Patent GB-1177110-A (GB)
Title: Automatic Freezing Point Indicator and method of Indication
Inventor: DAVIS VINCENT C
IPC: G01N25/04
Filed: 1967-05-03
Abstract: 1,177,110. Measuring freezing point electrically. CHEVRON RESEARCH CO. May 3, 1967, No.20645/67. Heading G1N. In the purification of liquids using a batch distillation unit (14, Fig. 1 not shown), through which a ratio weir (15) recycles the liquid until a required purity is attained, the purity is determined from a measurement of the freezing point of a sample of the liquid taken from the reflux conduit (17) and fed via conduit (19), valve (25) to the freezing point measuring apparatus 21, Fig.2. The apparatus comprises an inner pipe 51 through which the sample flows and which is provided with a thermistor 20 encased in an aluminium tube 59, and a concentric outer tube 50 defining a space between the two through which steam or cooling air flows, respective inlets 26, 27 and outlet 35, being provided in the outer tube. The thermistor 20, Fig. 3, is connected in a double self balancing bridge having a common branch including thermistor 20 and resistors R6 R7 and separate branches comprising resistors R3, R4, R9 and balancing potentiometer 67 and resistors Ri, R2, R9 and balancing potentiometer 68, the potentiometer slides 65, 66 being adjusted by motors 71,72 to balance the two bridges. The motors 71,72 also drive the pens 39,40 of a chart recorder and, through switches 75, 78, cause the opening and closing of valves 29, 31, 33, 37 controlling the supply of steam, air and the sample to the apparatus. The elements of the double bridge are so chosen that one part of it is sensitive to temperatures over a range of 30 degrees and the other over a range of 1 degree centred on the freezing point of the pure sample. Thus for the purification of phthalic anhydride the parts of the bridge are sensitive over the ranges 110‹-140‹C. and 130À2‹- 131À2‹C. For certain liquids, e.g. aromatic liquid hydrocarbons and their derivatives it is found that the liquid can be undercooled below its actual freezing point by such an amount that, when crystallisation occurs, the latent heat of crystallisation is such as to bring it back locally to its freezing point. This property is utilized to control the recording of the temperature of the sample in the range of its freezing point. In operation, at the beginning of the cycle the sample is below the threshold temperature of the potentiometer 67, valves 29 and 33 are open permitting steam and sample to flow into the apparatus. As the temperature of the sample rises above 110‹C. the pen 39 starts to move and at 130-2‹C. the pen 40 starts to move, the chart over which they move being stationary. When the latter pen reaches full scale at 131-2‹C., a relay K is energized causing valves 29, 33 to be closed and valves 31, 37 to be opened allowing cooling air into the apparatus to cool the sample below its freezing point until crystallisation occurs adjacent the air inlet and the thermistor 20 when heat is generated locally causing the sample around the thermistor to rise in temperature, the resulting reversal of motor 71 ( and pin 39) causes the energization of motor M1 of a timing device 96 and motor M2 driving the chart of the recorder. The latter motor M2 is energized for the time of operation of motor M1 which is sufficient for the recording of peaks in the temperature of the sample after crystallisation. The motors M1, M2 are deenergized at the end of one revolution of cams 100, 100 at which point switch 95 is opened resulting in the de-energization of relay K and the reversal of valves 29, 31, 33, 37 for the start of the next cycle of measurement.