Method of separation of gaseous hydrocarbons from gasoline

BACKGROUND OF THE INVENTION

This invention relates to a separation process. It also relates to a method for separating the effluent from a hydrocarbon conversion zone into normally gaseous products and normally liquid products. It particularly relates to a method for recovering gasoline and normally gaseous hydrocarbons from the effluent of a hydrocarbon coking unit.

It is known in the prior art to subject relatively heavy hydrocarbonaceous materials to heat soaking or thermal cracking conditions in order to convert the materials to coke and upgraded hydrocarbon products, such as LPG, gas oil, and gasoline.

The prior art processes of coking generally take the form of delayed coking, fluid coking, etc. Since the standard coking processes are well known in the prior art, it is not considered necessary to show the coking process in detail. The units presently available for coking operate by heating a feed to a temperature from 750° F. to 950° F. and under a pressure of from about 10 to 100 p.s.i.g. Sufficient residence times from a few seconds to perhaps several hours areutilized to convert the heated material to coke and lower molecular weight products. The coke is removed from the coking drums and the fluid effluent is sent to hydrocarbon recovery facilities.

The prior art schemes for separating the effluent from a conversion process, such as a coking unit, include broadly the use of a fractionation column, an absorption column, and a stabilizer column. Conventionally, coker gasoline, coker gas oil, and normally gaseous hydrocarbons are obtained in these prior art processes.

The present invention provides improvement over the prior art schemes by utilizing a combination of processing techniques which achieve a substantially greater recovery of desired normally gaseous hydrocarbons in high purity while at the same time maintaining the quality of the coker gasoline at a desirably high level.

SUMMARY OF THE INVENTION

Accordingly, it is an object of this invention to provide a method for separating the effluent of a hydrocarbon conversion zone into normally gaseous hydrocarbons and normally liquid hydrocarbons.

It is another object of this invention to provide an improved method for separating the fluid effluent of a coking conversion zone into gasoline and normally gaseous hydrocarbons.

Therefore, the present invention provides an improved method for recovering gasoline and normally gaseous hydrocarbons from the fluid effluent of a hydrocarbon conversion unit which comprises the steps of: (a) fractionating said effluent in a first fractionation zone into a light distillate fraction and gas oil fraction, said light distillate fraction containing gasoline and normally gaseous hydrocarbons; (b) separating said light distillate fraction in a separation zone into a first gaseous stream ⁵ and a first liquid stream; (c) introducing at least a portion of said first gaseous stream into a first absorption zone maintained under absorption conditions including at least a portion of said first liquid stream as absorp,θ tion medium; (d) removing from said first absorption zone a first rich oil fraction and a second gaseous stream; (e) passing said second gaseous stream into a second absorption zone maintained under absorption conditions including the presence of at least a portion of said gas oil 15 fraction as absorption medium; (f) removing from said second absorption zone a second rich oil stream and a first product stream comprising C₂ and lighter components; (g) passing said first rich oil into said separation zone of Step (b) and passing said second rich oil stream to said 20 fractionation zone of Step (a); and, (h) introducing the remaining liquid stream of Step (b) into a second fractionation zone and recovering therefrom a second product stream comprising normally gaseous hydrocarbons and a third product stream comprising gasoline.

Another embodiment of this invention includes the method hereinabove wherein a portion of said third product stream is passed into said first absorption zone as absorption medium.

Thus, it is seen from the broad embodiments presented 30 hereinabove that the present invention provides a facile and economical separation process for recovering gasoline and normally gaseous hydrocarbons, such as C₃ and C₄hydrocarbons, from the effluent of a hydrocarbon conversion zone. The critical features of the invention in35 elude, in combination, a first fractionating column, a liquid-gas separation zone, dual absorption zones, and a second fractionation zone.

It is to be noted that the present invention has been described as being broadly applicable to separating the ⁴⁰ fluid effluent from a hydrocarbon conversion unit. More particularly, however, the present invention is uniquely applicable to separating the fluid effluent from a hydrocarbon coking unit. However, the present invention is not to be limited to the coking operation; rather, it is to be 45 limited only by the scope of the appended claims since such separation scheme can be utilized to separate any fluid effluent containing the various components to be separated herein.

To produce a fluid effluent which is separated according 50 to the practice of the preferred embodiment of the present invention, a high boiling hydrocarbon charge, such as crude bottoms from the vacuum distillation of reduced crude, vacuum tar, reduced crudes, topped crudes, blends thereof, etc. is charged into a conventional conversion op55 eration which is maintained under conventional conversion conditions, such as coking conditions of either the delayed type or fluid type. The charge material is first preheated and the preheated charge is fed into the bottom portion of, for example, a coker drum or plurality of drums 60 which is subjected to elevated temperatures and usually moderate pressures. Under coking conditions, destructive distillation of the charge material occurs and results in the formation of lighter boiling hydrocarbons and coke. The lighter boiling hydrocarbons are withdrawn from the 65 coker drum while the coke is usually caused to remain therein. When the desired amount of coke is formed, the conventional manner of operation at that point discontinues the introduction of the preheated charge material and the deposited coke is removed from the drum. The 7° lighter boiling hydrocarbon materials which have been withdrawn from the coker drum comprise the preferred

3,470,084 fluid effluent to be separated according to the present invention. Although reference is made to the fluid effluent as being “lighter boiling hydrocarbons,” it is recognized that such fluid effluent may also contain nonhydrocarbon gaseous materials, such as hydrogen, acid gases etc. _g

The operation of the various steps in the combination method of the present invention will be better understood with reference to the appended drawing which is a schematic representation of apparatus for practicing the preferred embodiment of the present invention. ₁₀

DESCRIPTION OF THE DRAWING

Referring now to the drawing, a suitable high boiling charge material is passed via line 10 into hydrocarbon conversion zone 11 which may be a conventional coking 15 unit. Fluid effluent from the conversion zone is withdrawn via line 12 and passed into fractionation zone 13. Suitable fractionation conditions are maintained in zone 13 to separate the fluid effluent, initially, into a light distillate fraction which is withdrawn via line 14, a side-cut gas oil 20 fraction which is withdrawn via line 15, and a heavy gas oil fraction which is withdrawn from the lower portion of zone 13 via line 16. Depending upon the characteristics of the fluid effluent in line 12, there may be more or less product streams withdrawn from zone 13. 25

The light distillate fraction containing gasoline boiling range material and normally gaseous hydrocarbons such as C₂, C₃, and C₄ hydrocarbons admixed with other gases, such as hydrogen, is passed via line 14 into first separation zone 17 which is maintained under relatively low 30 pressure. Suitable separation conditions are maintained in zone 17 in order to separate the light distillate fraction into a first gaseous stream which is removed via line 18 and a first liquid stream which is withdrawn via line 19.

The material in line 18 comprises a broad mixture of 35 normally gaseous hydrocarbons, including the previously mentioned hydrogen and acid gases, if any, and is at a relatively low pressure. According to the preferred embodiment of this invention, the first gaseous stream in line 18 is passed into compressing means 20 for raising ⁴⁰the pressure thereof to a relatively high level. The compressed gaseous stream is removed from compressing means 20 via line 18(a), admixed with hereinafter specified liquid stream from line 21, and the admixture passed via line 22 into separation zone 23 which is maintained 45 under a relatively high pressure. Other suitable operating conditions are maintained in zone 23 to separate the admixture into a second gaseous stream which is withdrawn via line 24 and a second liquid stream which is withdrawn via line 25. 50

The second gaseous stream is introduced via line 24 into first absorption zone 26, preferably, at the lower end thereof. Absorption zone 26 is maintained under absorption conditions including the presence of at least a major portion of the first liquid stream withdrawn from 55 separation zone 17 as at least one part of the absorption medium therein and which is introduced into absorption zone 26 via line 19. As more fully discussed hereinafter, an additional part of the absorption medium may also be introduced into absorption zone 26 via line 35. Other 60 operating conditions are maintained in absorption zone 26 in order to absorb substantially all of the C₃ and C₄hydrocarbons as well as some C₂ hydrocarbons into the absorption medium. Thereafter, the first rich absorption oil is withdrawn from zone 26 via line 21 and passed into 65 admixture with the compressed gaseous material from line 18(a), as previously mentioned.

A third gaseous stream comprising C₂ and lighter gaseous components contaminated with C₃ and C₄ hydrocarbons is withdrawn from absorption zone 26 via line 70 27 and passed into second absorption zone 28. The absorption medium utilized in absorption zone 28 is a portion of the lighter gas oil stream which was removed from fractionation zone 13 via line 15. This lighter gas oil absoiption medium is passed via line 29 into the upper por- 75 tion of absorption zone 28 for contact therein with the gaseous material being introduced via line 27. A first product stream comprising C₂ and lighter components including hydrogen and acid gas, if any, is withdrawn from absorption zone 28 via line 30 and, preferably, sent to fuel. The second rich absorption oil is withdrawn from absorption zone 28 via line 31 and returned to fractionation zone 13, preferably, as reflux therein at a locus above the withdrawal locus for the lighter gas oil material in line 15.

Returning now to separation zone 23, the second liquid stream in line 25 is now passed into second fractionation zone 32 which is maintained under fractionation condi tions sufficient to produce a gaseous stream comprising, preferably, C₂ hydrocarbons contaminated with C₃ and C₄ hydrocarbons which is withdrawn via line 33. It is distinctly preferred, in order to maximize the recovery of C₃ and C₄ hydrocarbons, that the material in line 33 be sent via line 37 into separation zone 23 in admixture with the previously mentioned first rich absorption oil in line 21. However, if desired, another product stream comprising these light hydrocarbons may be removed from the system via line 33. A second product stream comprising C₃ and C₄ hydrocarbons is removed from zone 32 via line 36 and a third product stream comprising a conversion gasoline is withdrawn from the system via line 34. As previously mentioned, it is preferred that a portion of the third product stream in line 34 be passed via line 35 into first absorption zone 26. However, a portion of the material in line 35 may also be sent to zone 13, or zone 18, or zone 23, by means not shown, if desired. A still further preferred embodiment of this invention is characterized by having the material in line 35 introduced into absorption zone 26 at a locus above the locus of introduction of the first liquid stream in line 19.

By operating in the manner set forth hereinabove it was found that extremely high recoveries (i.e., quantity and quality) of the normally gaseous hydrocarbons, such as C₃ and C₄ hydrocarbons, was achieved. In addition, a desirably stabilized gasoline product and, in the preferred embodiment, two gas oil fractions, were obtained as products.

It is further noted that the preferred embodiment is characterized by having the rich oil from the first absorption zone being passed into the second separation zone maintained at relatively high pressure thereby providing enrichment of the liquid with normally gaseous components which are subsequently separated in fractionation zone 22. Similarly, the preferred embodiment is characterized by having not only two absorption zones, but by having also two distinct absorption mediums which are introduced into the first absorption zone. By operating in this manner, excellent recovery of C₃ and C₄hydrocarbons is obtained.

The description of the drawing has not included specific operating details for each of the pieces of equipment contained in the combination method. It is deemed within the skill of those experienced in this art to choose the proper operating conditions to effectuate the various separations required by the description of the invention. The fractionation steps, the absorption steps and the separation steps are basically conventional operations which have been combined in a novel manner to produce an improved result. Those skilled in the art will also recognize that the required separating conditions will be, of course, influenced to a considerable extent by the characteristics of the fluid to be separated in line 12.

However, the following example is furnished to set forth the best mode contemplated for practicing the preferred embodiment of the invention.

EXAMPLE

A commercial size coking unit was operated according to conventional practice. The fluid effluent, after

3,470,084 separation of the coke, was passed into apparatus schematically arranged as in the attached drawing.

The charge to the coking unit (line 10) was a vacuum reduced crude having the following properties:

Gravity, ⁰ API_____________________________ 19.115

Sulfur, wt. percent θ·16

Con Carbon, wt. percent 10.9 “UOP” K factor12.25

After suitable pre-heat, the feedstock was charged at a ₁₀rate equivalent to 6000 barrels per stream day into the coking zone at a transfer temperature of about 900° F. and a coke chamber pressure of about 75 p.s.i.g. After separation of the coke, the fluid effluent was ultimately passed via line 12 into fractionation zone 13. jg

The fluid effluent or vapors from the cake chamber are passed into the lower section of fractionator 13 at a temperature of 820° F. and a pressure of about 75 p.s.i.g. An overhead stream comprising normally gaseous material and gasoline is withdrawn via line 14 at a ₂o temperature of about 302° F., a side-cut stream comprising light gas-oil is withdrawn via line 15 at a temperature of about 585° F., and a bottoms stream comprising heavy gas-oil is withdrawn via line 16 at a temperature of about 750° F. Under these conditions the fol- ₂5 lowing product streams were separated (composition data are in mols per hour):

‘ Line No.

Component 15 16 οθ ° API____________ 39.029.0

BPSD____________ 1,7941,554

Wt. percent S_____ 0.10.1

Component

114.00 .58 24.86 99.14 13.55 54.86 31.55 55.44 39.17

9.43 30.50 39.41 18.69 37.90

229.48

HsO______

H’S-.....

H₂........

Ci________

C₂-.......

C’—.....

C₃-_______ c₃........

Cr.......

1C₄_______ nCi______ c₅-_______ iCs_______ nCs______

Ce-338⁰ F.

__________________________35

---------------------------------------- 40

The material in line 14 is cooled by condensing means, such as an air-fin condenser, to a temperature of about 100° F. and passed into separator 17 which is under ⁴⁵a pressure of about 60 p.s.i.g. A gaseous stream is withdrawn from separator 17 via line 18 and a liquid stream withdrawn via line 19. These withdrawn streams had the following composition (data are in mols per hour):

Line NoComponent 18 and 18(a)19

<td rowspan="2"> H’O.................... HaS____________________ Ha---------------------- Ci---------------------- C₂“.....................</td><td> 3.59 —...... .46 27.74 95.39 11.90</td><td> .04 .04 1.25 .55</td><td> 55</td>

<td> 46.10</td><td> 2.92</td><td></td>

<td> Ct---------------------- Ca----------------------</td><td> 20.48 34.29 14.69</td><td> 3.69 7.05 8.16.</td><td></td>

<td> iC&lt;_____________________ nCi___________________- Ca-_____________________ iCs--------------------- nCs........-........... Ca-338° F_____________-</td><td> 3.91 10.40 6.62 3.39 5.56 1.42</td><td> 1.84 6.70 10.93 5.10 10.78 76.02</td><td> 60</td>

<td> Total_____________</td><td> 282.94</td><td> 135.07</td><td> 65</td>

The gaseous stream in line 18 is passed into centrifugal compressor means 20 wherein the pressure is raised from about 60 p.s.i.g. to about 225 p.s.i.g. and then admixed, as previously mentioned, with recycle streams from lines 21 and, preferably from line 37, respectively. The ad- 70 mixture is then introduced into separator 23 at a temperature of about 100° F. and a pressure of about 215 p.s.i.g. Under these conditions a gaseous stream is separated and withdrawn via line 24 and a remaining liquid stream is withdrawn via line 25. The composition of these 75 withdrawn streams from separator 23 had the following composition (mols per hour):

<td rowspan="2"> Component</td><td colspan="3"> Line No.</td>

<td> 22</td><td> 24</td><td> 25</td>

<td> HaO__________________</td><td> 3.59</td><td> 1.37.....</td><td></td>

<td> HaS__________________</td><td> 1.74</td><td> .98</td><td> .76</td>

<td> h₂____________________</td><td> 25.67</td><td> 25.12</td><td> .55</td>

<td> Ci____________________</td><td> 122.37</td><td> 106.15</td><td> 16.22</td>

<td> c₂-___________________</td><td> 26.94</td><td> 17.85</td><td> 9.09</td>

<td> C₂____________________</td><td> 140.32</td><td> 83.49</td><td> 56.83</td>

<td> C₃-—-_______________</td><td> 85.75</td><td> 32.07</td><td> 53.68</td>

<td> C₃____________________</td><td> 121.48</td><td> 42.11</td><td> 79.37</td>

<td> C₄“___________________</td><td> 40.47</td><td> 7.36</td><td> 33.11</td>

<td> iC₄_____________ ---</td><td> 10.79</td><td> 2.22</td><td> 8.57</td>

<td> nC₄_„...............</td><td> 28.69</td><td> 4.64</td><td> 24.05</td>

<td> c₅-..................</td><td> 40.07</td><td> 2.98</td><td> 37.09</td>

<td></td><td> 19.28</td><td> 1.62</td><td> 17.66</td>

<td> nCa___________________</td><td> 37.33</td><td> 2.49</td><td> 34.84</td>

<td> C₆-338^oF.__........-</td><td> 179.07</td><td> .55</td><td> 178, 52</td>

<td> Total__________</td><td> 883.56</td><td> 331.00</td><td> 550.34</td>

The liquid stream in line 25 is introduced into fractionation zone 32 which preferably is divided into two separate distillation columns, the first one being typically a gasoline stripper for the removal of C₂’s and lighter material from the feedstock and the second one being a debutanizer column for the recovery of C₃ and C₄ hydrocarbons and gasoline as product streams. For convenience sake, however, the dual fractionation zones have been shown in the attached drawing as a single column. By operating the fractionation zones under conventional conditions of temperature and pressure, an overhead stream comprising the C₂ and lighter components contaminated with C₃ and C₄ hydrocarbons is withdrawn via line 33. In the preferred embodiment of the invention the material in line 33 is passed via line 37 and 21 into admixture with the compressed vapors in line 18(a) as previously mentioned. However, if desired, the material in line 33 may be withdrawn from the system and no recycle to high pressure separator 23 being employed. Obviously, a small amount of material or large amount of material or no amount of material may be withdrawn from the system via line 33. Typically, a gasoline stripper would operate with a flash zone temperature of about 140° F. and a pressure of about 242 p.s.i.g. Similarly, the debutanizer column could operate with a flash zone temperature of about 400° F. and a flash zone pressure of about 195 p.s.i.g. Under these conditions a stream comprising C₃ and C₄ hydrocarbons is withdrawn from fractionation zone 32 via line 36 and a gasoline product stream is withdrawn from the bottom of the debutanizer column via line 34. As illustrative of the type of separation which may be accomplished in fractionation zone 32, the following composition data is provided (mols per hour):

Line No.

Component 34 36 33 and 37 h₂o_.....

HaS______

Ha......Ci__......

Ca-.......

Ca________

C₃“------C₃________

Cr-......

iC₄_______ nC₄______

Cr—. iC₅.....nC₈______

Ce-338° F.

.50 .12 .48

13.34

6.26

12.78

76.54 .25 18.48 34.51 21.21

5.27 15.85 .43 .21 .32

Total Π0.02 96.54 201.09

Referring again to high pressure separator 23, the gaseous material in line 24 is passed into a first absorption zone 26 at a temperature of about 100° F. and a pressure of about 212 p.s.i.g. Sufficient operating conditions are maintained in absorption zone 26 to dissolve substantially all of the C₃ and C₄ hydrocarbons into the

3,470,084 lean oil which comprises at least in part the material in line 19 the composition of which has previously been given. The lean oil is introduced into absorption zone 26 so that effective counter-current contact may be obtained between the liquid and vapor in the zone. If desirable or _ deemed necessary, suitable packing material may also be placed in zone 26, the type and quantity of which are well known to those skilled in the art. Additionally, another portion of the absorption medium comprises gasoline from line 34 which is introduced into zone 26 via line 35 (the composition of the material in line 34 has also been previously given). Under the operating conditions imposed in zone 26 the C₂ and lighter materials are effectively rejected from this zone and are withdrawn from absorber 26 via line 27. The combined rich oil 15 having C₃ and C₄ hydrocarbons absorbed therein is similarly withdrawn from zone 26 via line 21, admixed with the material coming from line 37, and further admixed with the gas leaving compressor 20 via line 18(«) for recycle and liquid enrichment in high pressure separator 20 23. Illustrative of the separation obtained in absorber 26 are the following composition data (mols per hour):

Line No. 25

Component

27

HjO______

HjS______

H,-------Ci-------cr------c,-------c₃-------c₃-------c&lt;-.......

iC&lt;------nCi______

Cs-------1C_S_______ nCs______

Ci-338° F.

.53 .38

10.76

5.95 37.64 30.07 42.33 15.03

3.86 11.19 27.46 12.84 26.62

174.87

1.37 .49 24.78 96.64 12.45 48.77

5.69 6.83

1.14 .36 .77 3.78 2.02 3.24 .90

Total--------------------- 399.53 209.23

The rejected C₂ and lighter material contaminated with C₃+ components is passed via line 27 into second absorber 28 at a temperature of about 118° F. and a pressure of about 205 p.s.i.g. The gas in line 27 is introduced into the lower portion of absorber 28 and contacted in counter-current manner therein with lean oil introduced ⁴³ into the upper portion of absorber 28 via line 29. As previously mentioned, the absorber oil or medium for use in zone 28 is a portion of the light gas-oil product which was removed from fractionating column 13 via line 15, the composition of which has previously been given. The C₂ and lighter material is ultimately rejected via line 30 and sent preferably to fuel. The rich oil from second absorption zone 28 is withdrawn via line 31 and preferably returned to the upper portion of fractionating column 13 as additional reflux thereon. A typical composition data of the separation obtained in absorber 28 is shown as follows (mols per hour):

Line No.

Component 30 31

<td> h₂o_________________________</td><td colspan="2"> 1.37 ....................</td>

<td> H₂S_________________________</td><td> -.38</td><td> .11</td>

<td> H₂---------------------------</td><td> 24.65</td><td> .13</td>

<td> Ci---------------------------</td><td> 93.12</td><td> 3.52</td>

<td> cr--------------------------</td><td> 10.96</td><td> 1.49</td>

<td> Cs---------------------------</td><td> 40.83</td><td> 7.94</td>

<td> Ca---------------------------</td><td> 3.43</td><td> 2.26</td>

<td> Cs---------------------------</td><td> 3.83</td><td> 3.00</td>

<td> Cl---------------------------</td><td> .20</td><td> .94</td>

<td> iC&lt;__________________________</td><td> .09</td><td> .27</td>

<td> nC&lt;__..........-............</td><td> .10</td><td> .67</td>

<td> Ci-__________________________</td><td> .04</td><td> 3.74</td>

<td> iCs--------------------------</td><td> .03</td><td> 1.99</td>

<td> nCs__________---------------</td><td> .02</td><td> 3.22</td>

<td> Ce-338° F____________________</td><td> .00</td><td> .90</td>

<td> 338° F .-650° F_______________</td><td> .01</td><td> 81.93</td>

<td> Total__________________</td><td> 179.06</td><td> 112.11</td>

PREFERRED EMBODIMENT

Therefore, in summary, the preferred embodiment of the present invention includes the method for separating the fluid effluent from a coking reaction zone which comprises: (a) passing said effluent into a first fractionation zone under conditions sufficient to produce a first distillate fraction comprising hydrogen, normally gaseous hydrocarbons, and gasoline, a second distillate fraction comprising light gas-oil, and a third fraction comprising heavy gas-oil; (b) introducing said first distillate into a first separation zone maintained under conditions including relatively low pressure sufficient to produce a first gaseous stream and a first liquid stream; (c) compressing said first gaseous stream and admixing the compressed stream with a hereinafter specified rich oil stream; (d) passing the admixture of Step (c) into a second separation zone maintained under conditions including relatively high pressure sufficient to produce a second gaseous stream and a second liquid stream; (e) passing said second gaseous stream into a first absorption zone maintained under conditions sufficient to absorb C₃+ hydrocarbons into an absorption medium comprising at least in part said first liquid stream of Step (b); (f) introducing a third gaseous stream comprising C₂ and lighter gaseous components contaminated with C₃, and C₄ hydrocarbons into a second absorption zone maintained under conditions sufficient to absorb said contaminants into an absorption medium comprising at least in part a portion of said light gasoil from Step (a); (g) removing from said second absorption zone a first product stream comprising hydrogen, and C₂ and lighter hydrocarbons; (h) passing said second liquid stream into a second separation zone under conditions sufficient to produce a second product stream comprising C₃ and C₄ hydrocarbons and a third product stream comprising gasoline; (i) introducing the rich oil containing absorbed C₃ and C₄ hydrocarbons from Step (c) into admixture with said compressed gaseous stream as specified in Step (c); and, (j) returning a portion of said third product stream to said first absorption zone as another part of said absorption medium.