WO1997008266A1

WO1997008266A1 - Visbreaking process using plastics as co-feed

Info

Publication number: WO1997008266A1
Application number: PCT/US1996/012703
Authority: WO
Inventors: Mark Peter Bartilucci; William Joseph Tracy, Iii; Gerald Joseph Teitman
Original assignee: Mobil Oil Corporation
Priority date: 1995-08-22
Filing date: 1996-08-05
Publication date: 1997-03-06

Abstract

A visbreaking process for the conjoint disposal of polyolefin plastic materials and production of fuel oil basestock is disclosed that comprises co-feeding a feedstream comprising between 0.01 and 5 weight percent of plastic materials and a heavy petroleum feedstock into a visbreaking zone under high severity visbreaking conditions. As a result, the visbroken product contains an enhanced yield of gas oil fraction of lower viscosity plus a reduced yield of carbonaceous sediment and bottoms fraction of substantially lower viscosity.

Description

VISBREAKING PROCESS USING PLASTICS AS CO-FEED

This invention relates to a novel process for the disposal of plastic waste material. The invention especially relates to a process for the disposal of polyolefin plastic material as part of the feedstream to visbreaking processes and to the discovery of the synergistic effect on visbroken product properties of the conjoining of polyolefin with a heavy petroleum visbreaking feedstream.

Where once material recycling and resource recovery were common words and practice in but a few industries, in a surprisingly short time they are now common in virtually every home and industry in the land. Responding to the new awareness of the fragility of our ecosystem with passion and prudence, society has set about the rectification of past material excesses by falling upon the more ubiquitous materials used in modern civilization for corrective recovery. There is none more ubiquitous than plastics and no more ubiquitous plastics than polyethylene and polypropylene. Most plastic materials, particularly polyolefins, are produced from petroleum derived raw materials. Thus, polyolefins are made by polymerizing a-olefin monomers such as ethylene and propylene which in turn are made by cracking of petroleum derived hydrocarbons. Petroleum derived hydrocarbons are the main component of petroleum derived fuels and petrochemicals. It would therefore be desirable to reconvert the polyolefins into products which are more like their petroleum based starting materials in order to be able to reuse the carbon and the hydrogen content of the plastics.

There have been many attempts made in the past to convert all sorts of waste materials into petroleum type products, either fuels, lubricating oils, coke, or other products. In United States Patent No. 1,950,811 there is disclosed a process for recovering oil and coke from oil bearing residues in combination with non-petroleum raw materials such as coal, peat or sawdust by treating a suitable feed at high temperatures of about 900 to 1,000°F. In United States Patent No. 2,412,879, a process for producing coke is disclosed wherein the feed to the coker is a mixture of conventional petroleum based coker feed and about 1-10 % cellulosic material.

In United States Patent No. 3,909,364, a carbonizable waste, such as garbage and sanitary sludge, is mixed with coal. This mixture is then devolatilized to produce a char which is mixed with residual oil to produce a solid fuel product.

None of these prior processes has taken into account or been designed to treat plastic waste materials. Pyrolysis processes have been described as offering the possibility of conversion of some solid organic wastes. Reference is here made to "Industrial Solid Wastes

Management", pp 356-406, Proceedings of the National Industrial, Solid Wastes Management Conference, for a discussion of some of the conventional means for carrying out this desirable work. In United States Patent No. 4,108,730, there is disclosed a means for disposing of solid polymeric wastes, such as rubber tires, plastic wares, plastic packaging, scrap plastic, etc. The process dissolves these materials in heavy petroleum oils such as FCC heavy cycle oil in the absence of added hydrogen. The feeds are dispersed and dissolved with little or no gas evolution. The resultant liquid is said to resemble crude oil and to make an excellent feed to a catalytic cracker. The cracking of this feed results in the usual array of products from a cat cracker.

In United States Patent No. 4,118,281, a process has been disclosed for the dissolution of organic waste materials, including garbage, plastic, paper, wood, rubber, etc. in a conventional feed to a delayed coker unit process. Thermal decomposition of this mixture under conventional coker operating conditions is said to convert this feed into oil, water, gases and coke. The waste material which is being used to augment the conventional coker feed is suitably dissolved in a refinery fraction such as catalytic cracker recycle, FCC main column bottoms, TCC syntower bottoms, and the like. The preferred dissolving materials for these wastes are set forth to be fresh or recycle petroleum coker feed. This patent holds that carrying out the process described therein produces more oil than gas phase pyrolysis of organic waste materials.

As noted above, the high conversion petroleum refining processes of fluid catalytic cracking and coking have been adapted as means to dispose of plastic materials conjointedly with hydrocarbon processing; but less vigorous processes have not been so exploited. For instance, no prior interest has been directed to the use of visbreaking processes as a means for the disposal of plastics. The artisan, perhaps, has been dissuaded from investigating visbreaking for plastics disposal by the relatively mild energetic conditions of the process that, at first blush, seem to offer no compelling advantage over FCC and similar high conversion processes for upgrading petroleum feeds.

Also, the heavy fuel oil product produced in the visbreaker is frequently used for heavy diesel fuel, a ship-fuel product with demanding quality specifications. Visbreaking, or viscosity breaking, is a well known petroleum refining process in which heavy oils including residual fractions or reduced crudes are pyrolyzed, i.e., cracked, under comparatively mild conditions to provide products having lower viscosities. The process reduces the amount of less viscous and more valuable blending oils required to make the residual visbroken stocks useful as fuel oils. The visbreaker feedstock usually consists of one or more refinery streams derived from sources such as atmospheric residuum, vacuum residuum, vacuum gas oil (VGO) furfural extract, propane-deasphalted tar and catalytic cracker bottoms. Most of these feedstock components, except the heavy aromatic oils, behave independently in the visbreaking operation. Consequently, the severity of the operation for a mixed feed is limited greatly by the least desirable, i.e, highest coke-forming, components.

For a typical visbreaking process, the crude or resid feed is passed through a heater and heated to between 425°C (797°F) and about 525°C (977°F) at about 450 to about 7000 kPa. Light gas-oil may be recycled from the product fractionator to quench the visbreaker reactor effluent to about 260°C to about 370°C. Cracked products from the reaction are flash distilled with the vapor overhead being fractionated into a light distillate overhead product, for example gasoline and light gas-oil bottoms, and the liquid bottoms are vacuum fractionated into heavy gas-oil distillate and residual tar. Examples of such visbreaking methods are described in Beuther et al, "Thermal Visbreaking of Heavy Residues," The Oil and Gas Journal, 57:46, Nov. 9, 1959, pp. 151-157; Rhoe et al, "Visbreaking: A Flexible Process, " Hydrocarbon Processing, January 1979, pp. 131-136; and U.S. Pat. No. 4,233,138. Visbreaking processes are one of the more commonly utilized processes for upgrading heavy petroleum feedstock and are available in many refineries, large and small. However, the mild operating conditions of visbreaking suggests that the process would be relatively unforgiving with respect to the potential for upsets caused by changes in the feedstream composition to the process such as cofeeding plastics. Nevertheless, the availability of the process invites a conclusion that visbreaking may have great merit as a refinery process for disposal of certain plastic waste material, especially polyolefins, if plastic waste do not upset the process or product slate.

Therefore, it is an object of the present invention to provide a visbreaking process for the combined disposal of plastic waste and upgrading of heavy petroleum feedstock.

Additionally, it is an object of the present invention to provide a visbreaking process for the conjoint disposal of plastic waste that also provides a visbroken product of superior quality compared to conventional visbroken product quality.

The discovery inherent in the present invention relates to a range of visbreaking process conditions that permit the conjoint disposal of polyolefin plastic material as a portion of the largely heavy petroleum feedstock to the process. Surprisingly, the discovered conditions not only permit the use of visbreaking for plastic material disposal but lead to products of lower viscosity that are more compatible with conventional gas oils as blended to provide heavy fuel oil. Especially surprising is the fact that the products of the novel visbreaking process when blended to produce fuel oil provide a product with a significantly lower amount of sediment. Feeding polyolefins such as polyethylene and polypropylene to a visbreaker has revealed a decrease in the viscosity of the visbreaker gas oil (VBGO) as well as visbreaker (VB) bottoms. Also, there is a reduction in the viscosity of the VBGO/VB bottoms mixture. More definitively, a visbreaking process for the conjoint disposal of polyolefin plastic materials and production of fuel oil basestock has been discovered that comprises cofeeding a feedstream comprising between 0.01 and 5 weight percent of plastic materials and a heavy petroleum feedstock into a visbreaking zone under visbreaking conditions. As a result, the visbroken product contains an enhanced yield of gas oil fraction, particularly the 270-600°F gas oil fraction, of lower viscosity plus a reduced yield of carbonaceous sediment and bottoms fraction, especially 600°F bottoms fraction, of substantially lower viscosity. The visbreaking conditions comprise an equivalent reaction time, ERT, between 200 and 1000 seconds at a temperature between 650°F and 950°F.

The invention includes an improvement in the process for visbreaking a heavy petroleum feedstock to produce fuel oil basestock which comprises cofeeding a feedstream comprising between 0.01 and 5 weight percent of polyolefin plastic materials and heavy petroleum feedstock into a visbreaking zone under visbreaking conditions comprising an equivalent reaction time, ERT, between 200 and 1000 seconds at a temperature between 650°F and 950°F. The visbroken product is separated by distillation to recover an enhanced yield of low viscosity 270-600°F gas oil fraction and a reduced yield of low viscosity 600°F bottoms fraction. The combination of the fractions provides basestock containing less than 0.50 weight percent carbonaceous sediment. Feedstream

The heavy oil feeds used in the present upgrading process in combination with polyolefins may be a single refinery stream or a mixture of refinery streams derived from various sources. The present process is suitable for upgrading a wide variety of heavy liquid hydrocarbon oils in which at least 75 weight percent of the components boil over 370°C. Included in this class of feeds are residual fractions obtained by catalytic cracking of gas oils, solvent extracts obtained during the processing of lube oil stocks, asphalt precipitates obtained from deasphalting operations, high boiling bottoms or resids obtained during vacuum distillation of petroleum oils, tar sand bitumen feedstocks, and the like. These oils may contain hetero- atom impurities such as nitrogen or sulfur as well having relatively high metal contents.

Plastic materials which may be included with the heavy oil feedstream in the process of the invention include those materials which can be cracked or depolymerized under visbreaking conditions such as polyolefins and polystyrene. However, the preferred materials are polyolefins produced as homopolymers or copolymers of lower olefins, i.e., ethylene, propylene and butylene, especially polyethylene, polypropylene, polybutylene and the like. The plastic materials can be fed to the visbreaking process in quantities between 0.01 and 5 weight percent of the heavy oil feed. Even larger amounts of plastic feed can be employed but considering the disparity between the great size of a visbreaking reactor and the quantity of waste plastic realistically available to the refinery the range of 0.01 to 5 weight percent will suffice. Plastic feed ranges between 0.1 and 2 weight percent are preferred. Visbreaking Conditions

In most hydrocarbon processes, there is a tradeoff between reaction temperature and residence time of reactants. Because visbreaking is a well-known and widely practiced process, however, correlations have been developed so that it is possible to express precisely the severity of the visbreaking process. An expression of the "severity" of a particular visbreaking operation does not mean that a certain degree of conversion can be predicted or obtained or that a certain amount of coke or sediment will be formed; rather, it means that it is possible to predict that if all other reaction parameters are unchanged (e.g., feed composition, reactor pressure) except for the temperature and residence time in the reactor, two operations can be compared and it can be determined whether one process is more severe than the other. Equations and tables have been developed for comparing reaction severities. Typical of such presentations is the discussion of "soaking factor" in Petroleum Refinery Engineering-Thermocracking and Decomposition Process- Equation 19-23 and Table 19-18, in Nelson-Modern Refining Technology, Chapter 19. Although that text uses the term "soaking factor", the term "ERT" or "Equivalent Reaction Time" in seconds as measured at 427°C (800°F) is used in this specification to express visbreaking severity; numerically, soaking factor is the same as ERT at 427°C.

ERT refers to the severity of the operation, expressed as the equivalent number of seconds of residence time in a reactor operating at 427°C (800°F) . In very general terms, the reaction rate doubles for every 12° to 13°C increase in temperature. Thus, 60 seconds of residence time at 427°C is equivalent to 60 ERT, and increasing the temperature to 456°C would make the operation five times as severe, i.e. 300 ERT. Expressed in another way, 300 seconds at 427°C. is equivalent to 60 seconds at 456°C, and the same product mix and distribution should be obtained under either set of conditions. The visbreaking process conditions which may be used can vary widely based on the nature of the heavy oil material and other factors. In general, the process is carried out at temperatures ranging from 350° to 485°C, preferably 425° to 455°C, at residence times ranging from 1 to 60 minutes, preferably 7 to 20 minutes. For normal severity visbreaking (NSVB) , expressed as ERT, the process of the invention generally operates at an Equivalent Reaction Time of 250 to 1500 ERT seconds and preferably 400 to 1200 ERT seconds and more preferably 500 to 800 ERT seconds at 427° C. In many cases, severity will be up to 800 ERT seconds at 427° C.

Although less common, a few refineries practice thermal cracking or high severity visbreaking (HSVB) of non-residual gas oil such as heavy vacuum gas oil (HVGO) . HSVB is similar to NSVB but operates at an ERT between 1,000 and 2,500 seconds.

The limit of severity is determined primarily by prod¬ uct quality. Visbreaking is an inexpensive process. Once a visbreaker has been installed it does not cost much more to run it at high severity in order to achieve the maximum viscosity reduction possible with a given feed stock. However, the two limiting factors in the visbreaker operation are the formation of coke (which tends to plug the coil and/or soaking drum used in the visbreaker and also take the product out of specification) and sediment formation in the product. Sediment formation is a complicated phenomenon. As a generalization, it can be stated that, if the composition of an oil is changed enough, the asphaltic materials may no longer dissolve in the product and hence settle out as sediment. The problem becomes worse when cutterstocks or blending stocks of a less aromatic nature are added to the visbreaker product; the asphaltics or other materials that would remain dissolved in the visbreaker product are no longer soluble upon blending the visbreaker product with other, less aromatic materials.

An important aspect of the invention is the improvement of visbreaker performance by optimizing operational severity for heavy oil feedstocks. In general, as severity is increased, increased yields of distillate and gaseous hydrocarbons are obtained with a reduction in the viscosity of the visbroken products so that the amount of cutter oil required for blending to obtain - specification-viscosity residual fuel oil is also reduced. At high severities, however, there is an increased tendency to form coke deposits which results in plugged heater tubes and/or the production of unstable fuel oils as measured by sediment formation. Consequently, an unexpected finding of the process of the present invention is that polyolefin plastics reduce coke and sediment formation despite their non-aromatic nature. This is the exact opposite of what normally would be predicted. The pressure employed in a visbreaker will usually be sufficient to maintain most of the material in the reactor coil and/or soaker drum in the liquid phase. Normally the pressure is not considered as a control variable,although attempts are made to keep the pressure high enough to maintain most of the material in the visbreaker in the liquid phase. Some vapor formation in the visbreaker is not harmful, and is frequently inevitable because of the production of some light ends in the visbreaking process. Some coil visbreaker units operate with 20-40% vaporization material at the visbreaker coil outlet. Lighter solvents will vaporize more and the vapor will not do much good towards improving the processing of the liquid phase material. Accordingly, liquid phase operation is preferred, but significant amounts of vaporization can be tolerated.

In general, the pressures commonly encountered in visbreakers range from 170 to 10450 kPa, with a vast majority of units operating with pressures of 1480 to 7000 kPa. Such pressures will usually he sufficient to maintain liquid phase conditions and the desired degree of conversion.

Visbreaker

The visbreaker unit itself may be conventional in form, typically of the coil type, i.e. a tubular reactor which is entirely in the heater, or drum type or with a combination of coil and drum in order to provide the requisite residence time under the temperature conditions employed. As far as product type and distribution is con¬ cerned, it is of no great significance whether the resi¬ dence time is obtained in a coil, drum, or combination of both. Typical of the coil/drum combinations is the unit disclosed in U.S. Pat. No. 4,247,387.

The nature of the concept is to co-feed up to several per cent polyolefin with the normal feed to the visbreaker. The polyolefin preferably may be polyethylene, polypropylene, and/or polystyrene, as previously noted. However, a broader range of plastic materials including old automobile tires (natural and artificial rubber) may also be included as part of the feedstream to the visbreaker. There are three possible methods for injection of the combined plastic and heavy oil feed into the visbreaker: slurry cofeed, solution co-feed, and direct co-feed of solids. The preferred method is slurry co-feed.

The process of the instant invention was illustrated in two exemplary Examples 2 and 4 wherein two heavy oil feedstreams to a visbreaker were combined with high density polyethylene (HDPE) and the mixture visbroken. The VB bottoms and VBGO were blended with gas oil to produce heavy fuel oil (HFO) of specification viscosity. By way of comparison. Examples 1 and 3 were carried out to visbreak the heavy oil feedstreams without mixing with HDPE and the results were compared to Examples 2 and 4. The feedstocks were Arab Light Vacuum Resid for NSVB (normal severity visbreaking) for Examples 1 and 2, and HVGO for HSVB (high severity visbreaking) for Examples 3 and 4 Feedstock properties are presented in Table 1.

Table 1

HVGO Vac Resid

API Gravity 17 8.5

Pour Point, °F 120 115

KV @ 100°C 21.1 1040

KV @ 50°C 690.3 na

Distillation

IBP 411

5 771

10 819

20 874

30 906

40 936

50 960

60 980

70 1002

80 1027

90 1056

95 1080

EP 1162 Table 2 presents the results of the visbreaking experiments carried out on the petroleum feedstock described in Table 1.

Table 2 shows an increase in yield for the gas oil fraction of the product of the process of the invention and a reduction in the yield of the bottoms fraction. For the given feedstream and product separation conditions, the gas oil fraction is the (270-600°F) fraction and the bottoms fraction is the 600+°F fraction. However, as the artisan well knows, the temperature range employed to define a gas oil or bottoms fraction varies by refinery or refinery practice but generally falls within the range of 270°F- 700°F for gas oil and 600+°F or 550+°F for the bottoms fraction. These are the broader ranges of the products whose compositions are improved by the process of the invention, illustrated by the specific ranges of the examples presented in Table 2.

Table 2

EXAMPLES 1

FEED

Oil Vacuum Resid (NSVB) HVGO (HSVB)

HDPE, wt % 0 2 0 2

OPERATIONS ERT, seconds, est. 600 1300 Coil outlet, °F 844 841 869 869 Coil outlet, press, psig 406 402 403 404

YIELDS, wt %

Gas (C4-) 1.6 1.5 2.5 2.6 Naphtha (C5-270°F) 2.4 2.2 4.3 5.2

Gas Oil (270-600°F) 3.8 4.4 12.1 13.4

Bottoms (600°F+) 92.1 91.9 81.1 78.8

VB GAS OIL KV @ 40°C,cSt 1.806 1.667 1.879 1.639

VB BOTTOMS

KV @ 180°F, cSt 1,533 1,022 28.76 35.41

KV @ 300°F, cSt 51.93 37.03 5.07 5.358

VB BOTTOMS + VBGO

KV @ 50 °C cSt 13,471 5,510 1,649 1,247

COMPATIBILITY(1) (2) Sediment @ nominal 180 cSt HFO, wt. %

1.19 0.04 0.74 0.39

Sediment @ nominal 460 cSt HFO, wt. %

0.15 0.04 1.07 0.04 (1) HFO prepared by blending VB Btms and VBGO in yield proportion and diluting to viscosity specification with gas oil.

(2) Sediment measured per Shell Hot Filtration Test.

Certain high quality HFO grades must contain less than 0.15 wt. % sediment.

Claims

Claimed:

1. A visbreaking process for the conjoint disposal of polyolefin plastic materials and production of fuel oil basestock, comprising; cofeeding a feedstream comprising between 0.01 and 5 weight percent of said materials and a heavy petroleum feedstock into a visbreaking zone under visbreaking conditions whereby the visbroken product contains an enhanced yield of gas oil fraction plus a reduced yield of carbonaceous sediment and bottoms fraction.

2. The process of claim 1 wherein the visbreaking conditions comprise an equivalent reaction time, ERT, between 200 and 1000 seconds at a temperature between 650°F and 900°F.

3. The process of claim 1 wherein the equivalent reaction time is between 1000 and 2000 seconds at a temperature between 750°F and 950°F.

4. The process of claim 2 wherein the equivalent reaction time is between 400 and 900 seconds at a temperature between 700°F and 865°F.

5. The process of any of claims 1 to 4 wherein said plastic materials comprise polyethylene, polypropylene or mixtures thereof.

6. The process of any of claims 1 to 5 wherein said polyolefin feedstream comprises between 0.1 and 2 weight percent.

7. The process of any of claims 1 to 6 wherein said feedstock comprises heavy vacuum gas oil, vacuum residua or furfural extract.

8. The process of any of claims 1 to 7 wherein said feedstock comprises heavy vacuum gas oil and includes the further step of distilling said visbroken product to recover said basestock comprising at least a 10 percent increase in yield of said gas oil having a kinematic viscosity at 40°C of less than 1.700 cS plus a visbroken bottoms fraction having a kinematic viscosity at 180°F of less than 40 cS.

9. The process of any of claims 1 to 8 including the further step of blending said basestock with gas oil to produce fuel oil having less than 0.15 weight percent sediment as determined by the Shell Hot Filtration test.

10. The process of any of claims 1 to 9 wherein said feedstock comprises vacuum residua and the process includes the further step of distilling said visbroken product to recover said basestock comprising at least a 10 percent increase in yield of 270-600°F gas oil having a kinematic viscosity at 40°C of less than 1.700 cS plus said visbroken bottoms fraction having a kinematic viscosity at 180°F of less than 1,100 cS.