Patent Application
20240208897
The invention relates to improvements of certain equipment of a urea production plant. Particularly the invention relates to an improved design applicable to urea strippers and evaporators.
Urea is produced industrially by reacting ammonia and carbon dioxide under suitable urea-forming conditions, which typically include a high pressure of 100 to 200 bar and a high temperature around 200° C. An overview of the industrial processes for the synthesis of urea can be found in Meessen, “Urea”, Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH Verlag, 2010.
The reaction mixture that can be obtained from a synthesis reactor is an aqueous solution including urea and a significant amount of unconverted reagents, typically in the form of ammonium carbamate. In a urea stripping process, the reaction mixture is processed in a urea stripper wherein ammonium carbamate contained in the mixture is thermally decomposed to obtain a purified urea solution and a gaseous stream including ammonia and carbon dioxide removed from the mixture.
The purified urea solution still contains come carbamate and is normally sent to one or more recovery sections operating at a lower pressure to further decompose and remove the ammonium carbamate; the ammonia and carbon dioxide emerging from the stripper are normally condensed and recycled to the reactor.
The stripping process may be performed with the help of a stripping medium.
The known CO2 stripping process uses gaseous CO2 as a stripping medium. In the alternative ammonia-stripping process, the reaction mixture contains excess of ammonia so that ammonia can act as a stripping aid.
The urea stripper is normally a vertical shell-and-tube apparatus wherein the reaction mixture flows downwards inside the tubes of a tube bundle, usually in a falling-film regime. The tube bundle is heated by hot steam traversing the shell side to provide the heat for decomposition of carbamate. The stripping medium, if used, is generally fed counter-current to the solution.
The urea solution effluent from the tubes, i.e. after the stripping process, is collected in the lower part of the stripper. Particularly, the known strippers have a bottom liquid holder wherein the purified urea solution is collected. The bottom liquid holder is directly in communication with a liquid outlet port of the stripper. The ammonia and CO2 vapours on the other hand are generally withdrawn from top of the stripper.
The bottom holder is usually equipped with a vortex breaker to avoid the formation of a vortex in proximity of the outlet nozzle.
The applicant has noted that urea strippers suffer from a problem of gas carry-over. This term denotes the undesired occurrence that some process gas contained in the stripper (predominantly ammonia and/or carbon dioxide) is carried by the liquid effluent. It is desirable to maintain a good separation between the liquid phase and the gaseous phase to avoid said gas carry-over. However the applicant has noted the bottom part of the stripper, wherein the processed liquid is collected and directed towards the liquid outlet, is particularly exposed to the risk of gas carryover.
The applicant has further noted that gas carryover may occur in the evaporators of urea plants. An evaporator is provided to remove water from the aqueous solution produced in a recovery section. Said aqueous solution contains urea, water and unavoidable impurities. By removing water, the solution is converted into a more concentrated solution or a urea melt, for example a urea melts suitable for a finishing process like prilling or granulation. The urea melt may have a concentration greater than 96 wt % of urea or greater than 98 wt % or greater than 99 wt % according to the use. For a prilling process a purity greater than 99.5 wt % may be required. The applicant has noted that the concentrated solution or urea melt may entrain some process gas, predominantly ammonia and carbon dioxide or inert gas.
The invention addresses the technical problem of how to avoid or reduce the drawback of gas carryover in a urea stripper or urea evaporator, as above disclosed. Accordingly the invention aims to increase the separation between the liquid effluent and process gas contained in the stripper or evaporator. This separation is also termed phase disengagement. Accordingly the patent aims to improve the phase disengagement at the liquid outlet of the stripper or of the evaporator.
The applicant has realized that a common feature of urea strippers and evaporators is that the volume of the liquid holder (hold up volume) is comparatively small to reduce the residence time of the liquid (typically around 5 to 20 seconds) and related problems of formation of undesired by-products such as biuret. This is particularly true for large items designed for a large capacity, wherein the size of the main pressure vessel is comparatively larger. As a consequence the liquid flow entering the holder has a high velocity and momentum which may promote the carry-over of process gas, typically of ammonia, carbon dioxide and inert gases. The idea underlying the invention is to provide the bottom of the apparatus, particularly the liquid holder, with fittings adapted to oppose this effect.
In accordance with the above, the invention may include at least one of the following:
The above items represent fittings adapted to reduce the velocity of the liquid stream inside the bottom liquid holder and to limit the undesired formation of liquid-gas recirculation. More precisely the above items can break the liquid flow and reduce its velocity and momentum in the lower part of the apparatus and particularly at the liquid outlet. The applicant has found that breaking the liquid flow and reducing its velocity and momentum can greatly reduce the undesired gas carryover.
Further aspects of the invention are a urea plant including the inventive urea stripper and/or the inventive evaporator, and a method for revamping such items. Particularly the various embodiments of the invention can be applied to the provision of a new stripper or evaporator as well as to the modification of an existing stripper or of an existing evaporator.
The stripper is normally included in a high-pressure synthesis loop including a reactor and a condenser and optionally a scrubber.
The modification of an existing item is an interesting embodiment of the invention. A relatively inexpensive modification of the lower part of the apparatus may give a significant improvement in terms of performance. For example referring to the modification of a stripper, the invention gives a better separation of phases thus improving the recycle of the gaseous phase to the condenser and the operation of the recovery section downstream, where the liquid effluent of the stripper is processed.
The above options a) to d) are now elucidated in a greater detail.
The option a) may include a plurality of perforated screens arranged at different elevation in the bottom liquid holder. The plurality of perforated screens may include disc-shaped screens and/or ring-shaped screens. Said screens may be stacked one above the other.
In an embodiment each of the perforated screens has a surface area substantially equal to the cross section of the liquid holder.
In an interesting embodiment, the plurality of perforated screens includes screens with a surface area smaller than the cross section of the liquid holder and the screens are arranged to cover different portions of the cross section.
Particularly, at least two consecutive screens may be staggered or offset to provide a sort of labyrinth path for the liquid stream flowing into the liquid holder.
In another interesting embodiment the option a) includes a horizontal screen proximal to a liquid outlet duct of the liquid holder. Said horizontal screen has preferably an unperforated central region adapted to act as a vortex breaker and a perforate annular region around said central region. Said screen may be spaced apart from the outlet duct by suitable supports so that the liquid flows into the outlet duct through passages between the supports. The supports may be substantially shaped like blades.
In an interesting embodiment, one or more perforated screen may be integral with a vortex breaker installed in the liquid holder. A vortex breaker is usually provided in the bottom liquid holder of a stripper.
The option b) may include a perforated cylindrical wall placed in the bottom liquid holder, so that the inside of the bottom liquid holder is divided into an annular chamber around said wall and a central chamber inside said wall. Said chambers are communication through the perforations of said wall and only the central chamber is in communication with the liquid outlet. The liquid stream (e.g. aqueous solution from a stripper) is collected predominantly in the annular chamber; from here it passes into the central chamber through the perforated wall and eventually into the liquid outlet.
In a further embodiment the option b) includes a vertically arranged perforated cylindrical wall extending downward from an unperforated horizontal screen. Said vertically arranged perforated wall may be realized with the known technique of Johnson screens or equivalent. In some embodiments it can be a slotted wall with a number of slots providing a suitable passage area.
According to yet another embodiment the option c) includes a closed cylindrical chamber placed in the bottom liquid holder, so that the only region of the bottom liquid holder where liquid can be collected is an annular region around said sealed chamber. Said closed chamber may be delimited by a section of tube fitted in the liquid holder.
Said closed chamber reduced the volume available to the liquid in the bottom holder, so that for a given amount (flowrate) of liquid a higher level will form in the holder. A higher liquid level improves the phase disengagement.
The option d) may include a plurality of blades of a suitable shape and/or vertical plates projecting from the inner surface of a bottom domed end of the apparatus.
Said option d) is particularly interesting for a stripper. In operation, the solution effluent from the tubes flows over the inner surface of the domed end towards the bottom liquid holder. The provision of a suitable number of blades or vertical plates reduces the velocity and momentum of the liquid flow travelling towards the bottom holder.
The invention is applicable to all kinds of urea strippers. The urea stripper may be adapted to operate according to any of: the self-stripping process, the ammonia-stripping process, the CO2 stripping process. A stripper for the CO2 stripping process includes a CO2 inlet and a CO2 distributor in the lower part. The CO2 distributor is adapted to provide an uprising flow of gaseous CO2 in the tubes of the tube bundle, counter-current relative to the falling film of liquid.
The urea stripper is generally part of a high-pressure synthesis section or high-pressure synthesis loop. Said loop may include, in addition to the stripper, a urea synthesis reactor and a high-pressure condenser. The high-pressure condenser typically receives the gaseous stream from the stripper and a recycle carbamate solution from a recovery section. The loop may also include a high-pressure scrubber when vapours and inert gas withdrawn from top of the reactor are scrubbed with a carbamate solution.
The evaporator to which the invention is applied may be part of an evaporation section comprising more than one evaporator.
A further aspect of the invention is a method for revamping a stripper or an evaporator of a urea plant according to the claims.
A particularly interesting embodiment provides the modification or replacement of the vortex breaker. A urea stripper is normally equipped with a vortex breaker which includes unperforated screen positioned above the entrance of the liquid outlet and a suitable number (e.g. four) of lateral blade supports. The liquid flowing from above can enter the liquid outlet only by flowing laterally between the support blades as the screen prevents the liquid from falling vertically and directly into the liquid outlet.
According to the invention said vortex breaker can be modified or replaced providing a perforated horizontal annular screen around said unperforated vortex-breaker screen. As a result, a first horizontal screen is provided with a unperforated central region surrounded by a perforated annular region. In some embodiments, one or more additional perforated screens, horizontally arranged, may be placed above said first screen.
Some embodiments of the inventive method of revamping can be put into practice by a modification or replacement of the existing vortex breaker. A vortex breaker can be modified for example by adding one or more perforated screen(s) to the conventional structure of the vortex breaker, as will be elucidated below with some examples.
In another embodiment a modified vortex breaker includes an unperforated horizontal first screen and a vertically arranged annular perforated screen extending down from said first screen.
Embodiments of the revamping method based on modified vortex breakers may be of particular interest due to easy and inexpensive implementation. Particularly, the vortex breaker is relatively easy to access by opening the lower part of the pressure vessel and therefore the modification can be carried out on site in a relatively short time, which is a considerable advantage. Usually urea strippers and evaporators have a bottom manhole so that operation inside the lower part of its vessel is possible without complication.
The above features of a urea stripper are known to the skilled person and need not be described in detail.
The liquid holder 11 is basically a cylinder vessel with a diameter smaller than that of the stripper, i.e. smaller than the main pressure vessel 1.
In operation, the solution effluent from the tubes 8 flows over the inner surface 19 of the domed end 10 and is collected in the liquid holder 11 forming the liquid level 18. Flowing laterally around the vortex breaker 16, the liquid finally enters the outlet 12 and leaves the stripper.
Accordingly, the solution flows into the outlet 12 through the perforated screens 20 which reduce the velocity and momentum of the liquid flow and increase phase separation.
The various embodiments of the invention are not mutually exclusive and can be combined. In principle, all embodiments described herein can be freely combined. For example some embodiments may include the screens or plates 30, 31 of
Embodiments of
In operation, the urea solution of line 43 traverses the tube side of the shell-and-tube exchanger 41, wherein the tubes are externally heated by steam 45 in the shell side. Water is then removed in the vessel 42 under vacuum and leaves the vessel via line 47.
The above embodiments and combinations thereof for reducing the gas carryover are applicable to the liquid holder of the evaporator E. This applies in particular to options a) to d) as above described and to embodiments of
| Number | Date | Country | Kind |
|---|---|---|---|
| 21170937.3 | Apr 2021 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2022/060995 | 4/26/2022 | WO |
