EP0591856A1

EP0591856A1 - Pulsed air decoking

Info

Publication number: EP0591856A1
Application number: EP93115874A
Authority: EP
Inventors: Dennis A. Duncan
Original assignee: Stone and Webster Engineering Corp
Current assignee: Stone and Webster Engineering Corp
Priority date: 1992-10-05
Filing date: 1993-10-01
Publication date: 1994-04-13
Also published as: KR940009317A; FI934349A; CA2107541A1; FI934349A0; NO933547L; BR9303999A; JPH06316696A; MX9306082A; AU4871593A; AU656485B2; NO933547D0; CN1085593A

Abstract

A method for decoking a hydrocarbon reaction process pathway without the need to shutdown or cool the reactor, including the steps of interrupting the hydrocarbon feed flow, introducing a series of air pulses of limited duration into a steam matrix for combustion of the coke until the coke is removed, and resuming the hydrocarbon feed.

Description

FIELD OF THE INVENTION

The present invention relates to the field of hydrocarbon cracking and more specifically to the efficient maintenance of hydrocarbon reaction systems which require periodic decoking.

BACKGROUND OF THE INVENTION

Cracking hydrocarbons from long chain molecules to gasoline and other useful short chain hydrocarbons results in the production of significant secondary by-products of the reaction, most notably carbonaceous deposits or coke. The effects of the coke produced in the reaction are considerable, especially in fired tubular furnace reactors and the like utilizing process reactor tubes in which the cracking reaction occurs.
During the reaction, the coke by-products form deposits, inter alia, as a layer on the inside of the process tubes of the reactor. The coke deposits increase the pressure drop and inhibit heat transfer across the tubes thereby penalizing the process. The coke must, therefore, be removed periodically to restore the pressure drop and heat transfer to normal levels.
The coke removal process, generally called decoking, is commonly carried out by combustion (also known as steam-air decoking), by steam reaction or by mechanical removal. Steam-air decoking is performed by introducing a small quantity of air in a steam matrix, usually starting at about 1-2% and increasing in stages to about 20% by weight of air relative to steam, into the fouled process tubes to burn-off the coke from the inside of the tubes. In contrast, steam reaction decoking requires the introduction of steam to the tubes at high temperatures to react with the coke. An example of the steam reaction decoking is found in United States Patent No. 4,376,694. Finally, mechanical decoking makes use of physical means for breaking loose and scouring the coke from the inside of the process tubes, generally a high pressure water jet, i.e. at a pressure of from about 700 to 1000 bar.
Of these methods, steam-air decoking can be the most efficient, however, to be efficient requires close monitoring of the process to maintain a reasonable burning rate. If the burning rate is too low the decoking operation will require a longer period of time, resulting in the processing operation remaining off stream for a longer period. On the other hand, if the burning rate is too high the excessive temperature will damage or even burn through the process tubes.
In conventional steam-air decoking the burning rate is monitored by measuring the concentration of carbon dioxide from time to time in the decoking effluent gas. In conventional decoking of the fired tube reactor, overheating of the process tubes is safeguarded against by visual inspection of the tubes in the firebox and/or observing tube temperatures by means of a pyrometer.
It is therefore an object of the present invention to provide a method of decoking which can be performed over a short period of time, thereby reducing reactor downtime.
It is another object of the invention to provide a method of decoking which can be automatically monitored to maximize combustion while avoiding damage to the reactor, thus limiting operator involvement.

SUMMARY OF THE INVENTION

These and other objects are achieved by the present invention which is directed to a method of decoking a coke fouled hydrocarbon reaction pathway in a hydrocarbon reaction system comprising the steps of:

(a) interrupting the hydrocarbon feed;
(b) injecting an air pulse into a steam matrix into the reaction pathway for combustion of the coke, said air pulse being of sufficient concentration for vigorous combustion of the coke for a period of time limited in duration so as not to raise the temperature of the reactor components above their design temperature, thereby producing an effluent gas;
(c) interrupting the injection of the air into the pathway for a period of time to subdue combustion and allow the temperature of the reactor components heated during combustion to decrease;
(d) sequentially repeating steps (b) and (c) until the effluent gas has a CO₂ content of less than about 0.2 volume %; and
(e) resuming the hydrocarbon feed. The additional step of continuously monitoring the CO₂ content of the effluent gas produced during decoking by using sensing means such as a gravitometer, infra-red analyzer or other means is also contemplated.

The above process allows the use of higher weight concentrations of air in steam, i.e. in the range of from about 20% to about 50% by weight. The increased concentration of air results in vigorous burning that might seriously overheat the tubes but for the interruption of the air pulse, allowing the combustion and heat generation to subside or extinguish. Another air pulse is then injected into the steam matrix after a short time interval to rejuvenate or reinitiate the coke combustion, with the on/off sequence repeated until decoking is complete.
The frequency of the air pulse and the concentration of the air in steam during the pulse in a particular environment varies with both the geometry of the reactor pathway and the characteristics of the feedstock being processed. As the decoking progresses, the concentration of air in the steam would normally be increased to ensure rapid and thorough combustion of the coke. The frequency of the air pulses would be regulated to prevent large, damaging temperature swings in the tube metal.
Further, the process of alternately heating and cooling the coke and the tube metal by the pulsed burning will create a tendency for some of the coke to spall from the tube walls, thus further accelerating the decoking process.
The rate of burning is established by a CO₂ sensing means such as a RANAREX gravitometer or a direct reading CO₂ analyzer such as an infrared analyzer. The gravitometer measures the specific gravity of the effluent gas relative to air and thus senses the concentration of carbon dioxide in the effluent, as the molecular weight of CO₂ is high relative to that of air. Similarly, the infra-red CO₂ analyzer measures the CO₂ content of a gas stream such as the effluent gas directly. The CO₂ content and the rate of air injection establishes the rate of coke burning.
The CO₂ sensing means is closely linked to the decoking effluent by means of a sampling line. The CO₂ sensing means is adapted to generate an electrical signal which is used to terminate the pulsing once the CO₂ level has dropped below the desired point. Automatic means to control either the total volume of air being injected during a pulse or the frequency of the air pulses, as appropriate, is contemplated. The choice of air volume or frequency controls depends on the mechanical means chosen to regulate the air flow.
For example, decoking of a fired tubular furnace for cracking hydrocarbons to ethylene and other olefinic compounds under the present invention is achieved by injecting a pulse of air at a weight concentration relative to the steam flow for a period of time to initiate combustion and burn the coke. At the end of this period the air would be interrupted either totally or partially, subduing or extinguishing the combustion, to avoid raising the temperature of the tubes beyond their design temperature. After the end of the air pulse interruption, a subsequent air pulse is injected into the steam matrix with the on-off cycle repeated many times until the CO₂ content of the effluent gas during the air pulse is less than about 0.2 volume % indicating that the effluent is substantially free of CO₂.
The period of time that the air flow is on and the period it is interrupted, as well as the air concentration during the process are established by the CO₂ sensing means and generally change along the decoking process. An air flow is then left on for a period of time to "air polish" the pathway.

BRIEF DESCRIPTION OF THE DRAWINGS

The following drawings are included to better illustrate the present invention without limiting the invention in any manner whatsoever.
FIGURE 1 is a schematic view of a reactor furnace for use with the present invention.
FIGURE 2 is a graph of the specific gravity of the effluent during decoking under the present process.
FIGURE 3 is a graph of the specific gravity of the effluent during decoking under the present process from a downstream gravitometer.
FIGURE 4 is a plot of temperature at various points along the length of a process tube during decoking by the present method.
FIGURE 5 is a graph of temperature over time which follows the air pulses and the movement of coke burn toward and past a thermocouple.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

With reference to the figures, and specifically FIGURE 1, the present invention is shown in the environment of a fired tubular furnace (1) as used for cracking hydrocarbons to ethylene and other olefinic compounds. It is understood that the invention can be used with any suitable cracking furnace or hydrocarbon reaction system which needs periodic decoking.
The fired tubular furnace (1) has a convection section (2,) and a cracking section (3). The furnace (1) contains one or more process tubes (4) through which hydrocarbons fed through a hydrocarbon feed line (6) are cracked to produce product gases upon the application of heat, with carbonaceous deposits referred to herein as coke produced as a by-product. Steam from steam conduit 22 is used as a diluent during the reaction process. Heat is supplied by a heating medium introduced to the exterior of the process tubes (4) in the cracking section (3) of the furnace (1) through heating medium inlet (8), and exiting through an exhaust (10). As the product gases move through the process tubes (4) and out the product gas exit (12) the product gases are then passed through one or more quench/heat exchangers (14) and (16). The product gases are then directed along line (18) to downstream processing equipment such as a quench tower and separation means (not shown).
To implement the present invention, a valve (20) is placed in the hydrocarbon feed line (6) to interrupt the hydrocarbon feed. After interruption of the hydrocarbon feed flow, an air pulse from an air conduit (24) is injected into the flow of a steam matrix along a steam conduit (22). The relative volume and duration of air flow through the air conduit (24) and into the steam conduit (22) is controlled by a valve (26) placed in the air flow conduit (24). An alternative embodiment includes a nitrogen conduit (not shown) to deliver a nitrogen flow to the steam matrix in place of air when the air flow is interrupted.
During decoking downstream valving (34) controls direction of the effluent gas and coke spall away from the product separation system (not shown). When the hydrocarbon feed valve (20) is closed the valve (34) to the downstream processing system (not shown) is also closed and a decoking effluent valve (36) is opened to direct the decoking effluent along an effluent line (38) to the cracking section (3) of the furnace (1) exterior to the process tubes (4) for coke spall combustion, or optionally to a decoking drum (40) with gas vents (42). Since air cannot be introduced to the separation system, the valve (34) to the processing system is closed and the effluent valve (36) is opened to direct the effluent away from the separation system upon interruption of the hydrocarbon feed flow and introduction of air into the steam matrix for decoking. Automatic activation of the feed, steam, air, processing and effluent valves (20), (21), (26), (34) and (36) is contemplated to provide an automatic decoking system.
Additionally, CO₂ sensing means (28), such as a gravitometer with a range of from about 0.6 to about 1.2, or any CO₂ analyzer with a continuous read out, is closely linked to the decoking effluent by means of a sampling line (30). The sampling line (30) is preferably linked to the effluent line (38) downstream of the effluent valve (36) so that sampling takes place only during decoking. The CO₂ sensing means (28) preferably measures the content of CO₂ in the effluent gas continuously. The CO₂ sensing means (28) is adapted to generate an electrical signal which is used to determine the end of the pulsing or decoking cycle and possibly control the volume of air injected into the steam matrix during the air pulse and/or the frequency of air pulses, as appropriate. When the CO₂ content in the effluent gas is measured at less than 0.2 volume % it signifies that no more burning is taking place, i.e. that no coke is left to be burned, and the cracking process can resume.
Alternatively, the sampling line (30a) for the CO₂ sensing means (28a) can be taken from product line (18) or from a point closer to the furnace (1) such as at product gas/outlet (12) or between the heat exchangers (14) and (16). Although sensing means closer to the furnace (1) tend to be more responsive, the downstream sampling at lower temperatures is a simpler installation.
The decoking process of the present invention, utilizing the above structure, is put into effect after the cracking operation has been run for a period of time resulting in a coke buildup on the walls of the metal tubes (4) of the furnace (1), resulting in an increase in pressure drop and decreased heat transfer across the tube walls. The present process for removal of the coke, or decoking, is employed without the need to shut down or cool the furnace.
The present process is initiated by interrupting the hydrocarbon feed at valve (20), closing the product gas valve (34) and opening the decoking valve (36). A pulse of air is introduced into the steam matrix, in conduit (22) from air flow conduit (24) by the opening and closing of the air valve (26). The steam (and air pulse) is then heated in the convection section to about 900°F, up to about 1400°F. In the most preferred embodiment, the relative volume of air (i.e. concentration in the steam matrix) and duration of the pulse is automatically fixed by the mechanical means used. The signal from the CO₂ sensing means (28) is used to stop the pulsing process and initiate an "air polish", as later described, if desired.
The concentration of air in the steam matrix during the air pulse is contemplated to be in the range of from about 20 to about 50% by weight and varies loosely inversely with the duration of the pulse, i.e. the greater concentration of air in the steam matrix the shorter the duration. Generally, the duration of the pulse is long enough to initiate vigorous combustion of the coke but not so long as to raise the temperature above the predetermined design temperature of the metal process tubes (4) or other reactor components. The preferred duration is estimated to be in the range of about 10 to about 50 seconds when a concentration of about 40 weight % air in steam is used.
Additionally, the interruption of the air flow at valve (26) is for a period of time to subdue or even extinguish combustion and heat generation and allow the process tube temperature to decrease. The duration of the air flow interruption is generally less than the duration of the air pulse, i.e. in the range of from about 5 to about 30 seconds, however, the duration is controlled to ensure that damage to the tubes is avoided and the process is efficiently run.
The amount and duration of the air flow described above is contemplated from results achieved on a bench scale unit. The optimal values are determinable by experimentation on a full scale furnace by one skilled in the art.

EXAMPLE

Pulsed air decoking as described in the present disclosure has been tried three times in decoking a bench scale pyrolysis unit (BSU) following propane cracking operations of up to 5 hour duration.
A RANAREX gravitometer with a 0.3 to 1.3 range was close-coupled to the cracking tube outlet (12). As seen in FIGURE 2, rapid swings in effluent specific gravity are clearly visible, indicating that the swings in CO₂ are instantaneous upon the injection and interruption of an air pulse, virtually like turning on and off a switch. Similarly, the first set of data recorded on the usual RANAREX gravitometer (28) located much further downstream, shown in FIGURE 3, shows the same back and forth response of effluent specific gravity, or CO₂ concentration, as the air was pulsed on and off alternatively with N₂.
The air pulse duration used was essentially 45 seconds on and 45 seconds off, arbitrarily chosen for the present because the swings on both the upstream and downstream RANAREX gravitometers and on temperature indicators on the exterior of the tubes (4) suggested that duration for maximizing the amount of burn on each air pulse cycle. A constant air concentration to steam of 40 weight % throughout the burn was used. Although automatic operation is contemplated, the pulsing was done manually, switching instantaneously from air to nitrogen in the steam matrix thus inducing a swing in effluent specific gravity from 1.08 at maximum CO₂ (21%) to the nitrogen level of 0.966. The signal drop slightly below the nitrogen level was due to some steam-carbon reaction which continued when the air was cut off.
Each time the pulsed air technique was used, the progression of the burn through the coil was observed by means of the skin thermocouples (TI's) spaced along the tube length. As seen in FIGURE 4, the TI at the midpoint of the tube coil (4) where coke buildup usually begins, shows the first temperature blip and the other TI's further along the tube coil (4) show temperature increases (50-150°F) sequentially as the burn progresses toward the outlet.
Because the tube (4) has the usual rising temperature profile, the rate of burning speeds up significantly toward the coil outlet (12). Therefore, the burn time or air pulse may be shortened as the decoking progresses down the tube coil (4).
The TI's located inside the process stream at the outlet (12) show virtually no temperature change at the start of the decoking process when the burn is in the middle of the tube (4) (where the coke is first laid down). As the burn progresses the tube outlet temperature (TOT) is increasingly affected by the approaching burning front and ends up swinging more than the tube metal temperatures (TMTs). A potentiometer was attached to the key TOT measured on the process side, enabling the process effluent temperature swings to be followed very rapidly (see FIGURE 5). The combination of the steam flow (heat sink), the slow response of the 24 point strip chart recorder, and the automatic cutting back by the controllers of the electric heat input is enough to mask the process-side temperature swing (TOT) when the burn is well back in the tube.
The temperature swings induced on the tube (4) by the pulsed burn need not be too wide, i.e. about 200°F. At the roughly 1800°F operating temperature of the coil no metallurgical problems occur.
Some "peristaltic" effect of the temperature swings are inevitable, possibly inducing more coke spalling than during a normal decoking. If the coke deposit were unusually heavy, and the spalling were great enough to cause a tube blockage, this could be detrimental. In the usual case, however, the additional spalling, if it occurs, would speed up the decoking process and be a benefit. Coke spall was observed during this process example, as seen in virtually all decokings, both steam and air, but no measure of any difference in the amount of spall was noted.
When the decoking was complete the RANAREX signal settled at the level for air. At this point the pulsing process was stopped and the air left on for a period of about 10 minutes to "air polish" the tube coil (4). The chart of FIGURE 2 marked RANAREX No. 2 shows this. On this chart the decoking after 5 hours of cracking propane was finished in about 20 minutes. The coking rate in this 40 foot by $\frac{1}{4}$
and ⅜ inch I.D. swaged coil is relatively higher than in the commercial coil it simulates, probably proportional to the difference in surface to throughput ratio (about 19). Using the pulsed air system, less time was spent than normally necessary to complete decoking.
While the invention has been described in detail and with specific reference to embodiments thereof, various changes and modifications will be apparent to one skilled in the art without departing from the spirit and scope thereof.

Claims

A method of decoking a coke fouled hydrocarbon reaction pathway in a hydrocarbon reaction system comprising the steps of:
(a) interrupting the hydrocarbon feed;

(b) injecting an air pulse into a steam matrix directed into the reaction pathway for combustion of the coke, said air pulse being of sufficient concentration for the vigorous combustion of the coke for a period of time limited in duration so as not to raise the temperature of the reactor components above a predetermined temperature, thereby producing an effluent gas;

(c) interrupting the injection of the air into the pathway for a period of time to subdue combustion and allow the temperature of the reactor components heated during combustion to decrease;

(d) sequentially repeating steps (b) and (c) until the effluent gas produced during step (b) has a CO₂ content of less than about 0.2 volume %; and

(e) resuming the hydrocarbon feed.
The process defined in Claim 1 further comprising the step of continuously monitoring the effluent gas during decoking.
The process defined in Claim 1 wherein the concentration of air in the steam matrix during step (b) is from about 20 to about 50 weight %.
The process defined in Claim 3 wherein the concentration of air in the steam matrix during step (b) decreases over the decoking process.
The process defined in Claim 3 wherein the duration of the air pulse in step (b) is from about 10 seconds to about 50 seconds.
The process defined in Claim 5 wherein the duration of the air pulse is decreased over the decoking process.
The process defined in Claim 1 wherein the duration of interruption of the air during step (c) is from about 5 seconds to about 30 seconds.
The process defined in Claim 7 wherein the duration of the interruption is increased over the decoking process.
The process of Claim 1 further comprising the step of running an air stream through the tubes for a period of time after the CO₂ content in step (b) is reduced to less than 0.2 volume % to air polish the tubes.
The process defined in Claim 1 wherein combustion in the pathway is extinguished prior to the next successive air pulse.
The process of Claim 1 wherein the CO₂ content in the effluent gas is measured by comparing the specific gravity of the effluent to the specific gravity of air with a gravitometer.
The process of Claim 1 wherein the CO₂ content in the effluent is measured by direct means.
The process of Claim 12 wherein the direct means for measuring the CO₂ content comprises an infra-red analyzer.
The process of Claim 1 further comprising the step of directing the effluent gas away from a separation system for separating hydrocarbon reaction product gases upon interruption of the hydrocarbon feed.
The process of Claim 14 wherein the effluent gas is directed to a firebox of the reactor to ensure combustion of any coke spall and small amounts of carbon monoxide and hydrogen from steam-carbon reaction.