Reflux Samplers for Process Gas Chromatographs Their Use and The Importance of Product Design

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1 Yokogawa Corporation of America In today s industrial process world there is an on-going and overall reduction in the reliance in the use of manual monitoring and control and an increasing use of automation. This is driven by increasing labor costs and an increasing demand for accuracy and precision (or repeatability) in both the measurements and in the control systems. Analytical measurements are not immune to this trend, which can present a problem for the end user. It is well recognized that process analytical chemistry is frequently an environment that is detrimental to in-line sensing technology. Thus obtaining a representative and reproducible sample where in-line measurements do not work, is an often a fundamental pre-requisite to an accurate and precise measurement. ammonia reaction. Operators and vendors have all tried various approaches to solving the requirement for accurate sampling. Some examples and the problem associated with them are in-situ optical measurement (window clouding) and direct extractive techniques (equipment plugging). The problems associated with these efforts make them ineffective in the search for accuracy and precision in the analytical measurement. The solution preferred by many users and GC vendors world wide is the simple mechanical reflux sampler (See Figure 2). Figure 2 Yokogawa s Advanced Reflux Sampler Process gas chromatographs (GC) have been used since the 1950s and today there are literally tens of thousands of process GCs in use world wide Nevertheless this analytical workhorse for various on-line compositional measurements will simply either not work with hot, wet, particulate laden and corrosive samples or not work well or for long. Given that process GCs (see Figure 1) are frequently utilized on applications which are the most Figure 1 Yokogawa GC8000 Process Gas Chromatograph important from a process control perspective, this is not acceptable. Some critical GC applications are ethylene cracking, catalytic cracking, alkylation and The Yokogawa Advanced Reflux Sampler is offered in two styles either Manual or Automatic. Manual Samplers come in either fixed temperature or adjustable temperature. The Automatic comes in either Electronic with PID control or as a remote with DCS interface. It is designed to fulfill the requirement to remove liquids as a condensable component from low boiling point gases, by utilizing two cooling tubes.

2 Yokogawa Corporation of America The manual style is offered as a Self Acting Fixed (SAF) fixed outlet temperature unit and as a Self Acting Adjustable (SAA) model with adjustable outlet temperatures. Either style only requires instrument air for the vortex tube or cooler (or another cooling medium) to operate. No electrical power is required by either unit. The SAF model make use of a proportional controller with 10 F span (fully open to fully closed) for close temperature control. The SAA model uses a proportional control valve with adjustable temperature set-points. Neither model has any moving parts which helps ensure a long performance life expectancy. With the automatic style the Yokogawa Advanced Reflux Sampler cooling is provided by way of a vortex cooler controlled by a local electronic temperature controller local or remote DCS. The sample temperature input (RTD) for control of the cooling operation is a sensor placed directly in the sample path. How Does A Vortex Cooler Work? Figure 3 The Vortex Tube or Cooler as seen in Figure 3 was invented in 1933 by the French physicist Georges J. Ranque. How a vortex tube works is still debated by engineers, however simply put, vortex tubes behave in a very predictable and controllable way. Pressurized gas is injected into a swirl chamber where it accelerates into a high rate of rotation. One simple explanation is that the outer air is under higher pressure than the inner air (because of centrifugal force). Therefore the temperature of the outer air is higher than that of the inner air. Another explanation is that as both vortices rotate at the same angular velocity and direction, the inner vortex has lost angular momentum. The decrease of angular momentum is transferred as kinetic energy to the outer vortex, resulting in separated flows of hot and cold gas. However a vortex cooler or tube works, when compressed air is released into the tube through the vortex generator, hot air goes out of one end of the tube and cold air out the other. A adjustable control valve in the hot end allows for adjusting both the volume and temperature of air released from the cold end. Cold Fraction is the percentage of input compressed air that's released through the cold end of the tube. As a rule of thumb, the less cold air that is released, the colder the air will be. Cold fraction is adjusted with the control valve. Cold fraction is also a function of the type of vortex generator that's in the tube, i.e., a "high cold fraction" or "low cold fraction" generator. A high cold fraction tube can easily give you cold outputs F (28-50 C) below your compressed air temperature. High cold fractions give you a greater air flow, but they don't give the lowest possible temperatures. The high cold fraction combination of airflow and cold temperature produces the maximum refrigeration capacity, or greatest Btu/H (Kcal/H). A low cold fraction (below 50%) means a smaller volume of air coming out that's very cold (down to -40 F/-40 C). In short, the less air you release, the colder the air. The maximum Btu/H (Kcal/H) capacity (also called maximum cooling or refrigeration) occurs with a high cold fraction tube.

3 Yokogawa Corporation of America Yokogawa s Advanced Reflux Sampler The Yokogawa Advanced Reflux Sampler (ARS) has been designed specifically to meet nearly all of the sampling requirements of a modern petrochemical analyzer This is especially true for ethylene cracked gas effluents. The ARS also can be effectively employed in other applications such as: 1) Ethylene Plant, cracked gas effluent transfer line exchange (TLE) sampling. 2) Refinery FCCU, catalyst regeneration analysis sampling. 3) Iron/Steel Plant, blast oven furnace (BOF) & coke oven gas (COG) sampling. 4) TPA Plant, acetic acid removal for gas analysis using Titanium construction. 5) Others, primary sampling of high temperature, high particulate and saturated processes for subsequent analysis by an on-line process analyzer system. samplers share the same design features which give them superior performance. ARS Design Features Cooling Section All ARS models make use of two 360o stainless steel cooling coils which are located in the cooling section (See Figure 5). This dual cooling section, which is positioned above the filter section, ensures optimal sample cooling by providing substantially greater cooling capacity than that of competitive samplers. Figure 5 Figure 4 Figure 4 shows Yokogawa Advanced Reflux Sampler in three different configurations. While there are multiple options available to customize the ARS to a specific application requirement, all Yokogawa Furthermore all ARS models run cooling through the two coils, not the process sample.

4 Yokogawa Corporation of America This is extremely significant for several reasons. Why? 1) Since the sample gas now surrounds the two SS coils, the sample is evenly and uniformly cooled down in a very short time period. The reflux action can now occur in a homogeneous manner, across the cooling chamber and creates an environment which allows for complete removal of all liquids and any entailed solids from the gas stream. 2) Examine Figure 5 again. Just below the Coil One Outlet tube you can see the Diffusion Plate, which as a series of holes running around its outside edge. Its function is to cause turbulent flow into the Cooling Section. This turbulent flow helps to mix the hot sample gas into and between the two cooling coils, while at the same time not causing an obstruction to the liquid reflux from falling out the bottom of the sampler. 3) The dual coils wrap around the Inner Spacer Tube, whose sole function is to reduce the sample volume in the Cooling Section. This reduction in volume ensures the sample gas is forced up into coils as the gas has no other place to migrant to. Cooling is enhanced due to greater surface contact with the coolant coils. This in turn ensures that higher sample flow rates can be accommodated if required, without Figure 6 liquid carryover into the GC sample line. 4) First look at Figure 5 again. At the top, between Coil Inlet One and Coil Inlet Two is the top plate of the Inner Spacer Tube. This is shown more clearly in Figure 6, where one of Inlet Coils can barley be seen. This plate is important for several reasons. The obvious reason is that it seals the Inner Spacer Tube. Its second function is to give the TC which is mounted through the top of the Cooling Section a place to rest and measure the Sample Outlet Temperature (See Figure 7). This is a single point measurement and not an averaged temperature value for the entire Cooling Section. This single point value reflects the actual outlet temperature of the cooled sample gas, which is discharged out of the top of the Cooling Section. The field replaceable Demisting Pad is the final liquid vapor filter before the cooled gas sample is leaves the ARS and enters the process GC s sample line. 5) Knowing the actual outlet temperature of the cooled sample gas is important. See Figure 7.Yokogawa ARSs come in two styles, Manual and Automatic. Figure 7 shows the Self Acting Adjustable (SAA) model, manual style. No matter what style or model the Yokogawa ARS is, temperature control is always based on the actual measured temperature of the cooled sample outlet. It is not an averaged temperature based upon the hottest to coldest sample temperature. Temperature control is therefore accurate. 6) Usually the Yokogawa ARS is equipped with two Vortex Tubes or Coolers, although there is no reason that chilled water or any other coolant could not be used. The two coils are continuous metal tubing and no fittings or butt welds are used in their construction. Thus, there are no weak or leak points associated with the coils.

5 Yokogawa Corporation of America The Vortex Tubes are the High Cold Factions variety. High value, cold fractions tubes give you a greater air flow, but they don't give the lowest possible temperatures. The high cold fraction combination of airflow and cold temperature does however produce the maximum cooling capacity. Two vortex tubes address the needs for any possible low temperature requirements. Figure 7 capable of totally removing the particulate loaded liquids from the sample gas. Filter Section Just as all ARS models have common design features in the Cooling Section, the same is true in the Filter Section. Please see Figure 8. 1) The Filter Section is equipped with a Steam Injection Port. This port allows the end-user to steam clean the filter media without taking the ARS apart. Since this works effectively the majority of the time the steam injection port helps improves reliability and run time. 2) The Filter Section is flanged mounted to a block valve which is the process connection. The Filter Section is also flanged mounted to the Cooling Section. These flange connections ensure that the filter can be removed for a more thorough cleaning or replacement if that ever comes necessary. 3) The Steam Injection Port can serve an additional function other than one of maintenance. In some applications while there are undesirable liquids mixed in with the sample gas there can be situations in which the fluid levels are not always adequate to generate a reliable reflux action in the Cooling Section. By the addition of low volumes of steam via the injection port, the end-user can provide sufficient liquids in the Cooling Section to generate a reliable reflux action, which will then wash out all the unwanted liquids and particulates entailed in the gas sample. COMPARSION TO COMPETITIVE DESIGNS This is just as important as the use of two coils to run the coolant through. By using vortex tubes which produce maximum cooling capacity along with a Cooling Section designed to guarantee the fastest heat transfer you obtain a highly reliable and precise reflux action, Competitive samplers run the sample gas to be cooled through tubes stacked in bundles (See Figure 9) and the chilled coolant is blown across the tubes with the sample gas running up, down and across. While this is a historical method of construction, it is subject to several problems. They are:

6 Yokogawa Corporation of America Figure 8 1) In this configuration, only the first set of tubes experience full cooling. Those tubes further away from the coolant inlet experience less cooling due to obstructed coolant flow. This creates a vertical temperature gradient. Since heat rises the hotter coils experience a horizontal temperature gradient as well. 2) The first set of tubes removed some liquids and particulates. As the sample flows through the narrow tubes, the remaining liquid and liquid vapor can no longer hold the particulates in suspension. The remaining sample tubes provide the conditions which allow for solids to drop out of the sample. Those conditions are: First, the outside walls of the tubes are cooler relative to t Figure 9 the sample flowing through them. Second, the concentrations levels of liquid have been lowered by reflux action as the sample gas passed through the first set of tubes. Third, the remaining liquids and liquid vapor continue to cool as it passes up, down and across the sample tube bundle and eventually reach an energy state that doesn t allow for the particulates to be held in suspension and they start to drop out. As the particulates are dropped, a point will be reached where they start to stick to the narrow tube inner walls. On some hot days insufficient coolant capacity relative to sample temperatures results in sticky points building up

7 Yokogawa Corporation of America 3) These sticky points will over some period of operation, build up and cause plugs which constrict flow. The greater the constriction the greater the plugging. 4) Reflux samplers need to operate in series with the process GC, day and night and summer and winter. Outside ambient temperatures can vary hourly and will change daily. Temperature control is important to provide the required level of coolant flow. With the Yokogawa ARS a thermocouple is used to accuracy measure the temperature of the actual sample at a single point. As can be seen in Figure 9 there is no single point to measure the temperature of the sample gas and therefore a long thermocouple is wound around the sample tubes to get an average value. With competitive samplers, so long as the outside ambient temperature is the same as the day that the temperature control setpoint was selected then cooling will be adequate (this assumes sufficient coolant capacity) and the likelihood of particulate drop out is reduced. Changes in outside temperature will cause a change in the averaged measured temperature sample gas. Now the end-user can either always be changing the coolant setpoint or use DCS or PID control, which will still be working with an averaged measured temperature (Note: The large majority of reflux samplers on the market do not use PID or DC control). In either event the likelihood of sticky points forming increases with changes in outside ambient temperature due to the fact that averaged measured temperature does not really represent the actual sample gas temperature as it leaves the reflux sampler (Accuracy & Precision of the temperature input is low). 5) Tight coolant temperature control is critical to obtaining the fastest heat transfer so that a highly reliable and precise reflux action is obtained. When this condition does not exist the only options are to increase coolant flow rates or reduce sample flow rates to allow conditions for the reflux action to occur. Increasing coolant flow is feasible up to some point and but decreasing sample flows is never desirable as it creates control issues from the process GC measurement. When the above limits are reached a condition can occur in which insufficient reflux actions occurs within the sampler. The result is liquid, liquid vapor and particulate carryover into the process GC sample line. Now the process GC s sample conditioning system experiences an increased performance burden which increases its maintenance requirements. As was stated at the beginning of this document the process GC measurement will simply either not work with hot, wet, particulate laden and corrosive samples or not work well or for long; so keeping the GC s sample conditioning system operational is critical. 6) Finally, injecting hot steam is the common method used to remove particulate constrictions or plugs from a reflux sampler. The plugging could have built up over some period of operation or could have occurred suddenly due o a process upset. As can be seen in Figure 8 and 5, the Yokogawa ARS is easy to dissemble on site for a comprehensive cleaning and component replacement should than become necessary. Steam cleaning plugs out of sample tubes stacked in bundles (See Figure 9) is a cleaning issue raised to another magnitude. The only solution may be to return the sampler to the manufacturer for reconditioning. CONCLUSION A well designed reflux sampler will run coolant through tubes instead of running the sample gas through the tubes. Using coolant coils vs. sample coils provides the end-user the ability to

8 Yokogawa Corporation of America get a rapid and repeatable heat transfer rate and to obtain an accurate and precise temperature measurement. This is critical to obtaining tight temperature control of the coolant and the sample gas outlet temperate. Doing so ensures highly reliable and precise reflux action which removes all liquids and entrained particulates from the sample gas. A well designed reflux sampler will allow field replacement of components. It will allow the end-user to perform comprehensive cleaning on-site saving money and increasing availability.