follow the pattern in the table. The table also illustrates the pressure steps a single
adsorption bed progresses through during a cycle.
Let us start with a single target adsorber at full pressure being brought on line
and follow the steps:
• Adsorption – Raw feed is brought into the pressured vessel, normally in upflow.
Impurities in the feed adhere to the adsorbents from the bottom up, with hydrogen
adhering the least. Hence, hydrogen passes mostly through the bed and onto
production at nearly full pressure. The bed stays on line for a set time, determined
by the amount of impurities in the feed and the flow rate. This adsorption time can
be fine-tuned based on the product hydrogen purity. Nitrogen and CO will be the
first impurities to begin slipping out of the bed – normally setting the maximum
adsorption time. During this time, another adsorber bed is being brought up to
pressure using a little of the product hydrogen so it can be swung on line when our
current bed is taken off-line, without any pressure bumps.
• First equalization – At the end of the adsorption time, the feed and product
valves are closed on the vessel of interest, and the other vessel that was being
pressured up with product hydrogen goes on line. Then, the first equalization
valve opens on our target adsorber. This is the first of several depressurization
steps. The initially purged gases are fairly pure hydrogen, so flow in this first step
is upward and the hydrogen leaving is used to pressure up another adsorber bed
that will be brought on line next. There is no gas coming into the vessel. This is
called an equalization because at the end of the step, the pressure of our target
vessel and the pressure of the vessel it is repressuring are both about equal.
• Additional equalizations and providing purges – There are typically two more
equalization steps with other PSA vessels. In these steps, gas from our target
vessel is dropped in pressure. Flow out of the vessel is still upward. There is no
incoming flow. Flow from the current vessel, which is reasonably pure, is used to
push the impurities out of other, lower-pressure vessels. During these steps, the
actual impurities front in the target bed is continuing to rise.
• Blowdown – Now, we begin rejecting the impurities from the bottom of our
target vessel using downward flow. The vessel pressure is dropped to the low
tail-gas or off-gas drum pressure. This is where the impurities are largely
desorbed. Some hydrogen is unfortunately lost in the process. At this point,
there is no incoming gas, just depressurization out the bottom of the vessel.
• Purge – Now, the vessel is purged to blowdown using gas from one of the other
PSA vessels in the “provide purge” step. This is downflow at low pressure. The
impurities are reduced to their minimum in the bed.
• Equalization steps – The vessel is now repressured using equalization step gases
from other vessels that are being depressured. As the pressure is increased, the
purity of the repressuring gases increases. Flow is downward to avoid fluffing the
bed and there is no outgoing flow at this time.
• Repressurization – There is a final repressurization step using product hydrogen
in downflow to bring the bed pressure up to the product header pressure and
avoid a pressure bump when the vessel comes on line. There is no outflow.
628
S.A. Treese
adsorption bed progresses through during a cycle.
Let us start with a single target adsorber at full pressure being brought on line
and follow the steps:
• Adsorption – Raw feed is brought into the pressured vessel, normally in upflow.
Impurities in the feed adhere to the adsorbents from the bottom up, with hydrogen
adhering the least. Hence, hydrogen passes mostly through the bed and onto
production at nearly full pressure. The bed stays on line for a set time, determined
by the amount of impurities in the feed and the flow rate. This adsorption time can
be fine-tuned based on the product hydrogen purity. Nitrogen and CO will be the
first impurities to begin slipping out of the bed – normally setting the maximum
adsorption time. During this time, another adsorber bed is being brought up to
pressure using a little of the product hydrogen so it can be swung on line when our
current bed is taken off-line, without any pressure bumps.
• First equalization – At the end of the adsorption time, the feed and product
valves are closed on the vessel of interest, and the other vessel that was being
pressured up with product hydrogen goes on line. Then, the first equalization
valve opens on our target adsorber. This is the first of several depressurization
steps. The initially purged gases are fairly pure hydrogen, so flow in this first step
is upward and the hydrogen leaving is used to pressure up another adsorber bed
that will be brought on line next. There is no gas coming into the vessel. This is
called an equalization because at the end of the step, the pressure of our target
vessel and the pressure of the vessel it is repressuring are both about equal.
• Additional equalizations and providing purges – There are typically two more
equalization steps with other PSA vessels. In these steps, gas from our target
vessel is dropped in pressure. Flow out of the vessel is still upward. There is no
incoming flow. Flow from the current vessel, which is reasonably pure, is used to
push the impurities out of other, lower-pressure vessels. During these steps, the
actual impurities front in the target bed is continuing to rise.
• Blowdown – Now, we begin rejecting the impurities from the bottom of our
target vessel using downward flow. The vessel pressure is dropped to the low
tail-gas or off-gas drum pressure. This is where the impurities are largely
desorbed. Some hydrogen is unfortunately lost in the process. At this point,
there is no incoming gas, just depressurization out the bottom of the vessel.
• Purge – Now, the vessel is purged to blowdown using gas from one of the other
PSA vessels in the “provide purge” step. This is downflow at low pressure. The
impurities are reduced to their minimum in the bed.
• Equalization steps – The vessel is now repressured using equalization step gases
from other vessels that are being depressured. As the pressure is increased, the
purity of the repressuring gases increases. Flow is downward to avoid fluffing the
bed and there is no outgoing flow at this time.
• Repressurization – There is a final repressurization step using product hydrogen
in downflow to bring the bed pressure up to the product header pressure and
avoid a pressure bump when the vessel comes on line. There is no outflow.
628
S.A. Treese