Petrochemical Applications
Petrochemical Applications
Moisture Measurement Basics in the Petrochemical Industry
TIM 005 07.97
Many hygrometer manufacturers are capable of measuring in petrochemical applications. The General Eastern gold/aluminum oxide sensor is ideal because of its ability to offer continuous measurement from direct installation in the process stream, without a sampling system in many cases.
Continuous measurement is an important advantage over Karl Fischer titration. In addition, titration yields an "after the fact" reading and can often be subject to interference from other components that do not affect gold/aluminum oxide.
The gold/aluminum oxide sensor also has an advantage over lithium chloride and phosphorous pentoxide sensors in that the reading is not dependent on the flow rate or pressure. These sensors would indicate a change in the moisture level if these process parameters were to change. This means when using these sensors the sample stream flow rate and pressure must remain constant to achieve a proper reading.
Thegold/aluminum oxide sensor directly measures the water vapor of the stream and is not affected by changes in flow rate or pressure. Lithium chloride and phosphorous pentoxide sensors are susceptible to being washed away, so they cannot be used for liquid streams.
Liquid and gaseous petrochemicals are widely used in manufacturing. The presence of moisture in petrochemical feedstocks creates many different results, most of which adversely affect the process.
A good example of the problems caused by moisture in petrochemical processes is evidenced during the use of cyclohexane solvent in the preparation of plastics. When the moisture content rises above 100 PPMw significant changes can be seen. The integrity of the polymers often declines and, from that perspective, moisture adds to the cost of manufacturing through increased reject rates and the need for
additional quality control. As a result of the chemical reactions in these processes, unwanted byproducts are produced when excess moisture is present. These byproducts bring with them the burdens of additional material handling and proper disposal.
Many polymerization processes utilize a molecular sieve dryer to remove unwanted moisture when necessary. At times, the solvent supply to the polymerization process is dry enough to be used directly in the process, while at other times it must be routed through the dryer. A General Eastern hygrometer can be used to indicate when the solvent should be routed through the dryer. The diagram on the previous page shows an gold/aluminum oxide moisture probe installed in such a process.
The moisture probe is installed in a flow cell. This cell utilizes the pressure differential created by a pump to draw a sample into the cell, across the sensor, and back into the process.
Downstream from the pump, a three-way control valve can then be used to divert flow directly into the polymerization process or to route it first through the dryer. The 2850 series analyzers can be utilized in such an application. Both have the ability to output the moisture data in units of PPMw or dew point. Solubility data needed to calculate the PPM readings can be entered into these two analyzers directly through the front panel control keys. The solubility data can be found in published sources or can be calculated from solubility data found by laboratory methods.
By installing a General Eastern hygrometer, the moisture level of such a process can be continuously monitored. When the methods employed to remove the moisture from the process begin to fail, a moisture analyzer can generate an early warning. This can lead to significant reduction of material waste and manufacturing costs.
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