WO1999001193A1

WO1999001193A1 - Distllation columns for fractionation

Info

Publication number: WO1999001193A1
Application number: PCT/GB1998/001973
Authority: WO
Inventors: Frigyes Lestak; Derek William Reay
Original assignee: M. W. Kellogg Limited
Priority date: 1997-07-03
Filing date: 1998-07-03
Publication date: 1999-01-14
Also published as: AU8230798A; GB9714093D0

Abstract

A distillation column for fractionation of a feedstock into a plurality of fractions, the distillation column comprising a feed inlet located between the top and the bottom of the column, an overhead outlet at the top of the column, a bottoms outlet at the bottom of the column and at least one side stream outlet position between the top and the bottom of the column, the said column being provided with a plurality of plates, trays or packed section(s) arranged so that, in use, efficient contact between vapour and liquid is provided, the said column having a vertically extending wall which divides the column into a feed section into which the feedstock enters and a take-off section from which the side stream or streams are taken, characterized in that the said vertically extending wall is constructed from a plurality of wall segments, the wall segments being installed between trays or plates which extend horizontally across substantially the width of the column, or between the grids which support the packed sections.

Description

DISTILLATION COLUMNS FOR FRACTIONATION

The present invention relates to distillation columns for fractionation of a feedstock consisting of several components and, in particular, to an improved distillation column of a divided wall design.

It is well known in the art to separate a mixture of components of a fluid into the pure fractions by carrying out a plurality of distillation steps. In plants of the petrochemical industry a feedstock is separated into fractions by carrying out a so-called fractional distillation using a distillation column. The feedstock is split into a top fraction, a bottom fraction and in some cases a plurality of side fractions. Whilst it is possible using this method to obtain pure top and bottom fractions, it is more difficult to obtain pure side fractions.

Various proposals have been made for overcoming this problem. One such proposal is described in US-A- 2471134 and EP-A-0122367 and comprises the position of a vertical wall in the distillation column which effectively separates the column into two sections, namely a feed section into which the product enters and a take-off section from which the side fractions are drawn. Using such an arrangement it is possible to obtain side fractions which are substantially uncontaminated with top and bottom fractions. Distillation columns of this type have generally becomes known as "divided wall" distillation columns and are typically constructed with a single wall which is welded to the column shell, or which is formed by manufacturing the shell in two D-shaped pieces which are joined together. Welding is time consuming and expensive and for retrofitting to distillation columns already in use requires the removal of the internal devices which are used in such columns to obtain efficient contact between the vapour and liquid, such as plates or trays. Furthermore, distillation columns retrofitted in this manner may require a subsequent heat treatment to alleviate local stresses.

We have now developed an improved divided wall distillation column which overcomes the disadvantages of providing a single wall welded to the distillation column shell. The improved design of the present invention minimizes the welding to the column shell, enables easy installation, minimizes costs and in most cases requires minimum changes to existing tray designs. The concept can also be extended to packed distillation columns.

Accordingly, the present invention provides a distillation column for fractionation of a feedstock into a plurality of fractions, the distillation column comprising a feed inlet located between the top and the bottom of the column, an overhead outlet at the top of the column, a bottoms outlet at the bottom of the column and at least one side stream outlet positioned between the top and the bottom of the column, the said column being provided with a plurality of plates, trays or packed section (s) arranged so that, in use, efficient contact between vapour and liquid is provided, the said column having a vertically extending wall which divides the column into a feed section into which the feedstock enters and a take-off section from which the side stream or streams are taken, characterized in that the said vertically extending wall is constructed from a plurality of wall segments, the wall segments being installed between trays or plates which extend horizontally across substantially the width of the column, or between the grids which support the packed sections.

The distillation column of the present invention may either be constructed from appropriate new parts or may be constructed by appropriately modifying (retrofitting) an existing distillation column.

The wall segments used in the present invention may be constructed as a single piece, or may comprise a plurality of wall elements which are joined together to form a wall segment. The construction of the wall segment from a plurality of wall elements may make it easier to construct a divided wall when a distillation column is being retrofitted.

In one aspect, the distillation column design of the present invention may be used in columns with horizontally extending trays or plates. Generally the column will have trays with downcomers located at the side and at the centre. Every alternate tray will generally have central downcomers. The trays between which the wall segments are fitted generally have weirs (downcomers) located at the side edges thereof.

The receptors which are provided in or on the trays may be of channel-shaped cross-section, or T- shaped support attachment which are of a suitable dimension to receive at least one end of the wall sections therein. In a particularly preferred embodiment the top end of one wall and the bottom end of the wall section below it in the column are located in a multiple receptor provided on the tray positioned between the wall sections.

The vertically extending edges of each wall segment adjacent the column shell are preferably attached to the bolting bars which are generally attached to support rings fixed to the column shell. Preferably the dividing wall segments are sealed from the bolting bars and the bolting bars are sealed from the shell of the distillation column by means of gaskets. The bolting bars may typically be welded or bolted to the support rings. The concept of using dividing wall elements may be used in distillation columns fitted with plates or trays which are used for a single pass, or can be used with two, three and four pass trays.

In another aspect the concept of the present invention can be applied to packed columns. In this embodiment of the present invention the wall elements are positioned between the grids which support the packed sections. Generally, the vertically extending ends of the wall segments are attached to bolting bars which are attached to the packing support rings. The packing support rings are fixed to the column shell, typically by welding.

Preferably one or more of the wall segments is insulated. It will be understood that the distillation column design of the present invention makes it possible to thermally insulate only those portions of the dividing wall which require insulation. This provides an economic and efficient column design. Preferably, when the distillation column is provided with trays the insulation of one or more wall segments extends to a height which is above the height of the level of liquid which the tray and downcomer are designed to hold. Typically, the insulation will extend from the level of one tray to a height which is about three inches above the level of the tray above. The thermal insulation may be provided as a solid material, such as a concrete, or any other suitable material.

In another aspect of the present invention the vertical edges of the bolting bars to which the wall segments are attached are sealed from the shell by means of gaskets.

The distillation column of the present invention is of simple construction and obviates the necessity to weld to the column shell which is a time consuming operation. The provision of the divided wall in segments, together with the provision of the appropriate receptors in or on the column plates or trays, and the use of bolting bars, enables the revamping of a distillation column to be undertaken with a minimum of changes to an existing tray arrangement. This is less onerous than the installation of a dividing wall in a single piece, welded to the wall, which would require the removal and disposal of all trays in the column, or column section. A similar ease of construction is also encountered for packed columns .

Furthermore, since no welding operations to the column shell are involved in the formation of the distillation column of the present invention, there is no requirement for the dividing wall to be heat treated for stress relief.

The present invention will be further described with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of a prior art divided wall column;

Figure 2 is a schematic diagram of a central section of a divided wall column in a two-pass trayed column; Figure 3 is a schematic diagram of one manner in which the divided wall segments may be joined together in the present invention; and

Figure 4 is a schematic illustration of the manner in which the edges of the bolting bar segments may be sealed from the shell of the column; and Figure 5 is a schematic illustration of the concept of the present invention in relation to packed columns.

Referring to the drawings, Figure 1 illustrates schematically a prior art divided wall distillation column 1 which comprises a shell 2 divided into a prefractionator portion 3 and a main column portion 4 by means of a dividing wall 5. A feedstock comprising a mixture of hydrocarbon components A, B and C is fed into the prefractionator portion 3 of the column at 6. The column is operated according to known techniques so that a lower boiling point fraction A is taken off from the top of the column, the higher boiling point fraction C from the bottom of the column and a middle boiling fraction B from the side of the column. The dividing wall 5 may be partially or fully insulated, if desired.

As shown in Figure 1 the lower boiling point fraction A is passed through a condenser 7 and the higher boiling point fraction C is passed through a reboiler 8, with appropriate means to recycle a portion of each of the streams to the column.

Referring to Figure 2, the central section of a distillation column 10 has a wall 11 which divides the column into two. The column is of the so-called two pass tray type provided with a plurality of trays 12 which straddle the dividing wall 11, and a plurality of trays 13 which extend from the shell of the column. Trays 12 have weirs (downcomers) at the sides thereof (not shown) , whilst trays 13 have central weirs

(downcomers). Feedstock enters at 14, with the side product B being removed at 15.

Figure 3 illustrates one manner in which the divided wall segments may be joined together in accordance with the present invention. Dividing wall segments 21 and 22 abut a modified central panel 23 which is attached to the horizontally extending tray sections 24 and 25 which are secured by appropriate means, not shown. The upper wall segment 21 is attached to a wall support element 26. Alternatively, the wall may be bolted to the central panel 23 or inserted in a channel shaped central section (not shown) . Generally the insulation block 27 which is adjacent to or forms part of the dividing wall will be of a height extending from the level of trays 24 and 25 to the top of the downcomer of the tray which is above trays 24 and 25, not shown. Insulation will generally only be required on the section of the wall that contacts liquid since the heat transfer coefficient on the vapour side is significantly lower than on the liquid side.

The lower wall segment 22 is located adjacent wall support 28. The wall support may comprise a flange to which the upper end of the wall segment 22 may be attached by bolting or welding. The lower wall segment is not insulated.

In constructing a divided wall column in accordance with the present invention a plurality of wall segments will be mounted between adjacent trays in the manner as described with reference to Figure 3. In a distillation column of the two-pass or four-pass type it is only the centrally extending trays which require modification to retrofit a dividing wall in the distillation column as a plurality of wall segments. The other components of the distillation column will be constructed according to known design techniques. Dividing wall elements can also be installed in an orientation parallel to the flow path, using the technique of bolting bars and modified tray panels. The vertical bolting bars are welded or bolted to tray support rings.

Figure 4 illustrates one simple way in which the bolting bars may be secured using the tray support ring 31 that is welded to the shell of the distillation column. Referring to this Figure, a support ring 31 is welded to the shell of the column. The column shell abuts the edge 31a of the ring. A dividing wall segment 32 is spaced fractionally from the column shell at 33. The end of side 34 of the wall segment 32 is bolted thereto by a bolting bar 36. The bolting bar also has a gasket holder 37 bolted thereto which houses rope gasket 38 which presses against the shell of the column. Gasket 38 serves to provide a seal between the side of the wall and the column shell. This means of fitting the wall segments to the distillation column shell also avoids the necessity of welding the segments to the shell which may require heat treatment of the shell. Figure 5 illustrates the use of a divided wall constructed from a plurality of wall segments for packed columns. Referring to the drawing, packing support rings 39 are welded to the shell 40 of a distillation column. The packing support grids 41 are supported by the support rings 39. Bolting bars 42 are bolted or welded to the support rings 39 at the upper end and the lower ends thereof (not shown) . The bolting bars 42 are sealed from the column shell by means of gaskets (not shown) . Insulted wall panels 43 are fitted at the vertically extending edges thereof to the bolting bars 42. Appropriately shaped packing (not shown) is then placed on the support grids 41.

Claims

CLAIMS :

1. A distillation column for fractionation of a feedstock into a plurality of fractions, the distillation column comprising a feed inlet located between the top and the bottom of the column, an overhead outlet at the top of the column, a bottoms outlet at the bottom of the column and at least one side stream outlet positioned between the top and the bottom of the column, the said column being provided with a plurality of plates, trays or packed section (s) arranged so that, in use, efficient contact between vapour and liquid is provided, the said column having a vertically extending wall which divides the column into a feed section into which the feedstock enters and a take-off section from which the side stream or streams are taken, characterized in that the said vertically extending wall is constructed from a plurality of wall segments, the wall segments being installed between trays or plates which extend horizontally across substantially the width of the column, or between the grids which support the packed sections.

2. A distillation column as claimed in claim 1 wherein the wall segments are formed from a plurality of wall elements.

3. A distillation column as claimed in claim 1 or claim 2 wherein the column is provided with trays or plates and the horizontally extending ends of each wall segments are attached to or located in complementary receptors provided in or on the said trays or plates.

4. A distillation column as claimed in any one of claims 1 to 3 wherein additional trays attached to the shell of the distillation column are spaced between the horizontally extending adjacent central trays .

5. A distillation column as claimed in any one of the preceding claims wherein the receptor for at least one of the ends of each wall segment is of channel-shaped cross-section, or a T-shaped support attachment.

6. A distillation column as claimed in claim 5 wherein the top end of one wall segment and the bottom end of another wall segment below it in the column are located in a multiple receptor provided on a tray positioned between the said wall sections.

7. A distillation column as claimed in claim 1 or claim 2 wherein the column is a packed column and the vertically extending ends of the wall segments are attached to bolting bars which are attached to support rings fixed to the column shell.

8. A distillation column as claimed in any one of the preceding claims wherein one or more wall segments is/are at least partially thermally insulated.

9. A distillation column as claimed in claim 8 which is provided with trays or plates and wherein the insulation of the wall segment extends to a height which is above that of the level of liquid which the downcomer is designed to hold. W ^yvO^Kj 9^y9^y/^/0╧à1u19^ys3 PCT/GB98/01973

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10. A distillation column as claimed in claim 9 wherein the insulation of the wall segments extends from the level of one tray to a height which is about three inches above the level of the tray above.

11. A distillation column as claimed in any one of claims 8 to 10 wherein the insulation comprises a solid insulating material.

12. A distillation column as claimed in any one of claims 1 to 6, or 8 to 12 wherein the vertically extending edges of each wall segment adjacent the column shell are attached to bolting bars which are attached to support rings fixed to the column shell.

13. A distillation column as claimed in claim 12 wherein the wall segments are sealed from the bolting bars and the bolting bars are sealed from the shell of the distillation column by means of gaskets.

14. A distillation column as claimed in any one of the preceding claims wherein packing is used for vapour/liquid contact.