Vaporous carryover

Roller Water The steam purity limits define boiler-water limits because the steam cannot be purified once it leaves the boiler. For a once-through boiler, the boiler water must have the same specifications as the steam. A recirculating boiler is a still, and there can be considerable purification of the steam as it boils and is separated from the water in the steam dmm. The process of separation is not perfect, however, and some water is entrained in the steam. This water, called mechanical carryover, contains impurities in the same proportions as the boiler water, and its contribution to steam impurity is in those proportions. Typical mechanical carryover is less than 0.25% and often less than 0.1%, but operating conditions in the boiler can affect the mechanical carryover. In addition to mechanical carryover, chemicals can be carried into the steam because of solubility. This is called vaporous carryover. Total carryover is the sum of mechanical and vaporous carryover. The boiler-water specification must be such that the total carryover conforms to the steam purity requirements. For salts, such as sodium phosphate and sodium chloride, vaporous carryover is not a significant problem below approximately 15 MPa (2175 psia). As boiler pressures approach the critical point, vaporous carryover increases rapidly. Above 15 MPa (150 bar), boiler solids concentrations must be carefully controlled to minimize vaporous carryover. Most boilers operating over 18 MPa (180 bar) use all volatile treatment to prevent deposition of salts in turbines. Boiler-water limits for utility boiler are Us ted in Table 2. Recommendations from American Boiler Manufacturers Association (ABMA) for boiler-water limits for drum-type boilers and associated steam purity for watertube boilers are listed in Table 3. 

VAPOROUS CARRYOVER - Referring to impurities carried over with the steam and then forming a deposit on turbine bladings. This type of carryover is difficult to prevent. 

The first step in CTO distillation is depitching. A relatively small distillation column is used as a pitch stripper. The vapor from the pitch stripper is fed directiy into the rosin column, where rosin and fatty acids are separated. Rosin is taken from the bottoms of the column and fatty acids as a sidestream near the top. Palmitic acid and light neutrals are removed in the rosin column as heads. The operation is designed to minimize holdup and product decomposition. Care is taken to prevent carryover of some of the heavier neutrals, such as the sterols, from the depitcher to the rosin column (24). 

Vapor-liquid separators (drums) are used to separate a liquid from a vapor-liquid stream with a minimum of liquid carryover. The separator size is determined by the vapor velocity which depends on the entrainment method used. The working equation is ... 

The use of steam washing equipment. Apart from the removal of entrained solids and water, steam washers are quite effective at removing vaporous silica carryover (typically 60-70% removal). 

Essentially, the higher the pressure or temperature, the greater the potential for vaporous silica carryover and steam dissolution. When pressure or temperature falls, the steam becomes supersaturated and various forms of crystalline silica deposition begin to occur. As temperatures continue to fall, the deposits become increasingly amorphous in nature and also more insoluble. 

Entrainment Separators. In any process in which the product is volatilized, including both Gas Recycle and Liquid Recycle, ppm or ppb levels of metal catalyst may be entrained in the vapors leaving the separation system. Entrainment separators (Figure 2.9) are often included to recover the metal. Vaporous product effluent from a gas recycle reactor may be sent to a separator where it is passed through a demisting pad to return some aldehyde and condensation product and particularly to prevent potential carryover of catalyst. [6]... 

Fluidized bed reactors typrcally are vertical cylindrical vessels equipped with a support grid and feed sparger system for adequate fluidization and feed distribution, internal cooling coils for heat removal, and either external or internal cyclones to minimize catalyst carryover. Fluidizauon of the catalyst assures intimate contact between feed and product vapors, catalyst, and heat-transfer surfaces, and results in a uniform temperature within the reactor. Reaction heat can be removed by generating steam within the cooling coils or by some oilier heat-transfer medium. 

Steam Purity. The trend toward higher pressures and temperatures in steam power plant practice imposes a severe demand on steam-purification equipment for elimination of troublesome solids in the steam. Carryover may result from ineffective mechanical separation and from the vaporization of boiler-water salts. Total cany-over is the sum of the mechanical and vaporous carry-over of all impurities. 

Entrainment (Figure 6.14) is liquid transported fry the gas to the tray above. This liquid contains more of the less-volatile material than the tray above, and therefore it counteracts the mass transfer process and reduces tray efficiency. Other undesirable effects of entrainment are carryover of nonvolatile impurities upward to contaminate the overhead product and the possibility of damage to rotating machinery located in the path of tbe column overhead vapor. 

In the evaporation plant especially, glycerine can be carried out with the vapor during evaporation and lost in the vacuum system s condensing of water. The amount of carryover can be minimized in several ways ... 

The vapors from a chlorine vaporizer will contain some liquid droplets. The vaporizer consists of a vertical, cylindrical vessel with a submerged bundle for heating. A vapor rate of 2500 kg/h is required, and the vaporizer will operate at 6 bara. Size the vessel to restrict the carryover of liquid droplets. The liquid holdup time need not be considered, as the liquid level will be a function of the thermal design. 

The upper limit to vapor flow is set by the condition of flooding. At flooding, there is a sharp drop in plate efficiency and increase in pressure drop. Flooding is caused by either the excessive carryover of liquid to the next plate by entrainment or by liquid backing up in the downcomers. 

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