Chapter 1 Introduction Evaporators are kind of heat transfer equipments where the transfer mechanism is controlled by natural convection or forced convection. A solution containing a desired product is fed into the evaporator and it is heated by a heat source like steam. Because of the applied heat, the water in the solution is converted into vapour and is condensed while the concentrated solution is either removed or fed into a second evaporator for further concentration. If a single evaporator is used for the concentration of any solution, it is called a single effect evaporator system and if more than one evaporator is used for the concentration of any solution, it is called a multiple effect evaporator system. In a multiple effect evaporator the vapour from one evaporator is fed into the steam chest of the other evaporator. In such a system, the heat from the original steam fed into the system is reused in the successive effects.
1.1) Multi Effect Evaporator A multiple-effect evaporator, as defined in chemical engineering, is an apparatus for efficiently using the heat from steam to evaporate water. In a multiple-effect evaporator, water is boiled in a sequence of vessels, each held at a lower pressure than the last. Because the boiling point of water decreases as pressure decreases, the vapor boiled off in one vessel can be used to heat the next, and only the first vessel (at the highest pressure) requires an external source of heat. While in theory, evaporators may be built with an arbitrarily large number of stages, evaporators with more than four stages are rarely practical except in systems where the liquor is the desired product such as in chemical recovery systems where up to seven effects are used. The multiple-effect evaporator was invented by the African-American engineer Norbert Rillieux. Although he may have designed the apparatus during the 1820s and constructed a prototype in 1834, he did not build the first industrially practical evaporator until 1845 . Originally designed for concentrating sugar in sugar cane juice, it has since become widely used in all industrial applications where large volumes of water must be evaporated, such as salt production and water desalination. Multiple-effect evaporation plants in sugar beet factories have up to eight effects. In the pulp and paper industry, multi-effect evaporators are mainly used to evaporate water (1)
from black liquor solutions to allow its recycle as chemicals and fuel for the process.
1.2) Application of evaporators Evaporators are integral part of a number of process industries namely Pulp and Paper, Chlor-alkali, Sugar, pharmaceuticals, Desalination, Dairy and Food processing, etc (Bhargava et al., 2010). Evaporators find one of their most important applications in the food and drink industry. In these industries, evaporators are used to convert food like coffee to a certain consistency in order to make them last for considerable period of time. Evaporation is also used in laboratories as a drying process where preservation of long time activity is required. It is also used for the recovery of expensive solvents and prevents their wastage like hexane. Another important application of evaporation is cutting down the waste handling cost. If most of the wastes can be vapourized, the industry can greatly reduce the money spent on waste handling (Bhargava et al., 2010). The multiple effect evaporator system considered in the present work is used for the concentration of weak black liquor. It consists of seven effects. The feed flow sequence considered is backward and the system is supplied with live steam in the first two effects. In the system, feed and condensate flashing is incorporated to generate auxiliary vapour to be used in vapour bodies in order to improve the overall steam economy of the system.
1.3) Overview of Kraft Recovery Cycle Kraft pulping and the chemical recovery process is composed of the following units: cooking, washing, evaporation, burning, causticizing and calcining . Wood chips are cooked with white liquor (NaOH + Na2S) in a digester at about 170 ºC, to produce kraft pulp and weak black liquor. Weak Black liquor (WBL) , the by-product of the chemical recovery cycle in the pulp and paper industry, is composed of water, lignin, cellulose and inorganic sodium salts . These chemicals (2)
need to be recovered for the pulping process to be economically feasible. In order to do that, weak black liquor is separated from pulp in a washing unit. The black liquor is diluted by the wash water and generally contains 14-17% solids. 95-98% of chemicals are recovered in modern pulp washing units . For each ton of pulp, 8-10 tons of weak black liquor is produced. Weak black liquor is concentrated in a series of evaporators. The resulting concentrated black liquor is burned in the recovery furnace to produce an inorganic smelt of Na2CO3 and Na2S. The smelt is then dissolved in water to yield green liquor, an aqueous solution of Na2CO3 and Na2S, as shown in Reaction . The green liquor undergoes the causticizing process where Na2CO3 is converted into NaOH by reacting with Ca(OH)2, as in Reaction 3. At this point, the original white liquor required
for pulping is recovered. In order to provide lime for the causticizing process, lime mud (or precipitated CaCO3) is dewatered, dried and burned in a lime kiln to produce lime for
the causticizing reaction.
The main reactions in the kraft process are listed below:
Pulping Wood + NaOH + Na2S
Pulp + Weak Black Liquor
(Reaction. 1)
Combustion Black Liquor + O2 Na2CO3 + Na2S + CO2 + H2O
Causticizing (3)
(Reaction. 2)
H2O + CaO Ca(OH)2
(Reaction. 3)
Na2CO3 + Ca(OH)2 CaCO3↓+ 2NaOH
Calcining (lime kiln @ 800ºC) CaCO3 CaO + CO2
(Reaction. 4)
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1.4)
Problems associated with multiple effect evaporators
The problems associated with a multiple effect evaporator system are that it is an energy intensive system and therefore any measure to reduce the energy consumption by reducing the steam consumption will help in improving the profitability of the plant. In order to cater to this problem, efforts to propose new operating strategies have been made by many researchers to minimize the consumption of live steam in a multiple effect evaporator system in order to improve the steam economy of the system. Some of these operating strategies are feed-, product- and condensateflashing, feed- and steam- splitting and using an optimum feed flow sequence. One of the earliest works on optimizing a multiple effect evaporator by modifying the feed flow sequence was by Harper and Tsao in 1972. They developed a model for optimizing a multiple effect evaporator system by considering forward and backward feed flow sequence. This work was extended by Nishitani and Kunugita (1979) in which they considered all possible feed flow sequences to optimize a multiple effect evaporator system for generating a non inferior feed flow sequence. All these mathematical models are generally based on a set of linear or non- linear equations and when the operating strategy was changed, a whole new set of model equations were required for the new operating strategy. This problem was addressed by Stewart and Beveridge (1977) and Ayangbile, Okeke and Beveridge (1984). The developed a generalized cascade algorithm which could be solved again and again for the different operating strategies of a multiple effect evaporator system. In the present work, in extension to the modeling technique proposed by Ayangbile et al,(1984) feed and condensate flashing has also been included and it also considers the variations in the boiling point elevation and overall heat transfer coefficient.
1.5) Classification of Evaporators Weak black liquor leaving the washers contains 13 to 17% dissolved solids. In order to safely and effectively burn the black liquor to recover chemicals and heat, the solids content must be at least 60%. Increasing the solids content improves the
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recovery boiler thermal efficiency, stabilizes boiler operation and reduces sulfur emissions. Heating value of black liquor ranges between 5800-6600 Btu/lb of dry solids, which is low in comparison to other fuels, such as gas and oil .Thus, a large amount of water must be evaporated in order to increase the net fuel value of black liquor. To accomplish this task, the most common types of evaporators presently used in the industry are the rising film long tube vertical evaporators (LTV) and the falling film
evaporators (FF).
1.5.1)
Rising film long tube vertical evaporators (LTV)
A LTV evaporator is composed of two parts: a single pass shell-and-tube heat exchanger at the bottom and a vapour dome at the top. The tubes are typically 5 cm (2” OD), 6.7 – 9.1 meters long, held in place by a tube sheet at the top and bottom . Black liquor enters the tubes from the bottom of the unit, where it is heated by the steam on the shell side of the tubes. As the heated black liquor boils, the resulting water vapour helps push the black liquor upward, until it reaches a deflector at the top where it is separated from the vapour. All vapour domes have a deflector directly over the heating element to break foam and initiate downward flow to the liquor. The concentrated black liquor exits the unit through a liquor outlet at the bottom of the vapour dome. The fine black liquor
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droplets entrained in the water vapour are separated by means of a demister as the vapour passes through it at the top of the dome . Vapour
Demister Vapour Deflector
Dome
Condensate
Liquor Outlet
Steam Inlet
Manufacturer: Universal Process Engineers Private Ltd. (INDIA) Hangzhou Semya Machinery Co., Ltd. (CHINA)
Condensate Outlet
Vent
Tubes Liquor Inlet Rising film long tube vertical evaporator
1.5.2) Tubular falling film evaporator A tubular falling film evaporator is composed of a heating element similar to LTV and a vapour body at the bottom. Liquor is fed to the bottom of the evaporator where a fixed level is maintained. Liquor rises to the top by means of a recirculation pump, and flows down the tubes with gravity . The liquor and vapour mixture leaving the tubes enters the dome at the bottom of the unit. The vapour is then separated from the
liquor and is cleaned by a drop separator.
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Falling film evaporators have a significantly smaller risk of scaling than the LTV evaporators because no bulk boiling occurs inside the tubes; consequently, no dry spot formation exists where scaling can initiate . Falling film evaporators run at lower steam pressures than the LTV evaporators, since steam does not have to push the liquor upwards in the tubes. Consuming weaker steam minimizes the scaling caused by reverse solubility of compounds such as sodium sulphate or temperature-sensitive calcium
complexes.
Recirculating Liquor
Steam Inlet
Vapour Outlet
Vent Condensate Outlet
Drop Separato
Liquor Inlet
r Dome
Liquor Outlet
Recirculation Pump
Falling film tubular evaporator
Manufacturers : Universal Process Engineers Private Ltd. (INDIA) Anhui OECH Mechanical Equipment Co., Ltd. (CHINA)
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Chapter 2 Literature Survey Performance of parallel feed multiple effect evaporation system for seawater desalination. Hisham T. El-Dessoukya, Hisham M. Ettouneya, Faisal Mandanib a)
Department of Chemical Engineering, College of Engineering and Petroleum, Kuwait University, P.O. Box 5969,Safat 13060, Kuwait
b) College of Technological Studies, P.O. Box 42325, Shuwaikh 70654, Kuwait Received 9 December 1998; accepted 14 November 1999 Abstract Performance analysis is presented for the parallel feed multiple effect evaporation system. Two operating modes are considered in the analysis, which includes the parallel and the parallel/cross flow systems. Analysis is performed as a function of the heating steam temperature, salinity of the intake seawater, and number of effects. Results are presented as a function of parameters controlling the unit product cost, which includes the specific heat transfer area, the thermal performance ratio, the conversion ratio, and the specific flow rate of the cooling water. Results indicate that better performance is obtained for the parallel/cross flow system. However, the parallel feed system has similar characteristics and simpler design and operation procedures. Performance of both systems is consistent with literature data. Comparison of the two parallel feed systems versus conventional multistage flash desalination and the forward feed multiple effect evaporation schemes show that the forward feed system has better performance characteristics than the other three systems. All rights reserved. Keywords: Seawater desalination; Multiple effect evaporation; Parallel feed; Modelling
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Chapter 3 Working of Multi Effect Evaporators (MEE) To use steam efficiently, a series of evaporators are connected to each other so that the latent heat of vapour is used multiple times. Each evaporator is called an effect in this system. Live steam is only fed to the first effect, while the vapour generated in the
first effect is the heating medium in the second effect, and so on. In kraft pulp mills, evaporation occurs in multiple effect evaporators (MEE), where steam and black liquor flow counter-currently . Steam economy, defined as the ton of water evaporated over the ton of steam used, increases as the number of effects increases lists measured steam economies for a practical operation shows specific heat consumption decreases as the number of evaporators in a series increases. It is therefore desirable to have more evaporators connected in a series; however, in practice this number is limited to six to eight evaporators. ΔT of an evaporator body is defined as the temperature difference between the saturated vapour temperature and the liquor temperature. ΔT is critical for long tube vertical evaporators, where a ΔT of less than 9.5 ºC will often cause poor performance behaviour.
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The evaporators operate at different pressures and are connected so that the produced vapour in one is used as heating steam for the next one. Live steam is only fed to the first evaporator body/effect. The weak black liquor feed usually splits between the last two effects, where the liquor boils at lower temperature under vacuum. As the liquor flows through the evaporators (from sixth to first), the pressure of the evaporator, boiling temperature and % solids increase, while the volume of liquor decreases.
240 KPa
- 80 KPa
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The evaporation system starts with weak black liquor (WBL) from the brown stock washers entering No. 1 and No. 2 WBL storage tanks. Weak black liquor is received from the digesters after the pulp fibres have been washed in the brown stock
washers.
WBL contains about 14 - 16% dried solids, which contains the inorganic
compounds sodium hydroxide (NaOH), sodium carbonate (Na2CO3), sodium sulphide (Na2S), sodium sulphate (Na2SO4), calcium carbonate (CaCO3) and silicates. These chemicals are the result of chemical reactions, which take place in the digester cooking process. Disposing of these chemicals is undesirable both environmentally and economically because they are costly to replace. By utilizing the evaporation process the liquor can be concentrated to a density suitable for burning in the recovery boiler, where chemicals and energy are recovered.
Black liquor is useful as a fuel because it also contains organic compounds such as lignin and tannins. These compounds give the liquor a heating value of about 14.5 MJ/kg.
The liquor is concentrated most efficiently using a series of evaporators or
multiple effects. These effects are shell and tube heat exchangers, which are connected by vapour piping so that the water boiled off the liquor in the first effect, acts as heating steam in the steam chest of the following effect. The liquor basically follows a reverse flow to the vapour.
3.1) Black Liquor Flow: One of two transfer pumps to the weak liquor flash tank, where solids concentration is increased slightly, pumps the weak black liquor from the storage tanks. The liquor flows to the 5th effect vapour body where it is concentrated to 17% solids. The liquor flows through without a pump (by gravity) as the 5th effect vapour body is at a lower pressure than the flash tank. The 5th effect recirculation pump (13)
continuously circulates liquor through the 5th effect. From the 5th effect, the liquor is pumped through a level control valve on the 5th effect to the suction line of 4th effect
recirculation pump.
Black liquor flow path through evaporation system at Mill A
4th effect concentrates the liquor to 21% solids. A transfer pump draws liquor off the suction line of the 4th effect recirculation pump and pumps it through either the secondary reflux condenser or through the secondary reflux condenser bypass line to the 2nd effect vapour body. The secondary reflux condenser heats the liquor to 91°C. The 2nd effect concentrates the liquor to 27% solids. Liquor is recirculated through the 2nd
effect by the 2nd effect recirculation pump. From the 2nd effect the liquor flows to No. 2 (14)
product flash tank where it is concentrated to 28% solids.
The liquor is pumped from the flash tank to the soap skimming tank. The purpose of the evaporator soap system is to recover tall oil soap from various points in the black liquor system and to deliver this soap to the recovery boiler for incineration. This is necessary to maintain the efficiency of the evaporator and concentrator heating surfaces
which would otherwise foul, if soap is not removed.
After soap removal, the liquor is pumped to the 3rd effect vapour body. Liquor is recirculated through the effect with 3rd effect recirculation pump, concentrating the liquor to 39% solids. The 3rd effect transfer pump draws liquor off the recirculation pump suction line and pumps it through the primary reflux condenser to the 1st effect vapour
body. The condenser heats the liquor to 112°C.
The 1st effect concentrates the liquor to 58 - 62% solids. The liquor is circulated through the 1st effect by the 1st effect recirculation pump. Liquor, from the 1st effect, flows into the 58% flash tank where the liquor temperature is reduced to 115°C. Liquor is pumped from the flash tank by the 1st effect transfer pump to the 58% storage tank. The name of the flash tank is an indication of approximate solids content of the liquor inside
it.
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After the 58% storage tank, black liquor is further concentrated in the High Solids Concentrator. HSC is composed of two heathers and a flash tank. Liquor concentration
rises after HSC due to decrease in pressure in its flash tank.
3.2) Steam Flow: Saturated steam enters the system at 325 kPa (143°C) through the HSC and the 1st effect. The vapour from the 1st effect is used as a steam source to concentrate the liquor in the 2nd effect. The resulting vapour from the 2nd effect is, in turn, used to concentrate the liquor in the 3rd effect, and so on. The same process principle is carried through the 4th effect and the 5th effect. Steam pressure and temperature decreases as it travels through the effects. The final vapour from the 5th effect is condensed in the surface condenser to create a vacuum of –70 kPa to help drive steam and vapour through
the system (Figure 14).
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At the outlet of the vapour system, there is a surface condenser, a shell and tube heat exchanger that has cold mill water on the tube side and vapour out of the 5th effect on the shell side. In this case the primary purpose of the surface condenser is to cool and condense the vapour from the 5th effect and, in doing so, create a vacuum of approximately -70 kPa in the 5th effect. The other effects have progressively higher pressure where 1st effect operates at about 105 kPa of pressure. As the pressure decreases through the evaporators (first to last), so does the boiling point of the water in the liquor,
therefore, water will boil at temperatures significantly lower than 100oC.
3.3) Performance Measures: There are three main measures of evaporator performance: 1.
Capacity (kg vaporized / time)
2.
Economy (kg vaporized / kg steam input)
3.
Steam Consumption (kg / hr)
Note that the measures are related, since Consumption = Capacity/Economy. 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.
3.4) Boiling Point Elevation Since evaporators dealing with boiling solutions, and in particular with solutions with nonvolatile solutes, any calculations must account for the effect of boiling point elevation. The vapour pressure of an aqueous solution is less than that of pure water at the same temperature; so the boiling point of the solution will be higher than that of the water. This is called (17)
Boiling Point Elevation (BPE) or vapour pressure lowering. The boiling point of a solution is a colligative property -- it depends on the concentration of solute in the solution, but not on what the solute and solvent are. When working problems involving heat transfer to or from boiling solutions, it is necessary to adjust the temperature difference driving force for the boiling point elevation. The equilibrium vapour rising from a solution exhibiting boiling point elevation will exist at a temperature and pressure such that it is superheated with respect to pure vapour. The vapour rises at the solution boiling point, elevated with respect to the pure component boiling point. The vapour, however, is solute free, so it won't condense until the extra heat corresponding to the elevation is removed, thus it is superheated.
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3.5) Methods of feeding in Multi Effect Evaporators There are three feed operations - backward feed , forward feed and mixed feed operations. A brief explanation of these operations:
3.5.1) Backward Feed : In the backward operation, the raw feed enters the last (coldest) effect and the discharge from This effect becomes a feed for the next to last effect.This technique of evaporations is advantageous, in case the feed is cold, as much less liquid must be heated to the higher temperature existing in the early effects. The procedure is also used if the product is viscous and high temperatures are required to keep the viscosity low enough to produce good heat transfer coefficients.
3.5.2) Forward Feed: (19)
In the case of a forward feed operation, the raw feed is introduced in the first effect and is passed from effect to effect parallel to steam flow. The product is withdrawn from the last effect. This procedure is highly advantageous if the feed is hot. The method is also used if the concentrated product may be damaged or may deposit scale at high temperature.
3.5.3) Mixed Feed : In mixed feed the dilute liquid enters an intermediate effect , flows in forward feed to the end of the series , and is then pumped back to the first effects for final concentration. This eliminates some of the pumps needed in backward feed and yet permits the final evaporation to be done at the highest temperature.
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Chapter 4 Mass And Energy Balances 4.1) Single Effect Evaporators
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F : Feed flow rate (kg\hr) L: Product glow rate (kg\hr) S: Steam flow rate (kg\hr) V: Vapour flow rate (kg\hr) C: Condensate flow rate (kg\hr) Tf : Temperature of feed Ts : Condensing temperature of steam TBP : Boiling temperature of the liquid in evaporator qs : rate of heat transfer through heating surface from steam Hs : Specific enthalpy of steam Hc : Specific enthalpy of condensate Hv : Specific enthalpy of vapour Hf : Specific enthalpy of thin liquor (22)
HL : Specific enthalpy of thick liquor λ : ΔHvap: latent heat of condensation of steam.
ASSUMPTIONS MADE: 1)
There is no leakage or entrainment , that the flow of noncondensables is negligible, and that heat losses from the evaporator need not be considered.
2)
The steam entering the steam chest may be superheated, and the condensate usually leaves the steam chest somewhat subcooled below its boiling point. Both the superheat and subcooling of the condensate are small, however , and it is acceptable to neglect them in making an enthalpy balance.
4.2) Multi Effect Evaporators Typically, multiple effect evaporator calculations require an iterative solution procedure because so many of the required properties, etc., depend on unknown intermediate temperatures. Fortunately, the overall approach is basically the same for the majority of problems, requiring only minor adjustments to compensate for problem quirks. In a typical evaporator problem, you are given the steam supply pressure, the operating pressure of the final effect, values for the overall heat transfer coefficient in each effect, the feed pattern, and the feed and product compositions. You also know that the effects are all to have the same heat transfer area. You typically want to find the steam consumption and the heat transfer area, and one or more of the temperatures, flows, and compositions from within the system. The overall strategy is to estimate intermediate temperatures, solve the material balances for the solven t vapor flow rates, use these to determine the heat transferred in each effect, and from that information find the heat transfer area. If the areas are not equal, you revise the temperature estimates and repeat the procedure.
The steps in the procedure can be summarized as: 1)
Use the overall component balance to completely determine the feeds and product streams.
these numbers are fixed and are not changed by iteration. 2)
Calculate the total amount of solvent vaporized (another fixed number). Divide this up into
estimated amounts for each effect; usually it is convenient to split it equally. (23)
3)
Use component and material balance to get estimates for the remaining flowrates within the system and the compositions of the intermediate streams. These (and all the estimated quantities) will change each iteration.
4)
Use the compositions to estimate BPEs and other properties. Be sure to keep track of which
properties depend on composition, temperature, or both. 5)
Determine the overall temperature drop between the steam and the saturation temperature of the last effect (remember to subtract off the BPEs).
Note that the BPE values may depend on the concentrations, so the overall Delta T can vary with each iteration. 6)
Allocate the overall drop among the various effects. Since the areas are the same, the
temperature difference in each effect is roughly proportional to the overall transfer coefficients.
7)
Use the Delta T and BPE values to obtain estimates for all the temperatures in the system. Typically, you do this starting with the steam to the first effect, subtracting a Delta T, adding a BPE, etc.
You can use the saturation temperature of the last effect as a check -- it should match the value for your final effect operating pressure. 8)
Use the temperature and composition estimates to get enthalpy values. You can get these from specific heat calculations or from data.
Be sure to use the same reference temperatures for all streams, including those taken from steam tables, etc. (24)
9)
Set up the process side enthalpy balances. Use material balances to eliminate the liquid flows from the enthalpy equations. Do enough algebra so that the only unknowns left in the balances are the vapor flow rates and the steam to the first effect.
10)
Solve the set of equations that is made up of one enthalpy balance for each effect and the total vapor material balance for the unknown vapor flows (one off each effect and the steam to the first).
11)
Use heat transfer equations to calculate the heat transfer area for each effect.
12)
Compare the areas. If they are not equal, you need to repeat the calculation. Begin by using the areas you obtained to revise the temperature estimates. The recommended approach is to use the ratio of the calculated heat transfer area for an effect to the arithmetic mean of the calculated areas.
13)
Repeat the calculations (from step 7) until the system converges. If your BPEs, enthalpy data,
etc., depends on composition, you will need to include steps 3 and 4 in each cycle as well. 14)
Once the system has converged, answer questions. Be sure to use values from the final iteration
to calculate your answers.
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Chapter 5 Advantages and Limitations of Multiple Effect Evaporators 5.1)
Advantages
Two or more evaporator units can be run in sequence to produce a multiple effect evaporator. Each effect would consist a heat transfer surface, a vapour separator, as well as a vacuum source and a condenser. The vapours from the preceding effect are used as the heat source in the next effect. There are two advantages to multiple effect evaporators: •
Economy - they evaporate more water per kg steam by re-using vapours as heat sources in subsequent effects
•
Improve heat transfer - due to the viscous effects of the products as they become more concentrated
Each effect operates at a lower pressure and temperature than the effect preceding it so as to maintain a temperature difference and continue the evaporation procedure. The vapours are removed from the preceding effect at the boiling temperature of the product at that effect so that no temperature difference would exist if the vacuum were not increased. The operating costs of evaporation are relative to the number of effects and the temperature at which they operate.
5.2) Limitations in Multi Effect Evaporators
Scaling Problem Scaling is a persistent problem in evaporators in the kraft pulp mills. As black liquor is concentrated, dissolved salts begin to precipitate from the system as they reach solubility. Precipitated solids may deposit on the heat transfer surfaces, forming a layer of scale. Severe scaling can interrupt black liquor flow, creating a bottleneck in pulp
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production. Due to its low thermal conductivity, scale greatly reduces the heat transfer
efficiency, lowering the evaporator performance . The scaling in a kraft black liquor evaporator is of the following types: calcium carbonate scaling, burkeite scaling, soap or fibre scaling, aluminum silicate and oxalate
scaling. A brief description of each type is given below.
5.2.1) Calcium scaling: Calcium scales form mainly in the first effect. The rate of scaling strongly depends on temperature. Calcium binds to organic compounds such as lignin complexes, oxalate and soap. Calcium ions become free when temperature reaches 90 – 130 ºC, therefore causing calcium carbonate to form and precipitate on heating surfaces. Since calcium compounds are less soluble at higher temperatures, calcium scaling increases
rapidly as temperature increases.
5.2.2) Sodium carbonate and sulphate scaling: Sodium carbonate and sodium sulphate precipitate as a double salt, burkeite,
(2Na2SO4.Na2CO3). The solubility of sodium carbonate and sulphate decreases slightly when the liquor temperature is above 40 ºC . Heat transfer surfaces can be the host for nuclei formation as they have the highest temperature in the evaporator body. Studies show a great influence of calcium ions in the solubility of Na2CO3-Na2SO4-H2O system. Calcium ions restrain the nucleation of burkeite and dicarbonate, resulting in a higher degree of super-saturation . This type of scale is easily washable by circulating weak
liquor or vapour condensate through the evaporators.
5.2.3) Fibre and Soap scaling: (27)
Black liquor soap is a mixture of resin and fatty acids that is separated from weak and intermediate black liquors to avoid scaling and foaming in the evaporators and concentrators. High fibre content makes the separation of soap harder, since soap adheres to the fibre surface. This type of scaling is common in the 2nd, 3rd and 4th effects. To
reduce this type of scaling, soap is usually removed from the evaporators at the 3rd effect.
5.2.4) Aluminum silicate scaling: Sodium aluminum silicate scales are hard, glassy and persistent. This type of scale is usually found in first effect and final concentrators, and its amount is determined by aluminum and silicate concentrations [4]. Generally, in North American mills, silicate scaling is not a common problem due to its small input in the recovery cycle.
5.2.5) Oxalate scaling: The oxalate ions are formed in the cooking and bleaching process. Sodium oxalate particles can form in black liquor when the concentration exceeds 45% solids. Also when evaporation is performed under vacuum at about 90 ºC sodium oxalate can precipitate at 30-40% solids. To avoid this precipitation, the process temperature is raised to about 110 ºC . Calcium oxalate deposition is not a concern, since calcium is
removed in the form of calcium carbonate which is less soluble.
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Scale Samples collected from the 1st Effect, 58% flash tank outlet pipe, and high solids concentrator tube The mill’s initiative toward the scaling problem is to clean the evaporators and flash tanks with water or weak black liquor, a process called “boiling out”. Scaling is costly due to the price of cleaning and loss of efficiency, i.e. more steam is consumed in case of
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scaling due to
loss of heat transfer area. Cleaning the 1st effect during shut down costs
$137,000, with the additional cost of lost efficiency.
Chapter 6 Future Aspects in Multi Effect evaporators In the present work different energy reduction schemes (ERSs), used to reduce the consumption of steam for a multiple effect evaporator (MEE) system, are developed. These ERSs are condensate-, feed- and product- flashing and vapor bleeding. Further, a new scheme is proposed where condensate of vapor chest of an effect is used to preheat the liquor, which is entering into that effect using a counter current heat exchanger. This work also presents a comparative study between existing ERSs and selects the best ERS amongst these based on steam consumption as well as number of units involved. Further, in the present paper a simple graphical approach named “Modified Temperature Path (MTP)” is developed for the analysis of different feed flow sequences of a MEE system to screen best possible feed flow sequence. To study the effect of different ERSs on steam consumption and MTP analysis an example of septuple effect flat falling film evaporator (SEFFFE) system, employed for concentrating weak black liquor in an Indian Kraft Paper Mill, is considered. The results show that ERSs reduce the steam consumption up to 24.6%.
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Conclusion Liquor evaporation is an important energy consumer in a pulp and paper mill. The study focuses on the identification of actions to reduce the energy cost related to the evaporator section of wood pulping mill. The future energy saving methods concern the modification of the operation conditions of the decrease of the ∆Tmin (Temperature Drop) and increasing or decreasing pressures of evaporation effects allowed one to reduce by 20% the minimum energy requirement of the evaporation system with an associated utility cost reduction of 23%.
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Bibliography:1) Multivariate Analysis of Variables Affecting Thermal Performance of Black Liquor Evaporators By - Hamideh Hajiha A thesis submitted in conformity with the requirements for the degree of Master of Applied Science - Graduate Department of Chemical Engineering and Applied Chemistry, Faculty of Applied Science and Engineering, University of Toronto. 2) Energy integration study of a multi effect evaporator by Zoe Perin-Levasseur,Vanessa Palese, France 3) Energy reduction schemes for multiple effect evaporator systems By :- Shabina Khanam & Bikash Mohanty * Department of Chemical Engineering, National Institute of Technology Rourkela, Rourkela, India * Department of Chemical Engineering, Indian Institute of Technology Roorkee, Roorkee-, India. 4) Wikipedia – “Multi effect evaporators”. 5) Unit operations of chemical Engineering – 7th Edition
Warren L. McCabe, Julian C.Smith, Peter Harriott McGraw – Hill international edition 6) Through Net, www.google.com
On google – “RMP lecture notes” 7) Through Net, www.google.com
On google – “Energy balance in multi effect evaporations”.
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