Production of Xylidines by High Pressure Hydrogenation ALEXIS VOORHIES, JR., W. M. SMITH, AND R. B. MASON Esso Laboratories, Baton Rouge, La.
T h i s paper describes the adaptation of a semicommercial scale high pressure hydrogenation pilot plant to the production of approximately 8000 pounds per day of xylidines by the hydrogenation of mixed isomeric nitroxylenes. Included are a description of the process and equipment involved, personnel requirements, types of data taken, and results obtained. Optimum operating data previously obtained i n smaller laboratory soale equipment were checked and found to be essentially satisfactory for larger scale operation. This conclusion was later confirmed by actual commercial operation of the process on a scale approximately twenty times larger than the semicommercial plant scale.
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OhlPA4NIOSpapers (1, 3 ) outline the factors on which the production of xylidines was based, general development of the Esso Laboratories process for this purpose, and the properties and characteristics of the finished xylidines produced. Work on the nitroxylene hydrogenation process development ( 1) was initiated in batch autoclaves of , 1-liter capacity, proceeded through the usual laboratory and semicommercial pilot plant development in continuous units of three different sizes, small, medium, and large (production rates equivalent t o approximately 3, 30, and 8000 pounds of xylidines per day), and finally culminated in less than 10 months after the first autoclave runs in the commercial hydrogenation of nitroxylenes t o produce the equivalent of 150,000 pounds per day of recoverable specification xylidines. Continuous pilot unit equipment of small and medium size, with production rates equivalent t o approximately 3 and 30 pounds of xylidines per day has been described ( 2 ) . This type of equipment is suitable for the laboratory development of a process with respect to catalyst and feed stock testing and the effect of process variables on feed stock conversion, product yields, hydrogen consumption, etc. Because of its relatively small size, however, equipment of this type is usually overdesigned with respect to piping and vessel sizes as compared with commercial scale equipment. In small scale equipment, mass velocities are lower; pressure drop in the system is not as much of a problem. Heat losses, on the other hand, are much greater and, for highly exothermic or endothermic reactions, more nearly isothermal operation may be obtained. Adiabatic conditions, which prevail in commercial high pressure hydrogenation reactors of conventional design, are difficult t o approximate in small scale equipment. I n cases where heat of reaction is of significant influence on product yield or even on operating conditions necessary t o satisfactory operation of a given process, it is difficult t o predict the results to be obtained in commercial scale operation based on laboratory scale operation alone.
I n order to demonstrate that the laboratory process as developed by the Esso Laboratories was commercially feasible for the high pressure hydrogenation of nitroxylenes to xylidines, the large (semicommercial) pilot plant with a production rate equivalent t o approximately 8000 pounds of xylidines per day was put in operation. This unit was of sufficient size t o allow very close approximation of operating characteristics of commercial scale equipment, and thereby helped t o bridge the gaps existing between small scale laboratory equipment and commercial equipment. Subsequent to a satisfactory demonstration run for the ,Army Air Force, the unit was further continued in service in order t o supply Army Ordnance with finished xylidines in sufficient quantity t o enable the Petroleum Administration for War t o carry forward its experimental blending program involving the use of xylidine additive in aviation fuels. This paper describes in detail the semicommercial pilot plant employed for this purpose, the method of i t s operation, and the results obtained. OUTLINE OF PROCESS
The fundamental reaction involved in the process was the catalytic reduction of the nitro groups in mixed isomeric nitroxylenes (essentially monoderivative) t o amine groups, with the evolution of considerable quantities of heat of reaction (1). .4s the existing commercial hydrogenation equipment available t o the Esso Standard Oil Company a t its refinery a t Baton Rouge, La., was designed for high pressure operation (3000 pounds per square inch gage), laboratory work had been directed toward the development of hydrogenation conditions suitable for use a t high pressne, although the reaction could also be readily carried out at low hydrogen pressures. Owing t o the relatively high heat of reaction and the necessity for operating within a' narrow temperature range, liquid water injection with the nitroxylenes was used for temperature control, and a catalyst resistant t o t h e disintegrating action of hot liquid water was developed for use under these conditions.
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o P RCALIBRATED O u T AF EOE D AND F E E D AND CRUDE PRODUCT S T O R A G E FEED PUMPS
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Figure 1.
Layout of Serniconimercial High Pressure €1) drogeii:ition I nit
The flow plans of the process ( 1 ) may be brieflv outlined as follolvs: Nitroxylenes of satisfactory quality n ere contacted with gases containing hydrogen sulfide under pressures of 80 pounds per square inch gage in a n absorption system. This served as a means of introducing into the high pressure system (3000 pounds per square inch gaxe) the hydrogen sulfide necessary for maintaining catalyst activity. The nitroxylenes containing hydrogen sulfide in solution were then injected by means of a high pressure feed pump into the inlet of the reactor. Here, coining in direct contact with a preheated mixture of water and hydrogen under high pressure, the nitroxylenes were rapidly preheated. They were then passed down over the catalyst, hydrogenated, and removed from the bottom of the reactor. Exothermic heat of reaction was removed by vaporization of part of the LTater, by s3me increase in sensible heat content of the reactants, and finally, by injection of cold hydrogen directly into the reactor a t various levels in the catalyst bed. The total reactor effluent, after passing through a heat exchanger and water cooler, was discharged into a high pressure liquid-gas separator from which the excess hydrogen (over that required stoichiometrically) was removed and recycled to the reactor by means of a high pressure gas circulator. Hydrogen consumed in the rocess was replaced by addition of fresh hydrogen from a n outsiie source maintained a t a constant high pressure in order to keep the system pressure at the desired value. After removal of the total liquid product from the high pressure separator accompanied by release of pressure, s31ubility gas was removed in a low pressure gas-liquid separator and sent t o fuel.
The t u o liquid phases (crude xylidene arid rvaater) were subsequently separated in a settler. From the settler, the upper layer of crude xylidine was sent t o storage and the bottom waterlayer recycled to the unit. Provision was made for purge of a portion of the recycle water if desired in order to prevent any buildup of solids (soluble or suspended) ; water make-up required because of losses and purge was su plied from steam condensate. Provision was also made for recycye of a portion of the crude xylidines with the recycle water if desired, inasmuch as therc were indications that xylidine recycle Rith the water resulted in somewhat better maintenance of catalyst activity than recycle of ivater alone. Recycle water and xylidines, if any, were mixed under high pressure with excess hydrogen, preheated in the exchanger by heat exchange with the total reactor hot effluent, and finally brought to the desired temperature in a fired coil before being mixed with nitroxylene fresh feed a t the reactor inlet. Because of the presence of small amounts of inert materials in the hydrogen supply, s3me purge of recycle gas was necessary in order t o prevent buildup of these materials in the system with consequent decrease in hydrogen partial pressure. Ordinarily, as a result of their solubility a t high pressures in liquid hydrocarbon products or related materials such as xylidene, inerts such as nitrogen, and methane are removed continuously from the system without appreciable purge of recycle gas. I n this way, only moderate buildup in inert concentration is obtained and hydrogen par tial pressure is maintained a t a satisfactory level. However, rcuplacement with water of a considerable portion of the hydrocarbon fraction normally existing in the system, as was done in thci present case, necessitated deliberate purge of recycle gas in order
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to maintain satisfactory hydrogen partial pressure, owing to the lower relative solubility- of the inerts in water under high pressures. Crude xylidines from the hydrogenation step were mixed with approximately 10% by volume of a high boiling cracked gas oil a~ a flux and then topped in order to remove low boiling materials such as unnitrated hydrocarbons present in the nitroxylene feed stock. Bottoms from the topping tower were rerun in another column t o producc spcoification xylidene as an overhead product plus a bottoms fraction containing the added flux oil, diamines from any dinitroxylenes in the fresh feed, etc. DESCRIPTION OF PILOT PLANT
Figure 1 presents a layout plan of’the high pressure hydrogenation unit. This unit had been in existence for several years and operated under a variety of conditions in various adaptations of the high pressure hydrogenation process. Only minor revisions were necessary t o make i t suitable for the hydrogenation of nitroxylene. The following paragraphs list some of the main pieces of equipment in the high pressure plant together with a brief description of some of their characteristics.
FEEDAND CRUDEPRODUCT STORAGE.Nitroxylene feed and crude xylidine product were stored in horizontal steel drums (10 X 30 feet) of approximately 18,000-gallon capacity. CALIBRATED FEEDAND PRODUCT TANKS.Calibrated feed and product tanks of approximately 1500- to 2000-gallon capacity were used for measurement of over-all feed rates and product recovery. HIGHPRESSURE FEED PUMPS.The pumps were a direct steam drive, positive displacement, double action, outside packed, plunger type of Worthington manufacture. Auxiliary steamdriven booster pumps were used t o provide 50 t o 60 pounds per square inch positive suction pressure.
Figure 2.
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GASCIRCULATOR. The gas circulator was a direct steam drive, double action machine manufactured by Ingersoll-Rand. This circulator provided for recycle of excess hydrogen under full operating pressure with a n allowable pressure drop in the system not exceeding a maximum of about 500 pounds per square inch. PREHEATER COIL. The preheater coil consisted of a horizontal bank of 18-8 chrome-nickel alloy steel tubes radiantly heated by combustion of natural gas in a bricked-in furnace. PRODUCT COOLER. The product cooler consisted of several lengths of carbon steel pipe cooled by well water. The outlet sections of the cooler were jacketed (double-pipe sections) for better heat transfer; the inlet sections were simply submerged in the cooler tank, which provided a water reservoir with substantial heat capacity to allow time for plant control in any case of sudden failure of the cooling water supply. A few tubes at the inlet where temperatures were still relatively high (about 10 t o 15% of the total number of tubes) were of 18-8 chrome-nickel alloy steel. REACTORAND HEATEXCHANGER. The reactor and heat exchanger were kept inside an open-top concrete stall in order to localize any possible effects of a fire or explosion. Each vessel (26 feet X 20 inches and 26 feet X 12 inches, internal dimensions, respectively) consisted of a forged carbon steel shell to resist pressure stress and a carbon steel liner. The reactor was provided with internal insulation and a n additional liner t o hold the insulation in place. The insulatibn maintained the temperature of the reactor wall at a minimum level, thereby reducing temperature stresses in the thick shell and minimizing any tendency towards hydrogen attack on the metal under pressure stress. I n this manner, essentially adiabatic operation of the reactor was encountered in service. The catalyst within the reactor was divided into four individual beds separated by three short hollow, cylindrical cooling gas boxes placed between the individual beds. These boxes served t o gather together liquid and vapor reactants emerging from a preceding catalyst bed and subsequently redistribute them evenly over the top of the succeeding catalyst bed. In addition, the boxes provided a free space in the reactor into which
Compressor and Pump House, C o n t r o l House, Gas-Liquid Separators, Product Cooler, and Reactor S t a l l
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Figure 3.
Meactor Stall of H+ogenation
cold recycle gas could be injected and mixed viith thc reactants for control of the exothermic heat of hydrogenation. PIPIXG.All hot lines were constructed of 18-8 chrome-nickel alloy steel in order to minimize hydrogen attack; all others were of carbon steel. COXTROL HOLXE. Controls for the direct operation of the high pressure reaction system Tvere contained in a centrally located control house which was protected by a reinforced concrete n all on the side facing the high pressure system. The controls included t,hose for gas flow rates, preheater coil temperahre, product separator level, reactor emergency drain, and smothering steam to the reactor stall and fired coil.
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Hydrogenation Section
1 assist,ant operator
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Responsible for direct operation of the unit in accordance with specified operating conditions Pumping feed stock and product; catelling gas and liquid samples Controlling preheat and catalyst, tenipci,atures, gas rates, and liquid levels Care of feed pumps and gas circulatoi,; control of feed rates and pressure Product Distillation
1 assistant operator 1 helper
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Figure 2 s h o m the compressor and pump house in l,he left background, the control house, the high and low pressurc gasliquid separators, the product cooler, and a corner of the concrete reactor stall. Figure 3 s h o w the reactor stall in greater detail, the product cooler to the left of the stall, and a portion of the fircd coil in the left background. Figure 4 is a view of the comp.'essor and pump house interior Lvith the recycle gas circulator in the foreground. Figure 5 is a view of the crude xylidines distillation unit, which includes two fractionation towers (bubble cap plates) with attendant pumps and coolers. Not' shown are the rcboiler furnaces and run-do\?-n tanks.
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TABLE I. RESPONSIBILITIES OF OPERATISG PERSOSSEI. 1 head operator
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Pumping feed and product: catching samples Controlling reboiler and tower teiiiperaturea; feed, reboiler, a n d reflux pumps
OPERATION O F PILOT PLANT
Operation of the pilot plant was on a continuous basis, 24 hours a day, 7 days a week, interrupted only by t,emporary shutdowns for catalyst replacement or mechanical changes. Direct operation of the pilot plant was carried out with a total of 7 men per 8-hour shift. The responsibilities of the operating personnel are outlined in Table I. Over-all supervision of the operation was handled by onc fulltime chemical eiiginccr vho v a s directly responsible to the laboratory management, for ca ing out the research program. In this he was assisted by tivo other cheiiiical engineers, one concerned with the hydrogenation sect,ion and the other Ti-ith the distillation section. A fourth chcniical engineer was responsible for data workup and report writing. liissential services m r e also rendered by other personnel responsible for the mechanical
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maintenance of the plant and for the analytical work necessary for the control of the plant operation and product quality. Potential health hazards due t o the toxic nature of nitroxylenes, and especially xylidines, were successfully overcome by education of personnel in this respect and maintenance of conditions designed t o limit physical contact with these materials. Among the precautions taken may be mentioned: adequate ventilation of enclosed areas, provision of drip pans for pumps, etc., to miniGize saturation of floor areas by spillage, and daily provision of clean outer clothing for operating personnel. Rubber gloves, overshoes, etc., were also provided when necessary. COLLECTION O F DATA
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Operating data obtained directly at the unit may be divided into three general groups-pressures, temperatures, and flow rates. Pressures a t various points throughout the flow system were obtained by direct-reading Bourdou type pressure gages. Continuous pressure records of the high pressure fresh hydrogen supply t o the plant and the hydrogenation reactor outlet pressure were obtained with recording pressure gages, and the pressure drop across the reactor was recorded by a differential pressuremeasuring device. Temperatures throughout the system were obtained by using iron-constantan thermocouples in conjunction with a potentiometer type temperature indicator; temperature recorders vere used t o maintain records of the preheater coil outlet temperature and certain key catalyst temperatures. Provision was made in the catalyst temperature recorder to cut off the supply of nitroxylene feed and sound a n alarm if any
Figure 4,.
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with freedom from leakage. Over-all liquid flow rates were measured by the use of calibrated tankage; liquid feed rate control by orifice meter measurement a t relatively low pressure in the suction lines t o the high pressure feed pumps also provided a continuous record of the feed rates to the system. Most of the data pertaining to the direct operation of the unit were recorded every half hour; less critical data (feed and product temperatures, etc.) were recorded less frequently. Sampling and analysis of the gas and liquid feed and product streams entering and leaving the unit were carried out on a daily composite basis. Among these materials may be included nitroxylene fresh feed, fresh water, recycle-purge water, crude xylidine product, fresh hydrogen, recycle-purge gas, low pressure release gas, distilled xylidine product, light ends, and still bottoms. Daily spot or snap samples were taken of the recycle gas for hydrogen and hydrogen sulfide analysis, and of the inlet and exit gases a t the hydrogen sulfide absorber tower for determination of hydrogen sulfide content. Anal>-sx of samples from the top, middle, and bottom of each tank car of nitroxylenes received were also obtained before storage in order to assure satisfactory quality prior t o use. The flus oil used in the crude xylidine product distillation step was analyzed only from batch to batch. For direct control of the pilot unit operation, especially during periods of changing conditions, snap samples were taken as often as necessary. The analyses included : nitroxylene content of crude xylidine product t o ensure substantially complete conversion, and hydrogen and hydrogen sulfide concentrations in the
Compressor and Pump House Interior
catalyst temperatures exceeded a maximum value of 600' F. Gas flow rates were measured under pressure by means of orifice type flowmeters in conjunction with analyses for gas density. At the high operating pressures used, flowmeters operating on an electromagnetic principle were found t o give satisfactory service
recycle gas to ensure conditions for maintaining catalyst activity. Normally these analyses could be obtained in one or two hours. The analysis for nitroxylene in the hydrogeiiated product was based on titration with titanium chloride. Later work resulted in the development of a polarographic method for the determina-
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Figure 5 .
CI-iide Xylidines Distillation Unit
tion of both mono- arid diriitioxyleries n hicii \$as especially suitable for the high concentrations (of the monoderivative) normally met with in the analysis of the nitroxylene fresh feed. The titanium chloride titration method, however, was still used for the lower levels of nitroxylene concentration normally found in the hydrogenated product. RESIJL'TS
Operating conditions indicated by smaller scale pilot unit data t o be optimum were essentially duplicated in the semicommercial unit (Table IT-, 1 ) and were found, with a few exceptions, t o he entirely operable on the larger scale, The necessity for maintaining miniinum concentrations of hydrogen sulfide in the recycle gas was confirmed in the semicommercial unit. Below about 0.5 volume hydrogen sulfide in the recycle gas, very short catalyst life x a s found; and runs were prematurely terminatcxd bl excessive reactor pressure drop as a iesult of the deposition of carbonaceous mateiial on thct catalyst. I n such operations, crude xylidine products containing appreciable contents of riitrom lene u eie obtained, extremely black in color and very difficult to separate from thts recycle water. By maintenance of higher concentrations of hydrogen sulfide in the rervclc gas, very light colored, completel>converted products were obtained; and no difficulty was experienced in their subsequent seprxtion in good jield from the re-
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cycle liquid water diluent. At the relatively low reactor temperatures employed (350 O to 450" F.) organic sulfur compounds (especially carbon disulfide) added to maintain these high hydrogen sulfide concentrations appeared t o be incompletely hydrogenated t o hvdrogen sulfide and, in addition, appeared t o depress the catalyst activity. Bddition of hydrogen sulfide as such by absorption under pressure from a refinery gas stream seemed t o obviate this difficulty. Both water and nitroxylene, in the ratios utilized in the process, appeared t o have about the same capacity for hydrogen sulfide, and xylidine product was someLYhat better. How~ever, the use of nitroxylene as an absorbent appeared preferable for this purpose because both the recycle water and recycled xylidine already had a n appreciable content of hydrogen sulfide equivalent t o t h a t existing under the equilibrium conditions in the low pressure separator of the system. Numerous gasket materials tested in the xylidine product distillation section indicated that metal-clad asbestos gaskets were superior for allaround purposes, but lead gaskets were also satisfactory for look-boxes. Other gasket materials were softened and disintegrated by t h r vylidine product, and this resulted in very short service life. Other results of the semicommercial unit operation (corrosion experienced nTith mixtures of nitroxylenes and xylidines in contact with iron and elimination of heat-unstable sulfur compounds in the producl distillation section) have been described ( 1 ) . Engineering studies conducted during the operation of the semicommercial unit were primarily concerned a ith the evaluation of the magnitude of the heat of hydrogenation and thr extent to which this heat was absorbed by the various material streams in the reactor. Calculations based on heat balances indicated that the heat of hydrogenation of nitroxylene was about 200,000 t o 210,000 B.t.u. per pound mole a t 400" F. This result v-as in good agreement with the theoretical value of 212,000 B.t.u. per pound mole a t 400' F. computed for the hydrogenation of a closely related compound. These studies also indicated that approximately 5OC4 of the heat of hydrogenation was absorbed in vaporizing a ater for operation at 4 t o 1 ratio of reactor inlet water to nitroxylenes and 450" F. reactor outlet temperature. The vaporized water constituted 30 t o 40% of the water introduced a t the reactor inlet. CONC LU SIOV s
Fiom the operation of the laboratories' scmiconimcrcial pilot plant, it was concluded that the hydrogenation of nitroxglencs to xylidines was feasible in the available commercial high pressure hydrogenation equipment *under certain established optimum operating conditions. This conclusion was later confirmed b\ successful operation of the commercial cquipment a t conditions similar t o those found t o be optimum in the pilot unit equipment. LITERATURE
crrm
(1) Brown, C . L., Smith, W. M . , and Scharinann, W .G . , IND.1,;s~. CHEM.,40, 1538 (1948). (2) Brown, C. L.,Voorhies, A , Jr., and Smith. W .M., I b i d . , 38, 1 % 40 (1946). ( 3 ) Kunc, J. F., Jr., Howell, W.C., Jr., arid Starr, C . E., .Jr., Ihid.. 40,
1530 (1948). RECEIVED April 5 , 1948.
