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Title:
A SYSTEM AND A METHOD FOR ANALYSIS OF VENT GAS OF A UREA PLANT
Document Type and Number:
WIPO Patent Application WO/2018/051313
Kind Code:
A1
Abstract:
A system (2) for analysis of vent gas of a urea plant (1), comprising: a Raman spectroscope (22); a sampling conduit (24) that connects the spectroscope (22) to a main pipe (20) of the urea plant (1) for conveying a sample stream (25) to be analysed to the spectroscope (22); and a temperature-adjusting device (26), operated by a temperature controller (27) and acting on at least one thermal treatment portion (28) of the conduit (24) for adjusting the temperature of the sample stream (25) circulating in the conduit (24).

Inventors:
CERRONE, Cristina (Via Martiri Di Cefalonia 67, San Donato Milanese, 20097, IT)
CARLESSI, Lino (Via Pascolo 47, Dalmine, 24044, IT)
SERRAFERO, Alberto (Via Martiri Di Cefalonia 67, San Donato Milanese, 20097, IT)
VIOLA, Francesco (Via Martiri Di Cefalonia 67, San Donato Milanese, 20097, IT)
Application Number:
IB2017/055658
Publication Date:
March 22, 2018
Filing Date:
September 19, 2017
Export Citation:
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Assignee:
SAIPEM S.P.A. (Via Martiri di Cefalonia 67, San Donato Milanese, 20097, IT)
International Classes:
G01N21/65; C07C273/00; G01N1/10; G01N1/22; G01N21/01; G01N21/15; G01N21/85
Foreign References:
EP2955506A12015-12-16
EP1398617A12004-03-17
US3451895A1969-06-24
US5637809A1997-06-10
US20150377750A12015-12-31
DE4109688C11992-10-29
US5807750A1998-09-15
Attorney, Agent or Firm:
CERNUZZI, Daniele et al. (STUDIO TORTA S.p.A, Via Viotti 9, Torino, 10121, IT)
Download PDF:
Claims:
CLAIMS

1. A system (2) for analysis of vent gas of a urea plant (1), comprising a gas stream qualitative/quantitative analysis instrument (22) and a sampling conduit (24) having a connector (31) connectable to a main pipe (20) of the urea plant (1) for conveying a preferably continuous sample stream (25) of gas from the main pipe (20) to the analysis instrument (22); the system (2) being characterized in that the analysis instrument is a Raman spectroscope (22); and the system (2) comprises a temperature-ad usting device (26), operated by a temperature controller (27) and acting on at least one thermal treatment portion (28) of the conduit (24) for adjusting the temperature of the sample stream (25) circulating in the conduit (24) .

2. The system according to claim 1, wherein the device (26) comprises a cooling circuit (33), arranged around the thermal treatment portion (28) of the conduit (24) and in which a cooling fluid circulates; and at least one circulation member (34) controlled by the temperature controller (27) for circulating the cooling fluid in the cooling circuit (33) .

3. The system according to claim 1 or 2, comprising a reflux device (29) for collecting and returning to the main pipe (20) a condensate fraction of the sample stream (25) , condensed following a cooling of the sample stream (25) in the thermal treatment portion (28) of the conduit (24) .

4. The system according to claim 3, comprising a processing and control unit (30) configured so as to calculate the amount of condensable species returned to the main pipe (20) .

5. The system according to claim 3 or 4, wherein the reflux device (29) comprises a collecting portion (38) of the conduit (24); the collecting portion (38) being positioned above the main pipe (20) and beneath the thermal treatment portion (28) of the conduit (24) and being substantially vertical or slanted toward the connector (31) and hence towards the main pipe (20) such that the condensate fraction falls through the collecting portion (38) and into the main pipe (20) below under the effect of gravity.

6. The system according to one of the preceding claims, wherein the spectroscope (22) is positioned above the main pipe (20) and the conduit (24) departs vertically from the main pipe (20), and/or extends substantially vertical or slanted upwards from the connector (31) .

7. The system according to one of the preceding claims, wherein the temperature controller (27) is a "dual-loop" type controller and is connected to a pair of temperature sensors (35, 36) respectively positioned upstream and downstream, in a circulation direction of the sample stream (25) in the conduit (24), of the thermal treatment portion (28) of the conduit (24) .

8. The system according to one of the preceding claims, wherein the temperature controller (27) is set to a temperature lower than a threshold temperature that can damage components of the spectroscope (22) .

9. The system according to one of the preceding claims, wherein the device (26) also comprises a heating element (37), for example an electric resistance, positioned in the conduit (24) downstream of the thermal treatment portion (28) and the cooling circuit (33) with reference to a circulation direction of the sample stream (25) in the conduit (24), and operated by the temperature controller (27) for raising the temperature of the sample stream (25) after the cooling caused by the cooling circuit (33) and warding off saturation conditions of the sample stream (25) entering to the spectroscope (22) .

10. The system according to one of the preceding claims, comprising an automatic emergency shut-off valve (39), positioned at an inlet (32) of the spectroscope (22) or along the conduit (24) for interrupting the supply of the sample stream (25) to the spectroscope (22) in case of a thermal rise above a pre-set danger threshold.

11. The system according to one of the preceding claims, wherein the spectroscope (22) has an outlet (40) connected to a return conduit (41), which is connectable to the main pipe (20), in particular downstream of the connector (31) in a vent gas circulation direction through the main pipe (20), for returning the sample stream (25) conveyed through the spectroscope (22) back to the main pipe (20) .

12. A urea plant (1), comprising a system (2) for analysis of vent gas according to one of the preceding claims.

13. A method for analysis of vent gas of a urea plant (1), comprising the steps of: taking a preferably continuous sample stream (25) of gas from a main pipe (20) of the urea plant (1); and performing a qualitative/quantitative analysis of the sample stream (25) ; the method being characterized in that the analysis of the sample stream (25) is performed by Raman spectroscopy; and the method comprises a step of controlling the temperature of the sample stream (25) analysed by Raman spectroscopy for maintaining said temperature below a pre-set threshold temperature. 14. The method according to claim 13, comprising the steps of detecting the temperature of the sample stream (25) in a sampling conduit (24) that conveys the sample stream (25) to the Raman spectroscopy analysis step; and cooling the sample stream (25) if the detected temperature is higher than the pre-set threshold temperature.

15. The method according to claim 14, comprising, after the step of cooling the sample stream (25) , a step of slightly heating the sample stream (25) for warding off saturation conditions of the sample stream (25) entering the Raman spectroscopy analysis step.

16. The method according to claim 14 or 15, wherein the step of detecting the temperature of the sample stream (25) comprises the steps of detecting the temperature upstream and downstream, in a circulation direction of the sample stream (25) in the conduit (24), of a thermal treatment portion (28) of the conduit (24) in which the sample stream (25) is cooled.

17. The method according to one of claims 13 to 16, comprising a step of collecting a condensate fraction of the sample stream (25) , condensed in the step of cooling the sample stream (25) , and returning said condensate fraction back to the main pipe (20) of the urea plant (1) . 18. The method according to claim 17, comprising a step of calculating the amount of condensable species brought back into the main pipe (20) .

19. The method according to one of claims 13 to 18, wherein the analysis of the sample stream (25) is performed in a Raman spectroscope (22) positioned above the main pipe (20) and connected to the main pipe (20) via a sampling conduit (24) departing vertically, and/or extending substantially vertical or slanted upwards from a connector (31) joining the conduit (24) to the main pipe (20) .

20. The method according to one of claims 13 to 19, comprising a step of interrupting the supply of the sample stream (25) to the Raman spectroscopy analysis step in case of a thermal rise of the sample stream (25) above a pre-set danger threshold.

21. The method according to one of claims 13 to 20, comprising a step of returning the sample stream (25) analysed in the Raman spectroscopy analysis step back into the main pipe (20), in particular downstream, in a vent gas circulation direction through the main pipe (20), of a sampling point in which the sample stream (25) is taken from the main pipe (20) .

Description:
"A SYSTEM AND A METHOD FOR ANALYSIS OF VENT GAS OF A UREA PLANT"

TECHNICAL FIELD

The present invention relates to a system and a method for analysis of vent gas of a urea plant, as well as a urea plant equipped with a system for analysis of vent gas.

BACKGROUND ART

As is known, urea is produced on an industrial scale via processes based on the reaction, under high-temperature and high-pressure conditions, between carbon dioxide and ammonia to form ammonium carbamate, and the subsequent decomposition reaction of the ammonium carbamate to provide urea and water.

These processes are carried out in plants (urea plants) that generally comprise: a synthesis reactor, in which the reaction between carbon dioxide and ammonia takes place; purification and recovery sections, in which the aqueous urea solution produced in the reactor is progressively concentrated, with the recovery and recycling of unconverted reagents; and a finishing section, where the urea is solidified (for example, in a granulator or prilling tower) .

However it is configured, a urea plant generates continuous vent gas, containing inert unconverted substances and also small quantities of unreacted ammonia. Depending on the process used in the urea plant, this vent gas may also contain oxygen, introduced for passivation in particular.

For example, a urea plant that adopts the technology known as "Snamprogetti" discharges, more specifically from a so-called medium-pressure recovery section, a continuous gas stream typically containing nitrogen, oxygen, argon, methane, ammonia, hydrogen and water.

The need arises to monitor vent gas of the urea plant, especially when this vent gas contains hydrogen and oxygen, in particular for meeting safety requirements and, specifically, for remaining outside of the ignition limits.

If a risk situation is detected, action is taken on the vent gas stream, for example, by adding steam or another control fluid (inert gas, natural gas or the like) so as to avoid the formation of an explosive mixture.

It is also necessary to check when the risk situation has passed, so as to interrupt injection of the control fluid (steam or the like) in safe conditions.

The use of a gas chromatograph has been proposed for the analysis, with the plant running, of a vent gas stream coming from a urea plant and containing ammonia.

Gas chromatographic analysis is often used for monitoring processes as it enables identifying and quantifying a large number of components simultaneously. The individual sample components pass through a column at different speeds and are registered in succession by a detector. The time lapsing between introduction of the sample and the registration of a substance by the detector (retention time) is characteristic of the substance and is used to identify it. The magnitude of the detector signal is a measurement of the volumetric concentration of the component in the analysed fluid.

However, the use of a gas chromatograph has some drawbacks in the specific application of urea plant vent gas analysis. First of all, the presence of condensing steam or another control fluid in the gas stream to be analysed (a situation that occurs following injection of steam or another fluid to move outside the ignition limits) undermines the gas chromatography measurement of the gas mixture's components, especially the quantitative analysis of ammonia.

Furthermore, the phenomena associated with the condensation- evaporation of steam (or any other fluids) in the gas chromatograph can compromise the mechanical integrity of the instrument .

In addition, a gas chromatograph is a bulky instrument, also because it must be conditioned, and so, in general, it cannot be placed directly on the main pipe in which the gas stream to be analysed circulates, but must be placed in a suitable structure in a remote position and be connected to the main pipe by a specially provided line, which can also be relatively long. All of this, apart from obviously complicating the installation of the equipment, can also compromise measurement accuracy.

DISCLOSURE OF INVENTION

One object of the present invention is to provide a system and method for analysis of vent gas of a urea plant that enables overcoming the above-mentioned drawbacks of the known art.

In particular, one object of the invention is to provide a system and method for analysis of vent gas of a urea plant that enables quick, accurate and reliable vent gas analysis in all operating conditions of the urea plant, even in the event of significant variations due, for example, to the injection of steam or another control fluid into the vent gas.

The present invention thus relates to a system and method for analysis of vent gas of a urea plant as defined in essential terms in the appended claims 1 and 13, respectively, and to a urea plant equipped with a system for analysis of vent gas as defined in claim 12.

Additional preferred characteristics of the invention are indicated in the dependent claims.

In accordance with the invention, a spectroscopic analysis technique is used for the analysis of vent gas of the urea plant, in particular Raman spectroscopy. Raman spectroscopy is an analytical technique based on the vibrational and rotational energy of molecules and enables uniquely identifying the chemical species present in a sample and their relative amounts. In Raman spectroscopy, a sample to be analysed is exposed to monochromatic electromagnetic radiation, typically emitted by a laser source, and the frequencies in the radiation scattered by the sample are detected and analysed. By interacting with the electron molecules of the sample, the electromagnetic radiation induces variations in the vibrational and rotational energy of the molecules, with consequent scattering of the incident radiation. By analysing the scattered radiation, it is possible to identify components with different energy levels.

Raman spectroscopy analysis is performed by special instruments (Raman spectroscopes) . To date, this analytical technique does not appear to have ever been used in urea plants, in particular for the analysis of vent gas at risk of generating explosive mixtures, due to the structural limits of Raman spectroscopes, which are intrinsically unsuited for the purpose.

The inventors of the present invention have identified the following problems related to Raman spectroscopes:

a) the optical fibre probes that equip Raman spectroscopes have relatively low design temperatures ( indicatively about 80°C), and so the instruments cannot be supplied with samples at higher temperatures, which would cause mechanical damage upon contact with the probes. The vent gas of the urea plant to be analysed has, especially following the injection of steam or the like for restoring non-ignition conditions, temperatures decidedly higher than the damage threshold of the probes of Raman spectroscopes;

b) the vent gas of the urea plant to be analysed contains condensing water that carried ammonia with it: the condensable ammonia species is therefore present in the gas stream to be analysed, which can be absorbed by water and precipitate again in the main gas stream of the process. In consequence, the sample analysed by the Raman spectroscope would be affected by the quantitative precipitation of one of the components to be analysed, undermining the accuracy of the measurement. The invention makes it possible to analyse, with the plant running and even substantially continuously or with a high sampling frequency, the vent gas at risk of forming explosive mixtures in a urea plant. The analysis is carried out by a Raman spectroscope, opportunely engineered and installed in such a way as to provide totally reliable results, even in the event of deviations from the normal running conditions of the urea plant, for example, following injection of steam (or another control fluid) into the gas stream to be analysed in order to avoid the formation of possibly explosive mixtures.

In accordance with the invention, the Raman spectroscope is associated with a sampling interface positioned upstream of the Raman spectroscope and provided with a temperature control system. The invention enables achieving the following main advantages:

- analysis results, both qualitative and quantitative, which are accurate and reliable in any operating condition, even in the case of injecting steam or another control fluid into the gas stream to be analysed: the measurements, taken by the Raman spectroscope, are unaffected by the content of steam or another control fluid, as they are based on the atomic/steric properties of the chemical species and not their thermodynamic behaviour, as instead happens in gas chromatographic analysis; in general, analysis is possible with the plant running, guaranteeing measurement accuracy and mechanical integrity of the instrument even in the presence of deviations from the normal operating conditions of the plant and/or deviations in pressure, temperature and relative content of the chemical species ;

- with respect to measurements taken with a gas chromatograph, response times are quicker, measurements are more accurate and maintenance is less expensive.

- the Raman spectroscope is placed directly on the main pipe in which gas stream to be analysed circulates, and to which it is connected by a simple branch; auxiliary piping for carrying the gas stream to be analysed to a remote position is not needed .

BRIEF DESCRIPTION OF THE DRAWINGS

Further characteristics and advantages of the present invention will become clearer from the description of the following non-limitative embodiments, referring to the figures in the accompanying drawings, in which:

- Figure 1 is a schematic view of a urea production plant equipped with a system for analysis of vent gas in accordance with the invention; and

- Figure 2 is a schematic view of a system for analysis of vent gas of a urea plant in accordance with the invention. BEST MODE FOR CARRYING OUT THE INVENTION

Figure 1 shows, in a purely schematic and simplified manner, a urea plant 1, or rather a plant for the production of urea, equipped with a system 2 for analysis of vent gas produced in the urea plant 1.

The urea plant 1, as well as the urea production process implemented therein, can be one of several types. Here, reference is made, purely by way of example, to a urea production plant/process according to the known "Snamprogetti" technology. It remains understood that the invention is also applicable to other urea production plants/processes. In the non-limitative configuration shown, but not necessarily, the urea plant 1 comprises: a urea synthesis reactor 4, where the reaction of urea synthesis from ammonia and carbon dioxide takes place; recovery sections 5, 6 and 7, in particular a high-pressure recovery section 5, a medium- pressure recovery section 6 and a low-pressure recovery section 7, where a urea solution produced in the reactor 4 is progressively concentrated by the removal of unreacted ammonia and carbon dioxide and water, and the recovered components are recirculated; a vacuum concentration section 8, connected to a treatment section 9 for process condensates (basically water) ; and a finishing/solidification section 10, comprising, for example, in a granulator or prilling tower.

The reactor 4 is fed with NH3 and C02 through respective feed lines 12 and 13. A main urea line 14 connects a product outlet of the reactor 4 to the recovery sections 5, 6 and 7 and to the concentration section 8.

A recovery circuit 16 connects the recovery sections 5, 6 and 7 and the concentration section 8 to each other and to the NH3 feed line 12 for recirculating the unreacted components recovered by the recovery sections 5, 6 and 7 to the reactor 4.

The components of the various sections and the connection lines between them, which in themselves are known, are neither shown nor described in detail.

For example, but not necessarily, the main urea line 14 connects in series (by respective line segments) : the reactor 4, where the urea synthesis reaction from ammonia and carbon dioxide takes place; a stripper of the high-pressure recovery section 5, where unconverted NH3 and C02 and ammonium carbamate are recovered and recycled to the reactor 4 through the recovery circuit 16; a medium-pressure decomposer in the medium-pressure recovery section 6 and a low-pressure decomposer in the low-pressure recovery section 7, in which the carbamate is decomposed to give NH3 and C02, which are recycled to the reactor 4 through the recovery circuit 16; and one or more exchangers/concentrators in the concentration section 8, where the urea solution leaving the low-pressure recovery section 7 is concentrated before being sent to a granulator or prilling tower of the finishing/solidification section 10 for being granulated or prilled. In the urea plant 1, in particular in the recovery sections 5, 6 and 7 and in the concentration section 8, vent gas also containing ammonia is produced. The system 2 is connected to the urea plant 1 to receive and analyse at least one stream of vent gas produced in the urea plant 1.

In the non-limitative example shown, but not necessarily, the system 2 receives and analyses vent gas coming from the medium-pressure recovery section 6 and, in particular, that leaving the medium-pressure decomposer and conveyed in a main pipe 20. As, among other things, this vent gas contains hydrogen and oxygen, explosive mixtures may form, and so the main pipe 20 is connected to an injection device 21 for injecting a control fluid, for example, steam or another fluid, operated (manually by an operator via a control in the control room, or automatically by a control unit) when the system 2 detects potentially hazardous conditions for the formation of explosive mixtures. It remains understood that the system 2 can be used for analysing other effluents and/or gas streams produced in the urea plant 1 (which, as already pointed out, can also have a different configuration from that shown herein by way of example) . The system 2 can therefore be placed in a different position from that described and shown herein by way of example, as well as in a different type of urea plant.

Referring to Figure 2 as well, the system 2 comprises a Raman spectroscope 22 and a sampling interface 23 that connects the spectroscope 22 to the main pipe 20 in which the vent gas to be analysed circulates.

The spectroscope 22 is, in itself, a known instrument and, for simplicity, is therefore neither described nor illustrated in detail.

Basically, the spectroscope 22 comprises: at least one source of electromagnetic radiation, in particular a laser source, which directs monochromatic electromagnetic radiation on the sample to be analysed in an analysis chamber; at least one probe, in particular an optical fibre probe, which receives radiation scattered by the sample; and an analyser that analyses the frequencies present in the radiation scattered by the sample.

Advantageously, the spectroscope 22 is placed above the main pipe 20.

The sampling interface 23 comprises a sampling conduit 24, which connects the main pipe 20 to the spectroscope 22 to convey a continuous sample gas stream 25 from the main pipe 20 to the spectroscope 22; a temperature-ad usting device 26, controlled by a temperature controller 27 and acting on at least one thermal treatment portion 28 of the conduit 24 to adjust the temperature of the sample stream 25 circulating in the conduit 24; a reflux device 29 to return a condensate fraction of the sample stream 25, which has condensed following a cooling of the sample stream 25 in portion 28, back into the main pipe 20; and a processing and control unit

30 configured so as to calculate the amount of condensable species returned to the main pipe 20.

The conduit 24 connects the main pipe 20 to the spectroscope 22; in particular, the conduit 24 is connected to a connector

31 fitted on the main pipe 20 and to an inlet 32 of the spectroscope 22 and, more precisely, to the analysis chamber of the spectroscope 22.

Advantageously, the conduit 24 departs vertically from the main pipe 20, and/or extends substantially vertically or slanting upwards from the connector 31.

The temperature-adjusting device 26 can be of one of several types. In general, the device 26 is configured to reduce the temperature of the sample stream 25 when it is too high and could damage the components of the spectroscope 22, in particular its optical fibre probe.

For example, the device 26 comprises: a cooling circuit 33, arranged around portion 28 of the conduit 24 and in which a cooling fluid circulates (for example, air or water); and at least one circulation member 34 controlled by the temperature controller 27 for circulating the cooling fluid in the cooling circuit 33.

The temperature controller 27 (which can also be integrated in the processing and control unit 30 of the spectroscope 22) is preferably of the so-called "dual-loop" type and is connected to a pair of temperature sensors 35 and 36, respectively positioned upstream and downstream of portion 28 of the conduit 24 in the circulation direction of the sample stream 25 in the conduit 24 (for example, the temperature sensors 35 and 36 are arranged at an inlet of the conduit 24 and the inlet 32 of the spectroscope 22, respectively), to detect the temperature of the gas stream coming from the urea plant 1 through the main pipe 20 and of the sample stream 25 analysed in the spectroscope 22, respectively.

In particular, the temperature controller 27 is a controller of the proportional-integral-derivative type. The temperature controller 27 is set to a temperature (for example, 70°C) lower than a threshold temperature at which the components of the spectroscope 22 could become damaged (in particular, the optical fibre probe or probes) . Advantageously, the device 26 also comprises a heating element 37, for example an electric resistance, positioned in the conduit 24 downstream (always referring to the circulation direction of the sample stream 25) of portion 28 and of the cooling circuit 33, and in turn operated by the temperature controller 27 and/or by the processing and control unit 30 to raise the temperature (by a few degrees, indicatively 1-5°C) of the sample stream 25 after the cooling induced by the cooling circuit 33, in order to ward off saturation conditions of the sample stream 25 entering the spectroscope 22.

The cooling induced by the device 26 may cause the condensation of chemical species in the sample stream 25. The condensate fraction flows back into the main pipe 20 through the reflux device 29. For example, the reflux device 29 comprises a collecting portion 38 of the conduit 24; the portion 38 is placed above the main pipe 20 and beneath the thermal treatment portion 28 of the conduit 24 and is substantially vertical or slanted towards the main pipe 20 so that the condensate fraction falls through it into the main pipe 20 below, due to the effect of gravity.

The processing and control unit 30 is configured so as to calculate the amount of condensable species that are recirculated to the main pipe 20, via opportune computation algorithms based on thermodynamic models of the process implemented in the urea plant 1 and on the operating parameters of this process.

Optionally, the system also includes an automatic emergency shut-off valve 39, placed at the inlet 32 of the spectroscope 22 or along the conduit 24, to interrupt the supply of the sample stream 25 to the spectroscope 22 in the event of failure/malfunctioning of the temperature controller 27; the valve 39 intervenes automatically following any uncontrolled heat increase (above a preset danger threshold) to protect the spectroscope 22.

The spectroscope 22, or more specifically its analysis chamber, has an outlet 40 that is connected to the return conduit 41; the return conduit 41 is connected to the main pipe 20 and, in particular, is fitted on the main pipe 20 downstream of the connector 31 in the circulation direction of the vent gas circulating in the main pipe 20, to return the sample stream 25 that transited through the spectroscope 22 back to the main pipe 20. use, a gas stream constituted by vent gas produced in the urea plant 1 circulates in the main pipe 20.

The gas stream contains, purely by way of example, nitrogen, oxygen, argon, methane, ammonia, hydrogen and water, and has a temperature of approximately 50-170°C.

Part of the gas stream circulates through the conduit 24 and constitutes the sample stream 25 sent, preferably in continuously, to the spectroscope 22.

If the temperature of the sample stream 25 is lower than a preset temperature threshold (as in normal operating conditions of the urea plant 1), the temperature controller 27 does not operate the temperature-ad usting device 26 and the sample stream 25 reaches the spectroscope 22.

The spectroscope 22 performs qualitative and quantitative analysis on the sample stream 25. If the analysis detects conditions for the possible formation of explosive mixtures, the system 2 generates an alarm signal, on the basis of which the injection device 21 is (manually or automatically) activated for injecting steam ( indicatively at 200-250°C) or another control fluid into the gas stream circulating in the main pipe 20.

The injection of steam or another control fluid raises the temperature of the gas stream, and therefore also that of the sample stream 25, as well as varying its composition, but the system 2 is configured to effectively and correctly operate also in changed conditions.

In fact, if the temperature controller 27 detects, via the temperature sensors 35 and 36, a temperature of the sample stream 25 above the threshold temperature, the temperature controller 27 operates the temperature-adjusting device 26 to cool the sample stream 25 and bring the temperature of the sample stream 25 back below the threshold temperature.

Opportunely, the device 26 then intervenes, via the heating element 37, to slightly heat (a few degrees, indicatively 1- 5°C) the previously cooled sample stream 25 (in any case, keeping the temperature below the threshold temperature) , in order to ward off saturation conditions of sample stream 25. Any condensate fraction of the sample stream 25 falls back into the main pipe 20 through the reflux device 29.

The system 2 analyses the sample stream 25. The spectroscope 22 takes measurements at predetermined time intervals, or operates substantially continuously, depending on the measurement and response times of the instrument; the sample stream 25, after having travelled through the conduit 24 and reached the spectroscope 22, returns to the main pipe 20 through the return conduit 41, more specifically (preferably) downstream, in the direction of circulation of the vent gas circulating in the main pipe 20, of the connector 31, or rather of the sampling point where the sample stream 25 is taken from the main pipe 20 and sent to the spectroscope 22. When the system 2 detects that the conditions of possible formation of explosive mixtures have ceased, the system 2 generates an all-clear signal, on the basis of which injection of the control fluid (steam or the like) into the main pipe 20 is (manually or automatically) interrupted.

Finally, it is understood that further modifications and variants can be made regarding the system and method for analysis of vent gas of a urea plant and the urea plant described and illustrated herein without departing from the scope of the appended claims.