Hydrogen Purification

Key Features

Applications

The hydrogen required for the numerous industrial applications can be recovered from the hydrogen rich process streams. And that are readily available in refinery and petrochemical industries as "waste streams". The efficient Pressure Swing Adsorption technology uses this to purify and make available low cost high purity Hydrogen. Increasingly due to environmental considerations, PSA technology is breaking grounds especially in refineries as they aim to reduce sulphur and aromatics in their products. The process for Hydrogen purification is described here. The PSA process of purification basically means that the impurities in the feed gas get adsorbed to the fixed Adsorbent bed at high pressure. Subsequently the impurities(of the feed gas) gets desorbed at comparatively low pressure into an offgas stream. Thus an extremely pure hydrogen product with purities in excess of 99.999% can be achieved by this process.

Numerous hydrogen rich feedstocks can be treated, by this process, the list includes, methanol cracker syngas, ethylene cracker gas, steam reformer syngas, refinery gases etc. The PSA process can manage the purity requirements and feedgas composition by adjustments in the cycle and the type of adsorbents. The purification of Hydrogen is shown in the diagram that follows.


Diagram showing a Six Bed Purification System:
Here a six bed purification system is shown it is to be noted that for applications that demands higher hydrogen recoveries, additional adsorbers can be easily added. Multiple beds are effectively used for reducing the size of vessels and the quantity of adsorbent. For example a single train PSA makes possible Plant capacities that even exceeds 100,000 Nm3/h (90 Mmscfd).

As already discussed, several times though the process is a batch operation, but continuous product and offgas flows are got by employing multiple adsorbers that operates in a stepwise manner. Again it does not matter as to how intricate the PSA sequence is, the PSA process cycle boils down to the five basic steps. They are the following:

1. Adsorption Process
2. Co-Current Depressurization Process
3. Counter-Current Depressurization Process
4. Purge Process
5. Counter-Current Repressurization Process

Key Features of PSA operated Hydrogen Purification Unit:

• The Whole PSA process is under control of a PLC, which does the manipulation work by switching valves in a preordained sequence. This sophisticated control system is able to monitor and identify any problem within the system.
• Modular construction.

Applications of PSA Hydrogen Purification Systems:
The PSA Hydrogen purification systems has wide application in the following industries:

• Refinery
• Petrochemicals
• Vegetable Oils
• Paper & Textiles
• Steel Industry
• Fuel Cells

Selection of Hydrogen Purification Technology:
To conclude, the choice of the optimum purification method for a particular specific industrial applications must be considered very rationally. It should be perfectly based on the criterion of technical and economics. Given below is the table that shows, the degree of purity of hydrogen that is obtained from the different methods of separation. It could also be seen from the table that the amount of recovery of hydrogen also varies considerably. Now these can play a major role on process economics, especially in large scale applications.

A comparison between different separation techniques