GB1566525A - Method of and apparatus for fuel conversion - Google Patents

Description

(54) METHOD OF AND APPARATUS FOR FUEL CONVERSION (71) We, PULLMAN INCOR PORATED, a Corporation organized and existing under the laws of the State of Delaware, United States of America, of 200 South Michigan Avenue, Chicago, Illinois, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to the utilization of liquid fuel oils, particularly Fuel Oil No. 2 to 6, (U.S terminology) as an alternate source of fuel in natural gas fired equipment, such as boilers, gas turbines, various industrial furnaces and particularly for hydrocarbon steam reformers.With increasing demands placed upon natural gas reserves by use of natural gas in the production of petrochemicals and petrochemical products, a scarcity of natural gas has been created such that it is necessary to find sources of alternative fuels for gas fired equipment. Of course, it is possible to remove burners designed for natural gas service and replace them with burners designed for burning fuel oil but such is not always economic or advantageous to the operation of equipment where natural gas may be intermittently available for use as a fuel. This is particularly true with respect to hydrocarbon steam reforming furnaces, as described, for example, in U.S. Patent Nos.

3,257,172; 3,672,847 and 3,768,980, where many such burners are in operation and a conversion to fuel oil service would require days of down time with great loss of productivity.

While it has previously been disclosed, in U.S. Patent 3,393,964 for example, to atomize liquid hydrocarbons in the burner itself with a normally gaseous fuel, such operation would require the replacement of all existing natural gas burners with a specialized burner and, therefore, is impractical. U.S. Patent 3,291,191 describes the broad concept of vaporizing a normally liquid fuel to make it interchangeable with natural gas in the operation of a natural gas burner. It has previously been known to vaporize liquid fuel by heating. Such heating, however, results in heavy residue and consequent formation of coke requiring frequent shut downs for cleaning, and thus is generally unacceptable.

Other methods have beeen described wherein a portion of the fuel oil is burned with the gaseous products used to vaporize additional fuel oil for burning in a gas fired burner, as described in U.S. Patent 1,987,401. U.S. Patent 3,480,416 describes a method of preparing a gaseous fuel by cracking a portion of a liquid fuel and conjointly heating water to form a gas stream for further use either as a feed for flame cracking or for fuel. Again, there are many consequent disadvantages to such a method. Also, previously, naphthas and light petroleum distillates have been vaporized to replace natural gas as fuel but the prior art does not describe such a system for use of such heavier fuel oils.

Accordingly, it is the object of this invention to provide a system whereby fuel oil can be vaporized to form a gaseous product suitable for use interchangeably, and intermittently, with natural gas in a burner designed for natural gas combustion.

The invention provides apparatus for vaporizing fuel oil for use as fuel in a burner designed for operation with natural gas comprising: a. a heater for generating superheated steam; b. a fuel oil inlet; c. a mixer for mixing superheated steam from the superheater with oil from the fuel inlet; d. a heater for heating the mixture of steam and oil to produce a gaseous mixture of steam and vaporised oil; e. a separator for separating unvaporised oil from the gaseous mixture; f. a heater for heating the gaseous mixture to a temperature and pressure suitable for use in said burner; and g. an output from the last mentioned heater coupleable to said burner.

The invention will now be described further with reference to the accompanying drawings in which: Fig. 1 is a diagram showing in a simple form the vaporizing of fuel oil admixed with steam for use with a burner designed for operation with natural gas; Figs. 2 and 3 are diagrams showing apparatus according to the present invention.

In view of emission limitations placed upon processing plants, many tail gas streams are burned in conjunction with natural gas, both to take advantage of fuel values and to render such gases innocuous.

It is to be understood that when the term "burner designed for natural gas", or equivalent, is used, it includes a burner which uses natural gas and such combustible tail gases as a fuel since such tail gases must still be burned when the system of this invention is in operation.

In Fig. 1, fuel oil enters the system through line 10 and is contacted by a carrier gas, preferably superheated steam, entering through line 12 where they are mixed in a mixing means 14 which can suitably be a "Tee" in the lines 10, 12. The steam can be supplied from a plant utilities stream, from a package boiler or generated by equipment mounted with the unit itself. It is preferred that the steam be superheated so that a sufficient amount of latent heat would be available to prevent condensation of the steam when it comes in contact with the lower temperature fuel oil and thereby heat the fuel oil to some extent. This possible condensation can be partially alleviated by preheating the fuel oil through a suitable heat exchanger or the like.

Whilst the carrier gas is preferably steam, it is to be understood that combustible tail gases, rich in hydrogen, such as for example, the purge gas from an ammonia plant which generally includes hydrogen, ammonia, nitrogen and argon, can either be used alone, if a sufficient quantity is available, or in admixture with steam. Since these tail gases will generally be at an elevated temperature, the latent heat is valuable in aiding in the vaporization of the fuel oil. Tail gases suitable for use will preferably have a molecular weight closely approximating that of steam so that the partial pressure of the vaporized oil is reduced.

The mixture of carrier gas and fuel oil exits the mixing means through line 16 where it enters a vaporizer 18. The vaporizer 18 is a suitably designed heater wherein the materials to be heated entering through line 16 will course through tubes, shown schematically, in a radiant and/or convection zone of such vaporizer 18 wherein a major amount, preferably at least about 75 wt.%, of the fuel oil feed is vaporized to form a gaseous mixture of steam and vaporized fuel oil in admixture with any unvaporized fuel oil. Equipment suitable for use as vaporizer 18 can be designed by the skilled engineer once the disclosure of this system of this invention is known.

While many forms of heating can be used, vaporizer 18 is suitably fired with fuel and air entering through lines 20 and 22, respectively. The fuel used for the vaporizer may well be some of the very same fuel oil which is being vaporized. From the vaporizer 18 the gaseous mixture and any unvaporized fuel oil exits through line 24 and is conducted to separator 26 which may be any suitable gas/liquid separation device, such as a flash tank or drum wherein the gaseous mixture is separated and exits the separator 26 through line 28. The unvaporized oil exits the separator 26 through line 30 and may be recovered and used as fuel for the vaporizer 18.

The gaseous mixture is heated in the vaporizer 18 to such temperature and pressure conditions that the gaseous mixture exiting the separator 26 through line 28 will have temperature and pressure conditions suitable for the particular burner design. These temperatures will generally be within the range of from about 550"F. to about l,000 F. Usually, for a hydrocarbon steam reformer, these conditions involve temperatures from about 600"F. to about 800"F. and a pressure of from about 20 to about 65 psig, and preferably 25 to 45 psig.

Normally, the burner 32 is fired with natural gas, sometimes including a combustible tail gas, entering through line 34 supplied through line 36. The gaseous mixture is introduced into line 36 and enters the burner through line 34. This mode of operation is preferable in order to allow the quantity of natural gas being burned in the burner 32 to be gradually reduced while the amount of gaseous mixture being burned is gradually increased until vaporized fuel oil provides the sole fuel for the burner 32.

Gaseous fuel for the burner 32 can be introduced through line 38 into line 10 to aid in bringing the unit on stream by acting as a supplement to the carrier gas to provide sufficient velocity through the unit to prevent operational problems. As a steady operational state is being achieved, the gaseous burner fuel is reduced and vaporized oil replaces it.

Speaking in general terms, natural gas has a heating value of approximately 1000 BTU's per standard cubic foot while a vaporized fuel oil normally has a heating value of from about 2000 to about 5000 BTU/SCF. Thus, it is important to dilute the vaporized fuel oil. In the practice of this invention, the carrier gas, preferably steam, and most preferably superheated steam, is employed, not only as the diluent but to take advantage of the other benefits the carrier gas provides. One such advantage is that the carrier gas, particularly steam, helps to prevent coking both in the vaporizer 18 and in the burner 32. Steam also promotes a cleaner combustion gas when the vaporized fuel oil produced from operation of this invention is burned. Of particular advantage is that the carrier gas reduces the partial pressure of the fuel oil, aiding in the vaporization of the latter.To achieve such advantages, the carrier gas is mixed with the fuel oil in the mixing means 14 in the proportions of about 0.1 to about 1.0 pounds of steam per pound of fuel oil.

The upper limits of this ratio will normally be used for the vaporization of the heavier fuel oils with a range of from about 0.1 to about 0.7 normally being used. The optimum ratio can be determined by the degree of vaporization desired and the normal operating conditions of the burner.

For No. 2 grade fuel oil, for example, the preferred ratio is from about 0.3 to about 0.5 pounds per pound of fuel oil. Of course, this ratio would be adjusted within the above limits to provide for a proper balance depending upon the grade of fuel oil being used.

Turning to Fig. 2, a schematic diagram of an embodiment of this invention is shown, without detailing the common valves, fittings, controls and flow gauges known to those of ordinary skill in the art, wherein it is possible to both superheat the steam and the resulting gaseous mixture from the vaporization and to maintain some degree of portability of the equipment. While the apparatus described in Fig. 2 is primarily designed for erection on a portable platform, such as a skid, it is well adaptable to be used as a permanent fixture.

Fuel oil enters through line 110. Steam enters through line 112 and is mixed in a mixing means 114 which can be some suitable vessel or an ordinary pipeline "Tee". The mixed fuel oil and steam exit the mixing means 114 through line 116.

The steam entering through line 112 has passed through an upper coil in a superheater shell 118 which is suitably fired with fuel and air entering through lines 120 and 122, respectively. The superheater is designed to give the duty necessary to produce the temperature to pressure conditions desired for contacting and mixing the steam with the fuel oil entering the mixing means 114 through line 110. The steam enters the superheater through line 124 from any suitable plane source or package boiler also of standard design.

If mounted on a skid or movable platform, lines 110 and 124 would be fitted with suitable connecting means to attach the vaporizer of this invention to a source of steam and fuel oil. To compensate for differences in steam quality due to different locations, a bypass, not shown, with appropriate valves can be used to connect lines 124 and 112.

The mixed steam and fuel oil being carried by line 116 is passed through a vaporizer 126 which is of a design as aformentioned in discussion with Fig. 1.

Though other means of heating may be used, the vaporizer 126 is suitably fired by fuel which enters through line 128 and burns in the presence of air entering through line 130 creating radiant and/or convection zones in such vaporizer 126. The gases will suitably pass through tubes 132 in the vaporizer 126, such tubes 132 shown schematically on Fig. 2.

In the vaporizer 126 a major proportion of the fuel oil is vaporized, preferably at least about 75 wit.%, and, most preferably, from about 80 to about 98 wit.?/0. When No. 2 oil is used, substantially all of the liquid fuel oil can be vaporized. This vaporization in the tubes 132 of the vaporizer 126 results in the production of a gaseous mixture of steam and vaporized fuel oil. This gaseous mixture exits the vaporizer 126 through line 134 and enters separator 136 wherein the gaseous mixture is separated from any unvaporized liquid fuel oil, which exits through line 138 and can be suitably used as fuel for the superheater 118 or the vaporizer 126.

Otherwise, during use as a portable unit, line 138 may be provided with a suitable connecting means such that the residual liquid oil may be collected or disposed of in a proper manner.

The gaseous mixture, now substantially free of unvaporized fuel oil, exits the separator 136 through line 140 and thence through line 142 to a lower coil 144 in the common superheater shell 118. The gaseous mixture is raised to temperature and pressure conditions as hereinbefore stated suitable for fuel for a burner designed to normally operate on natural gas. The gaseous mixture exits at these conditions through line 146 which is joined by bypass line 148 connecting with the gaseous mixture outflow of the separator 136 through line 140 providing a bypass for the coil 144 of the superheater 118 should such operation be desired.

The combustible gaseous mixture to be charged to the burner passes through line 150 where it is connected to line 152, the natural gas feed line for the burner 154. The operation of the burner and the mix of gas feed and natural gas feed is as has been described for the broad embodiment of this invention and line 153 for introducing burner fuel into line 110 is provided for purposes previously discussed.

In the design of the portable unit line 150 would be provided with a fitting located in line 152 to provide for the change-over of the burner 154 to the vaporized fuel oil prepared through the practice of this invention. Of course, burner 154 may represent a single burner or a plurality of burners.

With respect to an example of a specific operation of the embodiment shown in Fig.

2, from which a scale-up can be made, and discussed above, Fuel Oil No. 2, previously heated in an exchanger, not shown, flowing at from 0.1 to about 0.5 gallons per minute, is mixed with steam at 5000F. supplied at the rate of 20 to about 150 pounds per hour.

This mixture enters the vaporizer 126 at 300"F. and 40 psig pressure and exits at a gaseous mixture through line 134 at about 30 psig pressure and 600"F. The gaseous effluent from the separator 136, any liquid material having been removed, is at 30 pounds and 600"F. also. In the superheater 118 the gaseous mixture is heated to 750"F.

and 20 psig and was successfully used to operate a burner designed for use in natural gas service.

In the practice of this embodiment of the invention, of course, the apparatus would also be provided with control means to vaporize the different fuel oils to the required conditions.

The embodiment shown in Fig. 3 provides an operating system having improved flexibility and reliability. This embodiment is substantially the same as previously described with respect to Fig. 2 with the exception that separate vessels are used for superheating the steam and gaseous mixture rather than one vessel as set forth above.

The reasons for this will become apparent from the following discussion. For purposes of simplication, the discussion of Fig. 3 will assume that a No. 2 fuel oil is used, it being understood that it is within the scope of this invention to use fuel oils heavier than No. 2, i.e., Fuel Oil No. 2 to No. 6. Steam is again be the carrier gas, it being understood that steam or a tail gas, as previously described, or mixtures thereof are within the scope of this discussion.

Fuel Oil No. 2 enters the apparatus through line 210 and is preferably heated in exchanger 220 which can be used to consume energy available at the operating plant location. While this heat exchanger 220 is optional, it is preferred so that it eases the mixing of the steam with the fuel oil in later operation of the method of this invention.When the heat exchanger 220 is used, the No. 2 fuel oil will exit the exchanger at approximately 3000F. through line 222, thence through valve 224 and a mixing means 226 which may, as has been described previously, be a mixing vessel or nothing more than an ordinary pipeline "Tee". Steam enters the system, preferably at 440"F. and 185 psig, through line 228 into a steam superheater 230 which is operated by burning a fuel in air which enters the superheater 230 through lines 232 and 234, respectively. Of course, the fuel entering through line 232 to be burned may also be fuel oil No. 2, such as is being vaporized in the practice of this invention.While the conditions of the carrier gas are flexible, depending upon availability and condition at a particular plant site, in the discussion steam exiting the superheater 230 through line 236 will be at about 800"F. and about 180 psig pressure.

As is seen on Fig. 3, line 236 branches into lines 238 and 240. The steam travels through branch 238 and proceeds through valve 242, which is open in normal operation of the system of this invention, and continues into the mixing means 226 to be mixed with the fuel oil. The mixture of steam and liquid fuel oil, in this example, 0.5 pounds steam per pound of oil, then passes through line 244 to the vaporizer 246 which, as discussed above, burns a fuel in the presence of air entering through lines 248 and 250, respectively. The vaporizer 246 is designed to have sufficient heating capacity to not only fully vaporize the fuel oil into a gaseous mixture but also provide some superheat to attain the conditions set for the burner in which the gaseous mixture is to be used. The purpose of this will become apparent from subsequent discussion herein.

Substantially all of the No. 2 fuel oil charged under these conditions is vaporized to a gaseous mixture containing the vapors of the fuel oil and steam. Of course, when the heavier fuel oils are used there will be somewhat more unvaporized material but conditions of the carrier gas, proportions thereof and heat in the vaporizer can be adjusted to optimize the conditions. The gaseous mixture leaves the vaporizer 246 through line 252 at conditions of from about 5000 F. to about 900"F. and from about 75 to about 130 psig pressure. For fuel oil No. 2 the preferred conditions are about 6400 F.

and 77 psig pressure. The gaseous mixture and any unvaporized fuel oil passes through valve 254, open during normal operation, through line 256, 258 and thence to separator 260 which can be any appropriate liquid/gas separation device, such as a drum separator.

As stated, substantially all liquid is vaporized when No. 2 oil, oi equivalent, is used but with the heavier oils some liquid will generally exit the vaporizer.

The unvaporized fuel oil exits the separator 260 through line 262 where it can be recovered and used to provide fuel for other use at the plant site, including possibly to fire the vaporizer 246 or superheater 230.

The gaseous stream exits the separator 260 through line 264 which branches into lines 266 and 268. Line 266 leads into valve 270, closed during normal operation, and line 272 which acts as a conduit for the vaporized oil to the gas fired burner.

Meanwhile, branch 268 proceeds through valve 274, open in normal operation, through line 276 to the vaporized oil superheater 278. The vaporized oil superheater 278 is fired by fuel burned in air entering through lines 280 and 282, respectively. The gaseous mixture, now free of unvaporized fuel oil, is heated in the gas superheater 278 to the conditions at which it will be introduced into the natural gas burner; i.e., about 750"F. at about 45 psig, and exits the gas superheater 278 through line 284 which branches into line 286 and 288. Line 288 leads to valve 290, closed in normal operation, which communicates with line 258, as previously described.The heated gaseous mixture is conducted through line 284 to line 286, proceeds through valve 292, open in normal operation, and line 294 which conducts the gaseous mixture to line 272 where it is further transmitted to the natural gas burner indicated on Fig. 3 as 296. As mentioned before, the burner 296 represents either a single burner or a plurality thereof and is not, of itself, an element of this invention.

It is preferable that the oil superheater have the same heat duty as the vaporizer previously described so that either of these two vessels may be used to independently vaporize the fuel oil in the presence of steam to create conditions wherein the gaseous mixture can be used in a burner normally designed to burn natural gas. This allows the vaporizer to be shut down for decoking or maintenance without disturbing the operation of the equipment utilizing the vaporized gaseous mixtures. For equipment, such as hydrocarbon reformers, this is very important since much economy is lost when it must be shut down.Accordingly, Fig. 3 shows a bypass operation of the system wherein the vaporizer 246 can be taken out of service and the gas superheater 278 be used in its place to act as both the vaporizer and the superheater supplying all the heat necessary to prepare the gaseous mixture for burning. Of course, this is not a preferable way of operating over an extended period of time, but it does allow for maintenance or decoking without forcing the equipment burning the gaseous mixture to shut down.

It is also possible, through the alternative piping shown in Fig. 3, to carry the entire heat load through the oil vaporizer and shut down the superheater for decoking, or repair, if necessary.

In order to bypass the vaporizer 246 branch 240 of line 236 would carry the superheated steam through the valve 298, closed in normal operation previously described, to communicate with line 276. It is at that point that the superheated steam and fuel oil would be mixed in an appropriate mixing means.

The heated fuel oil would bypass the vaporizer 246 by closing valve 224 and diverting the flow in line 222 through line 300, valve 302, closed in normal operation, line 304 to the mixing means in line 276 wherein the oil would be mixed with the steam entering through line 298.

During normal operation of the abovedescribed process to provide vaporized No.

2 fuel oil to a steam reformer requiring approximately 600 million btu's per hour, approximately 0.5 pounds of steam are used per pound of No. 2 fuel oil. As was described in previous modifications of the above-described invention, natural gas would be used to fire the burner 296 entering through line 306 and, alternatively, for purposes previously discussed, into line 210 through line 308. The gaseous mixture entering through 272 would gradually replace the natural gas as fuel for the burner 296 until all the fuel to the burner is vaporized fuel oil prepared in the practice of this invention.

As has been previously stated, the process of this invention operates equally well with the oil vaporizer 246 shut down or with the oil superheater 278 shut down. Table I illustrates the position of the valves shown in Fig. 3 and described above in these two alternative systems of operation as compared with normal operation, described in detail above.

Claims (1)

1. 3. A process as claimed in claim 1 or 2,

   wherein the time interval between the end of one pressure reduction and the start of the next pressure reduction is in each case from 5 to 500 seconds.

   4. A process as claimed in any of claims I to 3, wherein the residence time of the reaction mixture in the cooling zone is from 5 to 200 seconds.

   5. A process as claimed in any of claims 1 to 4 wherein the duration of the pressure reductions in the cooling zone is in each case from 0.1 to 10 seconds.

   6. A process for the manufacture of an ethylene polymer carried out substantially as described in the foregoing Example 2 or 4.

   7. A process as claimed in claim I whenever carried out in an apparatus as herein described with reference to and as illustrated in Fig. 1, Fig. 2 or Fig. 3 of the accompanying drawings.

   8. Ethylene polymers when manufactured by a process as claimed in any of claims I to 7.