Contact plant

Paraquat (l,r-dimethyl-4,4 -bipyridinium) and its dichloride salt (1,1 -dimethyl-4,4 -bipy-ridinium dichloride) are broad-spectrum contact plant killers and herbage desiccants that were introduced commercially during the past 35 years. Today, they rank among the most widely used herbicides globally and are frequently used in combination with other herbicides. The recommended field application rates for terrestrial weed control usually range between 0.28 and 1.12 kg paraquat/ha (0.25 and 1.0 pounds/acre) for aquatic weed control, it is 0.1 to 2.0 mg/L. Target plant species are unable to metabolize paraquat and tend to contain elevated residues paraquat-resistant... 

Contact plants, 23 768-769 Contact printing, in microarray fabrication, 76 386... 

In the contact plant for the manufacture of sulphuric acid, sulphuric acid is itself used for drying the air for the oxidation of the sulphur. When drying hydrocarbons such as benzene, it is sometimes convenient to pass the material through a bed of solid caustic soda, although, if the quantity is appreciable, this method is expensive. 

Iron Contact Plant.—The fundamental and secondary chemical reactions involved in this process having been considered, there remains only the plant, and the actual fuel consumption per 1000 cubic feet of hydrogen to be described. 

The Iron Contact plant is commercially manufactured in two distinct types —... 

In practice it was stated that in commercial iron-contact plants the consumption of blue gas was from... 

The contact process was invented by Phillips in England in 1831 but was not used commercially until many years later. Today 99% of all sulfuric acid is manufactured by this method. It was developed mainly because of the demand for stronger acid. All new contact plants use interpass absorption, also known as double absorption or double catalysis. This process will be described in detail in Fig. 2.3. 

Obviously, the concentration of the final product is governed by the quantity of water employed. In most contact plants, the quantity of water used is such that the final product is a concentrated acid (about 96% H2S04). 

Most sulfuric acid plants are double contact plants, Fig. 9.6, Tables 9.3, 19.3 and 23.2. They efficiently oxidize their feed S02(g) to S03(g) and efficiently make the resulting S03(g) into H2S04(f). Single contact plants (Fig. 9.1) are simpler and cheaper - but less efficient. 

Fig. 16.1. Schematic of single contact, 3 catalyst bed sulfuric acid plant. It is a single contact plant because it has only one H2S04 making step. Note gas cooling between catalyst beds. It permits additional S02 oxidation in the next catalyst bed.

This and the complexity of triple contact plants explain why none has been built. 

The most efficient double contact plants have one catalyst bed after H2S04 making, remainder before. 3 - 1 plants are more efficient than 2-2 plants. 4 - 1 plants are more efficient than 2-3 and 3-2 plants. 

The data are for FINAL H2SO4 making in double contact plants. ... 

The 65 to 70% sulfuric acid containing a residue of metal sulfates (3 to 5%) is either directly, or after further concentration to 80 to 90%, in admixture with 96% acid or oleum (from a double contact plant fed with sulfur dioxide from metal sulfate cracking) is further utilized in the digestion of titanium ore. 

With the double contact process it is unnecessary to purify the tail gases to reduce their sulfur dioxide content still further, whereas tail gases from single contact plants have to be purified. This can be realized either by scrubbing with ammonia or with an aqueous solution of sodium sulfite and sodium hydrogen sulfite (Wellman-Lord process), absorption on activated charcoal (sulfacid process from Lurgi) or by oxidative gas purification such as in the peracidox process (oxidation of sulfur dioxide with hydrogen peroxide or peroxomonosulfuric acid). 

Venturi concentrator The to be concentrated sulfuric acid is injected into the radiation scrubber and there brought into contact with a dry gas stream, which takes up and thereby removes the water vapor. The heat of evaporation is either supplied directly via hot gases (furnace gas) or indirectly by heating the acid to be concentrated (e.g. with tail gases from a double contact plant) or by heat exchange with hot sulfuric acid from sulfur trioxide absorption. In this preconcentration process waste heat can be utilized at low temperatures. The venturi concentrator is in particular employed when large quantities of dilute sulfuric acid or sulfuric acid strongly contaminated with solids has to be preconcentrated. 

Provision of the sulfur dioxide feed gas for a contact plant is usually obtained by sulfur combustion, but there are many other potential sources. Pyrite, FeS2, is burned (Eq. 9.27) and other sulfidic minerals are roasted (Eqs. 9.28 and 9.29), the latter primarily for their metal values rather than for the sulfur dioxide. 

Contact plant, 180 tonne/day Chamber process 50 tonne/day... 

Temperatures of about 1100°C are necessary in the presence of added air in order to promote complete oxidation of any organic impurities and thermolysis of the acid. Oxygen-enriched air may be used with advantage [71]. Subsequently, the sulfur dioxide is recovered and passed through a conventional contact plant for regeneration to sulfur trioxide and thence via rehydration to concentrated sulfuric acid (Fqs. 9.25 and 9.26). 

MATERIAL BALANCES. In a differential-contact plant such as the packed absorption tower illustrated in Fig. 22.9, there are no sudden discrete changes in composition as in a stage-contact plant. Instead the variations in composition are continuous from one end of the equipment to the other. Material balances for the portion of the column above an arbitrary section, as shown... 

The operating-line equation for a differential-contact plant, analogous to Eq. 

Cyclization of 2 in concentrated sulphuric acid [14-16] predominantly leads to p-ionone (17). The reaction proceeds rapidly even below room temperature and, to avoid secondary reactions, is carried out continuously. The precooled streams of sulphuric acid and the solution of 2 in petroleum ether or liquid CO2 are mixed in a reactor and then quenched with cold water. Small amounts of a-ionone (18) can be separated off by distillation during isolation of the product. In the cyclization step large amounts of approximately 40% aqueous sulphuric acid are produced. Treatment to deal with this is expensive but is essential for environmental reasons. Organic impurities are broken down to carbon dioxide in a cracking furnace with heavy oil burners. In the course of this process, sulphuric acid is thermally converted into sulphur dioxide, which is reoxidized in the contact plant. 

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