Measurement and Testing 37


and 100°C. A higher viscosity index indicates a smaller decrease in viscosity with increasing temperature of the lubricant. There are three ways to measure viscosity in refining processes: with a single viscometer, with interpolated dual viscometers, and with a viscosity analyser.


With a single viscometer, a temperature compensation model is applied. The instrument continuously measures the viscosity at process temperature and a processor calculates the viscosity at reference temperature (with variation law). When the difference between process and reference temperature is reduced to ±20°C, a compensation model is implemented. For known petroleum products and increased differences between process and reference temperature (e.g. 150°C), the ASTM D341 model is used. This implies that the reference product and its behavior must be known. This technique satisfies basic viscosity measurement requirements and presents several advantages like minimal front-end investment, instantaneous and continuous measurement, and extremely good reliability.


With two interpolated viscometers, viscosity measurement exists at two temperatures; one measurement is before the reference temperature, the other measurement is after. According to the end user’s reference temperature and identified parameters, a processor continuously calculates the viscosity according to the ASTM D341 model. Interpolated viscometers provide reliability with continuous viscosity measurement calculations. Like single viscometer use, this solution satisfies viscosity standards requirements.


The analyser method is the best method for controlling petroleum’s viscosity because viscosity is really measured at the reference temperature. With this viscosity principle, a sample is taken from the process and introduced to the analyser. The sample is prepared for measurement, viscosity at reference temperature is memorised, and the sample returned to the process. This procedure is repeated and the sample is continuously renewed. The analyser presents a supreme advantage as the measurement is made at the actual reference temperature, regardless of the product’s behavior. With the analyser, any effect of variable process temperature is eliminated. The correlation to ASTM standard is done directly, and accuracy is induced by the measuring principle as opposed to calculation approximation.


For single and dual viscometer applications, the viscosity will vary with product quality and temperature. For the analyser at reference temperature, a real viscosity measurement is made with constant reference temperature, whether higher or lower than the process temperature.


According to ASTM D445, inline measurement must be repeatable, simple to use and install, and require minimum maintenance in both time and cost. The instruments require ex-proof agreement to easily fit in every place of the refinery. Viscosity remains relevant to refining during mixing, blending, and separation operations. Viscosity is a quality control parameter and can be scrutinised in all phases. Superior quality for petroleum products and all its derivatives is dependent upon viscosity characteristics.


Burning


Many liquid fuels are used in the industry, with diesel and heavy fuel oils being the most common. Liquid fuels are used in boilers, burners, furnaces or engines in order to supply heat or mechanical energy. In each case, the burner introduces a spray into the combustion process. From simple to complex combustion formulas, it is known that acting on the viscosity interferes on the droplet size. By adjusting a spray’s droplet size to suit the application, a process viscometer in a combustion installation optimises energy production and reduces consumption. In addition, it reduces the un-burned residue as well as dirt accumulation in the combustion chamber and will allow avoiding corrosion. With its reliable and repeatable measure, the process viscometer also provides combustion efficiency. Maintenance, cleaning requirements, and atmospheric emissions are reduced.


In order to obtain superior operation in heavy fuel n°2 burners, the fuel spray must present defined characteristics linked to its viscosity. Those characteristics are provided by the burner manufacturer, and are reached while heating the fuel. Efficiency of the burner is optimum when viscosity of the fluid matches the specifications of the burner manufacturer. Installing a viscosity control system ensures the viscosity value reading and constant viscosity control. Controller interacts on the heater command, and determines the heating energy needed to maintain good heavy fuel viscosity.


In the past, temperature controls combined with viscosity and temperature charts were used because it was simple, and were efficient when heavy fuels had constant characteristics. This is no longer acceptable today as the relationship between viscosity and heavy fuel temperature presents higher dispersions due to the diverse origins of raw oils, different refining methods, and variations among additives. Temperature control alone does not guarantee permanent viscosity stability, as there is too much variation between the products and batches.


With petroleum products, viscosity is even more crucial, as it is a dedicated, burned energy source. Viscosity control is indispensable in the burning of heavy fuels in industrial motors, heaters, and marine engines. Viscosity control inside combustion engines is increasingly realised, measuring and improving power ratio. Refineries, power plants, and utility companies use burners, and manufacturers of burners and engines demand optimal viscosity value to improve their performance rates.


Figure 2: Viscosity vs temperature diagram for petroleum products Quality control


In petroleum operations, as in many industrial sectors, on-site control is of primary importance in regards to product delivery. Viscosity is a parameter that allows limiting the difference between the refineries ordered product and the effective delivered product.


Viscosity is also a point of security in distribution tanks. Instruments used for this measure need a very good repeatability and reliability. By checking viscosity, companies validate that tanks are supplied with the correct product. By verifying this step, potential mistakes with customers are avoided.


Conclusion


Petroleum-related operations present diverse applications. Viscosity is a key parameter in each phase, allowing or blocking production. In the petroleum industry, prices and volumes are huge; any viscosity related improvements are significant. This is why the global petroleum-related industry needs to pursue its investments in viscosity measurement, focusing on instruments providing long lasting satisfaction for productivity: robustness, repeatability, maintenance free capabilities, continuous measurement and resistance to high pressure and high temperature harsh environments. Manufacturers keep getting more sophisticated in their instrumentation technology as they develop new features and optimised characteristics to reach an optimum adaptation to industrial needs.


References


ASTM D2270-04 Standard Practice for Calculating Viscosity Index From Kinematic Viscosity at 40 and 100°C ASTM D341 - 09 Standard Practice for Viscosity-Temperature Charts for Liquid Petroleum Products


ASTM D445 - 11a Standard Test Method for Kinematic Viscosity of Transparent and Opaque Liquids (and Calculation of Dynamic Viscosity)


Researchers at UCF Automate Biodiesel Production Using In-line Viscometer


Automating the process of producing biodiesel was a challenge for researchers at the University of Central Florida’s School of Electrical Engineering and Computer Science. Engineers at Cambridge Viscosity (USA) implemented an in-line viscometer to help the researchers reach their goal of an efficient way to produce biodiesel.


To produce biodiesel, oil is mixed with alcohol to produce fatty acid methyl esters. The reaction, known as transeterfication, produces glycerol as a waste by-product. The viscosity of the glycerol is a key parameter, as high viscosity content can lead to potential problems such as clogged fuel filters or fuel pressure drops.


The researchers used a Cambridge Viscosity VISCOpro 1600 viscometer to monitor the viscosity of the glycerol by-product. Using potassium hydroxide as the catalyst for the transeterfication process, the team removed the glycerol by-product that sank to the bottom of the mixing chamber and routed it through pipes leading to the viscometer. If the glycerol’s viscosity reading was more than 6 mPas, the glycerol was directed to a waste container. After washing the biodiesel, the wastewater was also directed to the viscometer, and if viscosity was lower than 3 mPas, was discarded.


University researcher Kevin Brachle said the viscometer was simple to operate and produced accurate results. “The digital measurements we obtained from the VISCOpro were easy to integrate with our microcontroller and allowed us to determine if the biodiesel was on target. By monitoring the viscosity of the glycerol we were able to consistently produce quality biodiesel fuel,” says Brachle.


Reader Reply Card No 86 April / May 2012 • www.petro-online.com


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