DESC:FIELD
The present disclosure relates to the fractional distillation of crude oil.
DEFINITIONS
As used in the present disclosure, the following terms are generally intended to have the meaning as set forth below, except to the extent that the context in which they are used to indicate otherwise.
Off-gases refer to the gaseous by-product obtained from an industrial process.
Stripping in fractional distillation refers to a process of treating a distilled fraction, obtained from fractional distillation, with a stripping medium that leads to reduction in partial vapor pressure of the fractions. The step of stripping is carried out on each distilled fraction (side stream product) immediately after it leaves the main distillation tower.
Side stripping of fractions refers to the stripping of distilled fractions (side stream product) in side drawers in a distillation column.
C3+ hydrocarbons refer to saturated and/or unsaturated hydrocarbons having at least 3 carbon atoms.
Liquefied petroleum gas (LPG) stabilization unit refers to a unit involving C3/C4 separation from unstabilized naphtha.
BACKGROUND
The background information herein below relates to the present disclosure but is not necessarily prior art.
Fractional distillation is the separation of a mixture into its component parts, or fractions. Chemical compounds are separated by heating them to a temperature at which one or more fractions of the mixture will vaporize, by means of fractional distillation. Conventional methods employing steam as a carrier medium in fractional distillation tower generates considerable volume of sour water, which is highly corrosive. Sour water generated in the refineries comprises hydrogen sulfide and ammonia that needs to be removed before the water can be reused in the plant. Further, the use of the steam as the carrier medium increases water consumption, which makes the process energy intensive.
Therefore, there is felt a need for an alternative process for fractional distillation that mitigates the above mentioned drawbacks.
OBJECTS
Some of the objects of the present disclosure, which at least one embodiment herein satisfies, are as follows.
It is an object of the present disclosure to ameliorate one or more problems of the prior art or to at least provide a useful alternative.
Another object of the present disclosure is to provide a simple and economical process for distillation of petroleum fractions.
Yet another object of the present disclosure is to provide an energy efficient process for distillation of petroleum fractions that reduces the reboiling duty and generation of sour water.
Other objects and advantages of the present disclosure will be more apparent from the following description, which is not intended to limit the scope of the present disclosure.
SUMMARY
In one aspect the present disclosure provides a carrier gas composition for stripping at least one fraction obtained from crude oil distillation. The carrier gas composition comprises hydrogen gas in an amount in the range of 2 vol% to 80 vol% of the total composition and a gaseous hydrocarbon mixture in an amount in the range of 20 vol % to 98 vol % of the total composition, wherein the gaseous hydrocarbon mixture comprises hydrocarbons having carbon atoms in the range of 1 to 5. The hydrocarbons are saturated hydrocarbons, unsaturated hydrocarbons or combinations thereof.
In an embodiment, the gaseous hydrocarbon mixture comprises at least one C3+ hydrocarbon in an amount of at least 50 vol % of the total composition.
In an embodiment, the carrier gas composition comprises hydrogen gas in an amount in the range of 2 vol% to 45 vol% of the total composition and a gaseous hydrocarbon mixture in an amount in the range of 55 vol % to 98 vol % of the total composition.
In an embodiment, the saturated hydrocarbons are selected from straight chain alkanes and branched chain alkanes.
In another aspect, the present disclosure provides a process for fractional distillation of crude oil using the carrier gas composition as a stripping medium. The process comprises a step of heating a portion of crude oil to obtain a biphasic mixture comprising a vapor phase and a liquid phase. The biphasic mixture is introduced in the flash zone of a distillation column. The biphasic mixture is fractionated in an upper section of the distillation column above the flash zone, to obtain at least one fraction. The at least one fraction is stripped with a stripping medium comprising the carrier gas composition, to obtain at least one distillate product selected from the group consisting of heavy naphtha (HN), kerosene (kero), light gas oil (LGO) and heavy gas oil (HGO).
In an embodiment, step of stripping is carried out in a side stripping unit.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
The present disclosure will now be described with the help of the accompanying drawing, in which:
Figure 1 illustrates the apparatus (100) employed to carry out the process of the present disclosure;
Figure 2A illustrates comparative side stripping of heavy gas oil (HGO) fraction using refinery off gases (ROG) and steam as the stripping medium;
Figure 2B illustrates comparative side stripping of light gas oil (LGO) fraction using ROG and steam as the stripping medium;
Figure 3A illustrates comparative side stripping of kerosene (kero) fraction using ROG and steam as the stripping medium;
Figure 3B illustrates comparative side stripping of heavy naphtha side cut (HNSC) fraction using ROG and steam as the stripping medium;
Figure 4 illustrates side stripping of kero fraction with off-gases obtained from various industrial processes;
Figure 5 illustrates side stripping of kero fraction with different carrier gas composition in accordance with the present disclosure;
Figure 6 illustrates the change in overhead condenser temperature in side stripping of kero fraction with different carrier gas composition in accordance with the present disclosure; and
Figure 7 illustrates the change in overhead duty in side stripping of kero fraction with different carrier gas composition in accordance with the present disclosure.
DETAILED DESCRIPTION
Embodiments, of the present disclosure, will now be described with reference to the accompanying drawing.
Embodiments are provided so as to thoroughly and fully convey the scope of the present disclosure to the person skilled in the art. Numerous details are set forth, relating to specific components, and methods, to provide a complete understanding of embodiments of the present disclosure. It will be apparent to the person skilled in the art that the details provided in the embodiments should not be construed to limit the scope of the present disclosure. In some embodiments, well-known processes, well-known apparatus structures, and well-known techniques are not described in detail.
The terminology used, in the present disclosure, is only for the purpose of explaining a particular embodiment and such terminology shall not be considered to limit the scope of the present disclosure. As used in the present disclosure, the forms "a,” "an," and "the" may be intended to include the plural forms as well, unless the context clearly suggests otherwise. The terms "comprises," "comprising," “including,” and “having,” are open ended transitional phrases and therefore specify the presence of stated features, integers, steps, operations, elements, modules, units and/or components, but do not forbid the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. The particular order of steps disclosed in the method and process of the present disclosure is not to be construed as necessarily requiring their performance as described or illustrated. It is also to be understood that additional or alternative steps may be employed.
The terms first, second, third, etc., should not be construed to limit the scope of the present disclosure as the aforementioned terms may be only used to distinguish one element, component, region, layer or section from another component, region, layer or section. Terms such as first, second, third etc., when used herein do not imply a specific sequence or order unless clearly suggested by the present disclosure.
Conventional separation of the components of crude oil utilize steam as a carrier medium for stripping distilled fractions obtained from distillation, thus resulting in sour water generation. The sour water needs to be treated for removal of hydrogen sulfide and ammonia present therein, before re-using the water in the plant. Such treatment processes increase the overall operating cost of the distillation unit. Further, the fractional distillation process known in the art, that employ non-steam carrier media, reduces the generation of sour water, however, such processes make use of gas compressor and gas cooling system, which increases the capital cost.
The present disclosure provides a simple and economical process for fractional distillation of crude oil by using carrier gas composition instead of conventionally used steam. The process is energy efficient and reduces the quantity of sour water generation.
In one aspect the present disclosure provides a carrier gas composition for stripping at least one fraction obtained from crude oil distillation. The carrier gas composition comprises hydrogen gas in an amount in the range of 2 vol% to 80 vol% of the total composition and a gaseous hydrocarbon mixture in an amount in the range of 20 vol % to 98 vol % of the total composition, wherein the gaseous hydrocarbon mixture comprises hydrocarbons having carbon atoms in the range of 1 to 5. The hydrocarbons are saturated hydrocarbons, unsaturated hydrocarbons or combinations thereof.
In an embodiment, the gaseous hydrocarbon mixture comprises at least one C3+ hydrocarbon in an amount of at least 50 vol % of the total composition.
The saturated hydrocarbons are selected from straight chain alkanes and branched chain alkanes. In an embodiment, the saturated hydrocarbons are selected from the group consisting of methane, ethane, propane, isobutane, n-butane, isopentane and n-pentane.
In an embodiment, the carrier gas composition comprises hydrogen gas in an amount in the range of 2 vol% to 45 vol% of the total composition and, a gaseous hydrocarbon mixture in an amount in the range of 55 vol % to 98 vol % of the total composition, wherein the gaseous hydrocarbon mixture comprises at least one C3+ hydrocarbon in an amount of at least 50 vol % of the total composition.
In an embodiment, the carrier gas composition comprises hydrogen gas in an amount in the range of 3 vol % to 42 vol % of the total composition and a gaseous hydrocarbon mixture comprising methane in an amount in the range of 2 vol % to 40 vol% of the total composition, ethane in an amount in the range of 4 vol % to 8 vol % of the total composition, propane in an amount in the range of 10 vol % to 35 vol % of the total composition, isobutane in an amount in the range of 1 vol % to 7 vol% of the total composition, n-butane in an amount in the range of 5 vol% to 15 vol % of the total composition, isopentane in an amount in the range of 0.5 vol % to 1 vol % of the total composition, and n-pentane in an amount in the range of 0.5 vol % to 1 vol % of the total composition,.
It is desired that the dew point of the overhead vapor be greater than 30°C so as to enable condensation by heat exchange with cooling water in conjunction with air cooling. The variation in amount of C3+ hydrocarbon enables to control the overhead vapor dew point while variation in hydrogen gas content affects the control stripping efficiency of the carrier gas. The amount of H2 and C3+ in the composition determines tradeoff between stripping efficiency and overhead condenser temperature and in turn the condenser duty.
In an embodiment, the carrier gas composition comprises at least 50 vol % of C3+ hydrocarbons. With the presence of 50 vol% or more of the C3+ hydrocarbons, overhead condenser temperature (dew point) increases thereby leading to decrease in the condenser’s duty.
However, the condenser temperature is inversely proportional to hydrogen gas content in the stripping gas. In addition, the stripping efficiency is directly proportional to hydrogen gas content in the carrier gas composition. The reduction in the amount of hydrogen gas will require higher amount of gas for stripping.
Therefore, the best composition for carrier gas comprises optimum amounts of hydrogen gas and C3+ hydrocarbons.
In an exemplary embodiment, the carrier gas composition comprises hydrogen gas in an amount of 40.2 vol % of the total composition, and a gaseous hydrocarbon mixture comprising methane in an amount of 3.5 vol % of the total composition and ethane in an amount 4.5 vol % of the total composition, propane in an amount of 30.7 vol % of the total composition, isobutane in an amount of 6.4 vol % of the total composition, n-butane in an amount of 13.5 vol % of the total composition, isopentane in an amount of 0.6 vol % of the total composition, and n-pentane in an amount of 0.6 vol % of the total composition,.
The carrier gas composition is obtained from at least one source selected from the group consisting of refinery off gas, continuous catalytic reforming off gas, coker heavy gas oil hydro treating unit (CHTU) off gas, delayed coker off gas, deep catalytic cracker off gas and H2/C1-C5 gas mixture from hydrocarbon industrial process. In an exemplary embodiment, the carrier gas composition is obtained from coker heavy gas oil hydro treating unit (CHTU) off gas.
In an embodiment, the composition of the carrier gas is varied by a predetermined value, by introducing at least one C3+ hydrocarbon, to the carrier gas composition, wherein the at least one C3+ hydrocarbon is obtained from the liquefied petroleum gas (LPG) stabilization unit, LPG vapor or C3+ hydrocarbon vapor from hydrocarbon industrial processes.
In another aspect, the present disclosure provides a process for fractional distillation of crude oil using carrier gas as a stripping medium. The process is carried out using the apparatus as shown in Figure 1.
The apparatus comprises a desalter (102), a pre-flash drum (104), a charge heater (106), a distillation column (101), side stripping units (110), pump-around circuits (115), overhead receiver and condenser (117), LPG stabilizer unit (120) and a plurality of heating units (not shown in the figure). The distillation column comprises a flash zone (108) and a plurality of plates above as well as below the flash zone.
The process comprises a step of heating a portion of crude oil, to obtain a biphasic mixture comprising a vapor phase and a liquid phase. The biphasic mixture is introduced in the flash zone (108) of a distillation column (101).
In an embodiment, step of heating comprises heating the portion of crude oil in a pre-flash drum, to obtain liquid and vapors, wherein the vapors are directed to the flash zone (108).
In an embodiment, the liquid is heated in a charge heater (106), to a temperature in the range of 340 °C to 370 °C to obtain the biphasic mixture.
In an embodiment, prior to step of heating, the crude oil is treated in a desalter (102) to remove inorganic water soluble salts. The crude oil is mixed with water in the desalter (102) and separated using electrostatic forced aided gravity settling.
In the next step, the biphasic mixture is fractionated in an upper section of the distillation column above the flash zone (108), to obtain at least one fraction. The at least one fraction is stripped with a stripping medium comprising the carrier gas composition, to obtain at least one distillate product selected from the group consisting of heavy naphtha side cut (HNSC), kerosene (kero), light gas oil (LGO) and heavy gas oil (HGO).
Typically, the carrier gas composition comprises hydrogen gas in an amount in the range of 2 vol% to 80 vol% of the total composition and a gaseous hydrocarbon mixture in an amount in the range of 20 vol % to 98 vol % of the total composition, wherein the gaseous hydrocarbon mixture comprises hydrocarbons having carbon atoms in the range of 1 to 5. The hydrocarbons are saturated hydrocarbons, unsaturated hydrocarbons or combinations thereof.
In an embodiment, the gaseous hydrocarbon mixture comprises at least one C3+ hydrocarbon in an amount of at least 50 vol % of the total composition.
In an embodiment, the carrier gas composition comprises hydrogen gas in an amount in the range of 2 vol% to 45 vol% of the total composition and, a gaseous hydrocarbon mixture in an amount in the range of 55 vol % to 98 vol % of the total composition, wherein the gaseous hydrocarbon mixture comprises at least one C3+ hydrocarbon in an amount of at least 50 vol % of the total composition. The hydrocarbons are saturated hydrocarbons, unsaturated hydrocarbons or combinations thereof.
In an embodiment, the carrier gas composition comprises hydrogen gas in an amount in the range of 3 vol % to 42 vol % of the total composition, and a gaseous hydrocarbon mixture comprising methane in an amount in the range of 2 vol % to 40 vol% of the total composition, ethane in an amount in the range of 4 vol % to 8 vol % of the total composition propane in an amount in the range of 10 vol % to 35 vol % of the total composition, isobutane in an amount in the range of 1 vol % to 7 vol% of the total composition, n-butane in an amount in the range of 5 vol% to 15 vol % of the total composition, isopentane in an amount in the range of 0.5 vol % to 1 vol % of the total composition, and n-pentane in an amount in the range of 0.5 vol % to 1 vol % of the total composition.
In an exemplary embodiment, the carrier gas composition comprises hydrogen gas in an amount of 40.2 vol % of the total composition, and a gaseous hydrocarbon mixture comprising methane in an amount of 3.5 vol % of the total composition, ethane in an amount 4.5 vol % of the total composition propane in an amount of 30.7 vol % of the total composition, isobutane in an amount of 6.4 vol % of the total composition, n-butane in an amount of 13.5 vol % of the total composition, isopentane in an amount of 0.6 vol % of the total composition, and n-pentane in an amount of 0.6 vol % of the total composition.
Stripping in fractional distillation refers to a process of treating a distilled fraction, obtained from fractional distillation, with a stripping medium that leads to reduction in partial vapor pressure of the fractions. The step of stripping is carried out on each distilled fraction (side stream product) immediately after it leaves the main distillation tower.
In an embodiment, step of stripping is carried out in a side stripping unit (110).
In an exemplary embodiment, step of stripping is carried out in a single side stripping unit. In an embodiment, the step of stripping is carried out in a plurality of side stripping units, wherein each unit is used for stripping of one fraction.
In an embodiment, the at least one fraction is selected from the group consisting of heavy naphtha side cut (HNSC), kerosene (kero), light gas oil (LGO) and heavy gas oil (HGO).
The carrier gas is obtained from at least one source selected from the group consisting of refinery off gas, continuous catalytic reforming off gas, coker heavy gas oil hydro treating unit (CHTU) off gas, delayed coker off gas, deep catalytic cracker off gas and H2/C1-C5 gas mixture from hydrocarbon industrial process. In an embodiment, the carrier gas is obtained from coker heavy gas oil hydro treating unit (CHTU) off gas.
The off-gases used as carrier gas for stripping cause reduction in partial pressure of vapors in fractional distillation of crude oil fractions. This provides a relatively more energy efficient means of separation than steam stripping. Further, the quantity of sour water generated was reduced.
Typically about 0.10 - 0.15 kg of steam/ kg of sour water is used for stripping off H2S/NH3 from sour water generated in the overhead section of crude oil distillation column.
In an embodiment, the composition of the carrier gas is varied to a predetermined value, by introducing at least one C3+ hydrocarbon, to the carrier gas composition.
The C3+ hydrocarbon is obtained from the liquefied petroleum gas (LPG) stabilization unit, LPG vapor or C3+ hydrocarbon vapor from hydrocarbon industrial processes.
In an embodiment, the at least one C3+ hydrocarbon is obtained from liquefied petroleum gas (LPG) stabilization unit (120). In another embodiment, the at least one C3+ hydrocarbon is obtained from other suitable sources. In yet another embodiment, the at least one C3+ hydrocarbon is obtained from liquefied petroleum gas (LPG) stabilization unit (120) and other suitable sources.
The LPG stabilizers involve C3/C4 separation from unstabilized naphtha, wherein the overhead section (117) of the column condense C3/C4 rich vapors in the overhead condenser and route the liquid LPG for downstream LPG cleaning sections. The overhead reflux drums typically have a route up to fuel gas system for pressure control. The partial condensation in a given C3/C4 splitter overhead condenser and thereby routing the necessary C3/C4 rich vapor to carrier gas is leveraged to control C3+ content in the gas required for stripping.
In an embodiment, the step of stripping is carried out by using a mixture of two variable carrier gas compositions.
The reboiling duty saved during stripping of fractions with carrier gas is used to improve the preheat temperature of crude before entering the charge heater (106). This will reduce the heat requirement of the charge heater (106). Alternatively, the reboiling duty saved can also be used to generate steam which has high utility in industrial process for heating purposes and hence can assist in saving the equivalent heat requirement otherwise required to generate the same steam quantity. All these aspects make the overall process economical and energy efficient.
Further, the process of the present disclosure does not require significant change in column diameter of side stripping unit in comparison to the conventional steam stripping.
Thus, the process of the present disclosure is simple, economical and energy efficient that results in generation of relatively lesser amount of sour water.
The foregoing description of the embodiments has been provided for purposes of illustration and not intended to limit the scope of the present disclosure. Individual components of a particular embodiment are generally not limited to that particular embodiment, but, are interchangeable. Such variations are not to be regarded as a departure from the present disclosure, and all such modifications are considered to be within the scope of the present disclosure.
The present disclosure is further described in light of the following laboratory scale experiments which are set forth for illustration purpose only and not to be construed for limiting the scope of the disclosure. These experiments can be scaled up to industrial/commercial scale and the results obtained can be extrapolated to industrial/commercial scale.
Experiment 1: Fractional distillation of crude oil in accordance with the present disclosure
Crude oil was heated in a heat exchange network and then introduced in a desalter to remove water soluble salts, wherein crude oil was mixed with water and separated using electrostatic forced aided gravity settling. The crude oil was then introduced in a pre-flash drum which generated vapours and liquid. The vapors were directed to a flash zone of a distillation column, whereas the liquid was further heated at 350 °C, to obtain a biphasic mixture comprising vapour phase and liquid phase. The biphasic mixture was introduced in a flash zone of a distillation column. The biphasic mixture was fractionated in an upper section above the flash zone to obtain fractions comprising unstabilized naphtha, heavy naphtha side cut (HNSC), kerosene (Kero), light gas oil (LGO) and heavy gas oil (HGO). The fractions were stripped with a carrier gas composition in a side stripping unit to obtain distillate product comprising unstabilized naphtha, heavy naphtha side cut (HNSC), kerosene (Kero), light gas oil (LGO) and heavy gas oil (HGO). The experimental details, optimization data and other details are discussed as follows.
The details of the distillation column, with trays numbered from bottom to top are as provided in Table 1
Table 1: Details of distillation column used in the process of the present disclosure
Sr. No. Column configuration aspects Number
1 Crude throughput, m3/h 1200
2 No of trays 56
3 Feed entry tray 10
4 Heavy gas oil (HGO) Pump Around (PA) draw tray 19
5 HGO Pump Around return tray 21
6 HGO product draw tray 14
7 HGO stripper vapor return tray 16
8 HGO stripping medium Carrier gas
9 HGO Stripper column trays 4
10 LGO PA draw tray 28
11 LGO PA return tray 30
12 LGO product draw tray 22
13 LGO stripper vapor return tray 23
14 LGO stripping medium Carrier gas
15 LGO Stripper column trays 4
16 Kero PA draw tray 41
17 Kero PA return tray 43
18 Kero Product draw tray 31
19 Kero Stripper return tray 33
20 Kero stripping medium Carrier gas
21 Kero Stripper column trays 6
22 Heavy naphtha side cut (HNSC) draw tray 44
23 HNSC stripper vapor return tray 46
24 HNSC stripping medium Carrier gas
25 Unstablised naphtha PA draw tray 55
26 Unstablised naphtha PA return tray 52

The carrier gas composition was obtained as off-gases from various refinery processes, the composition of which is as provided in Table 2.
Table 2: Typical composition of various off gases obtained from refinery processes
Components Refinery Off Gas (ROG) CCR Off Gas CHTU FG Delayed Coker Offgas Deep Catalytic Cracker offgas
H2 16.4% 54.1% 70.3% 11.1% 13.2%
N2 0.1% 0.0% 0.0% 0.0% 6.1%
CH4 36.9% 10.7% 5.9% 57.7% 24.9%
C2H6 16.8% 11.7% 7.9% 24.3% 10.6%
C2H4 17.1% 0.7% 0.0% 4.0% 25.8%
C3H8 2.2% 12.5% 8.9% 0.6% 1.8%
C3H6 6.9% 0.8% 0.0% 1.3% 14.5%
i-C4H10 1.1% 5.3% 1.0% 0.2% 0.7%
n-C4H10 1.5% 3.4% 4.0% 0.3% 0.3%
1trans2-C4H8 0.1% 0.0% 0.0% 0.0% 0.4%
1-C4H8 0.1% 0.0% 0.0% 0.0% 0.3%
i-C4H8 0.1% 0.1% 0.0% 0.0% 0.6%
cis2-C4H8 0.1% 0.0% 0.0% 0.0% 0.4%
i-C5H12 0.4% 0.6% 1.0% 0.2% 0.4%
n-C5H12 0.2% 0.2% 1.0% 0.1% 0.1%
1,3Butadiene 0.0% 0.0% 0.0% 0.0% 0.0%
Molecular weight 22.19 18.01 12.99 19.35 25.18

Comparison data for side stripping of various fractions using RGO carrier gas or steam as the stripping media
Heavy gas oil (HGO) fraction’s effect of ASTM D86 5 vol% distillation by using ROG as carrier gas
The effect of ROG as the stripping medium in HGO fraction of ASTM D86 5 vol% distillation, was investigated, and the results were compared with the corresponding distillation fractions stripped with steam, as shown in Figure 2A. This was used as an optimization experiment for carrying out the distillation of other fractions including light gas oil (LGO), kerosene (kero), and HNSC as depicted in Figures 2B, 3A and 3B respectively.
As observed from Figures 2A, 2B, 3A and 3B, it was observed that the gas flow rate was lower in case of ROG stripping than steam stripping. Further, it was found that 32000 kg/h of neat ROG is required so as to attain 5vol% distillation conditions as per reference design.
In case of reduced coke oil (RCO) which is processed in the section below the flash zone, the ROG requirement was found to be reduced to half by using a minimum steam of 2000 kg/h as co-stripping media.
The optimal conditions for the use of ROG carrier gas as stripping media for each crude fraction distilled from atmospheric distillation column is as summarized in the Table 3 below:
Table 3: Reaction conditions for use of ROG as a stripping medium for distilled crude oil fractions
Distilled fraction Reference 5vol% ASTM D86 distillation, °C Stripping ROG optimal rate, kg/h Stripping steam optimal rate, kg/h
HNSC 137.8 500 830*
Kero 181.1 2000 1800*
LGO 234.9 600 500
HGO 263.4 600 500
RCO 324.4 32000 6000
15000 ROG+2000 Steam 6000
*side strippers are reboiled in reference design, steam stripping rate depicted for comparison purpose
The effect of variation in stripping media is reflected only in overhead condenser duty since the PA rate and PA temperature difference were fixed. The duty savings with ROG stripping for each crude fraction distilled from atmospheric distillation column is as summarized in the Table 4 below:
Table 4: Reaction conditions for use of ROG as a stripping medium for distilled crude oil fractions
Crude Oil Fraction Overhead Condenser duty, MMkcal/h ?Duty with respect to reference, MMkcal/h
Reference CDU with steam stripping 38.6
HNSC stripping with ROG 39.2 0.66
Kero stripping with ROG 39.9 1.31
LGO stripping with ROG 39.1 0.58
HGO stripping with ROG 39.1 0.56
RCO stripping with ROG 46.1* 7.56
42.4** 3.83
*RCO stripped with neat ROG of 32000 kg/h
**RCO stripped with ROG of 15000 kg/h along with steam of 2000kg/h
The overall benefit of the scheme is gauged after factoring the reboiling or stripping steam saved, after due considerations for effect of other parameters such as composition of the stripping or carrier gas, and the like.
The net energy savings due to employment of ROG as stripping media is as provided in Table 5.
Table 5: Reaction conditions for use of ROG as a stripping medium for distilled crude oil fractions
Entry No. ROG stripping for Additional overhead duty, MMkcal/h Stripping steam/ Reboiling duty saved, MMkcal/h Net energy saved, MMkcal/h Sour water reduction, kg/h
1 HNSC 0.66 0.69 0.03 0
2 Kero 1.31 4.80 3.49 0
3 LGO 0.58 0.36 -0.22 475
4 HGO 0.56 0.36 -0.20 426
5 RCO* 7.56 4.34 -3.22 5722
6 RCO** 3.83 2.89 -0.94 3969
*RCO stripped with neat ROG of 32000 kg/h
**RCO stripped with ROG of 15000 kg/h along with steam of 2000kg/h
In case of entries 1 and 2 in Table 5, the reference CDU involved reboiling of side strippers for HNSC and Kero fractions, there was no sour water reduction despite offering considerable energy savings. However in other CDU design, these streams were stripped with steam followed by coalescers and driers to remove moisture. In those designs, the stripping steam requirement was 830 kg/h and 1800 kg/h for HNSC and Kero respectively. During such cases, the expected reduction in sour water was 772 kg/h and 1659 kg/h respectively.
With respect to reference CDU design, reboiling duty saved in HNSC and Kero side strippers due to ROG stripping was used in the heat exchange network to increase the crude temperature preheat which in turn reduced the heat requirement of the charge heater. This provides a technical advancement over the conventional processes.
Further, it was observed that with increase in ROG requirement, the overhead condenser temperature dropped. This is because of reduction in dew point of the overhead vapors with increase in ROG flow. The overhead condenser temperature of CDU designed with ROG as stripping media for each of the crude cuts is presented below in Table 6.
Table 6: Overhead condenser temperature
CDU design configuration Overhead Condenser Temperature, °C
Reference CDU with steam stripping 46.1
HNSC stripping with ROG 36.0
Kero stripping with ROG 23.7
LGO stripping with ROG 34.8
HGO stripping with ROG 35.2
RCO stripping with ROG -0.4*
2.9**
*RCO stripped with neat ROG of 32000 kg/h
**RCO stripped with ROG of 15000 kg/h along with steam of 2000kg/h
To mitigate the reduction in condenser temperature requirement, conventionally, operating variables are varied that increase the distillation column operating pressure to cool the water or employment of coolants such as propylene instead of cooling water for the same column operating pressure. The increase in pressure causes relatively high energy consumption and usage of cold streams like propylene requires dedicated system analogous to that of refrigeration system, which is capital intensive. The present disclosure avoids these limitations and provides a remedy which is attractive in terms of both CAPEX and OPEX, while reducing the impact of overhead condenser temperature.
Comparative effect of various fuel gases as carrier gas:
In order to optimize the gas composition, various fuel gases were tested for side stripping. Since kero fraction requires more stripping gas relative to other crude cuts for attaining the same levels of 5% ASTM D86 boiling point, so in order to test the efficiency of stripping with varying gas composition, kero fraction is selected and preliminary optimization studies were carried out. The gases tabulated in Table 2 have been employed for stripping and the results are presented in Figure 4.
As observed in Figure 4, the stripping efficiency of CHTU fuel gas is relatively higher than the other gases while the dew point of the overhead vapors is lowest for the same. Also, the highest dew point is observed to be highest for PFCC off gas while the carrier gas required to achieve the given level of stripping is relatively higher. Thus various fuel gases compositions (derived from CHTU fuel gas) were inspected to understand the reasons for variation in stripping efficiency and overhead vapor dew points.
Optimization of carrier gas composition:
In accordance with the conclusion derived from Figure 4, CHTU fuel gas was chosen. Further, CHTU FG had highest H2 content while PFCC FG had highest C2+ and C3+ molecules. To understand the effect of these components, CHTU fuel gas was chosen, and the amount of H2, C2+ and C3 (vol%) were varied as in the Table 7 below to obtain carrier gas compositions (GC1-GC10) containing hydrogen gas and a gaseous hydrocarbon mixture (comprising C1-C5 hydrocarb0ns)

Table 7: Carrier gas composition
Composition(vol%) GC1 GC2 GC3 GC4 GC5 GC6 GC7 GC8 GC9 GC10
H2 68.8 67.4 61.1 55.1 49.0 40.2 30.2 20.2 10.0 3.4
CH4 5.8 5.7 5.2 4.7 4.1 3.4 13.5 23.5 33.7 40.2
C2H6 7.8 7.6 6.9 6.2 5.5 4.5 4.5 4.5 4.5 4.5
C3H8 10.0 11.0 15.6 19.9 24.4 30.7 30.7 30.7 30.7 30.7
I-C4H10 1.3 1.5 2.7 3.7 4.8 6.4 6.4 6.4 6.4 6.4
N-C4H10 4.4 4.9 6.9 8.8 10.7 13.5 13.5 13.5 13.5 13.5
I-C5H12 1.0 0.9 0.9 0.8 0. 0.6 0.6 0.6 0.6 0.6
N-C5H12 1.0 0.9 0.9 0.8 0.7 0.6 0.6 0.6 0.6 0.6
Summary, vol% GC1 GC2 GC3 GC4 GC5 GC6 GC7 GC8 GC9 GC10
H2 68.8 67.4 61.1 55.1 49.0 40.2 30.2 20.2 10.0 3.4%
C1 5.8 5.7 5.2 4.7 4.1 3.4 13.5 23.5 33.7 40.2
C2 7.8 7.6 6.9 6.2 5.5 4.5 4.5 4.5 4.5 4.5
C3 10.0 11.0 15.6 19.9 24.4 30.7 30.7 30.7 30.7 30.7
C4 5.7 6.4 9.6 12.5 15.6 19.9 19.9 19.9 19.9 19.9
C5+ 1.9 1.9 1.7 1.6 1.4 1.1 1.1 1.1 1.1 1.1

Further, for the same kero fraction D86 5vol% distillation temperature, the carrier gas requirement, condenser temperature (overhead vapor dew point) and condenser duty were studied. The results are as presented in Figure 5, 6 and 7.
As observed in Figures 5 to 7, it can be clearly understood that stripping efficiency is directly proportional to hydrogen gas content in the carrier gas. Also, the condenser temperature is inversely proportional to hydrogen gas content in the stripping gas. Interestingly, with increase in C3+ content, overhead condenser temperature (dew point) increased thereby leading to decrease in the condenser’s duty.
It is desired that the dew point of the overhead vapor be greater than 30°C so as to enable condensation by heat exchange with cooling water in conjunction with air cooling. The amount of C3+ is varied to control the overhead vapor dew point while H2 content is varied to control stripping efficiency of the carrier gas. The amount of H2 and C3+ in the composition determines tradeoff between stripping efficiency and overhead condenser temperature and in turn the condenser duty.
The preferred C3+ components vol% is more than 50%. The extent of H2 content in the same determines the stripping efficiency and marginally affects the overhead vapor temperature. Comparison between GC6 and GC7 data demonstrates the same. The preferred H2 composition is between 10-40 vol% in conjunction with C3+vol% of minimum 50 vol% above for effective tradeoff between stripping efficiency and condenser temperature. In case of compositions such as GC10, wherein the H2 composition in the carrier gas is as low as 3.4 vol%, it was observed that in order to maintain the overhead condenser temperature greater than 30°C, a higher carrier gas quantity was needed to attain the same stripping effect as compositions having relatively higher quantity of hydrogen.
In conclusion, hydrogen rich carrier gas provided good stripping efficiency whereas C3+ content helped to increase the overhead condenser’s temperature (by increasing the vapor’s dew point) in an atmospheric distillation column.
Effect of Simultaneous gas stripping:
The effect of stripping off more than one crude oil fractions simultaneously with carrier gas was investigated. For demonstrating the merit of claim on gas compositions, ROG or GC6 were used as the stripping media and the resulting data were compared. The combinations studied are simultaneous LGO & HGO stripping, simultaneous HNSC & Kero stripping and simultaneous HNSC, Kero, LGO & HGO stripping, using ROG as carrier gas as depicted in Table 8.
Table 8: Energy data and sour water generation – Simultaneous fractions ROG stripping
ROG stripping for ? Overhead duty, MMkcal/h Stripping steam/ Reboiling duty saved, MMkcal/h Net energy saved, MMkcal/h Sour water reduction, kg/h
HNSC & Kero 1.4 4.5 3.1 0
LGO & HGO 0.7 0.7 0.0 904
HNSC, Kero, LGO & HGO 1.1 6.2 5.1 901

Table 9 Stripping gas flow and condenser temperature – Simultaneous fractions ROG stripping
ROG stripping for Stripping gas flow, kg/h Overhead condenser temperature, °C
HNSC Kero LGO HGO
HNSC & Kero 500 2000 - - 21.2
LGO & HGO - - 600 600 29.2
HNSC, Kero, LGO & HGO 500 2000 600 600 17.7

As observed in Tables 8 and 9, it is clear that stripping using carrier gas in all side draw fractions in an atmospheric column is energy efficient. However, the overhead condenser temperature drops with increase in carrier gas flow as the stripping media.
For comparison, GC6 gas with around 40 vol% of H2 and greater than 50 vol% of C3+ content is employed for simultaneous stripping of crude oil fractions that are side stripped in a typical crude distillation column as presented in Tables 10 and 11.
Table 10: Energy and sour water data – Simultaneous fractions GC6 stripping
GC6 stripping for Column Top Pressure, kg/cm2 ?Overhead duty, MMkcal/h Stripping steam/ Reboiling duty saved, MMkcal/h Net energy saved, MMkcal/h Sour water reduction, kg/h
HNSC &Kero 2.4 0.7 5.5 4.8 53
LGO & HGO 2.4 0.3 0.7 0.4 932
HNSC, Kero, LGO & HGO 2.4 -0.1 6.2 6.3 1063

Table 11: Stripping gas flow & condenser temperature – Simultaneous fractions GC6 stripping
GC6 stripping for Stripping gas flow, kg/h Overhead condenser temperature, °C
HNSC Kero LGO HGO
HNSC &Kero 850 3200 - - 35.5
LGO & HGO - - 900 900 36.5
HNSC, Kero, LGO & HGO 1500 3200 900 900 39.1

As observed in Tables 10 and 11, GC6 demonstrated net energy savings with condenser temperatures above 30°C suitable for cooling in tropical conditions with cold utility in ambient conditions. Therefore, the employment of appropriate gas composition for stripping in side stripping unit of atmospheric distillation column as per the claim does not warrant increase in atmospheric distillation column pressure or usage of refrigerants like propylene for overhead condenser.
Impact of hydrocarbon gas side-stripping on column diameter:
In simultaneous stripping of crude fractions in side strippers with carrier gas composition, distillation column’s vapor-liquid traffic were altered with respect to the reference design, for understanding the need for change in column diameter of side stripping unit. The diameter calculations for reference design configurations were calculated with vapor-liquid traffic obtained from its process simulation. Similarly, for demonstration purpose GC6 gas composition was selected, and its vapor-liquid traffic while stripping all four crude oil fractions (HGO, LGO, Kero, HNSC) with GC6 in side strippers was used for comparing diameter calculations.
For a typical hole diameter of sieve tray and tray spacing employed in crude distillation, based on the vapor-liquid traffic and the corresponding transport properties, c-factor has been calculated using the Kister and Haas Correlation as provided below:
C_SB= 0.144[(d_H^2 s)/?_L ]^0.125 [?_G/?_L ]^0.1 v(S/h_ct )
wherein, dH = Hole diameter;
S = Tray spacing;
hct = Clear Liquid height at the transition from the froth to spray regime, in of liquid;
s = Surface tension (in dyne/cm);
?L = Liquid density; and
?L = Vapor density
From the c-factor calculated above, diameters were calculated for each of the fractionation section. The relative increase in column diameter as per the present process using carrier gas stripping over conventionally used column in steam stripping are tabulated and presented in Table 12:

Table 12: Impact on distillation column diameter

Crude Fraction Trays in Column % Increase in Column Diameter w.r.t. reference design
Unstabilised naphtha 52-56 1.4%
HNSC 44-51 1.4%
Kero 31-43 0.7%
LGO 22-30 0.1%
HGO 14-21 -
Feed 10-13 -
RCO 1-9 -
As observed in Table 12, it is clear that the design of the columns for fractionation of crude oil in the present process falls within practical margins, wherein it is known that the conventional crude distillation columns are designed with 10-20% margin. Hence, the distillation process of the present disclosure is relatively more economical and is not capital intensive.
The process of the present disclosure is simple, economical and energy efficient that results in generation of relatively lesser amount of sour water.
TECHNICAL ADVANCEMENTS
The present disclosure described herein above has several technical advantages including, but not limited to, the realization of a process for fractional distillation of crude oil:
that is simple and economical;
that is more energy efficient and has decreased water consumption;
involves almost no change in column diameter compared to conventional steam stripping;
that has steam/ reboiling duty reduction; and
generates relatively lesser quantity of sour water.
The embodiments herein and the various features and advantageous details thereof are explained with reference to the non-limiting embodiments in the following description. Descriptions of well-known components and processing techniques are omitted so as to not unnecessarily obscure the embodiments herein. The examples used herein are intended merely to facilitate an understanding of ways in which the embodiments herein may be practiced and to further enable those of skill in the art to practice the embodiments herein. Accordingly, the examples should not be construed as limiting the scope of the embodiments herein.
The foregoing description of the specific embodiments so fully reveal the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments herein have been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the embodiments as described herein.
The use of the expression “at least” or “at least one” suggests the use of one or more elements or ingredients or quantities, as the use may be in the embodiment of the disclosure to achieve one or more of the desired objects or results.
Any discussion of documents, acts, materials, devices, articles or the like that has been included in this specification is solely for the purpose of providing a context for the disclosure. It is not to be taken as an admission that any or all of these matters form a part of the prior art base or were common general knowledge in the field relevant to the disclosure as it existed anywhere before the priority date of this application.
The numerical values mentioned for the various physical parameters, dimensions or quantities are only approximations and it is envisaged that the values higher/lower than the numerical values assigned to the parameters, dimensions or quantities fall within the scope of the disclosure, unless there is a statement in the specification specific to the contrary.
While considerable emphasis has been placed herein on the components and component parts of the preferred embodiments, it will be appreciated that many embodiments can be made and that many changes can be made in the preferred embodiments without departing from the principles of the disclosure. These and other changes in the preferred embodiment as well as other embodiments of the disclosure will be apparent to those skilled in the art from the disclosure herein, whereby it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the disclosure and not as a limitation
,CLAIMS:WE CLAIM:
1. A carrier gas composition for stripping at least one fraction obtained from crude oil distillation, said composition comprising:
• hydrogen gas in an amount in the range of 2 vol% to 80 vol% of the total composition; and
• a gaseous hydrocarbon mixture in an amount in the range of 20 vol % to 98 vol % of the total composition, wherein said gaseous hydrocarbon mixture comprises hydrocarbons having carbon atoms in the range of 1 to 5, wherein said hydrocarbons are saturated hydrocarbons, unsaturated hydrocarbons or combinations thereof.
2. The carrier gas composition as claimed in claim 1, wherein said gaseous hydrocarbon mixture comprises at least one C3+ hydrocarbon in an amount of at least 50 vol % of the total composition.
3. The carrier gas composition as claimed in claim 1, comprising:
• hydrogen gas in an amount in the range of 2 vol% to 45 vol% of the total composition; and
• a gaseous hydrocarbon mixture in an amount in the range of 55 vol % to 98 vol % of the total composition,
wherein said gaseous hydrocarbon mixture comprises at least one C3+ hydrocarbon in an amount of at least 50 vol % of the total composition.
4. The carrier gas composition as claimed in claim 1, wherein the saturated hydrocarbons are selected from straight chain alkanes and branched chain alkanes.
5. The carrier gas composition as claimed in claim 1 or claim 4, wherein the saturated hydrocarbons are selected from the group consisting of methane, ethane, propane, isobutane, n-butane, isopentane and n-pentane.
6. The carrier gas composition as claimed in claim 1, comprising:
• hydrogen gas in an amount in the range of 3 vol % to 42 vol % of the total composition; and
• a gaseous hydrocarbon mixture comprising:
? methane in an amount in the range of 2 vol % to 40 vol% of the total composition;
? ethane in an amount in the range of 4 vol % to 8 vol % of the total composition;
? propane in an amount in the range of 10 vol % to 35 vol % of the total composition;
? isobutane in an amount in the range of 1 vol % to 7 vol% of the total composition;
? n-butane in an amount in the range of 5 vol% to 15 vol % of the total composition;
? isopentane in an amount in the range of 0.5 vol % to 1 vol % of the total composition; and
? n-pentane in an amount in the range of 0.5 vol % to 1 vol % of the total composition.
7. The carrier gas composition as claimed in any of the preceding claims, comprising:
• hydrogen gas in an amount of 40.2 vol % of the total composition; and
• a gaseous hydrocarbon mixture comprising:
? methane in an amount of 3.5 vol % of the total composition;
? ethane in an amount 4.5 vol % of the total composition;
? propane in an amount of 30.7 vol % of the total composition;
? isobutane in an amount of 6.4 vol % of the total composition;
? n-butane in an amount of 13.5 vol % of the total composition;
? isopentane in an amount of 0.6 vol % of the total composition; and
? n-pentane in an amount of 0.6 vol % of the total composition.

8. The carrier gas composition as claimed in any of the preceding claims, wherein the carrier gas is obtained from at least one source selected from the group consisting of refinery off gas, continuous catalytic reforming off gas, coker heavy gas oil hydro treating unit (CHTU) off gas, delayed coker off gas, deep catalytic cracker off gas and H2/C1-C5 gas mixture from hydrocarbon industrial process.
9. The carrier gas composition as claimed in any of the preceding claims, wherein composition of the carrier gas is varied by a predetermined value, by introducing at least one C3+ hydrocarbon, to the carrier gas composition, wherein said at least one C3+ hydrocarbon is obtained from the liquefied petroleum gas (LPG) stabilization unit, LPG vapor or C3+ hydrocarbon vapor from hydrocarbon industrial processes.
10. A process for the fractional distillation of crude oil using the carrier gas composition as claimed in any of the preceding claims, as a stripping medium, said process comprising the following steps:
(a) heating a portion of crude oil to obtain a biphasic mixture comprising a vapor phase and a liquid phase;
(b) introducing the biphasic mixture in the flash zone of a distillation column;
(c) fractionating the biphasic mixture in an upper section of the distillation column above the flash zone, to obtain at least one fraction; and
(d) stripping said at least one fraction with a stripping medium comprising the carrier gas composition, to obtain at least one distillate product selected from the group consisting of heavy naphtha (HN), kerosene (kero), light gas oil (LGO) and heavy gas oil (HGO).
11. The process as claimed in claim 10, wherein step (a) of heating comprises heating the portion of crude oil in a pre-flash drum, to obtain liquid and vapors, wherein the vapors are directed to the flash zone.
12. The process as claimed in claim 10, wherein the liquid is heated in a charge heater, to a temperature in the range of 340 °C to 370 °C to obtain the biphasic mixture.
13. The process as claimed in claim 10, wherein the fraction is at least one selected from the group consisting of heavy naphtha (HN), kerosene (kero), light gas oil (LGO) and heavy gas oil (HGO).
14. The process as claimed in claim 10, wherein step (d) of stripping is carried out in a side stripping unit.
15. The process as claimed in claim 10, wherein step (d) of stripping is carried out by using a mixture of two variable carrier gas compositions.