Patent Grant
7867379
This invention relates to a process of producing an upgraded product stream from steam cracker tar feedstream suitable for use in refinery or chemical plant processes or for utilization in fuel oil sales or blending. This process utilizes an ultrafiltration process for separating the steam cracker tar constituents resulting in a high recovery, low-energy process with improved separation and product properties.
The steam cracking processes for the production of ethylene are common petrochemical processes. These processes are practiced throughout the world with millions of tons of ethylene product being produced annually. The ethylene steam cracking process typically involves thermally cracking a feedstock (such as ethane, propane, naphtha, gas oils, or even higher boiling point feedstocks) in a furnace at very high temperatures normally in the range of about 1382 to about 1742° F. (750 to 950° C.) in the presence of steam followed by rapidly cooling the thermally cracked stream to produce a product stream comprising ethylene.
Although it is desired to maximize the amount of ethylene produced in the process, many other compounds are produced as a byproduct of the cracking process. These byproducts generally range from elemental hydrogen, H2, as well as light hydrocarbon components, such as C1-C4 hydrocarbons to heavier hydrocarbon compounds including naphtha range materials and heavy oils including steam cracker tar.
The steam cracker tar is a high molecular weight, highly aromatic product that results from the production of ethylene. The steam cracker tar is the heavy fraction or “bottoms” fraction of the product produced by a steam cracking process typically with an initial boiling point of about 550° F. (288° C.). The amount of steam cracker tar that is produced is dependent upon the actual configuration of the ethylene steam cracker unit, but is also dependent upon the initial feed content utilized in the process. In general, the heavier the feedstock to the ethylene steam cracking process, the more heavy oils including steam cracker tar is produced. While it is preferable to use a light feedstock such as ethane, natural gas, or naphtha, the more limited availability and higher costs of these lighter ethylene cracker feedstocks have driven the ethylene cracking economics toward the use of heavier, higher molecular weight feed compositions to meet the growing demand for ethylene production. One significant drawback to utilizing a heavy feedstock such as a gas oil as the feedstream, is that the amount of steam cracker tar produced by the process can be significant and the product stream from the ethylene steam cracker furnace can contain more than about 15 wt % to more than 40 wt % steam cracker tar boiling range material.
This steam cracker tar product generally boils in the range from about 550 to about 1500° F. (288 to 816° C.) and has significant asphaltene content. Steam cracker tar is preferably sold as a fuel oil or bunker fuel but due to its high aromatic and asphaltene content the steam cracker tar stream can encounter compatibility problems with being blended with other fuel oils and bunker fuels and therefore often needs to be segregated and sold as a separate downgraded fuel product. This presents significant additional storage logistics and costs to a refiner over being able to blend the steam cracker tar into the standard fuel oil or bunker fuel product pool.
Therefore, there exists in the industry a need for an improved process for improving the properties of steam cracker tars for use as a blend stock for fuel oils or bunker fuels.
The invention includes a process for improving the properties of a steam cracker tar feedstream for use in subsequent petrochemical processes or as a compatible blend stock for fuel oil or bunker fuels.
One embodiment of the present invention is a process for producing a product stream from a steam cracked hydrocarbon feed stream comprising:
In a preferred embodiment, the second fractionated product stream is comprised of an ethylene steam cracker tar. In yet another embodiment, the process produces a first permeate stream wherein the Insolubility Number (“IN”) is at least 20% lower than the Insolubility Number (“IN”) of the second fractionated product stream.
In still other preferred embodiments, the first membrane utilized in the membrane separations process has an average pore size from about 0.001 microns to about 2 microns, and in yet another embodiment, the transmembrane pressure across the first membrane in the process is from about 100 psig to about 1500 psig.
This invention is a process for improving the properties of an ethylene steam cracker tar feedstream for use in subsequent petrochemical processes or as a compatible blend stock for fuel oil or bunker fuels.
What has been discovered is the use of an ultrafiltration process to produce an ethylene steam cracker tar product stream that possesses a reduced Micro Carbon Residue (MCR) or Conradson Carbon Number (CCR) which herein are taken as equivalents, reduced asphaltenes, reduced average molecular weight and an improved Insolubility Number (“IN”) (as determined by the “Wiehe” Method) for use as a fuel oil or bunker fuel blend stock. Additionally, the increased MCR, increased asphaltene product produced as a retentate product of the current invention may be used is such petrochemical processes us partial oxidation (POX) or in carbon black production.
The term “Micro Carbon Residue” (or “MCR”) as used herein is a measure of carbon content of a sample as measured per test method ASTM D4530. The terms “Micro Carbon Residue” (“MCR”) and “Conradson Carbon Residue” (“CCR”) are considered as equivalent values as used herein and these terms are utilized interchangeably herein.
The term “average boiling point” as used herein is defined as the mass weighted average boiling point of the molecules in a mixture. The term “initial boiling point” as used herein is defined as the temperature at which 5 wt % of the mixture is volatized at atmospheric (standard) pressure. The term “final boiling point” as used herein is defined as the temperature at which 95 wt % of the mixture is volatized at atmospheric (standard) pressure.
The term “transmembrane pressure” as used herein is defined as the difference in pressure as measured across a membrane element being the difference in pressure between the higher pressure feed/retentate side of the membrane element and the lower pressure permeate side of the membrane elements.
In accordance with the present invention, the steam cracker tar stream (80) is then sent to a membrane system (90) wherein the stream contacts at least one membrane and a steam cracker tar retentate stream (100) and a separated steam cracker tar permeate stream (110) are removed from the membrane system. In a preferred embodiment, the steam cracker tar permeate stream, containing reduced levels of asphaltenes and hence producing improved blending compatibility characteristics is blended into a fuel oil or bunker fuel product. In a preferred embodiment, the composition of the steam cracker tar permeate stream is sufficiently improved by the current invention to be sold directly as a fuel oil or a bunker fuel product. In yet another preferred embodiment, the steam cracker tar retentate stream is sent as feed to either a partial oxidation (POX) process or for the manufacture of carbon black.
It is preferred that the membranes utilized in the present invention be constructed of such materials and designed so as to withstand prolonged operation at elevated temperatures and transmembrane pressures. In one embodiment of the present invention the membrane is comprised of a material selected from a ceramic, a metal, a glass, a polymer, or combinations thereof. In another embodiment, the membrane comprised of a material selected from a ceramic, a metal, or combination of ceramic and metal materials. Particular polymers that may be useful in embodiments of the present invention are polymers comprised of polyimides, polyamides, and/or polytetrafluoroethylenes provided that the membrane material chosen is sufficiently stable at the operating temperature of the separations process. In preferred embodiments, the membrane material has an average pore size of about 0.001 to about 2 microns (μm), more preferably about 0.002 to about 1 micron, and even more preferably about 0.004 to about 0.1 microns.
In a preferred embodiment of the present invention, the temperature of the steam cracker tar stream prior to contacting the membrane system is at a temperature of about 212 to about 662° F. (100 to about 350° C.), more preferably from about 212 to about 572° F. (100 to about 300° C.), and even more preferably from about 302 to about 482° F. (150 to about 250° C.). The transmembrane pressure may vary considerably depending on the selectivity and the flux rates that are desired, but it is preferred if the transmembrane pressure is from about 100 to about 1500 psig, more preferably from about 250 to about 1200 psig and even more preferably from about 500 to about 1000 psig.
In a preferred embodiment, the heavy hydrocarbon feedstream is flowed across the face of the membrane element(s) in a “cross-flow” configuration. In this embodiment, in the retentate zone, the heavy hydrocarbon feed contacts one end of the membrane element and flows across the membrane, while a retentate product stream is withdrawn from the other end of the retentate zone. As the feedstream/retentate flows across the face of the membrane, a composition selective in saturated compounds content flows through the membrane to the permeate zone wherein it is drawn off as a permeate product stream. In a cross-flow configuration, it is preferable that the Reynolds number in at least one retentate zone of the membrane separations unit be in the turbulent range, preferably above about 2000, and more preferably, above about 4000. In some embodiments, a portion of a retentate stream obtained from the membrane separation units may be recycled and mixed with the feedstream to the membrane separations unit prior to contacting the active membrane.
The Examples below illustrate the improved product qualities and the benefits of the current invention for producing an improved product stream from a steam cracker tar.
In this Example, a sample of a stream cracker tar was permeated in a batch membrane process using a 8 kD (kiloDalton) cutoff ceramic ultrafiltration membrane, which had an average pore size of approximately 0.008 microns. The transmembrane pressure was held at 700 psi and the feed temperature was 75° C. The flux rates as well as the feed, permeates and retentate wt % Micro Carbon Residue (MCR) values are shown in Table 1. The feed supplied in a closed batch cell which was constantly stirred and was tested at the beginning and the end of the test cycle for wt % MCR per test method ASTM D4530. The permeate samples taken at given intervals and select permeate samples were tested for wt % MCR.
As can be seen from Table 1, the initial Permeate Sample 1 as compared to the Initial Feed shows a 77.4% reduction in Micro Carbon Residue. It can be also be seen in the “MCR (wt %)” column in Table 1 that as the permeate was removed from the system the feed composition in the batch cell increased from 21.36 wt % MCR in the initial feed to a final 27.68 wt % MCR in the final feed. Comparing the last permeate sample tested for MCR (Permeate Sample 4) to the retentate wt % MCR shows the separation achieved an 82.4% reduction in Micro Carbon Residue reduction in the final stages. This shows that the separation process remained essentially stable (with a slight improvement) over the test period.
As seen from the test data of this example, a preferred embodiment of the present invention can achieve a wt % reduction in Micro Carbon Residue of the permeate as compared to the feed of at least 25%, more preferably 40%, and even more preferably 60%.
Another improvement in the steam cracker tar permeate overall boiling point range of the materials has been decreased. This is shown in
A similar steam cracker tar sample and 8 kD (kiloDalton) cutoff ceramic ultrafiltration membrane configuration to those utilized in Example 1 were utilized in Example 2 to test the effects of temperature on the steam cracker tar separation process of the current invention. In this Example, the transmembrane pressure was held at 700 psi and the feed temperature was ranged from 75° C. to 150° C. A similar testing configuration as utilized in Example 1 was used in this example wherein the feed was supplied in a closed batch cell which was constantly stirred during the testing cycle. The feed temperatures and permeate flux rates as well as the feed, retentate, and select permeate wt % Micro Carbon Residue (MCR) values are shown in Table 2. The permeate samples taken at given intervals and select permeate samples as well as the final retentate were also tested for wt % MCR per test method ASTM D4530.
As can be seen from Table 2, the initial Permeate Sample 1 as compared to the Initial Feed shows a 83.12% reduction in Micro Carbon Residue. This Permeate Sample 1 was run at a feed temperature of 75° C. The final Permeate Sample 10, which was run under a feed temperature of 150° C., shows a decrease in the % Reduction of Micro Carbon Residue of about 65.67% as compared with the final retentate. This indicates that there is some loss in efficiency of MCR reduction as the temperature of the feed increases. Conversely, it can be seen by viewing the “Permeate Flux Rate” column of Table 2, that at each temperature increase, a corresponding increase in the flux rate was experienced. Therefore, there is an optimal feed temperature that may be selected for a given feed depending upon the product quality desired and the volumetric recovery rate desired for the permeate and/or retentate product.
As seen from the test data of this example, a preferred embodiment of the present invention can achieve a wt % reduction in Micro Carbon Residue of the permeate as compared to the feed of at least 25%, more preferably 30%, even more preferably 50%, and even more preferably 70%.
In contrast, the average boiling point of the permeate run at a feed temperature of 150° C. shows little difference from the average boiling point of the feed stream (as shown by the similarity of the SIMDIS curves in
Selected permeate samples along with the initial feed and final retentate from Example 2 above were tested for a reduction in asphaltenes by determining the fraction of sample insoluble in cyclohexane. The data is shown in Table 3 and also graphically in
As can be seen in Table 3 above, the asphaltenes present in the steam cracker tar are significantly reduced in the permeate associated with the process of the present invention. In particular, an embodiment of the present invention results in a steam crack tar permeate product with at least a 25% reduction in cyclohexane soluble asphaltenes as compared to the steam crack tar feed. More preferably, the steam crack tar permeate product possesses at least a 50% reduction and even more preferably a 75% reduction in cyclohexane soluble asphaltenes as compared to the steam crack tar feed.
In this Example, the initial feed and selected permeate samples from Example 2 above were tested for compatibility by automated titration for the steam cracker tar initial feed as well as Permeate Samples 9 and 10. The Insolubility Number, “IN”, for a sample is determined in accordance with the method disclosed in U.S. Pat. No. 5,871,634 to Wiehe et al. which is incorporated by reference herein. The results of these tests for the initial feed sample and select permeate samples is shown as the Insolubility Number (“IN”) in Table 4.
As can be seen from the Insolubility Numbers in Table 4, as well as the asphaltene reduction data from Table 3 of Example 3, not only do the steam cracker tar permeate products of the present invention have a reduction in asphaltenes, but the degree of aromaticity required to dissolve the remaining asphaltenes has been significantly reduced. This results in a steam cracker permeate product that is significantly more compatible for use as a blend stream for either fuel oils or bunker fuels or as a final product.
As can be seen in Table 4, one embodiment of the present invention results in a steam cracker tar permeate product with a % decrease in the Insolubility, IN, of at least 20%, more preferably at least 25%, and even more preferably at least 30% as compared to the steam cracker tar stream prior to the ultrafiltration separation process.
Although the present invention has been described in terms of specific embodiments, it is not so limited. Suitable alterations and modifications for operation under specific conditions will be apparent to those skilled in the art. It is therefore intended that the following claims be interpreted as covering all such alterations and modifications as fall within the true spirit and scope of the invention.
This application claims the benefit of U.S. Provisional Application No. 60/966,472 filed Aug. 28, 2007.
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