Boiling point rise

Boiling point rise expresses the difference between the boiling point of a constant composition solution and the boiling point of pure water at the same pressure. For example, pure water boils at 100°C at one atmosphere and a 35% sodium hydroxide solution boils at about 121 °C at one atmosphere. The boiling point rise is therefore 21 °C. However, the vapor from the solution is superheated steam at 121°C, but the steam will condense at 100°C only. Consequently, boiling point rise represents a loss of total available temperature difference. 

Duhring s rule states that a linear relationships exists between the boiling point of a solution and the boiling point of pure water at the same pressure. Thus, the temperature difference between the boiling point in an evaporator and the boiling point of water at the same pressure is a direct measurement of the concentration of the solution. Two problems in making this measurement are location of the temperature sensors and control of absolute pressure. 

The temperature sensors must be located so that the measured values are truly representative of the actual conditions. Ideally, the sensor measuring liquor temperature should be just at the surface of the boiling liquid. This location can change, unfortunately, if the operator decides to adjust liquid levels in a particular effect. Many times the liquor sensor is installed near the bottom of the evaporator flash body where it will always be covered. This creates an error due to hydrostatic head which must be compensated for in the calibration. 

The vapor temperature sensor is installed in a condensing chamber in the vapor line. Hot condensate flashes over the sensor at an equilibrium temperature dictated by the pressure in the system. This temperature less the liquid boiling temperature (compensated for head effects) is the temperature difference reflecting product concentration. 

Changes in the absolute pressure of the system alter not only the boiling point of the liquor, but the flashing temperature of the condensate in the condensing chamber as well. Unfortunately, the latter effect occurs much more rapidly than the former, resulting in transient errors which may take a long time to resolve. It is therefore critical that absolute pressure be closely controlled if temperature difference is to be a successful measure of product concentration. 

---

The heat requirements in batch evaporation are the same as those in continuous evaporation except that the temperature (and sometimes pressure) of the vapor changes during the course of the cycle. Since the enthalpy of water vapor changes but little relative to temperature, the difference between continuous and batch heat requirements is almost always negligible. More important usually is the effect of variation of fluid properties, such as viscosity and boiling-point rise, on heat transfer. These can only be estimated by a step-by-step calculation. 

When the boiling point rise is appreciable, the economic number of effects in series with forward feed is 4-6. 

When the boiling point rise is small, minimum cost is obtained with 8-10 effects in series. 

As the rate of take-off is reduced near the end of a fraction, a slight lowering of the bath temperature may be necessary to avoid flooding of the column. Also as the boiling point rises during the collection of the intermediate fraction, the power input to the jacket must be increased in order to hold its temperature just below the boiling point. 

At 13.5 kN/m2 water boils at 325 K and in the absence of data as to the boiling-point rise, this will be taken as the temperature of evaporation, assuming an aqueous solution. The total enthalpy of steam at 325 K is 2594 kJ/kg. 

The boiling-point rise of the solution is 30 degK, the feed temperature is 291 K, the specific heat capacity of the feed is 4.0 kJ/kgdegK, the specific heat capacity of the product is 3.26 kJ/kgdegK and the density of the boiling liquid is 1390 kg/m3. 

Distilled water is produced from sea water by evaporation in a single-effect evaporator working on the vapour compression system. The vapour produced is compressed by a mechanical compressor of 50 per cent efficiency, and then returned to the calandria of the evaporator. Extra steam, dry and saturated at 650 kN/m2, is bled into the steam space through a throttling valve. The distilled water is withdrawn as condensate from the steam space. 50 per cent of the sea water is evaporated in the plant. The energy supplied in addition to that necessary to compress the vapour may be assumed to appear as superheat in the vapour. Calculate the quantity of extra steam required in kg/s. The production rate of distillate is 0.125 kg/s, the pressure in the vapour space is 101.3 kN/m2, the temperature difference from steam to liquor is 8 deg K, the boiling-point rise of sea water is 1.1 deg K and the specific heat capacity of sea water is 4.18 kJ/kgK. 

Solids (per cent by mass) Boiling-point rise (deg K) Specific heat capacity (kJ/kg K) Heat of dilution (kJ/kg)... 

A forward-feed double-effect standard vertical evaporator with equal heating areas in each effect is fed with 5 kg/s of a liquor of specific heat capacity of 4.18 kJ/kgK, and with no boiling-point rise, so that 50 per cent of the feed liquor is evaporated. The overall heat transfer coefficient in the second effect is 75 per cent of that in the first effect. Steam is fed at 395 K and the boiling-point in the second effect is 373 K. The feed is heated to its boiling point by an external heater in the first effect. 

A liquor containing 15 per cent solids is concentrated to 55 per cent solids in a doubleeffect evaporator operating at a pressure of 18 kN/m2 in the second effect. No crystals are formed. The feedrate is 2.5 kg/s at a temperature of 375 K with a specific heat capacity of 3.75 kJ/kg K. The boiling-point rise of the concentrated liquor is 6 deg K and the pressure of the steam fed to the first effect is 240 kN/m2. The overall heat transfer coefficients in... 

For the purpose of calculation, it may be assumed that the specific heat capacity is 4.18 kJ/kgK, that there is no boiling point rise, and that the latent heat of vaporisation is constant at 2330 kJ/kg over the temperature range in the system. The overall heat transfer coefficients may be taken as 2.5, 2.0 and 1.6 kW/m2 K in the first, second and third effects, respectively. 

Forward feed is used in each case, and the values of U are the same for the two systems. The boiling-point in the third effect is 325 K, and the liquor has no boiling-point rise. 

The feed enters the evaporator at 295 K and the concentrated liquor is withdrawn at the rate of 0.025 kg/s. The concentrated liquor exhibits a boiling-point rise of 10 degK. Heat losses to the surroundings are negligible. The nozzle efficiency is 0.95, the efficiency of momentum transfer is 0.80 and the efficiency of compression is 0.90. 

A single-effect evaporator is used to concentrate 0.075 kg/s of a 10 per cent caustic soda liquor to 30 per cent. The unit employs forced circulation in which the liquor is pumped through the vertical tubes of the calandria which are 32 mmo.d. by 28 mmi.d. and 1.2 m long. Steam is supplied at 394 K, dry and saturated, and the boiling-point rise of the 30 per cent solution is 15 degK. If the overall heat transfer coefficient is 1.75 kW/m2 K, how many tubes should be used, and what material of construction would be specified for the evaporator The latent heat of vaporisation under these conditions is 2270 kJ/kg. 

A double-effect forward-feed evaporator is required to give a product which contains 50 per cent by mass of solids. Each effect has 10 m2 of heating surface and the heat transfer coefficients are 2.8 and 1.7 kW/m2 K in the first and second effects respectively. Dry and saturated steam is available at 375 kN/m2 and the condenser operates at 13.5 kN/m2. The concentrated solution exhibits a boiling-point rise of 3 deg K. What is the maximum permissible feed rate if the feed contains 10 per cent solids and is at 310 K The latent heat is 2330 kJ/kg and the specific heat capacity is 4.18 kJ/kg under all the above conditions. 

The specific heat capacity may be taken as constant at 4.18 kJ/kgK, and the effects of boiling point rise and of hydrostatic head may be neglected. 

The temperature of liquor boiling in the first effect, assuming no boiling-point rise, is 373 K at which temperature steam is saturated at 101.3 kN/m2. 

Assuming that the liquor exhibits a 6 deg K boiling-point rise at all concentrations, then, with T[ as the temperature of boiling liquor in the first effect and 7 2 that in the second effect ... 

A salt solution at 293 K is fed at the rate of 6.3 kg/s to a forward-feed triple-effect evaporator and is concentrated from 2 per cent to 10 per cent of solids. Saturated steam at 170 kN/m2 is introduced into the calandria of the first effect and a pressure of 34 kN/m2 is maintained in the last effect. If the heat transfer coefficients in the three effects are 1.7, 1.4 and 1.1 kW/m2K respectively and the specific heat capacity of the liquid is approximately 4 kJ/kgK, what area is required if each effect is identical Condensate may be assumed to leave at the vapour temperature at each stage, and the effects of boiling point rise may be neglected. The latent heat of vaporisation may be taken as constant throughout. 

A value of the boiling liquor temperature T[ = 346 K obtained in (b) by heat balance must take into account the effects of hydrostatic head and of boiling-point rise. The true boiling-point rise is (346 — 341) = 5 deg K. 

---