By Edo Becker, Process Analytical Consultant for Specac Ltd

A petrochemical plant typically has several process sections, including reactors, distillation columns, and driers. Although simulation of chemical process has advanced in recent years, the most consistent benefits for process optimization are derived if regular analytical measurements are conducted on critical process streams within the unit.

The process stream can contain liquids, gases or even solids (catalyst particles, rust, dirt) and sometimes mixtures thereof. Due to these variable compositions, high pressures, temperatures and corrosivity, representative sampling of these streams for measuring is not straightforward.

With decades of successful installations in industry, NIR spectroscopy is still the most cost effective and standard method of choice for the application of process analytical technology.  In industrial organic chemistry most processes run at pressures of up to 100 bars and typically temperatures of up to 200 °C, sometimes even hotter.

In the following sections we present you with a few considerations which you are likely to encounter when a new sampling system has to be designed. This list is by no means complete and if you are new to the area we strongly advise you to speak to one of the many system integrators for on-line analysis for a detailed design.

Design considerations for sampling of liquid streams

For any NIR sampling system a Double Block and Bleed arrangement allows you to access the NIR Vortex^TM cell for maintenance in a safe manner if required. Maintenance could be required to clean the cell windows, to allow the safe removal of the cell or even to measure some additional samples for calibration.

Fouling of the cell windwos can occur if the process stream contains sticky material, which tend to drop out if the temperature of the cell has fallen to a low enough value. When the windows have clogged, it might be possible to increase the flow rate or the temperature or both to try and clean it in-situ, but if that is not possible a cleaning port like on the Specac Vortex^TM liquid cell would allow you to try and clean the windows in-situ (if this can be carried out in a safe manner).

Specac NIR cells are built using Sapphire optical windows, a material with one of the lowest sticking coefficient of all materials, surpassed only by diamond. Condensation or fouling is therefore very unlikely, but it is important that the temperature of the liquid cell is at least at the same temperature as the process to avoid these. Electrical heating tape or heating cartridges together with plenty of insulation can be employed to achieve a uniform temperature distribution. Once deposits and stickies have dropped out of solution it can take a long time and a lot of effort to clear up the liquid stream.

If no external pumps are available pressure drops can be used to drive the liquid through the cell with a sufficient flow rate, normally liquids are then routed to a safe location. If that is not possible pumps can be employed downstream to pump the liquid into the process stream again, however this solution requires more complex engineering and is therefore not the preferred one. If the flow rate is too low, the time-lag between the reactor take-off point and the point of measurement can become too long to achieve representative sampling.

Especially mixtures of hydrocarbons and water at times form emulsions which are very difficult to clear up, sometimes this occurs if the temperature falls below a certain value or the concentration of one component (e.g. water) is too high. You can try to break up the emulsion by using coalescing filters, but then one risks to change the composition of the stream.

Design considerations for sampling of gas/vapour streams

Specac Typhoon^TM gas cells normally have got path lengths of at least 10-20 cm and special care must be taken to avoid any droplet formation and subsequent condensation on the gas cell windows as the transmittance will drop rapidly. Therefore, in our view a high enough and uniform heating with no cold spots can be even more important for a gas cell than for a liquid cell.

Specac over the years has produced a number of different cells and the weight of the gas cells can be very high, as can be seen on the following picture of a custom-made gas cell by Specac.

The following installation shows a Specac gas cell with heating tape but no insulation for the analysis of a gas phase process. Achieving a uniform heating of the gas cell requires careful planning, including planning of the right locations for the control and trip thermocouples to avoid tripping the heating circuit due to excessive differences in the response of the two (e.g. if they are too far apart from each other). Once tripped it can take a long time to heat the complete system up again.

Particle filters can be useful to filter out dust, microscopic droplets, and rust particles, but they require regular maintenance. Here the installation of pressure transmitters before and after the filter can show you the extent of blockage inside and guide you as to when a filter replacement is required. Coalescing filters can also be employed, with the advantage that they might not have to be changed as regularly as filter s made from cellulose or Teflon.

If possible, a pressure drop in the sample loop using needle valves downstream of the cell can be used to achieve a reliable sample flow through the cell.   

The benefits

Using NIR spectroscopy together with industry leading spectroscopic accessories from Specac allows you to measure even the most hazardous streams on-line and derive massive benefits in terms of process control, safety and through-put of your plant.

The availability of continuous data will allow you to derive benefits by moving the plant control closer to the specification limit, reducing quality margins and unnecessary give-aways. By further reducing the variations in your final product concentration the internal quality specifications for the customer can be optimised as your on-line NIR systems allow you operate a tighter quality control scheme.

PAT support team

At Specac we are happy to help you by evaluating your specific application with our dedicated team of mechanical engineers, chemists and process experts. We have a wide range of contacts within the chemical industry, spectrometer manufacturers and fibre-optic producers to help and turn your initial idea into reality.

Share

• g
• k
• o
• 4