US2192214A - Cracking process and apparatus - Google Patents

Description

March 5, 1940. w 2,192,214

CRACKING PROCESS AND APPARATUS Filed Aug. 11, 1956 H07 6/15 on.

ZONE 4 TIA/61'- COLUMN 32 A 26 g w 6004 //V6 COIL INVENTOR HORACE M. I'VE/R ATTORN EY5 Patented Mar. 5, 1940 UNITED STATES PATENT OFFICE v 5 Claims.

It is generally recognized in the oil cracking art that the formation of coke is one of the principal difficulties that must be guarded against. It is well known that if the proportion of tarry materials rises above certain determinable values during the progress of a stream of hydrocarbons through a highly heated cracking zone, precipltation of these tars on the walls of the apparatus will occur and that an important, even though small, percentage of the tar will be transformed into adhering solid coke. Fouling of the surfaces with solid material necessitates interruption of the cracking operation to clean the apparatus. Because of this situation, it is common practice to give the hydrocarbon stream only a limited treatment and then withdraw it from the cracking zone,'when it is cooled and separated into its resultant components of tar, gas-oil, naphtha and gas. The residual gas-oil is usually recycled.

, This practice has two definite disadvantages. There is an appreciable operating loss resulting from the fact that on each cycle the-oil must be cooled and re-heated, and the withdrawal of the gas is a disadvantage; The gas produced in the cracking process tends to approach an approximate equilibrium and the presence of a large amount of such gas (particularly the unsaturated constituents) tends greatly to inhibit the formation of large additional quantities of gas. It is also known that a portion of the unsaturated constituents of the gas will be polymerized to form compounds which are liquids within the boiling range of the desired motor fuels.

4 I have discovered that coke is formed only if 7 liquid tar is present in the hydrocarbon stream and I have further found that this tar forms as a fog in the stream even though temperature and pressure conditions are such that the original charging stock should be completely vaporized. Molecular changes which take place during cracking produce this fog and the fog must form before sufllcient liquid will collect on the tube walls to cause coke formation.

Based on this discovery, I have found that the problem can be solved by the removal of thisfog at various stages during the crackingpperation. As a result, the dimculties above pointed out are avoided and definiteely improved results are obtained by intensifying the cracking factors of time and temperature very materially. The invention is applicable both to high pressure cracking, such as the operations carried on at 1000 pounds per square inch, as well as to low pressure cracking. The temperature and time factors which have heretofore been employed in these two different types of processes are quite diiferent one from. the other. In like manner the intensified conditions permitted by my process will differ. As a matter of fact, the degree of intensification which I can employ depends almost entirely on the economics, of the operations. Factors-such as the desired or necessary octane number of the gasoline, the balance between the value of gas and fuel oil, and other similar factors all will enter 'into the calculation. By removing the incipient l0 coke-forming material, I eliminate a physical limitation which has heretofore existed to prevent ideal performance, but in view of the many variable factors it is impossible to give any .precise and accurate definition of the benefits which flow tangential drum type separator such as frequently used at the discharge end of a cracking equipment ordinarily is substantially useless for this purpose. Later I describe one form of separator excellently suited to the purpose but it is probable that other types may also be employed. However, onefeature of design that must be taken care of in any separator useful for this process, is that means must be provided for quickly removing the tarry liquid from the actionv of the hot gases. If this is not done, the liquid will tend to coke in the separators and destroy \their efficiency. This criticism would apply to the use of most types of steam condensate sepa- 40 rating apparatus now on the market which rely upon impingement against vertical baffles along which the liquid trickles, whereas, I prefer to cause the heated hydrocarbons to impinge on stationary baflies to deflect the stream to give it a rotary motion and thereby I can cause the mate rial in the gaseous phase to traverse a path different from the liquid droplets in a denser phase in such a way as to causethe desired separation.

Preferably the cracking zone is divided into more than two sections and accordingly two or more separators will be required. In such case the tar discharge from all the separators may be brought to .a common point. However, I have found that this ordinarily will not be desirable, particularly with high pressure systems. Obviously it is essential that the pressure in the 'tar discharge line be maintained only slightly better to discharge the tar from one separator back into the main line just in advance of the next separator, with the last separator discharging into the line just in advance of the main pressure-control valve which regulates the pressure of the system. Thus each tar discharge pipe will by-pass a section of the cracking zone. This means that there will be some pressure drop but it will be so slight that it can readily be adjusted. Preferably the pressure is so controlled that only enough gas will flow through the tar discharge pipes to insure that they will be kept properly swept out.

By this simple arrangement the pressures are practically self-adjusting even though there be substantial variations in the over-all pressure of the system. I have also found that the injection of a liquid in advance of a separator is helpful in mechanically clearing the fog. -The tar from the previous separator may thus have a beneficial function, and this action may be supplemented by a further injection at this point of a liquid such as gas-oil or heavier hydrocarbon material such as a refractory high-boiling distillate. However, it is to be borne in mind that temperature equilibrium will not be established immediately after injection, and hence I may use a lighter cut and still obtain the desired liquid phase scrubbing effect on the hydrocarbons being processed. If desired, the liquids discharged from the separator may be promptly cooled somewhat, before being returned to the line.

This invention can readily be understood from the accompanying drawing which illustrates an example of the same in conjunction with a type of apparatus which has been found particularly suited to the purpose. In the drawing, Fig. 1

.is a diagrammatic flow sheet showing the pas- -As is well understood, the fluid will ordinarily have been brought (in a preheater not shown) almost to cracking temperature, and may, therefore, be partly or wholly in the gaseous phase. The original liquid is introduced under pressure by a pump not shown into the preheater and thence into the cracking zone. In the drawing, the cracking zone sections are indicated as coils,

but it is to be understood that this is only a 'conventionalized showing and any usual form of cracking apparatus may be employed, such as a series of parallel pipes connected to a pair of headers. It is also to be understood that the cracking zone indicated by the numeral I3 is heated by an appropriate furnace, not shown, to the desired cracking temperature.

I The hydrocarbon stream, substantially in vaporized condition, .enters section I2 of the cracking apparatus, where it is brought up to and maintained at cracking temperature. As a,

result, a portion of the oil is converted to gas, naphtha and tar'. The stream, now carrying some or all of the tar as a fog of fine droplets,

passes by pipe I5 to separator I6 where the bulk of this fog is separated preferably throu h centrifugal action as will later be described. The hydrocarbon stream consisting essentially of gas, naphtha and gas oil which has not been substantially cooled, passes through section 20 of the cracking apparatus where it is subjected to further cracking treatment, after which it passes by pipe 2I into separator 22. The tarry material separated' by separator I6 is conducted by pipe H to pipe 2I just before the latter enters separator 22. Pipe I1 is provided with a valve I9 set to compensate for the pressure drop in section 20, and allow the tar and a small amount of gas and vapor to pass.

In like manner, the hydrocarbon stream leaving separator 22 passes by pipe 24 to section 26 where it is given its final cracking treatment and it is to be understood that the aggregate cracking in sections I2, 20 and 26 may be considerably more intense'thanthat usually practised in a single cycle, so that very little, if any, unconverted gas-oil will remain, and so-called reforming treatments ordinarilywill be unnecessary. The discharge from section 26 passes through pipe 21 to valve 28 which maintains the desired pressure in the system, and thence at a lower or atmospheric pressure to an evaporator 30 which may be provided with a cooling coil 32. I

This evaporator is intended for separation of the tar which is discharged by pipe 36 controlled by valve 38. The gas and vapors of naphtha and any residual oil are taken to a fractionating col-. umn as is well understood in the art.

The tarry liquid from separator 22 is conducted by pipe 23 to pipe 2! just in advance of valve 28. Pipe 23 is provided with a valve 25 for controlling the pressure drop. Pipes 40 and 42are provided so that additional liquid may, if desired, be introduced into the hydrocarbon stream as it enters separators I6 and 22.

In Figs. 2 and 3, I show the detailed construction of th separator I6 and the separator 22 will be the same. The pipe I5 is connected by flange 44 with the cylindrical separator casing I6. Inside the casing I6 is the Venturi throat 46 carrying in its mouth the turbine plate 48s This plate is composed of radially arranged blades positioned with their edges slightly overlapping, somewhat like the blades in a blower, but in this case the plate 48 is stationary and the blades give a rapid swirling or rotary motiontothe vapors and fog of tarry droplets that pass through them. Preferably a streamlined member 5Il,'circular in cross section, is provided at the center of the guide to eliminate a vortex from the rapidly whirling vapors that emerge from the plate 48; 52 is the discharge mouth of the Venturi which is formed with a gradual increase in radius so that the fog droplets will follow along its surface. Cylindrical expansion nozzle 54 has its inlet end inside the mouth of member 52' with a slight clearance between them to provide an annular slot through which the droplets of tar will pass out to the casing I6. Pipe II conducts the collected tarry liquid from the casing, and ordinarily valve I9 should be set'to permit this liquid and a small amount of vapor to pass through.

The bulk of the gas and vapor substantially 60 discharge side of the plate 48 in order to act as a freed from tar will expand in nozzle 54 which converts a portion of the kinetic energy of rotation to pressure head prior to the emergence of the tar-free hydrocarbon stream into pipe I.

While I have principally described the process and equipment as useful for cracking petroleum hydrocarbons, it may also have other uses such as in the polymerization of hydrocarbon gases to liquids. In such case, tar is usually formed and its removal by themeans described above is advantageous. Also it is to be understood that the cracking process here selected for illustration as well as the details of the apparatus may be modified in many particulars without departing from the spirit of my invention.

What I claim is:

1. In a process of reacting hydrocarbons under conditions of heat and pressure to change their molecular structure, the steps of heating liquid hydrocarbons to convert them into the vapor phase, passing the vaporized hydrocarbons through a reaction zone at a cracking temperature and under pressure adapted to cause molecular changes to take place including the formation of a fog of extremely finely divided droplets of tarry material produced in liquid phase without substantial temperature drop; causing substantially immediately the stream of vaporized hydrocarbons and fog of tarry material to flow forward directly under substantially uniform pressure and without substantial drop in temperature and acquire a rapid whirling motion while moving continuously forward whereby said minute fog droplets are thrown to the periphery of the advancing stream, substantially immediately withdrawing a portion of the peripheral layer of such stream to effect separation of said fog droplets and avoiding recontact within subsequent portions of the cracking zone between said withdrawn fog of liquid droplets and the main portion of the stream advancing through the cracking zone.

v heating liquid hydrocarbons to convert them into the vapor phase, means for cracking the hydrocarbons at a cracking temperature and under pressureadapted to cause molecular changes to take place including the formation of a fog of extremely finely divided droplets of tarry ma-- terial produced in liquid phase without substantial temperature drop, means for causing the stream of vaporized hydrocarbons and fog of tarry material to flow forward substantially immediately under substantially uniform pressure" and without drop in temperature and acquire a rapid whirling motion while moving continuously forward so that the minute fog droplets are thrown to the periphery of the advancing stream, means for withdrawing substantially immediately a portion of the peripheral layer of such stream to effect separation of the fog droplets and for avoiding recontact within subsequent portions of the cracking zone between said withdrawn portion and the main portion of the stream advancing through the cracking zone.

4. An apparatus as defined in claim 3, in which the means for causing the vaporized hydrocarbons and vfog to whirl and effect separation of the fog droplets from the hydrocarbon stream cornprises a turbine plate having radially arranged, slightly overlapping blades which are adapted to give a. rapid swirling action to the stream of hydrocarbon vapors and fog of tarry droplets that pass through the turbine plate.

5. In a processof reacting hydrocarbons under conditions of heat and pressure to change their molecular structure, the steps of passing v'aporous hydrocarbons through a reaction zone at a cracking'temperature and under pressure adapted to cause molecular changes to take place including the formation of a fog of extremely finely divided droplets of tarry material produced in liquid phase without substantial temperature drop;

' causing substantially immediately the stream of vaporous hydrocarbons and fog of tarry material to flow forward directly under substantially uniform pressure and without substantial drop in temperature and acquire a rapid whirling motion while moving continuously forward whereby said minute fog droplets are thrown to the periphery of the advancing stream, substantially immediately "withdrawing a portion of the peripheral- A layer of such stream to eflect separation of said main portion of the stream advancing through 4 the cracking zone. HORACE M. WEIR.

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