Gas analysis in petrochemical production
Accurate gas analysis is critical in the production of PTA. Purified terephthalic acid (PTA) is, together with ethylene glycol, a key component in the manufacture of polyethylene terephthalate (PET), the most widely used of the polyester type of man-made fibres.
KEITH WARREN and KAREN GARGALLO
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PET is also a recyclable thermoplastic resin with US FDA approval for use as food and drink containers and bottles. As such, there is a continuing and growing demand for PTA throughout the world, particularly in fast-expanding economies such as those found in Asia.
Gas analysis plays a vital role in the PTA production process, delivering the measurements that support product quality, process efficiency, and safety.
The PTA manufacturing process
PTA is manufactured from p-xylene by careful and specific air oxidation in a reactor at high pressure and elevated temperature. Liquid acetic acid, which is highly flammable, is used as a solvent for this reaction. The crystalline PTA product is separated from the reaction liquor in separate crystalliser vessels and then recovered and purified.
There are two essential applications for gas analysis in the PTA plant – the oxidation reactors and the crystallisers. Firstly, air is passed into the oxidation reactors, oxidising the p-xylene methyl groups to terephthalic acid and generating carbon dioxide (CO2) and carbon monoxide (CO). Some oxygen (O2) remains unreacted, so the most critical gas analysis measurement is to monitor this residual O2 level in the off-gas, which should be around 4-5% O2. If the level rises too high, it means a dangerous situation could be developing in the reactor; sudden, runaway oxidation of the flammable materials could occur, resulting in an explosion.
However, if the O2 level is too low, then insufficient oxidation occurs, leading to poor efficiency and a low product yield. To achieve optimum results, the O2 level must be monitored with the best possible accuracy and the fastest response time.
A paramagnetic O2 analyser is recommended for this application, as this sensing technology is highly specific to oxygen, and so delivers high levels of accuracy in the reaction conditions. It also offers a fast response to changing O2 concentrations in the reactor.
Paramagnetic cells each consist of two nitrogen-filled glass spheres, mounted on a rotating suspension within a magnetic field (see Figure 1). Light shines on a centrally located mirror, and is reflected onto a pair of photocells. Because O2 is naturally paramagnetic, it is drawn into the magnetic field, and so displaces the glass spheres, causing the suspension to rotate. This motion is detected by the photocells which generate a signal to a feedback system. This, in turn, sends a current through a wire mounted on the suspension, creating a motor effect. The current produced is directly proportional to the concentration of O2 within the gas mixture, allowing an accurate and linear percentage reading to be made.
As this technology is non-depleting, paramagnetic cells never need changing, and the performance does not deteriorate over time, with significant benefits to ongoing maintenance costs and sensor lifespan.
A well-designed sample conditioning system is also required to ensure the analyser is able to cope with the high pressure, high temperature off-gas, which will contain trace p-xylene and significant levels of corrosive acetic acid vapour.
Additionally, many plants require a measurement of the CO2 level – and sometimes the CO level – in the off-gas, as this reveals more information about the progress of the oxidation reaction. An infrared gas analyser can be used for this measurement, ideally configured to deliver simultaneous measurements of CO2 and CO.
In the crystallisers, acetic acid vapour is driven off as the PTA product crystallises out of the solvent liquor. This vapour is extremely flammable, and so a measurement of the residual O2 in the crystalliser vapour is vital to provide a warning of any explosion risk. Monitoring the presence of CO2 in this vapour can also provide an indication of any post-oxidation that may be occurring, and so is a useful measurement.
Once again, paramagnetic and infrared sensing provides the most effective gas analysis for these O2 and CO2 measurements (see Figure 2).
Gas analysis requirements
The reactor off-gas is typically composed of:
• Nitrogen – approximately 90%
• Oxygen – 4%
• Carbon dioxide – 3%
• Acetic acid – 2%
• Water vapour – 1%
• Carbon Monoxide – <1%
• p-xylene, other organics, acid catalyst – trace
The off-gas is generally at a pressure of 20 barg and a temperature of 50-150°C, so the hazardous area classification required may be Zone 1 or 2 depending on the plant conditions.
Gas analysis measurements are normally specified on a dry basis with required ranges between 0-10% for O2, 0-5% CO2 and 0-2% CO. Speed of response is critical for gas analysis in this application, particularly for the O2 measurement. Typically, 30-45 seconds for overall T90 of the complete system is required.
The system must operate with minimal errors, so to ensure reactor safety a voting system may be used to monitor the O2 concentration. Voting systems use multiple analysers and the process relies on the measurement agreed upon by the majority of analysers. For example, in a three-analyser voting system, if one analyser detects a significant change, it is outvoted by the other two and no action is taken. However, if two of the three analysers (or all of them) detect a change, this reading is held as correct, and action may be taken, ranging from informing the operator to automatically halting the process.
Voting systems provide an extra layer of reliability in safety applications, and also allow a problem with an analyser to be detected at an early stage without endangering the process. If two analysers agree on a measurement and the third differs, it indicates a potential problem that can be investigated and corrected before the process is affected.
For the other measurement needs, a single infrared analyser is usually sufficient. Off-gas applications in the crystalliser are broadly similar and use identical analytical solutions.
For both reactor and crystalliser measurements, the sample conditioning system must be well designed to ensure a fast overall response time, and to handle significant levels of condensibles in the sample, removing them prior to measurement.
Sampling in the reactor off-gas stream also needs to correctly handle the high pressures involved. A water-washing ‘lute’ system may be used, and highly corrosion-resistant materials (titanium, Hastelloy, for instance) may be specified for construction, because of the presence of acetic acid and possible catalyst traces.
An experienced, expert gas analysis supplier will be able to provide the correct analyser and sample system packages to meet individual plant requirements.
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