This blog focuses on industrial, inline process refractometers and their use in industrial applications. Refractometry is used to measure the refractive index of a substance in order to determine its composition or purity. Posts include information on theory, construction, installation, new products and new markets.
Electron Machine Corporation | Umatilla, FL | PHONE: 352-669-3101 | ElectronMachine.com
Inline Process Refractometers for Industry
The MPR E-Scan™
Inline Process Refractometers for Pulp & Paper Mill Applications
Green Liquor Process Management with Inline Refractometers
Green liquor is the dissolved concentrations of sodium sulfide, sodium carbonate, and other substances from the paper-making process's recovery boiler. Measuring its density is an essential aspect of paper production quality.
The Electron Machine MPR EScan is used to measure the green liquor dissolved density, or TTA, at two different points in the process: after the green liquor dissolving tank and after the green liquor clarifier. With the refractometer sensing head positioned directly in the primary process lines, inline measurement enables real-time management of green liquor dilution to meet target TTA set-points. Excessive green liquid density and the accompanying harmful imminent crystallization within the dissolving tank are also indicated (and prevented) by the measurement.
One considerable challenge is sensor head scaling associated with green liquor. An optical coating forms on the refractometer sensing head. The coating must be dealt with efficiently and quickly to maintain the accuracy and with minimum maintenance. This is key for the refractometer's ability to provide an acceptable measurement cycle and duration. The maintenance necessary to keep the cleaning system running efficiently is challenging.
Controlling scaling is optimal when the variance of green liquor solids is reduced by automatically adjusting weak-wash dilution with the MPR E-Scan refractometer. Additionally, pressurized water, heated to the process temperature, rinses the refractometer optical components effectively, resulting in a further scaling reduction. The end outcome is advantageous for both control and acceptable maintenance scheduling.
By limiting thermal changes, minimizing maintenance, and providing a dependable measurement source for automatic inline control, refractometers with accompanying heated high-pressure water cleaning systems deliver excellent results in improving green liquor processing.
Visit www.electronmachine.com or contact 352-669-3101 for more information.
Inline Process Refractometers Used in Industry
Inline process refractometers are used in a wide variety of industrial and commercial applications including the measurement of the sugar content of food and beverages, monitoring the purity and concentration of ingredients in pharmaceuticals, analyzing the constituents in chemical used in pulp and paper processing, and purity control and concentration measurement of raw materials in the chemical industry.
Inline process refractometers provide a very reliable and accurate real time measurement, which is ideal for process loop optimization and control. With In-line process refractometers, product quality and batch times are more closely controlled, reducing costs as a result.
The Electron Machine Corporation has been designing and manufacturing in-line process refractometers since the early 1960s, with a focus on providing simple, rugged, and reliable instruments that provide value over time, with accurate measurement, minimal maintenance, and long service life. They are the pioneer in developing the industrial use of refractive index for safe, reliable, and accurate process measurement and control.
Electron Machine Corporation
https://electronmachine.com
352-669-3101
Process Refractometers
PROCESS REFRACTOMETERS FOR THE FOOD AND BEVERAGE INDUSTRY
- Quality control and purity determination of feedstock and end products.
- Determination of sugar concentration (Brix).
- Determination of the alcohol concentration in beer, wine and spirits.
- Quality control of milk-based products
- Dairy products, jams and jellies, tomato products, fruit juices, beer, wine, spirits.
PROCESS REFRACTOMETERS FOR THE CHEMICAL INDUSTRY
- Baseline development of concentrations in research and development.
- Quality control and purity determination of feedstock and end products.
- Chemical process monitoring during production.
- Hydrocarbons, organic solvents, alcohols, salt solutions, acids, bases, stains, paints and varnishes, industrial oils, resins, glue, polymers, silicones, hydrochloric acid applications, sulphuric acid applications, boiler cleaning chemicals.
PROCESS REFRACTOMETERS FOR THE PULP AND PAPER INDUSTRY
- Black liquor concentration and quality.
- Green liquor concentration and quality.
- Boiler cleaning chemicals.
- Green liquor, black liquor, boiler cleaning chemicals, red liquor, white liquor, tall oil, and resin.
Industrial Refractometers and Green Liquor Scale Mitigation
Industrial inline process refractometers, such as Electron Machine's MPR E-Scan, are used to measure the green liquor dissolved density, or TTA, at two stages in the process: after the green liquor dissolving tank and after the green liquor clarifier. The inline measurement, with the refractometer sensing head mounted directly in the main process lines, allows real-time control of green liquor dilution to meet target TTA set-points. The measurement is also used to indicate (and prevent) excessive green liquor density and the resulting dangerous impending crystallization within the dissolving tank, and lower the potential for scaling.
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Refractometer Optical Sensor Cleaning System |
With overall quality and safety in mind, the use of a refractometer sensing head cleaning system is compulsory. The use of ancillary inline cleaning systems, such as Electron Machine's HPC-2 High Pressure Cleaner, that use pressurized water heated to the process temperature, will clean the refractometer optical components and therefore mitigate scaling issues and the related quality, safety, and production problems in the kraft process.
Process Refractometers for Black Liquor and Green Liquor Processes in Pulp and Paper
A very fundamental explanation of the Kraft Process:
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MPR E-Scan and heated high-pressure cleaning system. |
Electron Machine's decades of effort and experience in the pulp and paper industry led to the development of their MPR E-Scan refractometer in tandem with their heated high-pressure cleaning system. The resulting combination ensures efficient optical coating removal and maintenance minimization so as to ensure a reliable measurement source for automatic online control.
Throwback Thursday - 1972 "Instrumentation Study - Black Liquor Solids Content" by the The Institute of Paper Chemistry
In 1972 The Institute of Paper Chemistry wrote a paper titled "Instrumentation Study - Black Liquor Solids Content" detailing a comparison between an Electron Machine Corporation refractometer and an NUS Corporation sonic velocimeter and their respective capability of measuring black liquor solids. Below is the document to view online, or you can download your own copy here.
The Three Major Causes of Refractometer Trouble in Black Liquor Recovery Boilers
Pulp and paper mill. |
- Loss of cooling water and its effect on the sensing head.
- Lack of reliability of the prism wash.
- Condensation in the sensing head.
Cooling Water Loss
It is of vital importance that the loss of cooling water be detected. This may be done through a temperature sensing element or flow monitor which shuts down the refractometer involved.
Damage to the sensing element of a refractometer does not occur instantaneously, but it is essential that the system detect abnormal temperatures due to cooling water loss, flow blockage, etc., and that the cooling water be promptly restored.
The individual refractometer manufacturer’s instruction and maintenance manuals shall be consulted with reference to: potential damage to the sensing element; identification of a damaged element; how and when to replace a damaged element.
Prism Wash
The time interval between prism washes may vary with the black liquor composition. It is recommended that the minimum wash period be 7-10 seconds of wash every 20 minutes. Short duration washes at more frequent intervals are more effective than long washes at long intervals. Ideally, steam pressure for prism washing should be 35 psig above the black liquor pressure, plus the pressure required to open the protective check valve.
Awareness must be maintained of the effect of changes to the prism wash programming variables. Various refractometer systems have the capability to adjust: condensate drain time, steam on time, recovery time and interval between wash time. It may be possible to configure the system to have the total time that both refractometers are in their wash cycle represent a significant percentage of operating time. If one refractometer is out of service for repairs and the remaining refractometer is in prism wash, black liquor solids are not being monitored. Prism wash should be minimized to that needed to maintain the system.
If high pressure steam is used, it may abrade the prism. If only high pressure steam is available, a reducing valve shall be used.
The refractometer prism must have a clear polished optical surface, and if it becomes abraded, it must be replaced.
If the prism wash system has not operated properly and the prism becomes coated, it must be removed and properly cleaned.
Condensation in Sensing Head
Condensate may build up in the refractometer sensing head and if this occurs, the instrument operation will be erratic.
The procedure for determining this condition and for the elimination of excessive moisture in the sensing head is not the same for all refractometers. The manufacturer’s instruction and maintenance manuals shall be consulted and followed carefully.
Reprinted from "Recommended Good Practice: Safe Firing of Black Liquor in Black Liquor Recovery Boilers" courtesy of the Black Liquor Recovery Boiler Advisory Committee.
Image by AlexiusHoratius [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)], from Wikimedia Commons
Applying Refractometers to the On-line Measurement of Green Liquor Density
Black Liquor Recovery Boilers
Recovery Boiler (courtesy of Wikipedia) |
Modern day BLRBs are designed similarly to industrial boilers, typically as two drum designs, for operating pressure under 900 psi, or single drum designs, for operating pressure over 900 psi. The combustion gases utilized by the boilers can be sticky, so the BLRB furnaces are taller than their utility or industrial watertube counterparts. The amount of pulp producible by a particular mill directly correlates to the size of the BLRB. Small BLRBs process about 750,000 pounds of dry solids per day, and larger BLRBs process about six million pounds of dry solids per day. Precise attention and vigilant maintenance are required in order to maximize investment return for each particular boiler.
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Black Liquor |
A particular risk of the BLRB process stems from the relationship between molten smelt and water. The pool of molten smelt that accumulates as a result of the reclamation process needs to be kept separate from water, because water and molten material mixing at high temperatures can result in a smelt-water explosion. These explosions can occur when black liquor water content is greater than 42% of the mixture. Additionally, there are numerous ways water can enter the process – as condensation from the soot blower, a faulty steam coil heater, wash hoses – so controller vigilance is absolutely key to explosion prevention.
The Black Liquor Recovery Board Advisory Committee has recently introduced an emergency shutdown procedure, where an emergency evacuation alarm signals as soon as suspected water enters the BLRB furnace. The operator, with corresponding training, shuts down all fuel flow and minimizes combustion until all but a minimal amount of water is drained rom the BLRB. Annual inspections of BLRBs mandate the testing of all pressure parts and safety systems, because utmost care must be assured in preventing risk of system damage or operator harm when dealing with BLRB processes.
Quality and Process Optimization with Inline Refractometers
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Process refractometer in plant. |
Process refractometer (Electron Machine Corp.) |
Process refractometers are used for monitoring and controlling process variables in the flowing process media (liquid) . These instruments are used for continual, extremely accurate, real-time substance identification. Through identifying critical factors such as the concentration and purity, manufacturer's can gain tight control over quality can consistency of product. Applications for process refractometers are found in commercial food & beverage, chemical, pulp & paper, and pharmaceutical industries. All share similar processes lines where process refractometry provides real-time, high value information about the product at critical points. These shared processes are:
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Process refractometers are used for food and beverage production. |
Concentration
The measurement of concentrations in compounds of organic chemicals, inorganic chemicals, and total dissolve solids are often required for product consistency. Process refractometers can be calibrated to detect a wide range of dilute chemicals and dissolved solids and be an excellent feedback mechanism for these process variables.
Mixing
Using process refractometers for ingredient mixing to control product quality and production reduces errors and limits variance. Comparing the process media to known reference values, through the use of an inline refractometer, optimizes consistency and maintains quality.
Crystallization
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Process refractometers are critical for making pulp and paper. |
Cleaning
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Process refractometers have many uses in chemical production. |
For more information on industrial process refractometers, contact Electron Machine by visiting https://www.electronmachine.com or call 352-669-3101.
Safe Firing of Black Liquor in Black Liquor Recovery Boilers: Refractometer Black Liquor Solids Measurement System
Recovery Boiler (Courtesy of Wikipedia) |
Information on the BLRBAC can be found here. The full document, as well as other important information, can be found here.
Refractometer Black Liquor Solids Measurement System
4.1 GeneralThe heart of the system for the safe firing of black liquor is the ability to correctly, accurately and reliably measure the solids in the black liquor stream immediately prior to the black liquor guns. To accomplish this solids measurement, refractometers have proven to be effective for black liquor recovery boiler service. As new techniques in measuring solids are developed and proven, they can be considered. For the solids measurements, two refractometers in series must be used. When both refractometers are in service, the requirement for an automatic black liquor diversion can be satisfied by either of the following options:
- If either refractometer reads dissolved solids content 58% or below (62% or below if firing >70% solids per guidelines in 6.4 of this document), an automatic black liquor diversion must take place.
- When both refractometers read dissolved solids content 58% or below (62% or below if firing >70% solids per guidelines in 6.4 of this document), an automatic black liquor diversion must take place.
If the instrument readings disagree on the percent solids by 2% absolute value, an audible and visual alarm must be given.
If one refractometer fails, or is removed from service, black liquor diversion must then be controlled by the remaining in-service instrument; and if this remaining instrument reads 58% or below solids, an automatic black liquor diversion must take place (62% or below solids if firing >70% solids per guidelines in 6.4 of this document). Black liquor shall not be fired if neither refractometer is in service. The refractometers should be part of a specifically integrated system adapted to the black liquor service, and include a system to monitor their operation and indicate trouble or failure of the individual refractometer. Refractometers used without such a monitoring system can fail unsafe and can give improper and unsafe dissolved solids readings under certain conditions.
4.2 Refractometer Control System Functions
The refractometer control system shall be capable of performing the following functions:
1. Monitor the positive (+) and negative (-) supply voltage of each refractometer independently. The refractometer's supply voltage shall be maintained within the predetermined minimum and maximum limits for safe operation.
2. Monitor the lamp voltage or lamp output of each refractometer independently. The refractometers’ lamp voltage must be within the predetermined minimum and maximum limits for safe operation.
3. Monitor the signal amplitude (if chopper circuit devices are used) of each refractometer independently. Each refractometer's signal amplitude must be maintained within the predetermined minimum and maximum limits for safe operation.
4. Monitor the liquor temperature at each refractometer’s sensing head independently assuring that each refractometer's liquor temperature is within the predetermined minimum and maximum limits for safe operation.
5. Monitor the automatic prism cleaning timer system of each refractometer. The sensor output circuit, prior to the hold circuit, should go negative or adequately decrease during the purge cycle.
6. Monitor the automatic prism cleaning timer system to assure that the purge occurs within the predetermined time.
7. Monitor the cooling water to each refractometer sensing head to assure that cooling water is not lost to a sensing head.
If any of these malfunctions (Items 1 through 7) occur, the following action shall be initiated:
a) An alarm shall be activated, identifying the refractometer and circuit at fault.
b) The refractometer shall be electrically removed from the refractometer control system.
c) The remaining “good” refractometer shall remain in service.
8. Compare the refractometer meter outputs. If a difference of 2% (absolute value) solids or greater exists between refractometer readings, an alarm shall be activated.
9. Performs a black liquor diversion, if one refractometer is removed from service or fails in prism wash, and the remaining refractometer fails or reads a solids of 58% or less.
10. Monitor all cables from the refractometer and the components of the control system. If any cable is cut or removed, an alarm shall be activated.
11. Provide primary alarm or diversion functions by a means other than the refractometer indicating meter’s contacts.
12. Have the capability to allow the manual removal of either refractometer from service retaining the remaining refractometer in full service for diversion purposes.
13. Require a manual reset following a black liquor diversion or malfunction of the refractometer control system.
14. Monitor the position of the sensing head isolation valves. A partially closed or closed valve shall activate an alarm and remove the refractometer from service.
15. Initiate a low solids alarm signal from each refractometer at 60% solids or at 70% solids if firing >70% solids per guidelines in 6.4 of this document.
16. Prohibit the simultaneous washing of the individual refractometers.
17. Require manual restoration of a refractometer which has been removed, either automatically or manually, from service.
18. Have provisions for manual prism washing.
19. Require an automatic switch to single refractometer diversion (for systems set to require both refractometers read low solids to divert – dual refractometer diversion) when one refractometer is in a prism wash cycle. Automatic return to the chosen dual refractometer diversion will occur after completion of the prism wash cycle.
All of the above functions may not apply to all refractometer control systems since some refractometers:
a) Do not utilize cooling water,
b) Have sensing heads that are not affected by liquor temperature, etc.,
c) May have differences in electronic circuitry.
4.3 Refractometer Control System - Controls & Indicators
The refractometer system shall be equipped with the following controls and indicators:
1. Reset switch.
2. Switch or other means to manually remove either refractometer from service.
3. Visual solids display for each refractometer.
4. Status lights indicating “in service”. “inoperative” and/or “malfunction” for the individual refractometer and status of diversion valve.
4.4 Refractometer Control System - Alarms and Indicators
The recommended alarms and indicators of the refractometer control system are:
4.5 Installation Requirements
1. The refractometers shall be installed in series.
2. The refractometer sensing heads shall be installed in such a manner that the individual sensing heads can be taken out of service or removed without having to valve off the liquor piping or open bypass valves.
3. All cabinets, wiring, etc., shall be suitable for the atmosphere and service conditions normal to a recovery boiler installation.
4. The refractometer sensing heads shall be installed so that the y are accessible and readily serviceable.
4.8 Prism Wash
The time interval between prism washes may vary with the black liquor composition. It is recommended that the minimum wash period be 7-10 seconds of wash every 20 minutes. Short duration washes at more frequent intervals are more effective than long washes at long intervals. Ideally, steam pressure for prism washing should be 35 psig above the black liquor pressure, plus the pressure required to open the protective check valve.
Awareness must be maintained of the effect of changes to the prism wash programming variables. Various refractometer systems have the capability to adjust: condensate drain time, steam on time, recovery time and interval between wash time. It may be possible to configure the system to have the total time that both refractometers are in their wash cycle represent a significant percentage of operating time. If one refractometer is out of service for repairs and the remaining refractometer is in prism wash, black liquor solids are not being monitored. Prism wash should be minimized to that needed to maintain the system.
If high pressure steam is used, it may abrade the prism. If only high pressure steam is available, a reducing valve shall be used.
The refractometer prism must have a clear polished optical surface, and if it becomes abraded, it must be replaced.
If the prism wash system has not operated properly and the prism becomes coated, it must be removed and properly cleaned.
4.9 Condensation in Sensing Head
Condensate may build up in the refractometer sensing head and if this occurs, the instrument operation will be erratic.
The procedure for determining this condition and for the elimination of excessive moisture in the sensing head is not the same for all refractometers. The manufacturer’s instruction and maintenance manuals shall be consulted and followed carefully.
4.10 Refractometer Calibration Standardization (Zero Offset) to Off-Line Test
A Refractometer Standardization (“zero shifting” or “bias adjustment”) is an adjustment of the refractometer calibration curve to an off-line test to account for un-dissolved solids and/or changes in the black liquor chemistry. This is normally performed while the instrument is actively measuring black liquor solids.
All refractometers shall be verified against a reliable periodic off-line test. (See Chapter 6 – Off-Line Black Liquor Solids Measurement)
The refractometers shall be standardized:
1. On initial start-up of the recovery boiler.
2. At any time it is felt or known that one of the refractometers may be deviating from the known black liquor solids content.
3. Any time there is a 2% difference between refractometers.
The reading of the refractometers shall be checked against the moisture analyzer or microwave analyzer at two hour intervals (8 hour intervals if firing above 70% solids), and the moisture analyzer or microwave analyzer shall be checked by the TAPPI Standard Method, T650-om-05, weekly.
All refractometer standardization changes shall be entered in the recovery boiler “log book.”
4.11 Refractometer Calibration
A Refractometer Calibration involves placing two or more “samples” onto the sensor to generate a refractive index vs. dissolved solids curve. This is typically performed utilizing calibration oils or electronically (depending on supplier) in a controlled environment, while the sensing head is off of the process line.
Calibration procedures shall be done in a manner that does not affect the system’s ability to automatically perform a black liquor diversion utilizing the remaining (active) in-service refractometer. Improper procedures, or those that defeat the monitoring system described in Chapter 4, can result in the system failing in an unsafe condition. Refer to the manufacturer’s appropriate procedures.
If the continuous solids monitor refractometer differs from the off-line test field measurement by more than 2% on an absolute basis, the off-line test results must be confirmed and then if required the continuous monitor refractometer should be standardized and/or recalibrated according to the manufacturer’s recommended procedures. Repeated errors may indicate a failure of a refractometer component. Refer to the manufacturer’s recommendations for repair or replacement.
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Bleaching of Pulp in the Paper Making Process
Typically, the pulp is treated with each chemical in a separate stage. Each stage includes a tower, where the bleaching occurs; a washer, which removes bleaching chemicals and dissolved lignins from the pulp prior to entering the next stage; and a seal tank, which collects the washer effluent to be used as wash water in other stages or to be sewered. Bleaching processes use various combinations of chemical stages called bleaching sequences.
The first stage in the bleaching process is the chlorination stage, whose primary function is to further delignify the pulp. Chlorine reacts with lignin to form compounds that are water-soluble or soluble in an alkaline medium, which aids in delignifying the pulp before it proceeds to the next bleaching stage.
The next stage after chlorination is typically the extraction stage. This stage and the remaining stages serve to bleach and whiten the delignified pulp. The extraction stage removes the chlorinated and oxidized lignin by solubilization in a caustic solution.
Chlorine dioxide is often used in bleaching, either in the chlorination stage (as a substitute for some of the chlorine usage - chlorine dioxide substitution) or as an additional chlorine dioxide stage. Chlorine dioxide has 2.63 times greater oxidizing power (on a pound per pound basis) than chlorine and is used for nearly all high brightness pulps.
The next stage is the actual bleaching stage. Hypochlorite is a true bleaching agent that destroys certain chromophobic groups of lignin. It also attacks the pulp so high cellulose degradation occurs in Kraft pulp. Application of hypochlorite to Kraft pulp is usually used only as an intermediate stage of the sequence or to produce semi-bleached pulps. In the bleach process, residual chlorine must be removed through washing in vacuum washers.
Air Toxic Sources and Emission
Estimation Methods
Sulfite Pulping
The two main types of chemical pulping are the more common sulfate pulping (most commonly known as Kraft pulping) and sulfite pulping. Kraft pulping accommodates a variety of tree species, recovers and reuses all pulping chemicals, and creates a paper with a higher sheet strength. Sulfite pulp, however, is easier to bleach, yields more bleached pulp, and is easier to refine for papermaking. The major difference between the two types of chemical pulping is the types of chemicals used to dissolve the lignin.
Sulfite Pulping
The concept of sulfite pulping was created in the United States in 1867, however it was not used in a mill until 1874 by a Swedish chemist who was probably unaware of the U.S. Patent (MacDonald, 277). Sulfite pulping produces a lighter pulp than Kraft pulping. It can be used for newsprint, and when bleached can be used for writing papers and for the manufacture of viscose rayon, acetate filaments and films, and cellophane.Sulfite pulping follows many of the same steps as Kraft pulping. The major difference in sulfite pulping is that the digester “cooks” with a mixture of H2SO3 (sulfurous acid) and HSO3 ion in the form of calcium, magnesium, sodium, or ammonium bisulfate). The pulp continues on through the same processes as in the Kraft pulping process.
However, the chemicals separated from the pulp in the washers may or may not go into a recovery process. Chemical recovery in sulfite pulping is practiced only if it is economical. If chemical recovery does occur the liquor goes through an evaporator and then to a recovery furnace. Here, smelt is not formed, but ash and SO2 are formed.
Paper Manufacturing: Kraft (Sulfate) Pulping
and Emission Estimation Methods
The two main types of chemical pulping are the more common sulfate pulping (most commonly known as Kraft pulping) and sulfite pulping. Kraft pulping accommodates a variety of tree species, recovers and reuses all pulping chemicals, and creates a paper with a higher sheet strength. Sulfite pulp, however, is easier to bleach, yields more bleached pulp, and is easier to refine for papermaking. The major difference between the two types of chemical pulping is the types of chemicals used to dissolve the lignin.
The Kraft process was developed in Germany in 1879 and was first applied to a Swedish mill in 1885. The resulting paper was much stronger than any paper previously made, and therefore the process was named “Kraft”, (German and Swedish for “strength”). Kraft pulping creates dark brown paper which is used for boxes, paper bags, and wrapping paper. Kraft pulp can also be used for writing paper and paperboard when bleached, and for diapers when fluffed.
The three main steps involved in Kraft pulping are:
- Digestion: wood chips are cooked
- Washing: black liquor is separated from the pulp
- Chemical recovery: chemicals are recovered from the black liquor for reuse
- Digestion
Relief gases are vented continuously from the digester, which helps remove air and other non- condensable gases and reduce the pressure at blow, when the pulp is discharged to the blow tank. After the cooking process, the pulp and black liquor (the chemical mix left after the cooking process) are discharged to a blow tank.
By-products can be recovered from the digestion process. For example, turpentine distills with water out of the blow tank and the evaporators and is separated to be used. The resin acids and fatty acids dissolved from the wood form sodium soaps which are skimmed off the black liquor from storage tanks, evaporators, and black liquor oxidation tanks, and then acidified with sulfuric acid to form tall oil.
Before the washing process, the pulp is usually sent to deknotters, screens used to remove knots (large pieces of fiber not completely broken down in the digester).
- Brownstock Washing
All the washer types use water (fresh or recycled) and are usually placed in series to achieve higher removal efficiency.
The rinsed pulp is screened for oversize particles and then excess water is removed. This is done in a gravity thickener (more commonly known as a decker).
- Chemical Recovery
The first step in recovering the chemicals from the black liquor is evaporation. This removes excess water from the black liquor and maximizes the fuel value for the recovery furnace.
There are two types of evaporators generally used in the chemical recovery process: direct (DCE) and indirect (NDCE) contact evaporators. Some types of DCE include the multiple-effect evaporator (most common), flash evaporation and thermocompressor evaporation. DCE use heat from direct contact with the recovery furnace flue gases, while NDCE uses indirect contact.
Black liquor oxidation is needed after DCE, but not after NDCE. After DCE, the black liquor is normally oxidized with air to control the sulfide level and prevent the release of odorous compounds. This is done by countercurrently passing the black liquor through an air stream using a porous diffuser, sieve tray tower, packed tower or agitated air sparge. The oxidation reaction converts sodium sulfide to sodium thiosulfate.
After NDCE or black liquor oxidation, the black liquor is then forced through spray nozzles into the recovery furnace, where it is burned providing heat to generate steam. This also conserves the inorganic chemicals, which create a molten smelt on the floor of the furnace.
The molten smelt, composed of sodium sulfide and sodium carbonate, is drained from the recovery furnace hearth through smelt spouts. In a smelt dissolving tank, the smelt is quenched with water, producing green liquor.
Sodium carbonate from the smelt is then converted to sodium hydroxide in the causticizer by adding calcium hydroxide. The calcium carbonate resulting from the reaction precipitates from the solution and is collected and sent to the lime kiln where it is converted to lime (calcium oxide). The calcium oxide is then slaked to produce calcium hydroxide for reuse in the causticizer.
Overview of Chemical Recovery Processes in Pulp & Paper Mills
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Figure 1 |
The production of kraft and soda paper products from wood can be divided into three process areas:
- Pulping of wood chips
- Chemical recovery
- Product forming (includes bleaching)
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Figure 2 |
The purpose of the chemical recovery cycle is to recover cooking liquor chemicals from spent
cooking liquor. The process involves concentrating black liquor, combusting organic compounds, reducing inorganic compounds, and reconstituting cooking liquor.
Cooking liquor, which is referred to as "white liquor, is an aqueous solution of sodium hydroxide (Na01) and sodium sulfide (Na2S) that is used in the pulping area of the mill. In the pulping process, white liquor is introduced with wood chips into digesters, where the wood chips are "cooked" under pressure. The contents of the digester are then discharged to a blow tank, where the softened chips are disintegrated into fibers or "pulp. The pulp and spent cooking liquor are subsequently separated in a series of brown stock washers: Spent cooking liquor, referred to as "weak black liquor, from the brown stock washers is routed to the chemical recovery area. Weak black liquor is a dilute solution (approximately 12 to 15 percent solids) of wood lignins, organic materials, oxidized inorganic compounds (sodium sulfate (Na2SO4), sodium carbonate (Na2003)), and white liquor (Na2S and Na0H).
In the chemical recovery cycle, weak black liquor is first directed through a series of multiple-effect evaporators (MEE's) to increase the solids content to about 50 percent. The "strong. (or "heavy") black liquor from the MEE's is then either oxidized in the BLO system if it is further concentrated in a DCE or routed directly to a concentrator (NDCE). Oxidation of the black liquor prior to evaporation in a DCE reduces emissions of TRS compounds, which are stripped from the black liquor in the DCE when it contacts hot flue gases from the recovery furnace. The solids content of the black liquor following the final evaporator/concentrator typically averages 65 to 68 percent.
Concentrated black liquor is sprayed into the recovery furnace, where organic compounds are combusted, and the Na2SO4 is reduced to Na2S. The black liquor burned in the recovery furnace has a high energy content (13,500 to 15,400 kilojoules per kilogram (kJ/kg) of dry solids (5,800 to 6,600 British thermal units per pound {Btu/lb} of dry solids)), which is recovered as steam for process requirements, such as cooking wood chips, heating and evaporating black liquor, preheating combustion air, and drying the pulp or paper products. Particulate matter (PM) (primarily Na2SO4) exiting the furnace with the hot flue gases is collected in an electrostatic precipitator (ESP) and added to the black liquor to be fired in the recovery furnace. Additional makeup Na2SO4, or "saltcake," may also be added to the black liquor prior to firing.
Molten inorganic salts, referred to as "smelt," collect in a char bed at the bottom of the furnace. Smelt is drawn off and dissolved in weak wash water in the SDT to form a solution of carbonate salts called "green liquor," which is primarily Na2S and Na2CO3. Green liquor also contains insoluble unburned carbon and inorganic Impurities, called dregs, which are removed in a series of clarification tanks.
Decanted green liquor is transferred to the causticizing area, where the Na2CO3 is converted to NaOH by the addition of lime (calcium oxide [Ca0]). The green liquor is first transferred to a slaker tank, where Ca0 from the lime kiln reacts with water to form calcium hydroxide (Ca(OH)2). From the slake, liquor flows through a series of agitated tanks, referred to as causticizers, that allow the causticizing reaction to go to completion (i.e., Ca(OH)2 reacts with Na2CO3 to form NaOH and CaCO3).
The causticizing product is then routed to the white liquor clarifier, which removes CaCO3 precipitate, referred to as "lime mud." The lime mud, along with dregs from the green liquor clarifier, is washed in the mud washer to remove the last traces of sodium. The mud from the mud washer is then dried and calcined in a lime kiln to produce "reburned" lime, which is reintroduced to the slaker. The mud washer filtrate, known as weak wash, is used in the SDT to dissolve recovery furnace smelt. The white liquor (NaOH and Na2S) from the clarifier is recycled to the digesters in the pulping area of the mill.
At about 7 percent of kraft mills, neutral sulfite semi-chemical (NSSC) pulping is also practiced. The NSSC process involves pulping wood chips in a solution of sodium sulfite and sodium bicarbonate, followed by mechanical de-fibrating. The NSSC and kraft processes often overlap in the chemical recovery loop, when the spent NSSC liquor, referred to as "pink liquor," is mixed with kraft black liquor and burned in the recovery furnace. In such cases, the NSSC chemicals replace most or all of the makeup chemicals. For Federal regulatory purposes, if the weight percentage of pink liquor solids exceeds 7 percent of the total mixture of solids fired and the sulfidity of the resultant green liquor exceeds 28 percent, the recovery furnace is classified as a "cross-recovery furnace.'" Because the pink liquor adds additional sulfur to the black liquor, TRS emissions from cross recovery furnaces tend to be higher than from straight kraft black liquor recovery furnaces.
Industrial Refractometers in Action: Pulp & Paper Mill
The Electron Machine Corporation pioneered the use of refractometers to accurately measure black liquor dissolved solids nearly 50 years ago. Our long history with this application has resulted in numerous design features that specifically address problems associated with this harsh process measurement. Electron Machine refractometers have been accurately measuring green liquor solids in the paper industry for more than 30 years.
For more information visit http://www.electronmachine.com or call 352-669-3101.