ON-SITE SOLVENT GENERATION AND MAKEUP FOR TAR SOLVATION IN AN OLEFIN PLANT

Abstract

A system and process for generating, on-site, a sustained C.sub.6+C.sub.7 aromatic rich solvent stream for tar solvation within the olefin plant employing a two-fuel oil tower system receiving a hydrocarbon feed from a quench water separator drum, where the two-fuel oil tower system is configured to make a sufficient solvent stream containing C.sub.6+C.sub.7 aromatic rich hydrocarbons that is recycled and mixed with quench water going to the quench water separator drum to assist in removing tar molecules out of the quench water.

Claims

1. A system for generating on-site a C.sub.6+C.sub.7 aromatic rich solvent stream, the system comprising: a quench water separator drum having sequential compartments including a last compartment, a primary fuel oil separation tower, and a secondary fuel oil separation tower; where: the quench water separator drum is connected to the primary fuel oil separation tower by a line directing a hydrocarbon stream from the last compartment of the quench water separator drum to the primary fuel oil separation tower; there is a recycle line connecting an overhead stream of the primary fuel oil separation tower to a feed stream of the quench water separator drum; and there is a line connecting a bottoms stream of the primary fuel oil separation tower to the secondary fuel oil separation tower.

2. The system of claim 1, further comprising a makeup stream connecting a portion of a C.sub.6+C.sub.7 rich bottoms stream of a debutanizer to the line directing a hydrocarbon stream from the last compartment of the quench water separator drum to the primary fuel oil separation tower.

3. The system of claim 1, where the quench water separator drum is a two phase separator having tar in a top light hydrocarbon phase.

4. The system of claim 1, where tar present in the feed stream of the quench water separator drum is dissolved in a hydrocarbon phase in the quench water separator drum.

5. A process for generating on-site a C.sub.6+C.sub.7 aromatic rich solvent stream, the process comprising: directing a hydrocarbon stream from a last compartment of a quench water separator drum to a primary fuel oil separation tower; separating C.sub.6+C.sub.7 aromatic hydrocarbons from C.sub.8+ hydrocarbons in the primary fuel oil separation tower to form the C.sub.6+C.sub.7 aromatic rich solvent overhead stream and a C.sub.8+ hydrocarbon bottoms stream; and recycling the C.sub.6+C.sub.7 aromatic rich solvent overhead stream to a feed stream of the quench water separator drum in an effective amount to separate tar from a water phase in the quench water separator drum and dissolve the tar in a hydrocarbon phase of the quench water separator drum.

6. The process of claim 5, further comprising directing the C.sub.8+ hydrocarbon bottoms stream to a secondary fuel oil separation tower and producing a C.sub.8-C.sub.10 steam cracked naphtha (SCN) product and a heavy fuel oil product in the secondary fuel oil separation tower.

7. The process of claim 6, where the primary fuel oil separation tower is operated at low pressure conditions ranging from about 12 psia (83 kPa) to about 17 psia (117 kPa).

8. The process of claim 6, where the primary fuel oil separation tower is operated at a temperature ranging from about 180° F. (82° C.) to about 320° F. (160° C.).

9. The process of claim 6, where the secondary fuel separation tower is operated under vacuum conditions ranging from about 2 psia (14 kPa) to about 7 psia (48 kPa).

10. The process of claim 6, where the secondary fuel separation tower is operated at a temperature ranging from about 200° F. (93° C.) to about 280° F. (138° C.).

11. The process of claim 5, further comprising mixing the hydrocarbon stream from the last compartment of the quench water separator drum with a portion of a C.sub.6+C.sub.7 rich bottoms stream from a debutanizer prior to entering the primary fuel oil separation tower thereby increasing the amount of C.sub.6+C.sub.7 aromatic hydrocarbons in the C.sub.6+C.sub.7 aromatic rich solvent overhead stream.

12. The process of claim 11 where the C.sub.6+C.sub.7 rich bottoms stream from the debutanizer is free of C.sub.5 hydrocarbons.

13. The process of claim 5 where the quench water separator drum is a two phase separator and the method further comprises dissolving tar into a top light hydrocarbon phase.

14. An ethylene plant comprising: a system for generating on-site a C.sub.6+C.sub.7 aromatic rich solvent stream, the system comprising: a quench water separator drum having sequential compartments including a last compartment, a primary fuel oil separation tower, and a secondary fuel oil separation tower; where: the quench water separator drum is connected to the primary fuel oil separation tower by a line directing a hydrocarbon stream from the last compartment of the quench water separator drum to the primary fuel oil separation tower; there is a recycle line connecting an overhead stream of the primary fuel oil separation tower to a feed stream of the quench water separator drum; and there is a line connecting a bottoms stream of the primary fuel oil separation tower to the secondary fuel oil separation tower.

15. The ethylene plant of claim 14, further comprising a makeup stream connecting a portion of a C.sub.6+C.sub.7 rich bottoms stream of a debutanizer to the line directing a hydrocarbon stream from the last compartment of the quench water separator drum to the primary fuel separation tower.

16. The ethylene plant of claim 14, where the quench water separator drum is a two phase separator having tar in a top light hydrocarbon phase.

17. The ethylene plant of claim 14, where tar present in the feed stream of the quench water separator drum is dissolved in a hydrocarbon phase in the quench water separator drum.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] FIG. 1 is a schematic illustration of a non-limiting embodiment of a two-fuel oil tower system for producing a C.sub.6+C.sub.7 aromatic rich solvent stream for recycling to the quench water separator drum; and

[0014] FIG. 2 is a schematic illustration of a non-limiting embodiment of a debutanizer system with a C.sub.5-rich side draw stream for production of a C.sub.5-free and C.sub.6+C.sub.7 rich bottoms stream for use as a tar solvent makeup for a quench water separator drum.

DETAILED DESCRIPTION

[0015] It has been discovered that, in circumstances in which having access to an external aromatic rich stream for tar solvation is difficult or expensive, an indigenous, sustained C.sub.6+C.sub.7 aromatic rich solvent stream may be produced within the olefin plant by a two-fuel oil tower configuration (one primary fuel oil tower and one secondary fuel oil tower) designed to also make specification grade steam cracked naphtha (SCN) product (C.sub.5-C.sub.10 range molecules) and specification grade heavy fuel oil product (C.sub.11+ range molecules) while also generating the desired solvent stream containing C.sub.6+C.sub.7 aromatic rich hydrocarbons that is mixed with quench water going to quench water separator drum to assist in removing tar molecules out of the quench water. In this two tower configuration, both the primary fuel oil separation tower and the secondary fuel oil separation tower operate at very low pressure to keep the operating temperature low, and so that steam is used as a stripping medium to control flash point in the fuel oil product. The desired stream containing C.sub.6 and C.sub.7 aromatic molecules are fractionated and produced intentionally, and this C.sub.6+C.sub.7 stream is mixed with quench water going to a quench water separator drum where it helps in removing tar molecules out of the water—thus achieving tar solvation. Directly addressing the presence of tar is important to manage fouling of exchangers and equipment in the quench system. Although it is a desirable goal to remove 100 wt % of the tar from the water, the process will be considered successful if at least 99.9 wt % of the tar is removed from the water, alternatively if at least 99.5 wt % of the tar is removed, and in another non-limiting embodiment 99.0 wt % of the tar is removed.

[0016] In other words, the amount of C.sub.6 and C.sub.7 aromatic molecules are made in small quantities from an ethane feed, in this process and system, the C.sub.6+C.sub.7 rich aromatic molecules are recycled in a closed loop. However, there will be at times loss of these molecules from the system due to equipment maintenance and draining. To avoid losing inventory of these desirable molecules, a makeup stream of debutanizer bottoms is provided to the primary fuel oil separation tower.

[0017] It has also been discovered that a C.sub.6+C.sub.7 aromatic rich solvent makeup stream may be generated by taking a portion of the bottoms stream of a debutanizer column located downstream in the olefin plant, which is comprised of C.sub.6+C.sub.7 aromatic rich hydrocarbons and is essentially free of C.sub.5 hydrocarbons, after a C.sub.5-rich side stream is drawn out of the debutanizer and providing the portion of the bottoms stream to the primary fuel oil tower to maintain inventory of C.sub.6+C.sub.7 aromatic rich tar solvation molecules in quench system. If some or all of this makeup stream is not used or if there is any surplus, the molecules are blended with pyrolysis gasoline product.

[0018] The schematic illustration in FIG. 1 shows one non-limiting embodiment of a system 10 and process for generating on-site a sustained C.sub.6+C.sub.7 aromatic rich solvent stream on the site of the olefin plant where the system 10 resides. More specifically referring to FIG. 1, process gas 12 from the pyrolysis furnaces (not shown), in a non-limiting embodiment at about 450° F. (about 232° C.) is fed to quench tower 14 where the process gas 12 is further cooled by direct contact with quench water 16. Typical quench tower tar limits without tar solvation are about 1 wt %, typical of butane cracking, in the process gas feed to the tower 14. Tar solvation dramatically improves the quench water quality also for feedstocks that make <1 wt % tar, like ethane and propane. Quench water 16 may be cooled against cooling water 18 (CW) in heat exchanger 20. For sites that recycle dilution steam (not shown), the water may be sent to the steam generators. Advantageously, in such cases, the tar solvation greatly reduces steam generator fouling. Benefits may also be realized for gas cracker systems that do not recycle steam. Due to the use of tar solvation, the water leaving quench water separator drum 26 should be clear and clean, and thus avoids downstream or later fouling of the quench circuit typically attributable to tar. Tar solvation turns the quench water separator drum 26 from a three phase separator with tar on the bottom, to a two phase separator with tar in the top light hydrocarbon phase.

[0019] Quench Tower Overhead vapor 22 is sent to a process gas compressor (PGC; not shown), in one non-limiting embodiment at a temperature of about 100° F. (about 38° C.), whereas the quench tower bottoms stream 24 containing condensed hydrocarbons is fed to quench water separator drum 26, which has a plurality of compartments. A hydrocarbon phase stream 28 from the last compartment of quench water separator drum 26 is sent to the primary fuel oil separation tower 30. Hydrocarbons 32 condensed in the process gas compressor (PGC; not shown) second stage may be also combined with hydrocarbon feed 28 from the quench tower separator drum 26.

[0020] The primary fuel oil separation tower 30 separates C.sub.6+C.sub.7 aromatic rich hydrocarbons in an overhead stream 34 from a C.sub.8+ hydrocarbon bottoms stream 36 through fractionation. Upon separation, the C.sub.6+C.sub.7 aromatic rich hydrocarbons in an overhead stream 34 from the primary fuel oil separation tower 30 overhead is recycled back to the quench tower bottoms feed 24 to create an effective concentration of C.sub.6+C.sub.7 aromatics in the quench water separator drum 26 to keep tar dissolved in hydrocarbon phase. The lower density of C.sub.6+C.sub.7 aromatic rich solvent overhead stream 34 helps in separating the water phase of the quench water in the quench water separator drum 26 from the hydrocarbon phase stream 28.

[0021] It will be appreciated that the primary fuel oil separation tower 30 is operated at low pressure to keep operating temperature low and steam is injected to lower the boiling point of the tower bottoms. Non-limiting examples of the pressure that may be used ranges from about 12 psia (83 kPa) independently to about 17 psia (117 kPa); alternatively from about 15 psia (103 kPa) independently to about 17 psia (117 kPa). When the term “independently” is used herein with respect to a range, it is meant that any threshold maybe used together with any other threshold given to provide a suitable alternative parameter range. Non-limiting examples of the operating temperature that may be used ranges from about 180° F. (82° C.) independently to about 320° F. (160° C.); alternatively from about 200° F. (93° C.) independently to about 280° F. (138° C.).

[0022] Referring again to the non-limiting embodiment shown in FIG. 1, the C.sub.8+ hydrocarbon bottoms stream 36 of the primary fuel oil separation tower 30 is fed to a secondary fuel oil separation tower 38 (running under deep vacuum for the reasons above) to produce a C.sub.8-C.sub.10 steam cracked naphtha (SCN) stream 40 and a C.sub.11+ heavy fuel oil product stream 42. In more detail, the secondary fuel oil separation tower 38 may be operated at a pressure in the range of from about 2 psia (14 kPa) independently to about 7 psia (48 kPa); alternatively from about 2 psia (14 kPa) independently to about 4 psia (28 kPa), and at an operating temperature that may be used ranges from about 200° F. (93° C.) independently to about 280° F. (138° C.).

[0023] It is further appreciated that, despite being produced and recycled in a closed loop as shown in FIG. 1, since C.sub.6+C.sub.7 aromatic molecules may originate only in small quantities from the ethane stream cracker furnace in the plant, upsets in the system 10 described herein due to pump draining and equipment maintenance may lead to problems with the material balance in the system 10 and losses of C.sub.6+C.sub.7 molecules. To avoid a loss in on-site supply of these molecules, a C.sub.6-C.sub.7 rich tar solvent makeup stream 44 of C.sub.6+C.sub.7 may be generated from a debutanizer column 48 that is further downstream of the quench and two-fuel oil tower systems in the olefin plant.

[0024] More specifically, a C.sub.6+C.sub.7 makeup stream 44 may be generated by, in the embodiment illustrated in FIG. 2, taking a portion of the C.sub.6-C.sub.7 rich bottoms stream 46 of a debutanizer column 48 receiving hydrocarbon feed 58 located downstream in the olefin plant, which is comprised of C.sub.6+C.sub.7 aromatic rich hydrocarbons and is essentially free of C.sub.5 hydrocarbons (which are not understood to be useful for dissolving tar at the quench water separator drum 26 operating temperature), after a C.sub.5-rich side draw stream 50 is drawn out of the debutanizer column 48 and providing the portion of the C.sub.6-C.sub.7 rich bottoms stream 46 to the primary fuel oil separation tower 30 as C.sub.6-C.sub.7 rich tar solvent makeup stream 44.

[0025] Referring again to the non-limiting embodiment of the system and process shown in FIG. 2, the debutanizer column 48 is configured to produce a C.sub.5-rich side draw stream 50. The operating conditions of the debutanizer column 48 include an overhead pressure of from about 29 psia (200 kPa) independently to about 130 psia (896 kPa) depending on condensing media (i.e., refrigerant, cooling water) and an overhead operating temperature ranging from about 1° F. (−17° C.) independently to about 113° F. (45° C.); and a bottoms temperature from about 180° F. (82° C.) independently to about 280° F. (138° C.). It will be appreciated that the location of C.sub.5-rich side draw 50 should be carefully evaluated through simulation of the debutanizer column 48 to ensure that the C.sub.5-rich side draw 50 has the highest concentration of C.sub.5 range hydrocarbons in order to minimize the amount of C.sub.5 range hydrocarbons concentration in C.sub.6-C.sub.7 rich bottoms stream 46 of debutanizer column 48. In accordance with this non-limiting embodiment, the C.sub.6-C.sub.7 rich bottoms stream 46 of debutanizer column 48 mainly consists of C.sub.6+C.sub.7 range hydrocarbons, a portion of which, based on the C.sub.6-C.sub.7 rich tar solvent makeup stream 44 needs of the two-fuel oil tower system and process shown in FIG. 1, is withdrawn as suitable C.sub.6-C.sub.7 rich tar solvent makeup stream 44 for tar solvation.

[0026] After drawing the necessary C.sub.6-C.sub.7 rich tar solvent makeup stream 44 for the quench system, net debutanizer bottoms stream 52 may be mixed with C.sub.5-rich side draw stream 50 in a pyrolysis gasoline drum 54 as net pyrolysis gasoline 56.

[0027] In the absence of an external source of aromatic rich solvent, such as is required in U.S. Pat. Nos. 7,560,019 and 8,025,773, the above-described system and process provides a distinct advantage over past invention to conduct tar solvation and avoid potential fouling of heat exchanges, quench tower packings, and other equipment.

[0028] In the foregoing specification, the invention has been described with reference to specific embodiments thereof. However, the specification is to be regarded in an illustrative rather than a restrictive sense. For example, fuel oil separation tower conditions and configurations, quench system conditions and configuration, debutanizer conditions and configuration, and the composition and amount of the various hydrocarbon and water streams falling within the claimed or disclosed parameters, but not specifically identified or tried in a particular example, are expected to be within the scope of this invention.

[0029] The present invention may be practiced in the absence of an element not disclosed. In addition, the present invention may suitably comprise, consist or consist essentially of the elements disclosed. In a non-limiting embodiment, there may be provided a system for generating on-site a C.sub.6+C.sub.7 aromatic rich solvent stream, where the system comprises, consists essentially of, or consists of a quench water separator drum having sequential compartments including a last compartment, a primary fuel oil separation tower, and a secondary fuel oil separation tower; where the quench water separator drum is connected to the primary fuel oil separation tower by a line directing a hydrocarbon stream from the last compartment of the quench water separator drum to the primary fuel oil separation tower, there is a recycle line connecting an overhead stream of the primary fuel oil separation tower to a feed stream of the quench water separator drum; and there is a line connecting a bottoms stream of the primary fuel oil separation tower to the secondary fuel oil separation tower.

[0030] There may be also provided in another non-restrictive version a process for generating on-site a C.sub.6+C.sub.7 aromatic rich solvent stream, where the process comprises, consists essentially of, or consists of directing a hydrocarbon stream from a last compartment of a quench water separator drum to a primary fuel oil separation tower, separating C.sub.6+C.sub.7 aromatic hydrocarbons from C.sub.8+ hydrocarbons in the primary fuel oil separation tower to form the C.sub.6+C.sub.7 aromatic rich solvent overhead stream and a C.sub.8+ hydrocarbon bottoms stream, and recycling the C.sub.6+C.sub.7 aromatic rich solvent overhead stream to a feed stream of the quench water separator drum in an effective amount to separate tar from a water phase in the quench water separator drum and dissolve the tar in a hydrocarbon phase of the quench water separator drum.

[0031] There may be further provided in a different non-limiting embodiment an ethylene plant comprising a system for generating on-site a C.sub.6+C.sub.7 aromatic rich solvent stream, where the system comprises, consists essentially of, or consists of a quench water separator drum having sequential compartments including a last compartment, a primary fuel oil separation tower, and a secondary fuel oil separation tower; where the quench water separator drum is connected to the primary fuel oil separation tower by a line directing a hydrocarbon stream from the last compartment of the quench water separator drum to the primary fuel oil separation tower, there is a recycle line connecting an overhead stream of the primary fuel oil separation tower to a feed stream of the quench water separator drum; and there is a line connecting a bottoms stream of the primary fuel oil separation tower to the secondary fuel oil separation tower.

[0032] The words “comprising” and “comprises” as used throughout the claims, are to be interpreted to mean “including but not limited to” and “includes but not limited to”, respectively.

[0033] To the extent used herein, the word “substantially” shall mean “being largely but not wholly that which is specified.”

[0034] As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise.

[0035] To the extent used herein, the term “about” in reference to a given parameter is inclusive of the stated value and has the meaning dictated by the context (e.g., it includes the degree of error associated with measurement of the given parameter).

[0036] As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.