

Secondary reformer burner

Casale has developed a new type of burner specifically for the secondary reformer. Its shorter flame length and more even gas distribution improve the operating efficiency of the secondary reformer and reduce methane slip; consequently, the inert content in the synthesis make-up gas is lower. Casale normally recommends replacement of the burner where the objective is capacity increase to improve the performance of this unit and to avoid flame impingement on the catalyst layer. In addition, the Casale burner imposes a lower pressure drop than the original one on both air and gas sides.

Fig. 2. Casale “trumpet” secondary reformer burner in workshop

At the moment Casale has 26 secondary reformer burners on stream. As an example, a Casale burner was successfully installed in a Kellogg ammonia plant located in the People’s Republic of China. Since its start-up in March 2008 this burner has been operating satisfactorily according to design and has reduced the methane slip at the outlet of the secondary reformer from the 0.45% (dry basis) obtained with the previously installed burner to 0.29% (dry basis). The low methane slip and the temperature reading in the catalyst bed and at the reformer outlet are a clear indication of the uniform temperature at the bed inlet.

Exchanger reformer

Another technology to debottleneck primary reformers is to incorporate a high temperature heat-exchange reactor that uses the residual heat in the effluent from either a primary or a secondary reformer to produce hydrogen from methane (steam reforming). This high-grade heat (usually at a temperature of about 900°C) is, in conventional plants, used in the waste heat boiler to produce HP steam. Casale has access to the high temperature heat exchange reactor technology of TechnipFMC, named TPR® (TechnipFMC Parallel Reformer®).



The aim is to increase the reforming capacity of the plant or to reduce the fuel consumption. In other cases, the target could be to open chances for energy substitution, by reducing the steam production of the plant. Three scenarios can be depicted:

• CHEAPER FUEL: steam saved is produced in an auxiliary boiler fed with coal or any other fuel cheaper than NG.

• IMPORT: instead of driving the machines with steam turbines, an electrical motor is used, and the electrical energy required is imported from the grid.

• COMBINED CYCLE: similarly, to the previous solution, in this case the electricity, instead of being imported, is generated with a more efficient cycle (e.g. a combined gas turbine-steam turbine power cycle or a bottoming cycle), thus reducing the overall energy consumption of the plant.

TPR® has a so-called 3 nozzles configuration (“Two-in, one-out”), as shown in figure 3, with open tubes and one tubesheet. TPR® is used in parallel with primary reformer: the mixed feed is split in two parts, one of which is fed to the TPR®. The parameters, such as T and S/C, of the feed to the two reformers can be adjusted independently.

Fig. 3. TechnipFMC Parallel Reformer®

Technip commercializes TPRR as a revamp solution for H2 plants, and has 6 applications in the last 15 years, with a capacity increase ranging from 20 to 30%. Other 3 will come on-stream in the next years.

Primary waste heat boiler

One of the latest modifications developed by Casale is replacement of the old-fashioned bayonet tube waste heat boiler, located downstream of the secondary reformer, with a brand-new double-pipe waste heat boiler offered in co-operation with Arvos (formerly part of Alstom); this design has been successfully used by Arvos for the last 50 years. The purpose of this replacement is to solve the mechanical issues which led to frequent failures in the old bayonet tube boiler and to improve control of the gas temperature at the inlet of the downstream high-temperature shift converter, even at high plant loads. These new boilers overcome the previous limitations and provide safer operation of the plant.



Fig. 4. Old bayonet type waste heat boiler replaced by double-pipe boiler from Arvos

Shift converters

The design developed by Casale is based on an axial-radial catalyst bed. Catalyst beds laid out in the new axial-radial configuration have an inherently low pressure drop which also remains stable over time, and this makes possible the use of a small-sized catalyst, which is more active and more resistant to poisons. Catalyst life is extended, and it is possible to achieve a lower CO slip or a significant increase in capacity in existing plants. Casale axial-radial technology in both HT and LT shift converters guarantees, in addition to the above-mentioned features, the following advantages:

• Protection of the catalyst from water droplets carried over from the secondary reformer heat recovery train or elsewhere

• Loading different volumes of catalyst with no mechanical modifications

• Easy operation (same as axial design). In an existing plant, the shift converters can easily be transformed to the axial-radial design by installing new vertical perforated walls, which are cylindrical and form the inlet and outlet walls of the catalyst bed. They are introduced in prefabricated sections, which are assembled inside the existing converter vessel. Casale successfully installed HT shift axial-radial internals as part of a revamp of three ammonia lines in the same complex in India, undertaken in 2013. In this case the pressure drop was reduced from 1.2 kg/cm2 with the previous axial design to less than 0.5 kg/cm2 with the Casale design. At the moment Casale has 27 revamped shift converters on stream.

CO2 removal section

According to the design technology of the section, different strategies can be applied to improve this part of the plant. To date Casale has modernized 21 CO2 removal sections world-wide.



1. Hot potassium carbonate systems

For hot potassium carbonate systems, Casale has a good collaboration with Giammarco-Vetrocoke and can implement the GV low-energy process (see Fig. 8) to improve the CO2 removal section. In this way the regeneration consumption is decreased to 650-750 kcal/Nm3°CO2.

Fig. 5. GV Low-Energy CO2 removal scheme. The GV low-energy process is a two-stage scheme with two separate strippers working at different pressures – a high-pressure (HP) stripper and a low-pressure (LP) stripper. Part of the rich solution from the absorber (about 60% of the total circulation) is introduced into the top of the HP stripper, while the remainder, let down in pressure by about 1 kg/cm2, passes into the LP stripper. The pressure difference between the HP and LP strippers is such that sufficient flashed steam is produced to strip out the CO2 from the rich solution fed to the top of the LP stripper, achieving the same quality as the semi-lean solution withdrawn from the HP stripper. As a reference example, this scheme was successfully applied in the three ammonia lines of a single complex in India, which was revamped in 2013 with a saving of about 0.2 Gcal/MT of ammonia. Other applications are in Russia, Romania and Ukraine. As most of the work can be done while the plant is in operation, the modification can be accomplished in a normal turnaround. Even easier is the case in which the plant is already equipped with two solution regenerators.

2. Amine-based systems

When revamping amine-based CO2 removal units, Casale mainly co-operates with BASF. To improve the efficiency of this section it is normal to install an additional LP flash tower, which adds equilibrium stages to the regeneration section, saving reboiling duty on the process and steam reboilers.



Another modification aimed at reducing the specific energy consumption of the CO2 removal section entails replacing the existing shell-and-tube rich/lean solution exchanger with a new plate-and-frame exchanger. The main advantage is the better temperature approach between the hot and the cold side. The pressure drop on the hot side is also somewhat lower in the plate heat exchanger, which improves the operation of the solution pump (on account of the higher net positive suction head).

Fig. 6. Improvement of amine-based CO2 removal section

In summary, rich solution coming from the CO2 absorber is first depressurized in the LP flash tower, after which partially regenerated “semi-lean” solution is pumped to the existing regenerator, where it is completely regenerated thanks to the energy supplied by the bottom reboilers. The calculated energy saving resulting from the installation of the LP flash tower and the rich/lean solution plate-and-frame exchanger is about 0.2 Gcal/MT of ammonia, as proved in a modification of this kind in Central America.

3. Solution swap from hot potassium carbonate to amine-based solution

This modification provides potentially greater energy savings but requires much more extensive modifications and investment. Owing to the bigger solution circulation flow and the different physical characteristics of the solution, the lean and semi-lean solution pumps will probably have to be replaced completely. New HP and LP flash vessels will be required as well, while the existing absorber and stripper can probably be reused after internal revamping (depending on the size of the existing columns). Final energy savings can be as high as 0.4-0.5 Gcal/MT of ammonia. This scheme is under implementation in four ammonia plants in India.



Synthesis loop

1. Ammonia washing unit (AWU)

An ammonia washing unit is a device installed between the stages of the synthesis gas compressor to remove water from the make-up gas (MUG). Part of the liquid ammonia coming from the HP separator is injected into the MUG from the LP stage of the synthesis gas compressor in a patented Casale ejector. The pressure difference between the HP separator and the third stage of the synthesis gas compressor is sufficient to drive ammonia through the ejector without any additional pumping. The dehydrated synthesis gas is then fed to the high-pressure stage of the compressor, while the liquid ammonia containing water removed from the gas is sent to the let-down separator. In many plants the make-up gas is introduced into the synthesis loop upstream of the ammonia chiller, so that its moisture content is condensed and absorbed into the liquid ammonia. The AWU removes the water from the synthesis gas before it ever enters the loop, so the make-up gas no longer needs to be chilled before it is sent to the ammonia converter.

Fig. 7. Typical Casale AWU installation

Casale has 14 references for ammonia wash units. With this modification the energy saving achievable is typically about 50,000 kcal/MT of ammonia. This is a quite simple modification, involving very few items, and has an interesting simple payback in the range 3-5 years, although that is heavily influenced by the cost of energy.

2. Synthesis converter

The ammonia converter is one of the most critical items when planning a revamp for either energy saving or capacity increase, and in most cases it is the first item to be revamped thanks to the relatively low cost and very high return. Casale is very active in this field and has introduced fundamental innovations in converter design and revamping. Most recently it has introduced to the market the next generation of ammonia reactors, so-called pseudo-isothermal plate-cooled converters. Casale’s Isothermal Ammonia Converter (IAC) replaces the commonly used multiple adiabatic catalyst bed design and offers higher per-pass conversion. The design is based on the use of cooling plates, directly immersed in the catalyst, to remove heat continuously as the reaction proceeds. Four of these converters are already on stream. The most recent was installed in a plant in North America in 2013 and brought an energy saving of about 0.3 Gcal/MT of ammonia with respect to the previous design at end-of-life conditions.



Fig. 8. Isothermal revamp of an adiabatic bottle-shaped ammonia converter



MODIFICATIONS IN THE UREA PLANT With a large number of revamping projects successfully carried out in the last decades, Casale is

now the world leader in urea plant revamping, with a reference list comprising more than 150 plants

revamping ranging from replacement of single equipment to massive increase in capacity. The same

approach is being adopted for the recently acquired melamine technology where this concept was

never investigated.

Clients may benefit from the wide experience that Casale has achieved through the revamping of

urea plants of any technology, ranging from stripping plants (i.e. CO2 stripping and Ammonia

stripping) to total recycle ones. Each technology has different peculiarities which need to be

approached in a specific manner. In view of that, for each process scheme Casale has developed a

set of tailored technologies in order to fulfil the required aims of the revamping for the best return.

The more representative revamping schemes specifically for major urea technologies are detailed

in the next paragraphs.

CO2 STRIPPING PROCESS

In CO2 stripping plants urea is synthesized at a pressure of 140 bar (gauge pressure is assumed

within the entire paper), a temperature of 180-185°C and a molar NH3/CO2 ratio of 2.95-3.0 in the

reactor. Carbon dioxide conversion under such conditions is 58-60%. The reactor effluent, containing

unconverted ammonia and carbon dioxide in the form of ammonium carbamate, is subjected to a

stripping operation at essentially the reactor pressure, using carbon dioxide as the stripping agent.

The stripped-off ammonia and carbon dioxide are then partially condensed and recycled to the

reactor. The heat produced by this condensation is used to generate 3.5 bar steam. The unreacted

carbamate from the stripping section is thoroughly decomposed in a decomposition stage operating

at 3 bar and is subsequently condensed to form a carbamate solution, which is recycled to the 140

bar synthesis section. Further concentration of the urea solution leaving the 3 bar decomposition

stage takes place in the evaporation section.

1. Split Flow LoopTM and Full CondenserTM: boosting capacity/efficiency

The Split Flow Loop™ process is an improved CO2 stripping process. A minor portion of the

gas flow leaving the HP stripper (sufficiently large to maintain the correct heat balance and

provide the amount of passivation air needed in the reactor) is routed directly to the reactor,

while the remainder is sent first to the carbamate condenser and then, after separation of the

carbamate solution, directly to the inert scrubber. In this way the volume of inert gases

passing through the reactor is reduced, which improves CO2 conversion efficiency because

inert gases are detrimental to CO2 conversion).

The Split Flow LoopTM scheme (see fig. 9) is provided in combination with Full

CondenserTM, Casale's proprietary design of HP carbamate condensers for CO2 stripping

plants. In contrast with traditional designs, it operates as a submerged condenser, which is

more effective at carbamate formation. Due to its high efficiency, it would be a total

condenser, were it not for the inert gases, which preventing complete conversion of the

reactant.

Thanks to its performance, the combination of Full CondenserTM with Split Flow LoopTM is

a very powerful tool for debottlenecking CO2 stripping plants with minor modifications. A

traditional CO2 stripping plant is actually transformed into a Split Flow LoopTM / Full

CondenserTM process rather easily by:

- Addition of specifically designed internals to the existing HP carbamate condenser.

- Modification of a few lines to implement the new process flow scheme.



Fig. 9 - Split Flow LoopTM process flow diagram

In combination with other Casale technologies such as High-Efficiency Trays in the reactor,

the Split Flow LoopTM / Full CondenserTM configuration can be used to debottleneck CO2

stripping plants for a very low investment. In combination, those technologies can increase

the HP loop capacity by as much as 30-50%.

Several plants have been revamped based on Split Flow LoopTM technology which is

nowadays a consolidated process scheme for CO2 stripping urea plants. It is a mature

technology with easy operation during normal conditions as well as during start-up/shut-down

phases.

AMMONIA STRIPPING PROCESS

In the ammonia stripping process the synthesis section is operated at slightly higher pressure than

in CO2 stripping plants. In fact, ammonia and carbon dioxide are converted to urea via ammonium

carbamate at a pressure of 160 bar and a temperature of 185-190°C. A molar ratio of 3.2-3.4 is

typically maintained in the reactor, achieving a CO2 conversion of up to 62%. The urea-carbamate

solution enters the stripper, where a large part of the unconverted carbamate is decomposed by the

stripping action of the excess ammonia. Residual carbamate and carbon dioxide are recovered

downstream of the stripper in two decomposition stages operating at 17 bar (medium pressure) and

3.5 bar (low pressure) respectively. Ammonia and carbon dioxide vapors from the stripper top are

mixed with the recovered carbamate solution from the medium-pressure (MP) and low-pressure (LP)

sections, condensed in the HP carbamate condenser and recycled to the reactor. The heat of

condensation is used to produce LP steam. The urea solution leaving the LP decomposition stage

is later on concentrated in the evaporation section.



1. MP predecomposer: capacity/efficiency enhancement

Part of Casale's portfolio of technologies for ammonia self-stripping plants is the addition of

a medium-pressure pre-decomposer upstream of the existing medium-pressure

decomposer. If, as is often the case, this is installed together with a vacuum pre-concentrator,

this has the combined effect of debottlenecking the medium-pressure decomposition section

and optimizing the steam network.

The heat recovery step performed in the vacuum pre-concentrator drastically reduces the

demand for low-pressure steam. As a consequence, quite often more low-pressure steam is

generated in the HP carbamate condenser than the plant requires and it has to be vented to

atmosphere, reducing the efficiency gain associated with the vacuum pre-concentrator. With

an MP pre-decomposer, the plant steam balance can be optimized leading to full exploitation

of the heat recovery steps.

Casale has designed and successfully put into operation several MP pre-decomposers as

part of major plant revamping projects. Casale's latest design, which is currently being

installed in a revamp project at an Indian plant whose main goal is to drastically reduce the

plant's steam consumption, consists of a vertical upflow decomposer in which the upper

section operates as a gas-liquid separator.

Fig. 10 - Flow scheme of MP pre-decomposer in a NH3 stripping plant

The urea-carbamate mixture from the HP stripper enters the upper section of the new pre-

decomposer, equipped with specifically designed internals, where gaseous and liquid

streams are efficiently separated. The vapors join the MP decomposer vent stream whereas

the liquid phase is routed to the bottom section of the pre-decomposer. As it passes along

the tubes carbamate is decomposed, generating ammonia and carbon dioxide. At the upper

end of the tube bundle the resulting mixture is conveyed to the MP decomposer separator.

The main advantage in comparison with other traditional designs (pure upflow pre-

decomposers) is that the amount of water in the vapor stream from MP decomposition is 25-

30% lower because of the separation step performed in the upper channel of the pre-

decomposer. That drastically reduces the amount of high temperature vapors routed to the

separator of the MP decomposer. Water scrubbing taking place in the packing of the MP

separator is therefore much more effective.



Because there is less water in the vapors from MP decomposition, the carbamate liquor

recycled to the synthesis section is more concentrated, which noticeably improves CO2

conversion. Additionally, in Casale's design, the duty demand is lower than in pure upflow

designs, since there is no need to provide heat for the vapors entrained in the solution from

the HP stripper, which are diverted upstream the pre-decomposer.

Smaller equipment can thus be used, and the steam demand of the pre-decomposer is

reduced by 8-10%.

TOTAL RECYCLE PROCESSES

Total recycle processes have a high-pressure synthesis section followed by medium- and low-

pressure steps for decomposing unreacted carbamate and recovering carbon dioxide and ammonia.

In comparison with modern CO2 or ammonia stripping processes they lack a high-pressure

decomposition and condensation stage. Ammonia and carbon dioxide are recycled separately in the

form of pure liquid ammonia and aqueous carbamate solution. Total recycle processes are typically

operated with high N/C molar ratios (3.8-4.5) in the synthesis reactor to maximize carbon dioxide

conversion.

1. High Efficiency Combined (HEC) process: increase efficiency and capacity

This process was specifically developed for revamping total recycle plants where the required

capacity increase demand is within 50-60% of the original nameplate value. In addition, the

process scheme is particularly suitable for integration with melamine production facilities.

The base concept of the HEC process is to recast the existing synthesis reactor as a once-

through synthesis step which is fed only with ammonia and carbon dioxide. Because no water

is recycled to it, it is possible to achieve a CO2 conversion of up to 75-77%. To maximize

conversion, this "primary" reactor is operated with rather high N/C ratios (3.5-3.8) and at 240

bar. A kettle-type HP condenser is lined up upstream of the once-through reactor to remove

part of the heat of carbamate formation and avoid excessive amounts of vapors in the reactor.

LP steam up to 8 bar is generated during carbamate formation.

The small amount of residual carbamate is decomposed, condensed and recycled as

aqueous solution to a synthesis section which is added in parallel to the once-through reactor.

This section is equipped with a "secondary" urea reactor and HP decomposer. The "primary"

reactor effluent is decomposed in the HP decomposer with 20 bar steam and the developed

vapours are routed to the "secondary" reactor, whereas the urea solution from the bottom is

sent to the existing downstream MP and LP decomposition stages. The "secondary" reactor

is fed only with vapours from the HP decomposer and recycled carbamate. The outlet urea

and carbamate solution joins the urea solution from the stripper bottom. The "secondary"

reactor is operated at 150 bar. At the typical N/C ratio of 4.5; CO2 conversion is 55%.

The combination of the once-through synthesis with the "secondary" process yields a

combined CO2 conversion of up to 72%.

The advantage of the HEC process is that, due to its high efficiency of conversion, the load

on the back-end equipment is not significantly increased. Therefore, even for a capacity

increase of 60% of the original capacity, major interventions are limited to the high-pressure

section; the back-end equipment is less affected. The resulting revamping program thus

requires only short downtime, since the new section can be installed with the existing reactor

operating conventionally.

The efficiency of the process, in terms of steam consumption, is also significantly improved

in comparison with conventional total recycle processes. The typical specific consumption for

a HEC process is in the range of 900 kg/MT urea.



Fig. 11 - Block flow diagram of total recycle urea plant revamped with HEC technology

2. Conversion to ammonia stripping synthesis

When the demand is for a really massive capacity increase (up to 100% of the nameplate

capacity) and/or energy reduction is of paramount importance, Casale's approach is to apply

ammonia stripping technology to the synthesis section of a conventional total recycle plant.

The HP section is provided with all the equipment of the ammonia stripping process: HP

stripper, kettle-type HP condenser, HP carbamate separator and carbamate ejector.

Depending on the required capacity increase, the existing reactor may be maintained as it is

or additional reaction volume may be added downstream.

The resulting CO2 conversion of the revamped HP section is in line with the typical value for

ammonia stripping processes (80%), which compares very favorably with the conversion

achieved by total recycle reactors (typically 60%). So the urea solution going to the back end

has a lower carbamate content and, even in case of a massive capacity increase, only minor

changes need be made in the back end and normally all existing equipment items will be fully

part of the revamped scheme. The steam consumption of the revamped plant broadly similar

to that of a modern ammonia stripping plant: expected values are in the neighborhood of 700

kg/MT urea, which is drastically lower than the usual consumption of total recycle plants.

Conversion of the HP section of existing total recycle plants to ammonia stripping technology

is a proven strategy for achieving drastic capacity increase combined with a drastic

improvement in plant performance.



DIGITAL PRODUCTS

Casale also aims at supporting the customers during operations, to ensure that the potential of the

plant is fully exploited. This is why Casale has developed a collection of innovative digital products

that make the unique know-how of Casale engineers available to its customers.

This results in improved plant efficiency, enhanced stability and reduced downtime.

Casale Remote Engineering Services (CaRES)

Casale Remote Engineering Services (CaRES) is a plant monitoring service that enhances the value

of plant operating data.

With CaRES, Casale puts at Clients disposal expert know-how and simulation capabilities for the

analysis of plant operation and production optimization using world-wide standard performance

indicators, so clients get the full benefit from their existing wealth of information. Casale proprietary

models are the key to reach the highest fidelity in reproducing plant behavior and perform an

accurate analysis.

The first step is to define the scope of the service. It is possible to start by analyzing a single section

and scale up later to the whole plant. Typical scopes could be the back end of an ammonia plant

(syngas compression, synthesis loop, refrigeration section), the back end of a methanol plant, a full

nitric acid plant, and many others.

Once data is stored in Casale Data Center, it will be processed using unique algorithms. Data will

be first cleaned using statistical techniques. Mass and energy data reconciliation will be performed:

this is an essential step to further analyze plant data, to minimize the impact of measurement

inaccuracies. Using the same proprietary thermodynamic and kinetic models used to design the

plant, the algorithms will calculate key performance indicators (KPI) for the overall plant, the sections,

and the single equipment, in order to monitor all of them. These values are often non-trivial and

cannot be simply inferred from measurements.

Plant data will be monitored to generate alerts for anomalous equipment status. This is what we call

proactive maintenance, the perfect way to prevent failures and costly downtime. All this information

will be delivered with a dedicated web-based dashboard. This will be Client’s personal and

customized interface with Casale.

Fig. 12 – Example of web-based dashboard



Casale Model Predictive Control

Operating a plant is a very challenging task. Not only because there are several parameters to be

tuned, but also because many internal and external plant variables (ambient conditions, feedstock

changes) are quickly changing. Even the most experienced operator cannot adjust the operating

parameters rapidly enough to respond to those oscillations. As a result, in order to avoid instabilities

or unexpected behavior, plants are often operated far below their maximum achievable performance.

A model predictive control (MPC) system is an automation system that complements Distributed

Control System (DCS) to overcome these problems by constantly tuning the set points to stabilize

plant performance. Once stabilization is achieved, it is then possible to safely increase the plant

performance and achieve the benefit of increased production and/or reduced consumption.

Fig. 13 – MPC principle

Casale partners with Rockwell Automation, a long-term pioneer of automation systems to offer a

dedicated MPC product for all Casale technologies. The product is based on the established Pavillion

MPC automation system of Rockwell, which has been successfully applied to several chemical

industrial sectors.

Most available MPC systems are simple automation system, which apply purely mathematical

algorithms to optimize selected plant variables, by fine tuning of a set of given input parameters

(Controlled Variables CV). While this approach might seem sufficient, it can be successful only to

correct small plant perturbations or when dealing with simple linear systems. Complex systems, as

several equipment of many fertilizer plants, can be properly stabilized and optimized only using a full

kinetic and thermodynamic model. This is why Casale and Rockwell have partnered to offer a unique

MPC product, which combines best-in-class neural network techniques of the Pavillion algorithms

and incorporates Casale proprietary rigorous models to achieve a greater MPC performance.

Casale Model Predictive Control can be applied to either single plant sections, or to a whole plant.

A fully modular approach is offered, so that initial pilot tests can be performed on a small significant

area, scaling up later on.



THE “SAVeNG” CONCEPT Casale has developed the process revamping scheme named “SAVeNG” with the aim of achieving major energy savings in existing fertilizer complexes, bringing their competitiveness up to the same level as that of more recent plants, built according to the most up-to-date technologies. Casale’s SAVeNG concept is based on a “total approach” towards revamping fertilizer complexes, adopting technologies from Casale’s huge portfolio in both the ammonia and the urea field, enhanced by integration of the process plants with the complex offsites. This concept can be summarized with the following keywords:

“Lean steam” – limit steam generation to the extent strictly required for efficient heat recovery and minimize steam generation in auxiliary boilers, favoring the shift of machine drives from turbine to electric motor;

“Power unit” – maximize in-house power generation with an efficient gas turbine (new unit or upgrade of existing units) to feed the new drives;

”Integration” – modify the process plants to allow efficient utilization of the waste heat from turbine exhaust.

Fig. 14 – Casale SAVeNG concept applied to urea complex In order to decrease at the maximum steam generation, it also possible to install a so-called “Gas Heated Reformer”. GHR is an equipment that uses the residual heat in the effluent from either a primary or a secondary reformer to produce hydrogen from methane (steam reforming). This high grade heat (usually at a temperature of about 900°C) is, in conventional plants, used in the waste heat boiler to produce HP steam. The GHR allows to use this duty to produce hydrogen. Casale has access to Technip GHR technology, named TPR® (TechnipFMC Parallel Reformer).

The Casale SAVeNG concept can be applied to any ammonia plant, from 600 MTD to over 2000 MTD and any urea plant, either stripping (NH3 or CO2) or total recycle technology, even in the case of multiple production trains.

Studies conducted on different urea complex configurations showed that the achievable energy saving by application of the SAVeNG concept can be as high as 1 Gcal/MT of urea (depending on the plant technology and actual energy consumption), with final energy consumption lower than 5.0 Gcal/MT of urea.



CASE HYSTORY #1: IFFCO PHULPUR REVAMPING, AN EXAMPLE OF “SAVeNG” APPLICATION In 2014 IFFCO, a major fertilizer producer in India, selected Casale to develop the basic engineering for revamping its three fertilizer complexes located in Aonla and Phulpur (Uttar Pradesh) and Kalol (Gujarat). The revamp of the Phulpur urea complex is the most comprehensive and significant example of the application of the Casale SAVeNG concept, amply demonstrating Casale’s flair for integration between the different production units and the utilities system. IFFCO’s Phulpur complex consists of two ammonia plants (by MW Kellogg and Haldor Topsoe) and three urea plants (by Snamprogetti), producing altogether 3040 MTD of ammonia, fully converted to 5210 MTD of prilled urea. The on-site utilities include cooling towers, steam and power generation plants meeting the overall requirements of the process units. One of the main advantages of having a single licensor in charge of the whole complex, including the various process units and interconnecting networks, is the possibility of effectively integrating the requirements of all the units and of the common services to achieve the maximum efficiency out of the overall system. Besides process modifications in the production units, Casale’s revamp featured changes in the utilities sections (steam and power generation) with the aim of optimizing the integration of interconnected networks and maximizing the overall energy efficiency of the complex. Modifications in Ammonia plants In both ammonia plants, most of the modifications have been made in the front end mainly because of the reduction in S/C molar ratio at the primary reformer inlet. These conceptual process modifications resulted in a reduction of the overall front-end pressure drop and steam consumption. Because of the reduction in S/C ratio, less heat is available for recovery from steam condensation in the process gas. This mainly affects the CO2 removal system, which has had to be upgraded to improve its energy-efficiency. Switching from hot potassium carbonate to BASF OASE® amine technology was the choice to meet this target, reducing the heat demand of the section to about 500 kcal/Nm3 of removed CO2, from the previous figure of 850 kcal/Nm3. In Ammonia 2, a synthesis gas drying system based on liquid ammonia washing has been introduced after the second stage of the synthesis gas compressor, allowing the MUG to be fed directly to the synthesis converter (synloop reversal). Benefits arise from the reduced load on the ammonia chillers and the lower loop pressure drop. In Ammonia 1, a new MP boiler has been installed between the first and second synthesis converters, reducing the second converter inlet temperature to 300°C, which not only optimizes conversion but also increases heat recovery. To optimize steam consumption and properly balance the plant’s networks, the performance of the main turbomachinery has been analyzed to identify the major sources of inefficiency, and corrective modifications have been applied. In Ammonia 1, a new VAM chiller has been provided at the suction of the process air compressor, while the drives of both the process air and refrigeration compressors have been replaced by the OEM. In Ammonia 2 the synthesis gas compressor has been revamped by the OEM, while the turbine drives of one BFW pump, one semi-lean pump and one CW pump have been replaced by electric motors.



Modifications in Urea plants Both lines of the urea plant have been revamped with the purpose of improving CO2 conversion within the HP section and optimizing the steam balance of the facility. Better conversion efficiency has been achieved by replacing the existing internals of the urea reactors with a complete set of Casale’s High-Efficiency Trays. The other major steam saving has been achieved through installing an ammonia preheater in each line to recover heat from the condensation of process vapors from the LP decomposers. The existing CO2 compressors, of both lines, have been replaced with new centrifugal machines. Existing steam turbines and CO2 chillers have been retained. The steam network of the plant has also been optimized by a minor intervention in the MP section. Modifications in Utility plant In the common utilities systems, steam and power generation have been integrated with the installation of a new 31.2 MW gas turbine generator (ISO rated power), equipped with a new HRSG for HP steam generation. With the aim of effective utilization of low temperature heat source, VAM has been installed to maintain the GTG suction air temperature at 12 °C against the maximum ambient of 45 °C, utilizing heat from hot condensates from both urea plants. This has helped the GTG to provide increased constant electrical power in the complex. Thanks to the leaner steam balance and the new HRSG, producing about 60 ton/h of HP steam, it has been possible to switch off the gas-fired auxiliary boiler and two out of three coal-fired auxiliary boilers, reducing the average steam generation cost from 0.95 to 0.83 Gcal/ton. Steam imports and exports in individual plants have been balanced to match the expected consumptions with no surplus from each pressure level. Achieved results After commissioning and start-up of the revamped plants, the guaranteed performances have been fully matched clearly demonstrating the success of the project. The following summarizes the achieved performances, as recorded during plant’s guarantee test runs in January 2018. It is remarkable to notice that a consistent energy saving is obtained even in plant 2, which was already operating with a fairly good consumption.

Phulpur 1 Phulpur 2

Before After Before After

Specific energy (Gcal/MT of Ammonia)

8.84 7.75 7.55 7.10

Energy Saving (Gcal/MT of Ammonia)

- 1.09 - 0.45

Specific energy (Gcal/MT of Urea)

6.44 5.50 5.36 4.84

Energy Saving (Gcal/MT of Urea)

- 0.94 - 0.52

Specific energy including off-sites (Gcal/MT of Urea)

6.67 5.66 5.56 5.01

Energy Saving (Gcal/MT of Urea)

- 1.01 - 0.55



CASE HYSTORY #2: ENERGY SAVING IN A STAND-ALONE AMMONIA PLANT A typical example of energy saving revamp was carried out in 2015 in Central America, where an aged plant designed more than 40 years ago was modified to decrease energy consumption by 1 Gcal/MT. The modifications were focused on the sections where changes could provide significant energy saving at a reasonable capital cost. The main plant limitations were:

• High primary reformer stack temperature

• High specific energy consumption in the CO2 removal section

• Overloaded synthesis loop

• Machines close to their limits.

Modifications in Ammonia plants To above overcome limitations Casale carried out the following changes.

• A new saturation section was installed

• A minor revamp was done on the air compressor

• The CO2 removal section was revamped

• The LP case of the synthesis gas compressor was revamped

• A new additional ammonia converter bed was installed

• The refrigerant compressor turbine was replaced.

The existing primary reformer is a Foster Wheeler Terrace Wall and the flue gas temperature at the stack was quite high and close to 300ºC. The installation of an air pre-heating system was immediately discarded due to the structural impact that this could have on the existing primary reformer. To recover the heat available in the flue gas was then decided to design a new saturation tower, provided with a new coil installed in the top part of the primary reformer. The steam to carbon ratio downstream the saturator is about 0.5÷0.55 mol/mol, providing MP steam saving in the steam system. The new scheme allowed a reduction of the stack temperature up to 190÷195ºC and the relevant energy saving was about 0.15÷0.2 Gcal/MT. The existing CO2 removal section is based on a rich/lean design based on activated MDEA solution. To improve the efficiency of this section was decided to install an additional LP flash tower; this modification is going to add equilibrium stages to the regeneration section saving reboiling duty on the process and steam reboilers. Another modification focused to reduce the section specific energy consumption, was the installation of a new plate and frame solution rich/lean exchanger to replace the existing shell and tube one. The main advantages of this solution is based on a better temperature approach between the hot and the cold side, moreover the pressure drop on the hot side was kept rather low to improve the operation of the solution pump (higher NPSH available). The main benefits of the previous modifications were a significant LP steam saving to the steam reboilers that, through a throttling of the let-down valve between the LP and MP steam headers, corresponded to a saving in MP steam. A lower duty requirement allowed also a significant steam to carbon ratio reduction, actually this parameter was decreased from 3.3 (mol/mol) of the base case to 3.0 (mol/mol) of the revamped case. The calculated energy saving thanks to the installation of the LP flash tower and to the rich/lean solution plate and frame exchanger was about 0.2 Gcal/MT. The synthesis loop was one of the main bottlenecks of the plant, as the loop was working quite loaded and close to the PSV synloop set pressure. To improve the plant safety and to provide a significant energy saving to the plant was decided to install an additional ammonia converter downstream the existing ones. The new converter was based on a single adiabatic bed having the Casale axial-radial internals.



The new operating synthesis loop pressure at circulator discharge was decreased from 163 barg up to 152 barg despite the capacity increase, while the outlet ammonia concentration from the converters was increased from 15% (mol) up to 20% (mol). The significant synloop circulation reduction allowed a specific reduction of the refrigerant section power requirement of the plant, providing additional saving. The energy saving achievable thanks to the installation of the additional ammonia converter was more than 0.2 Gcal/MT. The ammonia plant is a part of an industrial complex and was importing medium pressure steam (36 barg steam) from the common utilities; additionally, the ammonia plant is not equipped with an auxiliary boiler making harder the steam network management. More specifically this plant is the farthest from the common utilities (where the package boilers for the entire complex are located) and is located at the bottom of the distribution system with the lowest working MP steam pressure; as a consequence of the previous arrangement, in case of MP steam excess inside the ammonia plant, this “energy” could not be exported from the ammonia plant MP header to whatever external MP steam user (steam from utilities is supplied at 37-38 barg); in other words this was the main limitation for the plant and for the execution of a profitable revamping. To overcome the previous limitation, Casale noticed that the Refrigerant compressor MP steam condensing turbine was operating with very low efficiency; for this reason and considering the previous limitation on the steam system, it was decided to completely replace the existing refrigerant compressor turbine with a brand new one working between HP steam and condensation and with the capability to export MP steam at 40 barg (the provision for LP steam re-injection was also provided).

Fig. 15 - Old steam system vs. new steam system This solution allowed an important energy saving for the following reasons: - The refrigerant compressor turbine efficiency was significantly increased; - The steam import was completely removed by steam export; - The Utilities boiler load was correspondently reduced saving NG and allowing a more efficient operation. The installation of this new turbine was the key for the plant revamping success and allowed this vintage plant to change its (utility) status from energy “consumer” to energy “feeder”.



Achieved results The total duration of the project was two years from award of the contract to ammonia production. The major milestones were as follows:

• June 2013, Casale was selected to perform the engineering phase of the revamp.

• November 2013, the basic engineering was completed.

• December 2014, HAZOP was carried out for the revamp

• September 2014, the detailed engineering phase was completed.

• April 2015, the plant was shut-down for execution of the revamp.

• June 2015, ammonia product to storage.

Final plant performance exceeded guarantees, making the performance of this old plant similar to that of a modern plant:

Plant production and energy consumption

Pre-Revamp Post-Revamp

Guarantee Test Run

Production 979 1040 1071

Energy Saving, Gcal/MT

0.95 1.05

Sp. Energy, Gcal/MT(*) 9.15 8.20 8.10

(*) Based on LHV

The previous calculations have been done considering the ammonia plant only; the decreased load of the utilities boilers allowed them to operate closer to their maximum efficiency, moreover due to the higher ammonia plant efficiency, the cooling water system, that is part of the utilities, worked at a lower load and more efficiently too.

In other words, the better efficiency of the ammonia plant allowed also the utilities to operate more efficiently.

Documents

ENERGY SAVING IN AMMONIA-UREA COMPLEXES