Distillation column reactor and process for carrying out reaction therein

DISTILLATION COLUMN BEACTOE AND PROCESS FOR CARETING OUT ! REACTIONS THEREIN

ריאקטוך כעל עמוד לזיקוק ותהליך לביצוע תגובות בעמוד

The use of a distillation column for both separation and reaction is known. In particular, use of a column in s®!h 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 vapor from the top of the column. Such a ’process is disclosed in U.S. 2,384,793 where high molecular weight acids and high molecular weight alcohols are esterified. Also of interest is the process and equip10 ment disclosed in U.S. 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 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 1 to 5 minutes.

It is an object of the present invention to provide a distillation column reactor which enables long residence times 20 to be obtained and, further, which does not significantly increase the pressure drop throughout the column. In accord with the invention an improved distillation column reactor comprises a distillation column fitted with trays and downcomers and having a liquid reservoir between said downcomers and trays, each of said liquid reservoirs being provided with vapor ports and a conduit to a lower tray, whereby liquid from said downcomer enters said reservoir and then flows through said downcomer in said reservoir to a lower tray, thereby providing increased liquid Due to the construction of the residence time in said column. distillation column reactor according to ;the invention, the liquid remains substantially out of contact with’the vapour. The invention also provides a process for carrying out simultaneously fractionation and reaction where conversion of the reactant species requires 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 favorably 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.

In order to better understand the invention, reference is now made to the figure. As shown in the drawing, a column (11) contains the usual standard tray (12) and weir (13). Below the tray (12) is the liquid reservoir (14) which is filled by the liquid which spills from over the weir above it. It will be understood that a reservoir need not necessarily be blow 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. Each reservoir has one or more vapor ports (15) through which vapor rises from the lower tray to the upper tray and a vapor distributer cap (16) may be

A vapour by-pass permits vapor to flow through said by-pas^_rfro!g_e|he 20 employed in the conventional manner if desired./ A conduit 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 capacity. Furthermore, the downcomer from the standard tray (12) can be designed so as to impart a tangential velocity to the liquid entering the A reservoir thereby effecting a modicum of agitation'to enhance mixing.

The improved distillation reactor described above^will be useful, as indicated, in equilibrium limited reactions vjpich are exemplified by reactions such as esterification, etherification, hydrolysis and the like. In the operation of the distillation reactor of the invention the regional fractionation time (i.e., the residence time per tray) is short in accord with the usual short contact time between the vapor and the liquid on the standard trays. However, the liquid reservoirs in the column provide a relatively long contact time for the region where the 10 reaction occurs. Furthermore, in the reservoir the vapors do not have an opportunity to recombine and thus the reaction is driven to completion.

In order to further 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 containing:

<td></td><td> 19132</td><td> Ib-moles/hr</td><td>^h2°</td>

<td></td><td> 765</td><td> Ib-moles/hr</td><td> potentially free NH*</td>

<td> 20</td><td> 383</td><td> Ib-moles/hr</td><td> non-volatile reactive material</td>

The column has 30 trays with the feed to tray number 25. The distillate product is 50 mole % H^O and contains 99% of the available NH*.

Assuming constant molal overflow, the molar vapor traffic, V, in the column is given by

V = D (R_d + 1) where: ;

D = distillate product rate

R, = reflux ratio

Setting and noting from the above that D = 1515 lbmoles/hr._׳ the vapor traffic is

V = 5833 Ib-moles/hr.

The density of the vapor,, at the average column temperature 3 of 474°F. is approximately 1.15 Ibm/ft, . Setting the molecular weight of the vapor at 17.5 (50 mole % 61.,0/50 mole % ׳ (^66א the volumetric flowrate, V, is

V - V x 17.5/e^ = 88763 ft.3/hr.

For a superficial velocity, .χ,-׳, of 1 ft./sec. the required column diameter, D^, is

D. = /_4 V'

V1T3600V^ = 5.60 ft.

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, A^. Thus, the active area, A , is a ר 20 A = 0.75 A. 0.75 ־־ΤΠ3.__ ^{a 1} 4 ¹ = 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.,

ר (1515/20208)10 = 7.5 mole %) the weir crest is 3.71 (calculated by the Francis Weir Equation, Perry 4th Edition, p. 18.9). Thus, the total effective height on a tray is about 6.71. This leads to an effective volume, H, of

H - 6.71 x 18.5 = 10.34 ft. .

12 I;

4Since the feed is saturated, the liquid traffic, 1, in the stripping section is

L = R _ר X D + F d o = 24598 Ib-moles/hr.

As the system is extremely water rich, the molecular weight can be set at 18. The liquid density,^!., at the average column temperature is 51 lbm/ft.³. Thus, the volumetric liquid traffic, L' , in the stripping section is

L' = L x 18/5^ ג10 = 8682 ft. /hr .

Therefore, the liquid residence time per tray, ^e_tray׳ ^is= H/L¹tray = 4.3 secs.

and for 25 trays in the stripping section the total residence ^tirae9 ׳total׳ ^is θ 1.79 = _י minutes for a column of standard design, total

EXAMPLE II

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

Setting the diameter of the port, D , at 16, the active area of the reservoir becomes

A - 18.5 -JEDpZ_ = 17.1 ft.

and the volume, H , of a 5' deep reservoir is

H = 5 x 17,1 85.5 ־ ft. r

The liquid residence time, ©_r׳ for the reservoir is the

Θ = 35.5 secs, r

And the total residence time for 25 trays and 25 reservoirs,

Θ,. x ,, is (tray +r)tot'

Θ,. ,,,.= 16.6 min.

(tray +r)tot

The major results of the above calculations are summarized in the following table. For completeness the pressure drops associated with the standard column andthat 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

<td></td><td colspan="2"> Example I Standard Column</td><td> Example II Column of The Invention</td>

<td></td><td> Superficial Vapor Velocity ft/sec</td><td> 1</td><td> 1</td>

<td> 10</td><td> Liquid Depth, in.</td><td></td><td></td>

<td></td><td> Trays Weir Weir Crest Reservoirs</td><td> 3.0 3.7</td><td> 3.0 3.7 60.0</td>

<td></td><td> Pressure Drop, psi Tray Reservoir Total/Stage</td><td> 0.21 0.21</td><td> 0.21 0.14 0.35</td>

<td> 20</td><td> Residence Time in Stripping Section, sec. Tray, Θ _ tray״ Reservoir, Θ Total/Stage</td><td> 4.3 4.3</td><td> 4.3 35.5 39.8</td>

<td></td><td> Total for Column, min.</td><td> 1.79</td><td> 16.6</td>

<td></td><td> The benefit derived</td><td> from this increase</td><td> in residence</td>

time is demonstrated by the following typical system:

A + H₂^צ B + NH₃

When this equilibrium mixture is fed to a distillation column the volatile reaction product, NH^, is removed from the reaction 30 zone and the equilibrium is shifted to the desired product B.

This reactive distillation sequence can be approximated as a first order irreversible reaction

A + H₂Q > B׳+ NH₃with a rate constant, 0.295 min. \ Employing 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 th^' invention conversion is 99%. Thus, the advantage for increased conversion of the column of the invention is clearly demonstrated.