# US 5,710,357 A · Process for recovering olefins from cat-cracked gas without accumulating undesirable oxides of nitrogen

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- Number: US 5,710,357 A
- Document ID: US5710357A
- Status: Granted
- Kind: A
- Application number: 08/464,492
- Filed: 1995-06-05
- Granted: 1998-01-20
- Earliest claimed: 1992-10-28
- Assignees: Exxon Chemical Patents Inc.
- Inventors: Dane Clark Grenoble; Roy Thomas Halle; William Douglas Thomson
- Web page: https://patentgrep.com/patent/US5710357A

## CPC

- C10G 70/041: {by distillation}
- C10G 70/06: by gas-liquid contact
- F25J 3/0209: {Natural gas or substitute natural gas}
- F25J 3/0219: {Refinery gas, cracking gas, coke oven gas, gaseous mixtures containing aliphatic unsaturated CnHm or gaseous mixtures of undefined nature}
- F25J 3/0233: {separation of CnHm with 1 carbon atom or more}
- F25J 3/0238: {separation of CnHm with 2 carbon atoms or more}
- F25J 3/0242: {separation of CnHm with 3 carbon atoms or more}
- F25J 3/0247: {separation of CnHm with 4 carbon atoms or more}
- F25J 3/0252: {separation of hydrogen (production of hydrogen containing gas in general C01B3/00, e.g. separation of hydrogen or hydrogen containing gases form gaseous mixtures at low temperatures C01B3/506)}
- F25J 2205/30: using a washing, e.g. "scrubbing" or bubble column for purification purposes
- F25J 2205/50: using absorption, i.e. with selective solvents or lean oil, heavier CnHm and including generally a regeneration step for the solvent or lean oil
- F25J 2210/12: Refinery or petrochemical off-gas
- F25J 2215/62: Ethane or ethylene
- F25J 2215/64: Propane or propylene
- F25J 2220/02: Separating impurities in general from the feed stream
- C10G: CRACKING HYDROCARBON OILS; PRODUCTION OF LIQUID HYDROCARBON MIXTURES, e.g. BY DESTRUCTIVE HYDROGENATION, OLIGOMERISATION, POLYMERISATION (cracking to hydrogen or synthesis gas C01B; cracking or pyrolysis of hydrocarbon gases to individual hydrocarbons or mixtures thereof of definite or specified constitution C07C; cracking to cokes C10B); RECOVERY OF HYDROCARBON OILS FROM OIL-SHALE, OIL-SAND, OR GASES; REFINING MIXTURES MAINLY CONSISTING OF HYDROCARBONS; REFORMING OF NAPHTHA; MINERAL WAXES
- F25J: LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS {OR LIQUEFIED GASEOUS} MIXTURES BY PRESSURE AND COLD TREATMENT {OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE (cryogenic pumps F04B37/08; gas storage vessels, gas holders F17; filing vessels with, or discharging from vessels, compressed, liquefied or solidified gases F17C; refrigeration machines, plants, or systems F25B)}

## Abstract

A safe, effective, and economical method is provided for recovering olefins from cat-cracked gases without accumulating dangerous amounts of nitrogen oxides. A stream of cat-cracked gas first is scrubbed to remove acid gases, including nitrogen dioxide (NO.sub.2), and then is passed through a depropanizer fractionation tower. Hydrocarbons having four or more carbon atoms are recovered in the bottoms of the depropanizer, and the overhead from the depropanizer--which is composed of hydrocarbons having three or fewer carbon atoms--is sent to an absorber demethanizer tower. Hydrocarbons having two or more carbon atoms are recovered in the bottoms from the absorber demethanizer tower, where temperatures are no lower than about -45.56.degree. C. (-50.degree. F.) The overhead from the absorber demethanizer tower--which is composed of methane, hydrogen, and trace amounts of nitrogen oxide, C.sub.2, and absorbent (C.sub.3)--then is chilled to condense and recover trace amounts of C.sub.2 and heavier gases, including trace amounts of the C.sub.3 absorbent, at temperatures of about -101.11.degree. C. (-150.degree. F.) or higher. Thus, recovery of desired hydrocarbons from the cat-cracked gas is conducted at temperatures that are high enough to prevent the oxidation of nitric oxide (NO) to form nitrogen dioxide (NO.sub.2) and high enough to prevent the accumulation of unwanted nitrogen oxides.

## Claims

### Claim 1

A process for preventing the accumulation of undesirable oxides of nitrogen during the recovery of one or more desired hydrocarbons from cat-cracked gas comprising:
passing said gas through an alkaline solution to remove acid gases;
passing said gas through a depropanizer at a temperature between about -28.89.degree. C. (-20.degree. F.) to -40.degree. C. (-40.degree. F.) to form a first portion and a second portion;
passing said first portion through an absorber demethanizer at a temperature above about -45.56.degree. (-50.degree. F.) to form a third portion and a fourth portion;
recovering at least one desired hydrocarbon from said third portion at temperatures above about -106.67.degree. C. (-160.degree. F.).

### Claim 2 (depends on 1)

The process of claim 1 wherein said recovering step comprises chilling said third portion to a temperature between about -78.89.degree. C. (-110.degree. F.) and -101.11.degree. C. (-150.degree. F.) whereby said third portion is separated into a fifth and a sixth portion.

### Claim 3 (depends on 2)

The process of claim 2 wherein said chilling step comprises heat exchanging said sixth portion with said third portion after expansion of said third portion.

### Claim 4 (depends on 3)

The process of claim 3 wherein said expansion is a Joule Thomson expansion.

### Claim 5 (depends on 1)

The process of claim 1 wherein said first portion primarily comprises hydrocarbons having no more than three carbon atoms and said second portion primarily comprises hydrocarbons having at least four carbon atoms.

### Claim 6 (depends on 4)

The process of claim 4 wherein said first portion primarily comprises hydrocarbons having no more than three carbon atoms and said second portion primarily comprises hydrocarbons having at least four carbon atoms.

### Claim 7 (depends on 1)

The process of claim 1 wherein said third portion primarily comprises compounds selected from the group consisting of methane, hydrogen, nitrogen-containing compounds, and a small proportion of hydrocarbons having two and three carbon atoms, and said fourth portion primarily comprises hydrocarbons having at least two carbon atoms.

### Claim 8 (depends on 6)

The process of claim 6 wherein said third portion primarily comprises compounds selected from the group consisting of methane, hydrogen, nitrogen-containing compounds, and a small proportion of hydrocarbons having two and three carbon atoms, and said fourth portion primarily comprises hydrocarbons having at least two carbon atoms.

### Claim 9 (depends on 2)

The process of claim 2 wherein said fifth portion comprises a fraction enriched in hydrocarbons having two and three carbon atoms and said sixth portion primarily comprises compounds selected from the group consisting of hydrogen, methane, and nitric oxide.

### Claim 10 (depends on 8)

The process of claim 8 wherein said fifth portion primarily comprises hydrocarbons having two and three carbon atoms and said sixth portion primarily comprises compounds selected from the group consisting of hydrogen, methane, and nitric oxide.

### Claim 11 (depends on 1)

The process of claim 1 wherein said recovering step comprises absorbing said at least one desired hydrocarbon from said third portion using a hydrocarbon absorbent having more than three carbon atoms.

### Claim 12 (depends on 1)

The process of claim 1 wherein said first portion is passed through said absorber demethanizer at temperatures between about -28.89.degree. C. (-20.degree. F.) and -40.degree. C. (-40.degree. F.).

### Claim 13 (depends on 12)

The process of claim 12 wherein said recovering step comprises chilling said third portion to a temperature between about -78.89.degree. C. (-110.degree. F.) and -101.11.degree. C. (-150.degree. F.) whereby said third portion is separated into a fifth and a sixth portion.

### Claim 14 (depends on 13)

The process of claim 13 wherein said chilling step comprises heat exchanging said sixth portion with said third portion after expansion of said third portion.

### Claim 15 (depends on 14)

The process of claim 14 wherein said expansion is Joule Thomson expansion.

### Claim 16 (depends on 12)

The process of claim 12 wherein said first portion primarily comprises hydrocarbons having no more than three carbon atoms and said second portion primarily comprises hydrocarbons having at least four carbon atoms.

### Claim 17 (depends on 13)

The process of claim 13 wherein said first portion primarily comprises hydrocarbons having no more than three carbon atoms and said second portion primarily comprises hydrocarbons having at least four carbon atoms.

### Claim 18 (depends on 16)

The process of claim 16 wherein said third portion primarily comprises compounds selected from the group consisting of methane, hydrogen, nitrogen-containing compounds, and a small proportion of hydrocarbons having two and three carbon atoms, and said fourth portion primarily comprises hydrocarbons having at least two carbon atoms.

### Claim 19 (depends on 17)

The process of claim 17 wherein said third portion primarily comprises compounds selected from the group consisting of methane, hydrogen, nitrogen-containing compounds, and a small proportion of hydrocarbons having two and three carbon atoms, and said fourth portion primarily comprises hydrocarbons having at least two carbon atoms.

### Claim 20 (depends on 18)

The process of claim 18 wherein said fifth portion primarily comprises a fraction enriched in hydrocarbons having two and three carbon atoms and said sixth portion primarily comprises compounds selected from the group consisting of hydrogen, methane, and nitric oxide.

### Claim 21 (depends on 19)

The process of claim 19 wherein said fifth portion primarily comprises hydrocarbons having two and three carbon atoms and said sixth portion primarily comprises compounds selected from the group consisting of hydrogen, methane, and nitric oxide.

### Claim 22 (depends on 12)

The process of claim 12 wherein said recovering step comprises absorbing said at least one desired hydrocarbon from said third portion using a hydrocarbon absorbent having more than three carbon atoms.

### Claim 23 (depends on 13)

The process of claim 13 wherein said temperature of said third portion is reduced to between about -78.89.degree. C. (-110.degree. F.) and -90.degree. C. (-130-.degree. F.) during said chilling step.

### Claim 24 (depends on 19)

The process of claim 19 wherein said temperature of said third portion is reduced to between about -78.89.degree. C. (-110.degree. F.) and -90.degree. C. (-13020 F.) during said chilling step.

### Claim 25 (depends on 12)

The process of claim 12 wherein said desired hydrocarbon is an olefin.

### Claim 26 (depends on 24)

The process of claim 24 wherein said desired hydrocarbon is an olefin.