Mineral oils: processes and products – Fractionation – Distillation
Reexamination Certificate
2001-11-29
2004-04-20
Caldarola, Glenn (Department: 1764)
Mineral oils: processes and products
Fractionation
Distillation
C208S347000, C208S354000, C208S355000, C208S358000
Reexamination Certificate
active
06723231
ABSTRACT:
The invention is directed to a process to separate propene from the product stream obtained in a fluidized catalytic cracking process.
In a fluid catalytic cracking (FCC) process a mixture of hydrocarbons is prepared by means of catalytic cracking of a petroleum distillate or residue fraction. The hydrocarbon reactor effluent is separated in a separation section into gasoline, light and heavy cycle oil and gaseous products, for example methane, LPG, propene and butene. In a conventional separation section the reactor effluent is first separated in a so-called main fractionator. The top product obtained in the main fractionator will comprise next to the so-called permanent gases like hydrogen, methane and nitrogen a certain amount of ethene, ethane, propene, propane and other saturated and unsaturated hydrocarbon compounds having a boiling point of below 220° C. The valuable hydrocarbon compounds boiling in the gasoline range are recovered from this top product in a so-called unsaturated gas plant as described in U.S. Pat. No. 4,605,493.
U.S. Pat. No. 4,605,493 describes a process in which the top product of the main fractionator is first compressed in one or more stages to a higher pressure level. This section is also referred to as the recontacting section. After the recontacting section the gaseous compounds having a boiling point of ethane and below are separated from the hydrocarbon products having a boiling point of at least propene and above by means of distillate separation step. This distillate separation step comprises in that the compressed top product is separated in a gaseous fraction and a liquid fraction by means of a flash operation. The gaseous traction is sent to an absorber section and the liquid product is sent to a stripping section. In the combined absorber and stripping section propene, propane and higher boiling hydrocarbon compounds are separated from the gaseous compounds including ethane and ethene and lower boiling compounds. In the embodiment illustrated in U.S. Pat. No. 4,605,493 the absorber and stripping sections are represented by two separate vessels. Embodiments in which both sections are present in one column are also known from the prior art.
Propene has become an important by-product of a FCC unit operation. The importance of this by-product is for example illustrated by the fact that dedicated ZSM-5 containing catalyst additives are used to enhance the propene yield in a FCC unit operation. A problem often associated with the increase in propene yield above the design value of an existing plant is that the above described rectifying absorber becomes a bottleneck. This bottleneck may for example result in that the part of the extra propene prepared in the FCC reactor will not be separated from the gaseous products in the rectifying absorber. Recovery of propene from the gaseous product downstream of the rectifying absorber is economically less attractive.
The object of the present invention is to provide a process in which propene can be separated from the gaseous FCC products with a higher efficiency than is possible with prior art processes.
This object is achieved by the following process: Process to separate propene from gaseous fluid catalytic cracking products by performing the following steps:
a) separating d feed mixture comprising the gaseous products, propene and other saturated and unsaturated hydrocarbons obtained in a fluid catalytic cracking process into a hydrocarbon-rich liquid fraction and a hydrogen containing gaseous fraction,
b) separating the hydrogen containing gaseous fraction into a hydrogen-rich gaseous fraction and a hydrocarbon-rich gaseous fraction by means of a membrane separation,
c) supplying the hydrocarbon-rich gaseous fraction obtained in step (b) to an absorber section and obtaining in said absorber section a lower boiling fraction rich in gaseous products having a boiling point of ethane or below and supplying the hydro-carbon-rich liquid fraction obtained in step (a) to a stripper section and obtaining in said stripper section a higher boiling fraction comprising propene and hydrocarbons having a boiling point higher than ethane.
Applicants have found that by separating of part of the hydrogen present in the feed to the rectifying absorber in step (c) that the efficiency of the propene recovery is improved. This improvement enables one, for example, to either make use of smaller distillate units for new FCC units or to de-bottleneck existing FCC units enabling a higher propene production. An additional advantage is that hydrogen is obtained having a reasonable good quality in a relatively simple manner when compared to methods which can recover hydrogen from gaseous fractions at a point more down stream of the rectifying absorber. A further advantage is that in step (b) part of the sour gasses like H
2
S and HCN are removed with the hydrogen, thereby reducing the amount of corrosive compounds in step (c).
The feed mixture of step (a) is a mixture obtained in a FCC main fractionator comprising gaseous products and saturated and unsaturated hydrocarbons ranging from methane to hydrocarbons having an atmospheric bolting point of about 253° C. and preferably to about 220° C. Apart from hydrogen the gaseous FCC products comprise various components like NH
3
, H
2
S, CO, CO
2
, and H
2
O. The feed mixture has a pressure typically between 11 and 25 bars. The content of hydrogen in the hydrogen containing gaseous fraction will suitably be 3 vol. % or higher. In a typical FCC process the hydrogen content in the hydrogen containing gaseous fraction will be between 5-20 vol. %.
The separation in a liquid and gaseous fraction in step (a) can be achieved by a conventional flash operation, for example in a knock out vessel. It has been found advantageous to reduce the contaminant level (especially NH
3
and H
2
S) of the hydrogen containing gaseous fraction sent to step (b). This leads to a more hydrogen-rich gaseous fraction in step (b). Removal of sour gasses can be achieved by conventional methods. For example by contacting the feed prior to step (a) with water and separating in step (a) the feed mixture into a sour water fraction and the above mentioned hydrocarbon-rich liquid fraction and hydrogen containing gaseous fraction. Such a separation may be suitably performed in a three phase separation vessel.
Step (b) can be performed by making use of conventional membrane separation means, which are known to be selective of separating hydrogen from small hydrocarbons. Selective separation occurs when a pressure gradient across the membrane is applied. Preferably a hydrogen separation selectivity greater than 20, more preferably greater than 50, is required, wherein the selectivity is defined as the permeability ratio of hydrogen over methane. Permeability is defined as the number of moles of a compound which permeates a membrane per square meters per day per bar of pressure difference.
It is also advantageous for the propene recovery in step (c) when membranes are used through which methane and ethane will permeate significantly faster than the heavier hydrocarbons (C
3
+). Preferably the membrane has a methane separation selectivity of greater than 5, wherein the selectivity is defined as the permeability ratio of methane over propane. Suitable membranes should further have a sufficient permeation rate for the hydrogen and should have a sufficient life time. Preferred membranes further show a good resistance to liquid hydrocarbons. The membranes can be made from either inorganic or organic material. Examples of inorganic materials are ceramic, carbon and molecular sieve materials. An example of a ceramic membrane is described in U.S. Pat. No. 5,827,569. Organic membrane materials are preferably of a polymer material, for example polyaramid, polyetherimide and polyimid. Examples of commercial membrane systems which can be used in the process according to the invention are Medal of L'Air Liquide, Prism alpha of Air Products, Polysep of UOP and Membrane Systems (e.g. module B-H) of Ube.
The membrane is suita
Geus Eduard Rudolf
Harris John William
Pek Johan Jan Barend
Caldarola Glenn
Nguyen Tam M.
LandOfFree
Propene recovery does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Propene recovery, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Propene recovery will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3241574