Essay: Wastewater treatment by multiple effect evaporator

ABSTRACT
The main goal behind this project is to treat highly generated TDS, TSS, BOD, etc. The utility model discloses a multiple-effect evaporation waste water treatment system which comprises at least seven effect evaporators in series connection, wherein an air outlet of each effect evaporator is connected with a separator, an air outlet of each effect separator is connected with a heat source port of a next effect evaporator, and an air outlet of a tail effect separator is connected with a cooler. The multiple-effect evaporation waste water treatment system is characterized by further comprising a water ring vacuum system, wherein the water ring vacuum system is mainly formed by a vacuum pump and a condensation tank, an air inlet pipeline of the vacuum pump is connected with the cooler, and an air outlet of the vacuum pump is connected with the condensation tank. According to the multiple-effect evaporation waste water treatment system, by means of arrangement of the water ring vacuum system, decompression evaporation separation is achieved, boiling points of materials are reduced, and energy consumption of the whole system is reduced.
TABLE OF CONTENTS Page No.
Acknowledgement 3
Abstract 4
Chapter 1: Introduction 7
Chapter 2: Principle of multiple effect evaporator
2.1 Plant selection of MEE
8
Chapter 3: Working of multiple effect evaporator
3.1 Advantages of MEE
3.2 Need of MEE
3.3 Types of waste treated in MEE
3.4 Application of MEE 10
Chapter 4 : Industry example
4.1 Properties of black liquor
4.2 Composition of black liquor 17
Chapter 5 : Schematic diagram of seven effect multiple effect evaporator
5.1 Description of process 20
Chapter 6 : Result and Conclusion 23
Chapter 7 : References 24
LIST OF TABLE
Sr. No. Name of table Page no.
1 Organic constituents of black liquor 19
2 Inorganic constituents of black liquor 19
CHAPTER 1: INTRODUCTION
Over the decade the pulp and paper industry has entered a new era:
‘ The new environmental regulation has been put in place resulting in a substantial tightening of the liquor cycle to reduce colour and BOD discharge and also in the demand for higher firing solids in the recovery boiler to reduce air emission.
‘ Due to increased in energy costs have become a major concern for each and every and all mill operation resulting in special attention to be paid to optimizing on side energy production process and usage to become more efficient competitive.
‘ Due to all these factor have placed increase emphasis on improving the performance and energy efficiency of the evaporate plant as it is among biggest consumer of steam and cooling water within the plant.
‘ Gaining additional black liquor solids throughout and increasing final liquor concentration, often substantially above the original design capacity of the equipment.
‘ Reducing the generation of foul condensate though better segregation in order to minimize the operating cost associated with post treatment of condensate.
‘ In a multiple effect evaporator the vapour from one evaporator is fed into the steam chest of the other evaporate.
‘ Finally, multiple effect evaporator is one of the system which reduces the steam costs and reuse the waste utilization.
‘
CHAPTER 2: PRINCIPLE OF MULTIPLE EFFECT EVAPORATOR
‘ Multiple effect evaporator is a thermal evaporation process, widely used for concentration of various liquids in the form of solution, suspension, and emulsions concentration is accomplished by boiling out of a solvent, normally water, from the liquid. In most cases, concentration resulting from the evaporation process is the final product of the process.
‘ Multiple effect evaporation process fall in to two general categories:
(1) Film type evaporation
(2) Suppressed boiling type evaporation
‘ Film type evaporation involve the generation of a thin product film to heat transfer surface to generate minimal resistance to heat transfer. Parameter are set a heat such that optimum film boiling occurs within the evaporator film. Caution must be used to ensure a continuous film hydraulic condition (wetting rate) and to prevent unclear boiling and fouling, otherwise, the rate of transferring heat will fall off typically, while the rate of scaling on the heat transfer surface will typically increase.
‘ A major requirement of the evaporation process is to maintain the quality of the liquid during evaporation and to avoid heat damage to the concentration. This normally necessities exposing the liquid to the lowest possible boiler temperature for the short period of time. Protecting concentrate temperature and other physical requirement of the finished product have resulted in development of many different evaporator types. Demand of energy efficiency and to reduce the minimal impact on environment and steam economy to reduced. ‘
2.1: PLANT SELECTION OF MEE
Design of evaporation plant requires consideration of numerous, and sometimes contradictory, requirement. The most important requirement are as follows:
‘ Product characteristics, including heat sensitivity, viscosity, corrosiveness, forming tendency, fouling and precipitation, boiling behaviour.
‘ Capacity and operation data, including quantities, concentration, temperatures, annual operating hours, change of product, controls automation, required operating media, such as steam, cooling water, electric power, cleaning agent, spare parts.
‘ Capital and financial costs
‘ Personnel costs for operation and maintenance
‘ Standards and condition for manufacture, (for outdoor sites), connections for energy and product, service platforms
‘ Legal regulations covering safety, accident prevention, sound emission, environmental requirements
‘
CHAPTER 3: WORKING OF MULTIPALE EFFECT EVAPORATOR
‘ A multiple effect is an evaporation unit which consist of several evaporators thermally linked and thus reducing considerably energy consumption.
‘ Evaporators can be used at different operating pressure: high pressure, atmospheric pressure, reduced pressure (vacuum). Consequently, they can be linked up so that the vapour given off in an evaporator can be used to heat a subsequent one, which operates at a lower pressure, and so on. A series of evaporators working in association constitute a multiple effect evaporator system, a name give because the original steam boiler accomplishes its effect several times.
‘ In such arrangement effects can be evaporators and crystallisers, of any type and also stripping column and flash vessels.
‘ The first effect is heated directly and next effects are heated by vapours from the previous effects. The last effect process vapours are condensed on a heat sink. Heating is archived commonly with live steam, but can also be done with a heat transfer fluid such as thermal oil or a molten salt, or waste heat from other parts of the plant like dryer vapour for example.
‘ In considering the heat balance for a single effect evaporator, the enthalpy of the evaporated vapour is approximately equal to the heat input on the heating side. In the common case of water evaporation, about 1T/h of vapour will be produce by 1T/h of live steam, since the values for the specific heat of evaporation on the heating and product side are about the same. In a case of a triple effect evaporation plant 1T/h of live steam will be able to evaporate 3T/h of water, so only 33% of the energy requirement of a single effect. Say of a triple effect, with the use of maximum only 33% of live steam in the triple effect evaporation plant compared to the single effect evaporation plant you are still able to do the same evaporation.
‘ The live steam consumption based on the number of effect has to be computed since variation are possible change in the feeding methods, circulation rates as well as temperature of the initial process solution fed in the process solution withdrawn.
‘ Co-current, counter, parallel or mixed flow of process solution with respect to live steam are possible based on process requirement and energy optimization. In case of a co-current multiple effect the concentrated process solution leaves at the cold end, in case of a counter-current flow, the concentrated process solution leaves at hot end of the evaporation plant.
‘ The maximum number of effects that can be installed is governed by the heating temperature, the heat sink temperature and the boiling point elevation of the product.it dictates the temperature difference within each evaporator’s heat exchanger. The total temperature difference is distributed between the individual effects and therefore the larger the number of effect, the smaller the temperature difference for each effect. This in turn increases the heating surface required to achieve a given evaporation rate.
‘ Increasing the number of effect increase the complexity of the plant arrangement and renders the operation and control more difficult. The product residence time will also increase.
‘ With the number of effects the capital cost increases, therefore the optimum capital/operating cost balance has to be calculated.
‘ For large evaporation capacity the number of effect can also be governed by maximum construction size of an evaporators well as profitability of energy recovery.
‘ Multiple effect are very popular in alumina refineries where in order to achieve the overall water balance of the plant they are used to concentrate sodium aluminate liquor know as spent liquor (mainly 5 or 6 effect falling film plant with or without salting out and associated flash tank, but also 3 or 4 effect).
‘ They are also used for black liquor concentration in the pulp and paper industry (7 or 8 effect), caustic soda concentration (3 effect is almost standard).
The main feature of multiple effect evaporators are as follows:
‘ Live steam consumption decreases as number of effects increases
‘ Cooling water consumption in condenser decreases as number of effects increases
‘ Co-current, counter-current, parallel or mixed flow
‘ Any type of evaporator or crystallizers, stripping column or flash vessels can be used
MODELING OF MULTIPLE EFFECT EVAPORATOR
In a model based on equations, the equations are developed for each effect and for every operating condition separately and are solved for the unknown variables.
The following equations are written for each evaporator in a mathematical model of a five effect evaporator system.
i. The enthalpy balance equations
ii. The heat transfer rates
iii. The phase equilibrium relationship
iv. The mass balance equations
3.1 ADVANTAGES OF MULTIPLE EFFECT EVAPORATOR
‘ Suitable for large scale and continuous operation.
‘ Highly economical when compared to single effect
‘ Multiple effects or stages, are now used to minimize the energy input required to evaporate or boil off undesirable water content.
‘ The energy consumption for multiple effect evaporator is less.
‘ By removing moisture through vaporization, industry can greatly reduce the amount of waste product that must be processed.
‘ The total evaporation achieved in these systems is approximately the number of effects times the energy input to the first effect.
‘ It can be used to remove water or other liquids from mixtures.
‘ The process of evaporation is widely used to concentrate foods and chemical as well as salvage solvents.
‘ They are used for the recovery of sodium hydroxide in kraft pulping.
.
3.2 NEED MULTIPLE EFFECT EVAPORATOR
‘ Reduces transportation cost.
‘ Reduces storage costs prepare for the next unit operation- drying crystallisation etc.
‘ Reduces deteriorative chemical reactions.
‘ Recovery of solvents.
3.3 TYPE OF WASTES TREATED IN MULTIPLE EFFECT EVAPORATOR
Generally all types of waste generated by mankind are treated by multiple effect evaporator:-
DEFINITION: Wastes are substances or objects, which are disposed of or are intended to be disposed of or are required to be disposed of by the provisions of national law.
Following are the different sources of industrial waste water they are as follows:
Industrial wastewater treatment covers the mechanisms and processes used to treat water that have been contaminated in some way by anthropogenic industrial or commercial activities prior to its release into the environment or its re-use.
‘ SOURCES OF INDUSTRIAL WASTE WATER ARE:
‘ Industrial and steel industry
‘ Mines and quarries
‘ Food industry
‘ Pulp and paper industry
‘ Complex organic chemicals industry
‘ Nuclear industry
‘ Water treatment
‘ TREATMENT OF INDUSTRIAL WASTE WATER ARE:
‘ Brine treatment
‘ Treatment of toxic materials
‘ Oil and grease removal
‘ Removal of biodegradable organics
‘ Treatment of acids and alkalis
‘ Treatment of other organics
3.4 APPLICATION OF MULTIPLE EFFECT EVAPORATOR
There is very wide application of multiple effect evaporator, since it treat all type of waste.
‘ Following are the application of multiple effect evaporator:-
‘ Milk / dairy industry ( milk, yoghurt ,serum)
‘ Beverages ( juices, fruits concentrate , tea )
‘ Wine industry ( self-enrichment of grapes must, dealcoholisation of must)
‘ Paper and pulp industry
‘ Food industry
‘ Pharmaceutical industry
‘ Petrochemical industry
CHAPTER 4: INDUSTRY EXAMPLE
(1) PAPER AND PULP INDUSTRY:
INTRODUCTION:
‘ In industries, black liquor is the waste product for paper and pulp industries by Kraft process. Approximately 7 tonnes of black liquor are produced in the manufacture of one tonne of pulp.
‘ Black liquor is quite toxic to aquatic life, and causes a very dark caramel colour in water.so, in order to reduce the toxic effect of this black liquor industries are treating this wastewater with multiple effect evaporator.
‘ In the pulp and paper industry, multi-effect evaporators are used to evaporate water
‘ From black liquor solutions to allow it’s recycle as chemicals and fuel for the process. Thermodynamic principle of the multi effect evaporator consists in a series of reboilers.
‘ Operating at different pressures; the water evaporated at one stage is condensed and
used as the heat source for another stage. Due to its strong integration with the
process, it is worth to analyse the integration of the multiple effect evaporator with the
rest of the process.
BLACK LIQUOR:
Black liquor is the spent liquor that is left from the Kraft process. It is obtained when pulpwood is digested to paper pulp and lignin, hemicelluloses and other extractives from the wood is removed to free the cellulose fibres.
The black liquor is an aqueous solution of lignin residues, hemicelluloses, and the inorganic chemicals used in the process and it contains more than half of the energy of wood fed to the digester.
One of the most important uses of black liquor is as a liquid alternative fuel derived from biomass.
4.1 PROPERTIES OF BLACK LIQUOR:
Some of the properties of black liquor are as follows:
1. Black liquor is distinctly alkaline in nature with its pH varying from 10.5 to 13.5 but it is not caustic in nature. The reason behind this is that most of the alkali in it is present in the form of neutral compounds.
2. The lignin has an intense black colour and the colour changes to muddish brown, when it is diluted with water and even when it is diluted to 0.04 % with water, it still retains yellow colour.
3. Black liquor is foamy at low concentrations and the foaming of black liquor increases with the increase in resin content in it.
4. The amount of total solids in black liquor depends on the quantity of alkali charged into the digester and the yield of the pulp. Under average conditions, black liquor contains 14 ‘ 18 % solids.
5. The presence of inorganic compounds in black liquor tend to increase the specific heat, thermal conductivity, density, specific gravity, viscosity but it has no effect on the surface tension of the black liquor.
4.2 COMPOSITION OF BLACK LIQUOR:
1. ORGANIC CONSTITUENTS OF BLACK LIQUOR:
SR.NO ORGANIC CONSTIUENT
1. Alkali lignins and thio lignins
2. Iso-saccharinic acids
3. Low molecular weights polysaccharides
4. Resins and fatty acids
5. Sugars
Table 4.2.1
2. INORGANIC CONSTIUENTS OF BLACK LIQUOR:
SR.NO. INORGANIC COMPOUND gm/litre
1. Sodium hydroxide 4-8
2. Sodium sulphide 6-12
3. Sodium carbonate 6-14
4. Sodium thio sulphate 1-2
5. Sodium sulphite 0.5-1
6. Sodium polysulphides 1-2
7. Sodium sulphate 1-2
Table 4.2.2
CHAPTER 5 : DIAGRAM OF SEVEN EFFECT MULTPLE EFFECT EVAPORATOR
FIGURE: DIAGRAM OF SYSTEM
Figure 5.1
5.1 DESCRIPTION OF PROCESS:
The schematic diagram of the system shows that it has seven effects. Live steam is supplies to the first and the second effect and the black liquor flows in the backward direction. The feed initially enters the feed flash tank (FFT) and after undergoing flashing, it enters the seventh effect. Seven condensate flash tanks are present. Out of these, first, second and third flash tanks are primary condensate flash tanks and the fourth, fifth, sixth and seventh flash tanks are secondary flash tanks. The final concentrated product comes out of the seventh effect.
EXAMPLE CALCULATION:
A seven-effect black liquor evaporation plant having a conventional steam stripping column (66) and an additional steam stripping column (38) according to the invention is provided in which:
Amount of evaporated water: 500 t/h
Amount of clean condensate: (streams 24-26, 32-35) 456.7 t/h
Contaminated condensate (21-23, 28-31) to conventional stripper (66): 43.3 t/h
Amount of methanol in feed waste liquor: 300 kg/h
RESULT:
Amount of methanol in clean condensates:
Conventional method: 60 kg/h (131 ppm), 80% removal efficiency.
According to present invention: 4 kg/h, (8.8 ppm) (very clean condensate), 98.7% removal efficiency.
CHAPTER 6: RESULT AND CONCLUSION
In the present work, Multiple effect evaporator is used to treat effluent of highly generated TDS, TSS, BOD, etc. Thus, the harmful effects of effluent on human life and aquatic system is reduced because of using these multiple effect evaporator system. In Paper and pulp industry, black liquor is generated for 1 tonnes production of paper near about 7 tonnes black liquor is obtained. Thus, Black liquor by using multiple effect evaporator it can be recycle as chemicals and fuel for the process. MEE (Multiple Effect Evaporation system), which is famous in each and every industry, is accordingly modified for treatment of effluent and there are re- using the generated condensate for cooling towers, which is helping in saving of plenty of water. Thus by using these system, we can save our environment and prevent it from less polluting.
CHAPTER 7: REFERENCES
1. Chemical Recovery in the Alkaline Pulping Processes , Third Edition, Prepared by the Alkaline Pulping Committee of the Pulp Manufacture Division Committee Assignment No. 6500; Co edited by Robert P. Green and Gerald Hough; TAPPI Press 1992.
2. Radovic, L. R., Tasic, A. Z., Grozanic, D. K., Djordjevic, B. D., & Valent, V. J. (1979), Computer design and analysis of operation of a multiple effect evaporator system in the sugar industry. Industrial and Engineering Chemistry Process Design and Development, 18318’323.
3. Ray, A. K., Rao, N. J., Bansal, M. C., &Mohanty, B. (1992), Design data and correlations of waste liquor/black liquor from pulp mills. ISPPTA Journal, 4 1’21.
4. Bremford, D. J., & Muller-Steinhagen, H. (1994). Multiple effect evaporator performance for black liquor. I. Simulation of steady state operation for different evaporator arrangements. Appita Journal, 47, 320’326.
5. Itahara, S.,&Stiel, L. I. (1966). Optimal design of multiple effect evaporators by dynamic programming. Industrial&Engineering Chemistry Process Design and Development, 5, 309.
6. Lambert, R. N., Joye, D. D., & Koko, F. W. (1987). Design calculations for multiple effect evaporators. I. Linear methods. Industrial & Engineering Chemistry Research, 26, 100’104.
7. http://www.googlepatent.com