lowest temperature nodes are termed as unstable nodes, as all trajectories leave from them, while the highest temperature points in the region are termed stable nodes, as all trajectories ultimately reach them. The point that the trajectories approach from one direction and end in a different direction (as always is the point of intermediate boiling component) is termed saddle point. Residue curves that divide the composition space into different distillation regions are called distillation boundaries.
The separation of components of similar volatility may become economical if an entrainer can be found that effectively changes the relative volatility. It is also desirable that the entrainer be reasonably cheap, stable, non-toxic, and readily recoverable from the components. In practice, it is probably this last criterion that severely limits the application of extractive and azeotropic distillation. The majority of successful processes, in fact, are those in which the entrainer and one of the components separate into two liquid phases on cooling if direct recovery by distillation is not feasible.
A further restriction in the selection of an azeotropic entrainer is that the boiling point of the entrainer be in the range 10 to 40°C (18 to 72°F) below that of the components. Thus, although the entrainer is more volatile than the components and distills off in the overhead product, it is present in a sufficiently high concentration in the rectification section of the column.
See also: Azeotropic Distillation, Distillation.
Azeotropic Distillation
Azeotropic distillation is the use of a third component to separate two close-boiling components by means of the formation of an azeotropic mixture between one of the original components and the third component to increase the difference in the boiling points and facilitates separation by distillation.
All compounds have definite boiling temperatures, but a mixture of chemically dissimilar compounds sometimes causes one or both of the components to boil at a temperature other than that expected. For example, benzene boils at 80oC (176oF), but if it is mixed with hexane, it distills at 69oC (156oF). A mixture that boils at a temperature lower than the boiling point of either of the components is called an azeotropic mixture.
Chemical bonding between the components of the mixture creates properties unique to the mixture. If the system forms azeotropes, as in a benzene and cyclohexane system, a different problem arises - the azeotropic composition limit the separation, and for a better separation, this azeotrope must be bypassed in some way. At the azeotropic point, the mixture contains the given component in the same proportion as the vapor, so that evaporation does not change the purity, and distillation does not affect separation. For example, ethyl alcohol and water form an azeotrope (azeotropic mixture) at 78.2°C (172.8°F).
If the separation of individual components from petroleum itself or from petroleum products is required, there are means by which this can be accomplished. For example, when a constant-boiling mixture of hydrocarbons contains components whose vapor pressure is affected differently by the addition of, say, a non-hydrocarbon compound, distillation of the hydrocarbon mixture in the presence of non-hydrocarbon additive may facilitate separation of the hydrocarbon components.
In general, the non-hydrocarbon additive is a polar organic compound and should also have the ability to form a binary minimum constant-boiling (or azeotropic) mixture with each of the hydrocarbons. Thus, it is often possible to separate compounds that have close boiling points by means of azeotropic distillation.
See also: Azeotrope, Distillation.
B
Bacteria
Bacteria are microscopic, single-celled organisms that thrive in diverse environments. These organisms can live in soil, the ocean, and inside the human digestive system. Bacteria are classified into five groups according to their basic shapes: (i) spherical shapes, also known as cocci, (ii) rod shapes, also known as bacilli, (iii) spiral shapes, also known as spirilla, (iv) comma shapes, also known as vibrios, and (v) corkscrew shapes, also known as spirochaetes which can also exist as single cells, in pairs, chains, or clusters.
Bacteria occur individually or grow as groups ranging from two to millions of individual cells. Individual bacteria cells are small and may be observed only through a microscope. Most bacteria fall into the size range of 0.5 to 3.0 microns (1 micron = 1 m × 10-6). However, considering all species, a size range of 0.3-50 microns is observed. In general, it is assumed that a filter with a pore size on the order of 0.45 micron will remove all bacteria from water passing through it.
The metabolic activity of bacteria is greatly influenced by their small size. Their surface-to-volume ratio is extremely large, so that the inside of a bacterial cell is accessible to a chemical substance in the surrounding medium. Thus, for the same reason that a finely divided catalyst is more efficient than a more coarsely divided one, bacteria may cause rapid chemical reactions compared to those mediated by larger organisms. Bacteria excrete enzymes that can act outside the cell (exoenzymes) that break down solid food material to soluble components which can penetrate bacterial cell walls, where the digestion process is completed.
Bacteria obtain the energy needed for their metabolic processes and reproduction by mediating redox reactions. Nature provides a large number of such reactions, and bacterial species have evolved that utilize many of these. As a consequence of their participation in such reactions, bacteria are involved in many biochemical processes in water and soil.
Bacteria are essential participants in many important elemental cycles in nature, including those of nitrogen, carbon, and sulfur. They are responsible for the formation of many mineral deposits, including some of iron and manganese. On a smaller scale, some of these deposits form through bacterial action in natural water systems and even in pipes used to transport water.
The presence of coliform bacteria, specifically E. coli (a type of coliform bacteria), in drinking water suggests the water may contain pathogens that can cause a variety of diseases and even death.
See also: Bacterial Mining.
Bacterial Mining
Bacterial mining, or bio-mining, represents the use of microorganisms to leach metals from ores or mine tailings (wastes), followed by the subsequent recovery of metals of interest from the leaching solution. The term has also been applied to the recovery of oil from reservoirs where the reservoir energy has been depleted over time. This could well be a process of the future for recovering energy from alternate sources that have been difficult-toimpossible to reach or recover and develop.
The use of microbes to extract metals from ores is simply the harnessing of a natural process for commercial purposes. Microbes have participated in the deposition and solubilization of heavy metals in the earth’s crust since geologically ancient times. Most of this activity is linked to the iron and sulfur cycles. Anaerobic sulfate reducing bacteria generate sulfides that can react with a variety of metals to form insoluble metal sulfides. There are two main types of processes for commercial-scale microbially assisted metal recovery. These are irrigation-type and stirred tank-type processes. Bio-mining involves a chemical process called leaching which is actually oxidation reaction and maybe called bio-oxidation. Bio-leaching processes can be carried out at a range of temperatures, and as would be expected, the iron- and sulfur-oxidizing microbes present differ depending on the temperature ranges. In mineral bio-oxidation processes that operate at 40°C or less, the most important microorganisms are believed to be a consortium of gram-negative bacteria such as Acidithiobacillus
