PPC

Introduction to Particle Size and Concentration Measurements for Real-Time Process Applications:

Introduction to Particle Size and Concentration Measurements for Real-Time Process Applications:

The Process Metrix PPC (Process Particle Counter) is designed to provide in-situ measurements of airborne particles for low concentration (< 5 gm/m3) industrial gas/particle flow systems. This sensor is capable of providing particle concentration, size distribution, and velocity information in hostile environments. PPC can (when enclosed in a water-jacketed probe) measure in gas temperatures up to 1400C or high pressures (up to 100 ATM) in extractive mode. Measurements at combined high temperature and pressure (400C and 20 Atm) have also been achieved.

Industry Applications:

Gas Turbines

Turbo-expanders

Filtration Efficiency measurements

Stack and Duct monitoring (CEMs)

Industries:

Petroleum refineries

Gas Turbines

Gas Distribution Systems

Cement, Steel

Foundry

Aluminum

Boilers

Powder Production/collection

Pulp and Paper

Powder milling and Processing

Laboratory and Field Research

Filter Testing, and more

Particle Size and Concentration Measurements for Real-Time Process Applications

Over a 20 year development period Process Metrix (and its predecessor, Insitec) have developed three types of optical in situ real-time particle instruments, which fall under two technical categories. The two types are ensemble (measuring the light scattering from a group of particles) and single particle counting (SPC). Instruments within these categories have differing performance characteristics depending primarily on size and concentration. Basically, ensemble methods are needed for high concentration applications, while SPC methods are required for low concentrations, with the dividing line typically being 1-10 ppm by volume. The choice of which type of instrument is most suitable for a given application is the subject of the following note.

The determining factor for the optimal instrument choice is primarily particle concentration, and secondarily particle size. This is certainly an absolute requirement for in-process measurements, where it is not possible to use dilution to match instrument operating requirements. In contrast, for laboratory measurements the user generally has the latitude to provide sufficient dilution to match limited instrument range. The downside of this laboratory flexibility is the time delay and the need for uncertain sample acquisition and handling, which can create uncertainties regarding sample integrity.

Figure 1. Instrument operational map as a function of particle size and concentration for four in-process instruments developed by PMC. PPC and Stilletto are single particle counters, while STAR and EPCS are based on scattering information from an ensemble of scattering particles.

Single particle counting (SPC) optical instruments rely on passing one particle at a time through the optical sensing volume. For laboratory-based SPC instruments, this limit is approximately 10³-10⁴ particles/cm³. This corresponds to a volume concentration of about 1 part per million (PPM) for particles with a 10 micron median diameter, leading to an upper limit mass concentration of 0.3 gm/m³ for unit density particles. For particles of 1 micron median diameter, this limit would be more in the range of 0.3 part per billion (PPB) by volume or 0.3 mg/m³.

Process Metrix instruments emphasize the developing and future trend toward automated real-time in-process measurements. Figure 1 shows a general operating map for Process Metrix single particle counting and ensemble instruments. Note that the particle concentration range for all these instruments exceeds 15 orders of magnitude, along with 4 orders of magnitude in size range! It is no surprise that different instrument configurations will be required to accommodate this broad range of potential applications.

The PPC SPC technique for in situ measurements was developed to accommodate particle number concentrations up to 10⁷/cm³ by using a small optical sample volume with a beam diameter of 10 microns. Based on our experience, this # concentration appears to be a practical upper limit for SPC instruments. Cost, complexity, and practical physical limitations all conspire to put the maximum achievable limit for SPC instruments at less than 10⁸ particles/cm³. Nevertheless, this is 3-4 orders of magnitude higher concentration than laboratory instruments! On a mass basis, this corresponds to a general limit of less than 5 gm/m3 (5 ppm by volume) for typical particle distributions with median diameters less than 10 microns.
Primary applications include high temperature or pressure measurements in power generation boilers, gasifiers, and gas turbines. Other applications include petroleum processing with catalysts, and large scale industrial filtration processes. For a summary discussion of new applications to gas turbines and refinery expanders being developed by PMC in conjunction with DOE, click on the following Brochure and Application note, (PPC brochure June 2003.pdf, PPC App Note, August, 04.pdf ).

PPC Manual (February 2007), Must have user ID and password to access this document.

For information regarding the Process Particle Counter (PPC) contact:

Donald Holve, Ph.D.
dholve@processmetrix.com
voice: (925) 460-0385 x116