Degassing

There are two main times during the processing of polyurethane where vacuum degassing may be required. The first is the prepolymer as supplied by the manufacturer. The second is after mixing the prepolymer with the rest of the system (curative, pigment, extenders, and fillers). 

The manufacturer of the prepolymer endeavors to supply a low-moisture, low-free isocyanate, gas-free material. Moisture may be introduced with repeated opening of the prepolymer container. Contaminants will form bubbles in the final material. Any moisture or free isocyanates will have an effect on the reaction ratios as well as change the chemistry to a degree. 

There are two main methods to de-gas the prepolymer. The first is a simple batch system composed of a vacuum pump and a pot designed to 

For safety reasons, the pot must be specifically designed for vacuum use and any replacements be made from the same materials and the same size and design. A simple replaceable liner such as plastic wrapping in the pot will help keep the system in good condition. 

A second degassing may be carried out after the addition of all the ingredients to remove any bubbles introduced into the mix. It must be remembered that this takes up part of the available casting time. 

The presence of some water-soluble gases in water, for example, CO3, O3, HjS is undesirable since they cause corrosion of metallic and concrete components of the systems used in water technology and industry. Thus, water degassing is one of the most efficient anticorrosion measures. This process is also necessary for feed waters used in boilers. For water degassing both thermal and chemical procedures are required. 

Depending on the pressure in the system, pressure or vacuum thermal degassing processes are distinguished. 

Chemical degassing is based on addition of appropriate chemicals, and is used for oxygen removal only. The reduction of oxygen content in the feed waters for high pressure boilers is a stringent requirement. 

In plants where the pressure in boilers is up to 6 MPa, sulphites are used for degassing of feed waters hydrazine is more convenient for higher pressures [3, 4, 58]. 

Gases need to be removed prior to the die when they produce defects in the extrudate. The most common defect caused by gas is foaming. Of course, foamed extrusions are often produced by design, but when foaming is unwanted, the gas must be removed before the die. Defects also take the form of bubbles and surface roughness. 

Unwanted gas maybe present in the polymer due to a variety of reasons. Moisture that has turned into steam is a common cause. Also, some material components release gas as they degrade at relatively high temperatures. Finally, the gas maybe the by-product of a chemical reaction designed into the system to provide a particular product property. 

The screw must be designed to prevent polymer from escaping through the vent, often called vent flow. This is normally accomplished by having a greater pumping capability downstream of the vent than upstream. This condition ensures that the polymer is removed from the extraction section before it can build up pressure. However, proper operation often depends on factors such as polymer viscosity, head pressure, and screw speed. It is not unusual for vent flow to occur under off-design conditions, such as start-up. 

A most important preliminary to the accurate measurement of an adsorption isotherm is the preparation of the adsorbent surface. In their usual state all surfaces are covered with a physically adsorbed film which must be removed or degassed before any quantitative measurements can be made. As the binding energy in physical adsorption is weak van der Waals forces, this film can be readily removed if the solid is maintained at a high temperature while under vacuum. 

The degree of degassing attained is dependent on three variables, pressure, temperature and time. In test and control work, the degassing conditions may be chosen empirically and maintained identical in all estimates since only reproducibility is required. For more accurate measurements, conditions have to be chosen more carefully. 

Although it is advisable to outgas at as low a pressure as possible, due to considerations of time and equipment the degassing pressure is kept as high as is consistent with accurate results. The pressures usually recommended are easily attainable with a diffusion pump. Emmett [171], for example, recommends 10-5 mm Hg, while Joy [112] recommends 10- mm Hg, since under this condition the rate of degassing is controlled largely by diffusion from the interior of the particles. 

For routine analyses Bugge and Kerlogue [172] found that a vacuum of 10-2 to 10-3 mm Hg was sufficient and the differences in surface areas so obtained was smaller than 3% of those obtained at 10-5 mm Hg. 

Vacuum should be applied slowly to prevent powder from being sucked into the vacuum line. Cleaning contaminated equipment is a time-consuming chore and contamination can be prevented provided care is taken. The introduction of a plug of cotton wool into the neck of the sample tube can reduce the possibility of powder loss. At the end of the degassing cycle the sample cell is isolated from the vacuum 

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Chamber B is filled with partially degassed sample material at 0°C. Chamber A is filled with air at 37.8°C and at atmospheric pressure. 

Kinetic measurements were performed employii UV-vis spectroscopy (Perkin Elmer "K2, X5 or 12 spectrophotometer) using quartz cuvettes of 1 cm pathlength at 25 0.1 C. Second-order rate constants of the reaction of methyl vinyl ketone (4.8) with cyclopentadiene (4.6) were determined from the pseudo-first-order rate constants obtained by followirg the absorption of 4.6 at 253-260 nm in the presence of an excess of 4.8. Typical concentrations were [4.8] = 18 mM and [4.6] = 0.1 mM. In order to ensure rapid dissolution of 4.6, this compound was added from a stock solution of 5.0 )j1 in 2.00 g of 1-propanol. In order to prevent evaporation of the extremely volatile 4.6, the cuvettes were filled almost completely and sealed carefully. The water used for the experiments with MeReOj was degassed by purging with argon for 0.5 hours prior to the measurements. All rate constants were reproducible to within 3%. 

Reppe s work also resulted in the high pressure route which was estabUshed by BASF at Ludwigshafen in 1956. In this process, acetylene, carbon monoxide, water, and a nickel catalyst react at about 200°C and 13.9 MPa (2016 psi) to give acryUc acid. Safety problems caused by handling of acetylene are alleviated by the use of tetrahydrofuran as an inert solvent. In this process, the catalyst is a mixture of nickel bromide with a cupric bromide promotor. The hquid reactor effluent is degassed and extracted. The acryUc acid is obtained by distillation of the extract and subsequendy esterified to the desked acryhc ester. The BASF process gives acryhc acid, whereas the Rohm and Haas process provides the esters dkecdy. 

The above batch process has undergone numerous refinements to improve yields, processing characteristics, purity, and storage stabiUty, but it remains the standard method of manufacture for these products. Recentiy a continuous process has been reported by Bayer AG (6) wherein the condensation is carried out in an extmder. The by-products are removed in a degassing zone, and the molten polymer, mixed with stabilizers, is subsequendy cracked to yield raw monomer. 

This principle is illustrated in Figure 10 (45). Water adsorption at low pressures is markedly reduced on a poly(vinyhdene chloride)-based activated carbon after removal of surface oxygenated groups by degassing at 1000°C. Following this treatment, water adsorption is dominated by capillary condensation in mesopores, and the si2e of the adsorption-desorption hysteresis loop increases, because the pore volume previously occupied by water at the lower pressures now remains empty until the water pressure reaches pressures 0.3 to 0.4 times the vapor pressure) at which capillary condensation can occur. 

Liquid Effluents. Recycling of acid, soda, and zinc have long been necessary economically, and the acid—soda reaction product, sodium sulfate, is extracted and sold into other sectors of the chemical industry. Acid recovery usually involves the degassing, filtering, and evaporative concentration of the spent acid leaving the spinning machines. Excess sodium sulfate is removed by crystallization and then dehydrated before sale. Traces of zinc that escape recovery are removable from the main Hquid effluent stream to the extent that practically all the zinc can now be retained in the process. 

HBI is an effective trim coolant for molten steel in ladle metallurgy faciUties, ladle refiners, ladle furnaces, and vacuum degassers. It provides cold iron units in an ideal size and density for penetrating the ladle slag and cooling the metal. 

Lithium is used in metallurgical operations for degassing and impurity removal (see Metallurgy). In copper (qv) refining, lithium metal reacts with hydrogen to form lithium hydride which subsequendy reacts, along with further lithium metal, with cuprous oxide to form copper and lithium hydroxide and lithium oxide. The lithium salts are then removed from the surface of the molten copper. 

Chevron s WWT (wastewater treatment) process treats refinery sour water for reuse, producing ammonia and hydrogen sulfide [7783-06-04] as by-products (100). Degassed sour water is fed to the first of two strippers. Here hydrogen sulfide is stripped overhead while water and ammonia flow out the column bottoms. The bottoms from the first stripper is fed to the second stripper which produces ammonia as the overhead product. The gaseous ammonia is next treated for hydrogen sulfide and water removal, compressed, and further purified. Ammonia recovery options include anhydrous Hquid ammonia, aqueous Hquid ammonia, and ammonia vapor for incineration. There are more than 20 reported units in operation, the aimual production of ammonia from this process is about 200,000 t. 

Ladle metallurgy, the treatment of Hquid steel in the ladle, is a field in which several new processes, or new combinations of old processes, continue to be developed (19,20). The objectives often include one or more of the following on a given heat more efficient methods for alloy additions and control of final chemistry improved temperature and composition homogenisation inclusion flotation desulfurization and dephosphorization sulfide and oxide shape control and vacuum degassing, especially for hydrogen and carbon monoxide to make interstitial-free (IF) steels. Electric arcs are normally used to raise the temperature of the Hquid metal (ladle arc furnace). 

R. J. Fmehan, Vacuum Degassing, Iron and Steel Society, Warrendale, Pa., 1990. 

Dispersed mixtures of boron and another metal are used as deoxidizing and degassing agents to harden steel (qv) (5,6), to increase the conductivity of copper (qv) in turbojet engines, and in the making of brass and bronze (see Copper alloys). Two examples are alloys of ferroboron and manganese boron. 

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