Corrosion in Isomerization of Light Hydrocarbons U
J
BY ALURIINURI CHLORIDE-HY DROCARBON COMPLEX CATALYST S . FR-IGES. C.
N.. SYSER--i;ISDERl. 1\11 \I7.K. HERTWIG
S t u n da rd Oil Co rnpctny (Indio 11 u), W'hi t i n g : Inrl. Corrosion i n the liquid phase isomerization of light
ant1 the iwiiainclc~rhighly polyinerizeci : t i ~ dcyclicizeci 1iydroc:trbons. This catalyst, also tcrnicd coniples in this paper, is imniiscil~kw i t h the liquid hydrocsrt~onst8hatare to be isomrs,,izcd. Pretleated h.yc1rocarhon feed enters t h e bottom of the rcactor, is distributed across n j h t e with niultiple sinnll holes, and bubbles u p through the caiaI?-,?t n-hwe it large part of the i i ~ ~ r ~ iparial :iffins w e converted to tlwir iwnicric forms. Sormally, the rektctor temperature is 215" to 250" F. Anhydrous hydrogen chloride is employed as a catalyst promoter arid is added continuously t o the reactor charge while it is stripped out of the coolccl rcactos effiuent and recycled again t,o t~hereactor charge. Figure 1 shows a simplified f t n v diagram of the catalyst scction of a typical liquid phasc isomerization unit. More detailed descriptions of this process havc already been published ( I , 2 ) . .i trace of catalyst tcnds t o carry o u t of the reactor with the reaci'or hydrocarbon effluent, both as finis cirop1cxts of complex, and as aluminum chloride in solution. Therefort,, corrosion due e c q e c t e d in the reactor and connecting lines, , pumps, and lines of the systrni between t h e reactor and the hydrogen chloride stripper. After removal of the hydrogen chloride, the complex is no longer corrosive t o any important extent. Parts of the system containing only clean hydrocarbons, or hydrocarbon plus anhydrous hydrogen chloride, present no unusual corrosion problems. Some corrosion difficulties have b w n found in the neutralixatio~isystem handling the hydrogen chloride stripper bottoms stream, but this is only a problem of completely hydrolyzing and neutralizing the traces of dissolved aluininurii chloride found in this stream, and is of no intercst in this discuPsion of corrosion by complex.
hj drocarbons by aluminum chloride-h~-ilrocarbon complex catalyst causes penetration of carbon steel from 1 to 5 inches per year in the reactor, ancl reaches 15 inches per year at points of high turbulence. The liner arid vessels handling the products of reaction exhibit penetration rates about one tenth of those in the reactor. Corrosion rates vary with the catalyst turbulence, hydrogen chloride concentration, catalyst activity, and temperature. This attack by aluminuni chloride complex has been studied 0 1 1 laboratory and commercial scale, arid sereral means have been found for safeguarding steel equipment. The most successful of these iiiyolves the use of gunited Luiiinite cement reactor liners and Hastello>-B reactor nozzle. flange, and valve protection.
used in tlie wirtinie productiim of nigh line involvcd the isomeriz:ttirin of Ionoctnne nuniber nnrrilal palaffin hydrocnrboris to their high ociitilt. iiiini1,w isomeric forme. ri-l3utaw, pentane, and hcranc, ~ y r iwiiierizetl e in the presence of a liquid aluniinunl chlorideDuring the h~-t]rugeiic.ii10ride-liydrocarl~oii cuniples c:it:ilys:. pilot plant development and large scale optxition oi t!iis process serious corroeioit dificulties 11 ere encnuntereil. The alumi~iuiii chloride bearing c a t a l p t proved t o be extrenielj- corrosive t o steel :ind other c o n i n i ~ i ijtructural metals. Corrosion resulttd in leaks in the reactor, coniicct iny piping, :%ridlieat eschangcrs doiviistrc3:tin fi,oiii the reactor, arid in failurcb O f block and ccintri>l teni to function propc.rly. These c o r i v i o n probl~.miwere ~ ( ~ l ~1-i ~e tlic. -d suit:ihlt> use of reactor linPr.3, pipe and v d v r alloy trim, and equipment design to minimize turbulcarice in steel lines and vessels. HaLstelloy B was found to 11c the onlj- satisfactorily resistant metal for continued coniact ivitli the catalvst, and ,,.as used for lining pipe and trimnlirlg valves. Higll ~~~~~i~~ gunited on the inside of tiie steel best reactor liners and several in ?elvice reactors, for over a year. This is presented because ,,f iIlc~easillgillterest ill the commercial applicarion of organic reactions enlploJ.ing alunlinum rhloiitle ancl aluminum chloride-organic complex eatal) It is hoped that the corrosion experience revieired, the discussion of controlling factors in this type of corrosion, and the methods of eliminating corrosion ill be of some assistance t o future users of aluminum chloride-hj-drocarhon-h'drogen chloride coniplexes for catalyzing chemical reactions.
CORROSION PROBLEMS IN THE PROCESS
The r,iitalyst section of thc isonierization unit is suhjcct to corrosion in the reactor, connecting piping, and the system over to the hydrogcn chloride stripper. Corrosion of steel in this system logically falls into three general classifications. First, general over-all corrosion is found within the coniplex phasc in the reactor. I t is characterized by a general loss of met,al rather similar to t h a t experienced with aqueous acids. IIowever. i t is
I N D I A N ISOMERIZATIOS PROCESS
The Btanda.rd Oil Company (Indiana) light hydrocarbon isomerization process makes use of a vertical tower-type reactor wherein the liquid phase feed contacts the catalyst. The latter is a viscous dark brown corrosive liquid containing about 60% aluminum chloride, several per cent anhydrous hydrogen chloride, 1
HIDR
F
Figure 1.
Present address, D t a h Oil Refining Company, Salt Lake City, Utah.
1133
Process Flow Diagram
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
1134
Vol. 40, No. 6
01-20
05
I 5
R E C Y C L E HCI
, EA:K
I
FPESEIISE
-1 C O N T R O L
2
VALVE
01-03
-15
i ?
COMPLEX SETTLER
.t 3:
I
STRl PPE R FEED PUMP
I
i DRAIN Figlire 2.
Commercial Equipnietit Corrosion I'ctietratioii
Rates Catalytic reactor section. Numbers refer t o corrosion rates on carbon steel expressed as inches penetration per year (7900 hours).
soniewliat selective in action, and some areas are corroded a n d pitted t o a greater extent than adjoining areas, although irequently without any apparent reason. Secondly, a t turbulent comples-hydrocarbon interfaces, such as in some reactor nozzles and lines, extreme grooving action occurs on steel; this gives the appearance t h a t the metal has been cut out by a t,ool. The third class of corrosion is a variable pitting action in the reactor overhead system caused by the impingement on steel surfaces of fine complex droplets carried in the reactor product stream. Corrosion measurements were made in both the pilot and cornnlcrcial units by observing the loss in !wight of test strips hung in the vessels for periods of 100 t o 1000 hours. .Ilso, measurenients m r e made of the change in n.all dimensions of vessels and pipes. .ill corrosion penct,ration rates in t'his article a r e expressed as inches per operating year (7900 hours). Figure 2 s h o w the general part of t h e unit subject t o sevcre corrosion by complex, together v i t h t h e typical corrosion rates on unprotcctcd carbon steel in various locations of the unit. Ranges of values are reported because of the variation in corrosion observed x i t h different operating condit,ions, and the differences resulting from isomerizing butane, pentane, or hexanes. T h e last employs a more active cat,alyst,ivhich has a greater tendency t o carry over with the reactor product, thereby causing greater corrosion in the reactor effluent lines and reactor product cooler. Unprotected carbon steel is penetrated a t the rate of 1 to 5 inches per year in the main reactor bed of comples, depcnding upon the exact, location and operating conditions. The higher rate is found in the more turbulent 1on.er portions of the reactor near t,he hydrocarbon feed distribut,or plate. Table I s h o w the effect of height in the reactor on the corrosion of steel tcst strips hung in the pilot plant reactor within the comples phase.
Figwe 3 shows typic,al corrosion iii n Iioi~izorital pipe. C a h o n steel flange faces \\-ere also swiously scored and pcnetratcd as s h o n n in Figure 4. Tnblc I1 lists a fe\v of thcsc tqiical corroxiiiii espci,icnccLs. Corrosion rates in t h e bystem bern.ec,n the rcactor and hydrogen chloride stripper (Figure 2) are primarily a matter of the degree of turbulence, and secondly, the temperature at any given point. The most severe troubles are locatedin the line Icaving the
TABLE 11. CORROSIOS AT COXPLEX-HYDROC~RROS IXTERFACES C.IT.\LYST L-IYER I 9 C A R B O X STEEL SOZZLES TYITHIS RE-ICTOR Penetration Rate,
In. / Y r .
Location of Corro-ion
(7900 Hr.)
Botroiii of reactor catalyst overflow
11.4
I ' r o c e s Plant
L o i n a t e pilot
plant Isoillare pilot plant P e n t a n e pilot plant P e n t a n e pilot plant Coiiiinercial iqnniate u n i t , R-hiting Cominercinl biitane u n i t , V-hi t ini. Coinmercial butane u n i t , Wood River
line Bottoin of n a p h t h a inlet line t o re-
5.3
actor
Fide wall of reactor c n r a l x a t sample nipple Side rail of reactor carall-bt sample nipple Horizontal corroqion on niid-reactor nozzle flange iace
16.0
Comules charge nuzzle flange face
15 (approx.)
Ring gasket on l o v e r reactor man-
13 (approx.)
9.9 13.2
hole
T.\RLEI. EFFECTOF
H E I G H T IX PILOTPL.\ST REACTOR ON STEEL C O R R O S I O S PESETR.4TIOS
In. from B o t t o m of Reactor11
Penetration on Carbon Steel, In. /Yr. (7900 X r . )
18 3.9 2.3 50 1.8 84 1.: 102 1.4 I n all runs a catalyht height of 156 inche,s m a s employed. 23
a
Similarly, higher corrosion rates were found at, the center of the 1.5-inch diameter pilot plant reactor than at the reactor n-all. I n one test run, therniowlls at the renter of the reactor showed from 1.5 t o 4 times the corrosion esperienced by the steel wactor wall a t the same level. During the run from which the
Figure
3. Corrosion in Pilnt Reactor Feed Inlet Line
Plant
Corrosion resulting from agitated pool of complex i n bottom of line. The 5.3 in./yr. penetration rate was determined during 455 hoursof operation.
June 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
1135
F-iCTORS AFFECTISG CORROSION RhTES
Figure 4. Vhiting Saphtha Isomcrizstion Reactor Carbon Steel Faced Flange on Horizontal Nozzle within Catalyst Level Depth of corrosion was 0.25 inch in one week, representing a 13.2 in./yr. penetration rate.
reactor, the entrance t o the product cooler, and the ports and srating surfaces of the reactor hack pressure control valve n-here turbulence is extremely high. The catalyst apparently is carried R.< very fine suspended droplets in the h>-drocaibonstream, t o the cstent of 0.01 to O.lyc,and the impingement of these particles upon a steel surface leads t o high corrosion rates on the metal. Thc. lower cross section of lines and the outside of bends s h o n much greater corrosion than the remainder of the pipe section, :is shon-n in Figures 5 and 6. Agreement between the pilot plant and comniercial unit, corrwion d a t a n-as fairly good within the reactor complex level. H o n e r e r . more uniform corrosion rates x e r e found throughout the large scale reactor than in the pilot plant vessel. The same magnitude of penetration \Tas found a t turbulent complex-hydrocarbon interfaces in the pilot reactor and in the commercial reactor nozzles, a s sliovn in Table 11. On the other hand, much lower flow rates werc.eniployed in the pilot, unit reactor product system a n d correspondingly lower corrosion rates were observed. Batch isomerization experiments were carried o u t in a 13-1iter carbon s t w l stirring bomb reactor. The conditions in the stirring bomb were not, on the averapc, the s0me as those maintained in the continuous process ton-er reactor and the corrosion observed in the stii,ring bomb was less than half of t h a t in the pilot plsnt. Corrosion tests were atteniptcd in a steel shaking bomb: using activc catalyst repressured with several hundred pounds of hydrogrn chloridt:. However, less than 0.1 inch per year penetr:ition d. Thus, it n-as entirely satisfactory from the corrosion standpoint t o carry o u t the batch scale laboratory experiments in carbon steel.
to those found in the large scale plant.
The corrosion of carbon steel by aluminum chloride-hydrorarbon complex is affected to a very considerable degrce by the general operating conditions and specific local conditions. By i:tr the most important single factor affecting corrosion is the turbulence of the comples n-ith respect to the metal surface. €Iiphturbulence has resulted in corrosion rates up to 16 inchesper year, whereas quiet pools of complex, such as found in the complex settler of the reactor effluent system, have corroded < t w l no more than 0.1 to 0.2 inch per year. Tests have been conducted t o shon- the effect of turbulence nithin the commercial reactors. A carbon steel test strip was placed inside a Hastelloy box of 2 X 12 inch cross section, 18 inches below the open top, The box \vas bolted t o the react'or distributor plate. After c.xposure t o this relatively quiet complex inside the box for 5 days, measurements on the strip indicated a penetration of 1 inch ~ r. a r . A similar strip outside the box n-as com-iew - placed _ pletely disintegrated; thip indicated a minimum penetration of 4.1 inches per year. One of the early solutions proposed to the reactor corrosion problem was the use of a metallic reactor liner such as Hastelloy. However, it is very difficult t o keep a metallic liner absolutely tight, as it will tend t o leak along welds and cracks. Therefore, it was necessary to determine the corrosion rate behind a n imperfect liner, and investigate the effect of turbulence behind the liner. A steel plate was protected by Hastelloy plates in n-hich were drilled two sets of three holes each. One set consisted of three 0.04-inch diameter holes in the Hastelloy, spaced 3 inches apart along a horizontal line, and t h e other consisted of three similar holes spaced 1.25 inches apart, vertically. \Then this plate had hung in the reactor for 2340 hours, the complex w n t through the holes and attacked the steel; small individual pits behind the horizontally arranged holes indicated 0.42 inch per year penetration of the steel. However, behind the vertically placed holes considerable steel area was corroded away between the three holes by the action of turbulent complex behind the Hastelloy liner, and the indicated penetration r a t e was 0.54 inch per year. Thus adjacent, or large cracks in a liner, may permit serious corrosion nhere the liner does not form a continuous bond with the steel to be protected. Second in importance to the state of turbulence of the complex is the concentration of anhydrous hydrogen chloride in the catalyst. Both complex and hydrogen chloride must be present to cause appreciable corrosion, and all cases of severe penetration have occurred nhere the complex n-as in equilibrium with a hydrocarbon stream bearing several per cent of hydrogen chloride under several hundred pounds pressure. Complex in equilibrium n-ith anhydrous hydrogen chloride at atmospheric pressure, or a hydrocarbon free of hydrogen chloride, is essentially noncorrosive, and carbon steel storage drums may be used satisfactorily under these conditions. -1drum used in the laboratory for pre-
Figure 5. Corrosion in Horizontal Bend of Carbon Steel Reactor Product Effluent Line after 4144 Hours of Operation Rlaximum penetration rate was 0.8% n./yr.
Figure 6.
Cross Section of Pipe Shown in Figure 5
Greater corrosion ia evident towards the hottom of t h e pipe and the outside of the curve.
INDUSTRIAL AND ENGINEERING CHEMISTRY
1136
34 33 30
Ilastelloy B Haatello, .i h-ickel Inconel Xichrome 11onel Worthite Toncan iron Wrought i r o n Stainless steela Carbon steel CopiJer-nickel alloh Red brnss Yellow brass 1Ierchrome A Steltite 50, 6 Tantalum Ihrigized steel Chemical lead Tin Zinc Silver Aluminum Antirnonial lead
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0.87 0.42
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\Yitliin the operating range of IS0 t u 280 F., temperature has some effPct upon complex corrosion i n thc rewtor, but it plays a s u b o d i n a t e role to the above-mrntioned factors, and variations iii the other factors have overshadowed most esperimental data that might show this effect of tenipemture. C'orrosion is soniewhat greater a t the higher temperature, but n-ithin this entire i'angc steel corrosion b y coniples is severe. On the other hand, the temperature range of 100" to 150" F. appears to be r:tther critical, particularly in the lines and vrssels between the r('nctiir outlet and the entrance to the hydrogrn chloride s t r i p p w Thc pilot plant reactor product cooler consisted of a vc~rtical4.3-fooidouble pipc cooler. Corrosion rates of 0.2 t o 0.26 inch pcr yenr n-ere Eouiid a t the top entrance; only 0.02 to 0.04 inch per year were o l i w r v d a t the 80" F. loner end. I n a commercii1 cooler the horizontal tubes at the inlet, operating a t 240" F., shon-ed around 0.6 inch per year penetration. This tapered off gradually to 0.2 inch per year in thc outlet tubes a t 100" E'. I n these horizontal tubes, roughly 857; of the corrosion occurred in the lower r1'oss section of the tube. Commercial d a t a on pumps in the rcictor overhead system have shon-n a v r r y marked increase in corimion on raising the temperature from 70-80 F. to 1401.50' F, Therefore, although the tcmperature effect on cori~osiori railnot be clearly defined, it may- be concluded that belon- 100" F. the corrosion rate is ver>- considerably reduced; at any tcmpcrature much above 100" F., coinplex attack upon steel m w t be considered severe. II-eld stresses in carbon steel have shown it variable tciitic e corrosion locally around the n-elti. T h e quantita not bern cstahlishcd, b u t strws-relieved pipina and thin the reactor stripper syqt(,in itre prcfcrrc~cl,and in gerieritl have giver1 h > t t c rcoinnic~rcinl COltI in the one reactor compared with that in the other receiving thc. fresh aluminum chloride. S o d a t a from the second reactor :ti'(: tabulated in this report; all t h e pilot plant test strips r e f w e d to were exposed in the reactor receiving the fresh aluminum chloride fortification. The rate of addition of fresh aluminuni chloride to the reactor catalJ-st also affects its corrosion activitj-. With a n aluminum chloride rate of 0.1 t o 0.2 pound of aluminum chloride per barrel (42 gallons) of reactor charge, corrosion penetration in the pilot plant reactor ranged from 0.1 to 1.0 inch per year; additions up to 1.0 pound per barrel gave the previously mentioned penetration of 1 to 5 inches per year. Limited d a t a oii commercial plant experience evidenced a smaller variation of corrosion with change in catalyst fortification rate, showing the higher range of corrosion rates throughout the reactor with anywhere from 0.1 to 1.0 pound per barrel aluminum chloride fortification rate. When t h e pilot plant w i s operated without any fresh aluminum chloride additions, the average corrosion rate over a given run was Iess than indicated above, and the longer runs shoxed lower average penetration rates on test strips as shown in Table 111. Extrapolation of the data back t o the beginning of the run s h o w an initial penetration rate of approximately 2 inches per year, which is comparable Kith other corrosion results determined with continuous catalyst fortification.
( - 1 1 t i , t ! i i q p ) i i i t . thi. tli-ruwion l:ns crntcLrd al)out tlie conip k x c.nt:il\..t nilacii iipoii ca1~1)onrtcol. T I I o&r to find the t~ot~rosioi~ 1)c.iic.tratiori o n otlic.1. nwtals lvliicli might wi'vc as possitjlc~,siihstitutos foi, c:il~iion s t w l , a si&s of v:iiiouO test .-triiis n.as esposcd to comples i n the pilot plant i,eactor. T h s e strips wvrv 1,8x 1 X 6 inch?., had initially bright surfacrs, arid w i ~ ciposetl t to thc rc,ac.tion conditions a t 215' t o 250" I'. for :iri~~n-Iierc f r o m 100 to 1000 hours. Talilc IT- inc*ludes all the m
