GB1571120A - Distillation column reactor and process - Google Patents

Description

(54) DISTILLATION COLUMN REACTOR AND PROCESS (71) We, SUN VENTURES INC., a eorporation organised under the laws of the State of Pennsylvania, of 100, Matsonford Road, Radnor, Pennsylvania, 19087, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to a distillation column reactor and process.

The use of a distillation column for both separation and reaction is known. In particular the use of the column in such a dual role is common in esterification where reactions which are equilibrium limited are driven to a high degree of conversion by the continuous removal of one of the reaction products from the reaction zone; e.g.

removal of water as vapour from the top of the column. Such a process is disclosed in U.S. Patent No. 2,384,793 where high molecular weight acids and high molecular weight alcohols are esterified. Also of interest are the process and equipment disclosed in U.S. Patent No. 3,634,535 where etherification is carried out.

In using such a distillation column reactor it may be desirable that the residence time for the liquid phase should be considerably longer than that obtainable with conventional equipment. This situation would occur when the kinetics of the reacting system dictate a total holding time in excess of the usual I to 5 minutes We have sought to provide a distillation column reactor which enables long residence times to be obtained and, further, which does not significantly increase the pressure drop throughout the column.

The invention provides an improved distillation column reactor which comprises a distillation column fitted with conventional trays with downcomers and having a single liquid reservoir of substantial depth between the trays, each of the liquid reservoirs being provided with a vapour port and a conduit extending through the reservoir to a lower tray, whereby liquid from the downcomer enters the reservoir and, after being held in the reservoir for an extended period of time, flows through the conduit in the reservoir to a lower tray, thereby providing increased liquid residence time in the column. The invention also provides a process for carrying out simultaneously fractionation and reaction where conversion of the reactant species requires a longer than usual residence time. In a preferred and specific embodiment, the process is employed for equilibrium limited reactions; e.g. those reactions which are favourably affected by removal of one of the products from the reaction zone and requiring a long residence time for the reactants in order to drive the reaction to completion.

According to one aspect of the invention we provide a distillation column reactor which comprises a distillation column containing a plurality of superposed trays each provided with a downcomer, a plurality of liquid reservoirs each of which is between a respective pair of adjacent trays, each reservoir being provided with vapour by-pass means which in use permit vapours to flow therethrough from the tray immediately below the reservoir to the tray immediately above it, an open-ended conduit extending through the bottom of each reservoir to a substantial height thereabove and towards the tray immediately below, the conduit in use, allowing overflow liquid to flow from the top of the liquid surface of the respective reservoir through the conduit to the tray below the reservoir, each conduit providing its respective reservoir with substantial depth to hold liquid substantially out of contact with vapour, each reservoir occupying the entire cross-sectional area of the column save for the area occupied by the vapour by-pass means and the conduit, and the top end of each conduit being above the bottom of the downcomer of the tray immediately above so that, in use, the downcomer extends below the surface of the liquid in the reservoir immediately below it.

According to another aspect of the invention we also provide a process for carrying out a reaction in a distillation column reactor, which comprises effecting said reaction in a distillation column containing a plurality of superposed trays each provided with a downcomer, a plurality of liquid reservoirs each of which is between a respective pair of adjacent trays, each reservoir being provided with vapour by-pass means which permit vapours to flow therethrough from the tray immediately below the reservoir to the tray immediately above it, an open-ended conduit extending through the bottom of each reservoir- to a substantial height thereabove and towards the tray immediately below, the conduit allowing overflow liquid to flow from the top of the liquid surface of the respective reservoir through the conduit to the tray below the reservoir, each conduit providing its respective reservoir with substantial depth to hold liquid substantially out of contact with vapour, each reservoir occupying the entire cross-sectional area of the column save for the area occupied by the vapour by-pass means and the conduit, and the top end of each conduit being above the bottom of the downcomer of the tray immediately above so that the downcomer extends below the surface of the liquid in the reservoir immediately below it.

The process according to the invention provides increased residence time for reactants with a minimum drop in pressure in the column.

The downcomer of each tray preferably extends to near the bottom of the reservoir immediately below it.

For a better understanding of the invention, reference is now made to the accompanying drawing which shows a multiplicity of superposed conventional trays with downcomers and interdigitated single reservoirs. As shown in the drawing, a column 11 contains the usual standard tray 12 with downcomer 12a and weir 13. It will be noted that the downcomer 12a extends below the surface of the liquid in the reservoir - and preferably will extend, as shown, to almost the bottom of the reservoir 14. In this way, the liquid flowing over the weir 12 is brought to the bottom of the reservoir and thus completely avoids any liquid by-pass which might occur if the downcomer did not extend below the liquid surface; i.e., liquid flowing on the surface of the reservoir to the conduit 17. If desired, baffles may be provided within the reservoir (not shown) so as further to ensure against by-passing, and to create an orderly progression of liquid from the bottom of the downcomer to the top of the conduit 17.

The downcomer extends below the liquid surface in particular to avoid liquid by-pass from the surface of the reservoir to the conduit when the column diameter is relatively small. It will be understood that, for large diameter columns, the downcomer need not extend almost to the bottom of the reservoir, but only somewhat below the height of the conduit 17 which conducts liquid from the reservoir to the lower tray as explained below. Below the tray 12 is the liquid reservoir 14 which is filled by the liquid which spills over the weir above it. It will be understood also that a reservoir need not necessarily be below every tray, but that the number of reservoirs will be dictated by the hold-up time desired, the capacity of each reservoir, and other parameters of the equipment or process. The reservoir, as indicated by the drawing, has a substantial depth in order to hold a significant volume of liquid, thereby providing a significant increased residence time of the liquid in the system. Each reservoir has vapour by-pass means in the form of one or more vapour ports 15 through which vapour rises from the tray immediately below the reservoir to the tray immediately above the reservoir and vapour distributor caps 16 may be employed in the conventional manner if desired. Each standard tray 12 is perforated and thus enable vapour to pass from the space above the reservoir 14 to the tray above it. A conduit 17 of predetermined height within the reservoir 14 permits liquid to be returned to the standard tray below when the liquid level in the reservoir reaches the height at which spillover will occur. It will also be understood that adjustment of the height of the conduit 17 in the reservoir will also be a means of controlling reservoir depth and capacity. Furthermore, the downcomer from the standard tray 12 can be designed so as to impart a tangential velocity to the liquid entering the reservoir, thereby effecting a modicum of agitation so as further to enhance mixing. As will be observed from the drawing, each reservoir will occupy essentially the entire crosssectional area of the column, except for the area taken up by the vapour port 15 and the conduit 17.

It will be understood that vapour-liquid contact in the column occurs between the liquid in the trays and the vapour above them, but there is also contact of the vapour emanating from a port 15 with the surface of the liquid in the associated reservoir. Of course, the column functions in the usual manner in that the vapour from the port 15 is contacted with liquid above, heavy components in the vapour interchange on the tray with light components in the liquid and a new vapour emerges which contains a higher concentration of light components than was present in the vapour from the previous tray. Furthermore, the liquid from the tray traverses the downcomer into the reservoir.

The improved distillation reactor described above will be useful, as indicated in equilibrium limited reactions which are exemplified by reactions such as esterification, etherification and hydrolysis.

A particularly useful application of the system is for the distillation of a terephthalonitrile hydrolysate to remove ammonia from terephthalic acid salts and simultaneously to convert nitrogen-containing by-products of the hydrolysis to terephthalic acid. In the operation of the distillation reactor of the invention, the regional fractionation time (i.e., the residence time per tray) is short in accordance with the usual short contact time between the vapour and the liquid on the standard trays while the reservoirs provide a relatively long residence for the liquid. A major advantage of the system is that no by-passing (e.g., "weeping") of liquid between stages can occur since the only way liquid can pass from a reservoir to the tray below is through the conduit 17. This is extremely important for systems in which high conversion must be obtained.

In order further to illustrate the invention, the following comparative examples are given.

EXAMPLE I (Standard Column) The feed, F,, to a commercial high pressure/high temperature column is a saturated liquid at 4820F containing: 19132 Ib-moles/hr. H2O 765 Ib-moles/hr. potentially free NH3 383 Ib-moles/hr non-volatile reactive material The column has 30 trays with the feed to tray number 25. The distillate product is 50 mole percent H20 and contains 99 /" of the available NH3.

If we assume a constaht molar overflow, the molar vapour traffic, V, in the column is given by the equation: V=D(Rd+ 1) where: D=distillate product rate Rd=reflux ratio If the Rd value is set, i.e. Rd=2.85 and it is noted from the above that D=1515 Ib moles/hr., the vapour traffic is V=5833 Ib-moles/hr.

The density of the vapour, p,, at the average column temperature of 474"F is approximately 1.15 Ibm ft.3. If the molecular weight of the vapour is set at 17.5 (50 mole /n H2O/50 mole % NH3), the volumetric flowrate, V', is: V'=Vx 17.5/pv =88763 cubic ft./hr.

For a superficial velocity v of 1 ft./sec., the required column diameter, D1, is

It is assumed that the overflow weir on each tray will be a chord the length of which is 75 /" of the tray diameter. This standard configuration gives an active tray area of about 75% of the total cross-sectional area At. Thus, the active area Aa is: 0.757r Aa=0.75 A1 4 D,2 4 =18.5 sq. ft.

The weir height of the column is set at a usual value of 3". Due to the relatively large amount of liquid traffic (the distillate is only a small portion of the feed (i.e., 1515/20280)102=7.5 mole %), the weir crest is 3.71" (calculated by the Francis Weir Equation, Perry, 4th Edition, pp. 18-19).

Thus the total effective height on a tray is about 6.71". This leads to an effective volume H of: 6.71 11= 18.5=10.34 ft.3 12 Since the feed is saturated, the liquid traffic L in the stripping section is: L=RdxD+Fo =24598 Ib-moles/hr.

As the system is extremely water rich, the molecular weight can be set at 18. The liquid density p, at the average column temperature is 51 Ibm/ft.3. Thus, the volumetric liquid traffic, L' in the stripping section is: L'=Lx 18/Pt =8682 ft.3/hr.

Therefore, the liquid residence time per tray, strays is: Otray=H/L' =4.3 secs.

and for 25 trays in the stripping section the total residence time, 0stray)totals is: 0(tr,ytotaI=179 minutes for a column of standard design.

EXAMPLE II Using the distillation reactor of this invention with a deep liquid reservoir between below each pair of standard trays, vapour/liquid contact does not occur in the reservoir, but the liquid is held for a time sufficient for reaction while the vapour flows through the central large port with only a small drop in pressure. Of course, vapour/liquid contact does occur on the standard trays, thus providing the required fractionation capability.

We have set the diameter of each port 15, Dp, at 16", and the active area A, of each reservoir is: 7top2 A,=18 4 =17.1 ft.2 and the volume, H" of a 5' deep reservoir is: HT5x17.1=85.5 ft.3 The liquid residence time, Or, for the reservoir is then #r=35.5 secs., and the total residence time for 25 trays and 25 reservoirs, 0(tray+r)total' is: 8ttray+r)total= 16.6 min.

The major results of the above calculations are summarised in the following table. For completeness, the pressure drops associated with the standard column and that of the invention are also included. Thus, it is seen that, at the expense of only a marginal increase in pressure drop per stage, the column of the invention gives almost a 10-fold increase in residence time.

TABLE I Comparison of Standard Reactive Distillation Column with that of the Invention Example I Example II Column of Standard the Column Invention Superficial Vapour Velocity Ft/sec. 1 1 Liquid Depth, in.

Trays Weir 3.0 3.0 Weir Crest 3.7 3.7 Reservoirs 60.0 Pressure Drop, psi Tray 0.21 0.21 Reservoir 0.14 TotaVStage 0.21 0.35 Residence Time in Stripping Section, sec.

Tray, 0tray 4.3 4.3 Reservoir, 8, 35.5 TotaVStage 4.3 39.8 Total for Column, min. 1.79 16.6 The benefit derived from this increase in residence time is demonstrated by the following typical system: A+H20=B+NH3 When this equilibrium mixture is fed to a distillation column, the volatile reaction product, NH3, is removed from the reaction zone, and the equilibrium is shifted to the desired product B. This "reactive distillation" sequence can be approximated as a first order irreversible reaction: k A+H2O- > B+NH3 with a rate constant, k=0.295 min.-3. If we employ the residence times of the standard column (1.79 min.) and of the column of the invention (16.6 min.) as shown above, the conversion of species A in the standard column is 40% whereas in the column of the invention the conversion is 99%. Thus, the advantage for increased conversion of the column of the invention is clearly demonstrated.

WHAT WE CLAIM IS: 1. A distillation column reactor which comprises a distillation column containing a plurality of superposed trays each provided with a downcomer, a plurality of liquid reservoirs each of which is between a respective pair of adjacent trays, each reservoir being provided with vapour bypass means which in use permit vapours to flow therethrough from the tray immediately below the reservoir to the tray immediately above it, an open-ended conduit extending through the bottom of each reservoir to a substantial height thereabove and towards the tray immediately below, the conduit, in use, allowing overflow liquid to flow from the top of the liquid surface of the respective reservoir through the conduit to the tray below the reservoir, each conduit providing its respective reservoir with substantial depth to hold liquid substantially out of contact with vapour, each reservoir occupying the entire cross-sectional area of the column save for the area occupied by the vapour by-pass means and the conduit, and the top end of each conduit being above the bottom of the downcomer of the tray immediately above so that, in use, the downcomer extends below the surface of the liquid in the reservoir immediately below it.

2. A distillation column reactor as claimed in Claim I, wherein the downcomer of each tray extends to near the bottom of the reservoir immediately below it.

3. A process for carrying out a reaction in a distillation column reactor, which comprises effecting said reaction in a distillation column containing a plurality of superposed trays each provided with a downcomer, a plurality of liquid reservoirs each of which is between a respective pair of adjacent trays, each reservoir being provided with vapour by-pass means which permit vapours to flow therethrough from the tray immediately below the reservoir to the tray immediately above it, and openended conduit extending through the bottom of each reservoir to a substantial height thereabove and towards the tray immediately below, the conduit allowing overflow liquid to flow from the top of the liquid surface of the respective reservoir through the conduit to the tray below the reservoir, each conduit providing its respective reservoir with substantial depth to hold liquid substantially out of contact with vapour, each reservoir occupying the entire cross-sectional area of the column save for the area occupied by the vapour bypass means and the conduit, and the top end of each conduit being above the bottom of the downcomer of the tray immediately above so that the downcomer extends below the surface of the liquid in the reservoir immediately below it.

4. A process as claimed in Claim 3, wherein the downcomer of each tray extends to near the bottom of the reservoir immediately below it.

5. A process according to claim 3 or 4, wherein the reactants in said column include the nitrogen-containing by-products obtained in the hydrolysis of terephthalonitrile.

6. A distillation column reactor substantially as herein described with reference to the accompanying drawings and/or Example II.

7. A process for carrying out reactions in a distillation column reactor substantially as herein described with reference to the accompanying drawing and/or Example II.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (7)

**WARNING** start of CLMS field may overlap end of DESC **. employ the residence times of the standard column (1.79 min.) and of the column of the invention (16.6 min.) as shown above, the conversion of species A in the standard column is 40% whereas in the column of the invention the conversion is 99%. Thus, the advantage for increased conversion of the column of the invention is clearly demonstrated. WHAT WE CLAIM IS:

1. A distillation column reactor which comprises a distillation column containing a plurality of superposed trays each provided with a downcomer, a plurality of liquid reservoirs each of which is between a respective pair of adjacent trays, each reservoir being provided with vapour bypass means which in use permit vapours to flow therethrough from the tray immediately below the reservoir to the tray immediately above it, an open-ended conduit extending through the bottom of each reservoir to a substantial height thereabove and towards the tray immediately below, the conduit, in use, allowing overflow liquid to flow from the top of the liquid surface of the respective reservoir through the conduit to the tray below the reservoir, each conduit providing its respective reservoir with substantial depth to hold liquid substantially out of contact with vapour, each reservoir occupying the entire cross-sectional area of the column save for the area occupied by the vapour by-pass means and the conduit, and the top end of each conduit being above the bottom of the downcomer of the tray immediately above so that, in use, the downcomer extends below the surface of the liquid in the reservoir immediately below it.

2. A distillation column reactor as claimed in Claim I, wherein the downcomer of each tray extends to near the bottom of the reservoir immediately below it.

3. A process for carrying out a reaction in a distillation column reactor, which comprises effecting said reaction in a distillation column containing a plurality of superposed trays each provided with a downcomer, a plurality of liquid reservoirs each of which is between a respective pair of adjacent trays, each reservoir being provided with vapour by-pass means which permit vapours to flow therethrough from the tray immediately below the reservoir to the tray immediately above it, and openended conduit extending through the bottom of each reservoir to a substantial height thereabove and towards the tray immediately below, the conduit allowing overflow liquid to flow from the top of the liquid surface of the respective reservoir through the conduit to the tray below the reservoir, each conduit providing its respective reservoir with substantial depth to hold liquid substantially out of contact with vapour, each reservoir occupying the entire cross-sectional area of the column save for the area occupied by the vapour bypass means and the conduit, and the top end of each conduit being above the bottom of the downcomer of the tray immediately above so that the downcomer extends below the surface of the liquid in the reservoir immediately below it.

4. A process as claimed in Claim 3, wherein the downcomer of each tray extends to near the bottom of the reservoir immediately below it.

5. A process according to claim 3 or 4, wherein the reactants in said column include the nitrogen-containing by-products obtained in the hydrolysis of terephthalonitrile.

6. A distillation column reactor substantially as herein described with reference to the accompanying drawings and/or Example II.

7. A process for carrying out reactions in a distillation column reactor substantially as herein described with reference to the accompanying drawing and/or Example II.