FORM 2
THE PATENTS ACT 1970
(39 OF 1970)
&
THE PATENTS RULES, 2003
COMPLETE SPECIFICATION
(See section 10 and rule 13)
1. TITLE OF THE INVENTION :
"OPTIMIZATION FOR RECOVERY OF RICH GAS FROM CSU"
2. APPLICANT:
(a) NAME: Oil and Natural Gas Corporation Limited
(b) NATIONALITY: India
(c) ADDRESS: Institute of Oil & Gas Production Technology (IOGPT)
Phase-II, Panvel-410221, Navi Mumbai, Maharashtra India
3. PREAMBLE TO THE INVENTION
COMPLETE
The following specification particularly describes the invention and the manner in which it is to be performed.

OPTIMIZATION FOR RECOVERY OF RICH GAS FROM CSU BACKGROUND OF THE INVENTION
1. Field of the Invention:
The present invention relates to a process which enhances additional production of Liquefied Petroleum Gas (LPG) and Naphtha at Condensate Fractionation Unit (CFU) by means of recovery of rich associated gas from Crude Stabilization Units (CSUs) and further compression in off-gas compressors contributing additional condensate formation at off-gas compressors knock out drums which is processed at CFU.
2. Description of Prior Art:
Crude oil is received at a pressure of about 8 to 9 kg/cm2 (g) with a temperature of about 26 to 28 deg C from off-shore and is diverted to individual CSU trains. There are five identical trains of CSUs. Each CSU consists of a Pre-heater, High Pressure Separator, Crude-Crude Exchanger, Crude Heater, Degasser and Dehydrator and Low Pressure Separators.
At present Lean gas is used to maintain the separator pressure as well as to strip the crude oil rich gas components if any of before sending it to storage tanks. The lean gas injected along with the associated CSU gas is routed to CSU off-gas compressors to boost the pressure from 0.3 kg/cm2 to 51 kg/cm2(g) in three stages, viz, i) 0.3 to 3.6 kg/cm2,ii) 3.6 to 15 kg/cm2 and iii 15 to 51 kg/cm2. During compression and cooling, the condensed heavier hydrocarbons from the second stage as well as the third stage are routed along with the drop out condensate from slug catcher to Condensate Fractionation Units (CFU) for further fractionation to LPG

and Naphtha. Using of the lean gas which is predominantly methane (99%) reduces the richness of the CSU off-gas and reduces the condensate generation in the off-gas compressors knock out drums.
Detailed description of the invention:
A simulation model of entire five trains of CSU were developed along with three trains of Off-gas Compressors and CFU to study the various aspects of stripping gas and its effect on condensate generation in compressor knock out drums.
. Various flow parameters of CSU, Off-Gas Compressors and CFU are tabulated below in Table.No.3.1
Presently about 1887 m3/hr of crude is being processed in five trains of CSU and about 0.13 MMSCMD lean gas is injected. The CSU off-gas which is about 0.54 MMSCMD is routed to the Off-gas compressors plant. At the off-gas compressor plant at second stage (15 kg/cm2) & at third stage (51 kg/cm2) condensate generation is in the order of about 6.1 & 6.6 m3/hr. The off-gas compressors condensate along with the slug catcher condensate is routed to CFU for further fractionation into LPG and Naphtha.
From the Table No: 3.1, it is established that the simulation model is almost matched with the field flow parameters.
Table No. 3.1

Model match details
Parameter Field data Simulation data
CSU Average Oil (m3/hr) 1886.5 1890

Lean gas injection, MMSCMD 0.13 0.13
CSU Off-Gas (MMSCMD) 0.54 0.53
Compressor Discharge gas, MMSCMD 0.5 to 0.59 0.59
2nd stage condensate, m3/hr 6.1 6.8
3rd stage condensate, m3/hr 6.6 6.7
Average condensate to CFU, m3/hr 50.8 50.01
3.2. Discharge gas composition of Off-gas compressor plants are tabulated in Table.No.3.2
The CSU off-gas is routed to the Off-gas compressors plant to boost the pressure upto 51 kg/cm2.The following table illustrates the existing discharge gas composition versus the simulation model off-gas compressor discharge gas composition and concludes that the simulation model is almost matched with the gas composition of the field.
Table No. 3.2

Compressor Discharge Gas composition (mole %)
Components Field data Simulated data
C1 69.22 71.95
C2 12.04 11.89
C3 11.85 11.43
iC4 1.98 1.82
nC4 2.41 1.66
C5 + 0.94 0.25
C02 1.17 1.1
N2 0.39 0.4

Average Molecular 1 Weight 23.83 23.09
3.3. Condensate Fractionation Unit off-gas composition is tabulated below in Table.No.3.3.
The off-gas compressors condensate along with the slug catcher condensate is routed to CFU for further fractionation into LPG & Naphtha. The Condensate Fractionation Unit is having two columns. The first column, Light End Fractionation Column removes all the lighter hydrocarbons predominantly Methane and Ethane from the feed. The following table illustrates the existing CFU Off- gas composition versus the simulation model CFU off-gas composition and concludes that the simulation model is almost matched with the gas composition of the field.
Table No. 3.3

CFU Off Gas composition (mole %)
Components Field data Simulated data
C1 53.49 55.61
C2 27.89 25.3
C3 11.60 11:2
iC4 1.74 1.67
nC4 2.11 2.15
iC5 0.37 0.49
nC5 0.30 0.47
C6 + 0.12 0.8
C02 1.97 1.80
N2 0.41 0.5

Average Molecular Weight 25.89 26.4
3.4, Process parameters of CSUs, Off-gas Compressors & CFU plants are tabulated below in Table.No.3.4
At the Crude Stabilization Units, the High pressure separator is being operated at a pressure range of about 3 to 3.5 kg/cm2. The Dehydrator is being operated at a pressure of about 2.5 kg/cm2. The Low Pressure Separator is being operated at a pressure of about 0.1 kg/cm2.
Off-Gas Compressors are three stage reciprocating compressors where the gas is compressed from 0.1 kg/cm2 to 3.6 kg/cm2 in the First stage. The hot gas is passed through the cooler and then routed to second stage suction of the gas compressor where the pressure is boosted to 15 kg/cm2. The hot gas from the discharge side is routed to water coolers to cool the gas to about 40 .deg C and then passed through the second stage discharge knock out drums to remove the condensate formed. The second stage condensate is pumped to CFU. The cooled gas from the top of the second stage discharge knock out drums is routed to third stage suction of the off-gas compressors. The gas is compressed from 15 kg/cm2 to 51 kg/cm 2(g). The hot third stage discharge gas is routed to a water cooler to cool the gas to about 40 deg C and then passed through the third stage discharge knock out drums to remove the condensate formed and further routed to CFU.
Condensate Surge drum at CFU receives the Slug catcher condensate along with the Off-gas compressor discharge knock out drum Condensate which is further pumped into Light End Fractionation Column ( LEF) to remove lighter hydrocarbons The column is operated at a pressure of about 25 kg/cm2 with a top temperature of about 35 deg C. The bottom liquid from the LEF column is routed to LPG Column.

The LPG column is operated at a Pressure of about 10 kg/cm2 with a top temperature of about 55 deg C.
Table No. 3.4

Process Parameters of CSU, Off-gas compressors & CFU
Process Parameter Unit Field value Simulated value
CSU
HP Separator Pressure Kg/cm2(g) 3-3.5 3.3
Dehydrator Pressure Kg/cm2(g) 2.5 2.2
LP Separator Pressure Kg/cm2(g) 0.1 0.1

IHI Compressors
1st stage suction pressure Kg/cm2(g) 0.1 0.1
1st stage discharge pressure Kg/cm2(g) 3.6 3.6
2nd stage discharge pressure Kg/cm2(g) 15 15
3rd stage discharge pressure Kg/cm2(g) 51 51

CFU-II
Stripper over head pressure Kg/cm2(g) 25 25
Stripper over head temperature Deg C 35 38.5
LPG Column over head pressure Kg/cm2(g) 10 10.5
LPG Column over head temperature Deg C 55 53

LPG Column temperature bottom Deg C 156 159
Table Nos 3.1 to 3.4, establishes that Simulated CSU Model, Compressor model and CFU models almost matched with the existing field operating conditions. Hence, the model was used to find the suitable stripping gas and its effect on condensate generation.
Rich gas which is received from Off-shore is processed in LPG plants and the excess rich gas is routed to consumer line before extracting the heavier hydrocarbons along with the lean gas from the LPG plants and C2C3 plant. The lean gas is also used in the CSU for pressure maintenance as well as to strip the heavier hydrocarbons from the crude oil. The detailed gas compositions of rich and lean gas are tabulated below in Table. No: 3.5.
Table No. 3.5

Lean and Rich Gas composition (mole %)
Components Lean gas Rich gas
C1 98.56 82.6
c2 1.12 7.04
c3 0 4.92
iC4 0 0.99
nC4 0 1.31
iC5 0 0.32
nC5 0 0.26
C6 + 0 0.10
CO2 0 2.02

N2 0.32 0.40
Av MW 16.24 20.38
During the invention, in the established simulation model, instead of usage of the lean gas, the rich gas was tried and its effect was analyzed.
3.6 Effect on Separator Pressure Maintenance:
The lean gas is being spiked at High Pressure Separators as well as Low Pressure Separators to maintain the pressures. In the High Pressure Separators, the pressure is maintained in the range of 3 to 3.5 kg/cm2 and in the Low Pressure Separators, the pressure is maintained in the range of 0.1 to 0.3 kg/cm2. In the simulation model also after passing the rich gas instead of lean gas, it was observed that the pressure ranges are very much within the operating envelope.
The following Table No.3.6, establishes the pressure effect with lean gas as well as with Rich gas.
Table No. 3.6

, Simulation results of Pressure Effect of Lean and Rich Gas at CSU
Process Parameter Unit Field value Simulated value
HP Separator Pressure Kg/cm2(g) 3.3 3.3
LP Separator Pressure Kg/cm2(g) 0.1 0.1
From the above Table No: 3.6, it is established that the usage of Rich Gas instead

of Lean gas also maintains the separator pressure during Crude stabilization process.
3.7. Effect on Condensate Formation at Off-Gas knock out drums:
While using the Rich gas instead of Lean gas at CSU, apart from the pressure maintenance, the evolved CSU Off-gas was routed to Off-Gas Compressors to boost the pressure from 0.1 kg/cm2 to 51 kg/cm2 (g). It was observed that at the second and third stage discharge knock out drums, the additional condensate generation was increased to about 21 m3 / day. This experiment establishes that the additional advantage of usage of Rich gas instead of Lean gas. The results are reproduced at Table.No: 3.7.
Table No. 3.7

Simulation results of Effect of Condensate Generation at Off-gas Knock
out drums
Simulation Simulation
data data
Parameter with rich gas with lean gas Gain
injection at CSU injection at CSU
2nd stage condensate, m3/hr 7.33 6.87 0.46
3rd stage condensate, m3/hr 7.15 6.74 0.41
3.8. Effect on LPG & Naphtha production at CFU:
The additionally produced condensate was routed with the slug catcher condensate and the effect on production of LPG & Naphtha at CFU was analyzed. It was observed that by using the additional condensate (which was derived by means of

Rich gas), the additional production of LPG was about 4.2 TPD and the additional production of Naphtha was about 5.6 TPD. The results are tabulated in Table No: 3.8.
Table No. 3.8

Simulation results of Effect on LPG & Naphtha production at CFU
Parameter Simulation data
with rich gas injection at CSU Simulation data
with lean gas injection at CSU Gain
Average condensate to CFU, m3/hr 51.02 50.01 1.01
LPG production, TPD 163.7 159.5 4.2
Naphtha production, TPD 426.98 421.4 5.58
4. FIELD APPLICABILITY:
A 4 inch pipeline of about 700 meters long is needed to inject rich gas from rich gas header to the stripping gas header for CSU at Uran. The effect of rich gas stripping instead of Lean gas at CSU of Uran of ONGC is being implemented for additional condensate production at Off-gas compressors knock out drums and in turn additional LPG and Naphtha production at CFU apart from helping in the pressure maintenance at the Separators.
5.0 Summary of the invention:
Lean gas is used in the onshore oil plants to maintain the separator pressure as well as to strip the rich components if any from the crude oil before sending it to storage tanks. Lean gas which is rich in methane reduces the richness of the CSU off gas and also reduces the condensate generation in the off gas compressors knock out

drums. By using rich gas instead of lean gas the richness of the CSU off gas increases considerably and in the process increases the condensate generation in the CSU gas compressors knock out drums to about 21m3/day. Further processing of this additional condensate helps in increasing the production of LPG and Naphtha atCFU.

BRIEF DESCRIPTION OF THE DRAWINGS:
The above invention will be more clearly understood from the following drawings with the detailed accompanying description:
1. Fig-1 is a schematic diagram of "Crude Stabilization Unit"
2. Fig-2 is a schematic diagram of "CSU Off-gas compressors"
3. Fig-3 is a schematic diagram of "Condensate Fractionation Unit"
4. Fig-4 is a schematic diagram of "Modified process of Crude Stabilization Unit"

We claim:
1. A process for production of condensates from crude oil, the process comprising
the steps of:
a. supplying crude oil feed to a crude stabilization unit injected with rich
associated gas to remove heavier hydrocarbon components from said feed
to obtain crude stabilization unit off gas;
b. compressing the crude stabilization unit off gas received from the crude
stabilization unit in an off gas compressor unit to obtain condensates; and
c. fractionating the condensates in a condensate fractionation unit to obtain a
fractionated product.
2. The process as claimed in claim 1, wherein in step (a), the rich associated gas maintains the crude stabilization unit at an operating high pressure in the range of 3 kg/cm2 to 3.5 kg/cm2 and at an operating low pressure in the range of 0.1 kg/cm2 to 0.3 kg/cm2.
3. The process as claimed in claim 1, wherein in step (b), the crude stabilization unit off gas is compressed at a varying pressure in the range of 0.1 kg/cm2 to 51 kg/cm2.
4. The process as claimed in claims 1 or 3, wherein the crude stabilization unit off gas comprises the rich associated gas.
>
5. The process as claimed in claim 1, wherein in step (b), the condensates are
obtained through three stages.

6. The process as claimed in claim 5, wherein the condensates comprise a first stage condensate produced at a pressure in the range of 0.1 kg/cm2 to 3.6 kg/cm2.
7. The process as claimed in claim 5, wherein the condensates comprise a second stage condensate produced at a pressure in the range of 3.6 kg/cm2 to 15 kg/cm2.
8. The process as claimed in claim 5, wherein the condensates comprise a third stage condensate produced at a pressure in the range of 15 kg/cm2 to 51 kg/cm2.
9. The process as claimed in claim 5 or 6, wherein the first stage condensate is routed to step (a).
10. The process as claimed in claim 5 or 7 or 8, wherein the second stage condensate and the third stage condensate are routed to step (c).
11. The process as claimed in claim 1, wherein in step (c), the condensates comprise a slug catcher condensate.
12. The process as claimed in any one or more of the preceding claims, wherein the fractionated product comprises naphtha.
13. The process as claimed in any one or more of the preceding claims, wherein the fractionated product comprises liquefied petroleum gas.
14. The process as claimed in any one or more of the preceding claims, wherein the yield of the condensates is increased by at least about 2%.

15. The process as claimed in any one or more of the preceding claims, wherein the yield of naphtha is increased by at least about 1.3%.
16. The process as claimed in any one or more of the preceding claims, wherein the yield of liquefied petroleum gas is increased by at least about 2.6%.

OPTIMIZATION FOR RECOVERY OF RICH GAS FROM CSU