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
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
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
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
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.
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
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
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
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.
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
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
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
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
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
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
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
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.
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
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
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
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