If the analyzer in your analytical system requires gas but your sample is liquid, the only option is to convert the liquid to gas. This process is called vaporization or flash vaporization. The objective is to convert a sample of all liquid to all vapor instantly, without changing the composition.
It is not easy to vaporize a sample, nor is it always possible, so make sure it is really necessary and possible before you try. You should always analyze a liquid in a liquid phase unless there are strong reasons for analyzing in a vapor phase.
If you proceed with vaporization, it is important to understand the difference between evaporation and vaporization. Evaporation occurs gradually with an increase in temperature. Vaporization occurs instantly with a drop in pressure. Its not possible to vaporize a sample by increasing temperature. Heat causes evaporation, and adding more heat simply makes evaporation happen faster.
In a mixed sample, evaporation will allow some compounds to evaporate before others, resulting in fractionation. Vaporization, done properly, ensures that all of the compounds vaporize at the same time, preserving the samples composition.
However, it is possible for things to go wrong when vaporizing. Instead of flashing the whole sample into a vapor, you could unintentionally cause a combination of vaporization and evaporation. The result would be fractionation. Once a sample of mixed compounds fractionates, it is no longer suitable for analysis. With fractionation, a common scenario is for lighter molecules to evaporate first and travel on toward the analyzer, while the heavier molecules remain behind in the liquid phase. Even if at some later point in the process a fractionated sample appears to be all gas, the mixture will not be of the same molecular proportions as it was before fractionation. It will no longer accurately represent the product taken from the process line.
Lets take a closer look at the process of vaporization and how we can manipulate the variables (temperature, pressure and flow) to ensure proper vaporization and an accurate analytical result.
To vaporize a sample, one typically uses a vaporizing regulator, also called a vaporizer, which is a pressure-reducing regulator with the capacity to transfer heat to the sample at just the right location.
Vaporization consists of a three-stage process (Fig. 1). First, the sample enters the vaporizer as a liquid. At this point, the liquid should not be bubbling or boiling.
| Fig. 1. Drawing showing the three-stage |
Second, the liquid passes through the regulating orifice in the vaporizer, resulting in a severe and sudden pressure drop, which vaporizes the liquid. At the same time, heat is applied, which enables the vaporized liquid to remain a vapor.
Third, the sample, now a gas, exits the vaporizer and travels to the analyzer to be read. Due to the immediate transition to the vapor phase, the composition of the gas is unchanged from that of the liquid, ensuring an accurate reading.
In this delicate process, there are many variables or inputs that determine success or failure. For the purpose of this discussion, lets say there are two main sets of inputs.
The first set of inputs concerns the composition of the sample. Depending on the composition of the sample, it will begin to bubble and finish vaporizing at different pressures and temperatures. We will need to know what these pressures and temperatures are to successfully manage the process.
The second set of inputs concerns settings that you control in your sampling system: pressure, temperature and flow. Pressure and temperature are controlled at the vaporizer, while flow is controlled downstream at a rotameter (variable area flowmeter) and needle valve. We set these inputs based on what we know about the first set of inputs. Proper vaporization requires a delicate balance of all inputs.
Even when approaching vaporization in a systematic manner like this, the process does require some trial and error, so we will also talk about how to diagnose and address problems.
Understanding your sample.
The best way to understand the first set of inputs is with a phase diagram. A phase diagram plots pressure and temperature, showing at any pair of conditions whether a substance will be vapor, liquid or solid. The lines indicate the interfaces between two phases.
Phase diagrams for most pure gases are available on the Internet (one example is encyclopedia.airliquide.com). But diagrams for gas mixtures are very difficult to create without commercial software.
Fig. 2 represents a phase diagram for 20% hexane in pentane. When the sample is above the bubblepoint (blue line), its all liquid. We want the sample to be all liquid when it enters the vaporizer. When the mixture is below the dewpoint (gold line), its all vapor. The sample must be all vapor when it leaves the vaporizer.
| Fig. 2. Phase diagram showing 20% hexane |
in pentane, with temperature settings.
Between the bubblepoint and dewpoint lines is what we call the no-go zone. This zone is the boiling range of the sample. Here, the mixture is in two phases, part liquid and part vapor. Once a sample falls into the no-go zone, it is fractionated and no longer suitable for analysis. The objective in vaporization is to set the temperature, flow and pressure so that the sample skips instantly from the liquid side of the no-go zone to the vapor side of the no-go zone.
With pure and nearly pure samples, there is little to no boiling range or no-go zone. The bubblepoint and dewpoint lines are on top of each other or nearly so. Indeed, pure and nearly pure samples will convert to vapor of the same composition, whether through evaporation or vaporization. Some industrial samples approach this level of purity and convert easily.
On the other hand, some samples have such a wide boiling range or no-go zone that they cannot be successfully vaporized. There is no way to skip from the liquid side of the no-go zone to the vapor side of the no-go zone. We are unable to manipulate the variables (temperature, flow and pressure) in such a way as to avoid fractionation.
Most samples fall between these two extremes. For example, in Fig. 2, the band between bubblepoint and dewpoint is narrow enough that, with the proper settings, we can enable the sample to effectively skip from the liquid side of the no-go zone to the vapor side. At the same time, the band in Fig. 2 is wide enough that we cannot afford to be careless. Indeed, we will need to be skillful in our manipulation of the variables or we will end up with a sample in the no-go zone.
Setting temperature, pressure and flow.
Lets continue to work with the sample in Fig. 2 (20% hexane in pentane) and see how we can set our inputs to ensure successful vaporization.
In general, at the inlet, we want high pressure and low temperature. At the outlet, we want high temperature and low pressure. But there are limits as to how high and low these parameters can be, and not all of them are under our control completely. Vaporization is basically a balancing act between the variables.
Here is a four-step process for setting your inputs:
1. Determine the inlet pressure at your vaporizer. This pressure, which is fixed, is your process pressure, provided your vaporizer is located close to your sample tap. In Fig. 2, that pressure is 4 bar. Higher pressure is better because it allows you to keep the vaporizer temperature higher without boiling the incoming liquid.
2. Set your inlet temperature, or the temperature of your vaporizer. There are two objectives. First, the temperature must be low enough that, when the sample enters the vaporizer, it is entirely a liquid and isnt bubbling. In Fig. 2, the bubblepoint at 4 bar is 88°C, but we want to build in a cushion, so lets choose 80°C, a round number far enough away from 88°C to be safe. The second objective is that the temperature must be high enough to contribute to the complete flashing of the sample, ensuring that only vapor leaves the vaporizer. When you vaporize the sample, the temperature drops, in accordance with the laws of energy conservation. The sample temperature must be high enough at the outset so that after the pressure drop, the sample is not in the boiling range or no-go zone. In Fig. 2, the vapor temperature after the pressure drop is 60°C, just on the vapor side of the dewpoint line.
3. Set the outlet pressure at the vaporizer. Your objective is to drop the pressure below the gold dewpoint line. In Fig. 2, the outlet pressure is set to 1.5 bar. If the outlet pressure were any higher in this example, the sample would not vaporize entirely. It would fractionate.
4. Set your flow. Flow is set downstream at a valve and rotameter, not at the vaporizer. In a sampling system, high vapor flow is desirable because it moves the sample to the analyzer faster. However, high flow can be problematic, too, because with high flow, more heat is required to vaporize the sample. In other words, high flow results in a greater drop in temperature at the time of vaporization. In Fig. 2, the purple line illustrates the temperature drop. As flow increases, the purple line angles more sharply to the left.
Another variable influencing the temperature drop is the heat transfer capability of the vaporizer. Some vaporizers are constructed in such a way that heat transfers more efficiently to the sample. When the liquid sample converts to a vapor and its temperature drops, it draws heat from the stainless steel surrounding it. The critical question is how efficiently can the vaporizer replace that heat and keep it flowing to the sample. The more heat the sample can draw, the less its temperature drops during vaporization.
In some instances, it is possible for the vaporizer to be hot to the touch on the outside but cold at the core inside. Thats because the vaporized sample is drawing lots of heat and the vaporizer cannot transfer enough heat to keep up. The best solution is to reduce the flow.
In sum, the angle of the purple line in Fig. 2 is a product of the flow rate and the heat transfer capability of the vaporizer. With a good vaporizer and low flow, the line will become more vertical. Unfortunately, there is no easy way to calculate the location of the purple line, and it is not generated by any known software program. As a result, vaporization involves some approximation. As a rule of thumb, keep the flow rate as low as possible without causing an unacceptable delay in the samples travel time to the analyzer. Its better to start with a low flow rate and experiment with increasing it than to start with a higher flow rate.
Phase diagrams will enable you to approximate temperature, pressure, and flow settings, but some troubleshooting will still be required. One sure indication of a problem is poor repeatability in analyzer results.
There are two possibilities when the sample is fractionating instead of vaporizing, with Problem #1 being the more common:
Problem #1. Only part of the sample is being vaporized. Liquid is passing through the vaporizer and sitting in the tubing on the downstream side. Eventually, it evaporates. When it does, it draws heat from the surrounding tubing, making the tubing cold to the touch or causing frost or ice to form. Signs of the problem: Vaporizer outlet and downstream tubing is cold to the touch or has frost or ice on it. Note: In many cases, liquid on the downstream side of the vaporizer may pass beyond the area of the vaporizer and into other components, such as flowmeters and filters, where it can cause considerable damage.
Problem #1 solution. In the previously discussed approach, your best option would be to reduce the flow rate. Another option would be to lower the vaporizer outlet pressure, if that is possible. A third option would be to increase the heat to the vaporizer, but in this case you risk causing Problem #2.
Problem #2. The sample is boiling at the inlet to the vaporizer. It is fractionating before it can be vaporized. Lighter molecules evaporate and create a vapor wall, which pushes the liquid back into the process. A portion of that vapor wall then cools and condenses. Finally, the liquid sample moves again toward the vaporizer, where the lighter molecules evaporate, starting the cycle all over again. Meanwhile, the heavier molecules move on toward the analyzer for an inaccurate reading. Signs of the problem: The inlet tube to the vaporizer twitches, sometimes violently, and the measurement values oscillate.
Problem #2 solution. Lower the vaporizer temperature.
Know your variables.
Vaporizing a liquid sample is challenging. In many sampling systems around the world, vaporizers are fractionating samples and sending unrepresentative samples to the analyzer every minute of every day. You can dramatically increase your chances of success by researching a phase diagram of your systems particular mixture of compounds. You can further increase your chances of success by understanding what is occurring in the process; specifically, by knowing what the variables are (temperature, pressure and flow) and their role in influencing the process outcome. With this framework in place, you can come very close to the right settings, making adjustments in accordance with the signs and symptoms you observe. HP
Watching out for time delay
Time delay is another problem in vaporizing samples. It can be an issue on both the liquid and vapor side of the vaporizer. On the liquid side, the difficulty is caused by the samples degree of expansion when it is vaporized. A small amount of liquid creates a large amount of vapor.
Its easy for liquid on the upstream side of the vaporizer to be held up awhile before it is vaporized.
If your vaporizer is located near the tap, the best solution is to install a bypass on the liquid side of the vaporizer, so the sample being vaporized is always fresh. In addition, try to minimize the volume of the probe and tubing preceding the vaporizer.
To address time delay on the vapor side, you may want to increase flow. This may not be the best option. Many samples require low vapor flow rate for proper vaporization. High flow, in combination with insufficient heat at the vaporizer, could result in fractionation, with liquid passing downstream. Such a scenario would ruin the sample for analysis, evident by frost on the tubes downstream of the vaporizer.
A better way to reduce time delay on the vapor side is to minimize volume. For example, move the vaporizer closer to the analyzer and/or build a fast loop on the liquid side. HP
|The authors |
Doug Nordstrom is market manager for analytical instrumentation for Swagelok, and he focuses his efforts on advancing the companys involvement in sample handling systems. He previously worked in new product development for Swagelok and earned a number of Swagelok patents for products. Mr. Nordstrom graduated with a BS degree in mechanical engineering from Case Western Reserve University and earned a masters degree in business administration from Kent State University.
Tony Waters has 45 years of experience with process analyzers and their sampling systems. He has worked in engineering and marketing roles for an analyzer manufacturer, an end-user and a systems integrator. He founded three companies to provide specialized analyzer services to the process industries and is an expert in the application of process analyzers in refineries and chemical plants. Mr. Waters is particularly well known for process analyzer training courses that he has presented in many of the countries of Asia, Europe and the Middle East, as well as North and South America. His presentations have equal appeal to engineers and maintenance technicians.