AU703577B2 - Catalytic cracker riser termination device - Google Patents

Description

1 P/00/01l Regulation 3.2

AUSTRALIA

Patents Act 1990

ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT Invention Title: "CATALYTIC CRACKER RISER TERMINATION DEVICE" i.I i :The following statement is a full description of this invention, including the best method of performing it known to the Applicant:- FIELD OF THE INVENTION SThe invention relates an apparatus to separate hydrocarbon vapor and the catalyst used in a fluidised catalytic cracking device (hereinafter called FCC) in a riser terminating device (hereafter called an RTD) of the FCC.

BACKGROUND TO THE INVENTION In virtually all modern FCCs, hydrocarbon vapors and fine particles of catalyst travel vertically upwards together in a riser reactor pipe. During a short period of pneumatic transport, at temperatures normally in the range of 500-550 degrees C, the solid catalyst particles promote selective cracking of larger hydrocarbon feed molecules to more valuable product molecules such as LPG, petroleum and diesel components.

At the end of travel in the riser the vapor and catalyst must be quickly and efficiently separated in the RTD so that vapor can be discharged via a set of conventional cyclone devices to a distillation column for product cooling and recovery and catalyst can be directed via a steam-fluidised stripper bed to a catalyst regenerator vessel for removal by combustion of solid coke laydown (coke associated with) the catalyst.

The RTD desirably provides conditions where; residence time of hot product vapors in the RTD system (from riser 10 outlet to final cyclone inlet) should be below 1.5 seconds. RTD .contribution to post riser thermal cracking is negligible compared with the overall post riser thermal cracking which unavoidably downgrades some valuable distillate product to dry gas and coke precursors during a period of around 7-10 seconds. For a typical modern FCC, this is the total hot 15 vapor time spend in the RTD, transition ducts (if any) from the RTD to the cyclones, the reactor plenum chamber, the reactor overhead vapor line and the hot bottom volume of the main fractionator column below the quench tray section of the apparatus. Once the RTD residence time is only to 1.5 seconds efforts to shave further tenths of a second from this time tend to be counter productive giving negligible reduction of overall post riser thermal cracking while jeopardizing vapor/catalyst separation in the

RTD.

vapor discharge from the RTD must contain the least possible amount of catalyst so that for the final reactor cyclone stage catalyst carryover to the fractionator (which increases with inlet dust load from the RTD) is absolutely minimal. This in turn minimizes or avoids problems of; excessive erosion rates in the final cyclones, often leading to plant shutdowns due to holes or cracks in the cyclone/plenum system.

3 (ii) downgraded value of the slurry oil due to high catalyst contents, which demands costly recycle of slurry to the reactor for catalyst recovery (iii) high erosion rates of slurry lines and pumps leading to at the least expensive maintenance, and at the worst, loss of hot oil containment with consequent major plant fires, protracted off-stream time and even possibly loss of life.

catalyst discharge from the RTD dipleg/s must entrain the least possible hydrocarbon vapor to avoid problems of excessive injection of hydrocarbon vapor into the supposedly steam fluidised stripper bed of the unit, with the result that there is higher coke yields (or coke making factors) due to longer residence times of hot catalyst in the hydrocarbon-rich atmosphere. This in turn leads to lower catalyst/oil ratios and hotter regenerator temperatures. The result is potential S•mechanical damage to equipment plus accelerated catalyst deactivation with consequent loss of product yields. Over-cracking of entrained hydrocarbon vapor in the stripper bed also degrades product yield.

(ii) higher superficial velocities in the reactor dilute phase (due to the return of entrained dipleg vapor), thus increasing catalyst dust load to the final-stage cyclone inlets and aggravating any catalyst-in-slurry problems.

mechanical aspects of the RTD are that it must be robust, reliable and simple and should allow easy access for inspection and maintenance.

The RTD design should also facilitate neat, elegant integration with the final cyclone system, ideally employing a symmetrical overall layout for maximum uniformity of mechanical stresses and process mass fluxes.

Prior art RTDs tend to focus exclusively on above while little if any attention is given to above. The invention hereinafter

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described recognizes the importance of all of the above items and provides an apparatus for addressing all four.

Broadly stated the present invention provides in combination, a fluidised catalytic reaction vessel, a riser termination device within said vessel, and a cyclone within said vessel, wherein: said riser termination device comprises a swirl chamber defined by a wall with an inlet from a riser extending upwardly to a riser tip and through which vapor/catalyst mix can tangentially enter the swirl chamber, an axial outlet from the swirl chamber which is generally concentric with the axis of the swirl induced in the swirl chamber, which axis is disposed S below the level of the tip of the riser, a shave-off aperture in the wall of the swirl chamber for entry by catalyst travelling closely adjacent the wall of the swirl chamber, a dust box to receive catalyst exiting the swirl chamber through the shave-off aperture and a vapor aperture in the swirl chamber wall downstream in the direction of swirl of material in the swirl chamber from the shave-off aperture and providing communication between the swirl chamber and the dust box; said cyclone includes an inlet in the form of a horn directed towards and disposed closely adjacent the axial outlet of the swirl chamber to receive discharge from the swirl chamber, a discharge leg for cyclone separated catalyst and a discharge pipe for vapor having its inlet disposed at a level below that of said horn.

4A DESCRIPTION OF THE DRAWINGS.

A preferred embodiment of the invention and constructional variations are hereinafter described with reference to the accompanying drawings in which; Fig.l is a vertical cross-section through a reaction chamber housing the riser termination and cyclone of the invention, Fig.2 is a plan line 2-2 of Fig.l of another form of the invention in which there are three riser termination devices and cyclones in a common reaction chamber, 15 Fig.3 is a scaled down view of the arrangement of Fig.2, "i Fig.4 is a view similar to Fig.3 showing to the same scale an alternative arrangement for the components and illustrating that the alternate arrangement requires a larger reaction vessel, and Fig. 5 is a side view of a reaction chamber as shown in Fig. 2 with the side of the reaction vessel removed and some elements of the apparatus removed.

DESCRIPTION OF ILLUSTRATED EMBODIMENTS In Fig. 1 a reactor vessel is indicated 1. Within the reactor vessel I there is a riser termination device indicated generally 2. There is a riser/reactor pipe 3 entered into the reactor vessel 1 to supply a mixture of vapor and catalyst. Reaction takes place between the catalyst and the vapor at about 500 degrees C as the mixture passes at a velocity of about 15-20 meters per second through the reactor pipe 3. Due to a phenomenon called 'solids-slip' the catalyst speed of travel through 3 is significantly below the vapor speed of travel through 3.

The speed up the catalyst (because high catalyst speed is required for the operations follow) a riser nozzle 4 is provided and it is shaped at 5 with a diminishing cross-section to provide acceleration of the catalyst/vapor mix. The vapor speed will instantly inscrease as it passes through the nozzle however the catalyst speed 15 increase is slower as its acceleration is a function of its inertia and its large volume.

To accommodate the difference in acceleration a flow stabilization section identified 6 is provided. The result is that catalyst will have achieved its full potential exit velocity when it leaves the tip 7 of the riser which is, as shown in Fig. 1, located above tihe dotted centre line of the outlet The stream of vapor and catalyst tangentially enters a swirl chamber 28 defined by a wall 8. There many be several swirl chambers 28 in the reactor vessel I as will be hereinafter covered. The swirl action in the chamber 28 results in approximately 90-95% of the heavy catalyst being flung to the inner surface of the wall 8. The catalyst travelling along the surface of the wall 8 is "shaved-off" by means of a shave-off aperture 9 in the wall 8. There is a dust box 9 to receive the shaved-off catalyst.

-6- The remaining catalyst, which is of fine consistency and entrained in the vapor swirling in the chamber, spirals down as shown as a dotted line after at least two full revolutions in the chamber 28 and exits the chamber 28 through the axial outlet The cyclonic spin-up in the chamber 28 achieves 99 to 99.9% efficiency in separating the catalyst from the vapor.

Downsteam of the shave-off aperture 9 in the direction of swirl in the chamber 28 there is a vapor return aperture 15 in the wall 8. The aperture 15 allows the vapor which passes through the shave-off aperture 9 with the shaved-off catalyst and disengages from the down flowing catalyst to return to the chamber 28. The quantity of vapor shaved-off with the catalyst can be in the order of 10-20% of the vapor entering the chamber 28. The vapor return aperture 15 is positioned to tangentially introduce the return vapor into the chamber 28. Vapor return is not total and in the order of 2.5% of the vapor entering the chamber 28 will end up in the catalyst in the catalyst bed 16 which is kept fluidised by steam through a steam 15 supply pipe 17 to thereby strip substantially all of the remaining vapor from the S: catalyst in the bed 16.

As a preferred feature of the riser termination device of the invention there is •'"provided a catalyst drain means 11. The purpose of the drain means 11 is to overcome a tendency for an accumulation of catalyst at the bottom of the wall 8, an accumulation conveniently called a "catalyst dune", The drain means can be in the form of a series of holes or a slot or slots. There may be a downpipe system indicated 12 to enhance the pressure balance to promote catalyst downflow into the dust box 9. The cross-sectional area of the drain means is small compared to the vapor return aperture 15 so as to ensure vapors rising from the catalyst in the dust box 29 will not jet upwardly through the drains means 11 and interrupt the spin pattern in the chamber 28.

The dust box 29 includes has a discharge pipe 18, called a dip leg, which discharges below the level of the catalyst in the bed 16. This avoids a blow out of catalyst due to the pipe 18 being at an overpressure relative to the reactor dilute phase. A catalyst acceleration zone is provided to cater for the level of the bed 16 dropping to the extent that the discharge end of the pipe 18 becomes exposed. This zone converts surplus catalyst static 10 pressure to velocity head.

is to be noted that the pipe 18 slopes at an angle of about 4 degrees.

This facilitates the return to the dust box of vapors which did not disengage from the shaved-off catalyst and passed with the catalyst into the pipe 18.

*ee These vapors will escape from the catalyst if given an opportunity. The slope of the pipe 18 facilitates the vapor escape from the catalyst as it settles and defluidises in the pipe 18. If the pipe 18 were to be vertical :bubbles of vapor would tend to fill the pipe 18 and be pushed down with catalyst flow into the bed 16. The slope ensures catalyst flows down one face of the pipe 18 allowing the vapor to rise in that portion of the pipe 18 not occupied by the down flowing catalyst.

The catalyst leaving the pipe 18 contacts an impediment plate 20 which gently dissipates the kinetic energy in the catalyst flow. The result is a smooth entry of the catalyst into the bed 16.

The catalyst now substantially stripped of hydrocarbon vapors is directed via the pipe 21 to a catalyst regenerator.

8 In Fig.1 it will be seen that the discharge from the outlet of the chamber 28, after a preferred residence time in the chamber 28 of from 0.2 to 0.9 seconds, is directed into an inlet horn 22 of a cyclone 23. The cyclone 23 strips the catalyst still in the vapor stream and delivers it via the dip leg 24 into the catalyst bed 16. The substantially clean vapor is then drawn off through the cyclone discharge pipe 25 to a fractionator column of known form.

The horn 22 desirably includes a notch indicated 30 to promote dumping of catalyst pukes from the riser termination device direct into the catalyst bed. The notch 30 also facilitates entry of stripping stream (during operation) and for man access (during inspection shutdowns).

Fig.2 shows in plan a relationship between a gang of three riser termination devices 2a, 2b, 2c and three associated cyclones 23a, 23b, 23c. It is to be noted that the cyclones 23 are 'nested' between the riser termination devices. This arrangement provides minimal post-riser residence time and is far less complex than known designs which require elaborate manifolding with intermediate vertical flows to guard against delivery of catalyst accumulations, known as "pukes", into the cyclones 23.

It is the view of some persons in this field that the nested relationship illustrated is contrary to "good practice" procedures. The reason given is that with the nested cyclone arrangement illustrated there is a direction of rotation in the cyclones 23 which is contra to that accepted in the industry and believed to provide the most efficient cyclonic action in the final cyclone treatment of the vapor before delivery to the fractionator column.

The above has not proved to be the case in the experience of the inventor.

The essentials for the satisfactory operation of the devices is that there 9 should be an extended inlet horn 22 into the cyclone 23 and the pipe to the fractionator column should have the entry end well below the inlet from the horn 22 to permit adequate decay of the inlet gas spin.

The nested arrangement offers great commercial advantage as can be seen from the Figs.3 and 4. In Fig.4 the cyclones 23 are arranged to provide the rotation of the vapor accepted by the industry as being best practice. It will be seen that the size of the reactor vessel has to be quite large to accept this arrangement of components. In Fig.3 the same components are arranged in the cyclone nested arrangement proposed by the present inventor and the size of the reactor vessel is considerably smaller. This saving in size of the reactor vessel can represent very large saving in the construction cost of the reactor vessel. Another saving is that the there is a reduction in reactor vessel heat loss compared with the •arrangement of Fig.4.

Fig.5 shows a modified arrangement in which hot internal cyclones are ,suspended from a hot wall plenum chamber 27 of a cold reactor vessel.

There are other possible arrangements of riser termination devices and associated cyclones and the invention is not to be considered as limited to the arrangement described and illustrated herein.

The invention provides a number of desirable preferred and essential features. Amongst those features are the riser tip which has a flow acceleration length and a separate flow stabilization length to confer useful additional catalyst velocity at the exit from the riser tip.

A near cylindrical horizontal swirl chamber 28 of adequate volume and roundness to induce at least one and up to four full 360 degree revolutions of the vertically incoming flow of riser vapor. For optimal overall performance vapor residence time in the swirl chamber 28 is found to be in the order of 0.3 to 0.9 seconds.

A swirl chamber with either one or two horizontal generally central gas outlet nozzles protruding only a little distance into the swirl chamber so that the center of the chamber is totally free of all pipework and surface areas.

The absence of central wall friction then allows high spin-up velocities in the normal vortex area, with high G-forces enhancing catalyst separation.

This maximally exploits both inertial separation (for early bulk catalyst removal) and centrifugal separation (for trace catalyst removal) from the 10 riser vapor stream.

A swirl chamber with a full width rectangular catalyst shave-off slot, plus a full width rectangular vapor return opening.

A dust box of substantial volume to ensure maximal disengagement of riser vapors from the downwardly directed catalyst stream leaving the swirl 15 chamber above and to facilitate a smooth return of vapor to the swirl chamber in a manner non-disruptive to the gas swirl pattern in the swirl chamber.

A catalyst downpipe (dip leg) system with a flow acceleration zone to convert excess riser termination device pressure from a static head to a kinetic head, thus minimizing the risk of gas blow-out from an inadequately submerged or sealed downpipe tip.

A simple direct horizontal line up of riser termination device gas outlet with the final cyclone inlet with no need for intermediate manifolding and its complex or vertical flow sections to cope with potential catalyst pukes from the riser.

11 An inward scroll direction of the cyclones receiving vapor with the riser termination device gas outlets leading to a more space efficient integrated riser termination device/cyclone layout which can be housed in a relatively small diameter pressure vessel.

A set of catalyst drain orifices in the riser termination device drum bottom designed to discharge any catalyst dunes without allowing significant vapor upflow from the dust box.

A part open floor plan of cyclone inlet horn/s to promote dumping of catalyst pukes from the riser termination device direct into the catalyst bed 10 and also to facilitate entry of stripping stream (during operation) and for man access (during inspection shutdowns).

A riser termination device catalyst downpipe with a small slope off vertical to facilitate vapor return flow up the downpipe to the dust box and the swirl chamber 28.

A high riser termination device vapor/solid separation efficiency with more than 99% catalyst dust removal from the riser termination device vapor outlet and less than 3% vapor entrained with the riser termination device solids outlet.

Catalyst dust removal with less that 40ppm wt of catalyst/vapor leaving the final stage cyclones in the absence ot undue fines generated by avoidable attrition sources in the unit. Hence with 500 tonnes per day feedrate the reactor catalyst carry over would be well below 200 kg per day at the start of a run with very little deterioration expected after 3 or 4 year operating run. Overall riser termination device/cyclone efficiency of above 99.995 on catalyst circulation rate.

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12 A simple close coupled riser termination device/cyclone system with minimal post riser thermal, cracking of distillates to low value gas and coke,

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Claims (8)

1. "In combination, a fluidised catalytic reaction vessel, a riser termination device within said vessel, and a cyclone within said vessel, wherein: said riser termination device comprises a swirl chamber defined by a wall with an inlet from a riser extending upwardly to a riser tip and through which vapor/catalyst mix can tangentially enter the swirl chamber, an axial outlet from the swirl chamber which is generally concentric with the axis of the swirl induced in the swirl chamber, which axis is disposed below the level of the tip of the riser, a shave-off aperture in the wall of the swirl chamber for entry by catalyst travelling closely adjacent 15 the wall of the swirl chamber, a dust box to receive catalyst exiting the swirl chamber through the shave-off aperture and a vapor aperture in the swirl chamber wall downstream in the direction of swirl of material in the swirl chamber from the shave-off aperture and providing S 20 communication between the swirl chamber and the dust box; said cyclone includes an inlet in the form of a horn directed towards and disposed closely adjacent the axial outlet of the swirl chamber to receive discharge from the swirl chamber, a discharge leg for cyclone separated catalyst and a discharge pipe for vapor having its inlet disposed at a level below that of said horn.

2. A combination as claimed in claim 1 including a catalyst accumulation discharge opening from said swirl chamber where the accumulation discharge opening is disposed between the shave-off aperture and the vapor aperture. ,49 I, 14

3. A combination as claimed in claim 1 or claim 2 including a reactor pipe feeding said swirl chamber and said reactor pipe 'immediately prior to discharge into said swirl chamber is provided with a nozzle.-- which increases the speed at which catalyst is travelling at the time of entry into the swirl chamber.

4. A combination as claimed in claim 3 wherein the nozzle includes a tapered portion and downstream from the tapered portion a portion of constant cross-section to promote flow stabilization of said catalyst.

5. A combination as claimed in any one of the claims 1 to 4 including a discharge leg from the dust box where the 15 discharge leg lies at an angle to the vertical.

6. A combination as claimed in claim 5 where the discharge leg discharges against an energy dissipating plate. o 20

7. A combination as claimed in any one of the preceding claims wherein said horn has a notch in a lower face to allow any accumulations of catalyst in the stream discharged from the axial outlet of the swirl chamber to leave the stream before they can enter the cyclone.

8. A riser termination device substantially as hereinbefore described with reference to the accompanying drawings. Applicant PETER HADDON BARNES Date 19 January 1999 Attorney ROBERT G. SHELSTON of CARTER SMITH BEADLE