Adding relatively small amounts of a new additive to the feed stream of a steam cracker can inhibit coke formation on the metal surfaces of processing equipment and increase furnace run time.
The additive comprises a variable mixture of four to six inorganic salts in aqueous solution. The components of the additive mixture can be varied, as needed, for processing heavy feed materials such as heavy naphtha and gas oil.
The process was first tested it a Korean petrochemical plant and is now operating successfully at a commercial facility in Russia. The results of the Korean trial are presented here.
THE ADDITIVE
The additive was developed by Tetra International Inc., Brea, Calif.
Tetra president Henry Jo says the reactivity of the compounds during coke gasification is substantially greater than that of known catalysts, permitting a reduction in coke formation when the mixture is injected at concentrations of 5 20 ppm. At this low level, says Jo, the additive shows neither acidic nor basic characteristics.
The chemical oxidizes the produced coke to form car bon dioxide and a small amount of carbon monoxide. And some of the existing coke is removed, mostly as friable slag.
The additive components are very high boiling, and will not contaminate any products but heavy tar oil, says Jo. Both the chemicals and removed coke will end up in the heavy tar oil. As a result, the chemical causes no environmental problems, says Jo.
The heavy tar oil produced during the trial was sold for carbon black production with no problems, according to Jo. The oil also can be used as fuel oil. The only effect it will have on the fuel system, compared to other Bunker C fuel oils, is increased dust formation. Jo expects the chemical concentration in the oil to be about 100 ppm.
The additive system increases a processor's ability to vary hydrocarbon feedstocks. It also allows an increase in reaction temperature, says Jo, thus increasing ethylene production.
The technology is especially useful for "millisecond" furnaces, where coke is formed overly quickly.
FIELD DATA
Several sets of comparative runs first with the additive, then without were made in a commercial furnace in Korea. The furnace contains four coils and has a total rated capacity of 20,000 kg/hr hydrocarbon feed. Normal operating temperature is 835 C. (1,535 F.).
The first comparison used naphtha feed with an initial boiling point of 35 C. (95F.) and a final boiling point of 175 C. (347 F.). The composition of the naphtha was: 6.37 wt % aromatics, 22.03 wt % cyclic paraffins, 26.48 wt % isoparaffins, 45.08 wt % n paraffins, and 0.04 %wt sulfur.
In the test without the additive mixture, 5,000 kg/hr/coil of naphtha was mixed with 3,000 kg/hr/coil steam.
A comparative run was conducted under the same conditions using the additive. An aqueous mixture (a combination of three of Tetra's six additive components: A, B, C, D, E, and F) was added to the feed (for comparative purposes, the concentration will be called "N" ppm).
The addition of the mixture allowed about a 30% reduction in steam flow, compared to the first run. Over 180 days, the pressure drop remained essentially constant across the pyrocoils, and ethylene and propylene production were about 2% greater than that of the first run (Table 1).
The formation of coke in the transfer line exchanger (TLX) made it necessary to shut down during this first trial. Essentially no coke, however, was found in any of the furnace coils.
Upon completion of the run, the coils and TLX were inspected. No corrosion problems were noted.
Table 1 shows the composition of the pyrogas at the point of discharge from the furnace. Data to the left of the slash represent operation with the additive compound.
Data to the right represent operation without the additive compound.
Table 2 shows the temperature data for two additional runs using naphtha. These runs are analogous to, and the additive composition the same as, the first test.
Flow rates were 5,000 kg/hr/coil naphtha and 3,000 kg/hr/coil steam (without additive) and 5,000 kg/hr/coil naphtha and 1,900 kg/hr/coil steam (with additive). The temperature at the furnace exit was 835 C.
The additive concentration was varied between 1N and 2N, as defined earlier, depending upon the differential pressure across the pyrocoil.
Without the additive, the furnace had to be decoked after 40 days. With the additive mixture, the furnace operated 150 days, after which no coke had formed in the coils.
Throughout the 150 day run using the additive, no substantial change in the temperature of the pyrocoil walls of the furnace was noted (Table 2). In the run without the additive, the temperature climbed steadily, reaching a maximum after 40 days.
As the temperature of the pyrocoiI walls increases, the differential pressure across the pyrocoils increases.
Both effects indicate the deposition of coke on the inner tubular wall of the pyrocoils.
CARBON MONOXIDE
Carbon monoxide content at the beginning and end of the run using the additive was almost constant at 0.1% of mass. This value is only 25 30% greater than that of the run without the additive mixture.
Table 1 shows that, when using the additive, the concentration of carbon dioxide in the pyrogas, upon discharge from the furnace, increased 20 30%. This increase suggests that coke deposits are being gasified in the presence of the additive.
It is important to note that maintaining the proposed additive flow rate, based on the differential pressure of the pyrocoil, permits operation with relatively low releases of CO and CO2. The additive flow technology therefore is useful for the management of CO and CO2 formation.
Tetra has determined that the presence of Component F discourages the formation of unwanted CO and CO2 impurities. Components A and B loosen coke buildup on the tubular walls, permitting partial physical removal of coke from the pyrocoil via flow of the pyrolysis products. Components B, D, and E are corrosion protectors, as are Components A and C.
As seen in Tables 1 and 2, the use of the additive mixture increases furnace run time by a factor of 3 or 4. The output of high pressure steam from the heat exchangers of the TLX also increased by about 30% because of slower formation of coke and resin (2 3 times slower) in the heat exchanger tubes.
The additive mixture also effectively reduces coke deposition in the TLXS, especially in the inlet portion. The inlet (high temperature) portion, and as much as 60-70% of the TLXS, were completely free of coke during the entire 150 day run with the additive mixture.
Toward the exit (low temperature) portion of the TLX small coke deposits were found. The results are shown in Table 3.
Component A content, in terms of "A oxide," or AO, increased in the furnace using the additive compound, indicating, the presence of A in the TLX and its activity in the coke gasification reaction. Moreover, the absence of Fe, Cr, and Ni in the coke deposits of the furnace using the additive mixture indicates the absence of corrosion in the pyrocoils and tubes of the TLX.
ETHANE FEED
Comparative runs were made using ethane feedstock in an industrial furnace with four pyrolysis coils and a total rated capacity of 10,000 kg/hr hydrocarbon feedstock. The exit temperature from each coil was 855 C.
In the run made without the additive mixture, sufficient steam was added to the ethane to produce a mixture containing 40 wt % steam. The differential pressure across the pyrolysis coils, at an ethylene load of 2,500 kg/hr/coil and steam load of 1,000 kg/hr/coil, was approximately 1.5 kg/sq cm.
Formation of coke was indicated by the increase in differential pressure across the pyrolysis coil as the runs progressed.
After 40 days of operation, it was necessary to decoke the unit.
Significant levels of coke had formed on the inner surfaces of portions of the coil wall, and appreciable amounts of CO and CO2 were produced when the coils were decoked.
A comparative 180 day plant run was conducted under the same conditions, except that an additive mixture was introduced into the feed mixture.
The mixture was introduced as a concentration of "N" ppm during startup. The concentration was maintained at this level throughout the run because no noticeable increase in differential coil pressure was observed. Moreover, during the 180 day furnace cycle, the composition of the hydrocarbon/steam mixture remained constant at 20% steam.
As a result of these changes, ethylene output was 1.5% more than that obtained without the additive. In addition, the presence of Component F in the additive mixture lowered the formation of CO to a level comparable to that tonned in the absence of the additive mixture.
This effect can be seen in Table 4, which illustrates the composition of the pyrogas at the point of discharge from the furnace.
GAS OIL FEED
Comparative runs were made using gas oil with a density of 0.81 g/cc.
The gas oil boiling range was 190 350 C. (374 662 F.) and contained: 28,00 wt % aromatics, 32.00 wt % cyclic paraffins, 24.13 wt % isoparaffins, 15.6 wt % n paraffins, and 0.27 wt % sulfur, in sulfur containing hydrocarbons.
The furnace had four coils and a total rated capacity of 20,000 kg/hr hydrocarbon feedstock.
Pyrolysis was conducted at an exit temperature of 820 C. (1,508 F.). Flow rate during operation was 5,000 kg/hr/coil gas oil and 3,000 kg/hr/coil steam (without the additive). The steam flow rate with the additive mixture was increased to 4,500 kg/hr/coil.
The run without the additive mixture had to be stopped for decoking after 40 days.
The concentration of additive in the feedstream was varied as needed between 1N and 2N. The flow rate of additives was adjusted to control the pressure drop at a constant value throughout the run. When the pressure drop in the coil increased substantially the rate of additional injection was increased.
After 90 days of operation, the unit was shut down for summer. Even with the reduced steam flow no evidence of coke formation in the coils was found, nor was any coil corrosion. Further results are presented in Table 5, which shows the composition of the pyrogas as the point of discharge.
With a little practice, the process can be modified easily to adapt to various feedstocks and conditions.
The additive mixture can be prepared from its dry components for different hydrocarbon feedstock and furnaces.
Tetra also has developed special quench exchangers that can replace existing transfer line exchangers. The quench exchangers retard the reverse reactions of condensation and polymerization. According to Jo, the use of these exchangers with the additive will enable the operator to run an ethylene unit virtually without shutdown.
ECONOMICS
For a 1 billion lb/year ethylene plant, says Jo, the annual operating cost for the chemical mixture is $100,000 200,000. He estimates the technology would save a plant of that size more than $6 million/year.
The reduction in coke can increase ethylene yields and extend the lifetime of expensive alloy tubes, he says. And because decoking is not necessary, the technology will help reduce the air pollution problems associated with the decoking process.
Tetra International is negotiating with a company in Taiwan to test the additives in another ethylene unit. The technology is available for license.
Copyright 1994 Oil & Gas Journal. All Rights Reserved.
