Method for determining liquid-vapor interface via gamma radiation

Abstract

A method for determining the position of a liquid interface between a lower tray and an upper tray in a process apparatus such as a distillation column, using gamma scanning and comparing the maximum count rate of gamma photons measured between two trays with the count rate attributed to scanning through the apparatus filled with vapor. The method enables the operating parameters of the apparatus to be optimized and provides a manner of calculating the % flood of an individual tray.

Claims

1. A method for determining the position of a liquid interface between a lower tray and an upper tray in a process apparatus comprising the steps of: a) arranging a source of gamma radiation and a detector for said gamma radiation at opposed ends of a horizontal path P through said process apparatus, said path P being located at a vertical location between the height of said lower and upper trays and at height H above said lower tray; b) counting the amount of radiation emitted by the source which is detected by the detector; c) carrying out steps a) and b) N times to acquire N data points Dn, each consisting of the count rate Cn counted by the detector measuring radiation transmitted along path Pn at height Hn above said lower tray, each path Pn being of the same length and H.sub.1 being at or immediately above the height of the lower tray; d) defining the count rate CV to be the count rate when the path through the process apparatus passes through a vapour phase; e) identifying C.sub.MAX where C.sub.MAX=the maximum of measured count rates Cn between said lower tray and said upper tray; and f) estimating the height H.sub.L of the liquid interface by: i. if C.sub.MAXCV and C.sub.MAXCVx % of CV then H.sub.L=Hn at C.sub.MAX; or ii. if C.sub.MAX<CVx % of CV then extrapolate data points, including Dn at C.sub.MAX and at least Dn1 and Dn2, to find H.sub.L where extrapolated line meets CV.

2. A method according to claim 1 wherein, if more than one count rate fulfils the condition in paragraph f) i, then H.sub.L may be taken to be at the lowest height Hn, at which the condition is fulfilled.

3. A method according to claim 1, further comprising the step of calculating the liquid interface position from H.sub.L and the position of the lower tray.

4. A method according to claim 1, wherein N is at least 5.

5. A method according to claim 1, wherein the count rate is expressed as log.sub.10 (count rate) and step ii of part f) is carried out using a straight line fitting method.

6. A method according to claim 1, wherein the calculated liquid height, H.sub.L, is used to calculate the % flood by expressing H.sub.L as a percentage of the distance between the lower tray and the upper tray.

7. A method according to claim 1, further comprising the step of adjusting the operating parameters of the process apparatus based on the value of H.sub.L estimated in step f).

8. A method according to claim 1, wherein x is between 1 and 15.

9. A method according to claim 1, wherein said process apparatus is a distillation column.

10. A method according to claim 2, further comprising the step of calculating the liquid interface position from H.sub.L and the position of the lower tray.

11. A method according to claim 2, wherein N is at least 5.

12. A method according to claim 2, wherein the count rate is expressed as log.sub.10 (count rate) and step ii of part f) is carried out using a straight line fitting method.

13. A method according to claim 3, wherein the count rate is expressed as log.sub.10 (count rate) and step ii of part f) is carried out using a straight line fitting method.

14. A method according to claim 2, wherein the calculated liquid height, H.sub.L, is used to calculate the % flood by expressing H.sub.L as a percentage of the distance between the lower tray and the upper tray.

15. A method according to claim 2, further comprising the step of adjusting the operating parameters of the process apparatus based on the value of H.sub.L estimated in step f).

16. A method according to claim 2, wherein x is between 1 and 15.

17. A method according to claim 2, wherein said process apparatus is a distillation column.

Description

(1) FIG. 1 shows schematic view of a longitudinal section through a part of a distillation column having 3 trays.

(2) FIG. 2 shows a representation of a gamma scan of a distillation column.

(3) FIG. 3 shows a representation of a different gamma scan of a distillation column.

(4) In FIG. 1, a part of a distillation column 10 is shown with three trays, 12, each containing a liquid 14 in equilibrium with a vapour 16. The liquid level on each tray is shown by dashed line 18. A source 20 and detector 22 are mounted opposite each other at position A outside the wall of the column such that gamma radiation emitted by the source passes through the column to be detected by detector 22. The source and detector are moved after each measurement to positions shown by the dashed outlines arranged at different heights B-P along the column. The amount of radiation measured by detector 22 depends on the mass and density of the material through which the radiation passes. The count rate detected by the detector at position B, for example, is less than the count rate at position D, because at D the radiation path passes mostly through a vapour phase, whereas at B there is a layer of liquid in the path of the radiation.

(5) FIGS. 2 and 3 each show a gamma scan plot resulting from scanning a column similar to that shown in FIG. 1 at several different vertical positions on the column. Log.sub.10 count rate is plotted horizontally against the height (vertical position of the measurement). The maximum count rate measured over the column is found and a range of count rates within 4% of that maximum is plotted as a hatched bar 24 in FIGS. 2 and 3. This is known as the clear vapour bar. The width of the clear vapour bar depends on the value of x selected. For example, the clear vapour bar may represent a range of count rates within 5% of that maximum or a different % of the maximum. The minimum measured count rates coincide with the position of the metal trays. In FIG. 2, measured count rates at positions successively above tray 3 increase until the count rate coincides with the clear vapour bar, at which point the path of radiation is deemed to pass through vapour. The height at that position is shown ringed as H1 and H2 respectively for trays 3 and 2 on FIG. 2. In FIG. 3 the count rates for the trays shown do not meet the clear vapour bar above trays 3 and 2. In those cases, the maximum count rate C.sub.MAX and the preceding 3 data points are extrapolated to the clear vapour bar and the height at that position is deemed to be the height of the liquid level. Those heights are circled by H3 and H4.