Exploring Color in Liquefied Natural Gas with a Photometer

Given the massive volumes of LNG under transport, ROI on real-time measurement systems such as the ClearView® db can be measured in a few days. LNG is basically a natural gas that is cooled to a liquid state. Natural gas contains mainly methane, but may also include ethane, propane, and other heavier hydrocarbons as well as small residual quantities of O2, N2, CO2, H2O, and a few sulfur-containing compounds.

Before liquefaction, these residual materials are removed leaving behind only clean hydrocarbons. The natural gas composition can differ slightly from different producing locations. Usually, LNG is a colorless material, but at the time of distillation the carryover of larger hydrocarbons (C6+) can influence the color of the LNG (typical specification for C6+ in LNG is 0 to 0.05%).

Measuring the Saybolt or ASTM color online is an important parameter in a number of light hydrocarbon mixes for quality control to identify this carryover. Monitoring the color enables the refiner to confirm that the product falls within the specification. It is possible to reprocess any “out of spec” product before being sold.

Figure 1 is a flowchart of a typical natural gas to LNG process with target measuring points for online analysis.

Natural Gas Processing

Figure 1. Natural Gas Processing

Saybolt Color (ASTM D156) has a scale that spans from -16 (darkest) to +30 (lightest). Figure 2 illustrates the Saybolt measurement scale on a ClearView® db photometer fitted with a regular 50 mm pathlength cell. All materials darker than -16 can be measured with the ASTM Color scale (ASTM D1500). Using a ClearView® db dual beam photometer, both Saybolt and ASTM color can be measured online in real-time.

It is possible to configure the ClearView® db to measure either one or two sample points. Each unit can have 4-20 mA analog outputs (six outputs maximum on a one-channel system, and four outputs per channel on a two-channel system) together with corresponding contact closure outputs to alert fault states. The ClearView® db is also capable of Modbus TCP communication through Ethernet.

Saybolt Color as Measured by a ClearView® db Photometer

Figure 2. Saybolt Color as Measured by a ClearView® db Photometer

ClearView® db Enclosure Options

Zpurge Unit Class I, Division 2

Zpurge Unit Class I, Division 2

ExProof Unit Class I, Division 1

ExProof Unit Class I, Division 1

General Purpose Unit

General Purpose Unit

Image

This information has been sourced, reviewed and adapted from materials provided by Process Insights – Optical Absorption Spectroscopy.

For more information on this source, please visit Process Insights – Optical Absorption Spectroscopy.

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