DE69323285T2 - Compliance with the coke bond with phosphoric acid triamide - Google Patents

Description

Field of application of the invention

The invention relates to an antifouling process for treating heat transfer surfaces which heat or cool various hydrocarbon feedstocks, often in the presence of steam, under conditions which promote coke formation on the surfaces. The invention particularly relates to a process for preventing coke formation on heat transfer surfaces used to heat or cool a petroleum feedstock under coke forming conditions.

Background of the invention

The production of ethylene involves the use of pyrolysis or cracking furnaces to produce ethylene from various gaseous and liquid petroleum feedstocks. Typical gaseous output Materials include ethane, propane, butane and mixtures thereof. Typical feedstocks include naphthas, kerosene and atmospheric/vacuum gas oil. When gaseous or liquid hydrocarbon feedstocks are pyrolyzed in the presence of steam, significant amounts of ethylene and other useful unsaturated compounds are obtained. Steam is used to control the cracking reaction of unsaturated feedstocks to unsaturated products. The effluent products are cooled and fractionated in downstream columns and then further reacted or processed as required.

Fouling on cracking furnace coils, transfer line exchangers (TLEs), and other heat transfer surfaces occurs as a result of coke formation and polymer deposition. The problem of fouling is one of the major operational limitations encountered in the operation of an ethylene plant. Depending on the rate of deposition, ethylene furnaces must be shut down periodically for cleaning. In addition to periodic cleaning, emergency shutdowns are sometimes required due to dangerous pressure rises or temperature rises resulting from the buildup of deposits in the furnace coils and TLEs. Cleaning operations are performed either mechanically or by passing steam and/or air through the coils to oxidize and burn off the coke buildup.

A major limitation on ethylene furnace operating life is coke formation in the radiant zone or section and in the transfer line exchangers (TLE). Coke is normally removed by introducing steam and/or air into the unit, which burns off the carbonaceous deposits. Since coke is a good thermal insulator, furnace firing must be gradually increased to achieve sufficient heat transfer to maintain the desired level of conversion. Higher temperatures shorten the life of the piping and the The use of tubes is very expensive. In addition, coke formation reduces the effective cross-sectional area of the process gas, increasing the pressure drop within the furnace and TLE. Not only is valuable production time lost during coke removal, but the pressure increase resulting from coke formation also adversely affects ethylene yield. The operating life of ethylene furnaces averages from one week to four months, depending in part on the rate of fouling on the furnace piping and TLE. This fouling rate in turn depends on the type of feedstock, as well as on the furnace design and operating parameters. In general, however, heavier feedstocks and more severe cracking conditions lead to increased deposit formation in the furnace and on the TLE. A process or additive that can increase the operating life would result in fewer production days lost to decoking and lower maintenance costs.

Considerable efforts have been made over the last 20 years to develop phosphorus in numerous forms as a coke inhibitor (see US patents 3,531,394 Koszman; phosphoric acid); 4,105,540 (Weinland; phosphate and phosphite mono- and diesters); 4,542,253 and 4,842,716; (Kaplan et al., amine complexes of phosphate, phosphite, thiophosphate and thiophosphite mono- and diesters); 4,835,332 (Kisalus; triphenylphosphine) and 4,900,426 (Kisalus; triphenylphosphine oxide). Compared to other element-based additives, many of these phosphorus-based antifouling agents work extremely well in suppressing coke formation in both laboratory stimulation and industrial applications, but some result in adverse side effects that prevent prolonged use in many situations, such as contributing to corrosion, affecting catalyst performance, etc.

Convection zone corrosion is a problem with many of the state of the art phosphorus-based anti-coke additives. Along the path of the convection zone pipeline, conditions are constantly changing. Typically, heated steam and a heated hydrocarbon are introduced into the section separately and then mixed well before entering the radiant zone. During the numerous passes the streams experience, separately or mixed, temperatures, pressures and compositions may exist which enhance the conversion of anti-coke additives to detrimental corrosive byproducts. A product that is an excellent coke suppressant may also be an extremely corrosive product if it accumulates in the convection zone.

Once the additives have passed through the convection, radiation and TLE zones, they are subjected to the cooling conditions of the effluent. In a very simplified view, heavy products concentrate in the primary fractionator, in the water cooling tower and in the caustic tower and/or in the compressor separation drums, while the lighter components are fractionated in columns downstream of the compressors. The enrichment of coke inhibitors and their cracked byproducts is mainly determined by their physical properties. In brief, inhibitor byproducts with high boiling points are condensed early in the fractionation process, while lighter byproducts are carried to the later stages.

The buildup of antifouling agents and/or their byproducts in the radiant and TLE coke, in the primary fractionator or in the water cooling tower is mostly acceptable. These sections process and collect many other heavy products which are very impure and therefore trace amounts of an additive generally do not have a significant effect.

In contrast, additives and/or byproducts passing through the caustic tower and compressor zones can pose a significant problem. After these zones, purification becomes an important issue since downstream fractionation generally separates the unsaturated products into high purity chemicals. The presence of phosphorus-containing products, which could adversely affect the performance of the catalysts used to process these lighter components, is unacceptable.

Many phosphorus-containing products are good ligands and can adversely affect catalyst performance. The phosphorus byproduct of most concern is phosphine (PH3). This byproduct has an extremely low boiling point (-88ºC). In fact, it has essentially the same boiling point as acetylene (-84ºC), a hydrocarbon byproduct that is often catalytically hydrogenated to the more desirable ethylene.

Therefore, there remains a need for a phosphorus-based anticoking additive for cracking furnaces that is substantially free from contributing to corrosion and the formation of catalyst-impacting by-products.

Summary of the invention

The aim of the present invention is therefore to find a new phosphorus-based anti-coking additive for cracking furnaces which is substantially free from contributing to corrosion and forming by-products which degrade the catalyst.

It has surprisingly been found that the above-mentioned object can be achieved according to the invention by using specific phosphoric acid triamides of the formula (I) given below as anti-scale formers and as a coke suppressant in various high temperature applications including steam cracking furnaces for treating heat transfer/heat transfer surfaces used to heat or cool a petroleum feedstock under coke forming conditions.

The present invention provides a method for preventing coke formation on heat transfer/heat transfer surfaces used for heating or cooling a petroleum feedstock under coke forming conditions, which comprises

contacting the heat transfer/heat transfer surfaces with an effective amount of a phosphoric triamide of the formula

where:

X is a chalcogen, preferably sulphur and in particular oxygen, and

R¹ to R³ each represent an amino radical of the formula:

wherein R⁴ is hydrocarbyl and R⁵ is hydrogen or hydrocarbyl and

R⁴ and R⁵ taken together can form a heterocyclic radical and preferably do.

The heat transfer surfaces can be contacted with the inhibitor in a number of different ways, for example, by pretreating the heat transfer surfaces prior to heating or cooling the petroleum feedstock, by continuously or intermittently adding a trace amount of the additive to the petroleum feedstock while it is being heated or cooled, by adding the phosphoric triamide to the steam feedstock which is then mixed with the petroleum feedstock, to the petroleum feedstock itself, or to a feed mixture of the petroleum feedstock and steam.

When the petroleum feedstock which is heated or cooled is treated with the phosphoric triamide, the additive is preferably added in an amount of 0.1 to 1000 ppm based on the elemental phosphorus in the additive, especially 1 to 100 ppm based on the weight of the petroleum feedstock.

R⁴ and R⁵ in the above-mentioned phosphoric triamide formula are preferably each alkyl, aryl, alkylaryl or arylalkyl, the phosphoric triamide preferably having 3 to 48 carbon atoms, and R⁴ and R⁵ each particularly preferably having 1 to 15, especially 1 to 8 carbon atoms. Each heterocyclic radical particularly preferably has 4 to 12 carbon atoms and is selected from the group consisting of pyrrolyl, pyrrolinyl, pyrrolidinyl, dihydropyridinyl, tetrahydropyridinyl, piperidino, azepinyl, indolyl, isoindolinyl, indolinyl and carbazolyl.

A particularly preferred phosphoric triamide comprises tripiperidinophosphine oxide.

According to further preferred embodiments of the invention:

the petroleum feedstock includes ethane, propane, butane, naphtha, kerosene, gas oil or a combination thereof;

the heat transfer/heat transfer surfaces comprise cracking furnace coils or transfer line exchangers; and

the heat transfer/heat transfer surfaces are pretreated with the phosphoric triamide before the petroleum feedstock is heated or cooled.

Other preferred embodiments of the method according to the invention include:

Adding the phosphoric triamide to a petroleum feedstock and passing the resulting mixture through the convection and radiant zones or sections of a cracking furnace;

in addition, the fractionation of the furnace effluent and the catalytic treatment of a fraction thereof;

adding the phosphoric triamide to a petroleum feedstock or an ethylene furnace effluent upstream of a transfer line exchanger; and

passing the effluent from the cracking furnace containing the phosphoric triamide through the transfer line exchanger;

the further fractionation of the furnace effluent and the catalytic treatment of a fraction thereof;

the addition of phosphoric triamide to steam;

mixing the steam with a petroleum feedstock and

passing the mixture of starting material and steam containing the phosphoric triamide through a cracking furnace; and

adding the phosphoric triamide to a mixture of steam and petroleum feedstock; and

passing the resulting mixture through a cracking furnace. The phosphoric triamide derivatives of the above general formula (I) used according to the invention are already known from US-A-3 433 623 and EP-A-0 165 507, and they are described therein as additives for controlling weeds and as phosphorus dopants for use in the manufacture of integrated circuits, respectively. However, no other applications for these phosphoric triamides are described in either document. In particular, their use as anti-scale agents and as coke formation suppressants in various high-temperature applications is not mentioned therein.

For the purposes of the present invention, coke formation is defined as an accumulation of coke or coke precursors on the heat transfer surfaces, including the convection coils, radiant furnace coils, transfer line heat exchangers, cooling towers, or the like. Other phosphorus-containing compounds have been described in various patents and other publications as effective coke formation inhibitors. However, none of the phosphorus compounds provide the same performance as the phosphoric triamides used in the present invention. The performance is based not only on the ability of the anticoking agent to suppress and prevent coke formation, but, just as importantly, it is essentially free from causing any of the deleterious side effects associated with many of the prior art additives, such as contributing to corrosion or impairing catalyst performance.

The term "petroleum feedstock" as used herein refers to any hydrocarbon which is heated or cooled at the heat transfer/heat transfer surfaces generally, regardless of the degree of prior processing, and particularly when used in connection with an ethylene or other cracking furnace, it refers to the hydrocarbon before processing as well as the hydrocarbon during and after processing in the furnace itself, in the TLE, in the cooling zone, and the like. The feedstock may include ethane, propane, butane, kerosene, naphtha, gas oil, and combinations thereof.

Short description of the drawings

Figure 1 shows a graph showing the coke formation rates obtained by inhibition with tripiperidinophosphine oxide over a typical cracking furnace operating temperature range according to the present invention.

Description of the invention

The coking (coke formation) inhibitor used in the present invention is a phosphorus-based compound which is preferably substantially non-corrosive and preferably substantially free from phosphine formation under general hydrocarbon cracking conditions. The anticoking agent used in the present invention has the following general formula:

where:

X is a chalcogen, preferably sulphur and in particular oxygen, and

R¹ to R³ each independently represent an amino radical of the formula: R&sup4;

wherein each R⁴ independently represents hydrocarbyl, for example alkyl, aryl, alkylaryl or arylalkyl, and each R⁵ independently represents hydrogen or hydrocarbyl and R⁴ and R⁵ taken together can form, and preferably do form, a heterocyclic radical, for example pyrrolyl, pyrrolinyl, pyrrolidinyl, piperidino, dihydropyridinyl, tetrahydropyridinyl, azepinyl, indolyl, indolinyl, isoindolinyl and carbazolyl.

This class of nitrogen-containing phosphorus compounds are referred to here generally as phosphoric triamides, sometimes also known as phosphoramides or triaminophosphine oxides or sulfides.

The phosphoric triamide preferably contains 3 to 48 carbon atoms, and more preferably 12 to 24 carbon atoms. Each hydrocarbyl group R⁴ and R⁵ (if they do not form a heterocyclic amino group) preferably contains 1 to 15 carbon atoms. Each heterocyclic amino group (if R⁴ and R⁵ taken together form a heterocyclic group) preferably contains 4 to 12 carbon atoms. If the number of carbon atoms in the amino groups is excessively large, the economics of the additive are less favorable, the additive may lose its volatility and miscibility for proper mixing with the petroleum feedstock to be treated, or the desired stability. The hydrocarbyl groups may be substituted by or contain a heteroatom, for example a chalcogen or nitrogen atom, this is generally however, less preferred because of the inherent instability caused by the heteroatom. In some situations where the heteroatom imparts, for example, solubility in steam or water, the presence of a heteroatom may be useful, particularly when the heteroatom is located in a terminal portion of the hydrocarbyl group spaced from the amino moiety such that cleavage or other reaction of the heteroatom leaves the triaminophosphine moiety substantially intact for anticoking effectiveness.

The hydrocarbyl groups in each amino residue may be the same or different, for example when the phosphoric triamide has been formed from a mixture of different amines and/or has been reacted with different amines in stages. In many cases it is not necessary for the phosphoric triamide to be completely pure and a reaction product obtained using isomers or mixtures of amines, which may be more economically available than the pure amines, is generally suitable.

Specific representative examples of the anticoking additives include tripiperidinophosphine oxide, hexamethylphosphoramide, hexaoctylphosphoramide and hexaphenylphosphoramide. Phosphoric triamides in which the amino radicals are cyclic groups, particularly piperidino and pyrrolidinyl radicals, are preferred because of their non-corrosiveness and because they are substantially free of phosphine formation, as well as because of the suppression of coke formation. For the sake of clarity and convenience, and not by way of limitation, the preferred anticoking agent is generally referred to herein as tripiperidinophosphine oxide (TPyPO), it being understood, however, that this is only one example of the phosphoric triamides used in the present invention.

The phosphoric triamides are prepared by methods known per se and in some cases are commercially available. In general, the phosphoric triamides can be prepared by reacting phosphorus oxyhalide such as phosphorus oxybromide or phosphorus oxychloride with an excess of the desired amine such as piperidine in a suitable solvent, e.g. a heavy aromatic naphtha, toluene and benzene, to evolve the corresponding hydrogen halide. Other bases which are less nucleophilic (e.g. pyridine) can also be incorporated to effect the desired conversion. Alternatively, the phosphoric triamide can be prepared from the corresponding phosphorus triamide which is oxidized or sulfurized. In some cases it may be possible to effectively convert phosphorus triamide to the corresponding phosphorus triamide in situ in the cracking furnace according to the present invention.

The TPyPO is used to prevent coke formation on heat transfer surfaces, most commonly used for heating, but sometimes also for cooling, petroleum feedstocks under coke forming conditions, by treating the surfaces with an effective amount of TPyPO. The surface can be effectively treated, for example, by introducing the TPyPO into the petroleum feedstock before the feedstock comes into contact with the heat transfer surfaces.

Generally, the TPyPO can be used in an amount effective to achieve the desired coke prevention, typically in an amount of at least 0.1 ppm by weight in the hydrocarbon, preferably at least 1 ppm, based on elemental phosphorus. No additional benefit is generally obtained if the TPyPO is used in a relatively high concentration, and the economics are less favorable. Preferably, the TPyPO is used in an amount of from 0.1 to 1000 ppm, more preferably from 1 to 100 ppm, based on based on the weight of the hydrocarbon and calculated on the basis of elemental phosphorus.

The addition to the petroleum feedstock is preferably continuous, but it is also possible to carry out the petroleum feedstock treatment on an intermittent basis depending on the coking inhibition desired in the particular application. For example, if a shutdown of the heat transfer device is planned to perform maintenance for reasons other than the accumulation of coke deposits, the continuous addition of TPyPO to the petroleum feedstock can be terminated prior to the shutdown. However, the anticoking agent can also be used in the petroleum feedstock after the development of a pressure drop within the heat transfer device indicative of coke formation therein.

It is also possible to treat the heat transfer surfaces before they come into contact with the petroleum feedstock, for example by using TPyPO as a pretreatment or as a treatment between production runs. As a pretreatment, the TPyPO can be circulated through the heat transfer device, preferably in a suitable diluent. The heat transfer device can also be filled with the TPyPO solution and impregnated with it for a period of time to form a protective film on the heat transfer surfaces. Similarly, the petroleum feedstock may be added at a relatively high initial rate of, for example, 0.5 to 2.0 wt.% at the beginning of a run and after a period of, for example, 1 to 24 hours the amount may be reduced to the continuous dose levels as set out above.

When the petroleum feedstock that is heated or cooled is treated on a generally continuous basis, the TPyPO is preferably in the form of a solution to a master batch. The manner of mixing the TPyPO with the feedstock is not particularly critical and a vessel with an agitator is all that is required. Most conveniently, however, a master batch of TPyPO in a suitable solvent, such as an aliphatic or aromatic hydrocarbon, is metered into a feedstock stream and intimately mixed therewith by turbulence in the treatment device. The TPyPO may also be added to a steam or water stream injected or otherwise introduced into the petroleum feedstock stream, or the TPyPO may be added to a mixed stream of the petroleum feedstock and steam or water.

The TPyPO should be added to the feedstock upstream of the heat transfer/heat transition surfaces to be treated. The TPyPO addition should be carried out sufficiently upstream to allow sufficient mixing and dispersion of the additive in the feedstock, but preferably not too far upstream to avoid or minimize significant degradation or degradation of the TPyPO before it reaches the surfaces to be treated.

The invention is illustrated by the following examples.

Examples

In the following examples, various phosphorus compounds were evaluated and compared with each other in terms of their coking (coke formation) inhibition, their corrosiveness and their phosphine formation. The additives used have the names given in Table 1.

Table 1

Additive active component

A Tripiperidinophosphine oxide

B Amine-neutralized thiophosphate mono/diester*

C O,O,O-triphenyl phosphate

D Amine-neutralized phosphate mono/diester*

E Diphenylphosphinic acid**

F Triphenylphosphin

Borane-tributylphosphine complex

H Triphenylphosphine oxide

* The alkyl groups were C₆-C₁₀-paraffins; neutralized with morpholine

** neutralized with morpholine.

To determine coking suppression data, a laboratory reactor was used to simulate the conditions used in a typical ethylene furnace. The reactor is described in US-AA 835 332. All weights and percentages are by weight unless otherwise stated.

example 1

Tripiperidinophosphine oxide (TPyPO) was evaluated for coke suppression at a concentration of 200 ppm and 500 ppm in n-hexane. The hexane was fed to the test reactor at a rate of 38 to 40 g/h with a steam dilution of 0.5 kg/kg hexane and a V/F₀ of 41 to 43 l.s/mol, where V is the equivalent reactor volume (in l) and F₀ is the initial molar flow rate of hexane (in mol/s). The results shown in Figure 1 show that Additive A reduced the asymptotic coke rate over a temperature range above about 770°C. This performance is comparable to the performance of other additives at Phosphorus-based phosphorus-based pigments as described in US-A-4,842,716, 4,835,332 and 4,900,426.

Example 2

A high temperature axle bushing was used to determine the degradation properties of various additives over long periods of time. To accelerate the corrosion effects, Additive A was used at a concentration of 5% in heavy aromatic naphtha and other additives with an equivalent phosphorus content were used. The additive was placed in a high alloy vessel along with a hydrocarbon, an equivalent amount of water and pre-weighed carbon steel coupons. The contents were continuously rotated at temperatures representative of a typical convection zone of an ethylene furnace; mixing ensured that the coupons were exposed to both the liquid phase and the gas phase (consisting of water and the hydrocarbon). Exposing the additives to high temperature for long periods of time allowed for potential degradation to harmful byproducts. This method essentially simulated a worst case scenario, where there is a fairly high concentration of an additive in the convection zone with possible accumulation/degradation (e.g. by thermolysis, hydrolysis, disproportionation, etc.) of by-products, which may or may not be corrosive. Moreover, the occurrence of corrosion may not be the direct result of degradation, but may also be a property of the additive itself.

In Table 2, the test data for additive A are compared with three other compounds, two of which were amine-neutralized phosphate esters mono- and disubstituted with alkyl groups, ie known coking suppressants with an aggressive corrosivity. As can be seen, the tripiperidinophosphine oxide (A) showed an excellent recorded performance. This was not the case with the other phosphorus-based compounds.

Table 2

Additive weight loss (mg)

A(TPyPO) 9.2

B90

C115

D120

1.6

Example 3

A laboratory unit was constructed which simulated the dynamic (i.e. erosive and corrosive) conditions of a typical convection zone of an ethylene furnace. Corrosion is more likely to occur at or near the bends/curvatures of the convection zones because of the high erosion resulting from the velocity of the feed stream. Steam generated in one vessel was mixed with the hydrocarbon (hexane and toluene in a 50:50 weight percent ratio) from a second vessel (steam:hydrocarbon weight ratio 0.5-0.6). Heating to the desired temperature was accomplished by passing the mixture through two independent furnaces maintained at the specified temperatures (100 to 600ºC). Both furnaces were monitored and controlled by two separate temperature controllers. The pre-weighed corrosion coupons, made of carbon steel, were placed at the bends inside the furnace coil. Coupon A was placed in the process stream, ie exposed to the erosive and corrosive character of the process stream upstream and at a lower temperature (above 100ºC) relative to coupon B which was placed downstream at a higher temperature. (less than 600ºC). Thermocouples were used to record the temperature of both coupons and both oven zones.

The additives were added to the hydrocarbon feed and tested under identical conditions in the same manner as a blank (without additive). The coupon weight loss for several additives is given in Table 3. Tripiperidine phosphine oxide (A) at a concentration of 2.4 wt.% in the hydrocarbon gave excellent results compared to the other compounds tested with an equivalent phosphorus content. Table 3

Example 4

To determine the tendency of various phosphorus-based products to form PH3, a known catalyst poison, additives were evaluated in the apparatus described in Example 3. Additive A was used at 5 wt.% in the carbon and all other additives were used at an equivalent phosphorus content. To achieve the appropriate cracking temperature, a radiant zone (750 to 950°C) was placed immediately after the convection zone. To more accurately simulate a typical ethylene furnace downstream cooling process, the effluent gases were passed through several low temperature (0 to -78°C) maintained quench zones. containers, a caustic washer and a dryer containing 3Å molecular sieves.

The phosphine formation values given in Table 4 below are relative values (the value for the additive F = 100) and they were determined by the colorimetric value read from a gas detector placed downstream of all condensers (coolers). A low value indicates low PH3 formation, while higher values indicate higher amounts of PH3. As a further confirmation that PH3 was formed by the phosphorus-based chemicals, the cracked gas effluent was passed through deuterated chloroform at low temperatures (-78ºC) and analyzed by 31PNMR at -60ºC. The spectrum obtained was consistent with the spectrum of PH3 from the literature (-234 ppm, quartet with JPH192 Hz).

Table 4

Additive PH₃ formation rate

A(TPyPO) 4.7

F100

G > 250

H20

From the above data it can be seen that the tripiperidinophosphine oxide was as effective as the prior art phosphorus-based additives in suppressing coke formation, but was essentially free of contributing to corrosion and producing phosphine. It can also be seen that the other phosphorus-based additives evaluated either contributed to corrosion or produced phosphine under coking conditions.

The foregoing description of the invention is for the purpose of illustration and explanation only and is not to be taken as limiting in any way. Various changes in materials, apparatus, steps, procedures and specific parts and components will be readily apparent to those skilled in the art. All such changes are within the scope of the invention as defined by the following claims.

Claims (20)

1. A method for inhibiting coke formation on heat transfer surfaces used to heat or cool a petroleum feedstock under coke-forming conditions, comprising contacting the heat transfer/heat transfer surfaces with an effective amount of a phosphoric triamide of the formula where: X is a chalcogen and R¹ to R³ each represent an amino radical of the formula: wherein R⁴ is hydrocarbyl and R⁵ is hydrogen or hydrocarbyl and R⁴ and R⁵ together can form a heterocyclic radical. 2. Process according to claim 1, wherein X in the formula (I) is oxygen or sulfur. 3. A process according to claim 1 or 2, wherein the phosphoric triamide comprises 3 to 48 carbon atoms. 4. The process of claim 3, wherein the hydrocarbyls are free of heteroatoms. 5. The process of claim 4 wherein each of R⁴ and R⁵ independently represents alkyl, aryl, alkylaryl or arylalkyl having 1 to 15 carbon atoms. 6. A process according to any one of claims 1 to 3, wherein the amino radicals comprise heterocyclic radicals having 4 to 12 carbon atoms. 7. The process according to claim 6, wherein the amino radicals are selected from the group pyrrolyl, pyrrolinyl, pyrrolidinyl, dihydropyridinyl, tetrahydropyridinyl, piperidino, azepinyl, indolyl, isoindolinyl, indolinyl and carbazolyl. 8. A process according to any one of claims 1 to 3, wherein the phosphoric triamide comprises tripiperidinophosphine oxide. 9. A process according to any one of claims 1 to 8, wherein the heated or cooled petroleum feedstock is treated with 0.1 to 1000 ppm phosphoric triamide, calculated as elemental phosphorus in the phosphoric triamide, based on the weight of the feedstock. 10. The process of claim 9 wherein the heated or cooled petroleum feedstock is treated with 1 to 100 ppm phosphoric triamide, calculated as elemental phosphorus in the phosphoric triamide, based on the weight of the feedstock. 11. A process according to any one of claims 1 to 10, wherein the petroleum feedstock comprises ethane, propane, butane, naphtha, kerosene, gas oil or a combination thereof. 12. A process according to any one of claims 1 to 11, wherein the heat transfer surfaces comprise cracking furnace coils. 13. A method according to any one of claims 1 to 12, wherein the heat transfer surfaces comprise transfer line exchangers. 14. A process according to any one of claims 1 to 13, wherein the heat transfer/heat transition surfaces are pretreated with the phosphoric triamide prior to heating or cooling the petroleum feedstock. 15. A process according to any one of claims 1 to 14, which comprises adding the phosphoric triamide to a petroleum feedstock and passing the resulting mixture through convection and radiation sections of a cracking furnace. 16. The process of claim 15, further comprising fractionating the furnace effluent and catalytically treating a fraction thereof. 17. A process according to any one of claims 1 to 14, which comprises adding the phosphoric triamide to a petroleum feedstock or to an ethylene furnace effluent upstream of a transfer line exchanger and passing the cracking furnace effluent containing the phosphoric triamide through the transfer line exchanger. 18. The process of claim 17, further comprising fractionating the furnace effluent and catalytically treating a fraction thereof. 19. A process according to any one of claims 1 to 14, which comprises adding phosphoric triamide to steam; mixing the steam with a petroleum feedstock; and passing the mixture from the starting material and the steam containing the phosphoric triamide through a cracking furnace. 20. A process according to any one of claims 1 to 14, which comprises adding the phosphoric triamide to a mixture of steam and petroleum feedstock; and passing the resulting mixture through a cracking furnace.