Mineral oils: processes and products – Chemical conversion of hydrocarbons – Plural parallel stages of chemical conversion
Reexamination Certificate
2001-07-13
2003-11-04
Griffin, Walter D. (Department: 1764)
Mineral oils: processes and products
Chemical conversion of hydrocarbons
Plural parallel stages of chemical conversion
C208S067000, C208S072000, C208S074000, C208S075000, C422S144000, C422S145000, C422S241000
Reexamination Certificate
active
06641715
ABSTRACT:
The present invention relates to a process and apparatus for catalytic cracking of hydrocarbon feeds.
The petroleum industry routinely employs cracking processes in which hydrocarbon molecules with a high molecular weight and boiling point are split into smaller molecules with a lower boiling point.
In recent catalytic cracking processes such as that described in European patent EP-A-0 291 253, the cracking reaction takes place in an elongate vessel with a substantially circular cross section, the catalyst being admitted into the lower portion of the vessel along with the atomised hydrocarbon feed. Bringing the feed into contact with the hot catalyst vaporises the hydrocarbons, entraining the catalyst towards the upper portion of the reaction zone, with the introduction of an entrainment fluid assisting the upward movement. The products formed during the reaction have a very broad range of boiling points. In general, the products formed are distinguished by their boiling point and chemical nature:
Dry gases
H
2
, H
2
S, molecules containing 1 or 2 carbon atoms
LPG (liquid
Molecules containing 3 or 4 carbon atoms
petroleum gas)
Gasoline
Molecules containing at least 5 carbon atoms
with a boiling point of less than 220° C.
LCO (light
Molecules with a boiling point of more
cycle oil)
than 220° C. and less than 360° C.
Slurry
Molecules with a boiling point
of more than 360° C.
Coke
Heavy molecules (generally polyaromatics
remaining adsorbed on the catalyst
after the reaction)
The boiling points delimiting the cuts are given by way of indication and correspond to generally accepted standard values. Those cut points can vary depending on the refiner's needs, and in some cases can also produce intermediate cuts from the products formed.
The yields that are generally obtained naturally depend on the quality of the treated feeds. Typically, by way of indication, the yields observed (as a % by weight of the feed) for units are:
dry gas
1-5%
LPG
10-25%
gasoline
30-55%
LCO
15-25%
slurry
5-20%
coke
3-10%
In general, the coke formed is burned in one or more vessels termed regenerators towards which the catalyst circulates from the reactor outlet. The heat produced by combustion of the coke re-heats the catalyst, which is then re-introduced into the reactor inlet and brought into contact with the feed. The catalytic cracking process is an adiabatic process. The heat recovered by the catalyst during its passage into the regeneration zone is equal to the heat lost by the catalyst during its passage through the reaction zone. This constrains the operator to employ operating conditions that are not independent of each other. The operating conditions that most affect the yields and selectivities for a given reactor are essentially the catalyst flow rate, which is generally related to the feed flow rate by the term C/O (C for catalyst, and O for oil). The normal range for operating catalytic cracking units is generally:
C/O=4-8 (C/O=weight ratio of catalyst flow rate to feed flow rate);
T (at reactor head)=500-550° C.
Conversion is known to increase with temperature and C/O. However, using conventional techniques, that increase can, inter alia, be accompanied by a significant increase in the coke and dry gas yields. Increasing the coke yield by means of the regenerator-reactor thermal balance and dimensions of the unit frequently limits the operator to a restricted range of operating conditions and, for a given feed type, to a fairly fixed yield structure.
The sale price of different products can fluctuate with time, which may tempt the refiner to decide to maximise certain products to the detriment of some others. Further, the change in specifications imposed on the products in different states means that certain FCC products may no longer have an outlet (for example, LCO is highly aromatic and has a very poor cetane index, so its use in certain fuels in the gas oil pool poses a problem; the sulphur content of heavy gasoline (160° C.-220° C.) renders its use in gasoline pools difficult in some cases). It may thus be advantageous to minimise certain cuts as well.
Maximising propylene, a product with a high added value (molecule included in the LPG cut) is known to involve making the reaction conditions more severe (higher temperature, higher C/O). At the same time, making the conditions more severe means that the yields of other cuts (LCO and gasoline) decreases.
Mild conditions tend to maximise the LCO, which may be advantageous in states in which middle distillates are in great demand in the fuel market, but LPG (including propylene) and gasoline will probably not be maximised.
Thus, the operation of the reaction zone for a conventional unit is not always compatible with achieving the two aims such as the following non-limiting examples:
maximising propylene and LCO;
minimising heavy gasoline, maximising light gasoline.
There is thus a need for solutions that can enable the reaction zone to operate both under severe and mild conditions, for example using two reaction zones operating under different operating conditions.
The reaction zones generally used in the majority of catalytic cracking units in current use readily allow operations to be carried out under mild cracking conditions (C/O of 4 to 8 and reactor outlet temperatures of 500° C. to 550° C.). The residence time for the hydrocarbons in that reaction zone, the minimum constitution of which is a tube with a substantially circular cross section and elongate form in which the fluids flow in an overall bottom to top direction, usually termed a riser, and a system for separating the cracked vapours and the catalyst, is generally more than 2 seconds (s), of the order of about 2 s to about 10 s. The residence time for the hydrocarbons in contact with the catalyst is usually more than 1 s.
Juxtaposing two conventional reactors to obtain two types of operating conditions in the same catalytic cracking unit such as that described by Niccum, P. K., Miller, R. B., Claude A. and M. A. Silvermann in “Maxofin: a novel FCC process for maximizing light olefins using a new generation ZSM5 additive” (1998, NPRA annual meeting, San Francisco, Calif., USA, Mar. 16
th
, 1998), renders necessary the use of additives in the second riser where the reaction is carried out under more severe conditions to obtain a more favourable selectivity. Further, the more severe conditions in the second reactor cause a very large increase in the coke yield (more than 2% with respect to the feed). The arrangement of that type of system is thus not optimal.
The prior art is also illustrated by U.S. Pat. Nos. 4,424,116 and 4,606,810, which describe a concatenation of two riser reactors in series. U.S. Pat. No. 5,039,395 also illustrates the prior art.
In order to minimise a cut, with a unit possessing one or more conventional riser type vessels, it is also possible to recycle the products the production of which is to be minimised to the riser, in the case of heavy feeds, this has a huge advantage for the thermal balance of the units: vaporising the recycle consumes more heat and thus produces more heat in the regeneration zone and thus more coke in the reaction zone; further, since it is cleverly located downstream with respect to fresh feed injection, injection of the recycle encourages fresh feed vaporisation which then enables even heavier feeds to be treated (with higher median boiling points and end points). Such an apparatus has, for example, been described in French patent FR A-2 621 322 for cracking heavy cuts.
In that type of implementation, the recycled products are not exposed to very severe conditions and react only slightly. The aim of the recycles has more to do with the thermal balance and vaporising the feed than degrading the recycle into higher added value products.
It is also possible to use a recycle upstream of the feed, to expose the recycle to conditions that are more severe than those for the feed. Under those conditions, the products formed under the most severe conditions have the time to degrade above the f
Griffin Walter D.
Institut Francais du Pe'trole
Millen White Zelano & Branigan P.C.
LandOfFree
Method and device for catalytic cracking comprising reactors... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and device for catalytic cracking comprising reactors..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and device for catalytic cracking comprising reactors... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3117805