Hydrogenation unit control system

March 28, 1961 J. τ. cabbage 2,977,288

HYDROGENATION UNIT CONTROL SYSTEM

<img file="US2977288A_D0001.tif" />

A TTORNEYS

United States Patent Office

2,977,288

Patented Mar. 28, 1961

2,977,288

HYDROGENATION UNIT CONTROL SYSTEM

John T. Cabbage, Bartlesville, Okla., assignor to Phillips Petroleum Company, a corporation of Delaware

Filed Sept. 4,1958, Ser. No. 759,009

Claims. (Cl. 196—132)

This invention relates to hydrogenation of unsaturated organic oils. In one of its aspects, this invention relates to a hydrogenation control system.

It is well known in the art to hydrogenate unsaturated organic oils such as vegetable oils and fats, hydrocarbons such as aromatics and olefins, and the Eke. The hydrogenation of such compounds is frequently carried out in a liquid or fluid diluent and in the presence of a catalyst. Such hydrogenations are exothermic and sometimes, due to system upsets, the temperature in the hydrogenation units gets out of control necessitating shut I down. It is desirable that the hydrogenation unit op| erate above a predetermined minimum in order that | the reaction will proceed. On the other hand, it is de[ sirable to operate below a predetermined maximum to prevent unwanted side reactions. For example, in the conventional hydrogenation of benzene to cyclohexane in.the presence of n-hexane diluent and a nickel-supported catalyst, it is necessary to maintain the feed above about 380° F. to maintain the hydrocarbon in vapor phase since liquid is detrimental to the conventional catalyst and a reactor outlet temperature of not more than about 500° F. to prevent a demethylationhydrogenation reaction from taking place which would result in run-away temperatures.

It is an object of this invention to provide a novel control system for a hydrogenation process.

It is another object of this invention to provide a system for control of temperature in a hydrogenation process.

Still other objects, features and advantages of this invention will be obvious to those skilled in the art having been given this disclosure.

As has been indicated, this invention is applicable broadly to hydrogenation of unsaturated organic compounds such as vegetable oils, fatty oils and hydrocarbons. The invention is particularly applicable to hydrogenation of hydrocarbons boiling in the gasoline boiling range. In general, such hydrocarbons will be olefins of 4 to 30 carbon atoms per molecule and mononuclear [ aromatics containing up to 36 carbon atoms with up to r, 6 carbon atoms in any nuclear substituent. This latter । class of compounds can be represented by the formula: 1 X

Rif ηΐΐ rI! Jr

R wherein R is hydrogen, alkyl or alkenyl and wherein total carbon atoms in any one R does not exceed 6 and wherein the total carbon atoms in such substituents does not exceed 30.

While such unsaturated hydrocarbons are those usually employed, it will be recognized by those skilled in the art that the method and apparatus of this invention will be applicable to the hydrogenation of unsaturated hy2 drocarbons in general. Examples of such hydrocarbons include olefins, such as butene, isobutylene, butadiene, pentene-1, pentene-2, 2-methylpentene-l, 2,5-octadiene, 4-ethyloctene-l, nonene-3, 1,4,9-octadecatriene, tricon.5 tene-1, and the like; and aromatics such as benzene, paraxylene, metaxylene, orthoxylene, toluene, 1,3,5-trimethylbenzene, 1,2,3,4,5,6-hexylpentyl benzene, 2-hexyl benzene, l-methyl-2-ethyl benzene, styrene, and the like. Other materials which can be hydrogenated 10 using the control system of this invention include polynuclear aromatics, such as biphenyls, naphthalenes, and the like, heterocyclic compounds such as pyridines and quinolines, unsaturated acids such as maleic, fumaric, itaconic, methylethylmaleic, glutaconic, 15 alpha-methylglutaconic, alpha,beta,gamma-tributylglutaconic, 2-pentadeconic acids, and the like.

Any catalyst suitable for hydrogenation is applicable in this invention. Examples of such catalyst include nickel, platinum, tungsten, and molybdenum. These ma20 terials are generally finely divided and are on a porous support such as precipitated alumina, alumina-silica coprecipitate or kieselguhr.

The art is aware of the hydrogenation of unsaturated compounds in the presence of a catalyst and no 25 extensive discussion of these is needed here, this not being my invention. Such hydrogenations are frequently carried out in the presence of a fluid diluent and the diluent recycled in order to maintain the unsaturated compound at the desired concentration to maintain the 30 reactor temperature within the optimum operating limits.

As has been indicated, the hydrogenation reaction is exothermic and each mol of material being hydrogenated will have an effect on reactor temperature. It is believed this can best be explained by referring to a specific ex35 ample.

When benzene is hydrogenated to cyclohexane in the presence of a nickel on silica-alumina catalyst in the presence of diluent, each percent benzene in the feed causes a temperature rise of about 20° F. across the ⁴⁰ reactor. As has been indicated, the feed should be above about 380° F., preferably at about 400° F. for good hydrogenation and it is necessary to maintain the outlet temperature no higher than about 500° F. Therefore, the percent benzene in the feed should be maintained at about 5 percent. The percent benzene in the feed to the reactor can be controlled by regulating the amount of recycle, regulating the rate of fresh feed, of both.

According to this invention, the amount of recycle ^υ to a hydrogenation zone is controlled responsive to reactor effluent temperature up to full recycle after which the rate of fresh feed is controlled responsive to further increases in reactor effluent temperature. According to ₅₅ the apparatus aspect of this invention, means are provided for continuously detecting the temperature of the reactor effluent, means for regulating the flow of recycle liquid to the reactor responsive to changes in the detected temperature through a predetermined temperature ₆₀ range and means for regulating the flow of fresh feed to the reactor responsive to changes in the detected temperature above said predetermined range.

This invention will be further described with reference to the attached drawing which is a schematic flow dia55 gram of a typical hydrogenation process utilizing the control of this invention.

Referring now to the drawing, the unsaturated organic compound, feed, passes via conduit 1, orifice 2, pump 3, valve 4, conduit la, orifice 5, conduit lb, and heat exchanger 6 to heater 7. Flow recorder 38 is operably connected to orifice 2 and registers the flow .of fresh feed to the system. Flow recorder controller 39, with

2,977,288 a preset manual loading station, is operably connected to orifice 5 and valve 4 so as to regulate the flow of material at a rate determined as hereinafter explained. The reactor feed is heated in exchanger 6 by heat exchange with the reactor effluent and is varied to the desired reactor feed temperature in heater 7. It is within the skill of the art to use automatic controls in heater 7 if desired. Such control can regulate the heat source, e.g., fuel, to the heater responsive to temperature changes in the heater effluent in conduit 8. The heater effluent passes via conduit 8 to reactor 9 which is packed with a suitable catalyst and wherein the reaction takes place. It is within the scope of this invention and within the skill of the art to use a moving bed catalyst if desired. The effluent from reactor 9 passes via conduit 10 to heat exchanger 6 where it gives up some heat to the feed stream. The somewhat cooled effluent then passes via conduits 11 and 12 to separator 13 where gaseous hydrogen and liquid hydrocarbon are separated. The hydrogen, along with some low boiling hydrocarbon, passes overhead via conduit 14 to condenser 15 where the hydrocarbons are condensed and passed back to separator 13 via conduit 12. The hydrogen passes via conduit 16, compressor 17 and conduit 18 to conduit lb where it is admixed with the fresh feed and passed via heat exchanger 6, heater 7 and conduit 8 to the reactor 9. Since hydrogen is used in reactor 9, makeup hydrogen must be added. This make-up hydrogen is added to the recycle hydrogen in conduit 16 via conduit 19 and valve 20. This valve 20 can be operated responsive to changes in the system pressure such as by means of pressure-controller 21 operably connected to separator 13 to continuously detect the pressure therein and to valve 20 to regulate the flow of gas therethrough responsive to changes in separator 13 pressure. The liquid product from separator 13 is removed via conduit 22 and a portion is passed as recycle via conduit 23, pump 24, orifice 27, valve 25 and conduit 26 to conduit la where it is admixed with the fresh feed. The rate of recycle of product is controlled responsive to changes in temperature of the reactor 9 effluent. A temperature recorder-controller 28 is operably connected with conduit 10 to continuously determine the temperature of the flowing stream therein. This temperature recorder-controller is connected to flow recorder-controller 29 which in turn is operably connected to valve 25 to open valve 25 as the temperature increases and to close valve 25 as the temperature decreases. If the temperature continues to rise after valve 25 is fully opened, then the flow recorder-controller 29 is connected to flow recorder 39 to reset the same so as to reduce the flow of fresh feed through valve 4, thus reducing the total feed to the reactor to a lower value, below the original amount demanded by the original set point on flow controller 39, to thereby maintain the desired volume percent benzene in the total feed to the reactor. When the amount of benzene in the fresh feed starts to decrease, the temperature sensed by temperature recorder controller 28 starts to decrease, and temperature recorder controller 28 by way of the flow controller 29 actuates the flow controller 39 with its preset manual loading station, returning temperature recorder controller 39 toward its original set point. Ultimately, temperature recorder controller 39 reaches this set point, and remains at this original set point when the amount of benzene charged is at a sufficient low level. With a further decrease in benzene in the fresh feed, the temperature recorder controller 28 actuates a pinching down on recycle valve 25, and the temperature recorder controller 39 at the same time effects a further opening of valve 4, to maintain the desired total flow to the reactor at this original set point quantity. Product not demanded by recycle conduit 23 is passed via conduit 30 to product or further treatment as desired.

In the above description, valves, pumps and the like, except as needed to describe the system, have been omitted and can be supplied by those skilled in the art. Various modifications can be made as desired. For example, a condenser could be used in conduit 11, the hydrogen make-up, if under pressure, can be added to the recycle line downstream of compressor 17, the temperature recorder controller 28 can be of the split phase type and control flow recorders 29 and 39 separately rather than using flow recorder 29 for resetting flow recorder 39. Other modifications will be obvious to those skilled in the art.

I will further describe this invention by describing an embodiment wherein benzene is being hydrogenated to cyclohexane.

The benzene-containing fresh feed in conduit 1 has originally the following composition:

Volume percent

Benzene 6.8

Cyclohexane 0.3

Methylcyclopentane 29.0

Normal hexane 43.0

3-methylpentane 15.0

2-methylpentane 5.7

2,3-dimethylbutane 0.2

100.0

In hydrogenation of a benzene-containing stream, in this exothermic reaction, the temperature rise across the reactor 39 is about 20° F. per each percent of benzene in the total feed. The total feed to the reactor must be above 380° F., preferably about 400° F. to minimize liquid in the feed to the reactor which liquid feed is detrimental to the catalyst as is known in this field of operation. Also, the reactor outlet temperature, conduit 10, must not be above about 500° F., due to demethylation, preferably not above 480° F., in order to produce maximum quantity and quality of product.

Since the feed to the reactor in this 120° F. maximum difference between the inlet and outlet of the reactor must contain less than about 6 volume percent benzene, and practically about 4 volume percent benzene, it is necessary to dilute fresh feed stocks containing more than about 6 volume percent benzene with a hydrocarbon diluent free of benzene; e.g., a portion of the reactor effluent. Also, it is necessary to have a mol ratio of hydrogen to benzene of about 4:1 up to 12:1, preferably about 9:1 minimum.

In a specific operation using a conventional nickelkieselguhr supported catalyst, the feed to reactor 9 containing 6.8 volume percent benzene is heated to 402° F. in heater 7 and the reaction is at a pressure between 400-500 p.s.i.a., e.g., 450 p.s.i.g. in the example. The mol ratio of hydrogen to benzene is 9:1. The reactor effluent exits via conduit 10 at 480° F. and the fresh feed plus benzene-free diluent has a benzene content of 4.0 volume percent charged at the rate of fresh feed of 58.8 barrels per hour via conduit 1, and 41.2 barrels per hour of diluent via conduit 26, giving a total of 100 barrels per hour feed to reactor 9.

The temperature-controller-recorder 28 on the reactor effluent operates a split range flow-controller-recorder 29 on the diluent line. The air signal effected by the temperature recorder controller transmits a signal ranging from 0 to 15 p.s.i.g. In the lower range of 0 to 7 p.s.ig., air pressure only to valve 25 in the diluent line is controlled ranging from closed at 0 p.s.ig. to fully open at 7 p.s.ig. and above. Between 7 to 15 p.s.i.g. air pressure, the second portion of this conventional split range flow controller 29 takes over (the valve in the recycle line being now wide open). Since the recycle pump 24 and line 26 have a physical limit as to size, control of the reaction cannot be made using diluent only. A second control of fresh feed has to be used.

2,977,288 &lt;

*

This second range of the flow recorder controller acts to reset the flow recorder controller 39 on the fresh or benzene-containing feed.

While operating at the above conditions, there is a change in the benzene content of the fresh feed. This 5 amount of benzene, e.g., increases to 9.2 volume percent from the original 6.8 volume percent. Since the system was operating at 58.8/41.2 ratio of fresh feed to recycle, the effective volume percent benzene would be 5.42 volume percent and the effluent temperature would 10 try to rise above the 480° F. optimum, to about 510° F. As the temperature starts to rise, the air pressure signal from the TRC 28 increases to above 7 p.s.i.g. which first effects a full opening of the valve 25 in the recycle line. This line for economic purposes is limited in size 15 and in pump capacity (in this example of 100 barrels/hour total feed) to 50 barrels per hour maximum diluent flow. As the diluent increases from 41.2 to 50.0 (maximum) barrels per hour, the FRC on the feed at the same time effects a cutting back of fresh feed 20 from 58.8 to 50.0 barrels per hour. This resulting benzene volume percent in the reactor total feed would still be too high at 4.6 if the second part or 7-15 p.s.i.g. range of the FRC did not take over, and the temperature would try to go to about 495° F. 25

As the temperature is still attempting to rise, because of the 4.6 volume percent benzene feed to the reactor, the second or split range of the FRC (the recycle valve still completely opened) resets the FRC 39 on the feed to a lower value, allowing the total feed to be 88.5 bar- ³⁰rels per hour comprising fresh feed of 38.5 b./h. and recycle of 50 b./h. The hydrogen to benzene mol ratio becomes 10:1.

When the amount of benzene in the fresh feed starts to decrease, e.g., to 6.4, the 7-15 p.s.ig. range on FRC ³⁵29 (valve 25 remaining wide open) actuates the return of the set point from the above 88.5 b./h. to the original 100 b./h. set point on the FRC 39 (with its preset 100 b./h. manual loading station). This operation, at first, effects opening of valve 4 to flow 50 b./h. to main- ⁴⁰tain the original preset 100 b./h. total feed. However, the benzene content of the total feed is only 3.2, which quantity attempts to effect too low a temperature signal on TRC 29. The 0-7 p.s.ig. range then takes over on FRC 29 and causes valve 25 to pinch down to recycle ⁴⁵37.5 b./h., and FRC 9 effects an opening of valve 4 to flow 62.5 b./h. fresh feed to result in 100 b./h. total feed containing the desired 4.0 volume percent benzene. The hydrogen to benzene mol ratio is returned to the 9:1. __

This system uses optimum minimum recycle, while main- ⁵⁰taining optimum reactor effluent temperature and the fresh charge at the maximum allowed by the FRC preset as the maximum the equipment will handle whichever is smaller.