Oct-2010

Debottlenecking hydrogen plant production capacity

Retrofitting a steam methane reformer with oxygen enhanced reforming can substantially boost hydrogen production capacity at low capital investment

Gregory Panuccio, Troy Raybold, James Meagher, Ray Drnevich and Venkatarayaloo Janarthanan
Praxair

Article Summary

Stricter environmental emissions regulations and increased processing of heavier and more sour crude oil fractions are causing hydrogen demand to grow across the refining industry. Costs associated with new hydrogen plant construction have been escalating, so refiners are increasingly looking for low capital cost retrofit solutions that can expand hydrogen production capacity at their existing facilities. This article discusses Praxair’s oxygen enhanced reforming (OER) technology, which can expand hydrogen plant production capacity by enriching the steam methane reformer’s (SMR) combustion air with oxygen. An OER retrofit can be installed with low capital investment, quick turnaround and minimal plant downtime.

Options for increasing hydrogen supply capacity
A refinery manager has many options to consider when operations are limited by hydrogen supply from an on-site SMR. For example, hydrogen could be purchased from a liquid hydrogen supplier or the hydrogen supply capacity could be increased by constructing an additional on-site SMR or by replacing the existing SMR with newer and larger equipment. These solutions can be costly, so it is often most economical to debottleneck the existing hydrogen plant. Approaches typically considered for debottlenecking include increasing the firing rate of the reformer, replacing existing tubes with ones with a larger diameter or improved metallurgy, adding a low temperature shift (LTS) reactor downstream of the existing high temperature shift (HTS) unit, and adding a pre-reformer or a post-reformer.^1,2 Characteristics of these retrofit solutions are summarised in Table 1.

The first concept usually evaluated is to increase the firing rate of the primary reformer by burning more fuel and increasing the heat available in the radiant section of the reformer furnace. As a result, more heat is transferred to the reforming reaction within the tubes and additional feed gas can be processed. A new induced draft fan may have to be purchased to accommodate the additional flow of flue gases, and a significant increase in flue gas stack temperature can result unless the convective section heat recovery system is modified. Furthermore, the resulting increase in reformer tube wall temperature will reduce the life of the tubes and significantly add to plant maintenance costs. Increased maintenance costs can be defrayed by upgrading the reformer with new tubes made of better metallurgy that is tolerant to higher temperatures. Taking these steps can increase hydrogen production capacity by 5–15%, but will require significant capital investment and plant downtime.

The hydrogen production rate from an SMR can be increased by 3–5% by modifying the water-gas shift (WGS) reactor system in order to convert more of the residual carbon monoxide in the syngas into hydrogen and thereby improve hydrogen output without increasing the flow of process gas or flue gas, or raising the reformer firing rate. A two-stage HTS configuration can be installed with interstage cooling; the existing HTS reactor can be replaced with a medium temperature shift (MTS) reactor; or a LTS reactor can be installed immediately downstream of the HTS reactor. LTS units can be difficult to operate and additional make-up fuel is required to replace the lost PSA tailgas. Furthermore, modifying the WGS design will require a moderate capital investment and significant downtime for installation.

Retrofitting an existing SMR with a pre-reformer can boost production capacity by 8–10%. The pre-reformer reactor utilises heat from the hot flue gas that was previously used for steam production to instead reform a portion of the hydrocarbon and steam feedstock prior to introducing it into the reformer tubes. A pre-reformer retrofit will require a moderate capital investment and significant downtime, as both a new catalytic reactor and a flue gas convective section reheat coil must be installed. Pre-reforming will also result in lower production rates of valuable steam by-product.

A post-reformer retrofit (for instance, a secondary autothermal reformer or product gas heated reformer) can increase hydrogen production by as much as 30% if the SMR is not bottlenecked by the downstream syngas processing equipment. With a post-reformer, a significant portion of the feed is being reformed outside of the primary reformer, so hydrogen production is increased without increasing the firing rate in the reformer. However, installation of the retrofit will require significant investment of capital, plant downtime and plot space.

The characteristics of an OER retrofit are also shown in Table 1. With OER, oxygen is used to enrich combustion air in the reformer furnace, which can improve hydrogen production capacity by 10–15% without increasing maximum reformer tube wall temperatures, modifying the induced draft (ID) fan or convective section heat exchanger design, or appreciably affecting by-product steam production rates. Further-more, an OER retrofit can be installed with little plant downtime and capital investment.

Oxygen enhanced reforming
A simplified schematic of the radiant section of an SMR furnace that includes OER is shown in Figure 1. The combustion air is enriched with oxygen by one of two means: either oxygen is premixed with the air via sparger in the air feed ductwork or the oxygen is injected directly into the burner flame via a lance. For either delivery method, OER increases oxygen concentration and decreases the concentration of inert nitrogen in the combustion air. Since the oxygen concentration is higher than in normal air, the furnace firing rate can be increased without raising the volumetric flow rate of flue gas that is processed in the convective section of the reformer. And because the flue gas flow rate is unchanged, no modifications to the flue gas heat exchange equipment or the ID fan are required to maintain the reformer’s performance. A secondary effect of increasing the oxygen concentration in the combustion air is that the additional heat that is generated is released over the same distance or even over a shorter distance than with air. This results in an increase in the heat flux into the tubes near the inlet of the reformer where the tubes are coldest. Process gas flow to the reformer tubes is increased to utilise the additional heat that becomes available and to maintain the maximum tube wall temperature at the desired value. Typical reformer tube heat flux and maximum tube wall temperature profiles with and without OER are shown in Figure 2.

The extent to which oxygen enrichment of combustion air can increase hydrogen production capacity in an SMR depends on many factors, including whether other bottlenecks exist in the equipment upstream or downstream of the reformer furnace. Increasing the oxygen concentration in the combustion air to between 22% and 23% can typically result in a 10–15% increase in hydrogen production capacity.