Automatic Determination of the Density Application Note€¦ · Automatic Determination of the...

The density and derived quantities such as the specific gravity and API gravity are used worldwide to characterize liquid petroleum products and to compare their quality. Many crude oils, heavy oils, lubricating oils and paraffins are highly viscous or solid at room temperature. Their density is often still determined manually using hydrometers. The METTLER TOLEDO optional heating devices in combination with oscillating U-tube digital density meters allow density measurements to be automated and per-form ed much more rapidly and reproducibly.

Automatic Determination of the Density and API Gravity of Oils and Paraffins

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Probably more density determinations are performed in the petroleum oil industry than in any other industry. In the early days of the oil industry, most determinations were performed with hydrometers (also called aerom-eters) using the Baumé scale. Later investigations showed that a large majority of the hydrometers used in the petrochemical industry gave incor-rect results due to a manufacturing error. By then, however, hydrometers with the incorrect Baumé scale were in such widespread use that in 1921 the American Petroleum Institute (API) decided to adopt the incorrect scale as an official standard and to call it the API Gravity scale. The faulty hydrometers could thus still be used and new hydrometers measured API gravity directly.

About 60 years later, the first ASTM standards for density determinations were introduced using digital density meters (oscillating U-tube method). API gravity is still used in the USA to characterize the specific gravity of petrochemical products. Nowadays, most measurements are performed with digital density meters because the instruments are easy to use and provide more accurate results. API gravity is referred to as being in “degrees” although mathematically it has no units

G = specific gravity (60 °F / 60 °F)ρ = density(valid for liquids with a density < 1.000 g/cm3)

In many petrochemical laboratories, besides digital density meters, hydro-meters are still used to determine den-sity. The reason for this is nearly always the same: many samples are highly vis-cous or even solid at room temperature.

Such samples are usually first heated to liquefy them and then measured while they are hot. The measured density values are converted to a reference temperature of 15 °C or 60 °F using official API temperature com-pensation tables. Hot samples can usually only be injected into the mea-suring cell of a digital density meter (oscillating U-tube method) using pro-tective gloves. As a rule, the sample is heated together with a glass syringe and the hot syringe is filled with the sample. The measuring cell is then filled using the syringe. One must be careful not to inject too much sample because it will otherwise solidify at the cell outlet. Samples like crude oil and heavy oil are also very dark. This makes it difficult to see air bubbles in the measuring cell so that incorrect

Manually with hydrometersmeasurement results can hardly be avoided. Furthermore, larger volumes of solvents are usually needed to clean the cell after the measurement.

For these reasons, it often seems easier to heat a larger amount of sample in an oil bath and to determine the density as previously using a hydrometer.

However, density determinations of dark and viscous samples using hydrometers often yield results that are not very reproducible. The reasons for this are:

• Accuratethermostatingofthe samples is not possible.

• Thevaporizationofvolatile components cannot be avoided (open vessel).

• Itisdifficulttoreadtheresultbecause the hydrometer scale is often smeared with the dark sample.

• Theresults(densityatthereferencetemperature) must be determined from tables using linear interpola-tion (possible calculation error).

° Baumé = 140G

−130 ° API = 141.5G

−131.5 G=ρProbe (60 °F )

ρWasser (60 °F )

° Baumé = 140G

−130 ° API = 141.5G

−131.5 G=ρProbe (60 °F )

ρWasser (60 °F )

° Baumé = 140G

−130 ° API = 141.5G

−131.5 G=ρProbe (60 °F )

ρWasser (60 °F )

Sample (60° F)

Water (60° F)

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Figure 1: DM40 density meter with SC1 automation unit and optional heating device.

Automatically with oscillating U-tube digital density metersWith the optional heating devices for the SC1 and SC30 automation units, METTLER TOLEDO can now offer ideal systems for petrochemical laboratories that have to perform accurate density determinations of highly viscous sam-ples or even samples that are solid at room temperature.

These solutions allow determinations to be automatically performed with the DE density meters (oscillating U-tube method). The main advantages are:

• Thesamplesarefilledhotintovials.Hot syringes do not have to be handled.

• Thesamplesarealwaysinaheatedenvironment during the measure-ment. The measurement cell is filled automatically. Solidification of the samples at the cell outlet is not possible.

• Gasbubblesinthemeasurementcell are detected by an automatic error detection system. This elimi-nates the possibility of incorrect measurements.

• Themeasurementcelliscleanedautomatically with solvents at the end of the measurement. The rins-ing process is very efficient due to the powerful pulsed flow and

requires only small amounts of solvents. A further important advan-tage of automatic cleaning is that the user is not exposed to toxic sol-vents (toluene) or vapors.

• Themeasuredvaluesareauto-matically converted to the desired reference temperatures by means of the built-in API tables (according to ASTM D 1250). For example, it is possible to perform the measure-ments at 50 °C and to simultane-ously present the results of the determination as densities at 15 °C and as API gravity referred to a reference temperature of 60 °F.

• WiththeSC30samplechanger,even measurements of sample series can be completely automat-ed. The sample vials are placed on a heated turn-table. They are tightly sealed with screw caps and are not opened until just before the sample is measured. This eliminates the possibility of any changes occurring in the sample due to the vaporiza-tion of volatile components.

• Thesystemiselectricallyheated;anexternal thermostat is not required. The temperature of the optional heating device can be accurately controlled and adapted to the com-position of the measured samples.

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Density determinations in the petro-chemical industry are usually per-formed according to the following four standards:

1. ASTM D 4052-96

(Standard Test Method for Density and Relative Density of Liquids by Digital Density Meter). This standard method describes how to determine the density of petroleum oil distillates and viscous oils that are liquid at measurement temperatures of 15 to 35 °C and that have a vapor pressure of less than 80 kPa and a viscosity of less than 15,000 mm2/s (cSt). As a rule, no heating device is needed for density determinations according to this standard. If, however, series of viscous samples (e.g. viscous lubricating oils) often have to be mea-sured, the sample changer should be equipped with a heating device. The reason for this is that it is usually dif-ficult to ensure that viscous samples do not contain air bubbles after filling them into sample vials. Heating the samples before they are measured allows any trapped air bubbles to escape. If the measurements are not performed directly at 15 °C or 60 °F, the measured density values are usu-ally converted to one of these refer-ence temperatures.

2. ASTM D 5002-99

(Standard Test Method for Density and Relative Density of Crude Oils by Digi-tal Density Analyzer). This standard method describes how to determine the density of crude oils that are liquid in the temperature range 15 to 35 °C. It is also valid for crude oils that exhib-it a high vapor pressure. No heating device is needed for the measurement of relatively low-viscosity crude oils. A heating device must however be used for viscous crude oils or crude oils that contain solid paraffin and that can be liquefied by heating to a temperature of 35 °C. It is important

to note that crude oils usually contain volatile components, mainly pentane (boiling point 36 °C). This means that the samples must not be measured at temperatures above 35 °C because otherwise bubbles form in the mea-surement cell making it impossible to perform accurate density determina-tions. One must also make sure that samples are tightly sealed until just before they are measured.

3. DIN 51757 (1994-04)

(Testing of mineral oils and related materials;determinationofdensity).This standard method describes how to determine the density of crude oil, petroleum oil products and similar materials. The viscosity of the sam-ples at the measurement temperature must not be greater than 15,000 mm2/s and the vapor pressure not more than 80 kPa. According to this method, measurements can be per-formed at temperatures in the range 0 to 90 °C, that is, products that are highly viscous or even solid at ambi-ent temperature can be liquefied by heating and measured while they are hot. DIN 51757 describes the conver-sion of the measured density values to other reference temperatures. For petrochemical products, DIN 51757 refers to the „Petroleum Measure-ment Tables“ 53 A, 53B and 53 D according to ASTM D 1250 (and API D 2540), see below. Density deter-minations of highly viscous samples or samples that are solid at ambient temperature according to DIN 51757 can usually be automated without difficulty using the heating devices. It is very important to ensure that the samples are not excessively heated because otherwise the maximum allowable vapor pressure will be exceeded and volatile components could escape (gas bubbles in the measurement cell).

Standards

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4. DIN ISO EN 12185

(1996, Methods of test for petroleum and its products. Crude petroleum and petroleum products. Determination of density. Oscillating U-tube method). This standard is applicable to liquids with any vapor pressure assuming that the loss of volatile components can be excluded prior to measure-ment. The samples must exhibit only a single phase at the measurement temperature. According to the stan-dard, waxy crude oils and distillates must be heated and measured at a temperature of 3 °C above the cloud point or 20 °C above their solidifying point. If the measured density values are to be determined for a reference temperature of 15 or 20 °C, they must be converted according to ISO 91-1 and ISO 91-2 (or with the „Petroleum Measurement Tables“ 53 A, 53B and 53 D according to ASTM D 1250). The measurement temperature must always be as close as possible to the reference temperature. The heat-ing devices are excellent for density determinations of waxy petrochemical products according to this standard.

As a rule, density values are quoted for a reference temperature of 15 °C, 20 °C or 60 °F. If the samples have to be heated to perform the measure-

ments, the measured density values are converted using the “Petroleum Measurement Tables“ according to ASTM D 1250-04 (IP 200/04). Table 1liststhetableswhichareusedfor conversions depending on the reference temperature, type of mea-sured samples and density unit.

The above tables are all valid for mea-surements with hydrometers, that is, they include a correction for their ther-mal expansion. To use them to con-vert measurement results determined by digital density meters, the hydrom-eter correction must be calculated and subtracted from the tabulated values according to ISO 12185 (Annex A.1).

The METTLER TOLEDO DE density meters and DR combined meters have built-in temperature compensation tables that allow direct conversion of the measured density, the specific gravity or the API gravity to a reference temperature of 15 °C or 60 °F for the three product groups. Tables for the conversion of the measured values of the three product groups to a reference temperature of 20 °C can be entered manually. In the same way, the mea-surement values can be simultane-ously converted to two different refer-ence temperatures.

Unit Reference temperature

Table Product group

Density 15 °C 53 A

53 B

53 D

General crude oils

General petrochemical products

General lubricating oils

Density 20 °C 59 A

59 B

59 D

General crude oils

General petrochemical products

General lubricating oils

Specific gravity

(60 °F/60 °F)

60 °F 23 A

23 B

General crude oils

General petrochemical products

API gravity 60 °F 5 A

5 B

5 D

General crude oils

General petrochemical products

General lubricating oils

Table 1: Conversion tables according to ASTM D 1250-04 (IP 200/04).

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Practical applicationThe following points should be noted when the optional heating devices are used for the determination of the den-sity of samples that cannot be mea-sured at the reference temperature:

• Samplesshouldbeheatedaslittleaspossible;themeasure-ment temperature should be as close as possible to the reference temperature.

• Conversionofthemeasuredvalueat a particular temperature to the reference temperature is always associated with errors. The smaller the temperature compensation that needs to be applied, the smaller the error involved in the conversion.– Samples must not be heated to

above the boiling point of the most volatile component before or during the measurement (bubble formation in the mea-suring cell).

– As a rule, lubricating oils con-tainhardly any volatile compo-nents. It is advisable to heat samples of these products prior to measurement because it allows any gas bubbles con-tained in the samples to escape. If possible, the measurements should however be performed directly at the reference tempera-ture of 15 °C.

• Samplevialsshouldalwaysbetightly sealed until just before the measurement.– Evaporation of volatile compo-

nents causes errors.• Iftheviscosityofthesamplesat

the measurement temperature is more than 100 mPa·s, the density meter’s viscosity correction system must be switched on.– According to the above-men-

tioned standards, the measure-ment results are given to four decimal places (in g/cm3). A viscosity error makes itself noticeable on the fourth decimal place from a sample viscosity of about 100 mPa·s onward. Without viscosity correction, the density value determined for viscous samples is too high, and the API gravity value too low.

– With sample viscosities greater than 2,000 mPa·s, the viscos-ity correction should be set to “High“. This shortens the ∆measurementtime.

• Werecommendthattheautomaticerror detection system is always activated.– Samples can easily degas on

heating. Gas bubbles in the measurement cell cannot usu-ally be seen in dark products

Figure 2: DM40 density meter with SC30 automation unit and optional heating device.

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System configurationsAs a rule, density determinations in the petrochemical industry are performed with four-decimal-place digital density meters (DE40). First, the above-mentioned standards do not specify a higher absolute accu-racy, and second, the errors due to the use of temperature compensa-tiontables areusuallygreaterthan0.0001 g/cm3.METTLER TOLEDOoffers heating devices for the SC1 (for individual samples) and SC30 (sam-ple changer) automation systems. Our customers use different system configurations depending on the type of samples they analyze:

• Inmostrefineries,themajorityofsamples can be measured at room temperature without difficulty. A smaller number of samples (crude oil, etc.) cannot be measured at room temperature.– Since it takes 30 minutes for the

heating device to heat up and cool down, refineries usually operate two measurement sys-tems in parallel.

• DE40densitymeter+SC30 sample changer for the samples that are liquid at room temperature.

• DE40+SC1singlesampleautomation+heatingdeviceforextremely viscous and solid sam-ples (bunker fuel, paraffin oil, waxy crude oils).

• Inpetrochemicalcustomerservicelaboratories many samples are extremely viscous or solid at room temperature (waxy crude oils and heavy oils). Two automated mea-surement systems are therefore very often used, one with and one without the heating device (DE40 +SC30,DE40+SC30+heatingdevice).

• Intheproductionofmineraloils(lubricating oils, transformer oils, etc.) a heated measurement system is frequently used. An SC1 single sample automation unit or a SC30 sample changer is used for auto-mation depending on the number of samples.

Figure 3: The DE40-SC30-heating device in the customer service laboratory.

and cause large measurement errors.

– Viscous samples often contain air bubbles after filling them into the sample vials.

Remarks:(1) The Density meter DE40 has recently been replaced by the model DM40.(2) METTLER TOLEDO also offers the following digital density meters:

– DM45 DeltaRange, four-decimal-place instrument with five-decimal-place range from 0.70000 - 1.00000 g/cm3

– DM50, five-decimal-place instrument

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Mettler-Toledo AGPO Box VI-400, CH-8606 GreifenseeTel.+41-44-9442211Fax.+41-44-9443170

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