Urea 11th Symposium

11th Stamicarbon Urea Symposium19 – 22 May 2008, Noordwijk

Eleventh StamicarbonUrea Symposium 2008

Symposium 2008 - Paper 6w

Stamicarbon’s new urea plant concept

Authors:

Bart Gevers, Senior Engineer Urea ProcessJan Mennen, Principal Engineer Urea ProcessJo Meessen, Principal Engineer Urea Process

Stamicarbon B.V., the Netherlands


1.

Table of contents Page

1. Abstract ..................................................................................................................................22. Safurex and zero oxygen ...................................................................................................... 33. Low level synthesis arrangement............................................................................................ 6

3.1 Development of plant height of Stamicarbon urea plants ..................................................73.2 Maximum capacity for pool reactor concept....................................................................113.3. Further height reduction of a pool condenser urea synthesis section.............................11

3.3.1. Revamp of a PIC Kuwait plant ................................................................................113.3.2. Reactor located on ground level in pool condenser plant ........................................133.3.3. Conclusions for low level synthesis arrangement.................................................... 16

4. The full picture...................................................................................................................... 175. Summary..............................................................................................................................18Attachment: optimize stripping efficiency.................................................................................. 19

All technical and other information contained herein is based on general Stamicarbonexperience and within this limit is accurate to the best of our knowledge. However, no liabilityis accepted therefore and no warranty or guarantee is to be inferred. Copyright StamicarbonBV. All rights reserved. No part of this publication may be reproduced in any form or by anymeans without the permission of Stamicarbon BV. You will access its contents solely for yourown private use and will comply with all applicable laws and regulatory requirements relatingto your use of this information.


2.

1. Abstract

Stamicarbon is the world leader in urea technology. Leadership in this context also meansresponsibility. As process licensor we see this responsibility as a continuous effort to ensurethat our process plant designs fulfill our customer’s needs. We are proud to present our mostrecent achievements in this respect: the new plant concept that is available for licensing now.

The targets we have set for our new plant concept closely follow the requirements of ourcustomers as they are heard by us in our frequent contacts with them.

Minimize the Capital Expenditure (CAPEX) required for new plants. Minimize the Operational Expenditure (OPEX) required for new plants. Maintain high product quality, low emission figures and high on stream time. Have a maintenance free plant.

These targets have been achieved by the following elements, resulting from our investments inresearch and development and our experiences in revamp projects.

A low level arrangement for the synthesis equipment: minimizing CAPEX. Optimized choice of stripping efficiency, meaning minimizing steam consumption and so

minimizing OPEX. Zero oxygen introduction to the synthesis section, through the use of Safurex material

of construction. This element further paves the road for an intrinsically safe andmaintenance free plant and further simplification of the process design, among which:

eliminate the hydrogen removal; delete the HP scrubber; simplify the off-gas treatment section of the plant.


3.

2. Safurex and zero oxygen

Since urea processes became industrial, the very important issue to be solved was thecorrosion resistance of the applied materials under the extreme synthesis process conditions.When austenitic stainless steel materials have been used as construction material, continuouspassivation was required. Continuous passivation is usually done by adding air to the synthesissection, which has a negative influence on the process with respect to reaction conversion andprocess safety.

The excellent corrosion resistance properties of the Safurex construction material howeverpave the way for a synthesis section wherein passivation is not needed anymore. That uniqueproperty of the Safurex material is proved in the laboratory as well as under practical synthesisconditions.

In the laboratory the corrosion resistance properties for the Safurex was compared with BC.05(25.22.2) as well as the urea grade stainless steel material BC.01 (316L UG) in an oxygen freecarbamate solution at the prevailing extreme synthesis conditions. Under these conditions itappeared that the Safurex material did not become active, while austenitic stainless steelmaterials were severely actively corroded and even disappeared (figure 1).

Figure 1: zero oxygen test in autoclave for 75 days under synthesis conditions at 183 °C.

The laboratory experiments prove that the Safurex material is resistant against active corrosionin an oxygen free carbamate environment at synthesis process conditions, a pre-condition setfor the development of this material. Also in practice we proved over the years that Safurexhas met all the requirements set for the Safurex development. The material never showed anysign of active corrosion as you can learn from paper 2: Safurex ; it is not a dream. That makesStamicarbon fully confident to design urea plants where passivation of materials by using airsupply becomes history.

Safurex® 0.05 mm/y

BC.05 > 30 mm/y

BC.01 ? mm/y?


4.

A urea plant without the need of passivation of the applied materials in the synthesis sectionhas the following advantages.

Lower investment cost. Intrinsically safe process. Minimized ammonia emissions.

Lower investment costIf the air supply to the carbon dioxide feedstock becomes obsolete, the presence of inerts in thesynthesis section is decreased. Consequently the investment cost decreases for such plantsbecause of:

simplified inert vent scrubbing system; less and smaller equipment involved.

Since air as passivation agent is omitted and thus the need for combusting the hydrogen in thecarbon dioxide feedstock is rejected, the amount of inert in the synthesis section is furthersignificantly reduced. As a consequence, the usually applied high pressure scrubber, includingthe associated equipment and lines is replaced by a simple scruber system operating at areduced pressure before the inert vapor is purified in a low pressure absorber. A scrubbersystem operating at reduced pressure is already demonstrated at several occasions such asMangalore Chemicals & Fertilizers in India as presented in paper 3 of this symposium. In theStamicarbon new plant concept this scrubber system is even more simplified, since the heatexchanging part is not required here.

The absence of the passivation air supply and consequently the simplification of the inertscrubbing system leads to less equipment compared to a plant in which passivation air isneeded.

The following equipment becomes obsolete; The air blowers. The hydrogen converter. The high-pressure scrubber including pertaining conditioned cooling water system. The high-pressure ammonia ejector.

Furthermore the high pressure carbon dioxide compressor and the waste water treatmentsection become smaller, which further reduces the required investment cost for the zero oxygenplant.

Intrinsically safe processThe absence of the passivation air makes the urea process intrinsically safe, since no oxygen ispresent and thus no flammable mixtures can exist. Safety measures as combustion of thehydrogen to reduce the frequency of occurrence of flammable mixtures and installation ofpressure relieve volumes to minimize effects of an explosion are superfluous in the zero oxygenplant.


5.

Minimized ammonia emissionsAs the absence of passivation air leads to a decrease of inert in the system, the amount ofinerts to be vented is reduced drastically and thus the ammonia emission is reduced to aminimum. Consequently, absence of passivation air also benefits the environmental position ofthe zero oxygen plant. This was also successfully demonstrated in the low oxygen testperformed at the Shiraz Petrochemical Company urea plant in Iran (see also publication onAIChE Symposium in 2006).


6.

3. Low level synthesis arrangement

Since the introduction of the high pressure CO2 stripper in Stamicarbon urea plants in the sixtiesthe urea solution from the synthesis section has been directly fed to a so called low pressurerecirculation section. At that time this was a novelty, because up to then an in-line mediumpressure recirculation section extended with a pure ammonia recycle was generally requiredand from this medium pressure stage the resulting urea solution could be fed to a low pressurerecirculation section. Only the high stripping efficiency of the Stamicarbon CO2 stripper, inparticular with respect to ammonia, made it possible to leave out this medium pressurerecirculation and ammonia recycle section1.

Urea plant types other than Stamicarbon plants now also use high pressure strippers in variousways. But these strippers either do not use CO2 as stripping agent or are operated at a relativelyhigh pressure, limiting the maximum achievable stripping efficiency such that the solution fromthe stripper still has to go to an in-line medium pressure recirculation stage, often extended withan ammonia recycle.

Because Stamicarbon uses CO2 as stripping agent at a relatively low pressure (around 145 bar)the maximum achievable stripping efficiency is high and generally can go up to 80 % withrespect to ammonia and carbon dioxide. For this reason feeding the urea solution from thesynthesis section directly to a low pressure recirculation stage is something which can still onlybe applied in Stamicarbon urea melt plants, with the following advantages: lower investment cost for the recirculation section; easier operation because of a simple plant design; lower maintenance cost of the recirculation section.

A stripping efficiency of 80 % on the other hand results in a relatively high amount of excess lowpressure steam (about 4.5 - 5.5 bara) produced in the high pressure carbamate condenser, atthe cost of medium pressure steam consumption in the stripper. Finding good use for thisexcess low pressure steam can be a challenge. One typical application is in the steam turbinedriver of the CO2 compressor and in that case it replaces and thus reduces the high pressuresteam consumption in the turbine. This is not always done however. In a situation of excess lowpressure steam a lower stripping efficiency decreases the overall medium pressure steamconsumption of the urea plant (see also the attachment to this paper, in which the topic ofoptimizing stripping efficiency is discussed).

One other main feature of a Stamicarbon CO2 stripping plant is that the internal synthesisrecycle is on gravity flow only. That is still a major advantage, since gravity is 100 % reliable. Upto now however the gravity flow synthesis loop required a high elevated urea reactor.

1 An in-line MP recirculation section, receiving urea solution feed from a HP stripper, is not to be confused with anadd-on MP recirculation section. An add-on MP recirculation in a stripping plant receives feed directly from thereactor, thus unloading the HP stripper and the HP carbamate condenser, and is proven by Stamicarbon to be a verypowerful revamping tool.


7.

On the other hand Stamicarbon wants to be able to offer a low elevation plant concept to furtherreduce investment and maintenance costs. One of the issues there is how to get sufficientvapour entering the reactor without compromising too much the amount of CO2 available forstripping.

So, the challenge for Stamicarbon was how to design a urea synthesis section with a reactor onthe ground floor, the main synthesis recycle still on gravity flow and still being able to dischargethe urea solution from the stripper directly to a low pressure recirculation section.

3.1 Development of plant height of Stamicarbon urea plantsThe lay-out of the synthesis section has always been vertical, to be able to utilize gravity insteadof rotating equipment. Obviously, gravity is 100% reliable and not subject to wear and tear.

In the early CO2 stripping plants (1967- 1969) the high-pressure carbamate condenser, in whichammonia and carbon dioxide are condensed, was placed above the urea reactor (figure 2).

Figure 2: high pressure carbamate condenser above reactor

This was done to allow the condensed carbamate to gravitate to the reactor. The high pressurescrubber was built above the high pressure carbamate condenser so that the condensedcarbamate could flow by gravity to the condenser. This lay-out required a high steel structurealthough the carbamate condenser was a compact helicoil type, which was shorter than thefalling film condenser.

HP scrubber

Helicoilcarbamatecondenser

Reactor

Stripper

76000

38000

50000

64000

22000

0

14000

Total gravity


8.

As plant capacities became larger, the helicoil-type high pressure carbamate condenser wasreplaced by a shell and tube falling film-type high pressure carbamate condenser. This couldnot be erected above the reactor and was lowered to the level of the bottom of the reactor, stillmaking full use of gravity flow. The level of the high pressure scrubber could thus be lowered tothe level of the former high pressure carbamate condenser (figure 3).

Figure 3: high pressure scrubber above reactor

Around 1970 the high pressure ammonia ejector was introduced in the stripping process fortransporting the carbamate from the high pressure scrubber to the high pressure carbamatecondenser, using the ammonia as driving force. The amount of carbamate so transported wasrather small and in comparison with other processes the Stamicarbon stripping process was stilla ‘gravity driven’ process, although its structural height was reduced considerable because thehigh pressure scrubber could now be located at the same level as the reactor bottom (figure 4).

Figure 4: high pressure scrubber at reactor bottom level

Total gravityHP scrubber

Reactor andHP carbamatecondenser

Stripper

38000

50000

64000

22000

0

14000

Main recycle on gravity

ReactorHP carbamate condenserHP scrubber

38000

50000

22000

0

14000

Stripper


9.

Since the relocation of the high pressure scrubber to the reactor bottom level, the height of thesynthesis is dictated by the height of the reactor. In recent years the development of the poolcondenser and pool reactor further decreased the height of the reactor and so the height of thesynthesis (figure 5).

Figure 5: pool condenser replacing falling film-type carbamate condenser

The pool condenser is an improvement to the vertical shell/tube high pressure carbamatecondenser. Basically, it is a horizontal, submerged-type high pressure carbamate condenser inwhich a relatively large amount of urea is produced. In the downstream vertical urea reactor theurea reaction comes to completion. Since urea formation already for a large part took place inthe pool condenser, the downstream urea reactor volume is reduced significantly, reducing theplant height.

The success of the pool condenser concept was followed by the development of the poolreactor, now operated by two owners, DSM and Turkmenistan Fertilizers & Chemicals, while athird pool reactor plant is under construction, Fatima Fertilizer Company Pakistan. The poolreactor is a horizontal reactor with the high pressure carbamate condenser function integratedand the high pressure scrubber installed a few meters above it (figure 6). So, in the pool reactorconcept the vertical reactor is eliminated all together, again reducing the plant height. Theejector transporting the recycle carbamate from the HP scrubber can be omitted so that thewhole synthesis loop again operates by gravity alone.

Main recycle on gravityReactorHP scrubberPool condenser

Stripper

39000

22000

0

14000


10.

Figure 6: pool reactor replacing vertical reactor and high pressure carbamate condenser

As described above, during the life of the Stamicarbon CO2 Stripping Process there were thefollowing five main synthesis lay-outs, gradually reducing in height over the years (figure 7).

1. Total gravity: high pressure condenser above reactor.2. Total gravity: high pressure scrubber above reactor.3. Main recycle by gravity: high pressure scrubber at reactor bottom level.4. Main recycle by gravity: pool condenser replacing falling film-type high pressure carbamate

condenser.5. Total gravity: pool reactor replacing vertical reactor and high pressure carbamate

condenser.

Figure 7: Stamicarbon synthesis lay-out development up to today

Total gravityHP scrubberPool reactor

Stripper

22000

0

14000

Using 100% gravity the Stamicarbondesign came down from 76 meters tosome 27 meters for a poolreactor plant

76000

38000

50000

64000

22000

0

14000


11.

Without sacrificing gravity flow in the synthesis section and maintaining simple recirculationdesign, Stamicarbon has succeeded in lowering the plant skyline from 76 meter to some27 meter for a pool reactor plant and to some 39 meters for a pool condenser plant, while at thesame time increasing the single line capacity from 200 to 3700 MTPD (largest capacity onstream already).

3.2 Maximum capacity for pool reactor conceptThe pool reactor is in fact a low elevation urea plant. Combining the condenser with the reactorin one vessel however limits the plant capacity caused by constraints from up-scaling point ofview and in transport. The pool reactor concept is the Stamicarbon standard for plant capacitiesup to 2300 MTPD.

The largest plant capacity for a pool reactor plant designed so far is 1500 MTPD. However,nowadays urea plant capacities of over 3000 MTPD are no exceptions anymore. For these plantsizes a pool reactor concept can be considered not an option, at least for now. For capacitiesabove 2300 MTPD, the pool condenser concept is the Stamicarbon standard. This concept hasa vertical reactor additional to the pool condenser with the reactor dictating the total synthesisheight up to now.

3.3. Further height reduction of a pool condenser urea synthesis sectionSo far two advantages of the Stamicarbon stripping plants have been highlighted.

1. Having no in-line medium pressure recirculation section, as explained in the introduction ofthis paper.

2. Keeping the synthesis flowing on gravity flow, at least the main loop (the secondary loopbeing the transport of carbamate from the high pressure scrubber to further treatment in thesynthesis).

Before explaining how Stamicarbon can reduce the height of the synthesis of a pool condenserplant even further, while keeping these advantages, a relevant experience in this respect inrevamping urea plants will be discussed.

3.3.1. Revamp of a PIC Kuwait plantDuring already many years Stamicarbon has built up experience with revamps of urea plantswith reactors at grade floor. About 10 years ago a conventional Stamicarbon urea plant of PICKuwait was revamped to a CO2 stripping process (figure 8).


12.

Figure 8: synthesis section of PIC Kuwait plant after revamp

The revamp concept was to change the plant (1050 MTPD) into a CO2 stripping plant with adesign capacity of 1750 MTPD. This was done by, of course, adding a CO2 stripper and byadding a pool condenser (adding reactor volume and HP condensation capacity in one vessel).The reactor was installed at ground level, as in all conventional urea processes, but still the ideawas not to use a HP liquid ejector to feed the reactor with the solution from the pool condenser(to have the advantage of gravity flow).

This was possible by installing the pool condenser above the existing reactor.

Figure 8 shows that the HP stripper is elevated relatively high, in fact as high as possible withthe urea solution reactor/top separator vessel still being able to flow into the stripper. The highstripper elevation gave a reduction in HP piping cost, as the structure was already there for thepool condenser.

Since the PIC plant already had an in-line MP recirculation section, use was made of this in therevamp, to reduce the HP equipment cost for the revamp. This means that the applied revampconcept was based on a relatively low stripping efficiency.

VAPOUR AND INERTSTO ABSORPTIONCARBAMATE FROM

MP RECIRCULATIONSECTION

NH3

CO2

SOLUTION TOMP RECIRCULATIONSECTION

MP STEAM

MP STEAMCONDENSATE

LP STEAM / STEAMCONDENSATE

STEAM CONDENSATE

HP STRIPPER

POOL CONDENSER

CARBAMATEEJECTOR

STRIPPINGGAS EJECTOR

REACTOR

VapourLiquid

UtilitiesVapourLiquid

Utilities

41 mtr.

32 mtr.

29 mtr.


13.

Therefore CO2 directly from the compressor was available as vapour source for supplying heatto the reactor2. In first instance this was the only vapour source used.

As presented in the Stamicarbon symposium of 2004 the medium pressure condenser showeda too low capacity because of a suspected mechanical problem. The presented solution was toinstall a liquid ammonia driven stripgas ejector transferring stripgas directly to the reactor, givingroom to reduce the amount of CO2 to the reactor. This ejector was not in operation at the time ofthe symposium of 2004, but it is now and with success. Therefore the extra amount of CO2

becoming available for stripping increased the stripping efficiency, compensating for theunderperforming medium pressure carbamate condenser.

Now, in the case of PIC Kuwait the original reactor was relatively long, about 27 meter, and forthat reason the elevation of the pool condenser had to be relatively high. If on the other handthe reactor would have been shorter, the pool condenser would have been at a lower elevation,the stripper being equally lowered. Ultimately in this concept the pool condenser can be at thenormal elevation for a Stamicarbon pool condenser in a CO2 stripping plant, about 22 meters,with the stripper installed at grade floor, which is also the normal elevation for a Stamicarbonstripper.

This is an important aspect of this revamp, which contributed to the development of the newStamicarbon concept.

3.3.2. Reactor located on ground level in pool condenser plantFrom the above discussed example it has become clear that it is possible and proven oncommercial scale to have the reactor in a CO2 stripping plant located on ground level, whilemaintaining gravity flow in the synthesis loop. This can be done by installing a pool condenserabove the vertical reactor.

As already mentioned Stamicarbon already has been very successful with the pool condenser,which combines the functions of a HP carbamate condenser and a urea reactor in one vessel.Up to now in designing urea plants the downstream reactor was located above the poolcondenser, enabling also the overhead vapour of the pool condenser to enter the reactor. Now,Stamicarbon takes this one step further and has developed a new synthesis concept. In thisconcept the reactor is located on ground level with the pool condenser still located at about thesame height, but now above the reactor.

This implies that the overhead vapour of the pool condenser can no longer enter the reactor.Therefore CO2 directly from the compressor is used as vapour source for supplying heat to thereactor.

2 Since the urea formation reaction is endothermic a CO2 or a NH3/CO2 vapour feed is needed for supplying heat tothe reactor via carbamate formation, which is an exothermic reaction. This to prevent that the reactor temperatureends up too low.


14.

The required amount of CO2 for that can be kept low however, since the amount of urea to beformed in the vertical reactor is limited due to the fact that in the pool condenser a large amountof urea already is produced. An important feature here is that the pool condenser is extendedwith an adiabatic reaction part, reducing the required reaction volume of the vertical reactor.This aspect has been demonstrated to be successful in the Stamicarbon’s pool reactor plants.

The overhead vapour of the pool condenser is vented to a simple MP absorber. This is possiblebecause the overhead flow is small, thanks to the zero oxygen concept.

The new synthesis concept is presented in figure 9.

Figure 9: the new Stamicarbon synthesis concept

Because of the limited amount of CO2 required for reactor heating the achievable strippingefficiency in the new Stamicarbon plant concept is still high enough to be able to discharge theliquid from the stripper to a recirculation section operated at low pressure, so no in-line MPrecirculation section is required.

From figure 9 follows that the pool condenser and the reactor are directly communicatingvessels and so the pool condenser only needs to be a few meters above the reactor in order tohave the pool condenser overflowing to the reactor. A second criterion is that the top of thereactor is located sufficiently high above the liquid inlet of the stripper to have the reactoroverflowing to the stripper.

VAPOUR AND INERTSTO LP ABSORBER

NH3

SOLUTION TOLP RECIRCULATIONSECTION

MP STEAM

MP STEAMCONDENSATE

LP STEAM / STEAMCONDENSATE

STEAM CONDENSATE

HP STRIPPER

POOL REACTOR

REACTOR

CO2

VapourLiquid

UtilitiesVapourLiquid

Utilities

22 mtr.

14 mtr.

1 mtr.

CARBAMATE FROMMP RECIRCULATION

MP ABSORBER


15.

At this point it is clear that with the new Stamicarbon synthesis concept the challenge putforward in the introduction of this chapter is met.

- The reactor is located on the ground floor (reducing the total synthesis height).- The main synthesis recycle is on gravity flow.- The urea solution from the stripper is discharged directly to a low pressure recirculation

section.

Figure 10 shows an extension of the sky lines of the Stamicarbon plants in history.

Figure 10: the Stamicarbon synthesis height lowered further again

<2300mtd >2300mtd

With the new concept the height of a Stamicarbon pool condenser synthesis section has comedown from 39 meter to 25 meter.

76000

38000

50000

64000

22000

0

14000

Using 100% gravity the Stamicarbon designcame down from 76 meters to some 25 metersfor a pool condenser plant


16.

3.3.3. Conclusions for low level synthesis arrangementStamicarbon has developed a low elevation synthesis concept. This concept has the followingfeatures. The reactor is installed on ground level. The pool condenser remains at about its original height. The stripping efficiency is still high enough to be able to discharge the liquid from the stripper

to a recirculation section operated at low pressure. The synthesis recycle is still on gravity flow.


17.

4. The full picture

Adding up the effects of the two main topics of this paper, zero oxygen synthesis operation andthe low level synthesis arrangement, we arrive at the following features of the new Stamicarbonplant concept.

Minimum equipment- The air blowers are deleted.- The hydrogen converter is deleted.- The high pressure scrubber and pertaining cooling system is replaced by a simple

medium pressure absorber.- The high pressure ejector is deleted.

Maintenance friendly- Safurex material is maintenance free.- The reactor is installed on ground level.- All equipment is easy accessible.

Reliable operation, highest on stream time- The synthesis recycle is on gravity flow.- The downstream, low pressure part of the plant has not changed flow sheet-wise.- Intrinsically safe plant.

The overall medium pressure steam consumption is equal to that of our former plant concept,but can be reduced further, depending on the optimum stripping efficiency, adapted to the siterequirements. In the attachment of this paper the topic of optimizing stripping efficiency isdiscussed. In the new plant concept there is room for such an optimization, certainly if use ismade of the option for a stripgas ejector as demonstrated in the PIC Kuwait revamp. It isadvised that a stripping efficiency optimization is performed for each specific grass root ureaplant project.

With the deletion of required equipment and the low elevation of the synthesis the investmentcost for the new Stamicarbon urea plant concept is again reduced.


18.

5. Summary

In our continuous efforts to improve the performance of our urea plants, Stamicarbon now hasdeveloped a low elevation plant concept, whilst maintaining the trouble free principle of gravityflow for the main recycle within the synthesis section. All process elements used for this newplant concept are proven in revamp projects.

Through the use of Safurex material of construction, the process design can be furthersimplified since passivation of the synthesis materials of construction becomes history.Consequently the new plant concept in an intrinsically safe and maintenance free plant concept.

Overall, it can be concluded that with these new process developments, Stamicarbon oncemore shows its dedication to urea process design developments, which focus on our customersneeds.

Low CAPEX: don’t pay more for your plant then required. Low OPEX: don’t pay more for your energy then required. High product quality: assure trouble free acceptance of the product at your customers. High safety standards: no oxygen present, no explosion risk. High on-stream time: avoid production losses caused by non-scheduled production

interruptions. Low emission standards: assure your ‘license to operate’, and stay friends with your

neighbors!


19.

Attachment: optimize stripping efficiency

Reducing the energy consumption of a urea plant at first glance seems not too difficult. If welook at the simplified mass and heat balance as represented in figure11, then all we need to dois reduce the flow of MP steam to the HP stripper, which will result in a lower strippingefficiency.

Figure 11:simplified mass and heat balance of the Stamicarbon urea process

Let us look in a bit more detail, to see what happens if, for instance, the amount of heatintroduced in the form of MP steam to the synthesis section is reduced with say 1000 kW.Since the total amount of heat required to concentrate the urea melt to its final concentrationremains the same (as a first approximation), it follows that the amount of heat to be supplied tothe recirculation section then has to increase by 1000 kW. Moreover it should be observed thatthe amount of LP steam produced in the urea synthesis section will reduce by a heat equivalentof 1000 kW, since the amount of stripgas produced in the HP stripper is reduced by anequivalent of 1000 kW. So, as a net result, the amount of LP steam exported from the ureaplant will reduce by 2000 kW (1000 kW less produced, 1000 kW more consumed). Then thereare some secondary effects: if the stripping efficiency is reduced, then the carbamate recyclewill increase.

This has a negative effect on the achievable conversion in the reactor. The result of this will bean increase of the MP steam demand in the HP stripper, partially offsetting the initial 1000 kWequivalent MP steam reduction. Moreover, the increased carbamate recycle will result in morewater to be evaporated in the evaporation section, so increasing the LP steam demand in thissection. As an overall result, lowering the heat input to the stripper in the form of MP steam with1000 kW, will result in a net decrease of the LP steam exported from the urea plant by a heatequivalent of 2500 – 3000 kW.

Ureasynthesis

Ureasynthesis

LP Steam4.5bar

saturated

LP Steam4.5bar

saturated

Recirculationsection

Recirculationsection EvaporationEvaporationNH3

CO2

NH3CO2

CarbamateCarbamate

UreameltUreamelt

Strippedurea solution

Strippedurea solution

UreasolutionUrea

solution

OtherLP steam

consumers

OtherLP steam

consumers

strippingefficiency =

MP Steam23 bar330 C

LP Steam

Simplified heat balance Urea stripping plant


20.

Whether or not this is a good deal of course depends upon the relative value of MP steamversus LP steam at the site.A popular destination for LP steam export, used in many plant designs, is to send the LP steamexport to the turbine driving the CO2 compressor (figure 12)

Figure 12: LP steam used in CO2 compressor turbine

In this concept, HP steam (typically 40 bar, 400 °C) steam is used to drive this turbine. Therequired MP steam for the urea melt plant at a pressure level of 23 bar is extracted from thisturbine. The LP steam produced in the urea melt plant is submitted into the back end of thisturbine. The total required energy for the turbine is balanced by letting down part of the HPsteam to vacuum.

Whether or not it is a good deal to reduce the MP steam to the urea plant, on the cost of a muchhigher reduction in the LP steam produced, is not easily detected in this scheme.

Ureamelt plant

Ureamelt plant

HP Steam40 bar, 400°C

LP Steam4 bar, saturated

MP Steam23 bar, 330°C

Coolingwater

TurbineDriving CO2compressor

TurbineDriving CO2compressor


21.

It strongly depends on a number of boundary conditions, which can only be defined in closecooperation with the turbine manufacturer; to mention some.- The efficiencies of the different sections of the turbine.- The physical dimensions of the turbine may allow for only limited LP steam submission to

the back end.- Since extraction and admission steam flows are dictated by the urea melt plant, the only

parameter that is left for actual speed control of the turbine is the amount of HP steam tovacuum. In order to allow for proper speed control, this flow cannot be too low.

- Admission of LP steam in the back end of the turbine influences turbine dimensions as wellas dimensions required for the vacuum condenser. So, in the optimization process alsocapital investment should be taken into account.

As a final conclusion we may state that optimizing the stripping efficiency for a grass root plantrequires a careful evaluation of economic parameters, taking into account technical limitations.Such optimization can only be done in close cooperation between end-user, process licensor,engineering contractor and turbine supplier. Generic conclusions are not possible; it may wellbe that in some plants the optimum will be a ‘high’ stripping efficiency, whereas in other case a‘low’ stripping efficiency may turn out to be more beneficial to the plant operating company.

Urea 11th Symposium

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