EXTEND THE SEALS
Figure 1: In differential pressure (DP) applications, extended diaphragm seals can prevent plugging, and with equal-length capillaries and temperature compensation for ambient temperature and sun exposure variations, DP can give reasonable performance.
To consider nuclear sensors, you need an NCR license, an on-site certified radiation officer, and if you have heavy coating, it will still affect accuracy. In addition, you must also arrange for source disposal. For DP, extended diaphragms (Figure 1) with equal-length capillaries and temperature compensation for ambient temperature and sun exposure variations can also give reasonable performance.
While in batch reactors the residence time is measured by a timer, in continuous reactors, residence time is a ratio of reacting volume divided by the outflow (V/F), where V is a function of level. Therefore, in controlling continuous polymer reactors, level measurement is not optional, but a must.
Figure 2 illustrates such a control system. The selection of the level sensor should consider the comments I made in connection with the batch reactors, and some people might also consider the use of self-diagnosing laser (up to 300 °F, if the transmittance in the vapor space and the reflectance of the polymer surface is acceptable) or noncontacting and self-diagnosing radar (up to 500 °F, if there is no coating, condensation or crystallization on the antenna).
Béla Lipták
[email protected]
A: The biggest reasons not to use radioactive measurement are:
1. The instrument rays have to shine through the walls of the reactor so the receiver can absorb them, but the radioactive beam is not concentrated in one point like a laser, so sometimes you will have scattering of the radioactive beam, which could harm personnel.
2. Generally, reactors have extremely thick walls requiring a very high-energy source, which, over time, may make the reactor walls radioactive around the the beam area.
3. Most radioactive systems need to be close if not in contact with the surface of the walls of a hot reactor. This could damage the source, which could cause radiation to leak.
Alex (Alejandro) Varga
[email protected]
A: I have not come across DP level used in polymer reactors. Even with designs from 40 years ago, we were using nucleonic/radiation level, however, that is a small sample of the total number of polymer reactors in the world.
Most of the reactors I worked with did not use level. They just batched in a certain quantity and called it good. In the cases I am familiar with, there is no real benefit to knowing level in a batch reactor. Perhaps most of the modern polymer reactors are now semi-continuous.
There are regulatory difficulties involved with nucleonic installations. In the jurisdictions I know about, you have to have a trained and certified radiation officer on-site. There is also a perception that such devices are very dangerous and difficult to monitor/maintain/control.
Simon Lucchini, CFSE, MIEAust CPEng (Australia)
Chief controls specialist, Fluor Fellow in Safety Systems
[email protected]
