Meeting the challenges of increased hydrogen demand
With ever more stringent regulations for transport fuels, refiners increasingly have to invest in additional hydroprocessing capacity.
Mario Campi, Foster Wheeler
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Lower aromatics specifications for gasoline have, in some cases, resulted in lower achievable reformer through- puts and therefore lower reformer off-gas flows, — a traditional source of refinery hydrogen. As a consequence, there is an ever-increasing demand for hydrogen. The trend towards the processing of heavier crude feedstocks and the application of residue conversion technologies are further factors exacerbating the hydrogen shortfall.
Meeting these new hydrogen demands is a challenge all refiners must face, as the refining industry progresses into the 21st century. This article addresses the methods for meeting these challenges and focuses on an innovative, cost-effective solution developed by Foster Wheeler for the NIS Pancevo Refinery in Serbia.
In the modern competitive era, refiners have to look at integrated solutions to meet their hydro- gen demands. A new unit to supply the shortfall is rarely ideal from a cost point of view.
Every refinery is different, and understanding the intricacies of the current supply and demand is key to developing the optimal future solution. Foster Wheeler applies a proven approach to hydrogen management, which addresses:
• Current refinery balance
• Hydrogen recovery from various refinery off-gases
• Upgrading/revamping options for existing
hydrogen production unit, taking advantage of the latest technology and in-house fired heater expertise
• Additional new capacity
• Reliability of supply.
In order to analyse the existing hydrogen resources in refinery complexes, Foster Wheeler has developed an effective hydrogen pinch technique.
This analysis technique identifies the best opportunities for off-gas reuse and purification, and can be used to quantify the minimum size of new hydrogen production facilities, if required. The pinch technique aims to develop the optimal hydrogen recovery scheme, building in capex and constructability at an early stage. Key elements of the pinch analysis are:
• Confirmation of existing network headers
• Compression requirements
• Potential for reuse of off-gases containing hydrogen
• Potential application for hydrogen purification
The upgrading of an existing hydrogen production unit often includes updated micro-alloys for catalytic tubes and the higher activity of modern catalysts. This allows performance improvements in the steam reformer heater to be realised. By operating the reformer at higher outlet tempera- tures with lower pressure drops, hydrogen production can be increased.
Another option is to revamp the existing hydrogen production unit by installing the following equipment:
• Lower temperature shift.
Ultimately, the revamp of an existing unit will be limited to a capacity increase typically less than 20% due to hydraulic limitations. At this point, additional capacity becomes prohibitively expensive. The opportunity for revamping has to
be considered in context with the overall demand to ensure this is the most effective use of capital. Other solutions that demand consideration are opportunities to share hydrogen supplies with
neighbouring facilities or industrial gas suppliers.
Integrated off-gas utilisation
Recent trends in refinery hydrogen management have seen increasing application of integrated utilisation of refinery off-gases (ROG) through their potential link to on-purpose hydrogen generation units (HGU).
There are typically three options to be considered when utilising ROG in conjunction with additional hydrogen generation capacity:
• Option 1 Combine ROG with reformed gas upstream of a common pressure swing absorption (PSA)
• Option 2 Combine the ROG with feedstock upstream of the HGU
• Option 3 Send the ROG to a dedicated PSA, where tail gas is recompressed for either fuel export or utilised as reformer feed.
Option 1 can offer a cost-effective solution if the ROG provides a low H2 contribution to over- all demand. There are, however, a number of constraints associated with this option and hence its application is limited.
The ratio of ROG to reformed gas needs to be kept constant for optimum bed usage without risk of bed contamination
• If the HGU is shut down, the PSA operation on only ROG may result in adsorbent contamination by heavy hydrocarbons moving too high in the bed
• In many cases, the off-gas stream often contains less H2 and significantly higher concentrations of heavy hydrocarbons than originally predicted. This is difficult to foresee and can lead to irreversible adsorbent de-activation when undetected and if adequate measures are not taken to shorten the PSA cycle time
• The combined PSA tail gas has a much higher calorific value, which is often in excess of the steam reformer fuel requirements. A tail gas compressor is required to route the excess gas into the fuel gas network. Moreover, excess tail gas contains CO2, which can impair the opera- tion of existing burners in the refinery.
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