Jan-2006

Hydrogen management

Several possibilities exist for the recovery of hydrogen from offgases and the processing of different sources of refinery gases through the hydrogen plant. Catalysts play an important role in efficient hydrogen generation and recovery

Axel Düker, Süd-Chemie AG
Rainer Basse, Uhde GmbH

Article Summary

Refiners are currently facing strong competition, which compels them to lower operating costs wherever possible and to utilise all process purge gas and offgas streams. Stronger environmental regulations and the need to process heavier crude significantly increase hydrogen demand. The profitability of a refinery depends on the method by which hydrogen is produced and how hydrogen-containing gases are recovered.

The imperative for process optimisation and cost reduction leads to the application of a modern hydrogen management system. In the past, process optimisation and hydrogen recovery could avoid a new hydrogen production plant (HPP) investment. However, increasing hydrogen demand will soon require refineries to invest in new HPPs, taking into consideration the fact that most refinery processes involve catalytic reactions. For this reason, this discussion focuses on feed purification optimisation catalysts for the hydrogen-producing steam reformer.

Retrospective view
Refineries in the 1970s and early 1980s operated under quite different conditions than they do today. The main feedstock was a light and sweet crude, and this was available in sufficient quantity and easy to handle. As crude oil prices were low most of the time, investment costs governed overall production costs. Processes were not optimised with regard to operating costs in order to keep the investment cost low. Environmental regulations became more stringent from the 1980s onwards, with limitations on sulphur, NOx, VOCs or CO2 occurring only during the last 20 years. The demand on the quality of the products was also lower; petrol and diesel contained lead, sulphur, aromatics and others. During these times, hydrogen demand was quite low and, in case a refinery operated a hydrogen plant, the process scheme was simple, as shown in Figure 1.

Investment costs governed hydrogen plant considerations. The hydrogen plant was a simple utility unit, which processed natural gas or naphtha, depending on the availability, and produced hydrogen and steam as a by-product. The typical hydrogen process consists of desulphurisation, steam reforming, high-temperature CO shift and hydrogen purification. While the steam-reforming process remained in principle the same, except that the size of the steam reformers increased significantly, the purification process changed from a CO2-removal unit, followed by a methanation unit, to the pressure swing adsorption (PSA) unit, which can achieve a much purer hydrogen product and a more efficient hydrogen plant.

Refineries today
A refinery today operates in a significantly different environment. Raw material prices are increasing dramatically. The demand for refinery products is growing continuously, while the availability of light and sweet crude is decreasing, so refineries are forced to process more heavy and sour crude. Also, stronger environmental regulations and higher requirements on the products are already resulting in a change to the operating conditions. Due to higher crude oil and other feedstock prices, operating costs became more important and refiners had to optimise their processes.

These factors force refineries to focus on one additional refinery product: hydrogen (H). Its demand has increased significantly, and it is increasingly recognised that hydrogen is a valuable gas that needs special consideration. Utilising all hydrogen-containing streams is therefore essential for economical refinery operations.

Increased hydrogen demand
The first step in meeting the growing demand for hydrogen was the optimisation of plant processes and the application of hydrogen management tools like hydrogen pinch and network optimisation. This was followed by enhancing the efficiency of existing hydrogen plants and increasing their capacity. Installation of a pre-reformer or a reformer, such as a convective reformer or gas-heated reformer, operating in parallel to the steam reformer, adding an LT shift and raising the PSA efficiency are typical revamp measures.

The latest developments in processing heavy crude and producing ultra-low-sulphur fuel and diesel create a considerably higher hydrogen demand, which cannot be covered solely by the previously described steps. Now, refineries have no other choice than to look for new hydrogen sources, and the most obvious way is to build a new hydrogen plant.

The actual new demand drives the capacity consideration of new hydrogen plants. Most of the plants Uhde has built for refineries during the last several years are 90 000–160 000Nm_/h. This capacity can easily be reached by a single-train unit with a modern top-fired steam reformer with a single-train PSA unit.

The size of this hydrogen source results in a changing view of the hydrogen management system. A large-scale hydrogen plant today is not the minor supplemental hydrogen supplier it once was. On the contrary, it is now the major source of hydrogen, and more often the whole refinery depends on this source. If it fails, the whole refinery suffers.

Hydrogen process
A modern hydrogen plant can process not only natural gas or naphtha, but also other hydrocarbons and hydrogen-containing streams. Thus, refinery planners also consider a modern hydrogen plant to be a recovery unit for refinery offgases and tail gases. When evaluating the process, three major process steps can be defined as:
— Separation of hydrocarbons into H and C For this step, a steam reformer or partial oxidation unit is used, depending on the feedstocks. Steam reformers will be viewed from the perspective they still play as the major hydrogen production unit
— Conversion of CO with steam into hydrogen and CO2 This conversion takes place in a CO-shift conversion unit
— Hydrogen purification Depending on the required hydrogen purity, CO2 scrubbing followed by a methanation unit or a PSA unit can be used.
The refinery gases sent to a hydrogen plant for hydrogen recovery or production can be fed into the plant upstream of each of these process steps.

The major portion of refinery gases will be fed into the plant upstream of the reformer (Figure 2). These gases typically contain a higher amount of hydrocarbons; unfortunately, not only lighter paraffins, which can be mixed with the “main” feed gas, but also olefins and aromatics. Impurities like halogen compounds, heavy metals and sulphur do not make their processing easier.