During processing of hydrocarbon feeds, including petroleum hydrocarbons, petroleum feedstocks, petroleum processing intermediates, petrochemicals or petrochemical intermediates, including gas, oils and reformer stocks, chlorinated hydrocarbons, olefin plant fluids and deethanizer bottoms, the hydrocarbon feeds are customarily heated to temperatures of 40.degree. to 550.degree. C. The foregoing petroleum hydrocarbons or feedstocks are also used as heating media on the "hot side" of heating and heat exchange systems. Whenever petroleum hydrocarbon materials are subjected to high temperatures, a separate phase, known as fouling deposits, is produced from the petroleum hydrocarbon materials.
Fouling deposits are always considered to be highly undesirable by-products because:
(a) fouling deposits can reduce the diameter of pipes and conduits or vessels and cause a decrease in feed throughput, loss of capacity and decrease in process yields;
(b) fouling deposits impair thermal transfer, for example, in heat exchanger systems and furnaces, by forming an insulating layer on the heating surface, which layer both restricts heat transfer and results in the need for frequent shut-downs for cleaning; and
(c) fouling deposits can damage equipment, necessitating replacement thereof.
Although the nature of the fouling deposits is not completely understood, the deposits appear to contain coke-like carbonaceous deposits, polymers or condensates, solids and salt formations. The salt formations are primarily magnesium, calcium and sodium salts. The formation of carbon condensates is believed to be catalyzed by heavy metal impurities in the hydrocarbon feedstock, for example, by compounds of copper or iron. The heavy metal contaminants may also deleteriously affect hydrocarbon processing by increasing the rate of degenerative chain branching to form free radicals. The free radicals can initiate oxidation and polymerization reactions, which produce gums and sediments. It is also thought that the thus-produced gums and sediments entrain relatively more inert carbonaceous deposits to produce thicker fouling deposits and higher insulating effects than otherwise.
Fouling is also a problem in processes for making or purifying petrochemicals. For example, deposits which are primarily polymeric, are produced during processing of monomers such as ethylene or propylene or purification of feeds such as chlorinated hydrocarbons.
The formation of fouling deposits can be reduced by adding antifouling additives to the hydrocarbon feed prior to or during processing. Typical antifouling additives are those disclosed by Forester, U.S. Pat. No. 4,578,178, herein incorporated by reference. This reference describes a commonly-used test for testing fouling characteristics of feeds. The test comprises pumping process fluid from a pressure vessel through a heat exchanger, containing an electrically-heated rod. The process fluid is then cooled to room temperature in a water-cooled condenser and remixed with the fluid in the pressure vessel. The system is maintained under nitrogen pressure in order to minimize vaporization of the process fluid.
The temperature of the rod is controlled at a preselected temperature. However, as fouling occurs, less heat is transferred to the process fluid and the temperature of the fluid leaving the heat exchanger decreases. The decrease in effluent temperature therefore correlates empirically with the fouling tendency or fouling potential of the hydrocarbon feed.
In an alternative testing procedure, the temperature of the effluent process fluid from the pressure vessel is maintained constant by increasing the power to the rod in response to variations in the temperature of the effluent. The degree of fouling is proportional to the increase in rod temperature, compared to that of a control rod. Both methods of evaluating fouling are variations of the fouling rig test. Although results of the fouling rig tests provide a high degree of correlation with fouling tendencies of the material being evaluated, the test methods are neither rapid nor simple to operate.
Whitehurst, in U.S. Pat. No. 3856,664, has proposed determining the extent of removal of heavy metals from liquids, such as gasoline, by measuring the transmittance or absorbance at 425 nm.
Kitchen, III, et al. (U.S. Pat. No. 4,556,326) have recited testing fuel, subjected to accelerated ageing, by measuring transmittance at 527 nm.
Sien, in U.S. Pat. No. 4,388,408, has recited evaluating the suitability of coker feedstock by a process in which absorption chromatography and ultraviolet light absorptivity are employed.
Demers has proposed (U.S. Pat. No. 4,115,063) determining the total metal content of an organic solvent by a process including an extraction step and flame photometry or atomic absorption spectroscopy.
Saraceno, in U.S. Pat. No. 3,087,888, has proposed determining the vanadium content of an oil sample, using electron paramagnetic spectroscopy.
Doyle (U.S. Pat. No. 3,438,735) has proposed a colorimetric method for determining removal of metals from oily materials.
It will be apparent that existing methods for determining the fouling tendencies of petroleum oils are limited to the cumbersome, slow fouling rig tests or are directed to a specific impurity in the hydrocarbon material, rather than to cumulative fouling tendency of the sample.
It is the object of this invention to provide a simple method for determining the fouling tendency of hydrocarbon feeds, such as petroleum oils, as well as to use the information generated to control fouling tendency in refineries and other process contexts, by providing guidance as to the amount of antifouling agent, needed to produce acceptable operating conditions.