urea process

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The mosT efficienT process designfor modern urea planTs

urea process split flow looptmcasale

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The Split Flow Loop™ process is an improved CO2 stripping process. It uses the principle of directly routing to the reactor a minority 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, while sending the rest first to the carbamate condenser and then, after separation of the carbamate solution, directly to the inerts scrubber.

In this way the volume of inerts passing through the reactor is reduced, which has a beneficial effect on conversion efficiency.

In addition, condensation efficiency is enhanced by using a submerged condenser (the Full Condenser™) for HP carbamate condensation.

process ouTline

The Split Flow Loop™ is Casale’s process for new urea plantswith capacity up to 3,500 MTD.

Key feaTures•Proven design•Optimized plant cost•Low energy consumption

The main steps of the Split Flow Loop™ process are the following:

• The solution from the reactor is first treated in a stripper, operating at the same pressure as the reactor, where, using steam and CO2 as stripping agent, most of the unreacted NH3 and CO2 are driven off.

• Part of the unreacted NH3 and CO2 leaving the stripper is sent directly to the reactor, while the rest is recycled back to the reactor through an HP condenser.

• From the stripper, the urea solution, still containing unreacted NH3 and CO2 in the form of carbamate, is sent to a low-pressure single decomposition/condensation stage, where practically all the remaining unreacted NH3 and CO2 are separated and recovered in the form of a carbamate solution.

• The urea-water solution, containing only small quantities of NH3 and CO2, is then further treated in a two-stage vacuum evaporation section to obtain a urea melt for the prilling tower or the granulator.

• The process condensate obtained from the vacuum condensers is purified in two columns and one hydrolyser so as to eliminate all NH3, CO2 and urea.

• The carbamate solution obtained in the LP section is sent to the HP scrubber, where the inerts leaving the HP loop are washed.

Split Flow Loop™ process – Block diagram

reactor hp condenser

carbammate

hp Loop

inerts

inerts

inerts

hp stripper

hp scrubber

co2

nh3

Lpdecomposer

vacuumconcentrator

vacuumcondensers

condensatetreatment

Lpcondenser

urea

h2o

urea processspliT flow loopTm


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The synthesis section operates with very low inerts content with the following advantages:

• high CO2 conversion in the reactor (up to 62-63%);• high stripping efficiency;• high condensation efficiency.

The main elements characterizing the HP loop of the Split Flow Loop™ process are:

• the Casale Full Condenser™;• the Casale-Dente high efficiency trays;• the Casale high-efficiency hydrolyser used in the process condensate treatment unit.

casale full condenser™ The Casale Full Condenser™ is a submerged condenser with natural circulation. Both the gases and vapours from the stripper and liquid carbamate are introduced at the bottom and flow together upwards through the majority of the tubes. However, the gas flow is directed away from a small number of the tubes, and without the levitating influence of the gas the liquid in these tubes flows downwards. That produces the internal natural circulation.

The interfacial area between the two phases (liquid and gas) in the rising-flow tubes is significantly greater than in a conventional falling-film condenser, so the mass and heat transfer performance of the exchanger is highly improved. The HPCC is also better protected against corrosion in the new configuration, as all tube surfaces are better wetted. The new flow pattern of the HPCC, shown here, can be summarized as follows:

• Vapors coming from HP stripper are fed through one of the bottom nozzles and are distributed into the continuous liquid phase by a distributor on the bottom of the HPCC.

• As the two-phase flow rises through the tubes the vapors condense, although inerts remain in the gaseous phase.

• When the two-phase flow exits the tubes from the top tube sheet, the inerts separate from the condensed liquid and leave the condenser through the top nozzle.

• Fresh liquid (ammonia and carbamate mixture) enters the exchanger through the second nozzle in the bottom and is distributed in the tubes.

• A top weir defines the liquid level in the top part of the condenser. Overflowing liquid exits the exchanger through the second top nozzle.

Casale determines the appropriate internal circulation ratio needed to achieve optimal heat transfer in the two-phase upward tubes.

The Full Condenser™ is very efficient, with 50% higher heat transfer efficiency than a standard falling-film condenser. Also, a significant amount of urea is formed in the HPCC itself, reducing the load on the reactor.

In spite of all these advantages, the mechanical design is very simple, and it is proven in commercial use.

disTincTiVeTechnological elemenTsadVanTages

The Split Flow Loop™ process is a highly efficient technology and, for a given capacity, needs smaller, less costly equipment than less efficient conventional stripping processes. And the operating costs of the Split Flow Loop™ process are as low as those of the most advanced competing processes.

Because of the relatively small size of its critical equipment items, the Casale Split Flow Loop™ process is suitable for plants of very large capacity. The configuration of the HP loop of the Split Flow Loop™ process is shown below.

The solution leaving the reactor is treated in the HP stripper, where the carbamate present in the solution and containing unreacted NH3 and CO2 is decomposed with the help of the CO2 feed as strip-ping agent.

The vapor stream generated by the carbamate decomposition, containing NH3, CO2, some water and all the inerts present in the CO2 feed, is split after leaving the HP stripper. Only a portion of that vapor stream is condensed in the HP carbamate Full Condenser for return to the reactor.

Total condensation is not possible in the condenser because of the presence of inerts, so small amounts of uncondensed vapors leave from the top of the condenser and are sent directly to an HP scrub-ber together with the inerts. The rest of the vapors are directed to the reactor to provide the necessary amount of heat.

In this way, about two thirds of the total amount of inerts present in the CO2 are not sent to the reactor, and consequently urea con-version is maximized.

The HP scrubber recovers NH3 and CO2 from the inerts streams leaving the Full Condenser™ and the reactor using the carbama-te flow coming from the LP recovery section. Carbamate coming from the HP scrubber is sent to the Full Condenser™, to enhance condensation, using an ejector driven by the ammonia feed, which is also sent to the Full Condenser™.

To keep the correct N/C ratio in the Full Condenser™, part of the NH3 feed bypasses the HPCC and is sent to the reactor through another ejector, which enhances the driving force for the circula-tion by accelerating the carbamate leaving the Full Condenser™.

nh3

stripper

scrubber

hpcc fuLLcondensertm

co2 Lp decomp.

carbammate

reactor


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casale-denTe high-efficiency TraysCasale-Dente high-efficiency trays are the most efficient trays available in the market and are also an essential element in mak-ing the Split Flow Loop™ as efficient as possible, minimizing the required reaction volume and thus significantly saving on in-vestment cost.

The improved tray geometry of the Casale-Dente design promotes very efficient transfer of NH

3 and CO2 from the vapors into the liquid phase, where urea is formed.

The new trays are designed so that:

• Vapors and liquid follow separate but adjacent cocurrent paths through the space between the trays. This guarantees stable flow of the two phases and a better approach to an even, uni-form flow of the two phases throughout the whole reactor.

• These separated paths through the tray are chosen so that very efficient mixing takes place between vapor and liquid. Conse-quently there is a very high degree of both mass and heat tran-sfer within the liquid phase.

• It is possible to generate vapor bubbles with a far smaller dia-meter than with any previous design. In consequence, the in-terfacial surface for mass and heat transfer is increased.

• There is also a much larger interfacial surface for exchange between the vapor bubble emulsion and clear liquid.

• The relatively short path length of the recirculation streams into the emulsion phase significantly decreases transfer resistance.

high-efficiency hydrolyserThe High-Efficiency Hydrolyser makes it possible to treat the process condensate removing all of its ammonia and urea content, so it can be discharged without any environmental effects or can equally well be used as boiler feed water.

With the help (if necessary) of one or two supplementary stripping columns, Urea Casale’s High Efficiency Hydrolyser (HEH) can reduce the residual content of NH3 and urea in the process condensate to below 3 ppm, such that it can be used as boiler feed, which is particularly beneficial in locations where fresh water is scarce and/or expensive.

The High Efficiency Casale Hydrolyser makes efficient use of the stripping action of steam to enhance the hydrolysis of the urea content of the condensate by removing the NH3 and CO2 hydrolysis products, which would otherwise tend to slow the hydrolysis down. That is achieved by dividing the hydrolyser into two compartments, both provided with high-efficiency Casale trays.

The two zones have the following characteristics:

• The first zone, fed by the waste condensate to be treated, oper-ates in co-current, with injection of steam in the bot-tom. At the top of the first zone the vapors are finally removed from the liquid, which is then treated in the second zone.

• The second zone, fed by the liquid coming from the first zone, operates in counter-current, with liquid going downward and vapor going upward. Fresh steam is injected again in the bottom of this second zone. The driving force for the extraction of NH

3 and CO2 is, in this way, increased, allowing the urea content to be reduced to less than 3 ppm.

The vapors are separated from the liquid at the top of the zone and exit the hydrolyser, along with the vapors from the first zone. Steam at pressure lower than 25 bar can conveniently be used.

• CO2 conversion in the reactor 62.5 %• stripping efficiency 82 %• pressure of produced LP steam 5 bar a• specific carbamate recycle rate 450 kg/t• HP steam to turbine (41bar, 380°C) 1,000 kg/t (1)

• MP steam consumption (22 bar, superheated) 770 kg/t (2)

• Raw material consumption 564 kg/t (2)

• CW consumption 65 m3/t (2)(3)

• Electricity consumption 21 kWh/t

(1) Assuming standard steam turbine and CO2 compressor efficiencies (2) Urea solution unit coupled with granulation unit(3) Process users only

performance

By virtue of its high efficiency, the equipment size and investment cost for a Split Flow Loop™ plant of a specific size are lower than for other processes.

STEAM

STEAM

LIQ. + VAP.

VAP.

LIQ.VAP.

LIQ.

VAP.VAP. VAP.

LIQ.LIQ.

CASALEHIGH EFFICIENCY

TRAYS

VAP.

LIQ.LIQ.CASALE

HIGH EFFICIENCYTRAYS

LIQUIDOUTLET

VAPOROUTLET

LIQUIDINLET

STEAM

STEAM

LIQ. + VAP.

VAP.

LIQ.VAP.

LIQ.

VAP.VAP. VAP.

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TRAYS

VAP.

LIQ.LIQ.CASALE


LIQUIDOUTLET

VAPOROUTLET

LIQUIDINLET

STEAM

STEAM

LIQ. + VAP.

VAP.

LIQ.VAP.

LIQ.

VAP.VAP. VAP.

LIQ.LIQ.


TRAYS

VAP.

LIQ.LIQ.CASALE


LIQUIDOUTLET

VAPOROUTLET

LIQUIDINLET

VAP. VAP. VAP.

VAP. VAP. VAP.

LIQ. LIQ. LIQ. LIQ.

Casale-Dente high efficiency trays. High efficiency hydrolysers

The trays are plates corrugated into a series of parallel linear rid-ges and troughs. The ridges are flattened at the top and the trou-ghs are similarly flattened at the bottom.

Large perforations are provided in the trough bottoms for liquid to pass through and there are small perforations in both the sloping walls and the tops of the ridges for gases accumulating beneath them to pass through.

This unique design produces extremely small bubbles and, in con-sequence, a very high specific surface area for mass and heat transfer, enhancing the highly efficient mixing between vapors and liquid mentioned above.


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disTincTiVe feaTures

pressurized carbamaTe accumulaTor

The Casale process includes a tank located at the suction of the HP carbamate pump that collects the ammonium carbamate solution from the LP condenser. There are two main benefits.

Plant flexibility: During a normal run, the extra storage volume allows the amount of carbamate recycled to the synthesis loop to be kept constant for a long time, which contributes to the stability of operation. During a plant upset, this accumulator can be used as emergency storage – for example, in case the reactor has to be drained.

In comparison with an atmospheric tank, the same volume in a pressurized vessel accommodates a larger quantity of unreacted process components (NH3 and CO2 in the process solution), since the carbamate concentration in the liquid is proportional to the pressure.

Emissions control: The vent of the accumulator is usually sent to an atmospheric washing column to detect possible leakage, although during normal operations the vent is almost always closed. But during start-up or shut-down urea or carbamate solution has to be drained from the HP synthesis loop, and gaseous ammonia flashes off.

Operating under pressure significantly reduces the amount of ammonia evolved and, therefore, the possibility of overloading the downstream washing column, which would allow ammonia to be emitted to the atmosphere.

spillage recoVery sysTem

Instead of being simply lost in the plant, all liquid escapes from the process are collected in an underground tank.

From there a submerged pump sends them to the ammonia water tank, from which they are fed, along with the plant process condensate, to the waste water treatment system for recovery of nitrogen values.

That not only keeps atmospheric emissions of both ammonia and carbon dioxide, to a minimum but also allows more efficient use to be made of raw materials.

ammonia scrubbing of all process sTreams

Every continuous gaseous stream containing residual NH3 and CO2 is thoroughly washed before discharge to the atmosphere, with the dual aims of minimizing emissions and recovering unreacted materials. For each stream, the most suitable scrubbing conditions have been selected to optimize the ammonia absorption.

For this reason, off-gases from the HP scrubber, which are particularly rich in inerts originating from the CO2 feed and the passivation air, are scrubbed in a dedicated absorber, operating at medium pressure. The upper part is fed with clean and cold steam condensate for final washing to reduce the ammonia vented to atmosphere practically to zero.The vents coming from the vacuum systems, from the LP condensation system and from the reflux condenser are scrubbed in the LP absorber column,

Casale has drawn on its long experience in revamping urea plants of any technology to include certain features in the Split Flow Loop™ process aimed at improving its reliability and operating flexibility and at minimizing the environmental impact of the urea plant.

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scrubber

reactor

stripper

hpcc

The main distinctive features are:• provision of pressurized carbamate accumulator;• provision of a spillage recovery system;• ammonia scrubbing of all process streams;• collection of all process safety valves;• optimized lay-out.

collecTion of all processsafeTy ValVes

Discharges from all the process gaseous discharge points (safety valves and process vents) are collected in the blow-down system. Any liquid is separated in a dedicated drum, whereas the vapors are conveyed to the vent stack. By separating the liquid phase and recovering it into the process, the environmental impact of the emission can be minimized. And if the discharge of the vent is located at sufficient height, operating safety is also ensured.

opTimized layouT

The layout of the synthesis section for a Split Flow™ plant is vertical to provide gravity flow for fluid circulation in the HP loop. Using ejectors driven by the ammonia feed for transporting the carbamate from the HP scrubber to the Full Condenser™, as well as from the liquid outlet of the HP carbamate condenser to the reactor, reduces the structural height necessary to ensure proper circulation in comparison with a total gravity process. Therefore, at the moment, the height of the plant is essentially dictated by the height of the reactor.

Up to now, it has been necessary to build a supporting structure around the reactor strong enough to sustain it, including also the relevant stairs and platforms. But recently, a solution has been developed allowing the whole installation to be lightened while still providing the necessary support for the reactor.

Unlike typical arrangements, in this layout no dedicated structure is provided for the reactor over its full height. On the rare occasions when it is necessary to climb to the top, it is possible to do so using ladders installed directly on the reactor wall.

The highest platform is 38 m above ground level, so the whole structure is lowered and lightened, with consequent savings in investment for both materials and civil-erection works.

Combining Casale’s deep know-how of process technology with the its high-level engineering capability, it has also been possible to develop a very compact and efficient plant configuration.

The plot plan shown in the diagram is for a plant with capacity of 2,000 t/d and would require an area of approximately 50 x 55 m.

Typical elevation plan of Casale Split-Flow™ urea plant

operating at virtually atmospheric pressure, before being discharged to atmosphere. The LP absorber also utilizes clean and cold steam condensate for washing the gas.

It is not advisable to further wash the exhaust from the MP absorber in the LP absorber on account of the amount of inerts, which would both negatively affect the thermodynamic equilibrium and lead to ammonia entrainment.

As Casale’s recent experience grass-roots projects verifies, in plants where acid solution is available and where it is also possible to recover the salt solution from the washing column, the use of acid scrubbing enhances ammonia absorption significantly to reach the target concentration in the vent of 30 mg/Nm3.


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Applying a proven process technology for a production capacity much larger than referenced ones is always a challenging exercise. But in the case of the Casale Split Flow™ process there need be no doubts about reliability when scaling up to a jum-bo urea plant, thanks to the modular de-sign basis of each item of Casale critical equipment. Focusing our attention on the HP loop equipment it is easy to show that:

Reactor: Each high-efficiency tray crea-tes a series of mixing zones for the two phases that maximizes mass and heat transfer. The modular sequence of mixing zones on the tray makes it possible to de-sign it for very large capacities, since the number of mixing zones can easily be in-creased, enlarging the tray diameter and then, ultimately, the reactor diameter.

modular design concepT

Casale has experience in scaling up urea process technologies and has demonstrated the effectiveness of its concepts in this respect, reaching a scale-up factor of 2 without any problems. Casale Technology is suitable for the capacity of 3,500 t/d. Even higher capacities are achievable – up to 5,000 t/d – by providing an MP section comprising an MP decomposer and condenser working in parallel with the HP stripper.

VAP. TOSCRUBBER

VAP.LIQ. TOREACTOR

LIQ. FROMEJECTOR

VAP. FROMSTRIPPER

Full condenser

HP stripper distribution systemFull condenser distribution system

Stripper: Good performance of this item is guaranteed by uniform distribution of the reactor effluent into the tubes, the number of which is defined to give the opti-mum specific liquid load for each tube, proved by ex-perience in many applications. Thanks to its proprieta-ry liquid distributor, Casale is able to guarantee uniform distribution of the liquid into the stripper tubes at any capacity.

HPCC: A special modular vapor distributor is installed in the bottom of the Full Condenser™. Each tube is fed by a single metering tube.

Thanks to this special vapor distributor, the HPCC can be properly designed for any load by simply providing the appropriate number of tubes for the required ca-pacity.

Via Giulio Pocobelli, 6 | 6900 Lugano | Switzerland (CH)tel. +41 91 6419200 | fax +41 91 6419291

www.casale.ch | [email protected]

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