Optimizing Polymer Manufacturing with Process Gas Analysis

Process gas analysis is vital for the optimization and control of the polyethylene and polypropylene manufacturing method. The composition of the reactor gas streams offers the plant control system certain amount of production efficiency, and the data essential for making adjustments to process variables. Due to the requirement for polymer manufacturers to differ product formulations, the speed of inlet gas analysis becomes predominantly essential during process transitions.

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A MAX300-IG process control mass spectrometer was used to track polyethylene reactor gas to achieve a real-time measurement. This data was evaluated against a real-time measurement carried out using the conventional gas chromatography (GC) approach. The GC data validated the precision of the MAX300-IG, while additional comparison showed that the mass spectrometer was quicker, and capable of offering a better analysis.

The synthesis of polypropylene and polyethylene happens through polymerization of the gaseous monomer in the presence of stringently defined concentrations of the additives needed to create the preferred density and branching within the end product. Manufacturing facilities are built to produce varied polymeric materials based on market conditions.

These differences in the mode of operation of the plant happen in a semi-continuous method, with alterations to the composition of the gas-phase reactants establishing the end product. Consequently, there is a transition period between each "batch", as the reactor constituents change over from the first formulation to the composition that will yield its substitute. A batch can run for hours or many days, producing material of drastically diminished value, regarded as an “off-spec” product.

Normally, gas analysis of the reactor sample is done to automate process control according to production conditions. Certain gas mixtures are optimum for the generation of a specified class of polymer. Close observation of these concentrations enables the manufacturer to modify process variables if any variation is identified from the suitable set point. This ensures the plant retains the highest efficiency and is able to guard equipment from abnormal or uncontrolled reactions. Due to the high frequency of transition events, the quick detection of basic process conditions results in significant gains in efficiency; it reduces the material’s cost, which has to be sold as an "off-spec" product. For the highest efficiency, safety, and product yields, the polymer plant relies on rapid and precise gas analysis during formula transitions and stable process conditions.

Real-time Analysis in Polymer Production

Process gas analyzer, the MAX300-IG was applied to observe production at a polyethylene facility in parallel to a process GC. The mass spectrometer was able to test all of the gases in the process stream, while the GC determined ethane, hydrogen, and ethylene. The MAX300 analysis rate was 0.4 seconds per part, which enabled the analyzer to determine six streams at the facility at a speed of 10 seconds per stream, with a clearing delay. The rapid data acquisition offered a high level of accuracy compared to the GC data (Figure 1).

Figure 1. The MAX300-IG was used to monitor all components of the polyethylene process. Here, the hydrogen and the ethane trends from the mass spectrometer are shown along with 24 hours of GC data recorded on the same stream. Both instruments provide accurate measurements, while the real-time acquisition of the MAX300 yields a high-precision profile of the changing gas composition.

The speed of data reporting is crucial to plant operations when transitioning from one polymeric formulation to another. These transitions can happen numerous times per day and, if manufacturers are able to identify the end of the transition sooner, then they can sell a relatively higher-value product. The MAX300-IG has the ability to detect the endpoint of transition 5.3 minutes faster than the GC at the test site (Figure 2).

Focusing on the end of the transition is particularly crucial. For instance, on a production line manufacturing 5 tons of low density polyethylene (LDPE) per hour, 5.3 minutes represents 0.45 tons of product. The value of a product produced during 5.3 minutes, at the same LDPE production line is $618. The MAX300-IG has the required momentum to track numerous production lines within the same facility. At a site where three manufacturing lines experience a transition every 2 days, the value of spotting the completion 5 minutes earlier would save as much as $29,000 each month.

Figure 2. During product formula transition, the MAX300-IG identified the endpoint 5.3 minutes faster than the GC analyzing the same sample stream. The ability to quickly identify the onset of production conditions increased the volume of high-value product produced.

The MAX300-IG

For more than four decades, mass spectrometry has been extensively applied for industrial process control. With regard to polymer applications, the MAX300-IG (Figure 3) offers the real-time measurement of alkenes and alkane, in addition to nitrogen, hydrogen, and argon in the reactor sample streams (Table 1).

Figure 3. The MAX300-IG, Process Control Mass Spectrometer

Table 1. Standard polymer application measurements for the MAX300-IG, process gas analyzer.

The analyzer is capable of measuring all parts at concentrations from 100% down to 10 ppb. This dynamic range and the high linearity is predominantly significant during product transitions where component concentrations can alter considerably in a short period of time (Figure. 4).

Figure 4. The linearity of the MAX300-IG is demonstrated by the analysis of certified gas bottles. This dataset includes readings of propane, propylene, ethane, ethylene and hydrogen. Calibration for instrument response is obtained from a single standard and the analyzer is linear from 100% down to the low detection limit.

The quadrupole mass filter, measuring 19 mm, enables enhanced sample throughput, producing long term stability and high analysis repeatability, without any loss of sensitivity at the high-mass range. The analyzer performs quantitative investigation at a rate of 0.4 seconds per component. Basic maintenance involves replacing the plug-and-play ionizer unit and altering the pump oil biannually. A carrier gas is not required for the analyzer.

Conclusion

The MAX300-IG process gas analyzer is an efficient tool to control the polymer process. It requires minimal calibration support and maintenance, and offers a simultaneous, quantitative measurement of all compounds in the inlet and effluent streams of numerous reactors at a production unit. The control system gets analysis updates at the rate of 10 seconds per sample stream. This real-time testing is vital to sustain high-accuracy production control. Controlling the reactors to optimal conditions results in enhanced product yield, and equipment can also be maintained with the lowest amount of adjustment, decreasing turnover. Endpoint detection is specifically significant during formula transition. The rapid analysis of the MAX300-IG yields major economic increases compared to the relatively slower analysis processes.

This information has been sourced, reviewed and adapted from materials provided by Process Insights - Mass Spectrometry.

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