Tube Trailers
Sometimes, you need only a small amount of hydrogen of high purity. This is common
for semi-regeneration naphtha reformer start-ups. In these cases, hydrogen is often
supplied at high pressures in tube trailers. These are available from many industrial gas
suppliers. The high pressure eliminates the need for compression by the user.
A typical, large tube trailer from Air Products, for example, holds about 126,000
scf (3,570 Nm
3 ) of hydrogen at 2,640 psig (83 barg).
Pressure Swing Adsorption
Introduction
Throughout the discussion of hydrogen production, we have referred to pressure
swing adsorption (PSA) units. In this section, we will go a little deeper into the
design and operation of these units. The discussion will be limited to nonproprietary
information and available public literature. There is a great deal of proprietary
knowledge needed to design an effective and reliable PSA unit. Detailed design is
best accomplished by one of the PSA suppliers – such as UOP, Air Liquide, Linde,
Air Products, or CB&I. Industrial gas suppliers will also install and operate
recovery PSA systems at a refinery in an over-the-fence arrangement.
Also, note that hydrogen PSAs are not the same as PSAs for other purposes (like
instrument air drying or air separation). They require specific knowledge of the
hydrogen applications.
Process: Step by Step
A simplified process flow diagram for the business end of a generic 5-bed PSA is
illustrated in Fig. 22. To mimic a continuous process, a PSA unit consists of a series
of identical vessels filled with adsorbents. There are typically 4–12 beds.
The vessels are connected by several headers that carry the feed gas, product
hydrogen, purge and equalization gases, and off-gas or tail gas. The final off-gas
passes through a large surge drum that moderates pressure and composition surges
for the off-gas and minimizes the pressure swing impacts on the system receiving
the off-gas – normally an SMR furnace or fuel gas header/compressor.
Adsorbents in the vessels are selected based on the impurities in the feed gas. For
instance, in a typical SMR hydrogen plant, the bottom portion of the PSA bed will
contain activated carbon and the upper portion will contain molecular sieve. The
activated carbon can easily adsorb methane, CO 2 , water vapor, some CO, and other
trace compounds. The mole sieve adsorbs nitrogen and the balance of the CO.
It is critical that the feed gas be as dry as possible. If the gas has a high moisture
content, water adsorption may reach the molecular sieve where it will permanently
attach, reducing adsorption capacity. SMR hydrogen plant raw hydrogen feed is
limited to about 130
F to avoid this problem.
626
S.A. Treese
Sometimes, you need only a small amount of hydrogen of high purity. This is common
for semi-regeneration naphtha reformer start-ups. In these cases, hydrogen is often
supplied at high pressures in tube trailers. These are available from many industrial gas
suppliers. The high pressure eliminates the need for compression by the user.
A typical, large tube trailer from Air Products, for example, holds about 126,000
scf (3,570 Nm
3 ) of hydrogen at 2,640 psig (83 barg).
Pressure Swing Adsorption
Introduction
Throughout the discussion of hydrogen production, we have referred to pressure
swing adsorption (PSA) units. In this section, we will go a little deeper into the
design and operation of these units. The discussion will be limited to nonproprietary
information and available public literature. There is a great deal of proprietary
knowledge needed to design an effective and reliable PSA unit. Detailed design is
best accomplished by one of the PSA suppliers – such as UOP, Air Liquide, Linde,
Air Products, or CB&I. Industrial gas suppliers will also install and operate
recovery PSA systems at a refinery in an over-the-fence arrangement.
Also, note that hydrogen PSAs are not the same as PSAs for other purposes (like
instrument air drying or air separation). They require specific knowledge of the
hydrogen applications.
Process: Step by Step
A simplified process flow diagram for the business end of a generic 5-bed PSA is
illustrated in Fig. 22. To mimic a continuous process, a PSA unit consists of a series
of identical vessels filled with adsorbents. There are typically 4–12 beds.
The vessels are connected by several headers that carry the feed gas, product
hydrogen, purge and equalization gases, and off-gas or tail gas. The final off-gas
passes through a large surge drum that moderates pressure and composition surges
for the off-gas and minimizes the pressure swing impacts on the system receiving
the off-gas – normally an SMR furnace or fuel gas header/compressor.
Adsorbents in the vessels are selected based on the impurities in the feed gas. For
instance, in a typical SMR hydrogen plant, the bottom portion of the PSA bed will
contain activated carbon and the upper portion will contain molecular sieve. The
activated carbon can easily adsorb methane, CO 2 , water vapor, some CO, and other
trace compounds. The mole sieve adsorbs nitrogen and the balance of the CO.
It is critical that the feed gas be as dry as possible. If the gas has a high moisture
content, water adsorption may reach the molecular sieve where it will permanently
attach, reducing adsorption capacity. SMR hydrogen plant raw hydrogen feed is
limited to about 130
F to avoid this problem.
626
S.A. Treese