Types of reactor

Types of reactor

• The confines in which chemical reactions occur are called reactors.
• In chemical engineering chemical reaction or set of reactions is carried out in a reaction vessel that is in chemical reactors.
• All chemical processes are centered in a chemical reactor.
• The design of a chemical reactor is the most important factor in determining the overall process economics.
• While doing any operation we have to first decide what reactor type and reactor shape to select and whether it would be advantageous to operate in batch or continuous mode.
• Ideal reactors have three ideal flow or contacting patterns. They are main basic type of chemical reactor

1. Batch reactor
2. Flow reactor

• Continuous Stirred-Tank Reactor(CSTR)
• Plug Flow Reactor (PFR)

Batch reactor

• Whether the system does not exchanges mass with its surroundings, then the system is called a batch reactor.
• The batch reactor is simply a container with an agitator and an integral heating/cooling system.

• The reactants are initially charged into a container, are well mixed, and are left to react for a certain period.
• The resultant product mixture is then discharged.
• This is an unsteady-state operation where composition changes with time; however, at any instant the composition throughout the reactor is uniform
• During the operation, no addition or withdrawal is made.
• The experimental batch reactor is usually operated isothermally and at constant volume because it is easy to interpret the results of such runs.
• This reactor is a relatively simple device adaptable to small-scale laboratory set-ups, and it needs but little auxiliary equipment or instrumentation.
• Thus, it is used whenever possible for obtaining homogeneous kinetic data.

Advantages

• Suitable for small scale production
• Suitable for processes where a range of different products or grades is to be produced in the same equipment
• Suitable for reactions requiring long reaction times
• Suitable for reactions with superior selectivity

Limitation

• Not suitable for large batch sizes.
• It is a closed system in which once the reactants are added in the reactor, they will come out as products only after the completion of the reaction.

Application

• Batch processes are used in chemical (inks, dyes, polymers) and food industry.

Continuous Stirred-Tank Reactor (CSTR)

• The flow reactor is used primarily in the study of the kinetics of heterogeneous reactions.
• The first of the two ideal steady-state flow reactor is called the mixed reactor, the backmix reactor, the ideal stirred tank reactor, CSTR, or the CFSTR (constant flow stirred tank reactor)
• As its names suggest, it is a reactor in which the contents are well stirred and uniform throughout.
• In CSTR the reactants are fed to the reactor and the products or byproducts are withdrawn in between while the reaction is still progressing.

• The CSTR is normally run at steady-state and is usually operated so as to be quite well mixed.
• This type of flow is mixed flow, hence this reactor also called the mixed flow reactor or MFR.
• The exit stream from this reactor has the same composition and temperature as the fluid within the reactor.
• Continuous reactors are usually preferred for large scale production.

Advantages

• Highly flexible device
• By products may be removed in between the reaction.
• It is economically beneficial to operate several CSTRs in series or in parallel.
• Reaction can be carried out in horizontal as well as vertical reactors.

Limitation

• More complex and expensive than tubular units.
• All calculations performed with CSTRs assume perfect mixing.
• At steady state, the flow rate in must equal the flow rate out, otherwise the tank will overflow or go empty.

Application

• Chemical industry especially involving liquid/gas reactions.

Plug Flow Reactor (PFR)

• The other ideal steady-state flow reactors are variously known as the plug flow, slug flow, piston flow, ideal tubular, and unmixed flow reactor.
• It consists of a cylindrical pipe and is normally operated at a steady-state, as is the CSTR.

• This reactor is known as the plug flow reactor, or PFR because the flow in the reactor is highly turbulent and the flow field may be modeled by that of plug flow.
• It is characterized by the fact that the flow of fluid through the reactor is orderly with no element of fluid overtaking or mixing with any other element ahead or behind.
• Actually, there may be lateral mixing of fluid in a plug flow reactor; however, there must be no mixing or diffusion along the flow path.
• Hence in the tubular reactor, the reactants are continually consumed as they flow down the length of the reactor.
• The necessary and sufficient condition for plug flow is for the residence time in the reactor to be the same for all elements of fluid.

Advantage

• Higher efficiency than a CSTR of the same volume
• PFRs may have several pipes or tubes in parallel
• Both horizontal and vertical operations are common
• They can be jacketed
• Reagents may be introduced at locations even other then inlet

Limitation

• Not economical for small batches

Application

• The tubular reactor is especially suited to cases needing considerable heat transfer, where high pressures and very high or very low temperatures occur

AFFINITY LAWS OF PUMP

The Affinity Laws of pump

• The Affinity Laws of centrifugal pumps or fans indicates the influence on volume capacity, head (pressure) and/or power consumption of a pump or fan due to

1. change in speed of wheel - revolutions per minute (rpm)
2. geometrically similarity - change in impeller diameter

• Hence, the Affinity Laws are mathematical expressions that define changes in pump capacity,head, and BHP when a change is made to pump speed, impeller diameter, or both.
• Affinity laws are useful when an existing pump must be modified ti give a higher or lower head or different capacity
• The Affinity Laws are valid only under conditions of constant efficiency.

According to Affinity Laws:Capacity, Q: changes in direct proportion to impeller diameter D ratio, or to speed N ratio:

Head, H: changes in direct proportion to the square of impeller diameter D ratio, or the square of speed N ratio:

BHP: changes in direct proportion to the cube of impeller diameter ratio, or the cube of speed ratio:

• Where the subscript: 1 refers to initial condition, 2 refer to new condition
• If changes are made to both impeller diameter and pump speed the equations can be combined to:

POWER AND EFFICIENCY OF CENTRIFUGAL PUMP

Power and efficiency of centrifugal pump
Brake Horse Power (BHP) and Water horsepower (WHP)

• The work performed by a pump is a function of the total head and the weight of the liquid pumped in a given time period.

Pump input or brake horsepower (BHP) is the actual horsepower delivered to the pump shaft.

Pump output or hydraulic or water horsepower (WHP) is the liquid horsepower delivered by the pump.

• These two terms are defined by the following formulas.

where

• Q = Capacity in gallons per minute (GPM)
• HT = Total differential head, ft
• Specific gravity= Specific gravity of liquid
• Efficiency = Pump efficiency, %

• The constant 3960 is obtained by dividing the number or foot-pounds for one horsepower(33,000) by the weight of one gallon of water (8.33 pounds). 

The brake horsepower or input to a pump is greater than the hydraulic horsepower or output due to the mechanical and hydraulic losses incurred in the pump.

• Therefore the pump efficiency is the ratio of these two values.

Best Efficiency Point (BEP)

• The Head, NPSHr, efficiency, and BHP all vary with flow rate, Q.

Best Efficiency Point (BEP) is the capacity at maximum impeller diameter at which the efficiency is highest.

Significance of BEP

• BEP as a measure of optimum energy conversion
• When sizing and selecting centrifugal pumps for a given application the pump efficiency at design should be taken into consideration.
• The efficiency of centrifugal pumps is stated as a percentage and represents a unit of measure describing the change of centrifugal force (expressed as the velocity of the fluid) into pressure energy.
• The B.E.P.(best efficiency point) is the area on the curve where the change of velocity energy into pressure energy at a given gallon per minute is optimum; in essence, the point where the pump is most efficient.
• The operation of a centrifugal pump should not be outside the furthest left or right efficiency curves published by the manufacturer.
• Performance in these areas induces premature bearing and mechanical seal failures due to shaft deflection, and an increase in temperature of the process fluid in the pump casing causing seizure of close tolerance parts and cavitation.
• BEP is an important parameter in that many parametric calculations such as specific speed, suction specific speed, hydrodynamic size, viscosity correction, head rise to shut-off, etc. are based on capacity at BEP.
• Many users prefer that pumps operate within 80%to 110% of BEP for optimum performance.

Specific Speed

Specific speed as a measure of the geometric similarity of pumps

• Specific speed (Ns) is a non-dimensional design index that identifies the geometric similarity of pumps.
• It is used to classify pump impellers as to their type and proportions.
• Pumps of the same Ns but of different size are considered to be geometrically similar,one pump being a size- factor of the other.

Specific speed Calculation

• The following formula is used to determine specific speed:

where

• Q = Capacity at best efficiency point(BEP) at maximum impeller diameter, GPM(gallons per meter)
• H = Head per stage at BEP at maximum impeller diameter, ft
• N = pump speed, RPM

• As per the above formula, it is defined as the speed in revolutions per minute at which a geometrically similar impeller would operate if it were of such a size as to deliver one gallon per minute flow against one-foot head.
• Specific speed should be thought of only as an index used to predict certain pump characteristics.
• The specific speed determines the general shape or class of the impellers.

NET POSITIVE SUCTION HEAD

Net Positive Suction Head (NPSH)

• Power supplied to the pump depend on the difference in the pressure between discharge and suction and is independent of the pressure level.
• There is no effect on pump when suction pressure is below atmospheric pressure or well above it, as long as the fluid remains liquid.
• But if suction pressure is only slightly greater than the vapor pressure, some liquid may flash to vapor inside to pump, a process called cavitation, which greatly reduces the pump capacity.
• If the suction pressure is actually less than the vapor pressure, there will be vaporization in the suction line, and no liquid can be drawn into the pump.
• To avoid cavitation, the pressure at the pump inlet must exceed the vapor pressure by certain value called net positive suction head (NPSH)

Hence, Net Positive Suction Head or NPSH for pumps can be defined as the difference between liquid pressure at pump suction and liquid vapor pressure, expressed in terms of height of liquid column.

• Net positive suction head (NPSH) may refer to one of two quantities in the analysis of cavitation.

1. The Available NPSH (NPSHA): A measure of how close the fluid at a given point is to flashing, and so to cavitation.
2. The Required NPSH (NPSHR): The head value at a specific point (e.g. the inlet of a pump) required to keep the fluid from cavitating.

Net Positive Suction Head Required, NPSHr

• Pumps can pump only liquids, not vapors
• The satisfactory operation of a pump requires that vaporization of the liquid being pumped does not occur at any condition of operation.
• This is so desired because when a liquid vaporizes its volume increases very much.
• For example, 1 ft3 of water at room temperature becomes 1700 ft3 of vapor at the same temperature.
• This makes it clear that if we are to pump a fluid effectively, it must be kept always in the liquid form.
• Rise in temperature and fall in pressure induces vaporization
• The vaporization begins when the vapor pressure of the liquid at the operating temperature equals the external system pressure, which, in an open system is always equal to atmospheric pressure.
• Any decrease in external pressure or rise in operating temperature can induce vaporization and the pump stops pumping.
• Thus, the pump always needs to have a sufficient amount of suction head present to prevent this vaporization at the lowest pressure point in the pump.
• NPSH as a measure to prevent liquid vaporization
• NPSH required is a function of the pump design and is determined based on actual pump test by the vendor.
• NPSHr increases as capacity increases
• The NPSH required varies with speed and capacity within any particular pump.
• The NPSH required increase as the capacity is increasing because the velocity of the liquid is increasing, and as anytime the velocity of a liquid goes up, the pressure or head comes down.
• The NPSH is independent of the fluid density as are all head terms.

Net Positive Suction Head available, NPSHa

• Net Positive Suction Head Available is a function of the system in which the pump operates.
• It is the excess pressure of the liquid in feet absolute over its vapor pressure as it arrives at the pump suction, to be sure that the pump selected does not cavitate.
• It is calculated based on system or process conditions.
• The formula for calculating the NPSHa is as below

where

hps= pressure head i. e. absolute pressure at surface of reservoir converted into head
hs= static suction head
hvps= vapor pressure head
hfs= friction head 

Significance of NPSHr and NPSHa

• The NPSH available must always be greater than the NPSH required for the pump to operate properly.

PERFORMANCE PARAMETER OF CENTRIFUGAL PUMP

Performance parameter of centrifugal pump
The key performance parameters of centrifugal pumps are 

1. Capacity
2. Head
3. BHP(Brake horse power)
4. BEP (Best efficiency point)
5. Specific speed

Capacity

Capacity means the flow rate with which liquid is moved or pushed by the pump to the desired point in the process. 

• It is commonly measured in either gallons per minute(gpm) or cubic meters per hour (m3 /hr). 
• The capacity usually changes with the changes in operation of the process. 
• The capacity depends on a number of factors like: 

1. Process liquid characteristics i.e. density, viscosity 
2. Size of the pump and its inlet and outlet sections 
3. Impeller size 
4. Impeller rotational speed RPM 
5. Size and shape of cavities between the vanes 
6. Pump suction and discharge temperature and pressure conditions

• The effect on the flow through a pump by changing the outlet pressures is graphed on a pump curve.
• As liquids are essentially incompressible, the capacity is directly related with the velocity of flow in the suction pipe. 
• This relationship is as follows:

where

• Q= capacity
• V = velocity of flow
• A = area of pipe

Head

The pressure at any point in a vertical column of the liquid can be caused due to its weight. The height of this column is called the static head and is expressed in terms of feet of liquid. 

• The same head term is used to measure the kinetic energy created by the pump.

In other words, head is a measurement of the height of a liquid column that the pump could create from the kinetic energy imparted to the liquid. 

• The head is not equivalent to pressure. 
• Head is a term that has units of a length or feet and pressure has units of force per unit area or pound per square inch. 
• The main reason for using head instead of pressure to measure a centrifugal pumps energy is that the pressure from a pump will change if the specific gravity (weight) of the liquid changes, but the head will not change. 
• Since any given centrifugal pump can move a lot of different fluids, with different specific gravities, it is simpler to discuss the pumps head and forget about the pressure.
• A given pump with a given impeller diameter and speed will raise a liquid to a certain height regardless weight of the liquid. 

Pressure to Head Conversion formula 

• The static head corresponding to any specific pressure is dependent upon the weight of the liquid according to the following formula

There are different type of head, which are as follows


1. Static Suction Head (hS ) :

Head resulting from elevation of the liquid relative to the pump centerline is called static suction head. 

• If the liquid level is above pump centerline, hS is positive. 
• If the liquid level is below pump centerline, hS is negative. 
• Negative hS condition is commonly denoted as a “suction lift” condition 

2. Static Discharge Head (hd):

It is the vertical distance in feet between the pump centerline and the point of free discharge or the surface of the liquid in the discharge tank.

3. Friction Head (hf):

This head required to overcome the resistance to flow in the pipe and fittings. 

• It is dependent upon the size, condition and type of pipe, number and type of pipe fittings, flow rate, and nature of the liquid.

3. Vapor Pressure Head (hvp):

• Vapor pressure is the pressure at which a liquid and its vapor co-exist in equilibrium at a given temperature. 

When the vapor pressure is converted to head, it is referred to as vapor pressure head, hvp. 

• The value of hvp of a liquid increases with the rising temperature and in effect, opposes the pressure on the liquid surface, the positive force that tends to cause liquid flow into the pump suction i.e. it reduces the suction pressure head.

4. Pressure Head (hp):

Pressure Head must be considered when a pumping system either begins or terminates in a tank which is under some pressure other than atmospheric. 

• The pressure in such a tank must first be converted to feet of liquid. 
• Denoted as hp, pressure head refers to absolute pressure on the surface of the liquid reservoir supplying the pump suction, converted to feet of head. 
• If the system is open, hp equals atmospheric pressure head.

5. Velocity Head (hv):

Velocity head refers to the energy of a liquid as a result of its motion at some velocity ‘v’. 

• It is the equivalent head in feet through which the water would have to fall to acquire the same velocity, or in other words, the head necessary to accelerate the water. 
• The velocity head is usually insignificant and can be ignored in most high head systems.
• However, it can be a large factor and must be considered in low head systems.

6. Total Suction Head (HS ):

• The suction reservoir pressure head (hpS ) plus the static suction head (hS ) plus the velocity head at the pump suction flange (hVS) minus the friction head in the suction line (hfS ). 

HS = hpS + hS + hvS – hfS 

The total suction head is the reading of the gauge on the suction flange, converted to feet of liquid.

7. Total Discharge Head (Hd):

• The discharge reservoir pressure head (hpd ) plus static discharge head (hd) plus the velocity head at the pump discharge flange (hvd ) plus the total friction head in the discharge line (hfd). 

Hd = hpd + hd + hvd + hfd 

The total discharge head is the reading of a gauge at the discharge flange, converted to feet of liquid. 

8. Total Differential Head (HT):

It is the total discharge head minus the total suction head  

HT = Hd + HS (with a suction lift) 

HT = Hd - HS (with a suction head)

CENTRIFUGAL PUMP

Centrifugal pump

• A centrifugal pump is one of the simplest pieces of equipment in any process plant.
• Centrifugal pumps are the most common type of dynamic pump or kinetic pump and are used most often in applications with the moderate-to-high flow and low head.
• These pumps all rely on the centrifugal force as the fundamental principle by which they operate.

Centrifugal pump increase the mechanical energy of the liquid by centrifugal action

Its purpose is to convert the energy of a prime mover (an electric motor or turbine) first into velocity or kinetic energy and then into pressure energy of a fluid that is being pumped. 

 

The energy changes occur by virtue of two main parts of the pump, the impeller, and the volute or diffuser. 

• The impeller is the rotating part that converts driver energy into the kinetic energy.
• The volute or diffuser is the stationary part that converts the kinetic energy into pressure energy.

Working of centrifugal pump

The process liquid enters the suction nozzle and then into the eye (center) of a revolving device known as an impeller. 

• When the impeller rotates, it spins the liquid sitting in the cavities between the vanes outward and provides centrifugal acceleration.
• As liquid leaves, the eye of the impeller a low-pressure area is created causing more liquid to flow toward the inlet. 
• Because the impeller blades are curved, the fluid is pushed in a tangential and radial direction by the centrifugal force. 
• The faster the impeller revolves or the bigger the impeller is, then the higher will be the velocity of the liquid at the vane tip and the greater the energy imparted to the liquid.

Components of Centrifugal Pumps

• A centrifugal pump has two main components: 

A rotating component comprised of an impeller and a shaft.

A stationary component comprised of a casing, casing cover, and bearings. 

Stationary Components 

1. Casing

• Casings are generally of two types: volute and circular. 
• The impellers are fitted inside the casings. 
• Volute casings build a higher head.
• Circular casings are used for low head and high capacity. 
• A volute is a curved funnel increasing in area to the discharge port. As the area of the cross-section increases, the volute reduces the speed of the liquid and increases the pressure of the liquid. 
• One of the main purposes of a volute casing is to help balance the hydraulic pressure on the shaft of the pump. However, this occurs best at the manufacturers recommended capacity. 
• The circular casing has stationary diffusion vanes surrounding the impeller periphery that converts velocity energy to pressure energy. 
• Conventionally, the diffusers are applied to multi-stage pumps.
• The casings can be designed either as solid casings or split casings. 
• Solid casing implies a design in which the entire casing including the discharge nozzle is all contained in one casting or fabricated piece. 
• A split casing implies two or more parts are fastened together. 
• When the casing parts are divided by horizontal plane, the casing is described as horizontally split or axially split casing. When the split is in a vertical plane perpendicular to the rotation axis, the casing is described as vertically split or radially split casing. Casing Wear rings act as the seal between the casing and the impeller.

2. Suction and Discharge Nozzle

• The suction and discharge nozzles are part of the casings itself. 

3. Seal Chamber and Stuffing Box

• Seal chamber and Stuffing box both refer to a chamber, either integral with or separate from the pump case housing that forms the region between the shaft and casing where sealing media are installed. 
• When the sealing is achieved by means of a mechanical seal, the chamber is commonly referred to as a Seal Chamber. 
• When the sealing is achieved by means of packing, the chamber is referred to as a Stuffing Box. 
• Both the seal chamber and the stuffing box have the primary function of protecting the pump against leakage at the point where the shaft passes out through the pump pressure casing. 
• When the pressure at the bottom of the chamber is below atmospheric, it prevents air leakage into the pump. 
• When the pressure is above atmospheric, the chambers prevent liquid leakage out of the pump. 
• The seal chambers and stuffing boxes are also provided with cooling or heating arrangement for proper temperature control. 

3. Bearing housing

• The bearing housing encloses the bearings mounted on the shaft. 
• The bearings keep the shaft or rotor in correct alignment with the stationary parts under the action of radial and transverse loads. 
• The bearing house also includes an oil reservoir for lubrication, constant level oiler, jacket for cooling by circulating cooling water.

Rotating Components

1. Impeller

The impeller is the main rotating part that provides the centrifugal acceleration to the fluid. 

• The number of impellers determines the number of stages of the pump. 
• A single-stage pump has one impeller only and is best for low head service. 
• A two-stage pump has two impellers in series for medium head service. 
• A multi-stage pump has three or more impellers in series for high head service. 

2. Shaft

• The basic purpose of a centrifugal pump shaft is to transmit the torques encountered when starting and during operation while supporting the impeller and other rotating parts. 
• It must do this job with a deflection less than the minimum clearance between the rotating and stationary parts.

• PERFORMANCE PARAMETER OF CENTRIFUGAL PUMP

1. CAPACITY
2. HEAD
3. BHP(Brake horsepower)
4. BEP (Best efficiency point)
5. SPECIFIC SPEED

• NET POSITIVE SUCTION HEAD

FICKS LAW OF DIFFUSION

Fick's law of diffusion

• Mass transfer is the natural tendency to transfer a given component (species) in a mixture from a region of high concentration to a region of low concentration to bring about a uniform or equilibrium condition. 
• The mass transfer has three requirements: 

1. that transfer occurs only in a mixture,
2. that at least one substance within the mixture moves from region of high concentration to a region of low concentration,
3. that the rate of mass transfer—i.e., the “flux” of a given substance—be proportional to the concentration gradient of that substance. 

Fick's  law

• In 1855 FICK proposed a relation between the flux of the diffusing substance and concentration gradient as the first law of diffusion which named as Fick's law of diffusion.

Fick’s law states that the flux of a diffusing component A in the z-direction in a binary mixture of A and B is proportional to the molar concentration gradient.

• Hence Fick's law of diffusion for component A in a binary mixture of A and B for steady-state diffusion is

           Where

• The negative sign in the equation indicates that diffusion occurs in the direction of decrease in concentration. Hence the term dCA/dz is -ve and flux become positive
• JA = molar flux of A in the z-direction [kmol/(m2.s)]
• CA = molar concentration of A [ kmol/ m3]
• dCA/dz = concentration gradient in the z-direction
• DAB = proportionality constant, diffusion coefficient for component A diffusing through B [m2/s]
• Z = distance in the direction of diffusion

COMPARISION OF SINGLE AND MULTIPLE EFFECT EVAPORATOR

Comparison of single  and multiple effect evaporator

Single-effect evaporators 

• Single-effect evaporators are used 

1. when the throughput is low, 
2. when a cheap supply of steam is available, 
3. when expensive materials of construction must be used as is the case with corrosive feedstocks 
4. when the vapour is so contaminated so that it cannot be reused. 

• The feed and saturated steam with temperature at TF and TS respectively enter 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 T1 is the boiling point of the solution.
• The temperature of the vapor is also T1, since it is in equilibrium with the boiling solution.
• The pressure is P1, which is the vapor pressure of the solution at T1
• If the solution to be evaporated is assumed to be dilute and like water, then 1 kg of steam condensing will evaporate approximately 1 kg of vapor (if the feed entering has TF near the boiling point) 
• 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.
• However, for large-capacity operation, using more than one effect will markedly reduce steam costs
• The heat requirements of single-effect continuous evaporators may be obtained from mass and energy balances. 

Multiple - effect evaporator

• The single effect evaporator uses rather more than 1 kg of steam to evaporate 1 kg of water. 
• The latent heat of the vapor leaving in single effect evaporator is not used but is discarded.
• Much of this latent heat, however, can be recovered and reused by employing a multiple - effect evaporator, that is, vapor from one effect serves as the heating medium for the next.
• The economy of the system, measured by the kilograms of water vaporized per kilogram of steam condensed, increases with the number of effects.
• in multiple effect evaporator, the pressure in each effect is lower than that of the effect to which it receives steam and higher than that of the effect to which it supplies vapors
• Each effect, in itself, act as a single effect evaporator, and each has a temperature drop across its heating surface corresponding to the pressure drop in that effect.

Forward-feed multiple - effect evaporator 

• A simplified diagram of a forward-feed triple- effect evaporation system is shown in Fig. 

• If the feed to the first effect is near the boiling point at the pressure in the first effect, 1 kg of steam will evaporate almost 1 kg of water.
• The first effect operates at a temperature that is high enough that the evaporated water serves as the heating medium to the second effect.
• Here, again, almost another kg of water is evaporated, which can then be used as the heating medium to the third effect.
• As a very rough approximation, almost 3 kg of water will be evaporated for 1 kg of steam in a three-effect evaporator.
• Hence, the steam economy, which is kg vapor evaporated/kg steam used, is increased.
• This also holds approximately more than three effects.
• However, the increased steam economy of a multiple-effect evaporator is gained at the expense of the original first cost of these evaporators
• In forward-feed operation as shown in Fig. fresh feed is added to the first effect and flows to the next in the same direction as the vapor flow.
• This method of operation is used when the feed is hot or when the final concentrated product might be damaged at high temperatures.
• The boiling temperatures decrease from effect to effect. This means that if the first effect is at P1 = 1 atm abs pressure, the last effect will be under vacuum at a pressure P3.
• The concentration of the liquid increases from the first effect to the last effects
• This pattern of liquid flow is the simplest
• It requires a pump for feeding dilute solution to the first effect, since this effect is often at about atmospheric pressure, and a pump to remove thick liquor from the last effect.
• The transfer from effect to effect, however, can be done without pumps, since the flow in the direction of decreasing pressure, and control valves in the transfer line all that is required.

Backward-feed multiple - effect evaporator 

• In the backward-feed operation shown in Fig. for a triple-effect evaporator, the fresh feed enters the last and coldest effect and continues on until the concentrated product leaves the first effect. 

• This method of reverse feed is advantageous when the fresh feed is cold since a smaller amount of liquid must be heated to the higher temperatures in the second and first effects.
• However, liquid pumps must be used in each effect, since the flow is from low to high pressure.
• This reverse-feed method is also used when the concentrated product is highly viscous.
• The high temperatures in the early effects reduce the viscosity and give reasonable heat-transfer coefficients.
• Backward feed often gives a higher capacity than forward feed when the thick liquor is viscous, but it may give a lower economy than forward feed when the feed liquor is cold.

Parallel-feed multiple-effect evaporators

• Parallel-feed in multiple-effect evaporators involves the adding of fresh feed and withdrawal of the concentrated products from each effect.
• The vapor from each effect is still used to heat the next effect.
• 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

MEASURES OF EVAPORATOR PERFORMANCE

There are three main measures of evaporator performance:

• Capacity (kg vaporized / time)
• Economy (kg vaporized / kg steam input)
• Steam Consumption (kg / hr)

The performance of a evaporator is evaluated by the capacity and the economy.

Economy

• Economy is the number of kg of water vaporized per kg of steam fed to the unit.
• The rate of heat transfer q through the heating surface of an evaporator, by the definition of overall heat transfer coefficient, is product of three factors

1. The area of heat transfer surface A
2. The overall heat transfer coefficient U
3. The overall temperature drop ΔT

Q = U * A * ΔT

• Economy calculations are determined using enthalpy balances.
• The key factor in determining the economy of an evaporator is the number of effects.
• The economy of a single effect evaporator is always less than 1.0. 
• Multiple effect evaporators have higher economy but lower capacity than single effect.
• The thermal condition of the evaporator feed has an important impact on economy and performance. 
• If the feed is not already at its boiling point, heat effects must be considered. 
• If the feed is cold (below boiling) some of the heat going into the evaporator must be used to raise the feed to boiling before evaporation can begin; this reduces the capacity.
• If the feed is above the boiling point, some flash evaporation occurs on entry.

Capacity

• Capacity is defined as the no of kilograms of water vaporized per hour.
• If the feed to the evaporator is at the boiling temperature corresponding to the absolute pressure in the vapor space, all the heat transferred through the heating surface is available for evaporation and the capacity is proportional to q.
• If the feed is cold, the heat required to heat it to its boiling point may be quite large and the capacity for a given value of q is reduced accordingly, as heat used to heat the feed is not available for evaporation.
• if the feed is at a temperature above the boiling point in the vapor space, a portion of the feed evaporates spontaneously by adiabatic equilibration with the vapor-space pressure and the capacity is greater than that corresponding to q. This process is called flash evaporation.

Steam consumption

• Steam consumption is very important to know, and can be estimated by the ratio of capacity divided by the economy. 
• That is the steam consumption (in kg/h) is

                                          Consumption = Capacity/Economy.

Heat transfer in evaporators

• The rate equation for heat transfer takes the form:

Q = U * A * ΔT

where:

1.  Q is the heat transferred per unit time
2. U is the overall coefficient of heat transfer
3. A is the heat transfer surface
4. T is the temperature difference between the two streams. 

• In applying this equation to evaporators, there may be some difficulty in deciding the correct value for the temperature difference because of what is known as the boiling point rise (BPR) or boiling point elevation (BPE)
• If water is boiled in an evaporator under a given pressure, then the temperature of the liquor may be determined from steam tables and the temperature difference is readily calculated. 
• At the same pressure, a solution has a boiling point greater than that of water, and the difference between its boiling point and that of water is the BPR or BPE. 
• For example, at atmospheric pressure (101.3 kN/m2 ), a 25 per cent solution of sodium chloride boils at 381 K and shows a BPR of 8 deg K. If steam at 389 K were used to concentrate the salt solution, the overall temperature difference would not be (389 − 373) = 16 deg K, but (389 − 381) = 8 deg K. Such solutions usually require more heat to vaporise unit mass of water, so that the reduction in capacity of a unit may be considerable. 
• The value of the BPR cannot be calculated from physical data of the liquor, though Duhring’s rule is often used to find the change in BPR with pressure. 
• Duhring’s rule states that the boiling point of given solution is a linear function of the boiling point of pure water at the same pressure.
• Thus, if the boiling point of the solution is plotted against that of water at the same pressure, then a straight line is obtained.
• Thus, if the pressure is fixed, the boiling point of water is found from steam tables, and the boiling point of the solution from Duhring’s plot.
• Different lines are obtained for different concentrations.
• The boiling point rise is much greater with strong electrolytes, such as salt and caustic soda.

TYPES OF EVAPORATOR

Types of Evaporator

Single effect evaporator

When a single evaporator is used, the vapor from the boiling liquid is condensed and discarded. This method is called single effect evaporation, and although it is simple, it utilizes steam ineffectively.

• The solution to be concentrated flows inside the tubes. 
• The heating medium is steam condensing on metal tubes. 
• Usually boiling liquid is enter under a moderate vacuum.
• This increases the temperature difference between the steam and boiling liquid.

Multiple effect evaporator

Increasing the evaporation per kilogram of steam by using a series of evaporators between the steam supply and the condenser is called multiple effect evaporator

• If vapor from one evaporator is fed into the steam chest of the second evaporator and the vapor from second is then sent to a condenser, the operation becomes the double effect. 
• The heat in the original steam is reused in the second effect, and the evaporation is achieved by a unit mass of steam in the same manner.

Once through evaporator

• In once-through evaporation, the feed liquor passes through the tube only once, releases the vapor, and leaves the unit as thick liquor. 
• All the evaporation is accomplished in a single pass. 
• The ratio of evaporation to feed is limited in single pass units: the evaporators are well adopted to multiple effect operation, where the total amount of operation can be spread over several effects. 
• Agitated film evaporators are always operated once- through  falling film and climbing film evaporators can also be operated in this way. 
• Once through evaporators are specially useful for heat-sensitive materials. 

Circulation evaporators

• In circulation evaporators a pool of liquid is held within the equipment. 
• Incoming feed mixes with the liquid from the pool, and the mixture passes through the tubes. 
• Unevaporated liquid discharged from the tubes returns to the pool so that only part of the total evaporation occurs in one pass. 
• All forced circulation evaporators are operated in this way; climbing- film evaporators are usually circulation units.
• These are adapted to single-effect evaporation. 
• These are not suited for heat-sensitive materials.