IPP&T Magazine Online

Filtering Samples to On-line Analyzers

July 13, 2015  

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Increasingly precise process control strategies, often linked to computer control, have accelerated the use  of sophisticated on-line chemical composition analyzers in plant  applications. Instruments such  as gas and liquid chromatographs, ion chromatographs, laser optic instruments, atomic  absorption instruments  and specific ion analyzers, which were  rarities in laboratories ten years ago, are  now found routinely in plant  settings. While continuing miniaturization and “ruggedization” of the electronics are  making  the instrumentation circuitry  more  tolerant of the plant  environment, a problem that  will not diminish  is the level of contamination in plant  samples compared to laboratory samples.

Factors in plant operation that  magnify the difficulty of delivering acceptable samples to on-line process analyzers are:

• The sample stream must be processed and fed to the analyzer continuously, rather than  the batchwise method permissible in the laboratory.
• Test frequency in the plant  is far greater than  in the laboratory. Where a laboratory analyzer might perform 100 tests a month, an on-line plant  analyzer could do 100 tests a day. In the high frequency plant usage, trace contaminants that  would not be a problem in the laboratory can rapidly build up and cause instrument failure.

Filter requirements
It is not surprising that contaminants in the plant  samples are reported to be the most  frequent cause of problems with on-line analyzers. While the requirements for an effective filter in the sampling line is generally recognized, it is also important to recognize that  it is usually necessary to use  a filter specifically designed for sampling applications, rather than  trying to make  do with a general purpose  or homemade filter.

The characteristics that  a sample filter should have, in addition  to filtering  out contaminants, are:

• The filter must not change the composition of the sample, other than to remove unwanted impurities. Therefore, the choice of filter media generally is limited to a few chemically inert  materials: glass, stainless steel and PTFE.
• Since the sample filter often is in a remote or inconvenient location,  it must be capable of operating for a reasonable period  between scheduled maintenance checks. Even more  important, it should not be susceptible to unscheduled problems, such  as filter element plugging  or rupturing, between regular maintenance checks.
• Sample filter maintenance in the field usually  is performed under adverse conditions by personnel who are  not trained chemists; therefore, the filter should be designed for easy and uncomplicated maintenance. Filter elements should be rugged and not susceptible to handling damage; the unassembled housing should have a minimum number of loose  parts, and the housing should be designed so that  it is virtually impossible to install a filter element incorrectly.
• A filter should introduce minimum lag time into the system. Lag time can be dealt with in the sample system design (slip- stream sampling, for example), but sizable dead volume in the filter housing should be avoided. Since large reservoir volume is desirable in many filter applications – such as compressed air or water filters – filters not specifically designed for sampling usually are  not suited for analyzers.

Requirements for sample filters range so widely that  specifying a filter is best  done on a case-by-case basis. There  is, how- ever, one generalization that  applies to all sample filter requirements: the filter must be able to separate efficiently a noncontinuous phase contaminant from the continuous sample stream phase. Specifically, the filter must be able to make  the following separations, in addition to removing solid particles:

• Gas samples – remove liquid droplets
• Liquid samples – remove immiscible liquid droplets and gas bubbles.

Most filter media will do an adequate job of removing solid particles from liquids or gases, but the only practical commercial media  that  will separate liquids from gases, gas bubbles from liquids, and two immiscible liquids is resin-bonded glass microfiber media.  All recommendations in this paper are  based on resin-bonded glass fiber media.

Slipstream or bypass sampling
Instrument  sample use  rates are  invariably quite low, yet it is essential to minimize  lag time in the sample system. Since analyzers often are  located some distance from the sampling point, samples usually  are  trans- ported to the analyzer at a relatively high flow rate to minimize  lag time. The sample is divided at the analyzer, with the analyzer using the portion  it requires (usually a very small fraction of the total sample), and the balance recycled to the process or vented.

If the sample filter is located in the low-flow line to the analyzer, it will have good life between filter element changes because the solids  loading rate is very low; however, the filter must be carefully selected to avoid introducing unacceptable lag time. If the filter is located in the high-flow  portion  of the sample system, its effect on sample lag time can be relatively low, but the life between filter changes may be inconveniently short because the element is filtering  a much  greater volume of material than  the analyzer is using.

Ideally, a filter should be located at the point where the low-flow stream is withdrawn  to the analyzer. This arrangement permits the main volume of the filter to be swept  continuously by the high flow rate system, thus  minimizing  lag time. At the same time, only the low-flow stream to the analyzer is filtered, thus maximizing  filter life.

A slipstream filter requires inlet and outlet ports at opposite ends  of the filter element to allow the high flow rate of the by-passed material to sweep the surface of the filter element and the filter reservoir, and a third port connected to the low flow rate line to the analyzer, which allows filtered samples to be withdrawn from the filter reservoir.

If bubble removal from a liquid is a requirement; this function  may be combined with slipstream filtration, since  the recommended flow direction for bubble  removal is outside-to-inside, and the separated bubbles will be swept  out of the housing by the bypass stream. In this case the liquid feed should enter at the bottom  of the housing and the bypass liquid exit at the top of the housing.

A special problem arises in slipstream sampling if the filter is to coalesce and continuously drain suspended liquid from a gas stream or to coalesce liquid droplets from a liquid stream. As noted  previously, the coalesced liquid is removed in the form of large drops from the downstream side of the filter. Therefore, the coalescing filter requires two outlet  ports, one for the dry gas and one for the liquid drain.  To combine coalescing and slipstream filtration, a filter housing would need  four ports – two for inlet and bypass and two for filtered gas and coalesced liquid – which is not a practical design. Therefore, slipstreaming plus coalescing requires two stages of filtration. The second (coalescing) stage must be located in the sample line to the analyzer, and should be as small as possible to minimize  lag time. If the quantity of suspended liquid is not large, a miniature in-line  disposable filter unit may be considered as a trap  for the suspended liquid, to be replaced when saturated.

Stack gas sampling
When sampling hot, wet gas, such  as stack gas, a filter capable of withstanding the gas temperature can be installed in the stack at the beginning of the sample line to prevent solids  from entering the gas sample line. After the sample is cooled,  a coalescing filter is used to remove suspended liquids before the sample goes  to the analyzer. Flow direction is inside-to-outside. Filter housings with Pyrex glass bowls are often used  in this application to permit a visual check  of the liquid level in the filter housing. Since there is often a consider- able amount of liquid present at this point, steps must be taken to drain the housing to ensure that  liquid does  not build up and carry downstream to the analyzer.

The liquid coalescing filter should be located as close  to the analyzer as possible to minimize  the chance of condensation between the filter and the analyzer. Additional precautions that  can be taken to avoid downstream condensation include cooling the sample below ambient temperature upstream of the coalescing filter, and heating the line gradually between the filter and the analyzer.

Sampling liquid effluent streams
Liquid effluent analyzers usually deal with aqueous streams having a high solids  content. In addition,  the analyzers are  often located in remote areas of the plant and are  in- frequently serviced. Therefore, the sample filter system must have long life between filter tube changes even in a high solids situation. The general recommendation for this requirement is a two-stage filter system, a 75 micron  prefilter followed by a 25 micron  final filter. The filters should be oversized as much  as possible without casing  excessive lag time. Plastic filter housings are usually a good choice.

Hydrophobic membrane sampling
Many online instruments are  susceptible to corrosion and skewed analysis from any water and moisture contamination. Gas chromatoraphs, mass spectrometers, oxygen analyzers and other sensitive on-line instruments require complete removal of all moisture. Instrument sensitivity  levels range from PPM to PPB and “percent level” component concentrations.

As a result, it is good practice to install a hydrophobic membrane filter in line to the instrument to protect and safeguard it from any moisture contamination. A stain- less steel filter housing with a hydrophobic membrane allows the sample gas to flow on the upstream side of the membrane and exit through the outlet  port on the downstream side. Entrained moisture will not flow through the membrane and will exit out the bypass port on the upstream side of the membrane completely protecting the instrument from moisture.

If the sample gas contains excessive amounts of moisture and particulates it is recommended to use  a stainless steel filter housing that  incorporates a coalescing prefilter and the hydrophobic membrane. The coalescing prefilter will protect the membrane from premature blinding and extend its useful life.

Minimal panel space, permanent line mount sampling. As more  and more sample lines and instruments are  added  to instrument sheds, the need  to utilize the space in the most  efficient way has become critical. In addition,  the need  to maintain sample filters without having to “break” the sample line and expose it to ambient conditions has also become quite important to most  facilities. By not having to “break”  sample line the need  to flush the lines prior to start up is eliminated.

For this application, it is recommended a stainless steel filter designed to be horizontally mounted at a 10 degree angle be installed to minimize  the amount of foot print on the panel. This design incorporates the inlet, outlet and drain ports all in the head  of the filter enabling filter element change outs without disrupting the sample line.

Ken Perrotta, Division Engineering Manager
Allan Fish, Product Manager
Parker Hannifin Corporation
Filtration and Separation Division