CA1064645A - Process for the continuous polymerisation of lactams - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

The invention relates to a process for the continuous polymerisation of caprolactam in a reactor, wherein a reactor is used in which at least one third of the first 60% of the reactor volume is equipped with static mixing installations.

Description

1~646~
Thi~ inv~lltit)n r~:lat,~i l,o a l~r~c~s~ f'or the (~onl,inllolls r~)urlll,ion ol' I)ol,y~l~ni~ s~ ~)r~ ly l'rl~m ~ -c~p:ro.la~l,am ~;
I,h~ main or only (~olnporlellt~ l,o which, f'or ~xam~ , mattin~
u~(~nl,~, co:lol~rin~ nl,s or ~ hl; an(l hcnl, sl,a~)ili~,in~
t,~ S may l)~ a(l(l~
It i~ known l,o cr~t~ry ollt ~h~ cont,.inu~ lym~ri.~ial,ion ol'l)olyami(les in appallll.us o.l' variolls con~t,rucl,ions. ln th(~
.~ s.im~ t, cas~, the r~c~,or COIlsiSts ol' a ~implc, v~rt.ic~l . tubc .into ~hich caprolactam, ~OI~ ~xample, .is i.ntro~uc~d at :.
' 10 th~? I,op llnd frolll which polym(~r is removed at th~ bottom :: wh.ile w~at~r is d.~iven off as stoam at the top. The (les.ir~ t,o : control the ra-t~ of r~moval of water in accordanc~ wit~
reaction kinetic consideration~ has led to the deve~opment ,~ of multistage tubes, in which the various stages may be ,l 15 telescoped into each other. In the latter case, ascending ` and descending streams.o~ melt are obtained.
l 0-ther devices provided on and in the reactor have the object o.~ imparting a desired temperature profile to the ' I caprolactam which is undergoing polymerisation. This is '~', 20 achieved by simply heatin~ the l~all o~' the reactor or by i '' installing heat exchangers~ ~or example in the form of .~ plates or nests of tubes arranged parallel to the direction :~ - ..
.'~ of flow. It has been found sui-table to carry out the initial ', phase of the reaction at temperatures above 205C and the final phase below 250C, that is to say heat is appli~d to ll~ the top of a VK tube (e~g. the VK-process i~ described in `.~ "Eunststo~handbuch" Vol. VI, aarl Hauser Verlag Munich (1966) ., page 191) and removed in the lower part by a suitable heat ex-changer~ It i3 important to ensure that cooling ~ets in only when the conversion of caprolactam, whlch9 a3 i3 well known, a polyaddition reaction, has been substantially completed.
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:, Other developmants of the proce~s have the subject of ensuring more or less thorough mixing of the caprolactam T.

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wh.ich i~ to t)e po:lymerised, either :in order to stir additives into ih~ capro~ c-tam in the .ini-t:ial phase or in order to obtain a homogeneous melt. rrhe mechanical stirrers of various con-st.lllction ~requently used for this purpose are unsatisfactory7 .firstly because they are l.iable -to give trouble due to driving .
and sealing ~roblems and secondly because they do not help to produce -the required plug flow. Among the various solutions to overcome this dif~iculty, it has been proposed to use cascades of s-tirrer vessels which contribute to the formation ot plug :flow and the more so the larger the number oY vessels arranged in series. This solution, however, resulted in systems which were very complicated and therefore liable to . give trouble so that it was not possible to ensure an ~ absolutely constant reaction process which is necessary in .. 15 order to obtain high quality products.
It is also known to influence the flow in a VK tube by `` installing various devices in tho interior of the tube, for example V-shaped or perforated bafIle plates, conical hollow bodies or concentrically arranged annular surfaces extending parallel to the direction of flow to act as braking surfaces with a view to improve -the flow of the melt, but none of these devices has been found to be completly satisfactory.
The so-called static mixers which have recently become known provide a combination of a mixing action and an action .;l 25 which influences the flow.
It is an objcct of the present invention to improve the process for the continuous polymerisation of ~-caprolactam so that not only a thoroughly mixcd melt flowing as a plug flow is obtained but also the output is at least 20~ higher than can be achieved in prior art processes.
-` This problem is solved by carrying out the continuous polymerisation of caprolactam in a reactor in which the first ,' ~ T.Q A 1 C; a~7 2 1~;4645 60% of the reactor volume is equipped if posslble completely but at least to one-third with suitable static installatlons.
According to the invention there is provided a process for the con-tinuous polymerisation of caprolactam in a reactor, wherein at least one-third of the first 60% of the reactor volume is equipped with static mixing install- -ations including at least one mix m g inser~ having webs inclined at an angle to each other.
Suitable installations for this purpose are described in Canadian Patent 997,752 as devices for the static mixing of flowing media.
These reactor tubes contain at least one mixing insert consisting o webs inclined at an angle to each other. In one embodiment, these webs intersect, the inserts consisting of at least two slotted plates set at an angle to each other inside the housing and intermeshing through the slots. In another embodiment, the webs set at an angle to each other do not intersect -~
but contact each other along an edge to form a gable with its axis perpendi-cular to the axis of the tube. `~
' As the stream of material encounters the webs, it is split up into ~
i a number of individual streams separated from one another in time and space ~ -,~ due to the oblique positioning of the webs. On the downstream side of the ., , '1 20 webs there is a flow gradient in the transverse direction which ensures effi-cient exchange between the individual streams. Since division of the stream of material into separate streams is staggered in time and space, mixing also ~ ~-:;
takes place in the direction of flow and this is superimposed by radial flow ~ ~
'.! .,i components. With the transverse mixing which can be achieved it is possible ~ ~
. , -~' to obtain a good approximation to a plug flow profile so that a narrow spec~

`~ trum of dwelling times within the reactor can be obtained, which is advantage-' ous for caprolactam polymerisation. The inclination of the webs to the :
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lirection of the oncoming flow and to the wall of the tube may al~o be in more than one dimension. Ad(li-tional mixing effects are therel)y obtainc(l~
Accordin~ to one particular embodiment, the mixing insert compriscs a plurality of pairs of pla-tes which have a comb-like strllcture, and the pla-tes are arranged in two rows so that the wehs of at least two layers of one row intersect the webs of at least one laycr of the other row.
Several lines oi` intersection are thcrcby produced9 along which the partial streamsare further subdiv~ded.
The plates of each row are preferably arranged parallel to each other. ~his arrangement provide~ the po~sibility - of more rational manufacture of the pla-tes.

According to one particularly advantageous embodiment, several mixing inserts are provided and the webs of the pairs .
of plates o~ the mixing inserts are set at an angle to each other. This angle may be, for example, 90. With an angle of 90, the required length of the apparatus is particularly short because the p]ate~ of the puirs of plates or the plates of the outer pairs of plates of adjacent mixing inserts can then be pushed far into the gaps between the pairs of plates o~ the adjacent insert. The angular displacement between the mixing inserts in a row results in a subdivision ~, .
in space of the in~ividual streams which are produced by the webs .
With this embodiment, very powerful mixing effects can be ' obtained with only a few mixing inserts arranged in a row.
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The plates are preferably in the form of combs which have a generally elliptical outline, and in order to improve the flow in the marginal zones of the tube, the web con-. . , necting part is shifted from the wall of the tube towards the centre of the tube. In a rational manufacturing process~
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' 1~646~5 lhis eall l)e achieve(l l)y ~ nching ollt. It goes ~ithout sayingIhcl~ olhcl manllrrlctllre(l proces.ses nre also suitable for malllll`act~ irlg l;he mi~ing in~erts, Eor example large mixing inserlx can l)e f)ro(lllcell l)y welding. The plates may be con-strllcted~ for examl)le, by fixing the webs to a closed ring.
Thc we~)s and slots are preferably arranged parallel -to the main a~isi o-t the plates. This constitutes ano-ther advantage ~i-th regard to manufacture of the plates.
According to a particular embodiment, the webs have a ~ 10 special cross-sectional prol`ile, for example -they may be ; triangular, tcar-shaped or elliptical. Special flow effects which ensure vig~rous mixing in the -transverse direction, f`or example, are obtained in the case of a triangular section i~
one edge is at an angle against or in the direction of flow.
.~, `I 15 The webs may also have a hollow profile. In that case, they may be -traversed by a heating fluid, for example, so that they also perform a heat trans-fer function.
If it is dcsired to vary the velocity of flow of the medium at the centre of the tube as compared with the wall, i-t is advantageous to vary the width of the webs. The flow `~ is then displaced further towards the centre or towards the ::, periphery according to whether the webs are wider or narrower - towards the centre or the periphery.

The various possible variations of the apparatus for carrying out continuous caprolactam polymerisation afford ` the constructional engineer wide scope for optimising the apparatus. In particular, the mixing inserts can be constructed ~^ from variously formedwebs, or variously formed mixing inserts - with similar webs can be arranged behind one ano-ther in a :' suitable ~uence In this way it is possible to adapt the mixing inserts to special operating factors such as the flow ~ velocity, the viscosity and the residence time of the reactants `: Ie A 15 947 5 . .
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in the vaIious sect,ions.
V~triolls omllodiment,~ ot the al-paratus according to the invelltioll are ~hown purely diagrammatically in the accom-panying drawing~ and describcd below.
~`igures I to 5 rcprcsent various constrllctional cxamples Or -the appara-tus with di~ferent arrangements of the mixing inserts or plates and F'igure 6 shows an example in which the wbbs do not intermesh.
;; 10 In Figures 1 to G, similar parts have been indicated by reference numerals having the same digits in the unit place preceded by a digit representing the number of the figure.
Figure 1 shows several mixing inserts 12 arranged behlnd one another at angles of 90 to each other inside a tube 11.
The mixing inserts are formed hy intermeshing comb-like plates.
Figure 2 shows mixing inserts 22 arranged in a tube 21.
Each insert consists of ~ive pairs of plates 23, 23'. The , mixing inserts 22 are displaced by an angle of 90 relative to each other. Plates 23, 23' have the same configuration as in Figure 1.
The mixing inserts 32 arranged in tube 31 in Figure 3 `~ are similar in form to those shown in Figure 2 but in this case the oblique webs are in addition inclined to their longitudinal axis.
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Figure 4 shows a tube 41 containing mixing inserts 42 , which merge into each other in that the plates 43, 43' , in each case extend through several intersecting plates.
The plates are arranged that there are always two parallel : -:
i plates 43, 43' set close to each other while the distance '~ 30 between these two plates and the next following two plates is about twice as great as the distance of the first two plates to each other.

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~L~64~5 In the em~odiment shown in Figure 5, a tube 51 contains mixing inserts 52 in which plates 53, 53' in$ersect not only each othcr hut also the plates of adjacent mixing inserts 52, as in the e~ample shown in Figure 4, so -that individual mixing inserts cannot be exactly defined. In this embodiment, importance is attached to placing the lines of intersection outside the central axis of the tube 51.
Alternatively, the lines of intersection may bc dispensed with, as in the embodiment shown in Figure 6, and the individual webs may be connected along their edges to form V-shaped inscrts. In this embodiment, all the adjacent layers of webs may be displaced from each other in any way desired.
Figures 7 to 10 show diagrammatically reaction tubes for carrying ou-t the process according to the invention. Similar parts have again been indicated by the same unit digits preceded ~y the number of the figure.
3~ 71, 81 91 and 101 indicate the lactam supply. 72, 82, 92, and 102 indicate a condenser. The reaction tubes 73, 83, 93 and 103 are equipped with static installations 74, 84, 94, 20 and 104. In Figure 9, a hydrolysis apparatus 97 equipped i~ with a stirrer 95 is attached to thc upstream end of the reaction tube. In Figure 8, a stirrer 85 is provided inside the reaction tube. In this case, the lactam is preheated at 86.
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It will, of course, only be possible to apply the previously described apparatus in accordance with the invention in cases where -the stirrer or heat exchanger does not take up the entire volumc of the reactor. It is, of course, possible and in many cases advisable, in the inte-:1~ .
-~ 30 rests of obtaining a homogeneous melt with a plug flow to ~''5 instal static units also in the last 40% of the reactor volume, but this is not necessary for the purpose of the ~e A 15 947 7 ., I , .

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1~64~5 .inven~i~n On the o~her hand, it would be quit~ insu~ficien-t for the purpo~e of the invention to equip only the last 40%
o~ the .r~actor volum~ with th~s(~ irlstallations.
The known packing mixers which con~ists of a plurality of parallel, inters~cting channels open to each o-ther on one side ar~ also suitable ~or carrying out caprolactam poly-merisation in accordance with the invention, al-though for tubes of large diameter, in the case described here of the order oI up to 1500 mm, these mixers are not sufficiently :
economical owing to the manufacturing costs and the high . cost of thin sheet metal. Moreover, th~ pressure drop depends to a very great extent on the ~elected geometrical configuration o~ the channel, which is in some cases disadvantageous.
Statio mixers which have to be very long in order to achieve a good mixing effect, f~r e~ample one known embodi-ment consisting of a combinatio:n of alternately left-handed and right-handed helical elements, cannot be used in the process of the in~ention, with respect to the big diameters . i , ~, 20 of VK-tubes. Fo~ special constructions, e.g. by dividing the cross section of big VK-tube~ into many parallel tubes ; of smaller diameters, this mixing unit could be suitable . for carrying out caprolactam polymerisation, but with high ', costs of construction.
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The crfect of the preserlt inven-t,ion is illustrated by a comparison of t,he extract values o~ a polycaprolac-tam which has been obtained in a conventional polymerisation tu~)e in Example A with those of a polycaprolac-tam which has been oh-tained according to Example B in a tube which is completely identical with the tube used in Example A but in addition is equipped with the installations according to the invention. The quantities of chain regulator and water j and the reaction temperatures employed are also identical in the two cases, The lactam output from reactor A is 11.0 tons per day and from reactor B 12.1 tons per day. The extract values were obtained by the methanol method.

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Polymerisation -tube A Polymerisation tube B

11.0 tons per day output 12.1 tons per day output ' 11.95 % extract 9.54 ~0 ex-tract ~, 12.01 % " 9.84 % "
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512.28 % ~' 9.11 ~0 , 12.00 % ~' 9.85 % ~' llo90 % 1l 9.78 % " ~' 92 ~ ~ . 9~ 99 %

,"" 12.21 % " 9.23 ~ "
.,~i 10 For kinetic reasons, caprolactam conversions of 90% and .~,j ~' more can be achieved in hydrolytic caprolactam polymerisation.

It is clear that this conversion can easily be achieved in a ~`
~' reaction tube B operated in accordance with the invention ,` 15 whereas the conversion rate in the normal reactor is in the ,'~, region of 88% although the caprolactam output is about 10%.:~
"~"l lower.
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Claims (7)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. Process for the continuous polymerisation of caprolactam in a reactor, wherein a reactor is used in which at least one third of the first 60% of the reactor volume is equipped with static mixing installations including at least one mixing insert comprising webs inclined at an angle to each other.

2. Process according to claim 1, wherein the reactor is in the form of a tube and said mixing insert is in the form of a pair of plates which have the webs inclined at an angle to each other and to the axis of the reactor tube, the webs of the two layers either intermeshing like forks or abutting against each other along their edges in the centre of the tube to form a roof gable with the gable edge situated perpendicular to the axis of the tube.

3. Process according to claim 1, wherein the reactor is in the form of a tube and the mixing insert comprises several comb-like pairs of plates, the plates being arranged in two rows so that the webs of two layers of one row intersect the webs of at least one layer of the other row.

4. Process according to claim 3, wherein the plates of each row of the mixing insert are arranged parallel to each other.

5. Process according to claim 2, wherein the mixing insert consists of hollow section webs traversed by a heating medium, the webs being arranged at an angle to each other and to the axis of the reactor tube.

6. Process according to claim 3, wherein the mixing insert consists of hollow section webs through which a heating medium flows, the webs of two layers of one row intersecting the webs of at least one layer of the other row.

7. Process according to claim 2, wherein individual webs of the mixing insert are welded and adjacent webs can be arranged in any desired position in relation to each other.