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Powder Perspectives Article 5 Frequently Asked Questions, Part 1

Article 5, September 1, 2013 Appearing in Powderbulksolids.com, Powder Perspectives column

 

Frequently Asked Questions—Part 1

Having been a consultant for many years I have obviously been asked some of the same questions concerning solids flow, time and time again. Here are some more frequently asked questions and answers:

1. How can bulk solids flood or flush out of a bin?

Answer: Fine solids flowing in funnel flow (some material moving most material stagnant) typically experience flooding problems. In funnel flow, a preferential flow channel forms, usually directly over the outlet.  If fresh material is placed in the container, it will flow into the preferential flow channel and not have enough time to deaerate. The bulk solid now behaves as a fluid. As it reaches the outlet, the feeder which is designed to meter a solid cannot contrFloodingol a fluid, and the aerated product will flow uncontrolled from the vessel.

Additionally, if a “rathole” forms, where the preferential flow channel empties and forms a stable pipe, fresh product brought into the vessel or material that sloughs off the top of the pile will fluidize and flush from the bin.

2. What are the requirements for a properly designed mass flow bin?

Answer: There are two major considerations:

  • The      opening size required to prevent arching and
  • The      hopper slope required to ensure flow along the walls.

The definition of mass flow is that when material is removed from a bin, all the material is in motion. In order for all the material to remain in motion, it must flow at the walls of the bin.  There is friction that develops between the bulk solid and the wall surface of the hopper. If the walls are too rough or too shallow, the material will flow on itself rather than on the rougher/shallow walls. In mass flow, the walls are typically steep and smooth to ensure flow along them.

Interestingly, if the material is cohesive, it may arch (bridge) over the opening causing a flow stoppage, even in mass flow.

3. Am I restricted to steep conical hoppers to ensure mass flow?

Answer: Not necessarily. Wedge shaped hoppers are a great alternative to conical hoppers. Wedges, such as chisel or transition type are more forgiving from a flow standpoint.

Keep in mind that material in a conical hopper has to converge in 360°. While in a wedge configuration converges in only onWedge Hoppere direction. Wedges use slotted openings which require smaller widths to prevent arching and shallower hopper slopes to ensure mass flow. Because of the long slot, wedges will also provide a higher discharge rate.

One very important consideration is that the feeder design is as important as the wedge hopper. If the feeder (likely a screw or belt) is not designed to discharge material over the entire cross sectional area of the slot opening, the mass flow pattern will be destroyed.

4. My material’s angle of repose is 46.3°. How do I use this data to design a bin?

Answer: You cannot design a bin just from the angle of repose. What opening size do you use? What is the hopper slope, 46.3°?

You have to measure a materials flow properties in order to properly design a bin or hopper. The angle of repose can be used to determine the contour of a pile of material, perhaps to determine volume in a bin or silo, but not to design a bin.

5. I have been told that my material segregates, how does this happen?

Answer: Segregation can occur via sifting, fluidization, etc. Segregation requires: easy flowing material, interparticle motion and a range of particle sizes.

Sifting occurs when filling a bin or forming a pile, the fine particles sift through the coarse particles, allowing the fines to concentrate in the center while the coarse particles roll or slide to the outside.  This is considered side-to-side segregation and is typically resolved by ensuring mass flow.   Fluidization segregation occurs when solids are dropped into a bin from a conveyor such as a pneumatic conveyor. The coarse particles are driven to the bottom while the fine particles remain airborne and settle on top.  This is considered top to bottom segregation and in this case mass flow simply exacerbates the problem.

6.  What happens if my material is extremely difficult flowing?

Answer:  Unassisted gravity flow is typical the best way to handle powders; however, when your material is difficult flowing and gravity needs help, a flow aid device may be used.  There are many types of flow aid devices available to choose from such as vibratory, agitation, forced extraction, aeration, flow aid chemicals, and even explosives.

Flow AidYou should typically have your material’s flow properties measured to ensure that gravity is not achievable but the tests will also help you to choose a flow aid device, if required.  For instance, if your material is sensitive to excess pressures (caused by high drop heights into the bin or excess vibrations), do not use vibratory flow aids to solve your problem.

I will be presenting a workshop entitled: “Propensity for Particle Caking and Solutions Involving Flow Aids” at Powder & Bulk Solids’ Texas conference and exhibition on October 15, 2013 (Conference and exhibition is offered on October 14-16, 2013 at the Reliant Center, Houston, TX).

Powder & Bulk Solids Texas is Houston’s only exposition and conference for processing engineers, production engineers, plant managers, and operations managers who process, handle, package, transport, test, and store dry particulates and bulk solids.  Copy and paste the following link for information:  http://www.canontradeshows.com/expo/ptx_tx13/

 About our author
Joseph Marinelli is a consulting engineer and President of Solids Handling Technologies.  He has been providing testing and consulting services since 1972.  As a former consultant with Jenike & Johanson, Inc., he has years of experience testing powders and designing bins and feeders for reliable flow.  He lectures frequently on the topic of powder handling and has published several papers including and article in a chemical encyclopedia and two in a food powder book.

 

Joseph Marinelli

Solids Handling Technologies

1631 Caille Court

Fort Mill, South Carolina 29708

Telephone: 803-802-5527

E-mail:    joe@solidshandlingtech.com

 

How to Interpret a Solids Flow Report

 

How to Interpret a Solids Flow Report

By Joseph Marinelli, Solids Handling Technologies, Inc.

In order to determine if a new bin or silo will work reliably or to be able to make modifications to an existing troublesome bin or silo, you could use this guy or you could measure your solids flow properties.    To begin this process, you need to know: what type of flow pattern can develop (modes of flow) and your material’s flow properties.

Gorilla

Modes of Flow

There are two modes of flow that can develop in a bin or silo, funnel flow and mass flow (expanded flow is another but is simply a combination of a mass flow pattern and a funnel flow pattern).

Funnel Flow

Funnel flow occurs because the hopper is not sufficiently steep and smooth enough to ensure sliding along its walls. In funnel flow, material flows toward the outlet through a channel that forms within stagnant material caused by friction on the shallow and/or rough walls. With a cohesive solid, this channel expands upward from approximately the hopper outlet, potentially to silo cylinder walls Even if the outlet is fully live, the preferential flow channel may empty out and form a stable rathole. This may:

  • limit your live capacity
  • cause materials to agglomerate or spontaneously ignite
  •  cause powders to flood or flow uncontrolled
  •  enhance particle segregation
  •  cause silo failure.

Generally, a funnel flow pattern is only suitable for coarse, free-flowing, non-degrading solids when segregation is unimportant.

Mass Flow

In mass flow, the hopper is steep and smooth enough to ensure flow of all the material whenever any solid is withdrawn, thereby overcoming the friction that develops between the material and the hopper wall surface.  Where there are many disadvantages to funnel flow, mass flow has certain advantages, as follows:

  •  flow is uniform
  •  pressures acting at the outlet are practically independent of the head of solid in the bin
  •  segregation of particles is minimized by the first-in- first-out flow sequence associated with mass flow and segregated particles are re-mixed as they discharge from the outlet
  •  fine powders tend to deaerate and flooding is avoided, due to increased residence time

Generally, a mass flow pattern is recommended when handling cohesive materials, powders that can flood, materials which degrade with time and when segregation needs to be minimized.

Material Flow Properties

There are two major considerations for reliable flow; namely, cohesive strength and wall friction properties. Cohesive strength is measured using a bench scale laboratory testing device such as a direct shear tester (Jenike Shear Tester) seen here. This device is used to determine a material’s “Flow Function”, whereby the material’s cohesive strength is measured as a function of applied consolidation pressure (pressure/ strength relationship). The sample’s moisture content and particle size are controlled while the direct shear tester is capable of determining the effects of temperature and time of storage at rest. This information is then used to determine the opening size required to prevent arching and ratholing in a bin or hopper.

The Jenike Shear Tester  is also used to measure wall friction properties. Consider that friction develop between a solid and the walls of a bin or hopper. Wall friction determines whether the solid will slide on the wall (mass flow) or adhere to the wall forcing it to flow preferentially on itself (funnel flow) rather than at the walls.

Additionally, the material’s compressibility (bulk density/pressure relationship) is determined. A Flow Report is created describing the material’s flow properties. This report indicates values to be used to design a new bin or modify and existing one. A Flow Report typically consists of the following:

Page 1: Title Page

The first page is obviously a title page with a description of the project, company, etc.

Pages 2 & 3: Introduction and General Comments

This contains a General Comments section and is meant to provide general information regarding the flowability of the particular material being tested. These comments are given without any bin geometry in mind and serve to help explain tabulated data in the following pages and how to interpret it.

Pages 4 & 5: Cohesive Properties Test Results

Section 1: Arching and Ratholing Dimensions of the Flow Report indicates the arching and ratholing dimensions of your material as a function of time at rest, temperature, etc, as follows:

Arching Dimensions–the following are indicated as conditions your material was exposed to:

Time at Rest, hrs—In the example report, the material was tested to simulate 0 hrs storage (continuous flow) or as if the material was put in a bin and flow initiated immediately, shown on page 4.  As well, some period of storage at rest (shown further below) was simulated, such as overnight, 72 hr, etc.

 Temp., ºF—The material was tested at 90 ºF for 0 hrs and at 90 ºF cooling to room temperature after 3 days at rest to simulate actual storage conditions.

Particle size—An estimate of the material’s particle size is given here just to indicate whether the material is coarse or fine. In this case it would be considered fairly fine as it is considered-10 mesh.

Moisture content—The material’s total moisture content was measured according to an ASTM standard.

P-Factor—P-Factor is an estimation of the effect of excess pressure on your material. The magnitude of the excess pressure or overpressure factor can be estimated for vibration and impact during charging into the bin as follows:

Vibration:

Vibrators can affect flow two ways as follows: (1) While vibrators are commonly used as flow aid devices, they also pack materials in bins and hoppers. It is recommended that a P-Factor of 1.5 be used to calculate arching dimensions when vibrators are in use.  (2) Vibrators sometimes work well when your material gains strength with time but is easy handling during continuous flow (0 hrs storage). Vibrators should be used only to initiate flow and should be turned off once flow is initiated. The following equations can be used to estimate P-Factor due to vibrator use as described here:

P-Factor = (1 + x/g) or P-Factor = y/g, whichever is larger, where:

x = vertical upward component of acceleration y = horizontal component of acceleration

g = gravity constant

Impact on filling

If you are filling your bin with a  material and it drops close to the outlet, the P-Factor should be calculated as follows:

P-FACTOR = (1 = m) [w/(A B γ)] 2h/g where:

w = weight flow rate into bin h = height of fall

m = 0 for a rectangular outlet m = 1 for a circular outlet

A = impact area

B = outlet size of cone or slot

γ = bulk density

 Mass Flow Bc, ft—“B” is the hopper opening and “c” stands for conical, such that these dimensions are given for 0 hrs and for some period of storage at rest. These are the minimum arching dimensions for a conical hopper that is designed for mass flow.

Mass Flow Bp, ft—“B” again is the hopper opening, while “p” stands for planar or wedge type hoppers, such that these dimensions are given for 0 hrs and for some period of storage at rest. These are the minimum arching dimensions for the width of a slotted opening in a wedge hopper, designed for mass flow. Remember that the slot length should be at least three times the width.

Funnel Flow Bf, ft—“B” is the hopper opening and “f” stands for a funnel flow slotted opening, such that these dimensions are given for 0 hrs and for some period of storage at rest. These are the arching dimensions for a slotted opening in a funnel flow bin, such as a long slot on a flat bottom bin.

Ratholing Dimensions–the following are indicated as conditions your material was exposed to:

Time at Rest, hrs—Same as above

Temp., ºF—Same as above

σ1, psf—This is the major consolidation pressure acting on the material as it remains in a bin. This pressure is simulated in the laboratory tests.

EH, ft—Effective Head (EH) is determined as a result of material sliding on the cylinder walls. As it slides, the material loses some of its head pressure due to shear along the walls and is referred to as EH rather than the actual head of material. The effective head in the example report ranges from 5’ to 40’.

The critical rathole diameter DF is a function of the major consolidating pressure that acts on the solid in the bin, which is expressed in terms of EH, the effective consolidating head of solid in the bin, as follows:

EH = [R/(u k)] [ 1 – EXP(-uk H/R)]

R = hydraulic radius of the cylindrical portion of your bin, i.e. ratio of cross sectional area to circumference.  R= D/4 for a circular cylinder of diameter D.

R = W/2 for a long rectangular cylinder of width W.

u= tan (PHI-PRIME), coefficient of friction between the stored solid and the cylinder walls

k= ratio of horizontal to vertical solids pressure. A value of 0.4 is usually acceptable within cylinders.

h= height of the cylindrical portion of a bin.

Critical Rathole Diameters, Df, ft—“D” is the diameter of the opening required to collapse a rathole, while “f” stands for funnel flow. If your effective is 20’ after 3 days at rest the rathole dimension is 18.3’, meaning that an 18.3’ diameter opening is required to collapse a rathole, even at this low head.

Page 6: Compressibility Test Results

Section 2 Bulk Density/Pressure Relationship of the Flow Report on Page 6 indicates the bulk density of your material as a function of consolidation pressure or head of material as follows:

σ1, psf—Same as above

EH, ft—Same as above

γ, pcf—γ is the Greek symbol used for bulk density. There are a range of bulk densities when dealing with solids, not just loose density and packed density. In this case, γ ranges from 38.2 pcf to 48.9 pcf. γ is used in opening size and hopper angle calculations along with bin and feeder load calculations.

Page 7: Wall Friction Properties Test Results

Section 3 Recommended Hopper Angles for Mass Flow of the Flow Report beginning on Page 7, indicates the conical and wedge hopper angles required to ensure flow along the walls i.e. mass flow, as follows:

Outlet Dia., Cone, ft – Is usually interpreted as the conical opening size; however, it could be any span in the hopper, not just the opening.

Outlet Width, Slot, ft – Is usually interpreted as the slot opening size; however, it could be any span in the hopper, not just the opening.

ø’ – This is the wall friction angle generated during a wall friction on a particular wall surface. It is given in degrees from horizontal.

θc – This is the hopper angle (degrees from vertical) required for mass flow in a conical hopper.

θp – This is the hopper (degrees from vertical) required for mass flow along the sidewall of a wedge type hopper.

As an example of using these dimensions, you could design a mass flow conical hopper with a 2.1’ diameter opening (to prevent arching) or a slot that is 1.0 ft wide, that would require the following hopper slopes (depending on the wall surface preferred):

Wall Surface                     θc                     θp

2B stainless steel           16                      27

TIVAR 88                       23                     33

Carbon steel                    7                      16

To summarize, the Flow Report essentially describes the geometry required to ensure reliable flow. It yields opening sizes to prevent arching and ratholing, bulk density values, and hopper angles required for mass flow.

 

Handle Cohesive Products with Live Bottom Screws

Handle Cohesive Products with Live Bottom Screws
by Joseph Marinelli, Solids Handling Technologies

Those of you have read my column over the years know that I am a big proponent of screw feeders. Screws are reliable, robust and can handle a range of products, but most importantly will maintain a mass flow pattern if designed properly. Most bulk solids require a mass flow pattern as funnel flow is susceptible to ratholing, erratic flow, flooding, segregation, etc. As I have always stated, in order to evaluate whether a solid is going to flow properly, you need to measure its flow properties. Once its flow properties are determined, reliable bin and feeder designs can be developed.

Some materials are extremely cohesive and as such require large openings to prevent arching. Materials that come to mind are synthetic gypsum (sometimes referred to as FGD gypsum, flue gas desulferized gypsum), sludge, wet, heavy solids, etc. Mass flow is definitely required to maintain reliable flow. The feeder design is critical to maintaining a mass flow pattern. If the feeder creates a preferential flow channel, the material will flow in funnel flow and destroy the required mass flow pattern.

One approach that is commonly used is live bottom screws. As the name implies, the bottom (outlet) is fully live. Material discharges over the entire cross-sectional area of rather large outlets to maintain mass flow.

Assuming that your hopper is designed to give you reliable mass flow, a live bottom screw feeder must be capable of maintaining the mass flow pattern and withdraw material uniformly over the entire outlet’s cross-sectional area. The key to this approach is to ensure that the feeder increasesin capacity in the discharge direction.

Take, for example, the constant pitch, constant diameter screw shown here:

The material will be withdrawn preferentially from the back of the screw. The constant pitch flights do not allow any increase in capacity in the direction of feed. Therefore, the last flight fills with material and there is no more capacity to take material over the entire outlet length. Any modifications that you made to your hopper to ensure mass flow are now rendered useless. The improperly designed screw feeder creates a preferential flow channel that enforces a funnel flow pattern with its resulting problems of ratholing, erratic flow, flooding, segregation, etc.

The design shown here is a combination of conical shaft and increasing pitch to maintain
mass flow:

You might notice the following:
1. Trough Shape:  There is U-shaped trough to contain the screw. Vee-shaped troughs will not work because material usually only feeds directly above the screw diameter. A Vee-shaped trough will create stagnant material and destroy your mass flow pattern.
2. Hanger Bearings:  There are no hanger bearings to support the screw; therefore, the screw shaft has to be sized to withstand deflection. Hanger bearings will interfere with the full screw trough. Material will back up into the hopper, affecting flow. Your hanger bearings will also wear out quickly.
3. Finishes:  The screw flights should be smooth while the screw trough (walls) should be rough. If they are both smooth, the material will simply spin in the trough and not convey.
4. Speed:  The screw speed should be kept between 3 and 40 rpm. The screw speed should be 3 rpm on the low side to minimize speed reducer size and 40 rpm on the high side to maintain screw efficiency.
5. Clearances:  There is Ѕ” clearance between the screw and screw trough.

Remember to always:
• Measure your material’s flow properties.
• Design your bin for the correct flow pattern.
• Design your live bottom screw feeder properly, to ensure reliable flow.

For more information contact our author:

Joe Marinelli
Solids Handling Technologies, Inc.
1631 Caille Court
Fort Mill, SC 29708
Telephone: 803-802-5527
Fax: 803-802-0193
Email: joe@solidshandlingtech.com

Web site: http://www.solidshandlingtech.com/

Guest articles for the Ask Joe! Column are always welcome, for more information please contact Joe Marinelli directly at his email address:  joe@solidshandlingtech.com.

Requirements for Biomass Sampling

Requirements for Biomass Sampling
Guest article by Paul Janze, Sandwell Engineering

Introduction

With the current emphasis on the use of biomass for `green’ energy purposes, the importance of having good quality `hog fuel’, cannot be over-emphasized. And to ensure good quality fuel, good sampling procedures must be followed.

Woody biomass in chip form has been utilized by the pulp and paper industry for many decades and chip quality has long been recognized as having an important effect on pulp quality; to make good pulp you need good chips. Likewise for a biomass-fired plant to operate efficiently, it needs a reliable, constant supply of consistent quality fuel.

Good quality control relies on proper sampling, which must be accurate and precise and truly represent the main body of biomass. Without good sampling, quality control is based on false information. Bulk materials are difficult to sample properly on a production basis. Manual sampling can be done but it is labor intensive, prone to errors and does not easily fit into a production environment.

Collecting samples of biomass is one task not always done well, and unless the sample is taken properly, it will not be a true representation of the main product flow.

Biomass is not easy to sample. It appears in a myriad of species, forms and sizes; it knits together, doesn’t flow well, consolidates and packs easily; it can have a wide range of moisture contents, basic and bulk densities and calorific values; it will freeze; it is very dusty, catches fire easily and is self-combustible; it can contain all manner of contaminants; it can be quite fragile and care must be taken not to degrade the material.

To be of value, biomass samples need to be unbiased, accurate, precise and representative of the main lot or consignment of material.

The intent of this paper is to summarize the requirements that must be met in order to provide representative chip sampling and to do so in an easily understood and achievable manner. This paper does not consider chip classification or analysis.

1.  Features of Good Sampling

Accuracy – is the ability to obtain samples which represent the true nature of the material supply.

Precision – is a statistical term relating to the number of samples taken from a lot or consignment.

Biomass is a highly variable product and requires a large number of samples in order to establish sampling precision.  Precision can only be increased by increasing the number of spot samples.

A single spot sample generally is less likely to be representative of the main body of biomass than is a gross sample, which is a mixture of multiple spot samples.

2.  Sampling Bias

Poor sampling can induce systematic errors that skew the results. Two common errors are:

  1. Spot samples are taken where certain properties are over-represented. eg. – at tail-gate of a truck where fines have settled to the bottom.
  2. The sample device is not capable of taking a representative sample. eg. – the sample device is too small and either rejects large pieces or overflows.

Properly designed, operated and maintained automatic samplers can minimize systematic sampling bias.

Fractionation

Fractionation or particle separation can occur from the way in which biomass is loaded / stacked and the way in which it is transported. eg. – fines will settle down to the bottom of a biomass truck over a long haul.

Additionally, fractionation can be produced by the sampling device itself; either by rejecting oversize pieces, failing to pick-up small particles, selectively picking up small particles, or breaking up particles by the sampling motion itself.

Stationary material is not uniform due to fractionation where material stratifies according to size. In order to get a representative gross sample from stationary material, a very large number of increment or spot samples are required.

3.  Classification of Sampling Techniques

The quality of the sample depends upon the amount of human discretion involved, the sampling conditions, and the sampling location and timing.

  1. Generally, those samples which do not involve human discretion are more accurate.
  2. Accepted sampling conditions, include:
  • Stopped Belt Cut (Laboratory Reference Method) – taking a full cross-section cut of material from off of a stopped conveyor belt.
  • Full Stream Cut – taking a full cross-section cut of material from a falling stream.
  • Partial Stream Cut – only taking a part of the cross-section of falling stream.
  • Stationary Sampling – taking a sample from a stationary pile or container.

Generally, the stopped belt cut provides the best sample but is impractical to use in a production setting.  Full-stream cut and partial-stream cut samples can produce results which are representative of the main body of material, depending upon the sampling equipment design.  Stationary sampling produces the poorest sample.

  1. Sample location and timing
  • Systematic – samples are taken evenly spaced in time or location.
  • Random – samples are taken randomly spaced in time or location.

Systematic spacing generally provides better results. The challenge is to design a sampling system and procedure, which repetitively produces the most representative sample and is easy and practical to use.

4.  Establishing Sampling Procedures and Selecting Sampling Equipment

Establishing sampling procedures and selecting sampling devices requires an understanding of the following:

Material Being Sampled

Biomass is highly variable in size, configuration and moisture content. It is relatively fragile and size is often an important factor, so care must be taken not to break the particles unnecessarily during sampling.

Number and Size of Samples

This depends upon the variability of the biomass and is usually determined by the plant technical department based on historical statistical results.  Where new sources are coming on line, it can be expected that the number of samples required will be greater than for established sources, where historical data is available.  Normal sample size required in the lab is 8-10 litres (~0.35 ftі).

Sample Collection Method

The best practical method should be used. The `stopped-belt, full-cut’ method is the best, but is not practical.  The `falling stream, full-cut’ or `partial cut’ are the next best methods.

Sample Distribution Through Lot

Sample increments must be distributed through the whole volume of the lot, so that any one particle has an equal chance of being selected.  This is particularly important where `fractionation’ has occurred due to fines segregation to one part of the lot / consignment.

Characteristics and Movement of Sampling Device

The opening of the sampling device must be large enough so as not to reject the largest possible piece and the capacity must be large enough to completely contain the sample without spillage.  There should be no rejection by size of material or movement of the device through the material.  There should be no contamination of the sample by the device.

Preferably, the sample device will pass through the entire cross-section of the stream so that each particle has an equal chance of being selected; or at least through a partial section that will contain all particle sizes within the stream.

Device speed through the flow is critical as the device must not block the flow of material. The sampling device shall be non-clogging, self-cleaning and shall be designed to facilitate inspection and maintenance.  Ideally, it will not be complex or costly to purchase and maintain.

Location of Sampling Equipment

Equipment shall be located where:

  • It can effectively take a representative sample.
  • It is convenient and readily accessible for sample taking.
  • It is readily accessible for maintenance and inspection.

Maintenance of Sampling Equipment

Sampling equipment must be safely and readily accessible to enable inspection, cleaning and maintenance.  Worn mechanical sampling equipment can produce biased results, therefore it is imperative that equipment is inspected, repaired when necessary, and the performance measured regularly.

Preservation of Moisture Content

The sampling device / method shall neither dry the biomass out or add moisture.

Personnel

Sampling personnel shall be properly trained and qualified.

Criteria of Sampling Performance

Sampling procedures and equipment must be routinely monitored to ensure that the samples being collected are:

  • Unbiased
  • Accurate
  • Provide the degree of precision required
  • Representative of the whole lot

In addition, a constant sampling ratio should be maintained; ie. – constant volume or weight of sample as compared to the whole lot.

Sample Handling

Samples once collected must be:

  • Clearly labeled and identified
  • Sealed in moisture-proof containers
  • Stored in a cool, dry place

Samples properly stored in this manner can be kept in storage for up to 36 hours with no appreciable moisture loss.

Gross Sample Size Reduction

Normal increment sample size as collected will be 8-10 litres (average bucket size); therefore, combined multiple increments (the gross sample) must be reduced down to the laboratory sample size in a manner that retains biomass representative of the whole lot.

Large gross samples shall be thoroughly mixed before being reduced to laboratory sample size.
Large samples can be reduced in size by mixing thoroughly and coning and quartering or by the use of an automatic splitter.

5. Design of Sampling Equipment

There are many `off-the-shelf’ product samplers; most of which were designed for materials other than biomass. However, it is the author’s experience that the best sampling equipment is custom designed to meet the specific requirements of the biomass sampling application considering the sampling requirements described in this paper, the product being handled, and the physical and operational constraints.

(Photo:  Wood Chip Sampler)

Summary

To be of value, biomass samples need to be unbiased, accurate, precise and representative of the main lot or consignment of biomass.  Biomass is highly variable in size, configuration and moisture content and is prone to fractionation and stratification, which complicate the sampling procedure.

Single spot / increment samples tend not to be accurate or representative of the main lot / consignment, particularly those samples taken from stationary loads or piles where fractionation has occurred.

Two common sampling errors, which bias results are:

  • Spot samples are taken where certain properties are over-represented. eg. – at the tail-gate of a truck where fines have settled to the bottom.
  • The sample device is not capable of taking a representative sample. ie. – the sample device is too small and either rejects large pieces or overflows.

The best, practical sampling procedures and equipment have the following features:

  • Human discretion is minimized.
  • Full-stream cut or partial-stream cut sampling is employed.
  • Samples are taken systematically in time and /or location throughout the whole volume of the main lot / consignment.
  • They do not introduce bias or contaminants into the sample.
  • They are convenient to use.
  • Preferably, they automatically reduce the gross sample to the laboratory sample size.
  • Preferably, they have low capital and maintenance costs.

It is not easy to achieve all of these objectives, however, the primary goals of accuracy, precision and representativeness must take precedence.  Convenience and cost, while important, are secondary considerations.

About our author

Paul Janze is a senior material handling specialist with more than 30 years experience in engineering, equipment design and manufacture, project management and plant maintenance, primarily in the forest products industry. He is a specialist with difficult-to-handle materials such as wood chips, hog fuel, wastewood and bark, biosolids sludge and wet pulp, poultry litter and boiler ash which all have differing and unique handling characteristics.

For more information contact:

Mr. Paul Janze
Material Handling Specialist
Sandwell Engineering Inc.
885 Dunsmuir Street, Suite 600
Vancouver, BC,  V6C-1N5
Canada
Telephone: 604-684-9311
Fax: 604-688-5913
Email: pjanze@sandwell.com
Web site: http://www.sandwell.com/

Guest articles for the Ask Joe! Column are always welcome, for more information please contact Joe Marinelli directly at his email address:  joe@solidshandlingtech.com.

 

Models can help to address a Flow Problem, but Beware!

Models can help to address a Flow Problem, but Beware!
by Joe Marinelli, Solids Handling Technologies, Inc.

As consultants, we rely on established test methods and theory to identify, evaluate, and solve bulk solids flow problems. In this column we have written several articles covering proven solids handling test techniques such as shear testing, segregation testing and compressibility testing.

These articles explain that flow properties’ testing is absolutely necessary to ensure reliable flow from bins, hoppers and feeders. In this article we will discuss the benefits of modeling to aid in providing viable solutions to solids flow problems. But first, we need to remember the following when evaluating flow problems:

Material Properties

  • What materials are you handling?
  • What is their moisture content and how is it measured?
  • What is their particle size?
  • Are they exposed to temperature variations such as dried product entering a bin at 150 degrees F and cooling to room temperature?
  • How long does the material remain at rest in the silo? This is critical to determine the effect storage at rest on the cohesive strength of the product.
  • What is the bulk density?
  • What discharge rate do you require from the silo or bin?

Geometry

  • What are the current bin diameter or capacity requirements for a new bin?
  • What is the shape of the existing hopper?
  • What is the size of the opening?
  • How is it filled?
  • What is the silo or bin fabricated from, lined or coated with?
  • What type of feeder is used to control discharge rate?
  • What equipment is upstream and downstream of the bin or silo?

Modeling flow in bins and hoppers has been performed for years and has served a very useful purpose. I remember my early years as a consultant helping to build models, running and evaluating test results and using the information to lend confidence to the conceptual designs I recommended.

It is very useful when working with unusual geometries or complex handling equipment. Scaling becomes an issue at times especially when dealing with a cohesive product that has large arching and ratholing dimensions.

Here is a list of flow parameters that affect scaling:

  • Geometric similarity to the full scale bin or hopper
  • Material of fabrication, which affects wall friction values and subsequently hopper angles, should be similar to those in a full scale system and are critical to model success.
  • Can you use the actual product handled in the full scale application or do you need to find another material that will flow reliably in your model?

Here we discuss a success story using modeling to help resolve a nagging flow issue.

Our client was experiencing segregation problems handling their particular solid. We visited the site and determined from the visit and subsequent flow properties testing that the material was flowing in a funnel flow pattern whereby some material was flowing while most remained stagnant. The powder was subjected to sifting segregation as the fines concentrated in the center and the coarser particles rolled to the outside. As they initiated discharge from the bin, they noticed a high percentage of fines which negatively affected their product quality.

In order to solve the segregation problems, it was imperative that the material flow in a mass flow pattern, whereby all the material is in motion whenever any is withdrawn, i.e. product flows at the walls. Because of headroom constraints simply replacing the existing funnel flow hopper with a mass flow hopper, was not an option. We therefore recommended a cone-in-cone design similar to the one shown here.

The inner cone is fabricated from 2B stainless steel while the outer cone is lined with 2B stainless steel. This approach assures mass flow because the inner cone forces material to slide along the walls of the formerly too shallow outer cone. Notice the lower cone below the cone-in-cone set up as it will become important later.

After a few months of operation, it was determined that the bin was still experiencing segregation issues. This was puzzling since the cone-in-cone anti-segregating design should have solved the problem. When these type problems occur, it is usual due to material changes, installation and/or fabrication errors, etc. As far as we could determine, the fabrication and installation was as required and the material had not changed.

We then visited the site several time and we were able to observe the powder actually flowing. The bin was filled from empty with about 40 tons of product. The level of the product was just above the inner cone. The bin was then packed out at a rate of about 1000 lb/min. Immediately it was noticed that the inner cone only was flowing. The material flowed in mass flow (flow along the walls) in the inner cone, until it was empty. All the while, the annular region between the inner and outer cone did not flow. After the inner cone emptied, the annulus began to flow. Flow continued as mass flow in the outer cone area.

As a result, we fabricated a scale model of the bin in our laboratory using an existing 14″ diameter 60 degree, conical hopper modified with the addition of an inner 75 degree cone. This unit is scaled to exactly represent the existing bin. The model was filled with product supplied by client to the same level as was observed in the field (just to the top of the inner cone) and flow started.

We video taped the flow through the unit and observed that flow was as expected, uniform with both the inner cone and annular region moving towards the outlet, as shown in the video here.  We then tried several experiments to determine what could be possibly causing the inequality in the field. We raised the inner cone, we lowered the inner cone we enlarged the outlet, thinking that perhaps the field install was not exact. We noticed that while small changes in the lab equate to large changes in the field, there was only a minor effect on flow.

 

Modeling Test Video 1.

After several days of trials and head scratching, we finally noticed that the lower cone of the full scale bin, attached to the vertical section at the outlet, sloping at 75 degrees was fabricated from carbon steel instead of 2B stainless steel as recommended. We ran a wall friction test on carbon steel and discovered that the angle for mass flow is required to be 78 degrees to ensure flow along the walls of the lower cone.

We then roughened the walls of the lower cone of our laboratory model (installed sandpaper) and reran the test. The inner cone emptied first then the annular region emptied, just as in the field, as shown in the video here.  This meant that the lower cone was not capable of mass flow (in the field), creating a funnel flow pattern and causing the inner portion of the cone-in-cone to empty first.

 

 

 

Modeling Test Video 2.

We recommended that the lower cone be replaced with 2B stainless steel cone sloping at 75 degrees to ensure mass flow. The bin has been functioning properly for over one year now.

 

For more information contact:

Joseph Marinelli
Solids Handling Technologies, Inc.
1631 Caille Ct
Fort Mill, SC 29708
Telephone: 803-802-5527
Web site: http://www.solidshandlingtech.com/

 

Guest articles for the Ask Joe! Column are always welcome, for more information please contact Joe Marinelli directly at his email address:  joe@solidshandlingtech.com.

Should My Material Be Tested On-Site?

Should My Material Be Tested On-Site?
by Joseph Marinelli, Solids Handling Technologies, Inc.


In the past, we have discussed flow properties testing and the need for this type work to ensure reliable bin and hopper flow. The Jenike Shear Tester is a device that is recognized as the standard for testing bulk solids by ASTM (D-6128-97) in the US and in Europe and the test procedure is well adapted for a testing laboratory.

Jenike’s method has been scrutinized and perfected over the years and remains the preferred approach. It is classified as a direct shear tester that is capable of providing information on a solid’s cohesive strength as well as its wall friction properties. These are the two main considerations when designing a bin or hopper to ensure reliable material flow.

One key to reliable flow testing is to expose the solid being tested to conditions representative of actual environmental conditions it will be exposed in the field. Consider that not all bulk solids are handled at room temperature in a controlled environment. In order to run flow tests in the laboratory, an appropriate amount of the product to be tested is required to be shipped in a sealed container to the testing laboratory.

The testing laboratory should then ensure that the product is tested under representative conditions of moisture content, temperature, time at rest, etc. But, what happens if your material changes properties after a certain period of time. Perhaps the sample that was taken has been placed in a sealed container and shipped, typically requiring two to four days of travel depending on the shipping method.

What if the product’s moisture content, which at one time was surface moisture, now migrates into the particles and becomes inherent moisture. This may affect the material’s flow properties, especially its cohesive strength. When the sample is received, it is thoroughly mixed and then transferred to a jar for testing. Any cohesive strength due to moisture migration has been destroyed, and the results may be inaccurate, because in actual handling in a silo this would not be the case.

What can be done to overcome this particular problem?

One solution would be to run the flow properties tests on-site. This would allow access to fresh material and accurate simulation of environmental conditions. This is a common approach that we have used several times. We ship or drive the test equipment to the facility and set up somewhere near the process to be evaluated. We require about a 6’ long table and electricity to accomplish the tests. A fresh sample is taken from the process for testing.

This process is somewhat cumbersome in that the Jenike Shear Tester, shown here, weighs about 75 lb.  The rest of the equipment required for testing is shipped and totals 30 – 50 lb additional weight.

To simulate loads applied to the material, we require weights (several hundred pounds). These weights are applied to the shear cells in order to simulate the loads the material is exposed to in storage. Shipping becomes expensive and time consuming in that the equipment cannot be used while it is being shipped.

However, you must always keep in mind that the most important consideration is an accurate representation of the material flow properties. If it takes this type of approach to ensure accuracy, then it is absolutely necessary.

Having performed on-site tests many times over the years, I am still amazed at the attention the test setup receives. People wander by wondering what you are doing and luckily I enjoy talking about the shear test process.

For more information contact:

Joseph Marinelli
Solids Handling Technologies, Inc.
1631 Caille Ct
Fort Mill, SC 29708
Telephone:  803-802-5527
Web site:  http://www.solidshandlingtech.com/

Guest articles for the Ask Joe! Column are always welcome, for more information please contact Joe Marinelli directly at his email address:  joe@solidshandlingtech.com.