Method for Reducing Quench Oil Fouling in Cracking Processes

Abstract

Quench oil aging and its propensity to cause fouling may be evaluated by determining the amount of a precipitant necessary to cause the flocculation of polymer species present in the quench oil. The propensity of quench oil to cause fouling may be used as a basis to mitigate fouling in cracking processes.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This Application claims priority from the U.S. Provisional Patent Application having the Ser. No. 60/888,466 which was filed on Feb. 6, 2007; the contents of which are fully incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for reducing fouling in cracking processes. The present invention particularly relates to a method for reducing fouling from quench oil in cracking processes due to aging of the quench oil.

2. Background of the Art

Petrochemical plants, which include both Chemical Production Installations as well as Oil Refineries are known to employ two basic types of furnaces. The first of these is a steam cracker furnace. Steam crackers are used in applications including the production of ethylene. The second of these is a “steam reformer” furnace, which can be used to make hydrogen. Both types of furnaces include a number of tubes, generally arranged vertically, that form a continuous flow path, or coil, through the furnace. The flow path or coil includes an inlet and an outlet. In both types of furnaces, a mixture of a hydrocarbon feedstock and steam are fed into the inlet and passed through the tubes. The tubes are exposed to extreme heat generated by burners within the furnace. As the feedstock/steam mixture is passed through the tubes at high temperatures the mixture is gradually broken down such that the resulting product exiting the outlet is ethylene in the case of a steam cracker furnace and hydrogen in the case of a steam reformer furnace as well as other products including gasoline and coke.

During the cracking processes, the feed materials are heated to very high temperatures, in some embodiments, up to 900° C. This output is cooled by mixing it with a colder fluid and fed in a fractionating column where the separation of ethylene and light gasoline from a heavier oil takes place. The quality of the distillation, i.e. the amount of ethylene, light olefins and gasoline extracted from the top of the column, may be influenced by the temperature of the feed in the fractionating column. A higher temperature results in a higher yield of light products, which is often desirable. Attempting to handle such hot materials is usually not desirable and thus the need for a cooling step.

In some processes, the cooling step is implemented by admixing the very hot products from the cracking units with a comparatively cool fluid. The cool fluid, often an oil and most often a heavy oil, is typically referred to in the art as a “quench oil.” The heavy quench oil may be extracted from the process and is marketable as fuel oil.

In many processes, a minor amount of the quench oil is extracted to be used as a fuel, while the remaining part is recycled, sometimes back into the cracking process as a feed to the cracking unit or as reuse as a quench oil or both. During the course of its use, the heavy oil which is used as a quench oil may be continually exposed to temperatures ranging from 100 to 220° C. for extended periods of time.

Recycling quench oil may result in a number of serious unfavorable side effects. For example, viscosity increases of the recycled quench oil may be observed. In fact, the recirculating quench oil may remain in the circuit at relatively high temperatures for long periods of time, and this causes its aging. Symptomatic of this aging is the presence of unsaturated compounds, polymer and rubber formation, and a resulting viscosity increase. All of these side effects obviously may cause a negative impact upon the functioning of a production plant. Such negative impacts include an increase in the power required by the recirculation pumps, a reduction of the thermal exchange coefficients involved in steam production, and an increase of the maintenance costs involved in the cleaning of the plant components exposed to the quench oil.

SUMMARY OF THE INVENTION

In one aspect the invention is a method for reducing fouling from quench oil comprising treating a hydrocarbon feed using a cracking process having a quenching step, wherein: quench oil used in the quenching step has a known tendency to cause fouling; and the known tendency of the quench oil to cause fouling has been determined by measuring a tendency of the quench oil to precipitate polymeric species.

In another aspect the invention is method for reducing fouling from quench oil comprising treating a hydrocarbon feed using a cracking process having a quenching step, wherein process conditions in the cracking process have been adjusted based upon the tendency of quenching oil in the quenching step to cause fouling which is determined by measuring the tendency of the quench oil to precipitate polymeric species.

In one aspect the invention is a method for reducing fouling from quench oil in a cracking process comprising treating a hydrocarbon feed using a cracking process having a quenching step, introducing an additive to reduce fouling to the cracking process based upon a tendency of the quench oil in the quenching step to cause fouling which is determined by measuring a tendency of the quench oil to precipitate polymeric species.

In another aspect, the invention is a method for predicting the tendency for a quench oil to cause fouling in a cracking process by measuring the tendency of the quench oil to precipitate polymeric species.

In still another aspect, the invention is a method for measuring the tendency of the quench oil to precipitate polymeric species.

In another aspect, the invention is an apparatus for measuring the tendency of the quench oil to precipitate polymeric species.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed understanding of the present invention, reference should be made to the following detailed description of the preferred embodiments, taken in conjunction with the accompanying drawing(s) wherein:

FIG. 1 is graph showing the typical output of a transmittance probe in a quench oil sample during the addition of a precipitant to the quench oil sample.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In one aspect the invention is method for reducing fouling from quench oil in a cracking process comprising treating a hydrocarbon feed using a cracking process having a quenching step. Cracking processes are well known in the art of refining oil and other chemical processes. Such processes include, but are not limited to those disclosed in U.S. Pat. Nos. 6,096,188; 5,443,715; and 5,215,649; which are fully incorporated herein by reference. In the practice of one embodiment of the invention, a quench oil is contacted with an intermediate or even a final product of a cracking process.

The quench oils useful with some embodiments of the present invention may be selected from the group consisting of crude oil; the precursors of naphthalene, phenanthrene, pyrene, quinoline, and hydroquinone; alkyl derivatives of naphthalene, phenanthrene, pyrene, quinoline, and hydroquinone. The quench oils may also be selected from the group consisting of aromatic molecules containing phenol groups and aromatic molecules containing non-phenolic oxygen substitutes. Also useful as the quench oil in some embodiments of the present invention are those compounds selected from the group consisting of steam cracked quench oils, steam cracked tars, cat cracked tars, cat cracked cycle oils, cat cracked bottoms, coker gas oils, coal tar oils, and aromatic extent oils and cuts of steam cracked quench oils, steam cracked tars, cat cracked tars, cat cracked cycle oils, cat cracked bottoms, coker gas oils, coal tar oils, and aromatic extract oils.

The hydrocarbons feeds that can be treated using the process of the present invention include, but are not lime tied to crude oil and intermediate refinery products resulting from the refining of crude oil.

In the process of treating a hydrocarbon feed using a cracking process, many products may be made including ethylene, gasoline, diesel fuel, other fuel oils, and coke. Processes producing heavy oils and coke are often subject to fouling. For the purposes of this application, fouling is a condition wherein materials having a very high viscosity and mixtures of viscous materials and solids such as coke deposits from the quench oil and accumulate within process equipment causing reduced operational efficiency or even shutting down the processing equipment. For example, when fouling occurs, it may cause transfer pipes to clog which in turn may require the cracking unit to reduce process throughput or even shut down the unit. Such slow-downs and shut-downs often result in increased operating costs for the units affected and also any integrated units upstream or downstream of the affected unit.

In one aspect, the invention is a process for reducing fouling from quench oils by selecting quench oils that have a reduced tendency to produce fouling. In the practice of the invention, the tendency to produce fouling of a quench oil is determined by measuring the tendency of the quench oil to precipitate polymeric species. Stated another way, the difference in solubility parameters of candidate quench oils for use in a cracking process and for polymeric species present therein can be measured and this measurement used as a basis for evaluating the propensity of the quench oil to undergo a polymer phase separation which may cause the deposition of foulants during a cracking process.

The tendency of candidate quench oils to precipitate polymeric species may be determined by any means known to those of ordinary skill in the art of making such determinations to be useful. For example, in one embodiment of the invention, a sample of a quench oil candidate is placed in a container with a probe capable of measuring light scattering properties of the quench oil. In this embodiment, aliquots of a precipitant are added to the quench oil and the light scattering properties of the quench oil measured. A precipitant having a high light transmission level relative to the quench oil is used and the “dilution” effect of the precipitant will initially cause a reduction of light scattering in the sample until sufficient precipitant is added to the sample to cause precipitation of the polymer species thereby increasing light scatter. By comparing the amount of precipitant required to cause an increase in light scattering, sometimes also referred to as flocculation, quench oil candidates may be compared. In one embodiment of the invention, quench oil candidates requiring more precipitant to increase light scattering are considered less likely to foul than those candidates requiring less precipitant.

Precipitants useful with the invention include any which have a higher light transmission than the quench oil samples to be tested and which will cause a precipitation of polymer species from the quench oil. In one embodiment, these precipitants are selected from aliphatic solvents. Typical aliphatic solvents useful with the present invention may include pentane, hexane, heptane, octane, isobutane, cyclohexane, and the like. Any precipitant may be used as long as it meets the specified criteria.

In the practice of the invention, it may be desirable to dilute the quench oil with a solvent. For example, in the case of colored quench oil candidates, it may be desirable to dilute the quench oil candidates to a point that they are within a specified transmission scale for a particular type of probe. The solvents used should be selected so that they do not materially interfere with the precipitation of polymeric species. For example, in one embodiment of the invention, the solvents used with the present invention are aromatic solvents. Such solvents include, but are not limited to benzene, toluene, xylene, ethyl benzene, and mixtures thereof.

Once the amount of precipitant necessary to cause onset of flocculation is known, it may be desirable to repeat the experiment with differing amounts of solvent and determine the flocculation point by means of a linear regression calculation. Any method of comparing the results from the measurements may be used to evaluate the relative propensity of various quench oil candidates to precipitate polymer species.

In one embodiment of the invention, an automatic titrator is used in conjunction with a light probe to determine the flocculation point of a quench oil. An automatic titrator advantageously can dispense exact aliquots of precipitants and, when networked with suitable equipment, also record light scattering of sample therein. In an alternative embodiment, the automatic titrator, probe, and other equipment are networked to a controller. In many such embodiments, the controller is a personal computer.

The flocculation point of a quench oil is determined in some embodiments of the method of the invention by noting the point at which during a series of addition of precipitant to a quench oil sample, that light scattering starts to increase. The ability of a sample of quench oil to scatter light may be measured by any means known to useful to those of ordinary skill in the art of making such measurements. Preferably, the measurement is made using a probe and most preferably using a fiber optic probe. Exemplary fiber optic probes include transmission probes, reflectance probes, and attenuated total reflectance probes. Each of these probes has strengths and weaknesses that would make them more or less desirable for any given set of conditions. Those of ordinary skill in the art of making such measurements will know which probe to select for an application. For example, where the sample have a high level of opacity, it may be more desirable to use an attenuated total reflectance probe rather than a transmission probe. In one preferred embodiment, a fiber optics diffuse reflectance probe is used wherein a single fiber acts as a light source and 6 other fibers arranged around the source collect backscattered light.

The type of light employed by each probe may also be selected according to the conditions of the desired testing conditions. For example, the light employed may be UV, VIS or NIR. Such probes often employ silicon or germanium detectors. Any device useful for measuring light intensity may be used with the present invention.

The type of probe used will determine whether flocculation is observed by a decrease or an increase in light intensity at a detector. As a sample increases in ability to scatter light, less light passes directly through the sample. Transmittance probes function by measuring the amount of light passing through a sample. Using a transmittance probe, a measurement according to the invention would see an increase in the power of the light reaching the detector until the flocculation point at which time the power may rapidly decrease. For a reflectance probe, the observations would be the inverse with power decreasing until the flocculation point.

In additional to making single determinations, the method of the invention may be used continuously. In this embodiment of the invention, the flocculation point of recycled quench oil is measured as a function of time. As the amount of precipitant need to cause flocculation decreases, the likelihood of fouling increases. At some point in time, either based upon prior experience or use of a predictive model, the determined tendency of the recycled quench oil to foul is used as a basis to divert the quench oil from recycle to an alternative disposition such as use as a fuel oil or the like. In an alternative embodiment of the invention, rather than diverting quench oil as it reaches a certain tendency to foul, the process parameters may be changed to slow or prevent quench oil “aging.” For the purposes of the present application, “quench oil aging” means the phenomena where quench oil has a greater tendency to foul with time held at high temperatures such as is observed with quench oil that has been recycled. In still another embodiment of the invention, the measured tendency of the quench oil to foul can be used as a basis for a decision to introduce additives into the cracking process to reduce fouling.

Additives useful for quench oil viscosity fouling reduction and control include, but are not limited to, well known chemistries to those skilled in the art, such as dispersants, radical scavengers and fouling control additives made of overbased metal carboxylates and sulphonates. In some embodiments of the invention, additives these could include blends of the commercial dispersant/antifoulant product BPR34260 supplied by Baker Petrolite Corporation, antioxidants based on sterically hindered phenols and phenols, and their blends with amines such phenylene diamine and magnesium oxide overbase.

In the practice of the invention, the density, type and opacity of the quench oils to be evaluated will determine how the quench oils will be tested. Those of ordinary skill in the art of running a cracking unit are knowledgeable regarding the methodology necessary to test their processes. Still, generally, samples tested according to the invention may have sample sizes running from about 3 grams to about 50 grams. When diluted, the quench oils may be diluted in ratios (quench oil: Aromatic solvents) ranging from about 10:1 to about 1:20, and in some embodiments from about 2:1 to about 1:3. Typically, samples of quench oil are heated to from about 45 to about 60° C. prior to testing.

In an alternative embodiment of the invention, Hildebrand solubility parameters are determined for a sample of quench oil. The Hildebrand solubility parameters are determined by making several runs with the quench oil dissolved in varying amounts of aromatic solvent. The quantity of precipitant needed to reach the flocculation point is divided by the sample size of the quench oil and linearly correlated with the dilution ratio. From this relationship, the Hildebrand solubility parameters are then determined.

In some embodiments of the invention, it may be desirable to adjust process conditions in the cracking process based upon the tendency of quenching oil in the quenching step to cause fouling. While those of ordinary skill in the art are well aware of how to adjust a specific cracking process based upon a understanding of whether or not the quench oil used in the cracking process is likely to cause fouling, generally process parameters that could be adjusted include process temperatures, pressures, and residence times. For example, in at least some cracking processes, if an operator of the cracking process was aware that the quench oil used in the cracking process was likely to cause fouling, the operator may elect to decrease residence times, lower cracking temperatures, or increase pressures within the cracking process. In other embodiments, an operator may select to make the same or different adjustments based upon the specific characteristics of the subject cracking process. In one specific example, an operator may elect to change quench oil column (also known as Pyrolysis Column) bottom temperature, bottom column level, and rate of reflux of pyrolysis gasoline to the quench oil column.

While not wishing to be bound by any theory, it is believed that the polymer species that is precipitated from quench oils that result in the deposition of foulants within a cracking process are heavy aromatic polymers.

EXAMPLES

The following examples are provided to illustrate the present invention. The examples are not intended to limit the scope of the present invention and they should not be so interpreted. Amounts are in weight parts or weight percentages unless otherwise indicated.

Example 1

A sample of quench oil is placed into an automatic titrator. The reservoir of the automatic titrator is filled with normal heptane. A transmission probe is placed into contact with the quench oil sample and both the transmission probe and the automatic titrator are attached to a controller that records both light scattering and ml of n-heptane introduced into the sample. A curve showing a plot of this experiment is displayed in FIG. 1.

Example 2

Five quench oil candidate materials are tested on an apparatus substantially identical to that of Example 1. Each material is tested 5 times and the data compared. For each sample, the repeatability of flocculation point is less than 3 percent of the precipitant used.

Example 3 (Hypothetical)

The samples tested in Example 2 are evaluated for use with a steam cracker unit. The samples have a comparative value for flocculation point of:

• • Sample I: 1.2
  • Sample II: 2.9
  • Sample III: 1.7
  • Sample IV: 1.7
  • Sample V: 1.0
    Sample II is selected as the quench oil for the unit.

Example 4 (Hypothetical)

The recycle quench oil is tested substantially identically to Example 1 except that samples are removed from a cracking unit every 12 hours. The rate in decrease of the flocculation point is measured and compared against known conditions resulting in increased fouling. When the flocculation point decreases to the point that increased fouling appears likely to occur, the recycle quench oil is diverted for alternative disposition.

Example 5 (Hypothetical)

Example 4 is repeated substantially identically except that instead of diverting the quench oil from recycle, additives are introduced into the cracking unit to reduce fouling.

Example 6 (Hypothetical)

Example 4 is repeated substantially identically except that instead of diverting the quench oil from recycle, the conditions in the cracking unit are adjusted to extend the useful life of the quench oil.

Discussion of the Examples

Example 1 and FIG. 1 clearly show that from the beginning of the experiment until about 23.5 ml of precipitant had been introduced into the sample, light transmission increased, caused by the dilution effect of the precipitant. At about 23.5 ml, scattering stop decreasing and began increasing. This is the point at which flocculation occurred.

Claims

1. A method for reducing fouling from quench oil comprising: treating a hydrocarbon feed using a cracking process having a quenching step wherein:

quench oil used in the quenching step has a known tendency to cause fouling; and

the known tendency of the quench oil to cause fouling has been determined by measuring a tendency of the quench oil to precipitate polymeric species.

2. The method of claim 1 further comprising adjusting process conditions in the cracking process based upon the tendency of quenching oil in the quenching step to cause fouling.

3. The method of claim 1 wherein the quench oil is selected from the group consisting: of crude oil; the precursors of naphthalene, phenanthrene, pyrene, quinoline, and hydroquinone; alkyl derivatives of naphthalene, phenanthrene, pyrene, quinoline, and hydroquinone; and mixtures thereof.

4. The method of claim 1 wherein the quench oil is selected from the group consisting of: steam cracked quench oils; steam cracked tars; cat cracked tars; cat cracked cycle oils; cat cracked bottoms; coker gas oils; coal tar oils; aromatic extent oils; cuts of steam cracked quench oils, steam cracked tars, cat cracked tars, cat cracked cycle oils, cat cracked bottoms, coker gas oils, coal tar oils, and aromatic extract oils; and mixtures thereof.

5. The method of claim 1 wherein the hydrocarbon feed is selected from the group consisting of: crude oil, intermediate refinery products resulting from the refining of crude oil, and mixtures thereof.

6. The method of claim 1 wherein the hydrocarbon feed is used to produce ethylene, gasoline, diesel fuel, other fuel oils, or coke.

7. The method of claim 6 wherein the hydrocarbon feed is used to produce ethylene.

8. The method of claim 1 wherein the tendency of the quench oil to precipitate polymeric species is determined by measuring light scattering.

9. A method for reducing fouling from quench oil comprising: treating a hydrocarbon feed using a cracking process having a quenching step, and introducing an additive to reduce fouling to the cracking process based upon a tendency of the quench oil in the quenching step to cause fouling which is determined by measuring a tendency of the quench oil to precipitate polymeric species.

10. The method of claim 9 wherein the tendency of the quench oil to precipitate polymeric species is determined by measuring light scattering.

11. The method of claim 10 wherein differences in solubility parameters of candidate quench oils for use in a cracking process and for polymeric species present therein is measured and this measurement used as a basis for evaluating the propensity of the quench oil to undergo a polymer phase separation which may cause the deposition of foulants during a cracking process.

12. The method of claim 11 wherein the quench oil candidate is placed in a container with a probe capable of measuring light scattering properties of the quench oil.

13. The method of claim 12 further comprising introducing aliquots of a precipitant to the quench oil and measuring the light scattering properties of the quench oil.

14. The method of claim 13 wherein a quench oil candidate requiring more precipitant to increase light scattering is considered less likely to foul than a quench oil candidate requiring less precipitant.

15. The method of claim 13 wherein the precipitant is selected from the group consisting of pentane, hexane, heptane, octane, isobutane, cyclohexane, and mixtures thereof.

16. The method of claim 12 wherein the probe is a fiber optic probe.

17. The method of claim 16 wherein the fiber optic probe is selected from the group consisting of transmission probes, reflectance probes, and attenuated total reflectance probes.

18. The method of claim 17 further comprising using an automatic titrator to measure the light scattering properties of the quench oil.

19. The method of claim 9 further comprising using solvent to dilute the quench oil prior to measuring the tendency of the quench oil to precipitate polymeric species.

20. The method of claim 19 wherein the solvent is selected from the group consisting of: benzene, toluene, xylene, ethyl benzene, and mixtures thereof.