Reduction of fouling in thermal processing of olefinic feedstocks

Abstract

Reduction of fouling in a thermal process for treating feed streams comprising naphtha, pyrolysis oils or a mixture thereof said feed stream having a combined olefinic content from 10 to 50 weight % and the balance inert hydrocarbons at a temperature from 100° C. to 300° C. which comprises decreasing the amount of carbon steel in the apparatus contacting said feed stream and increasing the amount of stainless steel.

Description

FIELD OF THE INVENTION

The present invention relates to chemicals processing, more particularly chemical processing of hydrocarbon stream at elevated temperatures. In processing apparatus comprising a significant amount of carbon steel there is a tendency for the apparatus to become fouled. This fouling reduces the efficiency of such devices and leads to more frequent shut downs to clean the devices.

BACKGROUND OF THE INVENTION

A significant amount of information is available on the thermal stability of aviation fuels over various metal surfaces, however, most of the work done to date has been at temperatures where coking reactions predominate (>350° C.), using feedstocks that have negligible olefin and diene content.

L. E. Faith, G. H. Ackerman, H. T. Henderson, Heat Sink Capability of Jet A Fuel: Heat Transfer and Coking Studies, NASA Report CR-72951, Shell Development Company on contract, Cleveland, Ohio, July, 1971 evaluated the thermal stability of Jet A fuel using an Alcor JFTOT at 210° C. Jet fuel is predominantly a mixture of aliphatic, naphthenic and aromatic species, with very low olefinic content, which reacts predominantly via autoxidation. This is in contrast to pyrolysis gasoline, which contains upwards of 50% reactive olefins such as styrene, indene, and derivatives, and reacts via thermally and catalytically induced free-radical polymerization.

There have not been significant studies on the effect of the composition of steel on feed streams at temperatures below 300° C., typically from 100° C. to 300° C. preferably from 150° C. to 250° C. The present invention seeks to reduce fouling in the treatment of feedstocks having a significant olefin/diolefin content (e.g. from 10 to 50 weight %) at such temperatures.

SUMMARY OF THE INVENTION

The present invention provides a process to reduce fouling in a petrochemical thermal process for treating a feed stream comprising naphtha, pyrolysis oils or a mixture thereof said feed stream having a combined olefinic content from 10 to 50 weight % and the balance inert hydrocarbons at a temperature from 100° C. to 300° C. which comprises decreasing the amount of carbon steel in the apparatus contacting said feed stream and increasing the amount of stainless steel having an iron content from 60 to 75 weight %, a chrome content from 15 to 21 weight %, a nickel content from 1 to 15 weight %, a manganese content from 0 to 3 weight %, a molybdenum content from 0 to 3 weight % of and the balance comprising one or more elements selected from the group consisting of Si, Co, Cu, and other trace elements provided no single such element is present in an amount of greater than 1 weight %.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of the Alcor hot liquid process simulator (HLPS) used in the examples.

FIG. 2 shows the temperature profiles for heater tube for various alloys in the HPLS as a function of distance from the top of the heater tube.

FIG. 3 shows the propensity for fouling of pyrolysis gasoline over various alloys of heater tube at 250° C. at a nitrogen pressure of 150 psig and a flow rate of 3.0 ml/min.

DETAILED DESCRIPTION

Thermal process in accordance with the present invention involve process in which a feed stock is heated in an apparatus at temperatures below 300° C., typically from 100° C. to 300° C., preferably from 150° C. to 250° C. Such processes may be heat exchange processes, distillation, and the like. The pressures in the process may range from sub-atmospheric to several thousand pounds, typically from atmospheric to about 5,000 psig, preferably from about atmospheric to 5,000 psig, most preferably from about 15 psia to about 1,500 psig.

The feedstock is typically naptha or pyrolysis oil or pyrolysis gasoline. Generally the feedstock has a combined olefinic content from 10 to 50 weight % and the balance inert hydrocarbons. Typically the olefinic content comprises styrene monomers which are unsubstituted or substituted by one or more C_1-4, preferably C_1-2 alkyl radicals and C_9-12 bicyclic aromatic ring species which are unsubstituted or substituted by one or more C_1-4 alkyl radicals preferably C_1-2 alkyl radicals. Typically the bicyclic aromatic compounds are substituted indenes. The feedstock may have a content of styrene monomers which are unsubstituted or substituted by one or more C_1-4 alkyl radicals from 25 to 32 weight %. The feedstock may have a of C_9-12 bicyclic aromatic ring species which are unsubstituted or substituted by one or more C_1-4 alkyl radicals from 4 to 10 weight %. Preferably the combined content of styrene monomers which are unsubstituted or substituted by one or more C_1-4 alkyl radicals and C_9-12 bicyclic aromatic ring species which are unsubstituted or substituted by one or more C_1-4 alkyl radicals from 35 to 42 weight %.

Typically the stainless steel useful in accordance with the present invention steel has an average surface roughness (R_a) of less than 25 μm most preferably less than 20 μm. Generally the stainless steel of the present invention has an iron content from 60 to 75, preferably less than 73 weight %, a chrome content from 15 to 21, preferably from 16 to 21, most preferably from 16 to 18 weight %, a nickel content from 1 to 15, preferably from 0.3 to 13, most preferably less than 6 weight %, a manganese content from 0 to 3, preferably from 0.5 to 2, most preferably less than 1.5 weight %, a molybdenum content from 0 to 3, preferably 0.1 to 2.5 weight % of and the balance comprising one or more trace elements typically selected from the group consisting of Si, Co, Cu, and other trace elements provided no single such element is present in an amount of greater than 1, typically in an amount not greater than 0.5 weight %. The sum of the elements in the steel should add up to 100 weight %.

The present invention will now be illustrated, in a non-limiting manner by the following examples.

The HLPS used in the experiments is shown in FIG. 1. The feedstock is kept in a feed tank or reservoir 1. A feed line 2 passes from the feed tank 1 to a filter 3. The line then passes into the heater tube 4 made from the various alloys tested heated in a shell. The heater tube had thermocouples 5 and 6. Additionally, a temperature measuring device at thermocouple 7 was located internally in the heating tube. The fluid passed over the surface of the tube and then passed via line 8 through a filter 9. A pressure drop module 10 was placed across the filter and measured the pressure drop across the filter with time as the fluid cycled through the tube. When the pressure drop reached 200 mm Hg the experiment stopped (the cut off point). The time to reach the cut off point was recorded. After flowing through the filter 9 the fluid flowed through line 11 to a pump 12 and back to the feed tank 1. The feed tank was pressurized with nitrogen through line 13. The nitrogen pressure used in the experiments was 150 psig.

The feedstock for the experiments was pyrolysis gasoline obtained from NOVA Chemicals flexi-cracker at Corunna. The pyrolysis gasoline had an olefinic content shown in Table 1.

TABLE 1
Compound Type                   Weight %
Styrene                         17.4
Aromatic C1 substituted styrene 17.9
Aromatic C2 substituted styrene 1.4
Indene                          5.3
Aromatic C1 substituted indene  1.5
Aromatic C2 substituted indene  Less than 1
Total styrenics                 36.8
Total indenes                   6.7
Inerts                          55.7
Total                           99.2

Note the inerts are predominantly non-olefinic hydrocarbons other than styrene and indene.

The compositions of the various alloys used in the experiments are shown in Table 2.

	TABLE 2
	Element	1018CS	446SS	304SS	316SS
	Al	0.4	0.0	0.0	0.0
	Si	0.1	0.1	0.2	0.1
	S	0.0	0.0	0.0	0.0
	Cr	0.1	20.5	15.9	16.1
	Mn	0.7	0.8	1.1	1.7
	Fe	94.0	72.2	69.5	62.6
	Co	0.0	0.0	0.2	0.1
	Ni	0.1	0.4	8.9	12.4
	Cu	0.1	0.1	0.3	0.3
	Mo	0.0	0.1	0.3	2.5
	Total weight %	95.4	94.3	96.3	95.8

FIG. 2 shows the temperature profiles for heater tube for various alloys in the HPLS as a function of distance from the top of the heater tube. The average surface temperature for the various tubes is set forth in Table 3.

	TABLE 3
	316SS	304SS	446SS	1018CS
	Tsurface, ave (° C.)	199	195	195	193

The surface roughness for the various tubes was measured and the results are shown in Table 4.

TABLE 4
Surface Roughness of HLPS Heater Tubes
	1018CS	304SS	316SS	446SS
Average	0.27 ± 0.09	0.21 ± 0.03	0.23 ± 0.02	0.11 ± 0.02
Surface
Roughness
Ra (μm)

Tube surface roughness was analyzed using a Mitutoyo 402 Surftest Series 178 Instrument (Table 4). Surface roughness, measured as (R_a), is the arithmetical mean of the roughness profile. The data shows that surface roughness is similar for all heater tubes.

The test procedure used was as follows:

500 cc of pyrolysis gasoline was gravity filtered using 8 μm filter paper to remove pre-existing solids/gums, and was charged to the HLPS under 150 psig N₂. Liquid flow was set to 3.0 ml/min, giving the liquid a residence time of ˜11 seconds in the heated section. Fouling was monitored as a function of differential pressure (dP) across a 17 μm filter at the outlet of the test section. The test was stopped when the dP reached 200 mm Hg.

The fouling propensity for the various alloys is shown in FIG. 3. Relative to 316SS (relative rate=1.0), fouling rates over 1018CS, 304SS, and 446SS proceed according to the following scale:

	Alloy	1018CS	446SS	304SS	316SS
	Relative Initial	˜1.70	˜1.50	˜1.47	1.00
	Fouling Rate

In an earlier study by Faith and coworkers, the relative mass of deposits from Jet A fuel at 210° C. over the same alloys follows according to:

	Alloy	304CS	1015SS	316SS	446SS
	Relative Fouling	˜70	˜56.6	˜3.00	1.00
	Propensity

Results from the current work differ from work by Faith and coworkers because of the nature of the hydrocarbon fluid. Jet fuel is predominantly a mixture of aliphatic, naphthenic and aromatic species, with very low olefinic content, which reacts predominantly via autoxidation. This is in contrast to pyrolysis gasoline, which contains upwards of 50% reactive olefins such as styrene, indene, and derivatives, and reacts via thermally and catalytically induced free-radical polymerization.

The experiments show that stainless steel offers advantages over carbon steel in fouling mitigation for streams high in olefin and diene content.

Claims

1. A process to reduce fouling in a petrochemical thermal process for treating a feed stream comprising naphtha, pyrolysis oils or a mixture thereof said feed stream having a combined olefinic content from 10 to 50 weight % and the balance inert hydrocarbons at a temperature from 100° C. to 300° C. which comprises decreasing the amount of carbon steel in the apparatus contacting said feed stream and increasing the amount of stainless steel having an iron content from 60 to 75 weight %, a chrome content from 15 to 21 weight %, a nickel content from 1 to 15 weight %, a manganese content from 0 to 3 weight %, a molybdenum content from 0 to 3 weight % of and the balance comprising one or more elements selected from the group consisting of Si, Co, Cu, and other trace elements provided no single such element is present in an amount of greater than 1 weight %.

2. The process according to claim 1, wherein the olefinic content of said feed stream comprises styrene monomers which are unsubstituted or substituted by one or more C1-4 alkyl radicals and C9-12 bicyclic aromatic ring species which are unsubstituted or substituted by one or more C1-4 alkyl radicals.

3. The process according to claim 2, wherein said feed stream has a content of styrene monomers which are unsubstituted or substituted by one or more C1-4 alkyl radicals from 25 to 32 weight %.

4. The process according to claim 3, wherein said feed stock has a content of C9-12 bicyclic aromatic ring species which are unsubstituted or substituted by one or more C1-4 alkyl radicals from 4 to 10 weight %.

5. The process according to claim 4, wherein said feed stream has a combined content of styrene monomers which are unsubstituted or substituted by one or more C1-4 alkyl radicals and C9-12 bicyclic aromatic ring species which are unsubstituted or substituted by one or more C1-4 alkyl radicals from 35 to 42 weight %.

6. The process according to claim 5, wherein said stainless steel has a surface roughness (Ra) of less than 25 μm.

7. The process according to claim 6, wherein the thermal processing is conducted at a temperature from 150° C. to 250° C.

8. The process according to claim 7, wherein said stainless steel has an iron content of less than 73 weight %.

9. The process according to claim 8, wherein said stainless steel has a chrome content from 21 to 16 weight %.

10. The process according to claim 9, wherein said stainless steel has a Ni content from 0.3 to 13 weight %.

11. The process according to claim 10, wherein said stainless steel has a manganese content from 0.5 to 2 weight %.

12. The process according to claim 11, wherein said stainless steel has a molybdenum content from 0.1 to 2.5 weight %.