Process Refractometers for Black Liquor and Green Liquor Processes in Pulp and Paper

The only thing that a pulp and paper plant doesn't reuse is the "shade the building casts". When you consider reuse of energy and byproducts, the processes used in the production of pulp and paper are very efficient. Efficiency is costly however, because of the very hostile environments and demanding operating conditions endured by the plant's process equipment.

As an example, the "Kraft Process" (also known as the sulfate process) is the method of converting wood chips into pulp and cellulose fibers. The wood chips are mixed with sodium hydroxide and sodium sulfate, then soaked, cooked and processed.

A very fundamental explanation of the Kraft Process:

Wood chips are soaked and processed as "white liquor" in a mixture of sodium hydroxide and sodium sulphates. After impregnating the wood chips with white liquor, they are then cooked in digesters to break wood into cellulose. The solid pulp is then separated and the rest of the fluid is called the "black liquor". Black liquor is also processed for the removal of solids and chemicals to be reused during pulping. One of the final by-products is "green liquor" which contains both sodium carbonate and sodium sulfide and is then reacted into more white liquor with the addition of lime. All of these processes expose instruments, processing equipment, piping and valves to very harsh environments and chemicals. 

Electron Machine Corporation, a manufacturer of extremely robust process refractometers, has for over 40 years, been actively perfecting the use of refractometers for the measurement of green and black liquor density. 

Scaling of the sensor head: A key issue for keeping the system operational.

 MPR E-Scan and heated high-pressure cleaning system.

The problem of scaling in black liquor and green liquor applications is a major concern. It results in an optical coating on the sensor head of the refractometer the deteriorates performance. This scaling needs to be kept in-check in order to permit an acceptable duration of online measurement. With a means of effective removal at the time the the coating happens, the refractometer accuracy can maintained with minimum maintenance and downtime.

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.

For more information about the use of process refractometers in pulp and paper production contact Electron Machine Corporation by calling 352-669-3101 or visit their website at https://electronmachine.com.

Throwback Thursday - 1972 "Instrumentation Study - Black Liquor Solids Content" by the The Institute of Paper Chemistry

For all you Pulp & Paper historians out there, here's an "oldie but goodie" from the Electron Machine archives.

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.

Instrumentation Study: Black Liquor Solids Content - Electron Machine Corporation Refractometer vs NUS Corporation Sonic Velocimeter from Electron Machine Corporation

The Three Major Causes of Refractometer Trouble in Black Liquor Recovery Boilers

Pulp and paper mill.

The three major causes of refractometer trouble or failure in black liquor recovery boilers are:

1. Loss of cooling water and its effect on the sensing head.
2. Lack of reliability of the prism wash.
3. Condensation in the sensing head.

These may not apply to all refractometers due to differences in construction and circuitry.

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

A presentation to the Black Liquor Recovery Boiler Advisory Committee (BLRBAC) by C.A. Vossberg, President of Electron Machine Corporation.

Download the PDF version of "Applying Refractometers to the On-line Measurement of Green Liquor Density" from this link.

Applying Refractometers to the On-line Measurement of Green Liquor Density from Electron Machine Corporation

Black Liquor Recovery Boilers

Recovery Boiler (courtesy of Wikipedia)

Article courtesy of Electron Machine Corporation

“Black liquor” is a term used for the waste products that result from the pulping process. The black liquor recovery boiler (BLRB) allows for the chemicals in the waste products to be reclaimed via combustion. These reclaimed chemicals are then utilized to both meet steam demands in the process and to generate electricity.

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.

Black Liquor

In order to ensure stabilization of combustion, BLRBs are equipped with auxiliary burners which raise boiler temperature for the combustion process. The firing of the black liquor will eventually become self-sufficient. Combusting the black liquor allows for sulfur compounds used in the pulping process to be reduced to sulfide while inorganic chemicals essential to the process are melted down for reuse. The furnace vaporizes the black liquor as the liquor is sprayed into the furnace. Extra water is vaporized, and some of the combustion takes place as the black liquor falls to the furnace’s floor. The resulting molten smelt flows through spouts, which are operantly cooled via water, to a smelt dissolving tank.

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

Process refractometer
in plant.

Process refractometer
(Electron Machine Corp.)

Refractometry is a technology used to quickly, reliably, and very accurately identify a sample and determine the concentration and purity levels. This is done be taking a sample and measuring the refractive index and temperature of the media.

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:

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

Process refractometers are critical
for making pulp and paper.

Crystallization plays a key role in purification in many chemical processes, ranging from pharmaceutical manufacturing to food processing to liquor processing in pulp & paper production.  Keeping track of concentration levels is essential for the crystallization process and process refractometers provide real time information that allow process optimization.



Cleaning

Process refractometers
have many uses in
chemical production.

Changing product runs through existing lines is a major problem area for quality control. If a process line is used in the production of more than one product, it is important to ensure that no cross-product contamination occurs. To virtually eliminate this concern, process refractometers are used to check in for residual product presence (in real-time), providing assurance that purity levels are their highest.

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)

The following is reprinted from Chapter 4 of the Black Liquor Recovery Boiler Advisory Board (BLRBAC) Recommended Good Practice document titled "Safe Firing of Black Liquor in Black Liquor Recovery Boilers" (April 2016).

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 General

The 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:

1. 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.  
2. 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.  

Either option is satisfactory.

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.

5. The refractometer sensing heads may be installed in any position on a vertical pipe run. On a horizontal run of pipe, the sensing heads must be installed on sides of the pipe. The reason for this is to ensure that the prisms are always covered with liquor.  

6. The electrical power supply to the refractometer control system shall be from a dependable (stable) source.  

7. A dependable supply of cooling water of satisfactory capacity must be provided for refractometers requiring sensing head cooling water.  

8. Dry oil-free instrument air shall be provided to the refractometer sensing heads to prevent and control condensation in the heads.  

9. A steam supply source of sufficient capacity shall be provided to meet flow, and minimum and maximum pressures requirements.  All installation requirements may not apply to all refractometers and refractometer systems.  

4.6 Refractometer Problems 

The three major causes of refractometer trouble or failure are:  

1. Loss of cooling water and its effect on the sensing head.  

2. Lack of reliability of the prism wash.  

3. Condensation in the sensing head.  

These may not apply to all refractometers due to differences in construction and circuitry.  

4.7 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. 

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.

Get to Know Electron Machine Corporation

Electron Machine Corporation, headquartered Umatilla, FL, manufactures industrial, inline, process refractometers. As a vertically-integrated manufacturer, we have complete control over the time it takes to manufacture our instruments providing the highest levels of service and support to our customers. Superb quality control is attained by adapting modern technology and practices to existing designs. These include in-house microprocessor and DSP software design, surface-mount PC card design and assembly, 3D CAD/CAM designing, CNC machining, and MIG/TIG welding. Additionally, our founder's innovative nature is still with us as we continue to research and develop new products.

Learn more about Electron Machine at https://www.electronmachine.com or by calling 352-669-3101.

Bleaching of Pulp in the Paper Making Process

The purpose of the bleaching process is to enhance the physical and optical qualities (whiteness and brightness) of the pulp by removing or decolorizing the lignin. Two approaches are used in the chemical bleaching of pulps. One approach called brightening, uses selective chemicals, such as hydrogen peroxide, that destroy chromatographic groups but do not attack the lignin. Brightening produces a product with a temporary brightness (such as newspaper) that discolors from exposure to sunlight or oxygen. The other approach (true bleaching) seeks to almost totally remove residual lignin by adding oxidizing chemicals to the pulp in varying combinations of sequences, depending on the end use of the product. This creates a longer lasting (sometimes permanent) whiteness, but it weakens the fibers and reduces sheet strength. The most common bleaching and brightening agents are chlorine, chlorine dioxide, hydrogen peroxide and sodium hydroxide.

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.

Abstracted from Washington State
Air Toxic Sources and Emission
Estimation Methods

Sulfite Pulping

Pulping is the term used for the process which separates wood fibers. Chemical pulping, dissolving the lignin in the wood to create a pulp, is the most commonly used pulping process. Chemical pulping creates higher sheet strength than mechanical pulping; however, yields 40 to 50 percent pulp, where mechanical pulping yields 95 percent pulp.

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.

Description of Process 

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.

Abstracted from Washington State

Air Toxic Sources and Emission

Estimation Methods

Paper Manufacturing: Kraft (Sulfate) Pulping

Abstracted from Washington State Air Toxic Sources
and Emission Estimation Methods

Pulping is the term used for the process which separates wood fibers. Chemical pulping, dissolving the lignin in the wood to create a pulp, is the most commonly used pulping process. Chemical pulping creates higher sheet strength than mechanical pulping; however, yields 40 to 50 percent pulp, where mechanical pulping yields 95 percent pulp.

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:

1. Digestion: wood chips are cooked
2. Washing: black liquor is separated from the pulp
3. Chemical recovery: chemicals are recovered from the black liquor for reuse

Description of Process

• Digestion 

The first step in pulping wood is to “cook” the wood chips. A digester, heated by steam, “cooks” the wood chips in white liquor (a mix of sodium hydroxide (NaOH) and sodium sulfide (Na2S)) until done. The cooking process dissolves most of the lignin and only some of the hemicellulose , leaving mostly cellulose to hold the fibers together. The digester system may be a batch or a continuous process.

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 

Pulp from the blow tank and deknotter is washed with water in a process commonly called brownstock washing. Washing removes weak black liquor from the pulp which is sent to the chemical recovery process. This also prevents contamination during subsequent processing steps. Types of washers used include rotary vacuum washer (most common type of washer), diffusion washers, rotary pressure washers, horizontal belt washers, wash press, and dilution/extraction.

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 reason Kraft pulping is economically successful is that the used cooking liquor can be recovered and reused in the chemical recovery process.

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

Figure 1

The kraft process is the dominant pulping process in the United States, accounting for approximately 85 percent of all domestic pulp production. The soda pulping process is similar to the kraft process, except that soda pulping is a non-sulfur process. One reason why the kraft process dominates the paper industry is because of the ability of the kraft chemical recovery process to recover approximately 95 percent of the pulping chemicals and at the same time produce energy in the form of steam. Other reasons for the dominance of the kraft process include its ability to handle a wide variety of wood species and the superior strength of its pulp.

The production of kraft and soda paper products from wood can be divided into three process areas:

1. Pulping of wood chips
2. Chemical recovery
3. Product forming (includes bleaching)

Figure 2

The relationship of the chemical recovery cycle to the pulping and product forming processes is shown in Figure 1. Process flow diagrams of the chemical recovery area at kraft and soda pulp mills are shown in Figures 1 and 2, respectively.

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

This video below highlights various applications for inline refractometers in a pulp and 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.

Chemical Recovery in Black Liquor Processing for Pulp and Paper Production

Pulp and paper mill.

For economic and environmental reasons, pulp mills employ chemical recovery processes to reclaim spent cooking chemicals from the pulping process. At kraft and soda pulp mills, spent cooking liquor (referred to as weak black liquor), from the brown stock washers is routed to the chemical recovery area.

The chemical recovery process involves concentrating weak black liquor, combusting organic compounds, reducing inorganic compounds, and reconstituting the cooking liquor.

Residual weak black liquor from the pulping process is a dilute solution (approximately 12 to 15 percent solids) of wood lignin, organic materials, oxidized inorganic compounds (Na2SO4, Na2CO3), and white liquor (Na2S and NaOH). The weak black liquor is first directed through a series of multiple-effect evaporators to increase the solids content to about 50 percent to form “strong black liquor.”

Monitoring percent solids in black liquor
is an important part of chemical recovery.

The strong black liquor from the multiple-effect evaporator system is either oxidized in the black liquor oxidation system, or routed directly to a non-direct contact evaporator (also called a concentrator). Oxidation of the black liquor prior to evaporation in a direct contact evaporator reduces emissions of odorous total reduced sulfur compounds. 

The solids content of the black liquor following the final evaporator/ concentrator typically averages 65 to 68 percent. The soda chemical recovery process is similar to the kraft process, except that the soda process does not require black liquor oxidation systems, since it is a non-sulfur process that does not result in total reduced sulfur emissions.

The concentrated black liquor is then 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 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. 

The process steam from the recovery furnace is often supplemented with fossil fuel-fired and/or wood-fired power boilers. Particulate matter (primarily Na2SO4) exiting the furnace with the hot flue gases is collected in an electrostatic precipitator and added to the black liquor to be fired in the recovery furnace.

Refractometer for black liquor measurement.

The process of chemical recovery must be carefully managed. Process variables such as temperature, pressure, flow and level require robust instruments to ensure safety and accuracy. The measurement of black liquor solids content has relied upon the use of industrial inline refractometers for many decades. The Electron Machine Corporation, with it's ruggedly designed MPR E-Scan,  has established itself as the leader in this process. Incorporating a ruggedly designed sensing head with a 2205 S/S prism holder, sapphire prism, LED light source, and very sturdy electronics, the Electron Machine device delivers on it's claim as the "world's most rugged process refractometer".  Since the refractometer is specifically designed for the very harsh environment of a pulp mill, it promises years of low-maintenance and very reliable operation. 

For more information about inline process refractometers for black liquor processes visit this link or contact Electron Machine at 352-669-3101. 

Refractometers for Pulp and Paper Processing

The Electron Machine MPR E-Scan has numerous applications in the paper industry. The most common applications are on black liquor and green liquor. Electron Machine Corporation pioneered the use of refractometers to accurately measure black liquor dissolved solids nearly 50 years ago.

Black Liquor is the waste product from the process of digesting pulpwood into paper pulp by removing various elements to free the cellulose fibers. Electron Machine's long history with this application has resulted in numerous design features that specifically address problems associated with this harsh process measurement.

Green liquor is the dissolved concentration of sodium sulfide, sodium carbonate, and other compounds from the recovery boiler in the paper making process.  Electron Machine Corporation has been actively refining the use of refractometers for measuring green liquor density for over 30 years. The current system uses the MPR E-Scan refractometer combined with heated high-pressure water for cleaning. The resulting combination provides an effective removal of optical coatings by reducing thermal changes, minimizing maintenance to allow for a reliable measurement source for on-line automatic control.

The MDS Monitor Divert System is a BLRBAC compliant Black Liquor solids monitoring system designed specifically for Black Liquor recovery boilers. The MDS Monitor Divert System consists of two completely independent MPR E-Scan Hybrid-Digital refractometers with a separate monitor console that supervises the proper operation of each refractometer. The monitor constantly insures that all parameters remain within operational limits and applies the proper divert or alarm actions should a fault or low solids liquor be detected. A built-in printer records all actions with a date and time stamp. The entire system is designed to be user friendly with large daylight-readable color displays and an intuitive menu-driven interface.

The MPR E-Scan also gives paper companies the ability to accurately control the washing line, by detecting changes in the total dissolved solids coming off the washers. This precise measurement allows effective control of the fresh water flow to the washers, reducing excessive water usage. Combining the measurement with data- analysis tools, a company can monitor inefficiencies in the washing line and evaluate the washing results. Allowing improvements in washing efficiency and overall reduction in water. The MPR E- Scan will reduce the overall time needed to meet target dilution. With near instant readings of black liquor concentration and temperature, the instrument removes the reliance on offline testing. 

For more information, visit http://www.electronmachine.com or call 352-669-3101.

Industrial Refractometers in Brownstock Washing

Pulp and paper mill.

In pulp and paper production, brown stock washers are used to recover cooking chemicals from pulp production and are critical for maximizing chemical recovery which impacts the financial success and environmental compliance of a pulp mill.

The purpose of brownstock washing is to remove soluble matter from the pulp while using the least amount of water. Efficient washing improves the recovery of cooking chemicals, reduces the use of chemicals during bleaching, increase pulp quality and helps reduce deposit buildup. By utilizing a refractometer to measure the black liquor solids in the feed and outlet stock lines, and the incoming and outgoing filtrate lines, a paper company can experience increased control and cost savings.

The Electron Machine MPR E-Scan gives paper companies the ability to accurately control the washing line, by detecting changes in the total dissolved solids coming off the washers. This precise measurement allows effective control of the fresh water flow to the washers, reducing excessive water usage.

Combining the measurement with data analysis tools, a company can monitor inefficiencies in the washing line and evaluate the washing results. Allowing improvements in washing efficiency and overall reduction in water. The MPR E- Scan will reduce the overall time needed to meet target dilution. With near instant readings of black liquor concentration and temperature, the instrument removes the reliance on offline testing.

The MPR E-Scan is constructed of various alloys to ensure a long service life in a harsh chemical environment. With our customer pipeline adapters, the instruments can be implemented into any process. Due to the unique measurement principle, the instrument's readings are unaffected by bubbles, particles, fibers, color, flow, pressure or vibration. By utilizing the instrument to control and monitor the brownstock washing, paper companies can guarantee that proper dilution was met and maintained.

Electron Machine MPR E-Scan

KEY BENEFITS

• Increased washing efficiency
• Continuous accurate control of dilution factor
• Consistent pulp quality Increased evaporator efficiency
• Reduced wash loss and decrease wash water
• Error and Warning light indications Reduced time for correct wash concentration
• Continuous temperature readings 

Learn more about industrial refractometers and their application in the pulp and paper process by visiting the Electron Machine website at http://electronmachine.com or by calling 352-669-3101.

A BLRBAC Compliant Monitoring System Designed Specifically for Black Liquor Recovery Boilers

MDS Monitor Divert System

The Electron Machine Corporation manufactures a BLRBAC compliant Black Liquor solids monitoring system designed specifically for Black Liquor Recovery Boilers.

The Black Liquor Recovery Boiler Advisory Committee is a group that exists for the purpose of generating safety procedures and guidelines that govern the operation of Black Liquor Recovery Boilers.  The BLRBAC was formed in 1961 by several groups of concerned professionals that had become alarmed by the number of Black Liquor Recovery Boiler explosions. 

The MDS Monitor Divert System consists of two completely independent MPR E-Scan Hybrid-Digital refractometers with a separate monitor console that supervises the proper operation of each refractometer. The monitor constantly insures that all parameters remain within operational limits and applies the proper divert or alarm actions should a fault or low solids liquor be detected. A built-in printer records all actions with a date and time stamp. The entire system is designed to be user friendly with large daylight-readable color displays and an intuitive menu-driven interface.

Isolation Valves are also required to meet the BLRBAC guidelines and allow the refractometer sensing heads to be isolated from an active pipeline should maintenance be needed. The system closely monitors the position of these isolation valves to verify that the refractometers are in service.

All functions of the MDS Monitor Divert System are automatic. For example, should a refractometer fault occur the unit is electronically removed from service and an alarm is activated. Simultaneously the output signal is driven low to warn the operator to disregard this reading. The monitor system then isolates, displays the fault and provides a hard copy record on the built-in printer. Liquor diversion is now controlled by the refractometer in service.

• Intelligent purge function- ensures the proper cleaning and extends prism life 
• Divert trending - provides an actual time left before a diversion when liquor solids are declining 
• Password protection- to protect critical software areas

The MDS Monitor Divert System is pre-wired and mounted on a panel for easy installation. All customer connections are made to a single terminal strip. Standard output is a 4-20mA for liquor concentration with relay contacts for liquor divert, alarms, and system error indicators. A remote divert input is also available. The system can be provided in an optional stainless steel enclosure with either a vortex-cooler or fan ventilation depending on the specified mounting location.

For more information on the BLRBAC visit here.

For more information on the MDS Monitor Divert System visit here.

RSP Remote Status Panel for Process Refractometer

Electron Machine RSP Remote Status Panel for Process Refractometer from Electron Machine Corporation

Industrial Inline Refractometer for Green Liquor Density in Pulp & Paper Plant

Electron Machine Corporation has been actively refining the use of refractometers for measuring green liquor density for over 30 years. Their incremental efforts in this application has led to the current combination using the MPR E-Scan with their high pressure cleaner (HPC) adapter supplied with heated demineralized water. The removable nozzle provides for easy maintenance. This system ensures an accurate measurement in these difficult scaling conditions.

Industrial Inline Refractometer for Green Liquor Density in Pulp & Paper Plant from Electron Machine Corporation

The World's Most Rugged Inline Process Refractometers

Electron Machine manufactures the world's most rugged industrial, in-line refractometers used in pulp & paper processing, chemical production, and food and beverage processing.

The company is renowned for manufacturing industrial inline refractometers that hold up to the rigorous environments and the steady demands in these applications. These refractometers are built to withstand the most harsh conditions while delivering reliable and consistent readings and providing safe, reliable, and accurate process measurement and control.

Electron Machine inline refractometers are used in numerous applications in the paper industry such as black liquor, and green liquor sensing; in the food industry to detect sugar levels and properties of jams juices, beverages, and dairy product; and in the chemical industry to measure the strength of a chemical when diluted with water or another chemical.

For more information, visit http://www.electronmachine.com.