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

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 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.