Process for reducing fouling in higher olefin plants

An improved process for oligomerizing olefins is provided in which olefin feed is contacted with phosphoric acid catalyst to form an olefin oligomerization product containing phosphorous-containing foulant precursors. The olefin oligomerization product is then contacted with a foulant adsorbent comprising at least one member selected from the group consisting of alumina, activated alumina and magnesium oxide in an amount and under conditions sufficient to effect removal of at least a portion of said foulant precursors. The treated olefin oligomerization product stream can then be passed to downstream recovery and process equipment with minimized fouling.

BACKGROUND OF THE INVENTION 
Field of the Invention 
Higher olefins (e.g., acyclic alkenes of from 5 to 15+ carbon atoms) are 
produced commercially in large volumes by oligomerization over phosphoric 
acid catalysts using lower olefin feeds (for example, propylene, butenes 
and pentenes). Hydrocarbon Processing, vol. 47, page 170 (September 1968); 
Petroleum Refiner, vol. 36, no. 9, page 232 (1960). Prior art phosphoric 
acid catalysts which have been developed have employed a wide variety of 
catalyst supports, e.g. those disclosed in U.S. Pat. Nos. 2,586,852 and 
2,713,560 (each to Morrell), phosphoric acid on such supports as activated 
carbon, silica gel, diatomaceous earth, kiesulguhr, infusorial earth, 
bentonite, montmorillonite and similar adsorbent substances. This patentee 
also discloses that catalysts can be formed by baking at elevated 
temperature a mixture of kaolin and phosphoric acids (e.g., 
orthophosphoric, pyrophosphoric, metaphosphoric, and tetraphosphoric 
acids), and that the porosity of such kaolin catalysts can be improved by 
adding from 1 to 10% (based on the kaolin) by weight of calcium and/or 
magnesium oxides, or the corresponding carbonates, to the kaolin before 
mixing in the phosphoric acid. It is noted in these patents that mixtures 
of calcium oxide and magnesium oxide are preferred, because the former 
produces a more porous product but one which is softer and takes up water 
quite readily, whereas these features are corrected by the magnesium. 
Equipment fouling in such higher olefins plants has been a serious problem 
since the 1930's. Heretofore, equipment sparing was thought to be the only 
means to effectively combat this problem. This has necessitated 
maintaining multiple trains of vital equipment to enable continued plant 
operation while simultaneously cleaning fouled equipment that have been 
taken out of service. The debits associated with fouling for each higher 
olefins plant range from thousands to millions of dollars per year 
depending on several factors, among these, availability of spare heat 
exchangers, distillation towers, maintenance costs, frequency of re-tubing 
of reactors and exchangers, etc. 
Past attempts to minimize the degree of fouling (e.g., using 
neutralization, filtration and percolation) have met little success. Also, 
attempts have been made to introduce organic anti-foulants into process 
streams, but these have not created satisfactory solutions to the fouling 
in downstream equipment. 
Canadian Pat. No. 507,337 is directed to a method of inhibiting metal 
corrosion in a polymerization reactor employing a bulk liquid phosphoric 
acid catalyst wherein phosphoric acid esters (reaction products of 
phosphoric acid and the feed olefins and/or the olefinic oligomerization 
products) are maintained in the reaction mixture below about 0.08 mol of 
such esters per mol of free phosphoric acid. In the process, the olefin 
feed is dried (e.g., with silica or alumina), heated and then introduced 
into the reactor. The reactor's liquid effluent is withdrawn and phase 
separated to form a lower phosphoric acid phase (which is recycled to the 
reactor) and an upper hydrocarbon phase, which is treated for product 
recovery. A portion of the separated phosphoric acid phase is heated in a 
separate vessel to thermally decompose the phosphoric acid esters. 
However, this method can result in fouling of the heat treatment vessel 
and only addresses corrosion problems in the reactor. No provision is made 
for avoiding of fouling in downstream hydrocarbon processing equipment. 
Moreover, thermal treatment of the separated hydrocarbon phase could 
result in undesired loss of hydrocarbon product, due to increased 
polymerizations catalyzed by the phosphoric acid thermal decomposition 
by-products. 
SUMMARY OF THE INVENTION 
It has now been found that fouling in higher olefins plants can be greatly 
minimized by contacting the olefin oligomerization effluent from the 
oligomerization reaction zone with a solid alumina, activated alumina or 
magnesium oxide in an amount and under conditions sufficient to remove 
phosphoric acid esters therefrom. It has been surprisingly found that 
alumina, activated alumina and magnesium oxide efficiently reduce the 
foulant precursors, thereby greatly extending the useful service life of 
equipment downstream of the olefin oligomerization reaction zone. 
Without being bound thereby, it is believed that the lower olefins present 
in the olefin feed to the oligomerization reactor react with phosphoric 
acid present in the catalyst together with water in the olefin feed to 
produce phosphoric acid esters, and that these phosphoric acid esters pass 
into the reactor effluent and initiate further olefin polymerization on 
the walls of downstream process equipment. This additional polymerization 
of the olefins is believed to lead to high molecular weight fouling 
materials, which deposits on the walls of process equipment, impeding the 
ability of heat transfer equipment and impairing fluid flow therethrough.

DETAILED DESCRIPTION OF THE INVENTION 
In the conventional higher olefins process, the selected lower olefin is 
reacted over a solid phosphoric acid catalyst to produce branched 
mono-olefins of a higher carbon number. These mono-olefins so produced are 
used as feedstock for hydroformylation to form oxo-aldehydes (which can be 
subsequently hydrogenated to the corresponding oxo-alcohols and used as 
intermediates to form phthalate plasticizers, and which can also be 
employed as detergent intermediates, such as nonyl phenol and dodecyl 
benzene). The lower olefins which can be used can comprise propylene, 
butenes and pentenes, or mixtures of the foregoing. For example, propylene 
and butenes from steam cracking and catalytic petroleum cracking are 
suitable mixtures. Any of the isomeric olefins can be used, alone or as 
mixtures. The olefin feedstock is typically first treated to remove 
deleterious quantities of impurities such as organic sulfur, sulfur 
compounds, acetylenic compounds and diolefins (e.g., hydrogen sulfide, 
mercaptans, methyl acetylene, propadiene and the like). Such a feedstock 
pre-treatment can conventionally involve absorption of the impurities with 
mono- or diethanolamine and caustic wash stages for sulfur removal 
followed by selective catalytic hydrogenation to reduce the diolefins and 
acetylenes content. 
In addition to the olefin, paraffins and water are also generally 
introduced. The paraffins can suitably comprise propane, butane, and 
pentane, with the selected paraffin generally comprising a molecule of the 
same molecular structure as the selected olefin (e.g., propane for 
propylene feeds, butane for butylene feeds, and the like). The function of 
the propane is as a diluent of the olefin feed to prevent excessive 
catalyst temperatures from being achieved within the reactor, and thereby 
control undesired exotherms. In addition, water is typically employed in 
the olefin feed, and the water content is maintained at a level which is 
selected to control the hydration level of the phosphoric acid catalyst. 
Such a hydration level control is important to maintain activity and life 
of the phosphoric acid catalyst. Typically, olefin feeds to such an 
oligomerization reactor will comprise from about 20 to 60 wt. % olefin, 
from about 40 to 80 wt. % paraffin, and from about 0.01 to 0.07 wt. % 
water, and more typically from about 30 to 40 wt. % olefin, from about 60 
to 70 wt. % paraffin, and from about 0.02 to 0.04 wt. % water. However, 
the quantity of paraffin and water, and amounts of olefin, can vary widely 
depending on the olefin selected, the temperature and pressures to be 
employed in the oligomerization reactor, the precise products which are 
sought to be formed, the type of reactor which is employed and other 
factors. 
Generally, the oligomerization reaction is conducted at a temperature of 
from about 150.degree. to 230.degree. C., more typically from about 
165.degree. to 215.degree. C., and at a pressure of from about 4100 to 
8200 kPa, more typically from about 4800 to 7000 kPa. Again, the precise 
temperature and pressure employed in the olefin oligomerization reactor 
will depend on a large number of factors, among them the type of olefin 
which is fed, the olefin distribution of products which is sought to be 
formed, and other factors. 
The olefins can be passed to the reactor in either the liquid or vapor 
form, and feed rates are generally in the range of from about 1 to 3.5 
liters/kg-hr, more typically from about 2 to 3 liters/kg-hr. 
Since the oligomerization reaction is exothermic, the desired reaction 
temperature is conventionally maintained either by quenching with the 
selected paraffin gas, as by quenching between the catalyst stages when 
the reactor comprises a multistage vessel containing catalysts, or by 
conducting the reaction in a tubular reactor in which the phosphoric acid 
is contained within a plurality of parallel arranged tubes and around 
which cooling water is circulated for steam generation in order to remove 
the desired quantity of heat. 
The solid phosphoric acid catalyst is conventional and can comprise 
phosphoric acid on silica gel or of other materials of a silicous 
character, including diatomacous earth, kieselguhr and the like. Such 
conventional phosphoric acid catalysts are disclosed in U.S. Pat. Nos. 
2,586,852 and 2,713,560, the disclosures of which are hereby incorporated 
by reference. 
According to the improvement of the process of this invention, the 
oligomerization product stream, after withdrawal of the product stream 
from the catalyst bed/tubes, is contacted with alumina, activated alumina 
and/or magnesium oxide in an amount and under conditions sufficient to 
remove at least a portion (and preferably at least about 20 wt. %, and 
more preferably at least about 50 wt. %, and most preferably from about 50 
to 80 wt. %) of phosphorous-containing moieties in said product stream, 
preferably to achieve a concentration of phosphorous impurities in the 
treated product stream of less than about 100 ppm by weight, more 
preferably less than about 50 ppm by weight and most preferably less than 
about 20 ppm by weight. Generally, the amount of the alumina, activated 
alumina and/or magnesium oxides will typically comprise from about 0.01 to 
1.0 part by weight, more preferably from about 0.05 to 0.2 part by weight, 
per part by weight of the oligomerization catalyst. 
By "activated alumina" we mean alumina which has been activated by 
conventional methods of thermally treating granules of hydrated alumina. 
Illustrative of suitable activated aluminas are: activated alumina F1 
(Aluminum Company of America, Pittsburgh, Pennsylvania), and activated 
alumina A-201 (Kaiser Aluminum and Chemical Corporation, Baton Rouge, 
La.). The particle size of the aluminas, activated aluminas and magnesium 
oxides can vary widely and will generally range from about 325 mesh to 5 
cm in size, and the surface area of the activated alumina will generally 
range from about 200 to 400 square meters per gram. Suitable magnesium 
oxide solids are those containing at least 97 wt. % magnesium oxide, with 
the balance comprising calcium oxide, silica and iron oxide. 
The oligomerization product stream can be treated according to the foulant 
removal step of this invention in a separate vessel after removal of the 
product stream from the oligomerization reactor or within the reactor, but 
after the removal of the product stream from contact with the 
polymerization catalysts, e.g., by placing the selected alumina, activated 
alumina, and/or magnesium oxide foulant adsorbent of this invention as a 
separate, lower layer of solids within a conventional downflow chamber 
reactor, on top of which solids is placed the polymerization catalyst. 
When the foulant adsorbent is employed in the lower section of the 
reactor, the olefin feed will be introduced to the upper portion of the 
reactor for oligomerization over the phosphoric acid catalyst, and the 
resulting product stream then will contact the foulant adsorbent after 
removal from the oligomerization reaction zone in the reactor. Such a 
lower layer of foulant adsorbent should possess suitable particle size, 
and crush strength (e.g., .gtoreq.10 kg) properties to avoid undesired 
pressure drops over this lower layer. The defoulant adsorbent can also be 
positioned above the phosphoric acid for reactors which are operated in an 
upflow mode, with the olefin feed being introduced at the reactor's lower 
portion for withdrawal of the product stream from the reactor's upper 
sections. 
The process of the present invention can be further illustrated by 
reference to the following examples, wherein parts are by weight unless 
otherwise indicated. 
EXAMPLES 
In a series of batch experiments, 100 cc of an olefin oligomerization 
reactor effluent, produced by a conventional olefin oligomerization of a 
propylene feed over phosphoric acid on kieselguhr catalyst (at 205.degree. 
C. and 5 MPa) and comprising about 90 wt. % of C.sub.5 to C.sub.12 olefins 
and 10 wt. % C.sub.5 to C.sub.8 paraffins, and containing 134 wppm 
phosphorous impurities, was placed in a glass vessel containing the 
selected quantity of the indicated solid materials. The contents of the 
glass vessel were stirred for two hours. The liquid was then separated 
after allowing the solids to settle, and the liquids were analyzed for 
residual phosphorous impurities. 
The data thereby obtained are set forth in Table I below: 
TABLE I 
__________________________________________________________________________ 
Run Residual Fouling Precursor (ppm)* 
% 
No. 
Solid Material 
Grams 
(PA) 
(TMP) 
(DEP) 
Total 
Reduction 
__________________________________________________________________________ 
1 Activated Carbon (1) 
5.0 82 41 -- 123 9 
2 Activated Carbon (2) 
5.0 80 36 38 154 0 
3 Silica (3) 5.0 65 68 -- 133 0 
4 Alumina (4) 
0.5 16 -- -- 16 88 
5 Alumina (4) 
1.0 10 &lt;1 6 &lt;17 87 
6 Alumina (4) 
5.0 4 &lt;1 -- &lt;5 96 
7 Activated Alumina (5) 
0.5 -- -- -- -- 100 
8 Activated Alumina (5) 
1.0 -- &lt;1 -- &lt;1 99 
9 Activated Alumina (5) 
5.0 -- -- -- -- 100 
10 Magnesium oxide (6) 
0.5 &lt;1 -- -- -- 99 
11 Magnesium oxide (6) 
1.0 &lt;1 14 -- &lt;15 89 
12 Magnesium oxide (6) 
5.0 -- 6 -- 6 96 
__________________________________________________________________________ 
Notes: 
PA = H.sub.3 PO.sub.4 ; TMP = (CH.sub.3 O).sub.3 P; DEP = (C.sub.2 H.sub. 
O).sub.2 PO.sub.2 H 
(1) Filtrasorb 200 (950 m.sup.2 /gm); Calgon. 
(2) Filtrasorb 400 (1150 m.sup.2 /gm); Calgon. 
(3) Chromatographic grade silica (500 m.sup.2 /gm). 
(4) Chromatographic grade Fisher Scientific Company (200 m.sup.2 /gm). 
(5) Alcoa F1 alumina (92% Al.sub.2 O.sub.3 ; 210 m.sup.2 /g). 
(6) 0.2 m.sup.2 /gm; Technical grade MagChem 10 .TM. (Martin Marietta 
Chemicals). 
*Foulant levels remaining determined after contacting the absorbents with 
100 cm.sup.3 of the liquid feed. 
Therefore, it was surprisingly found that activated carbon and silica 
(despite these materials' high surface areas) are ineffective for removal 
of such fouling precursors; whereas alumina, activated alumina and 
magnesium oxide efficiently removed substantially all of these residual 
fouling precursors, with activated alumina being particularly effective. 
From the foregoing description, one skilled in the art can easily ascertain 
the essential characteristics of this invention and without departing from 
the spirit and scope thereof can make various changes and/or modifications 
to the invention for adapting it to various usages and conditions. 
Accordingly, such changes and modifications are properly intended to be 
within the full range of equivalents of the following claims.