1. Field of the Invention
The present invention relates to a chromatographic test method for determining the tendency of liquid hydrocarbon streams to foul equipment and more particularly to a method for determining oil-asphaltenes incompatibility and related fouling tendency. More specifically it is an improvement in a thin layer chromatographic method for determining the fouling tendency of said hydrocarbon streams.
2. Related Art
Different asphaltenes containing petroleum streams have different precipitating and fouling characteristics with regard to heated oil refinery surfaces. The problem of predicting the offending substances in a particular stream such as crude oil which foul heat exchanger equipment in oil refineries and petrochemical plants has been virtually unresolved. Equipment fouling by heated hydrocarbon streams which result in inorganic and carbonaceous deposits on heat exchanger surfaces leads to a blockage of flow and a decrease in heat transfer. Both resulting conditions severely reduce heat efficiency in the processing of the crude oil. If it can be predicted which crude oils or other petroleum streams are troublesome, measures can be taken in advance to prevent this fouling by either removing the offending substances causing the deleterious deposits, or by adding antifouling additives to the flow stream to reduce deposit formation. Therefore, it would be most desirable to be able to predict these streams with fouling tendencies.
There are a number of methods available for determining the rates of fouling of petroleum streams. Conceptually, they are all similar in that they attempt to measure the change in heat transfer from a heated surface to a test fluid.
One approach is to use a test unit which is configured to allow measurement of the fluid temperature at the exit of the heat-exchanger while the metal temperature of the heated tube is controlled. This configuration provides for close simulation of refinery and petrochemical plant heat-exchanger operations and provides for measurement of the significant effect of fouling which is indicated by the reduction of heat transfer. The test unit provides for a thermal fouling evaluation of the crude oil in an accelerated test which is designed to reproduce the fouling problem experienced in a refinery over several months. Acceleration is provided by carrying out test operating temperatures higher than those in a particular refinery unit, so that the prospective level of fouling can be produced in a reasonable period of time (usually 3-4 hours). Heat transfer data is obtained by holding the heater tube at a constant temperature while measuring the change in the liquid outlet temperature. As fouling progresses, i.e., a carbonaceous deposit build up on the heater tube surface, a decrease in the fluid outlet temperature results when using a constant outlet liquid temperature operation. The change in liquid outlet temperature with time provides the basic heat data required for comparative evaluation of untreated material and additive-treated material. The rate of change in outlet liquid temperature versus time shows relative fouling tendencies.
Current test equipment is only capable of measuring the overall tendency of heated petroleum streams to foul refinery equipment and cannot predict which are the offending substances or fractions.
In copending, commonly owned application U.S. Ser. No. 723,598 filed on 4/15/85, a chromatographic method for characterizing crude oil fouling was described which used thin layer chromatography (TLC). In some chromatograms the separation and difference in the asphaltenes and oil color is not visually clear. For those who are not familiar with TLC it maybe difficult to translate the poorly visual separation into fouling. In these cases, the asphaltene is not fully incompatible in the oil leading to the migration of some of the low molecular weight part of the asphaltenes with the oil and thus causing discoloration of the oil part. An improved method was therefore needed to clarify and simplify the interpretation and translation of the TLC chromatographic data into fouling characteristics.
Hence, it is an advantage of the present invention that an improved method which will predict the fouling tendency of asphaltene containing petroleum streams is provided. It is a particular advantage that the present invention (test) can be carried out in the refinery in a very short period of time by unit operators without extensive chemical training. It is a particular feature of the present invention that the visual demarcation line of the asphaltenes, if any is sharp and clear. These and other advantages and features will be apparent from the following text.