Process Sampling

Process Sampling Sentry Equipment Corporation

Objective • Gain an understanding of the methods to obtain representative samples • Review the fundamentals of gases, liquids and bulk solids • Overview of analyzing methods • Review the integration of sampling with on-line instrumentation

Why do we sample? • Product mix characteristics • Examples: Particle size, bacteria count, chemistry, etc. • Identify process variations • Process control • Environmental monitoring • Product quality assurance • Custody transfer

What kind of sample? • “True” overall average of the entire lot for one or many characteristics • Representative sample • Characteristics of the sample equal that of the entire lot or batch • Consistent, accurate, and repeatable

How to get a representative sample • Follow protocol that gets a consistent, accurate, and repeatable sample • Understand the material physical properties • Use techniques that have as little sample bias as possible • Profile the process and sampling system (i.e.. Liquids, slurries, gases, bulk solids, etc.) • Reduce sample to sample variation • Sample methods and results must be repeatable

Sampling Errors • Process analysis is only as good as the quality and representivity of the sample itself • Sampling errors are costly because they can lead to: • Unnecessary process changes • Additional analysis of more samples • Release of off-spec product to the customers • Scrap of good material • The Sampling Scapegoat

General Sampling Practices • Understand the process and the sampling objective to help select the proper device • I.e. Gas concentration, liquid chemistry, bulk solids particle size analysis • Determine the best sampler location • Take several increments and composite them to form the “sample” • Composite Sample – A sample obtained by combining several distinct sub-samples or increments • Sample frequently enough to identify process cycles

Direct Sampling • Typically includes dipping a cup or container into an open vessel or opening a spigot on a vessel that may be under pressure • May require the process to be interrupted in order to sample • This sampling technique is generally considered unsafe • Operator is exposed to any process or environmental hazards • Direct sampling introduces the ability for sample variations due to human error and sample contamination

Indirect Sampling • Indirect sampling isolates the operator from the process hazards • Maintains a closed system • Sample integrity preserved • Allows the process to be sampled during normal production cycle

A few general sampling practices • Mix the material prior to sampling (if possible) • Take several increments and composite them to form the “sample” • Composite Sample – A sample obtained by combining several distinct sub samples or increments • Collect the sample in a container that will not react with the sample • Sample frequently enough to identify process cycles

A few general sampling practices (cont.) • Determine the correct size required for the sample • Use a sampler and sampling process that removes bias • Understand the process • Sampling practices may vary from process to process

Three categories for sampling variation • Process variations/cycles • Material variation • Tool and techniques • Including sample handling

Process Variations • Be aware that the process and its conditions may vary over time • Examine the sampling results and the effects of sampling frequency • Use the sampler and sampling system to verify the process

Ways to reduce material variation • Determine the appropriate mass of the actual sample • Provides a better idea about the overall properties of the lot • Collect several increments from the lot and form a composite sample • Condition the material prior to sampling • Mix the sample if possible • Control the temperature, pressure, flow, etc. • Control the particle size

Product Characteristics • Identification of the product characteristics is critical for obtaining a representative sample • Every gas, liquid, and bulk solid has its own unique physical property • Many processes contain entrained gases that can expand when decompressed • Venting provisions must be taken into account • Bulk solid characteristics can be very unpredictable • Some may pack one moment and fluidize the next • Two grades of the same material may act differently

Flow Characteristics • Mass Flow • Funnel Flow • Arching/Bridging • Rat-holing • Flooding • Segregation

Material Properties • Abrasive – Materials that wear, grind, or rub on the inside of the sampler or any moving part • Arching/Bridging – Materials that tend to form a bridge-like structure • Free-Flowing – Materials that generally flow under the influence of gravity without the aid of agitation • Floodable – Materials that aerate to a point where they begin to flow like a liquid • Hygroscopic – Materials that tend to absorb and retain moisture from the atmosphere

Material Properties • Cohesive – Materials that tend to stick to itself • Adhesive – Materials that tend to adhere or stick to components of the sampler • Friable – Materials that are easily pulverized or broken apart • Static Generating – Materials that generate static from friction on itself or within the sampler

Bulk Density • Mass per unit of volume in powder form, including the air trapped between the particles • Measured in g/cc, kg/l, lb/ft3 • Examples: • Cake Mix = 44 lb/ft3 (0.705 g/cc) • Cement Powder = 85 lb/ft3 (1.361 g/cc) • Oats = 27 lb/ft3 (0.432 g/cc) • Parsley Flakes = 3 lb/ft3 (0.048 g/cc) • Sand = 99 lb/ft3 (1.586 g/cc) • Tungsten Carbide = 250 lb/ft3 (4.004 g/cc)

Tools and Techniques

Sampling Tool • The sampling tool is critical for providing a representative sample • The sampler must assure the sample integrity is preserved • Understand the analysis requirements to help determine which sampler to choose • Understand the process variables that may impact the sampler design • The sampler should keep the operator safe from any process or environmental hazards

Principle for correct sampling • Every part of the lot has an equal chance of being in the sample • Integrity of the sample is preserved before, during, and after sampling • Oxidation, abrasion, evaporation, and contamination are examples of improper handling • Vapor pressure is an important characteristic to take into account when sampling • The measure of the tendency of a material to form a vapor.

Manual “Grab” Samples • Typically, manual grab sampling is a poor, but common practice because it is taken from the most accessible part of the batch/process • Exception is when the sampler is located in a proper/acceptable sample point and other good practices are recognized

Gas Sampling • Maintain pressure and sample integrity • Monitor H2S, VOCs, water content • Speed loops and analyzers are common

Liquid Sampling • Control pressure, temperature, flow rate • Monitor pH, DO, Sodium, conductivity, TOCs, chemistry, etc. • On-line analysis is common

Bulk Solids Sampling • Material characteristics and flow properties vary • Monitor moisture, particle size, shape, etc • Difficult to analyze on-line • Material waste

Point Sample Strip Sample Cross Cut Sample Bulk Solids Sampling

Point Samplers • Takes a sample from a point in the material stream • Used when conducting chemical analysis or when material is homogenous

Strip Samplers • Takes a sample from a narrow portion of the stream • Used in situations where product segregation exists

Cross Cut Samplers • Takes a cross-section of the entire product stream • Provides the most representative sample, but requires a lot of headroom

Types of Measurements and Technology

At-Line On-Line Infrequent Samples sent to a remote location   Sample removed and tested locally Frequent Sample analysis conducted in real time Lab Testing Off, At and On-Line Off-Line 

Measurements/Analysis • Particle Size • Moisture • Chemical Composition • Surface Area • Shape • Temperature • Dissolved Oxygen • Effusivity • NIR • IR • Laser Induced Fluorescence • Light Scattering • Imaging

What you don’t know What you know What you don’t know that you don’t know Knowledge • Monitoring multiple properties provides insights into areas and mechanisms that have not been investigated

Summary • Sampling is a critical part of process control and is often overlooked • Indirect methods of sampling are the safest, yet most challenging • The goal is to get representative samples • Material and process variations as well as sampling techniques and tools effect our ability to get representative samples • Understand the process and sampling objectives 2 - New Rep Low E.ppt

Process Sampling