Qualitative Analysis of Black Liquor in the Kraft Process

Black liquor is a byproduct of the Kraft process of paper production. The Kraft process, also known as the sulfate process, is a method of pulping wood chips to produce wood pulp, which is then used to make paper. In the Kraft process, wood chips are cooked in a solution of sodium hydroxide (NaOH) and sodium sulfide (Na2S) at high temperatures and pressure. This chemical cooking process breaks down the lignin and cellulose in the wood chips, separating the fibers that can be used to make paper.

During the cooking process, a black, viscous liquid known as black liquor is produced. Black liquor contains lignin, hemicellulose, and other organic compounds that are dissolved or suspended in the cooking liquor. It also contains inorganic chemicals such as sodium hydroxide and sodium sulfide, as well as dissolved minerals from the wood chips.

Black liquor is separated from the wood pulp during the pulp washing and recovery process. It is then burned in a recovery boiler to generate steam and electricity for the mill. The heat from the burning black liquor drives a steam turbine, which generates electricity that can be used to power the mill and any excess can be sold back to the grid. The inorganic chemicals in the black liquor are also recovered and recycled back into the Kraft process.

Process refractometers are used in paper mills to analyze black liquor in order to determine its concentration and solids content. The concentration of black liquor can have a significant impact on the efficiency of the pulping process, so it is important to accurately measure it.

To use a process refractometer to analyze black liquor, a sample of the liquor is typically collected and placed in a sample cell. The refractometer then measures the refractive index of the sample, which is a measure of the way that light is bent as it passes through the sample. The refractive index of the sample is then used to calculate the concentration of the black liquor.

Process refractometers are widely used in paper mills because they are fast, accurate, and easy to use. They can also be used to measure the concentration of other process fluids, such as process water and spent cooking liquor.

For more information on the use of inline process refractometers for black liquor analysis, contact Electron Machine at +1 352-669-3101 or visit http://electronmachine.com.

Inline Process Refractometers for Pulp & Paper Mill Applications

Approximately 50 years ago, the Electron Machine Corporation became the first company to employ refractometers to precisely quantify the dissolved particles in black liquor. Our extensive experience with this application has resulted in several design characteristics specifically tailored to process measurement challenges in this demanding environment. For decades, the paper industry has also relied on Electron Machine refractometers to correctly measure green liquor solids in pulp and paper manufacturing.

Electron Machine

352-669-3101

https://electronmachine.com

Are You Interested in a Complimentary Black Liquor Solids Analysis?

How convinced are you of the accuracy of your black liquor solids measurement? 

Are you making the most of your time? Do you know the concentration of solids at each critical phase? Do you want to reduce the amount of time you spend gathering data?

Electron Machine Corporation is offering a complimentary black liquor study service for all pulp mill customers. 

Electron Machine has worked for years to develop and improve the speed and accuracy of both inline and offline solids concentration measuring equipment. This continual improvement for black liquor solids measurement allows Electron Machine to design and offer equipment that will significantly increase the speed of your data recording.

Take advantage of this great offer from Electron Machine now!

For more information, visit electronmachine.com/blackliquortesting.

Automatic Black Liquor Solids Content Measurement with Inline Refractometers

Until the early twentieth century, when recovery boilers first appeared, black liquor was a disposable by-product in paper production. In modern times, pulp mills use chemical recovery technologies to recapture spent cooking chemicals from the pulping process for economic and environmental reasons. An example is Black Liquor Recovery Boilers that enable papermakers to recover and reuse inorganic compounds while also extracting energy from pulping leftovers. 

The Kraft process is used in paper production, in which wood is transformed into wood pulp and ultimately into paper. However, the method generates a byproduct known as black liquor, a hazardous mixture of pulping wastes (such as lignin and hemicellulose), and inorganic Kraft process chemicals (such as sodium hydroxide and sodium sulfide). The weak black liquor is directed through evaporators to increase the solids content to form strong black liquor. Injection of the concentrated black liquor into the recovery furnace burns organic molecules and reduces Na2SO4 to Na2S. The high energy content of the black liquor burned in the recovery furnace recovers as steam for process requirements such as boiling wood chips, heating, evaporating black liquor, preheating combustion air, and drying the pulp or paper products.

The pulp industry places a high value on accurate, continuous measurement of black liquor solids concentration. Black liquor solid is an essential measurement operation influencing the economics of the pulping process. The mill's environmental load and the solids content of liquor injected into a recovery furnace can significantly impact the firing behavior and thus efficiency. Current trends toward better recovery furnace control for safety and lower air emissions necessitate more robust control over incoming solids content and proper modification of operating parameters to accommodate changes in solids content. Typical analyses of black liquor solids are conducted periodically throughout the mill, from the washers through the evaporator to the ring header. These analyses consist of taking many samples at critical locations and analyzed in the lab. Alternatively, as pulp and paper mills are modernized and automated, inline process refractometers are installed, providing dependable, accurate, and reliable solids measurement and contributing to the safe functioning of recovery boilers. Inline process refractometers are time-tested and proven to provide reliable and precise automatic monitoring of black liquor solids concentration and providing considerable time and labor savings.

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

Complimentary Black Liquor Testing Offer from Electron Machine Corp.

Electron Machine Corporation

Complimentary Black Liquor Testing Offer

https://electronmachine.com/blackliquortesting

352-669-3101

Where is Your Pulp Mill’s Black Liquor Bottleneck?

Black liquor solids is a key measurement operation influencing the economics of the pulping process, as well as the environmental load of the mill. Typical analyses of black liquor solids are conducted on a periodic bases throughout the mill, from the washers through the evaporator to the ring header. These analyses consist of many samples being taken at key location and analyzed in the lab. This process can take many hours or even days to collect and determine the solids. Electron Machine has worked for years to develop and improve the speed and the accuracy of both inline and offline equipment. This continual improvement for black liquor solids measurements allows Electron Machine the ability to offer equipment that can greatly increase the speed in which these studies are conducted.

How confident are you in your black liquor solids? Do you know the solids at every critical phase of the process? Would you like to decrease the time that solids analyses take?

LEARN MORE ABOUT THE COMPLIMENTARY ON-SITE BLACK LIQOR SOLIDS SERVICE

Electronmachine.com/BlackLiquorTesting

With the recent demand for continue uptime and reduced labor forces, Electron machine is offering a complimentary black liquor study service for all pulp mill customers to analyze the potential for efficiency improvement and to assist in the implementation of refractometers in this costly part of the pulping process. We can assist process control engineers with their black liquor sampling and with the use of our equipment can decrease the time required to complete sampling. The service includes a 1 day on-site evaluation of the mill’s current black liquor solids, as well as any equipment and control practices used, and the identification of potential areas for improvement.

You can learn more at the Electron Machine website here.  Take this opportunity now to schedule a complementary service to identify your Black Liquor bottleneck.

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.

MDS Black Liquor Solids Monitoring System Training by Electron Machine Corporation

Electron Machine Corporation offers free MDS training at our facility in Umatilla, Florida. The MDS Monitor Divert System is a BLRBAC compliant Black Liquor solids monitoring system designed specifically for Black Liquor recovery boilers. Classes cover theory, operation, maintenance, troubleshooting and correct usage of the isolation valve. Training is comprehensive, friendly, personalized and provides the hands-on know-how necessary to operate the MDS.

For more information contact Electron Machine Corporation at 352-669-3101 or visit https://electronmachine.com.

Electron Machine Attends 2018 Fall BLRBAC Meeting

C.A. Vossberg and Jack Wilburn of Electron Machine exhibiting at
BLRBAC 2018 Fall Meeting

The BLRBAC Fall Meeting was held in in Atlanta this past October and Electron Machine was pleased to attend and exhibit.

The BLRBAC is a  trade association that exists for the purpose of sharing knowledge, drafting guidelines, and maintaining those guidelines to help facilitate the safe and reliable operation of Black Liquor Recovery Boilers.

The sodium sulfate or kraft pulping process is used to produce pulp for the production of paper.  Black Liquor Recovery Boilers reclaim the spent combustible pulping chemicals in black liquor and create steam. If not for the advent of the black liquor recovery boiler (BLRB), this pulping process would not be economically viable.

BLRBAC’s objective is to promote improved safety of chemical recovery boilers and their auxiliaries through the interchange of technical knowledge, experience, and data on past and any future recovery boiler incidents.

Electron Machine has been an active member of the BLRBAC for over five decades and supports the very valuable work the BLRBAC does to improve safety, operation and efficiency of chemical recovery boilers.

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

Measuring Solids in Pulp & Paper Black Liquor

Pulp and paper plant.

Industrial paper manufacture involves a procedure known as the Kraft process, where wood is converted into wood pulp and then into paper. The process, however, produces a toxic byproduct referred to as black liquor. This primarily liquid mixture of pulping residues (such as lignin and hemicellulose) and inorganic chemicals from the Kraft process (such as sodium hydroxide and sodium sulfide) is toxic.

Until the invention of recovery boilers in the early 20th century, black liquor was often simply released into waterways. Black liquor recovery boilers allowed paper manufacturers to recover and reuse the inorganic chemicals and extract energy from the pulping residues.

Reliable, continuous measurement of black liquor solids content is a subject of considerable importance to the pulp industry. The solids content of liquor introduced into a recovery furnace can have a pronounced influence on firing behavior. Current trends toward better control of the recovery furnace for reasons of safety and reduced air emissions require a greater degree of control over incoming solids content and appropriate adjustment of operating conditions to handle variations in solids content. This, in turn, requires a reliable method for continuously monitoring solids content.

Instruments such as inline process refractometers successfully measuring black liquor solids concentration continuously. These instruments have the dependability, accuracy, and reliability to augment safe operation of recovery boilers. Refractive index-type instruments are more widely accepted and far more successful with less maintenance requirements than other types of instruments used for black liquor solids measurement, and have proven reliable and accurate for automatic monitoring of black liquor solids concentration.

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.

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.

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

Inline Refractometers Tough Enough for Paper Plant Black and Green Liquor Lines

Refractometer and HPC Adapter
with High Pressure Purge System

It's said the only thing a pulp and paper plant doesn't reuse is the "shade the building casts". The processes used in the production of pulp and paper are very efficient when you consider the reuse of energy and by-products. The efficiency comes at a cost though - through very hostile atmospheres and demanding operating conditions for process equipment.

For example, the "kraft process" (also known as the sulfate process) is the method to convert wood chips into pulp and then to cellulose fibers. This is done by mixing the wood chips with sodium hydroxide and sodium sulphate, soaking, cooking and processing.

Here's a very basic explanation of the kraft process. Wood chips are soaked and processed in sodium hydroxide and sodium sulphate mixture known as "white liquor". After the wood chips are impregnated with white liquor, they are then cooked in digesters to break the wood down into cellulose. The solid pulp is then separated and the remaining fluid is referred to as "black liquor". Black liquor is further processed to remove solids and chemicals which are to be re-used in the pulping process. One of the final by-products is "green liquor" which contains sodium carbonate and sodium sulfide and is then reacted with lime to regenerate more white liquor.

All of these steps expose instruments, process equipment, piping, and valves to very tough environments. Electron Machine Corporation, a manufacturer of extremely rugged inline process refractometers, has been actively refining the use of refractometers for measuring green and black liquor density for over 30 years.

The scaling associated with these applications results in an optical coating on the refractometer sensing head. If this scaling can be controlled to allow an acceptable duration of on-line measurement, and then effectively removed when coating occurs, the accuracy of the refractometer can be fully utilized with minimal maintenance. The primary issue then becomes the maintenance required to keep the cleaning system operating effectively.

Electron Machine's efforts led to a system using their "almost indestructible" 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 and minimizing maintenance to allow for a reliable measurement source for on-line automatic control.

If you're interested in refractometry in pulp and paper processing, look no further than Electron Machine. They have the history, the experience, and the toughest inline refractometer on the planet.

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

Inline Refractometers for Measuring Black Liquor Evaporation

Measuirng black liquor solids concentration.

Introduction:

Refractometers have long proven to be the instrument of choice for black liquor solids concentration. Electron Machine Corporation of Umatilla, Florida pioneered this technology to accurately measure black liquor dissolved solids 5 decades ago. Their MPR E-Scan inline refractometer provides digital accuracy with rugged components required for years of reliable use when installed in harsh industrial environments. This combination allows for easy integration into a mill-wide monitor / control system providing operators the information needed for efficient and easy-to-service control.

Application:

An important and overlooked area for refractometers is the pulp washing operation. By monitoring the concentration output the washers can be balanced to meet the needs of the evaporation process and a mill is able to regulate the mixing liquor. This control allows a consistent concentration supply to the evaporators and boiler. This installation is simple and greatly reduces hand sampling.

MPR E-Scan

Evaporation:

By installing the Electron Machine Corporation MPR E-Scan with Isolation Valve Adapters before and after each evaporator effect, the concentration and temperature output can be utilized to monitor efficiency, detect problems and operate economically. This monitoring allows control operators to adjust steam usage for optimal solids evaporation. Operators can determine if a quick flush cleaning can be performed to improve efficiency, prolonging the time between invasive, labor intensive cleanings. It also allows the detection of any problems before the liquor reaches the boiler, plus provides a consistent solids percentage for firing.

Conclusion:

With the addition of the Electron Machine Corporation MPR E-Scan to a mills process control system, a company can see increased profitability through the optimized control of steam usage, reduced labor cost from the reduction of offline hand sampling and the ability to prolong the time between labor intensive offline cleanings. This greater control provides more consistent firing solids allowing boilers to operator more efficiently.

Key Benefits:

• Consistent black liquor solids
• Minimize steam consumption
• Economical operation
• Optimize man hours
• Reduce costly evaporator cleanings