Oct-2011

Optimising and revamping a refinery hydrogen network

Pinch analysis and mathematical programming support hydrogen network design and retrofit. An increased demand for hydrogen often makes the supply of hydrogen a severe bottleneck for many modern refineries.

Zhao Jianwei, Luoyang Petrochemical Engineering Company
Lou Yuhang, Zhang Nan and Keith Guy, Process Integration Ltd

Article Summary

Much progress has been made in the last few years on the technological development of refinery hydrogen management, with two categories of approach: simple hydrogen pinch analysis for network targeting, and detailed mathematical programming for optimisation, design and retrofit. However, when applied to the revamping of existing refinery hydrogen networks, these approaches show common limitations caused by their rigid restrictions of constant hydrogen purity and hydrogen partial pressure in hydroprocessor reactors. In industrial practice, such restrictions are not followed exactly and make it extremely difficult to modify any complex hydrogen systems using existing techniques.

In this article, a modified hydrogen network model is proposed, which allows for marginal changes in hydrogen purity and hydrogen partial pressure in hydroprocessor reactors. Such changes have a minimal impact on reaction hydrogen consumption, product yields and quality, while providing extra degrees of freedom for refiners to exploit additional options for saving hydrogen. The developed approach has been applied to a hydrogen network revamping project for Sinopec, achieving a hydrogen utility reduction of 8.8% and annual operational cost savings of over $9.7 million, with a simple payback time of less than half a year.

Hydrogen addition
There is a worldwide trend towards processing heavier crude oils. To obtain the best value from these oils, refiners must be able to convert heavy-end compounds to lighter fractions that can be blended with gasoline or diesel. To achieve this, refineries are using more hydrogen addition than conventional carbon rejection for a better production yield. The reason for such a selection is not only to produce better-quality transportation fuels, but also to process lower-quality crude oil, which is heavier than conventional crudes and contains more sulphur and nitrogen. All of these facts are driving refineries to increase their levels of hydroprocessing, which places increasing demands on the supply of hydrogen. As energy prices rise, the cost of hydrogen production also increases. Currently, all major hydrogen production processes consume a significant amount of energy and generate a large amount of greenhouse gases. Therefore, better hydrogen management through hydrogen network optimisation is needed for energy savings and hydrogen generation cost reduction.

Existing methodologies
Hydrogen pinch analysis
Generally speaking, a refinery’s hydrogen network consists of three parts: hydrogen production, hydrogen consumption and hydrogen recovery through purification. Hydrogen producers generate hydrogen by means of continuous catalyst regeneration reformers (CCR), steam reformers and partial oxidation reformers. Major consumers include hydrocrackers, hydrotreaters and hydroprocessers. Hydrogen purifiers convert lower-purity hydrogen-containing gases into higher-purity product, which can then be reused in processes. Typical hydrogen purification technology includes membrane purifiers, pressure swing adsorption (PSA) and cryogenic separation. Interaction among hydrogen producers, consumers and purifiers determines the design of the hydrogen network in a refinery, as well as the demand for hydrogen.

In 1999, Alves1 presented hydrogen network pinch analysis and hydrogen network optimisation methods. The hydrogen network pinch analysis method is based on heat exchanger network pinch analysis technology.2 This method can determine the bottleneck of the whole hydrogen network and can systematically analyse hydrogen utilisation at different purification levels.

To carry out hydrogen network pinch analysis, it is necessary first to identify sources and sinks of hydrogen, which can be analogous to hot and cold streams in heat exchanger networks. Hydrogen sources are streams containing hydrogen, which can be used to provide hydrogen to the system. Hydrogen sinks are processes that consume hydrogen. A hydrogen consumer is a hydrogen source and also a hydrogen sink (see Figure 1). Recycle hydrogen, high-pressure purge, low-pressure purge and other purge gases can be regarded as hydrogen sources, and the reactor inlet should be seen as a hydrogen sink. With hydrogen pinch analysis, all conditions for hydrogen sources and sinks are fixed.

Once the conditions for all hydrogen sources and sinks are determined, the next step is to draw the hydrogen composite curve, which is a two-dimensional plot with the flow rate of total gas on the horizontal axis and purity on the vertical axis (see Figure 2).

Plotting the hydrogen demand profile and the hydrogen supply profile gives the hydrogen composite curves. This purity profile contains the hydrogen sinks and sources ordered by decreasing purity. Separately, the sink and source curves start at zero flow rate and continue until the lowest purity is represented. Where the hydrogen supply curve is above the hydrogen demand curve, the area between the two profiles is marked as surplus (+), which means the sources provide more hydrogen than is required by the sinks. If the hydrogen supply is below the hydrogen demand curve, the area between the two profiles is marked as deficit (-), which means sources do not provide enough hydrogen to the sinks. Calculating these surpluses (+) and deficits (-) and plotting them against the purity level constructs a hydrogen surplus diagram, or hydrogen pinch diagram (see Figure 3). For any existing network, the surplus curve is always on the right side of the vertical axis.

For an existing network, all parts of the surplus curve are always positive. The hydrogen utility can be reduced by moving the curve towards the vertical axis until a vertical segment between the purity of the sink and the source overlaps with the zero axes (see Figure 4). The purity at which this occurs is defined as the hydrogen pinch and is the theoretical bottleneck for how much hydrogen can be used from the sources to the sinks. The hydrogen utility flow rate that results in a pinch is the minimum target and is determined before any network design.

With hydrogen pinch technology, the minimum hydrogen demand of a hydrogen network can be determined with very basic information and simple data collection. Hydrogen pinch analysis technology also provides certain principles for hydrogen network design: