Claims:WE CLAIM:
1. A process for improving the yield of light olefins produced from heavy naphtha, said process comprising the following steps:
a. fractionally distilling said heavy naphtha feed having aromatic content below 20%, to obtain a first distillate (light cut) up to 90 °C, and a second distillate (medium cut) up to 110 °C;
b. vaporizing said first distillate at a temperature in the range of 100 °C to 150 °C to obtain a vaporized first distillate;
c. vaporizing said second distillate at a temperature in the range of 100 °C to 150 °C to obtain a vaporized second distillate;
d. mixing said vaporized first distillate with steam at a temperature in the range of ¬¬¬¬¬¬¬¬¬¬¬¬¬¬¬¬ 150 °C to 450 °C to obtain a first mixture;
e. mixing said vaporized second distillate with steam at a temperature in the range of ¬¬¬¬¬¬¬¬¬¬¬¬¬¬¬¬150 °C to 450 °C to obtain a second mixture;
f. cracking said first mixture in a cracking unit at a temperature in the range of 800 °C to 850 °C to generate light olefins comprising ethylene and propylene; and
g. cracking said second mixture in a cracking unit at a temperature in the range of 800 °C to 850 °C to generate light olefins comprising ethylene and propylene.

2. The process as claimed in claim 1, wherein step f) includes the step of introducing said first mixture and said second mixture in said cracking unit at a temperature in the range of 450 °C to 600 °C and discharging light olefins generated in said cracking unit at a temperature in the range of 800 °C to 850 °C.

3. The process as claimed in claim 1, wherein the fractional distillation in step a) is carried out at a temperature is in the range of 50 oC to 150 oC.

4. The process as claimed in claim 1, wherein the ratio of said vaporized first distillate to said steam is 2:1.

5. The process as claimed in claim 1, wherein the ratio of said vaporized second distillate to said steam is 2:1.
, Description:
FIELD
The present disclosure relates to a process for improving the yield of light olefins produced from heavy naphtha.
BACKGROUND
The background information herein below relates to the present disclosure but is not necessarily prior art.
Steam cracking of hydrocarbons to olefins such as ethylene and propylene is an important process in petrochemical industry. Hydrocarbons such as ethane, propane, butane, their mixtures and naphtha are cracked to olefins in tubular reactors in the presence of steam at higher temperatures in the range from 800-855 °C. The production volumes of such processes are extremely large and therefore, any small improvements in the process would be economical and commercially advantageous.
Developments in the thermal cracking processes include optimization of process conditions with change in feed stock quality and economic trade off of process conditions on furnace run length, energy, and decoking costs. However, not much work has been done in the upstream area to improve the naphtha feed quality although different cuts of naphtha are being processed at cracker plant based on cost as well as its availability.
The ethylene and propylene yield depends mainly on feed paraffin content. Generally, in cracker feedstock the aromatic content is in the range of 6-10% by weight. The aromatic content in naphtha feed increases coke formation, thus reducing furnace run length. One approach is the extraction of paraffins from naphtha feedstock for cracking that can result in improved ethylene yield. Another approach is the extraction of aromatics from naphtha feedstock to have raffinate with higher paraffins content in naphtha for further cracking, which results into improved ethylene yield. Since, the components in naphtha have very close boiling points as well as the mixture is azeotropic, the employed techniques are extraction and extractive distillation using solvent and absorption. There are different processes known for extracting high purity aromatics from hydrocarbon. Different technology uses different solvent. Different aromatic extraction processes are suitable for different ranges of aromatic content of feed. Azeotropic distillation is more suitable for feed having very high aromatic content. Extractive distillation is suitable for feed having 65-90% aromatic content, whereas for 20-65% aromatic content, liquid-liquid extraction is more suitable. However, for feed having below 20% aromatic content, these processes are not efficient. All these processes additionally require separation facility for solvent recovery from both extract and raffinate phases leading to higher capital investment and operating cost due to higher energy consumption and regeneration cost.
Therefore, there is felt a need to provide a process for improving the yield of light olefins produced from heavy naphtha that overcomes the drawbacks mentioned herein above.
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.
An object of the present disclosure is to provide a process for improving the yield of light olefins produced from heavy naphtha.
Another object of the present disclosure is to provide a process for producing light olefins having high yield of ethylene and propylene.
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
The present disclosure provides a process for improving the yield of light olefins from heavy naphtha. The process comprises the step of fractionally distillation of heavy naphtha feed having an aromatic content below 20% to obtain a first distillate (light cut) up to 90 °C, and a second distillate (medium cut) up to 110 °C. The so obtained first distillate and the second distillate are separately vaporized at a temperature in the range of 100 °C to 150 °C to obtain a vaporized first distillate and a vaporized second distillate, respectively. The vaporized first distillate is mixed with steam at a temperature in the range of 150 °C to 450 °C to obtain a first mixture. Further, the vaporized second distillate is mixed with a steam at a temperature in the range of 150 °C to 450 °C to obtain a second mixture. The so obtained first mixture is cracked in a cracking unit at a temperature in the range of 800 °C to 850 °C to generate light olefins comprising ethylene and propylene. Further, the so obtained second mixture is cracked in a cracking unit at a temperature in the range of 800 °C to 850 °C to generate light olefins comprising ethylene and propylene.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
The process of the present disclosure will now be described with the help of the accompanying drawing, in which:
Figure 1 illustrates the a schematic diagram of the batch distillation unit;
Figure 2 illustrates a schematic diagram of the bench scale cracker unit; and
Figures 3.1 to 3.6 illustrate graphical representation of various trends for feed quality parameters of light cut naphtha.
DETAILED DESCRIPTION
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.
There are different processes known for extracting high purity aromatics from hydrocarbons. Different technology uses different solvents. Different aromatic extraction processes are suitable for different ranges of aromatic content of the feed. Azeotropic distillation is more suitable for feed having very high aromatic content. Extractive distillation is suitable for feed having 65-90% aromatic content, whereas for 20-65% aromatic content, liquid-liquid extraction is more suitable. Below 20% aromatic content, these processes are not efficient. All these processes additionally require separation facility for solvent recovery from both extract and raffinate phases leading to the higher capital investment and operating cost due to higher energy consumption and regeneration cost.
To mitigate the aforestated drawbacks, the present disclosure provides a process in which distillation of heavy naphtha is carried out to separate the light naphtha cut as a distillate and the heavy cut is remained at the bottom of the column. The top light cut with improved paraffin content is cracked in a furnace and the bottom heavy cut can be sent back as a return stream. The process is described in detail.
The process for improving the yield of light olefins produced from heavy naphtha comprises the step of fractionally distillation of heavy naphtha feed having an aromatic content below 20% to obtain a first distillate (light cut) up to 90 °C, and a second distillate (medium cut) up to 110 °C. The so obtained first distillate is vaporized at a temperature in the range of 100 °C to 150 °C to obtain a vaporized first distillate. The so obtained second distillate is vaporized at a temperature in the range of 100 °C to 150 °C to obtain a vaporized second distillate. The vaporized first distillate is mixed with a steam at a temperature in the range of 150 °C to 450 °C to obtain a first mixture. Further, the vaporized second distillate is mixed with a steam at a temperature in the range of 150 °C to 450 °C to obtain a second mixture. The so obtained first mixture is cracked in a cracking unit at a temperature in the range of 800 °C to 850 °C to generate light olefins comprising ethylene and propylene. Further, the so obtained second mixture is cracked in a cracking unit at a temperature in the range of 800 °C to 850 °C to generate light olefins comprising ethylene and propylene.
In accordance with the present disclosure the fractional distillation of the heavy naphtha is carried out in a distillation unit. Figure 1 shows a schematic diagram of the batch glass distillation setup used for the distillation of heavy naphtha to light cuts. Referring to Figure 1, after introducing the heavy naphtha feed (F) into the distillation unit, the temperature of the heavy naphtha feed in the distillation unit is gradually increased in the range of 50 oC to 150 oC. The distillation unit has a temperature indicator (TI) at the bottom in the rector/flask and also at the top in the condenser. The distillation unit has a chilled water supply (W1) and vacuum switch (V) for obtaining a distillate. A first distillate is collected at a temperature below or up to 90 °C to obtain the first light cut. A second distillate is collected at a temperature below or up to 110 °C to obtain the medium light cut. A third distillate is collected at a temperature below or up to 130 °C to obtain third cut. The chilled water supply can be reused for further distillation.
The so obtained distillate cut is passed through a vaporizer to obtain a vaporized distillate. In accordance with the present disclosure, the so obtained first distillate is vaporized to obtain a vaporized first distillate. In accordance with the present disclosure, the so obtained second distillate is vaporized to obtain a vaporized second distillate. In one embodiment, the so obtained first distillate is vaporized at a temperature in the range of 100 °C to 150 °C to obtain a vaporized first distillate. In another embodiment, the so obtained second distillate is vaporized at a temperature in the range of 100 °C to 150 °C to obtain a vaporized second distillate.
The vaporized first distillate is mixed with steam at a temperature in the range of 150 °C to 450 °C to obtain a first mixture. Further, the vaporized second distillate is mixed with a steam at a temperature in the range of 150 °C to 450 °C to obtain a second mixture.
The so obtained first mixture is cracked in a cracking unit at a temperature in the range of 800 °C to 850 °C to generate light olefins comprising ethylene and propylene and the so obtained second mixture is cracked in a cracking unit at a temperature in the range of 800 °C to 850 °C to generate light olefins comprising ethylene and propylene, as illustrated in Figure 2. In an embodiment, each distillate cut is passed through a vaporizer and is mixed with steam in a dilution ratio in the range of 0.4 to 0.5 and 0.4-0.5 sec residence time.
The inlet temperature of the cracking unit is in the range of 450 °C to 600 °C and the outlet temperature is in the range of 800 °C to 850 °C.
In an embodiment the cracking unit is a bench scale cracking unit. A schematic diagram of the experimental set up is given in Figure 2. In an exemplary embodiment, naphtha in base run, distillate cuts in respective test runs are hereinafter called as cracker feed (10) and water (12) are stored in stainless steel (SS) tanks. The heavy naphtha is brought from the cracker plant and it is filtered. Distilled water is used for steam generation. The tanks are provided with level gauges to monitor the liquid level in the tank and flow rates of the feedstock at regular intervals of time. The tanks are placed on electronic weighing balances (14, 16). Metering pumps are also used for pumping the feed (18) and water (20). Cold water (CW in and CW out) is circulated through the pump head to maintain the temperature. Cracker feed and water are passed through respective vaporizers (22, 24) and are mixed in the mixer (26) also known as the convection section, before entering into the reactor (36) (radiation section). The temperatures of the vaporizers and mixer are approximately 100 °C and 500 °C, respectively. The combined cracker feed and steam from the mixer enters the reactor (36) from the top. The temperature at reactor inlet is in the range of 540-560 °C which is taken as the crossover temperature (XOT). The coil outlet temperature (COT) is maintained at 810 °C where the cracking occurs. In the cracking unit the temperature is checked and controlled by temperature indicator (TI) and temperature indicator and controller (TIC) in the appropriate places as illustrated in the Figure 2. The pressure gauges (PS) located at two different points: one at the mixer inlet and other at the top of the gas liquid separator for tracking the pressure drop within the system.
The furnace (38) is heated electrically. The cracked gases leaving the reactor pass through two transfer line exchangers: TLE’s (44, 46) where they are rapidly quenched to lower temperatures. This is done to avoid the losses of valuable products through secondary reactions. The TLE consists of two concentric tubes: the reactor effluent flows through the inner tube, while chilled water flows counter-currently through the outer tube. At the end of TLE’s, there is a gas-liquid separator (48) where heavy products like benzene, toluene and xylene along with condensed water are collected at the bottom and the gas product stream is connected to a gas flow meter, GFM (50) to measure the flow rate of the cracked gases and the gas flow meter outlet is connected to vent. There is a provision to collect sample for analysis before going to vent. The cracked gases are analyzed using gas chromatographic (GC) system. In one embodiment, the GC used is Hewlett-Packard (HP) 6890. The yields are expressed as (%wt. of component /wt. of naphtha). FID detector (flame ionization detector) (54) is used for hydrocarbon analysis and hydrogen is detected using a TCD detector (thermal conductivity detector) (56). CO and CO2 is analyzed using CO-CO2 analyzer (52).
In an exemplary embodiment cracking runs are carried out in the bench scale cracker unit at a coil outlet temperature of 810 °C, steam dilution ratio of 0.4 and 0.5 sec residence time. In an embodiment, each run is carried out for 6 h and the two runs are carried out with naphtha having 150 ppm sulphur. CO is reduced significantly in a run with DMDS (dimethyl disulfide). Two runs are carried out at a reduced temperature of XOT and COT respectively at 431 and 800°C and at identical conditions. Both the runs have good reproducibility of product yield.
The second base run is carried out at very low severity to compare ethylene and propylene yields from cracking of two distillate cuts (cut 1 up to 90°C; and cut 2 up to 110°C). The run is carried out at XOT of 432°C and COT of 800°C. At lower cracking temperature, lower yields of ethylene and propylene at around 18.11 % and 15.54 %, respectively are observed. Yields of ethylene and propylene from cracking of heavy feed, cut 1 and cut 2 are shown in Figure 3.1 and 3.2. Higher yields of ethylene and propylene are obtained from cracking of both cuts (cut 1 and cut 2) as compared to run with heavy feed. Propylene yield was lower in cut 2 as compared to heavy feed due to lower XOT temperature. Under identical conditions the yields are higher than the heavy feed based on the simulation.
The various trends for feed quality parameters which indicates that lighter cuts containing both undesirable aromatics and C7s+ content (Figure 3.1 & 3.2) content gets reduced, whereas total paraffin content increases (Figure 3.3) which is favorable for higher ethylene and propylene yields. Figure 3.4 shows recovery of each cut for all the three feeds. Depending upon the requirement, the lighter cuts can be taken out and cracked in the furnace. Further, the lighter the cut the higher is the yield of E+P (ethylene+ propylene); however, it requires higher amounts of heavier to be removed from bottom. Hence, based on the requirement the lighter cut can be taken from overhead.
The benefit in terms of production capacity due to improved naphtha quality in the furnace based on improved yield is provided in Table 3. These results clearly show increase in ethylene production by 7.72% and propylene production by 4.3%. The other significant benefit is the reduction in C9+s by 23.13%, which reduces the heating requirement significantly during cracking against the marginal increase of energy requirement per metric ton (MT) of naphtha for the upstream separation scheme.
The present disclosure is further illustrated herein below with the help of the following experiments. The experiments used herein are intended merely to facilitate an understanding of the 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 embodiments herein. These laboratory scale experiments can be scaled up to industrial/commercial scale and the results obtained can be extrapolated to industrial/commercial scale.
Experimental details:
Experiment 1: Distillation of heavy naphtha in accordance with the present disclosure:
The feed vessel capacity in the distillation unit was 20 liter. A multipurpose column was used for separation of the desired cut at atmospheric pressure. It has two packed bed columns with a height of 2.5 ft. each. Total column height was 5 ft. and the column diameter was 3.5”. The condenser was fixed at the top of the column to condense distillate. An additional condenser was also fixed on the top of the condenser. The feed vessel (20 liter capacity) was electrically heated. The reflux was adjusted by changing the opening of the valve manually. The bottom temperature was controlled by digital temperature indicator and controller (DTIC). Top temperature was measured by digital temperature indicator (DTI). The distillate product vessel was of 5 liter capacity with provision for cooling.
14000 ml of heavy naphtha was distilled to obtain light naphtha cut. The distillate cut up to 90°C (first cut), up to 110°C (second cut) and up to 130°C (third cut) were separated from heavy naphtha (IBP 49°C & FBP higher than 160°C) using the distillation system. The three cuts were analyzed using GC (gas chromatograph).
The results of different cuts are given in Table 1. Table 1 includes the total of iso and normal paraffins (i-P and n-P) and increase in total paraffins as compared to the feed. Table 1 also shows the aromatic content of different cuts.
Table 1: Composition of distillate cuts
Feed-1 Heavy Naphtha Cut – 1 90°C Cut -2 110°C Cut -3 130°C
*PIONA Analysis (% Wt.)
n-Paraffins Wt.% 27.08 30.50 32.47 30.68
i-Paraffins Wt.% 35.68 34.36 33.45 32.49
Total Paraffins Wt.% 62.76 64.86 65.92 63.18
Increase in Paraffins Wt.% 3.35 5.04 0.66
Naphthenes Wt.% 25.18 24.57 25.08 27.79
Aromatics Wt.% 11.66 9.69 8.72 8.93
Olefins Wt.% 0.40 0.89 0.28 0.10
C6 Aromatics Wt.% 2.99 9.67 6.17 5.16
C7 Aromatics Wt.% 2.55 0.01 2.55 3.35
C8 Aromatics Wt.% 5.08 - - 0.42
*PINOA Analysis refers to analysis of paraffins, isoparaffins, olefins, naphthenes, and aromatics
Table 2 gives the composition of three sets of naphtha feed having different aromatic content, i-P/n-P ratio and C7+ content that were subjected to simulation experiments to study the effect of feed composition. Feed -1 is heavy naphtha feed taken for experiment 1 discussed above.
Table 2: Feed composition of different heavy naphtha feeds
Particulars Feed-1 Feed-2 Feed-3
*PIONA Analysis (% Wt.)
n-Paraffins (n-P) wt % 27.08 26.59 29.96
i-Paraffins (i-P) wt % 35.68 36.62 27.51
Total Paraffins wt % 62.76 63.21 57.47
Naphthenes wt % 25.18 26.65 23.42
Aromatics wt % 11.66 9.35 18.73
Olefins wt % 0.4 0.79 0.39
C6 Aromatics wt % 2.99 0.02 0.04
C7 Aromatics wt % 2.55 0.02 0.07
C8 Aromatics wt % 5.08 0.03 0.05
i-P/n-P Ratio 1.32 1.38 0.92
C7+ 45.5 52.57 37.43
*PINOA Analysis refers to analysis of paraffins, isoparaffins, olefins, naphthenes, and aromatics

Experiment 2: Cracking of distillate cut in the bench cracker unit in accordance with the present disclosure
In the cracking unit/system, the length of the reactor tube was 36 cm and its inner diameter was 11 mm. The reactor was made up of Incoloy 800HT (Ni: 30-35%, Cr: 19-23 %, Fe > 39.5 % by wt). The process gas temperature was measured by means of a thermocouple. The pressure gauges (PG) located at two different points: one at the mixer inlet and other at the top of the gas liquid separator for tracking the pressure drop within the system. The furnace was 36 cm long and 26 cm wide. Cracking run was carried out with distillate cut 1 and steam. The feed flow rate of distillate cut was around 64 g/h and steam flow rate was 32 g/h. The run was carried out at coil outlet temperature of 800 °C. The run time was 6 h. The residence time was 0.4 s. The yields of ethylene and propylene in cracking run with cut-1(up to 90c) are 21.71 and 16.09% respectively.
For commercial furnaces operating with the average processing rate is 72.95 TPH. (The maximum processing rate is 75 TPH during 5 furnaces operation and minimum processing rate is 40 TPH for 3 furnaces operation). This experiment provides a scheme for heavy naphtha separation to reduce heavy content as well as improve paraffin content. The benefit in terms of production capacity due to processing improved naphtha quality in the furnace based on improved yield is provided in Table 3.
Table 3: Impact of improved naphtha quality on production
Basis 630960 **MTA Naphtha Processing Incr./Decr., Incr./Decr.,
Heavy feed
MTA Customized Cut
MTA MTA %
Hydrogen 4322 4541 219 0
Methane 69976 74099 4123 0
Ethylene 122604 132073 9470 7.72
Propylene 104695 109202 4507 4.31
Butadiene 20016 18957 -1059 -5.29
1-Butene 9186 9696 510 5.55
i-Butene 609 575 -34 -5.58
C5s 32459 36717 4258 13.12
Benzene 54869 52996 -1873 -3.41
Toluene 23006 26145 3139 13.64
Styrene 12740 10599 -2141 -16.81
C9+s 32588 25049 -7538 -23.13
**MTA = Metric tons per annum
The above results show increase in ethylene production by 7.72% and propylene production by 4.3%. The other significant benefit is the reduction in C9+s by 23.13% which with upstream separation scheme. This will reduce the energy requirement significantly per metric ton (MT) of Ethylene + Propylene against marginal increase of energy requirement per MT of naphtha against required in purposed scheme.
TECHNICAL ADVANCEMENTS
The present disclosure described herein above has several technical advantages including, but not limited to, the realization of:
• significant increase in the yield of ethylene and propylene; and
• energy efficient process.
The exemplary embodiments herein quantify the benefits arising out of this disclosure and the various features and advantageous details thereof are explained with reference to the non-limiting embodiments in the 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 reveals 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 has 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.
Any discussion of documents, acts, materials, devices, articles and 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.
While considerable emphasis has been placed herein on the particular features of this disclosure, it will be appreciated that various modifications 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 modifications in the nature of the disclosure or the preferred embodiments 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.