Production-Type Volatile Metal Fluoride Detector W. E. SHAW
AND
J. R . DEIRR’ATER
S a t i o n a l Lead Co. of Ohio, Cincinnati, Ohio
SENSISG element for deteiniining the presence of d. voling lights and annunriators. The relay could be used for atile metallic fuoiide in a gas stieam was needed to constopping flow if this were desired. trol aprocess in which a volatile metallic fluoride was converted to a nonvolatile metallic fluoride by reduction with dissociated amDETECTI’YG E L E l f E R T monia. This reaction is carried out in a long, vertical tube. A t the base of the tube the excess dissociated ammonia and the byThe element used for detection is a high purity graphlte sllecproduct hydrogen fluoride are separated in a disengaging section tioscopic electrode, Grade U2 (soft), ’/* inch in diameter. It from the nonvolatile metallic fluoride. Some of the small partiis activated by being immeised in a boiling solution of concaencles of product also pass off in this off-gas Etream. It is necessary to remove the fine, powdered product and, for economic operation] t o recover, by condensation, the hydrogen fluoride produced. The removal and collection of these economically desired materials are accomplished by inertia collectors, filters, and refrigeration. When a n occasional misoperation OCCUI’B in the process, the feed, a volatile metallic fluoride, may enter this off-gas system and cause considerable damage by plugging filters and the refrigeration system. It became necessary to devise a technique for detecting the release of any large amounts of feed into this system. A simple, rugged, and foolproof apparatus was desired that would monitor the system continuously. Because of the complicating factors that hydrogen fuoride and product are present in the off-gas line, none of the usual detection instruments could be used. One instrument, sensitive to small amounts of feed and operating on the principle of ultraviolet absorption, could be obtained, but this instrument was much too sensitive and delicate for the operation involved. It had been noticed in a similar pilot plant operation that when the volatile metallic fluoride entered the off-gas system the carbon tubes of the filter unit were attacked. Initial investigation of this phenomenon with the hope of obtaining an adequate detection device resulted in failure. Later investigation, in which a more extensive effort led to the use of sensitized carbon elements for detection] resulted in a unit that was successful and is nom installed in the plant. In principle, the sensitized detecting element is a conductor in an electrical circuit. When volatile metallic fluoride comes fn contact with this element deterioration occurs, and the resistance of the element is Figure 1. Elements used for detecting volatile metal fluorides increased. The increase of resistance throws Left. Used element out relays which serve to operate indicatRight. Element as prepared
792
INDUSTRIAL AND ENGINEERING CHEMISTRY
April 1956
793
possibly explosive spark in the off-gas line when the carbon rod breaks.
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TEST RESULTS
CODE
GIL= GREEN INDICATING LIGHT RIL= RED INDICATING LIGHT =ANNUNCIATOR R I = 6 VOLT RELAY R l - I , RI-2,8 R I - 3 R 1-4 ARE CONTACTS ON RELAY R - l *PUSH BUTTON TO START FCONTACTS NORMALLY OPEN CONTACTS NORMALLY CLOSED J-SWITCH %DETECTING ELEMENT RS=RELAY SHUNT
Equipment as shown in Figure 3 was set up t o test the unit. One minute after feed entered the test chamber, the voltage drop across CR reached 3 volts. This essentially cut the current through R1 approximately in half and reversed the contact points, thus activating the alarm system.
GI
4
Figure 2.
Diagram of detection circuit
trated nitric and hydrochloric acids mixed in the ratio of 10 to 1 for 8 hours. This element is mounted in a clip attached t o a Teflon plug which is screwed into the off-gas pipeline. Figure 1 shows the detection elements as they appear before and after use. The element on the left detected the presence of volatile metallic fluoride in the off-gas stream and was removed from the circuit before it had completely broken. The deterioration that occurs is evident on this element. DETECTION CIRCUIT
A single line drawing of the detection circuit is shown in Figure 2. The detecting element, CR,has a 6-volt alternating current potential impressed on it and is connected in series with a fourpoint relay, R1. This relay is an Allen Bradley relay, Type BX22A. When the resistance in CRincreases, R1 operates and opens contacts R1-1 and R1-4. The annunciator and alarm light contacts, Rl-2 and R1-3, are then closed and an alarm is consequently given. The voltmeter is used as a check instrument on the behavior of the unit and contact R1-4 serves to prevent the occurrence of a
r-l Figure 3, Diagram of test cell
After these tests had been completed, the unit was installed in the production plant and has performed satisfactorily many times. However, for some applications the quantitative aspect,s and sensitivity of this instrument would have t o be developed more completely. ACKNOWLEDGMENT
The assistance of R. M. Spenceley, W. J. Adams, and Fred Capuder in the development of this unit is acknowledged. RBCEIVED for review November 17, 1955.
ACCEPTED January 10, 1956.
