CA1296280C - Method for removal for furfural coke from metal surfaces - Google Patents

Abstract

ABSTRACT OF THIS DISCLOSURE
A method for removing furfural-derived coke from metallic surfaces by heating in an oxygen-containing gas, such as air, for a time and at a temperature sufficient to change the crush strength of the furfural coke to a point which will permit easy removal of such coke, without such an evolution of heat that the metallurgical properties of said metal surfaces are undesirably changed.

Description

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4 Background of the Invention 6 Field of the Invention 7 This invention relates to methods for removing petroleum coke deposits,8 and specifically, furfural coke. Coke can arise from a high temperature 9 condensation of hydrocarbyl species, i.e. materials containing primarily hydrogen and carbon with minor amounts of heteroatoms such as nitrogen, 11 oxygen, or sulfur. Furfurai coke, for purposeC. of this specification, means 12 hydrogen deficient hydrocarbyl species resulting from decomposition, 13 autoxidation, or polymerization of furfural.
14 Prior Art On page 181 of Hydrocarbon Processing published by Gulf Publishing 16 Company, May, 1978, an article entitled "Problems in Furfural Extraction"
17 discusses problems of coking in lube oil plants. Coke deposits apparently 18 arise due to decomposition, autoxidation or polymerization of furfural. Such 19 furfural decomposition is reported to be inhibited with sodium bicarbonate or tertiary amines which apparently neutralize acids formed during decomposition.
21 An example of a lube oil plant u~ilizing furfural extractions is reported in 22 the September, 1982, issue of Hydrocarbon Processing on page 184. However,23 regardless of the steps to inhibit coke from fùrfural decomposition which are 2~ presently ava~lable, there eventually results such an accumulation of such coke that its removal from the system is required.
26 Current methods for removing coke have all proven to be too troublesome 27 and time consuming. Scraping with water jets fails because furfural coke has ~ 2 ~ ~bV ., ,~
1 d very high crush strength. Simply scraping or chipping is unsatisfactor~
2 because when coke is on the shell side of exchangers, only the outer rows of3 tubes are accessible. Other methods for cleaning out furfural coke deposits~ include letting the coke weather in the open for several months, then cleaning with a jet of high pressure water (Texaco's method). This works if the 6 metallic surface is aluminum; it is not known if it will work if the surface7 is carbon or stainless steel.
8 Furfural coke deposits are particularly difficult to ~emove because 9 furfural coke is much harder and clings to metal surfaces more than conventional petroleum coke.
11 In general, there is no rapid and economically efficient method for 12 removing furfural coke deposits known to the prior art. Conventional methods ~3 have been found to be very difficult and inefficient in removing coke 14 deposits. Examples of traditional methods for removing coke deposits are chipping, water jet, steam cutting, and sawing.
16 Though furfural, as any coke, can be burned, simply burning coke out of a 17 metal heat exchanger is undesirable, because high temperatures from such 18 burning can lead to warping and the introduction of strains and stresses 19 within the metal~ Even the physical properties of specially prepared metals such as stainless steel or chrome in the presence of carbon at high 21 temperatures, such as above 800F (427~), often deteriorate so that they are 22 no longer as corrosion resistant as they would otherwise be had they been 23 properly treated.
2~ The ideal situation is to have a very quick, easy and cost effective method to remove furfural coke deposits. Specifically, it would be desirable 26 to have a method for quickly removing furfural coke from exchangers with l 3~ .Z.~)an 1 blockage ranging from 1/8" to completely plugged. This usuall~ represents 2 roughly two to four years of properly buffered operation.

3 Accordingly, an object of this invention is to provide a method for 4 removing furfural coke deposits, for example, from heat exchangers in a rapid and efficient manner without deteriorating any metallurgical properties of 6 surfaces from which such coke is removed.
7 A Brief Description of the Invention 8 Figure 1 is a furfural refining unit for preparing lube oils.
9 Figure 2 is the observed heat evolution at various temperatures when heating furfural coke.
11 Figure 3 includes a solid line graph of weight loss of a furfural coke12 sample as a function of temperature, wherein the sample is heated to 13 increasing temperatures at a constant rate. Temperature of sample as a 14 function of time is shown by dotted lines.
Broadly, this invention provides a method for cleaning furfural coke 16 deposits from metal surfaces by means of a unique heat treating process.
17 We have discovered that, unlike other forms of coke, it is possible to18 heat furfural coke in the presences of free oxygen at temperatures which do19 not lead to adverse metallurgical impacts on metallic surfaces, such as in heat exchangers, but yet will cause the character of the coke deposits to 21 change so as to become readily and easily removable. We have discovered, 22 surprisingly, that there exists temperatures in the range of about 400-800F
23 (204-427C), preferably 500-750F (260-399C), and still more preferably 24 550-700F (288~371C) wherein the crush strength of furfural coke deposits change after being heated in air at a temperature in this range for in excess 26 of at least one hour, preferably in ~xcess of two hours, and generally for a 27 time in the range of about four to six hours. Heating below 800F (427C) inl ''`.1 ~:~4~

1 an inert atmosphere does not result in a change, e.g. a decrease in crush 2 strength. in coke deposits which permit ready removal.
3 It is possible at these temperatures to heat the coke in air without causing such an excessive amount of heat to be given off that undesirable 6 metallurgic changes occur. When heating in air, we have discovered exotherms 6 occur at roughly 500F (260C) and 700F (371C) but do not know precisely 7 what process is causing these exotherms. No exotherms were observed when 8 heating in an inert atmosphere.
9 Somewhat different temperatures and periods of time are appropriate ~hen heating furfural coke, either in the presence of air at higher than 11 atmospheric pressure or in the presence of higher concentrations of molecular 12 oxygen than present in air to bring about a change in crush strength without 13 giving rise to too rapid or great an evolution of heat. Somewhat lower 14 temperatures and shorter periods of time for heating are required to bring about the same observed decrease in crush strength. Depending upon the 16 concentration of molecular oxygen, the heat treatment can be reduced by as 17 much as a third to a half.
18 To avoid pockets of furfural trapped within the coke getting out during 19 the heating process and suddenly bursting into flame in the presence of any oxygen in an oven, it is desirable to (1) apply a jet of water under pressure 21 to the tubes9 which is ineffective for removing substantial quantities of the 22 coke but does tend to remove most pockets of furfural, or (2) heat in the 23 absence of air or other oxidation-promoting materials, followed by the heating 24 required by thls invention. The initial heating in the absence of air can be at any temperature sufficient to vaporize all of the furfural, which would 2& certainly be above its boiling point at the pressures under which the material 27 is being heated, preferably in the range of about 400-500F (204-260C).

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1 Furfural coke forms predominantly in heater systems, high pressure flash 2 distillation sections, and high pressure furfural condenser/heat exchangers.3 In general, any time you have condensing furfural vapors or when 4 you are vaporizing furfural liquid, there is a tendency for furfural to polymerize and form coke deposits. Furfural coke is a unique type of coke in 6 that it is very difficult to remove, tends not to be particularly porous, and 7 has a very high crush strength determined in accordance with ASTM D-3313 of as 8 much as fourteen pounds. It also contains approximately 30% Carbon and 3~
9 Hydrogen, where the usual petroleum coke contains approximately 80% Carbon and 8% Hydrogen. It is highly resistant to the usual methods of coke removal such 11 as high pressure steam and chipping, cutting or other direct physical methods.
12 Detailed Description of the Invention 13 In Figure 1 a furfural refining unit is disclosed in which a waxy 14 distillate is extracted with furfural to yield a refined oil product or raffinate. The elements in Figure 1 are a deaerator (2), heat exchangers (4), 16 a counter-current extractor (5), pumps (6), three extract flash towers in 17 series (8), a furfural accumulator vessel (9), an extract stripper (10), an18 extract vacuum flash tower (11), a raffinate stripper (12), a raffinate vacuum 19 flash tower (13), a furfural tower (14), and a water tower (15).
The heat exchangers most prone to have furfural derived carbonaceous 21 (coke) deposits are: those exchangers (4) between lines (52) and (54), (54)~2 and (56), (56) and (58)~ t60) and (62), (73) and (74), (91) and (92), and hp 23 zone of vessel (8) and line (65).
24 Briefly, the operation of this system is as follows. An oil charge comprising, for example, 100 neutral or 330 neutral waxy distillate is 26 introduced through conduit (40) and heated by means of a heat exchanger (4) or 27 other heating element and transferred through line (42) to a deaerator (2) to ll 1 remove any air. Such step is not necessary if the material is previo~sly 2 deaerated and kept under an inert gas such as a nitrogen blanket. The 3 material after being deaerated is transferred from line (44) by pump (6) 4 through line (46) into heating element (4) or a heat exchanger (4) and then through line (48) into a counter-current extractor (5). Through line (51) is 6 introduced substantially pure or recovered furfural. A reflux emptying 7 tray (7) is circulated through lines (99) and (100) by means of a heat 8 exchanger (4) and a pump (6) to improve furfural extraction efficiency in 9 yielding a desirable raffinate produc~. The raffinate and furfural leave the top of counter-current extractor (5) through line (50), whereas heavier 11 aeromatic material not suitable or desirable for forming lubricating oils 12 leaves through line (52) as an extract mix.
13 The e~tract mix goes through a series of process steps to recover the 14 furfural for reintroduction into counter-current extractor (5). The first step of this series involves heating the extract through one or more heat 16 exchangers (4). The heated extract is introduced into the low pressure (lp) 17 zone of flash tower (8), wherein a series of flashes occurs. The lowest 1~ pressure is at the lowest temperature and the highest pressure at the highest 19 temperature.
Each flash tower overhead consisting primarily of furfural is removed and 21 sent through different heat exchangers (~).
2~ Bottoms through conduit (60) from the low pressure zone of flash 23 tower (8), comprising furfural and extract, are heated by an exchanger (4) 2~ followed by fired heater (7) and transferred through conduit (63) to a high pressure (hp) flash zone of flash tower ~8).

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1 Bottoms from high pressure zone are transferred through heat 2 exchanger (~) and conduit (65) to median pressure (mp) zone of flash 3 tower (8).
4 80ttoms fro~ median pressure zone of flash tower (8) are transferred through line (66) to vacuum flash tower (10).
6 The overheads (primarily furfural) from the median pressure (mp) flash 7 zone move throu~h line (67) then through exchanger (4) and line (69) to 8 furfural extractor and accumulator (9).
9 The overheads from the vacuum flash tower (1~), comprising primarily furfural, are transferred through line (73) to heat exchanger (4) and then 11 through line (74) to vessel (9) for reuse in the counter-current 12 extractor (5). Bottoms from extract vacuum flash zone (10) are transferred 13 through conduit (85) to the extract stripping zone (11).
14 Into stripping zone (11) steam is introduced to strip out furfural from the extract which leaves vessel (11) through a pump (6), a heat exchanger (4), 16 and line (41) to an appropriate storage zone not shown.
17 Stripped material from vessel (11) exits through line (84) for further 18 stripping in a two tower azeotrope stripping section, which consists of an 19 accumulator (14), a furfural tower (9) and water tower (15).
The stripped material from vessel (11) enters the accumulator (14). A
21 furfural rich stream is drawn through line (79) to the furfural tower (9) 22 where it is stripped with furfural vapors. The overhead of the furfural tower 23 is a furfural - water azeotrope and exits through line (76) where it combines 24 with the overhead of the water tower (also a furfural - water azeotrape). The combined stream flows through exchanger (~) back to accumulator (14).
26 Substantially pure furfural exits through the bottom of the furfural tower 27 (9), where it is recirculated back to the contactor (5). A second stream 1 (water rich) is drawn from the accumulator (14) through line (80) into the ~ water tower where it is stream stripped. The overhead being a furfural -3 water azeotrape combines with the overhead of the furfural tower as was stated 4 earlier. Substantially pure water exits through conduit (83) for reuse or discard.
6 The raffinate and furfural in line (50) first exchanges heat in a heat 7 exchanger (4) with furfural overhead in line (92) from raffinate vacuum flash 8 tower (13). The overhead in line (50) after heat exchange is further heated in 9 fired heater (7) prior to transfer in line (95) to raffinate vacuum flash tower (13), where most of the furfural is flashed overhead through 11 conduit (92). ~ottoms are routed through conduit (87) to the raffinate 12 stripper (12). In stripper (12), steam is used to strip out any remaining 13 furfural from the raffinate. The overhead conduit (86) combines with that of 14 the extract stripper (11) and enters azeotrope stripping section (14). Thebottoms (refined oil) is cooled then sent to storage for further processing.

17 Furfural-derived coke found in certain heat exchangers discussed herein 18 before, were heated at increasing temperatures in the presence of air. In 19 Figure 2, a graph of heat evolution versus temperature reveals two exotherms at roughly 560F (239C) and 750F (399C).
21 In Figure 3, the solid line shows a rapid weight loss in a sample of 22 furfural coke, beginning at around 452F and ending at around 660F. This 23 occurred while heating the sample at a constant rate (see broken line).
2~ The furfural coke tested had an initial crush strength of fourteen pounds as determined in accordance with ASTM D-3313. The ~inal crush strength after 26 heat treating was so low it did not register on our instruments, i.e. less 27 than one pound.

ll -9- ll 1 A water jet typically emits water at a pressure of four to five thousand 2 pounds per square inch (4,000 to S,000 psi). We have found that prior to heat 3 treatment of furfural derived coke in accordance with this invention, a water 4 jet of five thousand pounds per square inch was insufficient to remove such coke deposits. Even water jets as high as thirty thousand pounds per square 6 inch could not co~pletely remove coke deposits.
7 However, after heat treating in accordance with this invention, a water8 jet of S,000 psi easily removed all coke deposits. A water jet of as little9 as 1,000 psi workedO In fact, the nature of the coke had so changed that small vibrations such as from transporting a heat exchanger in a truck 1I resulted in removal of most of the coke deposits. The crush strength had 12 decreased from fourteen pounds to less than one pound, as measured in 13 accordance with ASTM D-3313.
14 Examples of commercially available water jets that can be used in thisinvention are: a Partek~ Liqua-Blaster model 610 DST and Jetpac~, model 1003, 16 sold by Adnrac, Inc. of Washington.
I7 Specific compositions, methods, or embodiments discussed herein are 18 intended to be only illustrati~e of the invention disclosed by this I9 Specification. Yariations on these compositions, methods, or embodiments, such as combinations of features from various embodiments, are readily 21 apparent to a person of skill in the art based upon the teachings of this 22 Specification and are therefore intended to be included as part of the 23 inventions disclosed herein. Any reference to literature articles or patents 24 made in the Specification is intended to result in such articles and patents being expressly incorporated herein by reference including any articles or 26 patents or other literature references cited within such articles or patents.

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Claims (10)

1. A process for removing furfural coke from metallic surfaces comprising: heating without causing an evolution of heat capable of undesirably altering metallurgical properties of said surfaces in the presence of a gas containing molecular oxygen at a sufficient temperature below 800°F (427°C) for a sufficient timeto change the crush strength of the coke so as to permit removal with a water jet at a pressure of five thousand pounds per square inch.

2. The process of claim one, wherein volatiles in said furfural coke are removed prior to said heating in the presence of molecular oxygen.

3. The process of claim two, wherein volatiles are removed by heating in an inert atmosphere.

4. The process of claim one, wherein said heating in the presence of molecular oxygen, is at a temperature in the range of about 400 to 800°F (204-427°C).

5. The process of claim two, wherein said heating in the presence of molecular oxygen, is at a temperature in the range of about 400 to 800°F (204-427°C).

6. The process of claim three, wherein said heating in the presence of molecular oxygen, is at a temperature in the range of about 400 to 800°F (204-427°C).

7. The process of claim three, wherein said heating in the presence of molecular oxygen, is at a temperature in the range of about 550 to 700°F (288-371GC).

8. A process for removing furfural coke from a metallic surface comprising:

A. Removing volatiles without igniting them;

B. Heating in the presence of molecular oxygen at a temperature and for a time sufficient to change the crush strength of the coke so as to permit easier removal of such coke than would otherwise be the case without damaging metallurgical properties of said metallic surface.

9. The process of claim 8, wherein said heating is at a temperature in the range of about 400°F to 800°F (204°C to 427°C).

10. The process of Claim 9, wherein said heating is for at least one hour.