SINGLE EFFECT EVAPORATOR AIM: To determine, the overall heat transfer co-efficient, capacity and steam economy of a single effect evaporator. THEORY: When one of the components in the solution is non volatile, the principle of evaporator is used to separate the volatile component from non volatile component. During evaporation process, concentration of the non-volatile component increases. The evaporator provides the necessary heat transfer interface to boil off the volatile component. Assuming that the heat losses to the surroundings are negligible compared to the extent heat transfer in the evaporator, the heat transfer coefficient can be established by setting up simple material balance and energy balance equations. Applying mass balance around the evaporator, Overall material balance: M f M V M p
(1)
Where Mf = Mass flow rate of feed solution into the evaporator Mv = Mass flow rate of vapor out of the vapor space of the evaporator Mp = Mass flow rate of product (concentrated solution) out of the evaporator Component balance NaOH mass balance M f x f
M V xv M p x p
(2)
where xf = Concentration of NaOH in the feed solution xv = Concentration of NaOH in the vapor = 0 (since vapor is pure water) xp = Concentration of NaOH in the Product solution Water mass balance M f (1 x f ) M V (1 xv ) M p (1 x p )
2. Energy balance, Q M f H f M s s Where,
M v v M f H s M p H p Qo
(3)
(4)
Q = Overall heat transfer rate in the evaporator Qo = Heat loss to the surroundings = negligible Mf = Mass flow rate of feed solution into the evaporator Mv = Mass flow rate of vapor out of the vapor space of the evaporator Mp = Mass flow rate of product (concentrated solution) out of the Evaporator. Ms = Mass flow rate of steam to the steam chest of the evaporator
Hf = specific Enthalpy of feed solution Hp = Specific Enthalpy of product solution Hs = Specific sensible enthalpy of feed solution associated with solution at boiling point ( Solution would be heated from room temperature to boiling temperature before boiling) λ s = Latent heat of steam λ v = Latent heat of vapor The enthalpies of vapor, product and feed respectively collected from data at corresponding temperature of streams (KJ/Kg), latent heats of steam and vapor can be obtained from steam tables CALICULATION OF OVERALL HEAT TRANSFER CO-EFFICIENT: Q U o Ao T
(5)
Q = Overall heat transfer rate of the evaporator Ao = Overall Heat transfer surface area of the total number of tubes (in the steam chest) Uo= Overall heat transfer coefficient of the evaporator based on total outside surface area of the tubes T T s T b =over all temperature drop across steam and vapor in the vapor space Ts = Steam condensing temperature= saturation temperature of steam Tb = Boiling temperature of solution Q (6) U 0 Ao T where, Ao
Do LN t
Ao = Overall Heat transfer surface area of the total number of tubes (in the steam chest) Do= Outside diameter of a tube in the steam chest L = Length of a tube in the steam chest Nt = Total number of tubes in the steam chest Evaporation capacity M v = mass rate of vapor generated = ………..kg/s Steam economy = Mass rate of vapor generated / Mass rate of steam consumed = M v M s
……… V/ ms
EQUIPMENT SET UP:
(7)
The equipment consists of an evaporator of 75mm ID. It consists of 12.5mm OD tubes of 750mm long, 13 in number. There is a external down comer of 50mm OD (2”). The product is collected with an inverted U module so that the level is self controlled. The feed is regulated by the feed valve with a rotameter. The feed solution is pumped to the evaporator with a ss 316 gear pump. The product and the vapor condensate are separately collected in their respective tanks. The condenser has 12.5 mm OD tubes of 500mm long 19in number. The entire setup is provided with a strong square section support. EXPERIMENTAL PROCEDURE: 1) Prepare 20 lits. Of 0.01N NaOH solution and standerdize with standard oxalic acid solution and fill the feed tank with the 0.01N NaOH solution. Keep the feed valve closed and open the by pass valve. Start the pump to give good recirculation so that the solution gets well mixed. 2) Fill in water into the boiler tank up to ¾. Switch on the heaters. Wait until the boiler pressure is 30 psi. 3) Partially open the feed valve and fix up some feed flow rate of the solution and maintain it at that flow rate throughout. Keep the product valve closed, so that the level in the evaporator is built up. 4) Slowly open steam outlet valve of the boiler and maintain constant steam pressure (2 – 3 psi) in the steam chest of the heat exchange section of the evaporator. Steam gets condensed by giving away the latent heat to the solution. Condensate gets collected in the condensate collection tank at saturation temperature. Now watch for the beginning of boiling in the evaporator as indicated by the dripping of condensate droplets out of the vapor condenser. Start the cooling water circulation by opening the inlet valve to the condenser and fix some inlet water flow rate and maintain it. 5) When steady state conditions are reached by observation of constant level in the evaporator, constant solution feed flow rate indicated by the feed solution rotameter, steady flow rate of both the top vapor condensate and steam condensate, steady flow of cooling water as indicated by water rotameter, and when all stream temperatures are steady note down the following. a) Flow rate of feed solution. b) Flow rate of top vapor condensate c) Steam condensate flow rate d) Temperature of all the streams (inlet temperature of steam to the evaporator, vapor space temperature, Inlet temperature of the feed solution, vapor temperature at inlet to the condenser, inlet, outlet cooling water temperatures of the condenser, e) Product rate (by actually collecting sample for known time)
At steady state it was assumed that the vapor stream leaving, product stream leaving the evaporator are in thermal equilibrium, ie temperature of vapor leaving the evaporator T v = temperature of the product leaving the evaporator T p = bioling temperature of solution Tb 6) Procedure for estimation of top vapor condensate flow rate: At steady state collect the condensate of vapor dripping out of the condenser in to a measuring jar for 10 min. Measure the temperature of the vapor condensate. At that temperature evaluate the density of water from property data hand book. Estimate volumetric flow rate of vapor condensate and Mass flow rate of vapor condensate. 7) Procedure for estimation of steam condensate flow rate: At steady state note down the time required for a rise in 1 cm level in the condensate collection tank. Estimate the density of condensate at saturation temperature of steam.(Steam is assumed to condense at saturation temperature) Estimate the volumetric flow rate of condensate into the tank by taking condensate tank dimensions and mass flow rate. 9)
Repeat the above procedures for different flow rates of feed solution. 10) Stop the heater, stop the pump, stop the cooling water circulation, drain all the solution from the evaporator, the feed tank and clean the evaporator by pumping fresh water into the evaporator system and drain it. 11) Analyze product solutions for NaOH concentration by titrating it against standard oxalic acid solution.
Note/Warning :
During the operation of the condenser, always see that the steam pressure inside the boiler shall not drop below 10 psi, and the water level in the boiler shall not drop below the level of immersion heaters.
OBSERVATIONS: Specifications of Evaporator: Diameter of evaporator = .095 m OD = .0125m Length L =.75m No of tubes, nt = 13 Vapor disengaging height, Lv = 100 mm Weight of NaOH taken = Amount of feed 20l Initial concentration of feed, xf = 0.01N (prepared, but to confirmed by titration) Normality of standard oxalic acid = N(use around 0.01N)
Sl Steam No. pressure Psi 2 Kgf/cm
Temp Of Steam, o Ts, C
Feed Volume of Rate product for LPm
10 min, Vp
P Cc/min
Volume of top condensate for V Cc/min
Volume of steam condensate, for
10 Time/1cm min raise Vsc Vsc
Temp at various o pts T, C 1 2 3 4 5 6
C
1 2 3 4
Sl. Titre reading T1 ml No. with feed solution
Titre reading T2 ml with product solution
Feed concentration xf
Product concentration, xp
N
N
g/l
Wt Frn
g/l
1 2 3 4
CALCULATIONS: 1. Calculate mass flow rate of product by using equations 1,2 &3. 2. Estimate overall heat transfer rate in the evaporator using eq. (4) 3. Estimate overall heat transfer co-efficient based on outside surface area of the tubes using eq.(6) 4. Estimate steam economy using eq (7)
RESULTS: Final concentration of NaOH = Over all heat transfer co-efficient of the evaporator Uo = Evaporator Capacity M v =………….. kg/s Steam economy =
M v M s
=………
2o
W/ m C
Wt Frn
