Carbon dioxide stripping urea plant with a natural-circulation synthesis loop and a method for retrofitting such plant

Abstract

A plant for synthesis of urea with a CO2-stripping process, comprising a natural-circulation synthesis loop, said loop including at least a urea reactor (1), a carbon dioxide stripper (2) and a condenser (3), said reactor, stripper and condenser operating substantially at the same elevated pressure, said loop comprising also a reactor effluent flow line (5), connecting said urea reactor to said stripper, which comprises means (15) for directly or indirectly detecting the flow rate and/or the direction of the flow through said reactor effluent flow line (5).

Claims

1. A plant for synthesis of urea with a carbon dioxide stripping process, comprising: a natural-circulation synthesis loop, said natural-circulation synthesis loop including at least a urea reactor, a stripper and a condenser, said urea reactor, stripper and condenser operating substantially at the same elevated pressure, said natural-circulation synthesis loop comprising also a reactor effluent flow line, connecting said urea reactor to said stripper; and, means for directly or indirectly detecting a flow rate and/or a direction of flow in said reactor effluent flow line.

2. The plant according to claim 1, said means being sensitive to difference between pressure in a first point of measure of said reactor effluent flow line and pressure in a second point of measure of said reactor effluent flow line.

3. The plant according to claim 2, said first point of measure being upstream a valve which is provided on the reactor effluent flow line for control of a liquid level in said urea reactor, and said second point of measure being downstream of said valve.

4. The plant according to claim 2, said means comprising a differential pressure sensor connected to said first point of measure and said second point of measure.

5. The plant according to claim 1, said means comprising a flow meter.

6. The plant according to claim 1, said means providing a signal which is related to the flow rate in said reactor effluent flow line.

7. The plant according to claim 1, said means providing a signal which is a flag signal, said flag signal having a first status when the flow rate in said reactor effluent flow line is within a normal operation range, and said signal changing to a second status when the direction of said flow rate is reversed or when the flow rate falls below a threshold value.

8. The plant according to claim 6, said signal being an alarm signal which is activated when the direction of the flow in the reactor effluent flow line is reversed, or when the flow rate in said line falls below a threshold value.

9. The plant according to claim 2, said means providing an alarm signal when said difference of pressure is deviating from normal valves, approaching zero or negative values.

10. A method for retrofitting a CO.sub.2-stripping urea plant, said plant comprising a natural-circulation synthesis loop, said natural-circulation synthesis loop including at least a urea reactor, a carbon dioxide stripper and a condenser, said urea reactor, stripper and condenser operating substantially at the same elevated pressure, said natural-circulation synthesis loop comprising also a reactor effluent flow line (5), connecting said urea reactor to said stripper, the method comprises a step of adding means for directly or indirectly detecting a flow rate and/or a direction of flow in said reactor effluent flow line.

11. The method according to claim 10, wherein said reactor effluent flow line comprises a valve for controlling a liquid level in the urea reactor, the method comprising adding a differential pressure sensor, said differential pressure sensor being arranged to detect the difference between pressure in a first point of the reactor effluent flow line upstream of said valve, and a second point of the same line downstream of said valve.

12. The method according to claim 10, comprising installing an alarm which is activated when said means detect a reverse flow in the natural-circulation synthesis loop, or when said means detect the flow rate in the natural-circulation synthesis loop falling below a threshold value.

Description

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

(1) FIG. 1 is a simplified diagram of the synthesis loop of a CO2-stripping urea plant. The diagram is simplified and some items such as valves, pumps and auxiliaries are not shown for the sake of simplicity.

(2) The loop comprises a urea reactor 1, a high-pressure stripper 2, a high-pressure condenser 3 and a high-pressure scrubber 4. In some embodiments, the loop may comprise more than one vessel for reaction, condensation or stripping, e.g. two condensers or two strippers. The urea solution produced in the reactor 1 and containing unconverted carbamate is directed to stripper 2 via the effluent flow line 5. Ammonia and carbon dioxide are stripped off the solution with the help of the fresh carbon dioxide feed 6. Concentrated urea solution 7 leaves the stripper 2 and is directed to a urea recovery section (not shown). The gaseous flow 8 leaving the stripper and containing ammonia and carbon dioxide is condensed in the condenser 3 and condensate 9 is sent back to reactor 1.

(3) A low-pressure carbamate solution 10 separated in the recovery section is scrubbed in the high-pressure 4 with overhead vapors 11 leaving the urea reactor, and then sent to the condenser 3 via an ejector 12. The ejector 12 is driven by the ammonia feed 13.

(4) The above features are known in the art and different implementations are possible. For example condensation in the condenser 3 may be partial or total; in case of partial condensation, the remaining gases are condensed in the reactor 1; in case of total condensation in the condenser 3, a gaseous feed of CO2 to reactor 1 is provided, e.g. splitting the main feed 6 into two lines, one to the stripper and the other to the reactor. Further embodiments include that the flow 8 is split into two parts, one part being sent to the reactor and the other part being sent to the condenser. These embodiments are cited as non-limitative examples of application of the invention.

(5) Circulation in the loop 1.fwdarw.2.fwdarw.3, that is inside flow lines 5.fwdarw.8.fwdarw.9, takes place naturally, being governed by gravity and by different values of the overall density inside the pressure vessels of the reactor, stripper and condenser. To this purpose, the stripper is usually below the reactor. A relevant driving force, in particular is given by the liquid level in the reactor, which is controlled by a valve 14. Said valve 14 receives a signal 20 from a sensor detecting the liquid level inside reactor 1, and operates by maintaining said liquid level within a desired range. It can be understood that the opening angle of said valve, introducing a certain pressure drop in the line 5, controls both the liquid level inside the reactor 1 and the flow rate in the line 5, which means that said valve 14 ultimately controls the overall circulation in the loop.

(6) The nominal flow through line 5 goes from reactor 1 to stripper 2 as shown by the arrows in FIG. 1.

(7) In the embodiment of FIG. 1, said line 5 comprises a differential pressure sensor 15 which is mounted across said valve 14. The sensor 15 is sensitive to the difference of pressure between a first point of measure 16 which is upstream the valve 14, and a second point of measure 17 which is downstream the valve 14. Preferably said points 16 and 17 are close to the valve, so that they have the same height and the measure of delta-p is not affected by one of the points of measure being above or below the other.

(8) The delta-p sensed by the sensor 15 is related to the flow rate [kg/s or m3/s] through the line 5. The delta-p is calculated as pressure in point 16 minus pressure in point 17 (p.sub.16p.sub.17). It can be noted that:

(9) said delta-p>0 means regular flow from reactor 1 to stripper 2,

(10) said delta-p<0 means reverse flow stripper 2 to reactor 1.

(11) Hence the sensor 15 makes available a signal 18 which is related to the current flow rate in the line or, in some embodiments, is a flag signal for triggering an alarm when the delta-p is negative (reverse flow) or when the delta-p is lower than expected. This signal is useful especially when the valve 14 is closed, before a start up. In such a condition, there is no current flowing in line 5, but detection of the pressure on both sides of the valve 14 allows predict the direction of the flow when the valve will open, and then allows prevention of a reverse start-up.

(12) In other embodiments, the delta-p measure can be taken between two generic points of the line 5, for example a first point closer to the reactor 1 and a second point closer to the stripper 2. As apparent, a positive delta-p will indicate regular flow while a negative delta-p will indicate that pressure is greater near the stripper, i.e. that circulation is reversed, or a low delta-p will indicate that the system is approaching the danger of flow reversal.

(13) In some embodiments of the invention, said signal 18 is fed to a control system of the plant. In this way, the control system is able to monitor the flow rate in line 5 and, indirectly, also the variations of density or gas/liquid ratio inside the equipments of the loop.

(14) The invention is also applicable to retrofitting of an existing urea plant. Referring again to FIG. 1, the invention can be carried out by adding the delta-p sensor 15 for measuring the difference of pressure between points 16, 17 across the existing valve 14.