Evaporator units

For each mol of urea produced in a total-recycle urea process, one mol of water is formed. It is usually discharged from the urea concentration and evaporation section of the plant. For example, a 1200 t/d plant discharges a minimum of 360 t/d of wastewater. With a barometric condenser in the vacuum section of the evaporation unit, the amount of wastewater is even higher. Small amounts of urea are usually found in wastewaters because of entrainment carry-over. 

In field evaporative units utilizing natural gas as the fuel source, the primary driving force is the heat supplied to the water. The theoretical evaporation rate for these units may be expressed as... 

The usual split package air-conditioner comprises one condensing unit connected by pipes to one evaporator unit (Figure 13.4). Twin condensing units are made to save on outdoor casings and reduce the number of pieces on a roof or wall. Such twins will be connected in the usual way to two separate indoor units. 

Evaporate unit mass of solvent from a large quantity of solution. 

It is decided to bleed off 0.25 kg/s of vapour from the vapour line to the second effect for use in another process. If the feed is still heated to the boiling-point of the first effect by external means, what will be the change in the steam consumption of the evaporator unit ... 

The design of an evaporation unit requires the practical application of data on heat transfer to boiling liquids, together with a realisation of what happens to the liquid during concentration. In addition to the three main features outlined above, liquors which have an inverse solubility curve and which are therefore likely to deposit scale on the heating surface merit special attention. 

As you may have observed water condenses on cold water mains or windows and ice forms on the evaporator unit in your refrigerator. This effect of condensation of gases and vapors on cold surfaces, water vapor in particular, as it is known in every day life, occurs not only at atmospheric pressure but also in vacuum. 

Figure 12.18 Simplified flow sheet of a double-effect evaporation unit.

Although the concentration of fluorine is the most important quantity in the control of the reaction rate and must be maintained within certain limits, in practice the stoichiometry, the molecular fluorine to substrate H-atom molar ratio, is used to determine the reaction parameters leading to a successful and efficient perfluorination. AF is most successful when sublimable solids are introduced into the hydrocarbon evaporator unit of the aerosol fluorinator as solutions by a syringe pump. This now common procedure emphasizes the individual molecule s isolation as it is fluorinated using AF. No intermolecular reactions between solute and solvent have been observed Choice of the solvent is important as it must not boil at a temperature below the melting point of the solute in order to prevent solid deposition in the tubes feeding the evaporator. It must also fluorinate to a material easily separable from the solid reactant after perfluorination. In most cases it has been found that aliphatic hydrochlorocarbons are excellent choices, but that carbon tetrachloride and chloroform and other radical-scavenging solvents are not (sec ref 6). 

Figure 7. Drawing of ammonium carbonate recovery unit and evaporator rated input capacity is 0.8 and 1.5 L/h with and without foaming respectively A, metering tank with preheater B, ammonium carbonate distillation, 0.5 kW C, foam level sensor (conductivity) D, condenser E, acid inlet, 6M HNOs F, evaporation unit, 1 kW G, Ca 2M (NHlt)2COi, to carbonate adjustment with C02 H, distillate, HzO + NOx I, metal nitrate concentrate, ca. 0.5M HNOs J, electrical immersion heater...

A triple effect evaporator is designed to reduce water from an incoming brine (NaCl + H2O) stream from 25 wt % to 3 wt %. If the evaporator unit is to produce 14,670 lb/hr of NaCl (along with 3 wt % H2O), determine ... 

Vs = estimated salvage value at end of service life, dollars X = number of evaporator units Y = time period for construction, years Z = time period land is owned before plant startup, years... 

During the time an evaporator is in operation, solids often deposit on the heat-transfer surfaces, forming a scale. The continuous formation of the scale causes a gradual increase in the resistance to the flow of heat and, consequently, a reduction in the rate of heat transfer and rate of evaporation if the same temperature-difference driving forces are maintained. Under these conditions, the evaporation unit must be shut down and cleaned after an optimum operation time, and the cycle is then repeated. 

CYCLE TIME FOR MINIMUM COST PER UNIT OF HEAT TRANSFER There are many different circumstances which may affect the minimum cost per unit of heat transferred in an evaporation operation. One simple and commonly occurring case will be considered. It may be assumed that an evaporation unit of fixed capacity is available, and a definite amount of feed and evaporation must be handled each day. The total cost for one cleaning and inventory charge is assumed to be constant no matter how much boiling time is used. The problem is to determine the cycle time which will permit operation at the least total cost. 

The vacuum evaporator unit produces urea melt at the required concentration either for prilling or granulation. 

The development of this important thermal separation process can be dated back to ancient times. Until the end of the 18 century, however, the stmcture of the distillation apparatus had remained almost unchanged. It consisted of an evaporation unit, a distillation flask heated by an oven, and a condensation unit with an air- and later water-cooled condenser. In the early 19 century, progress in distillation techniques was spurred by the necessity to produce sugar in Europe this politically motivated development resulted in numerous patents for the production of alcohol. Depending on the different starting materials, a number of distillation units and the first rectification columns were developed in various European countries [10]. The 19 and 20 centuries saw a rapid development of distillation technology prompted by increasing applications in the petrochemical, chemical and pharmaceutical industries. 

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