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Apr-2000

Revamp options to increase hydrogen production

Revamping of existing hydrogen plants to increase capacity is offered as an attractive possibility in many situations

Ib Dybkjær, Sandra Winter Madsen and Niels Udengaard, Haldor Topsøe
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Article Summary
When a refinery needs to increase hydrogen production – because of, for example, requirements for processing of heavy, sour crude, or more stringent specifications, and so on – several important decisions must be made, depending on the situation. Options may be governed by such considerations as the source of the additional hydrogen (over-the-fence supply from a gas supplier versus production in own facilities) or by raw material for the incremental capacity (hydrocarbon feedstock or recovery from hydrogen-containing offgases) and by technology to be used to generate the new production.

It is clear that the relevant options will depend on a number of factors, including:
- Design, capacity, and condition of the existing plant
- Desired objectives of the revamp project in terms of energy savings and/or capacity increase
- Availability and cost of feedstock and utilities
- Possible demand for co-production of power and/or steam
- Capital available for the revamp project, and the acceptable duration of downtime in relation to the revamp.
It is suggested that revamp projects are best handled in three phases:

Phase I
The purpose of the Phase I study is to identify revamp alternatives and to estimate the possible energy savings and capacity increase. The capital to implement the proposed modifications will be estimated with an accuracy of ±25 per cent. In order to compare the new operating conditions and energy consumption with the present performance, the study includes an evaluation of the existing plant operation.

An inspection of the plant to gather a set of operating data forms the basis of the feasibility study. The available operating data, such as actual flowmeter readings, measured pressures, temperatures, laboratory analyses, etc, are checked and smoothed for errors in instrument readings. This is done to establish a consistent set of heat and mass balances for the base case of the existing operation.

From the heat and mass balance the actual energy consumption is calculated, and new or modified equipment is identified as required to meet the project objectives. While establishing the base case, close contact is maintained between the Topsøe engineers and the plant personnel to ensure that the corrected data are as close as possible to the actual operating data.

Phase II
During this phase, detailed process engineering of the modifications recommended in Phase I and detailed mechanical engineering of certain new or modified critical items is completed. The engineering is sufficiently detailed to enable the client to obtain competitive bids on the complete revamp project.

The package will also contain a proposed time schedule with emphasis on the logistic arrangements necessary to minimise the interference with the existing operation. A summary of the scope of Phases I and II is given in Table 1 (on previous page}.

Phase III
In Phase III the final design and construction are completed under appropriate contracts. Revamp activities can, at the client’s request, be phased differently from that described above. This approach allows the client to review the project at predetermined intervals in order to decide whether to continue or reorient of the project.

Technical aspects
New, advanced technology relevant to revamp projects is abundantly available, including new process concepts, equipment design, and more effective catalysts, and new knowledge about the limits acceptable in operation of various units.

The extent to which advanced technology can be applied to a specific project varies depending on the specific situation, as indicated above. Therefore, revamp projects must be tailor-made from case to case, and where the phased approach is appropriate.

Layouts
The conventional layout of hydrogen plants, still used in many older plants operating today is shown in Figure 1. The plants comprise feedstock desulphurisation, steam reforming, high and low temperature shift conversion, removal of CO2, and methanation. The overall efficiency in this type of plant is typically around 80 per cent, corresponding to a net energy consumption (after credit for steam export) of around 3.5 Gcal/1000 Nm3 H2.

The reformer furnace is designed for a steam to carbon ratio between 5 and 6. This high ratio is required to obtain sufficiently high purity of the product hydrogen. The introduction of Pressure Swing Adsorption (PSA) systems in the 1970s simplified the process layout of hydrogen plants substantially. A typical lay-out of an early version of this lay-out is shown in Figure 2.

The PSA unit removes all impurities in the hydrogen-rich stream, producing a hydrogen product with nearly 100 per cent purity. The unit replaces low temperature shift, CO2 removal unit and methanator. The reformer furnace can now operate at low steam to carbon ratio as residual methane is removed in the PSA unit. Overall plant efficiency is increased to about 90 per cent.

The PSA unit separates the gas with a certain efficiency requiring a slightly larger reformer furnace compared with the conventional lay-out. Downstream units are, however, simplified substantially. The net energy consumption is typically 3.1 Gcal/1000 Nm3 H2. Revamp options provide more efficient desulphurisation of reformer hydrocarbon feed by replacing a possible activated carbon desulphurisation step with a hot zinc oxide adsorption step and by introducing a cobalt/molybdenum hydrodesulphurisation catalyst. This will lead to better performance and longer lifetime of downstream catalysts.
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