By David W. Spitzer, CONTROL Contributor
Flow, level, pressure and temperature instrumentation are commonly applied in the process industries, and also are used in plants that convert biomass to fuel. However, in contrast to the typical process plant, density/concentration and pH measurements are used extensively in processing biomass, and purifying its products. These physical properties are key to increasing yield, reducing processing time, and reducing energy consumption, so operators can control biomass process facilities profitably.
In North America, natural gas and hydroelectric power form the majority of thermal and electrical energy production. This is because of historically low fuel prices. In other locations, taxes and other disincentives to consumption make alternative energy sources more attractive. For example, windmills generate electricity in the Netherlands, geothermal energy provides about 50% of Iceland’s energy, and ethanol made from sugarcane fuels cars in Brazil.
It makes more sense to consider alternative sources of energy as energy prices climb. North America is focusing on generating electricity from solar cells and windmills, and producing alternative fossil fuels such as liquefied and gasified coal, as well as increasing fuel production from tar sands. In addition, throughout the cornbelt, small energy producers are using corn to produce ethanol.
The ethanol production cycle from biomass begins with plants that use atmospheric carbon dioxide to produce organic carbon. These plants are harvested, and converted into a useable feedstock, typically some form of sugar. The most commonly used plants are sugarcane and corn, but processes that use other crops and waste biomass are in development. Sugarcane typically is crushed before its juice is concentrated, and cooked prior to fermentation. Corn often is milled, and separated into its components to produce starch that’s further processed into sugar. After fermenting the sugar into ethanol and other byproducts, the ethanol is typically purified using distillation. Both biomasses use similar sugar fermentation and ethanol purification processes. More variations between processes are expected as the processes are improved.
|FIGURE 1: Tank Density/Concentration Sensor and Transmitter|
Differential pressure probes can determine sugar concentration.
The concentration of sugar can affect many parts of these processes. In general, too little sugar dilutes the process and reduces throughput. Conversely, too much over-saturates the process with sugar, so the it doesn’t function properly. This implies the process will operate best when its sugar concentration is between these two extremes, so sugar concentration should be controlled at its proper value within tight tolerances for the process to maintain peak or near-peak performance. So, sugar concentration is a key control parameter that should be accurately measured and tightly controlled.
Sugar is denser than water, and increasing its concentration in a sugar/water mixture causes its density to increase. The density of the mixture can be calculated when the sugar concentration is known. Conversely, the mixture’s sugar content can be calculated when the density is known. Sugar concentration measurements must be located both in flowing streams and in materials in vessels.
The density of material in vessels can be determined by measuring the differential pressure between two submerged pressure taps located at known elevations. In these installations, differential pressure transmitter diaphragm seals are used in both taps to reduce the possibility of plugging. However, to eliminate these nozzles and their potential for plugging, a differential pressure probe with differential pressure sensors, originally developed for processing sugar, can be inserted vertically into the vessel (See Figure 1). This approach “fixes” the distance between the sensors, and eliminates the requirement to precisely determine and control nozzle locations. Also, the height of the probe is adjustable after installation to help ensure that the sensors remain submerged, which contrasts with fixed-vessel nozzles that are difficult to alter due to errors or process changes.
Measuring Sugar Content
Sugar content in flowing pipelines can be inferred by measuring density using inline differential pressure probes, radiation, and U-tube densitometers. Inline differential pressure probes are inserted into a vertical section of pipe (See Figure 2 below), and operate similarly to those used in vessels. Radiation densitometers infer density by measuring radiation reaching the sensor from its source after the radiation travels through the material to be measured. U-tube densitometers infer density from frequency measurements of a U-shaped tube containing the material, whereby its natural frequency changes with changing material density. This same principle allows most Coriolis mass flowmeters to measure density in addition to flow.
“Density measurements are important in my sugar production plant because they infer sugar concentration,” says Clovis Massachi Muraishi, industrial engineering supervisor at Usina Açucareira Guaíra, São Paulo, Brazil. The plant has been making ethanol for over 20 years.
“Operating certain process streams at low sugar concentrations [inferred by relatively low-density measurements] can cause inefficient processing and poor crystallization. Streams operating at high sugar concentrations [inferred by relatively high density measurements] can cause inefficient processing, and create operational problems, such as crystal sugar deformation, machine and inversion problems,” adds Pedro Collegari, general manager at Grupo João Lyra’s Vale do Paranaíba plant, Capinopolis, Minas Gerais, Brazil.