PTT 205 HEAT AND MASS TRANSFER EVAPORATOR By

PTT 205 HEAT AND MASS TRANSFER EVAPORATOR By : Mrs. Noor Amirah Abdul Halim

TOPIC OUTLINE � Introduction � Types of evaporators � Processing Factors � Operation methods of evaporators � Overall heat transfer coefficient in evaporators � Calculations in single effect evaporator � Evaporation of biological materials

INTRODUCTION Purpose of evaporation : To concentrate solution by removing the vapor from a boiling liquid solution. Major cases : evaporation refers to the removal of water from an aqueous solution. In these cases the concentrated solution is the desired product and the evaporated water is normally discarded. Example: concentration of aqueous solutions of sugar, sodium chloride, sodium hydroxide, glycerol, glue, milk, and orange juice. Few cases : water, which contains a small amount of minerals, is evaporated to give a solids-free water to be used as boiler feed, for special chemical processes. Example : evaporation processes to evaporate seawater to provide drinking water.

TYPES OF EVAPORATOS Open kettle or pan: � � Simplest form of evaporators – heat is supplied by condensation of steam in a jacket or in coils immersed in the liquid In some cases paddles and scrapers for agitation are used Advantages – Inexpensive, simple to operate Disadvantages- Poor heat economy

TYPES OF EVAPORATOS Horizontal tube evaporator: � � � The horizontal bundle of heating tubes is similar to the bundle of tubes in a heat exchanger. The steam enters the tube, condense and leaves at the end of tube. The boiling solution covers the tube and the vapor leaves the liquid surface, goes through the baffle to prevent carry over of liquid droplets. Used for non-viscous liquids having high heat-transfer coefficients and liquids that do not deposit scales Advantage- relatively cheap Disadvantage - poor liquid circulation (and therefore unsuitable for viscous liquids)

Horizontal tube evaporator:

TYPES OF EVAPORATOS Short tube- vertical type evaporator � � Vertical of tubes are used where the liquid is inside the tubes and the steam condenses outside the tubes Not suitable for viscous liquid

TYPES OF EVAPORATOS Long tube- vertical type evaporator � � Heat transfer coefficient on the steam side is very high compared to the evaporating-liquid side – thus, high liquid velocities are desirable. The liquid is inside the tubes (3 -10 m long) The formation of vapor bubbles inside the tubes causes a pumping action which gives quite high liquid velocities Widely used in producing condensed milk

TYPES OF EVAPORATOS Falling-film type evaporator � � � A long tube-type evaporator Liquid is fed to the top of tubes and flows down the walls as a thin film. Vapor-liquid separation usually takes place at the bottom. Has small holdup time (5 -10 s or more) and high heat transfer coefficients Widely used for concentrating heat-sensitive materials such as fruit juices

TYPES OF EVAPORATOS Forced-circulation type evaporator � � The heat transfer coefficient can be increased by pumping to cause forced circulation of the liquid inside the tubes. This could de done in the long vertical or horizontal tubes -type evaporator by adding a pipe connection shown with a pump between the outlet concentrate line and the feed line However, the vertical tubes used are usually shorter than in the long-tube type evaporator Very suitable for viscous liquids.

TYPES OF EVAPORATOS Agitated-film evaporator � � � Agitators used to agitate the liquid to induce turbulence in film and hence, the heat transfer coefficient will be increase. This is done in modified falling film evaporator with only single, large, jacketed tube containing an internal agitator. Liquid enters the top of the tube and as it flows downwards, it is spread out into a turbulent film by the vertical agitator blades. Concentrated solution leaves at the bottom and vapor leaves out the top. Suitable for highly viscous and heat-sensitive material- e. g. rubber latex, gelatin, antibiotics and fruit juices Disadvantage-high cost and small capacity

AGITATED-FILM EVAPORATOR

PROCESSING FACTORS v Concentration in the liquid Ø Low viscosity: high heat transfer coefficient Ø High viscosity: low transfer coefficient Ø Adequate circulation and/or turbulence must be present to keep the coefficients from becoming too low v Solubility Ø Solubility increases with temperature Ø Crystallization may occur when a hot concentrated solution is cooled to room temperature v Temperature sensitivity of materials Ø Food and biological materials may be temperature sensitive and degrade at higher temperature or after prolonged heating.

v Foaming or frothing Ø Food solution such as skim milk and some fatty-acid solution form a foam or froth during boiling. Ø The foam is carried away along with vapor leaving the evaporator, thus losses might occur. v Pressure and temperature Ø High operating pressure: high boiling point Ø As the concentration of the dissolved material in solution increases by evaporation, the temperature of boiling may rise Ø To keep the temperatures low in heat-sensitive materials : operate under atmospheric pressure (under vacuum). v Scale deposition and materials of construction Ø Some solutions deposit solid materials called scale on the heating surfaces (fouling) Ø May cause a drastic decrease of the heat-transfer coefficient (U) and the evaporator must be cleaned. Ø The materials used in construction of the evaporator should be chosen to minimize corrosion.

OPERATIONS METHODS OF EVAPORATORS Single-effect Evaporator

Single-effect Evaporator q Based on figure: � The feed enters at TF Saturated steam at TS enters the heat- exchange section. Condensed steam leaves as condensate or drips. The solution in the evaporator is assumed to be completely mixed Hence, the concentrated product and the solution in the evaporator have the same composition. Temperature T 1 is the boiling point of the solution. The temperature of the vapor is also T 1, since it is in equilibrium with the boiling solution. The pressure is P 1, which is the vapor pressure of the solution at T 1. q q q A single-effect evaporator is wasteful of energy because the latent heat of the vapor leaving is not used but is discarded. It is simple but utilizes steam ineffectively. Single-effect evaporators are often used when the required capacity of operation is relatively small and/or the cost of steam is relatively cheap compared to the evaporator cost.

Multiple-effect Evaporator q Increasing the evaporation rate by using a series of evaporators between the steam supply and condenser. q The latent heat, can be recovered and reused (effective steam utilization) q For large capacity operations q Types of multiple-effect evaporator; Forward-feed multiple effect evaporator Ø The fresh feed is added to the first effect and flows to the next in the same direction as the vapor flow. Ø Used when the feed is hot or when the final concentrated product might be damaged at high temperatures. Ø The boiling temperature decrease from effect to effect. Ø If the first effect is at P 1 = 1 atm, so the last effect will be under vacuum.

Multiple-effect Evaporator

Multiple-effect Evaporator Backward-feed multiple effect evaporator Ø The fresh feed enters the last and coldest effect and continues on until the concentrated product leaves the first effect. Ø Used when the fresh feed is cold Ø However, liquid pumps must be used in each effect, since the flow is from low to high pressure. Ø This reverse-feed method is also used when the concentrated product is highly viscous. Ø Temperature increase from effect to effect Ø The high temperatures in the early effects reduce the viscosity and give reasonable heat-transfer coefficients.

Multiple-effect Evaporator

Multiple-effect Evaporator Parallel-feed multiple effect evaporator Ø Involves the adding of fresh feed and withdrawal of concentrated product from each effect. Ø The vapor from each effect is still used to heat the next effect. Ø This method of operation is mainly used when the feed is almost saturated and solid crystals are the product, as in the evaporation of brine to make salt.

OVERALL HEAT TRANSFER COEFFICIENT IN EVAPORATORS The overall heat-transfer coefficient, U in an evaporator considers on : q The steam-side condensing coefficient, which has a value of about 5700 W/m 2. K. q The metal wall, which has a high thermal conductivity and usually a negligible resistance q The resistance of the scale on the liquid side; and the liquid film coefficient, which is usually inside the tubes.

CALCULATIONS IN SINGLE EFFECT EVAPORATOR The calculation covers on: � Heat and Material Balances for Evaporators � Effects of Processing Variables on Evaporator Operation � Boiling-Point Rise of Solutions � Enthalpy - Concentration Charts of Solutions

Additional information: � λ = latent heat of steam (obtained from the steam table in appendix at Ts (saturated steam temperature) � An approximation for the latent heat of evaporation of 1 kg mass of water from aqueous solution can be obtained from steam table using temperature of the boiling solution, T 1. � If the heat capacity of the liquid feed (Cp. F) and the product (Cp. L) are known, they can be used to calculate the enthalpies.

EXERCISE A continuous single-effect evaporator concentrates 9072 kg/h of a 1. 0 wt % salt solution entering at 38ºC to a final concentration of 1. 5 wt %. The vapor space of the evaporator is at 101. 325 k. Pa (1. 0 atm abs) and the steam supplied is saturated at 150 k. Pa. The overall coefficient U = 1704 W/m 2. K. Calculate the amounts of vapor and liquid products and the heat-transfer area required. Assumed that, since it its dilute, the solution has the same boiling point as water. (Assume the heat capacity of the feed , Cp. F = 4. 19 k. J/kg. K) (The properties of steam can be obtained from the steam table

Effect of Processing Variables on Evaporators Effect of feed Temperature (TF) � The inlet temperature of the feed has a large effect on the evaporator operation. � When feed is not at its boiling point, steam is needed first to heat the feed to its boiling point and then to evaporate it. � Thus, feed must be at temperature greater or equal to the boiling point of the solution to improve the efficiency of evaporator � Preheating the feed can reduce the size of evaporator heat-transfer area. Effect of Pressure (P) � Pressure in the evaporator determine the boiling point of the solution (T 1). � Steam pressure determine the steam temperature (Ts). � Since q = U A (Ts – T 1), larger values of (Ts – T 1) is desirable since as (Ts – T 1) increases, the heating surface, A and cost of the evaporator decrease.

Effect of Processing Variables on Evaporators � Thus to obtain larger values of (Ts – T 1), lower T 1 is needed. � To obtain, lower T 1 the pressure in the evaporator can be reduced by operating under vacuum using a vacuum pump. Effect of Steam Pressure (Ps) � Another alternative to obtain larger values of (Ts – T 1), is using higher Ts � To obtain, higher Ts , high pressure steam can be used. � However, high pressure steam is more costly as well as often being more valuable as a source of power elsewhere. � Therefore, overall economic balances must be considered to determine the optimum steam pressure.

Boling-Point Rise (BPR) of Solution � In the majority of cases in evaporation, the solutions are not assumed to be dilute enough to be considered to have the same thermal properties as water. (as in exercise 1) � In most cases, thermal properties (heat capacity and the boiling point) of the solution being evaporated may differ considerably from those of water. � For strong solutions of dissolved solutes the boiling-point rise due to the solutes in the solution usually cannot be predicted. � � However, a useful empirical law known as Duhring’s rule can be applied. According to this rule, a straight line is obtained if the boiling point of a solution in °C or °F is plotted against the boiling point of pure water at the same pressure for a given concentration at different pressures.

Example: Duhring plot for boiling point of Na. OH

EXAMPLE 1 As an example of use of the chart, the pressure in an evaporator is given as 25. 6 k. Pa (3. 72 psia) and a solution of 30% Na. OH is being boiled. Determine the boiling temperature of the Na. OH solution and the boiling-point rise BPR of the solution over that of water at the same pressure. Solution: From the steam tables , the boiling point of water at 25. 6 k. Pa is 65. 9 °C. From the chart, for 65. 9 °C (150 °F) and 30% Na. OH, the boiling point of the Na. OH solution is 80 °C (175 °F). The boiling-point rise is 80 - 65. 9 = 14. 1 °C (57. 38°F).

Enthalpy-Concentration Chart of Solutions If the heat of solution of the aqueous solution being concentrated in the evaporator is large, neglecting it could cause errors in the heat balances. Should consider the heat-of-solution phenomenon. If pellets of Na. OH are dissolved in a given amount of water, it is found that a considerable temperature rise occurs; that is, heat is evolved, called heat of solution.

Example:

EXAMPLE 2 An evaporator is used to concentrate 4536 kg/h of a 20% Na. OH solution entering at 60ºC to a product of 50% solids. The pressure of the saturated steam used is 170 k. Pa and the vapor space pressure of the evaporator is at 12 k. Pa. The overall coefficient U is 1560 W/m 2. K. Calculate the steam used, the steam economy (in kg vapourized / kg steam used) and the heating surface area.

Solution:

EVAPORATION OF BIOLOGICAL MATERIALS � The evaporation of biological materials (e. g pharmaceuticals, milk, citrus juices, vegetable extracts) frequently differs from the evaporation of inorganic materials (e. g Na. Cl, Na. OH) and organic materials (e. g Ethanol, acetic acid). � Biological materials are heat sensitive, contain fine particles of suspended matter in solution and the equipments are designed for easy cleaning due to bacterial growth problem. Degradation of biological material is a function of the temperature and length of time. To keep temperature low, evaporation must be done under vacuum, which reduces the boiling point of the solution. To keep the time of contact low, equipment must provide for a low contact time for the material being evaporated.

Typical types of evaporator used for biological materials: 1. 2. 3. 4. Long-tube vertical evaporator: condensed milk Falling film evaporator: fruit juices Agitated-film evaporator: rubber latex, gelatin, antibiotics, fruit juices Heat-pump cycle evaporator: fruit juices, milk, pharmaceuticals

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