Radar level measurement
Disclosed is a method of measuring the level of a liquid in a vessel, such as a chemical reactor, by radar. The method particularly pertains to situations wherein a supercritical fluid is present above the liquid. More particularly, the method serves to cope with the typical vigorous circumstances of a chemical reaction, such as urea synthesis. The invention foresees the use of a tube extending into the liquid, so as to guide the radar waves to the surface level of the liquid.
1. A radar method for detecting the level of a liquid which liquid is a carbamate solution formed in urea synthesis in a urea synthesis reactor wherein a supercritical fluid is present above said liquid,
the method comprising transmitting electromagnetic waves into the direction of said liquid carbamate solution through a tube provided in said urea synthesis reactor, wherein the tube extends to a bottom end placed within said reactor above a desired minimum level of said liquid, and wherein said bottom end is defined by a reflective surface,
the tube further comprising at least one hole for venting gas, and at least one hole for allowing liquid to enter.
2. A method according to claim 1 , wherein the at least one hole for allowing liquid to enter the tube is positioned near or at the bottom end of said tube.
3. A method according to claim 1 , wherein the at least one hole for allowing liquid to enter the tube comprises 2-20 holes of diameter 2-20 mm.
4. A method according to claim 1 , wherein the tube extends substantially vertically into the liquid.
5. A method according to claim 1 , wherein the tube enters the vessel from the side and has a bend and extends further downwards into the liquid, wherein the tube contains a vent hole below the bend in the tube.
6. A method according to claim 1 , wherein the tube is made from a duplex ferritic-austenitic stainless steel having a high content of Cr and N, and a low content of Ni.
7. A method according to claim 6 , wherein the tube is made from a duplex, stainless steel alloy, containing, in percent by weight:
C maximally 0.05%;
Si maximally 0.8%;
Mn 0.3-4%;
Cr 28-35%;
Ni 3-10%;
Mo 1.0-4.0%;
N 0.2-0.6%;
Cu maximally 1.0%;
W maximally 2.0%;
S maximally 0.01%;
Ce 0-0.2%;
the remainder being Fe and normally occurring impurities and additives, the ferrite content is 30-70% by volume.
8. A method according to claim 1 , wherein the tube is cone-shaped on the inside adjacent to a radar cone.
9. A radar system suitable for being placed into a vessel, comprising a transmitter, a receiver and a tube connected at a top end to the transmitter and the receiver, and a bottom end that is closed off, the tube further comprising at least two holes at different distances from the bottom end, wherein the tube is made of a steel as defined in claim 6 .
10. The radar system of claim 9 wherein the vessel is a urea synthesis reactor.
11. The method of claim 3 wherein the at least one hole for allowing liquid to enter the tube comprises 5-15 holes of diameter 3-10 mm.
12. The method of claim 7 wherein:
C is maximally 0.03%;
Si is maximally 0.5%;
Mn is 0.3-1%;
Cr is 29-33%;
Mo is 1.0-1.3%; and
N is 0.36-0.55%.
13. The method of claim 7 wherein the ferrite content is 33-35% by volume.
14. The method of claim 1 wherein said reflective surface is adapted for detecting the bottom of the tube and provides a restriction for entry of said liquid.
15. The method of claim 8 which comprises heating the radar cone.
16. The system of claim 9 wherein said bottom end comprises a reflective surface adapted for detecting the bottom of the tube.
17. The system of claim 9 wherein said bottom end has a shape which is a ring covering part of said bottom, a grid, a closure plate comprising holes or a tight closure plate.