DISTILLATION CUT POINT CONTROL

Description

[0001] This invention relates to a method of controlling the composition of a liquid product in the product removal line at the bottom of a column of an apparatus including a multidraw distillation main column and at least one side stripper column having an input line receiving a draw from said main column, a stripper vapor inlet line, a plurality of trays, a product removal line and an overhead vapor removal line, distillation processes being carried out in said columns. More particularly, this invention relates to cut point control in petroleum crude towers.

[0002] It has been known to correlate side draw temperatures with cut points through simultaneous monitoring of numerous tower parameters (e.g., Nelson, "Petroleum Refinery Engineering", McGraw-Hill, Fourth Ed. 1958, page 473 ff). US-Patent 3,365,386 discloses providing two draws to a side stripper column to achieve a particular result, and using an initial boiling point (IBP) analyzer to monitor the IBP of the liquid removed from the bottom of a side stripper column and to use IBP in control. Partial pressure and temperature data are not used.

[0003] The invention, as claimed, solves the problem to provide an easy and accurate cut point control. It has been discovered that the cut point between any heavier cut to be withdrawn and lighter material may be controlled based on parameters around simply the bottom tray of a stripper for said heavier cut, in particular, that said cut point may be controlled through use of a characteristic of the liquid in said bottom tray.

[0004] In a preferred embodiment, said characteristic is the partial pressure of said liquid.

[0005] In a further preferred embodiment, said characteristic is the initial boiling point of the equilibrium flash vaporization curve ("IBP/EFV") of said liquid at atmospheric pressure.

[0006] "Cut point", means that temperature on a true boiling point ("TBP") curve (i.e., a batch process curve of percent of mixture--e.g., crude oil--removed in a heavily refluxed tower versus temperature reached to achieve that removal) at which a predetermined degree of separation is reached .

[0007] A preferred embodiment of the invention is described with reference to the drawings, in which:

[0008] Fig. 1 is a diagrammatic view with respect to practice of the method.

[0009] Fig. 2 is a pair of curves intersecting to give a cut point.

[0010] A crude tower of conventional arrangement, as shown in Fig. 1, and indicated generally at 10, and containing about fifty plates, was continuously supplied with heated crude oil through line 12. Emerging from tower 10 in order up its height were draw lines 14 (for atmospheric gas oils, 16 (for diesel oil), 18 (for kerosene), and 20 (for heavy naphtha). Said draw lines fed respectively into strippers 22, 24, 26, and 28 above the top plate of each thereof (each stripper having about six plates).

[0011] It was decided in advance that composition ranges desired to be manufactured would call for cut points between the atmospheric gas oil and diesel oil of 373°C (704° F), between diesel oil and kerosene of 255°C (492°F), and between kerosene and heavy naphtha of 161°C (322°F), My invention was used to maintain and control at these predetermined cut points each of the three.

[0012] The invention may be explained in particular detail with respect to the cut point between diesel oil and kerosene.

[0013] At startup, temperature in the draw tray from which draw line 16 emerged was monitored until about that expected to be associated with the desired cup point, about 268°C (515°F).

[0014] The present control method was then used to regulate actual cut point.

[0015] The following measurements were taken, then, each minute :

(1) Steam flow to stripper 24 (kg/hr.)

(2) Diesel oil flow from bottom of stripper 24 (barrels/day)

(3) Temperature in diesel oil draw line 16

(4) Temperature of diesel oil flowing from bottom of stripper 24

(5) Pressure in stripper 24 (treated as that at draw tray from which draw line 16 emerges, and determined by interpolating between bottom and top pressures of tower 10)

(6) Temperature of steam into stripper 24

(7) Pressure of steam into stripper 24.

[0016] Using these seven measurements, together with constants from laboratory data to give specific heat, partial pressure of diesel oil ("liquid") in the vapor above the bottom plate of stripper 24 is obtained; this is then used to determine atmospheric pressure IBP/EFV of the diesel oil. In making this determination, constants are desirably used which from most recent (usually daily) laboratory data update the apex of the two-phase region triangle defined by plotting EFV's for various vaporization percentages as shown in Fig. 3B3.1 of API Technical Data Book (August, 1963), pressure versus temperature graphs for each percentage mixture being a straight line. (Since partial pressure of the diesel oil and the temperature of the diesel oil on the bottom tray of stripper 24 define one point on the initial boiling point--i.e., 100% liquid, "IBP"--line and the apex the other, the atmospheric IBP/EFV may be easily picked off.)

[0017] Once daily the laboratory supplied an ASTM curve of temperature versus percent vaporized, for both the diesel oil and the kerosene. Using conventional conversions, these permitted establishment of true boiling point curves for each. Using these, plotted over widths reflecting their relative volumes (barrels/day), and with kerosene curve flipped, all as shown in Fig. 2, an intersection results at a temperature which is the cut point.

[0018] The difference between this temperature and the IBP/EFV temperature gives a correction factor that may be used with the IBP/EFV temperature to provide the running (minute by minute) cut point.

[0019] If the measured cut point is not exactly that desired, the flow rates in draw lines 16 and 18 are appropriately varied, in equal but opposite amounts.

[0020] In the same manner, the cut point between atmospheric gas oil and diesel oil was controlled using stripper 22 as the focus of control in the same way as was stripper 24 in the control above described, and, in the same way, the cut point between kerosene and heavy naphtha was controlled using stripper 26 as the focus of control. The cut point between heavy naphtha and light naphtha was controlled by prior art methods, although the method of my invention could of course have been used.

Claims

1. The method of controlling the composition of a liquid product in the product removal line at the bottom of a column of an apparatus including a multidraw distillation main column and at least one side stripper column having an input line receiving a draw from said main column, a stripping vapor inlet line, a plurality of trays, a product removal line and an overhead vapor removal line, distillation processes being carried out in said columns,
characterized in that said method comprises the steps
   calculating the partial pressure of components vaporized from liquid at the bottom of said stripper column,
   monitoring the temperature of liquid at the bottom of said stripper column,
   calculating by standard calculation techniques the initial boiling point (IBP) of the liquid at the bottom of said stripper column using said partial pressure and temperature to obtain calculated composition,
   comparing said calculated composition with a set point composition.

2. The method of claim 1 wherein vaporized components are returned to said multidraw column via said overhead vapor removal line.

3. The method of claims 1 or 2 wherein said initial boiling point is the initial boiling point of the equilibrium flash vaporisation curve.

4. The method of caim 3, wherein said initial boiling point of said equilibrium flash vaporisation curve is at atmospheric pressure.

5. The method of any of claims 1 to 4, further comprising the step varying the flow rates in the draw input lines of a pair of side stripper columns.

Ansprüche

1. Verfahren zum Steuern der Zusammensetzung eines flüssigen Produkts in einer Produktauslaßleitung am Boden einer Säule einer Apparatur, welche eine Fraktionierungs-Hauptsäule mit mehreren Auslässen und mindestens eine Stripper-Hilfssäule mit einer Eingangsleitung, welche eine Fraktion von der Hauptsäule erhält, einer Stripperdampfeinlaßleitung, einer Mehrzahl von Böden, einer Produktauslaßleitung und einer kopfseitigen Dampfauslaßleitung enthält, wobei in diesen Säulen Destillationsprozesse ablaufen, dadurch gekennzeichnet, daß
   der Partialdruck der Komponenten errechnet wird, die von der Flüssigkeit am Boden der Hilfssäule verdampfen,
   daß die Temperatur der Flüssigkeit am Boden der Hilfssäule überwacht wird,
   daß der Anfangssiedepunkt (IBP) der Flüssigkeit am Boden der Hilfssäule unter Verwendung des Partialdruckes und der Temperatur durch übliche Rechenverfahren errechnet wird um eine errechnete Zusammensetzung zu erhalten, und
   daß die errechnete Zusammensetzung mit einer Sollpunkt-Zusammensetzung verglichen wird.

2. Verfahren nach Anspruch 1, dadurch gekennzeichnet, daß verdampfte Komponenten über die kopfseitige Dampfauslaßleitung in die Fraktionierungssäule zurückgeführt werden.

3. Verfahren nach Anspruch 1 oder 2, dadurch gekennzeichnet, daß der Anfangssiedepunkt der Anfangssiedepunkt der Gleichgewichts-Flash-Verdampfungskurve ist.

4. Verfahren nach Anspruch 3, dadurch gekennzeichnet, daß der Anfangssiedepunkt der Gleichgewichts-Flash-Verdampfungskurve für Atmosphärendruck gilt.

5. Verfahren nach einem der Ansprüche 1 bis 4, dadurch gekennzeichnet, daß weiterhin die Durchsätze in den Fraktionseingangsleitungen zweier Stripper-Hilfssäulen geändert werden.

Revendications

1. Procédé de réglage de la composition d'un produit liquide dans la conduite d'extraction du produit au bas d'une colonne d'un appareil comprenant une colonne principale de distillation à soutirages multiples et au moins une colonne de rectification latérale qui comporte une conduite d'entrée recevant un produit soutiré dans ladite colonne principle, une conduite d'admission de vapeur de rectification, plusieurs plateaux, une conduite d'extraction du produit et une conduite d'évacuation de vapeur en tête, des traitements de distillation étant effectués dans lesdites colonnes, caractérisé en ce qu'il comprend les étapes consistant
   à calculer la pression partielle des composants vaporisés à partir du liquide au bas de ladite colonne de rectification,
   à surveiller la température du liquide au bas de ladite colonne de rectification,
   à calculer par des techniques de calcul bien établies le point initial de distillation (IBP) du liquide au bas de ladite colonne de rectification, en utilisant ladite pression partielle et ladite température, afin d'obtenir une composition calculée,
   à comparer ladite composition calculée avec une composition de consigne.

2. Procédé selon la revendication 1, dans lequel les composants vaporisés sont renvoyés dans ladite colonne à soutirages multiples par ladite conduite d'évacuation de vapeur en tête.

3. Procédé selon la revendication 1 ou 2, dans lequel ledit point initial de distillation est le point initial de distillation de la courbe de vaporisation par détente isenthalpique à l'équilibre.

4. Procédé selon la revendication 3, dans lequel ledit point initial de distillation de ladite courbe de vaporisation par détente isenthalpique à l'équilibre est à la pression atmospherique.

5. Procédé selon l'une quelconque des revendications 1 à 4, comprenant en outre l'étape consistant à faire varier les débits dans les conduites d'entrée de produit soutiré d'une paire de colonnes de rectification latérales.

Drawing