How to Keep Process Samples Out of the Lab

Inline or is it online? That is the question. No matter what it is referred to as, the idea is the same—get an accurate measurement of process "analyticals" at a point in or near the process stream. Even though the terms have been used interchangeably even in the not-so-recent past, they have evolved separate definitions.

02/01/2001


KEY WORDS

 

  • Process control & instrumentation

  • Inline instrumentation

  • Process analyzers

  • Process sensing

Sidebars:
On-line conductivity measurement ensures scrubber's process efficiency

Inline or is it online? That is the question. No matter what it is referred to as, the idea is the same-get an accurate measurement of process 'analyticals' at a point in or near the process stream. Even though the terms have been used interchangeably even in the not-so-recent past, they have evolved separate definitions.

Online process sensing has come to describe systems that are located near the process stream. Although they may move a media sample into an instrument with little or no human involvement, the fact remains that the sample must be transported some distance, whether manually or not. On the other hand, inline sensing has come to describe a process sensing system placed directly in the process stream. The 'sample' remains part of the process stream. In either case, the control engineer can be after the same process measurement. The sampling method makes the difference.

According to John Moore, vice president for e-business consulting services at ARC Advisory Group (Dedham, Mass.), there are often good reasons to go 'inline.' One, however, is usually not the cost of the equipment. In most cases, inline equipment is more costly to purchase, install, and commission than online instrumentation. But because labor costs of laboratory personnel and instrument technicians must be figured into total cost of ownership, process plants with lower overhead can present a better argument for the use of online equipment.

Other reasons are straightforward. Reliability of inline instrumentation-especially in the areas of gas chromatography and spectroscopy, says Mr. Moore-is excellent. The ability of inline instrumentation to sample and take readings at will allows processes to be optimized, resulting in higher product quality and lower production costs. Combine this with the ability to reduce human error and eliminate potentially hazardous exposure to technicians in the sample-gathering process and the promise of inline becomes attractive to process engineers.

Even with attractiveness of the technology, analytical instrumentation of this type still can be hard to find, much to the chagrin of many process engineers. According to John Becker, president of Analytical Technology Inc. (ATI, Oaks, Pa.), a company that makes gas sensors, 'Inline instruments are what everyone in the process industries wants, online is usually what they settle for.'

Hot and wet

Analyzers that must perform measurements on products of combustion usually have indirect access to the process stream. In measuring concentrations of gases, the presence of moisture and high temperatures are always problematic. Although keeping an analyzer an online unit is usually sufficient in protecting it from heat, this type of process penetration does nothing for moisture control.

There are two ways to collect combustion gas samples for analysis-cold and dry or hot and wet. Cold and dry requires that the moisture be removed from the sample using a chiller before it is sent to the analyzer. Hot and wet uses a heated probe and tube to convey the sample to the analyzer. To prevent condensation of the hot gas in the analyzer, the sample must be reduced to below atmospheric pressure downstream of the hot collection site. Eco Physics Inc. (Cleveland, O.) uses this technique in its high-end chemiluminescense-based NO/NO 2 /NO x analyzers.

Although both methods work for online analyzers, the hot and wet technique has an accuracy advantage. According to Tom McKarns, manager tech support for Eco Physics, hot and wet sampling gives a better picture of gas concentration, because NO 2 is soluble in water, and if the water is condensed out with a gas conditioner the concentration will be less than the true amount. Applications for this device include process control in boiler/burner and incinerator operations, power generation, and the metal processing industries.

Testing the waters

Obtaining meaningful measurement of moisture in continuously processed solid materials and emulsions (butter, sugar, soil, tobacco, etc.) requires technology that allows a noncontact sensor to determine moisture content of the moving materials. A number of different technologies have been adapted, including radio frequency absorption, infrared absorption, and microwave absorption techniques. Until these methods yielded practical solutions, however, determining moisture content was a slow and tedious lab procedure.

All three of these technologies have been put to use in continuous processes. According to Uwe Klotz, president of Industrial Products Inc. (IPI, McLean, Va.), a manufacturer of microwave absorption devices, these types of equipment have enabled manufacturers to streamline operations by taking the laboratory 'out of the loop,' improve quality, and help optimize plant operations. Accurately determining residual moisture in processed tobacco before storage, for instance, ensures consistency, aromatic quality, and shelf life of raw material prior to manufacture into finished products.

Accurate determination of water content prior to drying or hydrating product can help optimize operations. It gives the process engineer a benchmark by which energy consumption for the drying process or water/steam consumption for the hydrating process can be accurately predicted and controlled. Using this benchmark also helps facilitate energy and resource conservation, allowing a company to champion environmental responsibility.

Expanding limits

If infrared-based devices measure moisture, what are near-infrared devices (NIR) used for? According to NDC Infrared Engineering (Irwindale, Calif.), NIR methods measure moisture and other critical parameters (fats, proteins, sugars, etc.) in foods, chemicals, and pharmaceuticals. This technology uses dual detectors that are said to eliminate effects of changes in the source lamp or other system components. Additionally, in its MM710 gauge, NDC refined the process by splitting the beam to ensure that its two detectors track each other. The device features active diagnostic functions to automatically warn of any fault-window contamination, high internal temperature-and advise appropriate action

According to NDC, the gauge can be installed almost anywhere in a process line regardless of ambient temperature, factory-lighting levels, or variations in ambient relative humidity. The gauge measures and controls up to four components at once at a speed of one complete measurement every 7.5 msec. This 'virtually simultaneous' measurement reduces instrument noise and improves measurement accuracy. The device, which uses a proprietary full spectrum measurement concept, is said to make better use of the spectrum information and allows for more robust calibration to be derived, eliminating the need for regular recalibration.

Determining the mix

Oil and water do not mix, so the adage says. Actually, when oil is recovered from the ground it frequently has water mixed with it. Knowing and controlling the water/oil content improves the efficiency of oil recovery, maximizes quality via improved water removal (dehydration), minimizes water transport, and contributes to accurate custody transfer operation.

Honeywell IAC's (Phoenix, Ariz.) H 2 Oil Analyzer provides inline, real-time percentage water/oil measurement by determining intrinsic electrical properties (dielectric constant and conductivity) of the liquid flow. Using the electrical admittance calculation of the water/oil mixture, the instrument's analytical software model determines the percentage of water in the oil. Designed specifically for the petroleum recovery industry, this analyzer system uses an inline flanged sensor module and a remote module that serves as the human-machine interface and calibration unit.

Used for both oil well and flow station output testing, the device's high accuracy and inline functionality have reduced operating costs at the Petróleos de Venezuela, S.A.'s Lagotreco production unit in Venezuela, South America, says Eleazar Cardenas, automation support engineer at the site. Less dependence on laboratory measurements, fewer instrument calibrations, and improved oil production have helped maximize profits.

Water in oil is one thing. Pollutants in water, especially drinking water, are another. Water and wastewater industries have long made use of online instrumentation like pH/ORP devices. However, instruments for monitoring specific pollutants have been longer in coming, often due to their 'regional' nature. For example, hydrogen sulfide (H 2 S-infamous for that rotten egg smell and 'skunky' taste) occurs naturally, but not universally, in ground water supplies. In wastewater facilities, hydrogen sulfide in large concentrations can also damage concrete piping and remediation structures, such as holding ponds and settling tanks.

ATI offers an online system that measures low-level sulfide concentrations in water and wastewater applications. Sulfides are measured using a vapor-phase analytical system in which a sample is air-stripped into the vapor space above the reactor where measurement is made. The device uses a polargraphic sensor (essentially a gas-driven battery) which generates a linear signal proportional to H 2 S concentration. The A15/81 system handles concentrations from a few ppb to 20 ppm, with standard outputs and alarms that can be used to control chemical feed systems or provide process upset notification.


Keeping it clean

Disinfection of industrial wastewater involves complex chemical reactions. The 'bleach' smell often detected in disinfected water does not indicate an overdose of chlorine in a hit or miss manner. In reality, the process most often used for disinfecting drinking water and wastewater is called chloramination, a complex process that involves mixing chlorine and ammonia with the water at a neutral pH to form monochloramine, dichloramine, and trichloramine. Formation of all three are required for optimum disinfection. Early attempts at monitoring this process required three analyzers, two for chlorine and one for ammonia. This technique identified out-of-control conditions in the complex chemical reaction, but did not help fine-tune the chloramination process.

According to Richard Hofmann, research chemist at Hach Co. (Loveland, Co.) a better method of keeping the process in control is to determine the monochloramine and ammonia concentration in a single sample. Not only does this method directly measure the amount of monochloramine, the most effective and stable disinfectant, but also total ammonia. This allows control and optimization of the process using a single analyzer. Use of a single on-line analyzer permits the single process penetration to be placed as close to the instrument as possible, letting it respond faster to concentration changes.

In the crystal ball

Practical 'on the spot' analytical testing offers the process engineer the ultimate in process optimization opportunities by bringing more complex measurements into the realm of the 'usual suspects'-pressure, temperature, level, and flow. As cross-enterprise communication continues to improve, bringing all defining process measurements into the mix in real time can only help to provide an increasingly accurate picture of process, its quality level, and its profitability. As Mr. Becker of ATI says, 'Control engineers have gone about as far as they can in optimizing the process using only pressure, temperature, and flow. There is a need to accurately monitor more specific variables to do more.'




On-line conductivity measurement ensures scrubber's process efficiency

Numerous processes give off undesirable exhaust gases that must be collected, then treated. To meet clean air standards, fume scrubbers are widely utilized throughout industry to treat harmful process gases, vapors, dust, or odors. Scrubbers can be constructed to remove contaminants, such as NO and NO 2 , common fumes, and gases, such as chlorine, sulfur dioxide, hydrogen fluoride, hydrogen chloride, and others. They are also designed to remove particulate pollutants.

On-line conductivity sensing has been an integral part of high efficiency scrubber development, according to Marc Cartier, application engineer for +GF+Signet (El Monte, Calif.). A case in point is the Cloud Chamber Scrubber (CCS) designed by Tri-Mer Corp. (Owosso, Mich.). Within the scrubber, exhaust gases pass through 'clouds' of electrically charged water droplets, a technology developed by atmospheric physicist, Clyde Richards Ph.D., for Tri-Mer. The charge on the droplets attracts any submicron particulate, attracting it into the water. Fumes that touch the droplets are also removed. The ability to remove fumes, gases, and particulate simultaneously eliminated the maintenance and cost associated with conventional scrubber technologies. Efficiency of 99%+ is common for the CCS in the 0.1 to 100

'A critical performance spec in CCS operation is the conductance of the recirculated water used as the scrubbing medium,' says Mr. Cartier. 'Maintaining an application-specific conductivity range requires constant monitoring by an online conductivity sensor of the sump liquid and adjustment of water blowdown (dump) and make-up (replenish) rates. The severe duty of handling charged liquids requires sensors monitoring the process to be fully electrically isolated. High conductivity readings indicate high contaminant levels in the water resulting in automated blowdown. High contamination levels also form deposits that clog spray nozzles and pumps, so timely removal and replenishment of the working medium results in efficient scrubber operation, optimum reliability, low operating costs, and improved maintenance.



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