Production of propylene in a fluid catalytic cracking unit

Abstract

A process and apparatus for catalytic cracking of hydrocarbon feedstock employing circulating fluidized bed reactor-regenerator configuration for maximizing the yield of propylene (C3 olefin) is disclosed. The apparatus comprises two reaction zones operating under different temperature and weight hourly space velocity (WHSV), one primary zone for cracking of hydrocarbon feedstock and other as secondary zone for cracking of C4 fraction produced from the cracking of hydrocarbon feedstock in the primary reaction zone, optionally admixed with C4 stream from external source. Two dedicated conduits equipped with valves for control of catalyst flow rate are provided to supply the hot catalyst from a common catalyst regeneration zone wherein the catalyst flowing though conduit connected to the secondary reaction zone is cooled employing a heat exchanging device. The lower temperature achieved in secondary reaction zone on account of exchange of heat along with lower weight hourly space velocity (WHSV) selectively promotes oligomerization of C4 fraction before being cracked to produce C3 olefin in the subsequent portion of the reaction zone (primary).

Claims

1. A process for enhancing the yield of C3 olefin in fluid catalytic cracking of hydrocarbon feedstock-, the process comprising: a) contacting a hydrocarbon feedstock with a fluidizable solid micro-spherical cracking catalyst in a primary reaction zone of a riser to produce cracked hydrocarbon products and spent catalyst; b) separating the spent catalyst from the cracked hydrocarbon products and stripping the spent catalyst with steam in a reactor vessel to remove the cracked hydrocarbons entrapped inside the pores of the spent catalyst; c) separating a C4 hydrocarbon fraction from the cracked hydrocarbon products to obtain a recycle C4 hydrocarbon fraction comprising olefinic molecules; d) burning off the coke deposited on the spent catalyst in a catalyst regenerator to obtain a hot regenerated catalyst; e) recycling a part of the hot regenerated catalyst into the primary reaction zone and cooling a remaining part of the hot regenerated catalyst in a catalyst cooler to obtain a cooled regenerated catalyst, and recycling the cooled regenerated catalyst to a secondary reaction zone of the riser located upstream of the primary reaction zone of the riser; f) contacting the recycle C4 hydrocarbon fraction with the cooled regenerated catalyst in the secondary reaction zone of the riser operated at a temperature and weight hourly space velocity lower than that of the primary reaction zone for oligomerization reaction of the olefinic molecules of the recycle C4 hydrocarbon fraction to obtain oligomers; and g) lifting the oligomers into the primary reaction zone of the riser for cracking to produce C3 olefin.

2. The process as claimed in claim 1, wherein flow of the hot regenerated catalyst into the primary reaction zone is controlled to achieve a temperature at the exit of the riser in the range of 500? C. to 650? C.

3. The process as claimed in claim 1, wherein the solid micro-spherical cracking catalyst comprises not less than 5 wt % pentasil zeolite based additive.

4. The process as claimed in claim 1, wherein the flow of the cooled regenerated catalyst is controlled to achieve a temperature in the range of 350? C. to 450? C. inside the secondary reaction zone.

5. The process as claimed in claim 1, wherein weight hourly space velocity in the secondary reaction zone is maintained below 10 hr.sup.?1 to promote oligomerization of the olefinic molecules of the recycle C4 hydrocarbon fraction and the oligomers get cracked in the subsequent primary reaction zone to improve the yield of C3 olefin.

6. The process as claimed in claim 1, wherein a fresh C4 hydrocarbon fraction from external sources is optionally processed in the secondary reaction zone along with the recycle C4 hydrocarbons fraction.

7. The process as claimed in claim 1, wherein the fresh C4 hydrocarbon fraction and the recycle C4 hydrocarbon fraction are injected into the secondary reaction zone and acts as lifting media thus minimizing the requirement of lift steam.

8. The process as claimed in claim 1, wherein a part of the spent catalyst from the reactor vessel is directed to the catalyst cooler to maintain carbon content of the catalyst in the range of 0.3 wt % to 1 wt % in the secondary reaction zone.

Description

DESCRIPTION OF THE DRAWINGS

(1) FIGS. 1 & 2 illustrate the Schematic diagram of Riser-Stripper-regenerator section of FCC unit including the additional hardware of a secondary reaction zone and a dedicated cooled regenerated catalyst stand pipe along with a catalyst cooler.

DETAILED DESCRIPTION OF THE INVENTION

(2) The present invention provides a process and an apparatus to substantially increase the C3 olefin (propylene) yield through selective conversion of recycled or external C4 hydrocarbon fraction in a FCC unit.

(3) The invention will now be described in an exemplary and non-limiting embodiment as depicted in the accompanying drawings. There can, however, be other embodiments of the same invention, all of which are deemed covered by this description.

(4) The riser-stripper-regenerator section of the FCC unit is shown in FIG. 1. As in case of normal FCC units, regenerated catalyst is withdrawn from the regenerator (105) via a stand pipe (regenerated catalyst stand pipe) (106) and flows to riser (101) bottom via a control valve (107), which is controlled through a TRC (temperature recorder and controller) (108) in cascade with the temperature measuring element (109) placed at the top of the riser (101). Fresh feed is injected into the up-flowing catalyst in the riser (101) via suitable injectors (110). Vaporization and cracking of the feed takes place as it comes in contact with the hot regenerated catalyst and the whole mixture moves along the riser in upward direction. The hydrocarbon feed is cracked in the riser in presence of fluidizable solid micro-spherical cracking catalyst to produce hydrocarbon products and coke. The riser terminates inside the reactor vessel (102), where in the spent catalyst (catalyst laden with coke generated after reaction) is separated from the product vapors (103) and suitably steam stripped inside stripper (104) before being sent to the regenerator vessel (105) via another standpipe (111). The catalyst is regenerated inside the regeneration vessel by burning off of the coke deposited on the catalyst at a temperature in the range of 680 to 750? C. in presence of air.

(5) As an embodiment of the invention, the riser is elongated further towards the bottom portion to create an additional reaction zone called as secondary reaction zone (112) which is separate from the primary reaction zone (113) meant for cracking of the main hydrocarbon feed. The required quantity of catalyst flow to the secondary reaction zone is provided through an additional dedicated standpipe (114) connected with the common regenerator. This additional standpipe is equipped with a device (115) for cooling of the withdrawn catalyst followed by a control valve (116). Temperature of the cooled regenerated catalyst is targeted to be kept at around 350 to 450? C. which is the targeted temperature of the secondary reaction zone. A C4 hydrocarbon fraction is separated from the cracked hydrocarbon products (103) in the downstream section (not shown) to obtain a recycle C4 hydrocarbon fraction. The recycled/fresh C4 hydrocarbon fraction stream is injected into the secondary reaction zone. Weight hourly space velocity (WHSV) inside the secondary reaction zone (112) is maintained suitably below 10 hr.sup.?1, preferably between 5 to 10 hr.sup.?1, Lower temperature and weight hourly space velocity (WHSV) provided at this secondary reaction zone (preceding section of primary reaction zone) helps in promoting the oligomerisation reaction of the C4 olefinic streams with minimal cracking of the oligomers inside this reaction zone. The oligomers formed inside the secondary reaction zone immediately comes into contact with catalyst at higher temperature inside the primary reaction zone (upstream of the cracking of main hydrocarbon feed) to produce desired C3 olefins.

(6) In another embodiment of the invention, as shown in FIG. 2, an additional conduit (117) along with a control valve (118) is envisaged to mix some spent catalyst directly into the catalyst cooler device in order to maintain the carbon content of the catalyst in the range of 0.3 to 1 wt % inside the secondary reaction zone (112). This subside the undesired reactions thus to improve the overall propylene yield from the unit.

(7) In another embodiment of the invention, requirement of lift steam in the riser is reduced by injecting the C4 streams at the same position, where lift steam is injected. Purpose of lift steam is to give an upward thrust to the moving catalyst because of the expansion of steam after injection. Negative aspect of this steam is that it leads to thermal deactivation of the catalyst. By using C4 hydrocarbon stream as lift stream, the thrust effect takes place without any catalyst deactivation effect.

(8) In an embodiment of the invention, the invention provides a process for enhancing the yield of C3 olefin in fluid catalytic cracking of hydrocarbon feedstock, the process comprising: a) contacting a hydrocarbon feedstock in a primary reaction zone of a riser in the presence of a fluidizable solid micro-spherical cracking catalyst to produce cracked hydrocarbon products and spent catalyst; b) separating the spent catalyst from the cracked hydrocarbon products and stripping it with steam to remove the hydrocarbons entrapped inside the spent catalyst pores; c) separating a C4 hydrocarbon fraction of the cracked hydrocarbon products to obtain a recycle C4 hydrocarbon fraction; d) burning off the coke deposited on the spent catalyst in a catalyst regenerator to obtain a hot regenerated catalyst; e) recycling a part of the hot regenerated catalyst into the primary reaction zone and remaining part is cooled to obtain a cooled regenerated catalyst, which is recycled to a secondary reaction zone located at upstream of the primary reaction zone of the riser; f) contacting the recycle C4 hydrocarbon fraction with the cooled regenerated catalyst in the secondary reaction zone of the riser for oligomerization reaction of the olefinic molecules of the recycle C4 hydrocarbon fraction to obtain oligomers; and g) lifting the oligomers into the primary reaction zone of the riser in order to get cracked to produce C3 olefin.

(9) In an another embodiment of the present invention the flow of the hot regenerated catalyst into the primary reaction zone is controlled to achieve the temperature at the exit of riser in the range of 500 to 650? C.

(10) In an another embodiment of the present invention the solid micro-spherical cracking catalyst comprises not less than 5 wt % pentasil zeolite based additive. Pentasil zeolite based additive present in the solid micro-spherical cracking catalyst preferably in the range of 5 to 50 wt %, more preferably in the range of 10 to 40 wt %.

(11) In an another embodiment of the present invention the hot regenerated catalyst to the secondary reaction zone is cooled in a catalyst cooler and flow is controlled to achieve the temperature in the range of 350? C. to 450? C. inside the secondary reaction zone.

(12) In an another embodiment of the present invention weight hourly space velocity in the secondary reaction zone is maintained below 10 hr.sup.?1 to promote oligomerisation of the olefinic molecules of the recycle C4 hydrocarbon fraction and the oligomers get cracked in the subsequent primary reaction zone to improve the yield of C3 olefin in the overall unit. Weight hourly space velocity in the secondary reaction zone is preferably maintained between 5 to 10 hr.sup.?1.

(13) In an another embodiment of the present invention the fresh C4 hydrocarbon fraction from external sources is optionally processed in the secondary reaction zone along with the recycle C4 hydrocarbons fraction.

(14) In an another embodiment of the present invention the fresh C4 hydrocarbon fraction and the recycle C4 hydrocarbon fraction injected into the secondary reaction zone acts as lifting media thus minimizing the requirement of lift steam.

(15) In an another embodiment of the present invention a part of the spent catalyst from a reactor vessel is directed to the catalyst cooler to maintain carbon content of the catalyst in the range of 0.3% to 1% in the secondary reaction zone.

(16) Another aspect of the present invention also discloses an apparatus for enhancing the yield of C3 olefin in fluidized catalytic cracking of hydrocarbons, said apparatus comprising: a) a riser (101) having a primary reaction zone (113) for cracking of hydrocarbon feedstock, and a secondary reaction zone (112) for oligomerization of C4 hydrocarbon fraction; b) a reactor vessel (102) connected to the riser reactor (101) for separating cracked hydrocarbon products from a spent catalyst and stripping of the spent catalyst; c) a catalyst regenerator (105) for regenerating the spent catalyst; and d) a catalyst cooler (115) for cooling a part of hot regenerated catalyst.

(17) In an another embodiment of the present invention the riser has multiple injection points for injecting fresh C4 hydrocarbon stream and recycle C4 hydrocarbon stream, dilution stream, recycle heavy feed, fresh naphtha stream and recycle naphtha stream.

(18) In an another embodiment of the present invention the catalyst cooler (115) is connected to the secondary reaction zone (112) of the riser (101) by means of a conduit (114) having a plug valve (116) to control the flow of cooled regenerated catalyst.

EXAMPLE

(19) This example demonstrates the improvement in propylene yield in FCCU obtained through cracking of the product C.sub.4 stream generated out of cracking of residue feed in the riser using a residue feed having 4.4 wt % conradson carbon residue (CCR) and 941 kg/m.sup.3 density. The example given in this section is for illustration purpose only and don't construe to the claims as mentioned in subsequent section.

(20) The data shown in this example have been generated through preliminary engineering calculations based on experimental data from microreactor and pilot plant. Base case data is at a reaction temperature of 580? C. and without any C.sub.4 cracking. The catalyst used is a mixture of FCC catalyst having average particle size of 80 microns based on USY zeolite and an additive based on pentasil zeolite having silica to alumina molar ratio of 30. The catalyst was hydrothermally deactivated at 810? C. for 5 hrs. The composition of C.sub.4 stream used in the study is given in Table-I.

(21) TABLE-US-00001 TABLE I Composition of C.sub.4 recycle stream Components wt % i-butane 23.6 n-butane 6.5 1-butene 15 i-butylene 26.4 cis-2-butene 11.7 trans-2-butene 16 1,3-butadiene 0.8

(22) The improvements in propylene yield on fresh feed basis with recycle of C.sub.4 products using different schemes are presented in Table-II. Case I considers the recycle of C.sub.4 stream to the stripper operating at a temperature close to riser outlet temperature. In Case-II, stripper temperature is maintained at a higher temperature than Case-I. Case-III considers C.sub.4 recycle to the bottom of the riser so that the C.sub.4 cracking reaction is carried out at the upstream of main feed cracking. Case-IV considers the case of the invention, where in the C.sub.4 cracking reaction is carried out at two steps. The C.sub.4 injection zone temperature is maintained at 350? C.

(23) TABLE-US-00002 TABLE II Base Case-IV case Case-I Case-II Case-III (Invention) C.sub.4 recycle rate wt % 13 13 13 13 FF Riser Top ? C. 580 580 580 580 580 temperature C.sub.4 injection Stripper Stripper Upstream Secondary zone of reaction main feed zone (as per injection invention) at upstream of primary reaction zone (riser) C.sub.4 injection 575 600 610 350 zone temperature C.sub.4 injection hr.sup.?1 10 10 10 10 zone WHSV Propylene wt % 17.1 18.7 19.9 18.33 21.0 Yield FF

(24) Using the process of the present invention as depicted in Case-IV, it is found that the propylene production is found to be enhanced w.r.t. the base as well as the other prior art cases. In this example, a residue feed has been used with which ex-riser C.sub.4 quantity is coming to be 13 wt %. In the cases of using better quality feed, ex-riser C.sub.4 quantity will increase which in turn will make further increase in propylene yield using the present invention.