SAMPLING
The operator has two objectives as he makes petroleum products. First, he must make products which will perform satisfactorily for the customer. Second, he must control the refining processes so as to make products in the most economical manner. The operator cannot determine the quality of the products by seeing, tasting, feeling or smelling them. Therefore, he sends a small amount of each product to the laboratory for examination by various physical or chemical tests.
Likewise, knowledge of the properties of certain important process streams enables efficient and safe operation of the unit. If the results of these tests are to be useful, the small part tested must be identical to the stream that it represents. No sample result is any better than the care and correctness with which the sample is taken. In fact, samples which are not truly representative
can cause bad customer relations and mislead operators with serious consequences.
Petroleum products and certain process streams are required to meet certain specifications. A specification is a set of measurements or rules which define and describe a particular substance.
Types of Samples
Two types of samples are used in controlling the operation of refinery units, grab samples and composite samples. The grab sample is also sometimes called a spot sample.
In taking a grab sample, the entire sample is obtained at one time. The sample represents the composition of the total stream at that time. The operator uses the results of grab sample tests to make routine adjustments in operating conditions, if required. In order to follow the continuous operation of the unit, grab samples are sent to the laboratory at certain specified intervals.
To take a composite sample, equal portions of a stream are put in a large sample container at regular intervals over a period of time. For example, a pint of oil might be put into a gallon can once every four hours over a period of twenty-four hours. This would be called a twenty-four hour composite sample and would be representative of the composition of the stream for that period of time. Composite samples are used chiefly by the technical staff to :
(1) Determine the losses of valuable products into a less valuable stream.
(2) To evaluate the effects of changes in operating conditions.
(3) To make weight balances around the unit.
Tank Samples
Tank samples are taken several ways, but the most common basic procedure is to let a weighted sample bottle down into the tank at the end of a heavy cord. Sometimes, another string is attached to the cork in the bottle and the cork is pulled at the desired oil height in the tank.
Volatile liquids that are considerably above atmospheric temperature must sometimes be taken through sample coolers. These are commonly portable 'buckets' with tubing coiled in them and
sorrounded with water or ice. Distillate samples with flash points of 110°F to 200°F are frequently required from process streams at high temperature (200°F to SOOT). These must be taken through sample coolers which are usually located permanently at regular sample locations.
Heavy oils, even though they may have a high flash, must be taken through coolers to prevent injury to the personnel taking the samples. Rain or other extraneous water such as a wet sample bottle must not be allowed to enter heavy samples taken at temperatures above the boiling point of water (212°F). Other special samples such as catalyst, and certain chemicals will require special instructions which must be followed exactly.
Sampling Hazards
Sampling hazards can be of two basic types: (I) those which give bad samples and erroneous results and (2) those which are unsafe. Sometimes it is felt that the work of catching samples is a routine duty and something of a nuisance. Great care and extreme caution must be exercised when taking or handling samples. Every unit will have special instructions on how to take samples accurately and safely and these should be followed to the letter. However, no set of instructions can take the place of good judgment and common sense.
Remember that samples improperly taken can cause poor customer relations and cause unnecessary and expensive changes in operations. A poor sample may be caused by any of the following reasons:
1. Sample is taken from a "dead" line which contains the product but has no flow through it.
2. Sample is taken during a period of upset on the unit. During this time, operating conditions and product composition are changing.
3. Sample is taken without bleeding off or purging the sample cooler or connection. The sample in this case will be the same as the last sample rather than what is currently being processed.
4. Sample of a volatile sample is taken too hot. The lighter components will vapourise and leave a false heavy sample.
5. The sample container is not tightly sealed, allowing vaporization of the lighter components.
6. The sample is taken in a dirty bottle, or stoppered with a dirty cork or a rubber stopper.
7. The sample may be contaminated with heavy oil or grease from the operators hands.
8. Composite samples must be taken uniformly. If most of the sample is taken at one time, it is not representative.
9. Sample is improperly labelled by name, time, etc.
10. A light sensitive sample is taken in a clear bottle instead of a dark bottle.
Some of the safety precautions which must be followed when taking samples includes the following:
1. Use safety equipments as prescribed in the specific
instructions. This includes gloves, face shields, goggles,
oxygen masks, etc.
2. Check surrounding area for a source of ignition. Heavy
hydrocarbon vapours from the sample will travel along
the ground.
3. Allow a vapour space in the sample container for expansion
in the case of both bottles and bombs.
4. Keep complete control of the flow of sample.
5. Be careful and complete in tagging sample containers.
Wrong labels can cause accidents in the testing equipment
in the laboratory.
6. Check the sample containers for cleanliness or foreign
material. Water in the bottom or on the walls of a
container receiving hot oil can flash to steam.
7. Check sample coolers for water flow and freedom from
mechanical failure or plugging before using.
8. If necessary, to climb ladders or mount platforms for
samples, use both hands. If necessary, special sampling
equipment should be provided to allow both hands for
climbing.
9. Attach grounding devices for static electricity dissipation
where provided.
Using Sample Results
Test results are frequently received which are not within the .allowable limits of the specifications. The integrity of the laboratory is not to be questioned, but if the result seems unreasonable, additional available operating information should be checked before changing the unit operation. The single sample may truly indicate something is wrong, but it is possible that the sample was taken during upset conditions, was improperly labelled, or improperly tested or reported.
If the sample results indicate that corrective action should be taken, it is the duty of the operators to do so. After corrective action has been taken and the unit has lined out, it is equally important to resample and test the streams in question to see that the change in operations was proper and effective.
Testing
Many physical and chemical tests are run on hydrocarbons and the other materials used in processing them. A brief description of some of the more common and important tests should help the operator to understand better the importance of controlling the operating variables on the process units.
Gravity
The specific gravity of oil is defined as the ratio of the weights of equal volume of oil to equal volume of water at a standard temperature. The standard temperature used by oil refineries in India is 15°C. The gravity is measured by means of a hydrometer and the observed gravity reading is converted to 15°C by using standard tables. Hydrometer is an instrument which measures the volume of a liquid which corresponds to its weight. The basic principle in gravity determination by hydrometer is that a floating object in a liquid displaces a volume of liquid equal in weight to that of the floating mass. The hydrometer is graduated to read the gravity. The gravity of petroleum products is also expressed in degrees API. The standard temperature used in this case is 60°F. The relation between API gravity and specific gravity is as follows :-
141.5
Degrees API == ________________ - 131.5
Sp. gr. 60°/60°F
Gravity is a useful control test, but is not an index of product quality except in certain instances when considered in connection with other tests. The gravity must be known in order to correct
measured volumes to 60°F. It is also used extensively in blending operations.
Colour
Petroleum products may be divided, according to their colour, into two general classes - First, so called white products with range from practically colourless to "pale-straw" and second, pale-straw and darker. In general, however, the colour of most petroleum products is of little importance in connection with quality, except as an indication of contamination.
The so-called "white products" colours are measured by Saybolt chromometer. The colour is determined in this method by measuring depth of oil necessary to match a standard colour glass.
The results are reported as "Saybolt Colour". The colour scale ranges from—16 to + 30. The larger the number, the lighter the colour is. The ASTM colourimeter is used to determine the colour of
the "pale-straw" and darker products. Black oils can be evaluated by a dilution method which provides for diluting in the ratio of 15% dark ^oil to 85% of+21 Saybolt of lighter material (kerosene). Colour is measured by comparing a fixed thickness of oil with various glass discs. The results are reported as "ASTM Colour". The scale ranges from I to 8. The larger the number, the darker the oil is.
Doctor Test
The doctor test is essentially a very sensitive chemical test for hydrogen sulphide and / or mercaptans. It is made by shaking the product to be tested with a solution of sodium plumbite made by dissolving litharge in caustic soda. After shaking vigorously, a pinch of sulphur is added and the mixture is shaken again. If the sample is discoloured or the yellow colour of the sulphur film becomes brown or black the sample is called "sour", but if it remains unchanged in colour or only very slightly discoloured or flecked with black, the sample is reported sweet.
An immediate black precipitate before the sulphur addition indicates hydrogen sulphide. In the absence of hydrogen sulphide, the change is frequently gradual going through orange, red and brown to black. The test is used to indicate the presence of mercaptans which, in higher concentration, produce a characteristic objectionable odour.
pH
Water which is collected in reflux drums can become very corrosive if it is acidic or basic (alkaline). Samples of the circulating water are sent to the laboratory periodically to determine if the water
is either basic or acidic. The laboratory reports small concentrations of acids or bases in a solution as the pH of the solution. The pH scale ranges from I to 13 and actually starts in the middle, i.e., the pH of a neutral solution, distilled water for example is seven. As the solution becomes more acidic, the pH number decreases down to one, which represents a fairly strong acid solution. The scale goes upwards from seven towards thirteen, and the solution becomes more basic.
Distillation
The distillation test is one of the tests by which volatility is determined. It involves the general procedure of vapourizing the liquid under test and recording a set of figures indicating the
relation between vapour temperatures in the distilling vessel and quantities of distillate that have been liquefied by the condenser. The distillation results are very important for control purposes. Practically all distillation towers are operated to obtain streams with a specified distillation. Distillation results are an indication of certain performance characteristics of gasolines. Various boiling points in the distillation test of motor gasolines have been correlated with engine performance.
In general, ease of starting and a satisfactory warm-up period are assured by specifying a maximum temperature for 10% evaporated, or a minimum percentage evaporated at some moderately low temperature such as 158°F. To lesser extent, warm-up period is influenced by the
50% and 90% points. Excessive heavy ends may result in uneven distribution of the fuel to the various cylinders and may result in crankcase dilution. They are usually limited by specifying a maximum 90% evaporated point or maximum end point.
Flash
Flash point may be defined as the temperature to which an oil must be heated in order to give off sufficient vapour to form an inflammable mixture with air. Flash points are principally employed to
indicate fire hazards of petroleum products. In connection with light oils and fuels, flash points are specified (minimum and maximum) in order to ensure proper uniformity of product and ease of ignition in burners.
The measured flash point of a substance varies with the apparatus and procedure. Although a large number of so called "flash-testers" have been developed, only the closed cup testers will be discussed. These testers have two elements in common - One is a cup which is equipped with a stirrer for holding the sample. Another is a water bath or some other means for slowly raising the temperature of the sample.
Tog Closed Cup Tester
This instrument is designed and operated to yield the greatest possible degree of accuracy. It consists of a water bath and a cup without a stirrer. It is essential that the rate of heat input be carefully controlled. This is accomplished by using water as a medium for transmitting heat to the oil under test.
Penske-Martin Closed Cup Tester
This instrument is used almost exclusively, the world over for determining flash points of fuel oil and gas oil. Fuel oils frequently have high viscosities and stirring of the oil during the test is essential to ensure maximum accuracy. The test procedure is similar to the closed cup method. However, a water bath cannot be used because of the high temperature range of this instrument.
Reid Vapour Pressure
The vapour pressure of a substance is a measure of the tendency of the substance to vapourize. Therefore it is an indication of "light ends" in a stock. Vapour pressure is generally expressed
in pounds per square inch absolute. Vapour pressure of petrol has been correlated with vapour lock tendencies in automobiles and is more useful in this connection than the initial boiling point or other low distillation points. Vapour pressure is also used in connection with transportation safety regulations. The test is usually applied to petrol or to products containing naphtha such as jet fuels.
In the Reid method for measuring vapour pressure, the sample is placed in a bomb, connected to a sensitive pressure gauge and heated in a water bath to 100°F. The vapour pressure is determined in the presence of a volume of air four times the volume of gasoline. This standardizes the extent of evaporation, lessens the effect of dissolved gases, and permits direct reading of the vapour pressure in absolute units. Corrections to the pressure gauge readings are made for air expansion and change in vapour pressure of water intentionally added to ensure constant humidity for all tests.
Sulphur
Small amounts of sulphur in petroleum products can cause severe corrosion. Sulphur in gasoline can damage an automobile engine, especially in the winter time. Some of the products of combustion get into the crankcase and remain in the cylinders when the engine is stopped, causing serious corrosion of parts, such as bearing, starter chain, cylinder walls etc. Sulphur in products used for heating can cause corrosion of burners, boilers, stacks and auxiliary equipment.
For the purpose of this test, the sulphur content is considered as the total amount of sulphur present in the petroleum product regardless of how ic is chemically combined with other elements.
Petroleum products are analyzed for sulphur in the laboratory by the Lamp Method. The sulphur is oxidized by burning a sample of the material in a small lamp. The combustion products are passed
through a solution of hydrogen peroxide. The solution is analyzed with a sodium hydroxide solution and the sulphur determind by calculation.
Octane Number
The octane number, or rating, is a number used to measure the anti-knock properties of gasoline. It is the percentage by volume of ' iso-octane in a mixture of iso-octane and normal heptane that matches the gasoline being tested in knocking characteritics. The higher the octane number, the lower is the knocking tendency. Since pure iso-octane and normal heptane are very expensive, in many laboratories the gasoline being tested is compared with a reference fuel which has been
calibrated with the pure standards (iso-octane and n-heptane). There are two sets of operating conditions, or methods; the motor method and the research method. The motor method in general represents the fuel's behaviour in high speed performance. The results of neither method are completely satisfactory in predicting the overall road performance of a fuel, although the research method is considered better in this respect and it is the research octane number which is
generally quoted as the "octane number". Fuels of different compositions may have the same octane number yet respond quite differently to such engine variables as speed, engine design, spark timing, jacket and manifold temperatures, and air to fuel ratio. Road ratings, therefore, vary with different makes of cars and also between different cars of the same make.
B.S & W
The specifications for many of the finished products from refineries require them to be free from water and sediment. It is also important that there is no water or sediment in the charge to the
units. Sediment can cause exchanger and furnace fouling. Inclusion of water in feed can result in violent explosions when the water is converted to steam and expands to about 1600 times its original volume.
The water and sediment is determined in the laboratory as B.S & W which stands for Bottom Sediment and Water. In testing oil for B.S & W, it is first diluted with benzene into a 50-50 mixture.. It is then spun in a centrifuge for a specified length of time. The container for the oil is graduated so that the amount of B.S. & W may be read directly from the markings.
Carbon Residue
In certain types of oil burning equipment and in internal combustion engines where both the fuel and lubricant may deposit coke, it is important to know the relative coke forming tendencies of various fuels and oils. This tendency is indicated by the carbon residue test.
This test also indicates the yield of carbon which might be expected in cracking the oil. The carbon residue is reported in terms of weight per cent. This test can be used on fuel oil, gas oil and lubricating oil. To obtain a carbon residue on hydrocarbon samples, it is necessary to evaporate the oil in an apparatus so designed to exclude air, to provide for proper control of rate of heating, and to eliminate possible condensation of distillate in or on the oil contained. The unevaporated
material remaining is weighed and reported as carbon residue. Two methods are commonly used, Conradson and Ramsbottom.
Viscosity
Viscosity of an oil is a measure of its resistance to flow. Because viscosity changes with temperature, a numerical value of viscosity has no meaning unless the temperature is specified. In commercial work, viscosity is Usually expressed in seconds of time for a given volume of oil to flow through a certain orifice. Since viscosity is a measure of oil's resistance to flow, it is important in all movement of oil, whether it be through a wick in a kerosene lamp, or the pumping of oil through a circulating system or a pipeline. The ease with which a fuel oil may be atomized in a burner is also a function of viscosity.
Many instruments have been developed for measuring viscosity, but only one will be discussed in this text-Saybolt viscosimeter. The Saybolt method has two variations, Universal and Furol, differing in the size of the outflow orifice. The Universal orifice is used for gas oils and lubricating oil, the standard temperatures being 100, 130 and 210°F. The Furol orifice (Furol is a contraction of the phrase "fuel and road oils") is usually used for heavy oils and road fuels. The temperature
used for fuels is usually 122°F, though 77, 100 or 210°F are also used.
Cloud and Pour Points
As a sample of oil is cooled, the cloud point is the temperature at which paraffin wax or other solid substances begin to crystalize out. The Pour point of a petroleum is the lowest temperature at which the oil will pour or flow when it is chilled without disturbance under definite prescribed conditions. The cloud point is of some value when a haze or cloud in the oil would be objectionable for any reason. The pour point gives an indication of the temperature at which it may not be possible to pour or remove an oil from its container, or below which it might be dangerous to use the oil in gravity flow lubricating system. It is also a measure of the lower temperature limit for pumping fuel oil.
Corrosion -
Petroleum products are frequently used in contact with metal. When it is essential that the metal shall not be corroded, it is customary to require that the petroleum products pass an appropriate
corrosion test. The corrosive substance most likely to be present in such products as gasoline and kerosene is free sulphur which attacks copper readily. It is, therefore, customary to require that products shall pass a test involving contact with polished copper for a specified period of time, at a specified temperature. The Copper Strip Corrosion test involves determinations of the discolouration produced when a strip of sheet copper is immersed in the sample for half an hour at 212°F. It is applicable to kerosene and other transparent products. The exposed strip is compared to a series of 12 standard strips and the sample assigned a corrosion number of la to 4c depending upon the degree of discolouration.
Gum
Cracked gasoline, upon exposure to oxygen of the air as in ordinary storage, may undergo oxidation with formation of undesirable components. Upon evaporation of the gasoline, a resinous material is
formed which is called " gum ". This gum may cause intake valves to stick or it can interfere with carburettor operation. It is common practice to refer to the yield of residue obtained by an evaporation test as the gum content of the gasoline.
In the gum existent method, a sample of gasoline is evaporated rapidly at an elevated temperature (320-330°F.) in a current of heated air. The results are expressed as milligrams of gum per 100 cc of sample. Note: Existent gum is defined as the amount of the non-volatile residue present in the gasoline as received for the test.
The results are an index of the tendency of the gasoline to deposit gum in an engine before storage. In general however, only a fraction of the gum content of the gasoline as analyzed appears as deposits.
Orsat
The theory of gas analysis is simple. It consists of taking a measured volume of gas sample removing components one by one by suitable reactions and measuring the decrease in volume caused by each removal. Oxygen, carbon monoxide, and carbon dioxide are commonly determined by this method.
Aniline Point
The Aniline Point is an index of the paraffinicity of petroleum oils, and is to some extent an indication of the solvent power of .naphtha and solvents derived from petroleum. The procedure is to determine the temperature at which a solution of equal volumes of aniline and oil becomes cloudy upon cooling. The aniline point increases with an increase in the molecular weight of the hydrocarbons in a sample so it is necessary to know the approximate composition of the sample. This is accomplished by obtaining certain boiling range fractions. Then an equal volume of sample and aniline is stirred and cooled until a cloudy mixture is obtained, the temperature is observed and from this, the amount of aromatics may be determined from a correlation.
Bromine Number
The bromine number is a measure of the amount of olefins (unsaturates) in a hydrocarbon sample. As the amount of olefin increases the accuracy of the test, however, decreases somewhat.
Bromine will react with olefins in such a manner that one molecule of bromine will add to each double bond present. A known quantity in excess of the amount needed is added to the sample. The quantity of bromine which did not react with the olefins is determined by analysis. The amount of bromine which reacted with the olefins is determined by difference. This quantity of bromine which reacted is expressed as milligram of bromine per gram of sample. This is the bromine number.