Apr-2014

Revamping the hydrogen generation unit

Hydrogen plants can be modified for higher production by employing innovative process schemes

K V PATWARDHAN, V D DHUMAL and P BALARAMAKRISHNA
R&D Centre, L&T Hydrocarbon Engineering, India

Article Summary

Demand for refinery hydrogen is increasing rapidly due to the processing of heavy sour crudes and stringent fuel quality requirements. The existing capacity for hydroprocessing and the associated hydrogen network limits refinery throughput and operating margins. Innovative process schemes and reforming technologies are available for revamps of the hydrogen generation unit to enhance production substantially with minimum capital cost and energy consumption.

In this article, a simulation model is developed to study the impact of such upgrading schemes with optimum utilisation of existing reformer and downstream equipment. Installation of a pre- reformer and gas heated reformer in series or parallel combination with the existing reformer can enhance hydrogen generation capacity by 10-27% and reduce fuel consumption by 12%. For capacity increases up to 40%, an auto thermal reformer is more attractive but this scheme requires major modifications in the downstream sections.

An optimum revamp process scheme can always be devised depending on the plant’s specific requirements and capital/operational constraints.  

Future hydrogen balance
Hydrogen is usually regarded as a utility in oil refining. A modern 10 million t/y refinery will often require 150 000 Nm3/hr or more of hydrogen. Current trends in the refining industry are reducing the availability of hydrogen to the point where most refineries are concerned about their future hydrogen balance. New specifications for low sulphur fuels require increased hydrogen consumption in hydrotreaters. The use of heavier crude oils and more bottom of the barrel processing increase hydrogen demand in hydrocracking and heavy oil hydrotreating units. At the same time, limits on the aromatics content of gasoline, and requirements for oxygenates, have led to lower severity in the catalytic reformer. As a result, less hydrogen is produced in this unit. Demand for gasoline and middle distillates means that crude oil will require greater processing and thus more hydrogen.

In future, practically all fractions of sour crude will be subjected to catalytic processes that involve hydrogen. The total quantity of fractions directed to hydrotreating, hydrocracking and hydrodesulphurisation will amount to 90% of the total crude run. This demand can be met by applying a systematic, cost effective approach to hydrogen management. Options for the supply of hydrogen include increasing hydrogen plant production, revamping existing equipment, building a new hydrogen plant, purchasing hydrogen from outside suppliers, or recovery of hydrogen that was going to fuel by installing a hydrogen purification unit, or the lower cost alternative of optimising and revamping the hydrogen distribution network.

For many years, hydrogen has been produced commercially via steam methane reforming (SMR) of hydrocarbons. The technology has now reached a mature state and is the basis of a majority of plants built for generating hydrogen in refining and petrochemical complexes.
This article focuses on the revamp of the existing SMR based hydrogen plant, either for higher production or for lower specific energy consumption using modern reforming technologies available commercially from licensors.

Steam methane reforming
These plants consist of four basic sections (see Figure 1):
• The first is feedstock treatment where sulphur and other contaminants are removed
• The second is the steam methane reformer, which converts feedstock and steam to syngas at high temperature and moderate pressure. The reforming reaction between steam and hydrocarbons is highly endothermic and is carried out using a specially formulated nickel catalyst contained in vertical tubes situated in the radiant furnace of the reformer
• The third section is syngas heat recovery and incorporates CO shift reactor(s) to increase the hydrogen yield. In the adiabatic CO shift reactor vessel, the moderately exothermic water gas shift reaction converts carbon monoxide and steam to carbon dioxide and hydrogen
• The final section is raw hydrogen purification, in which modern plants employ a pressure swing adsorption (PSA) unit to achieve the final product purity. The PSA purification unit removes CO, CO2 and CH4 gases by adsorption from the hydrogen.
 
Modern reforming technologies
Pre-reforming
Pre-reforming is used for low temperature steam reforming of hydrocarbon feedstocks ranging from natural gas to heavy naphtha. Converting higher hydrocarbons in the pre-reformer results in stable and mild operating conditions for a downstream tubular reformer and thus ensures reliable operation of the tubular reformer. The pre- reformer is placed upstream of the tubular reforming unit. In order to obtain the required steam to carbon ratio, feedstock is mixed with process steam before entering the pre-reformer. In this, all higher hydrocarbons are converted to a mixture of carbon oxides, hydrogen and methane at equilibrium based on the methanation and water gas shift reactions. Pre-reforming allows operation at low steam to carbon (S/C) ratios and thereby reduces overall energy consumption. The pre-reformer also increases the lifetime of the tubular reformer and the shift catalysts as the sulphur present in the hydrocarbon feed and process steam is absorbed by the pre-reforming catalyst.

Gas heated reformer (GHR)
GHR utilises the heat available in the process gas at SMR exit for steam reforming in a heat exchanger type of reactor. This scheme is available in series or parallel combination with SMR. In a parallel combination with the available heat, up to 20% of the feed can be split and taken to GHR. In both combinations, a proportion of heat is utilised in the process side which reduces the steam production from the plant. GHR can also be used in series/parallel arrangement with a SMR-ATR combination of steam methane reforming and autothermal reforming. Other advantages of GHR technology compared with the fired steam reformer are:
• Small size and footprint, which can be essential when increasing the capacity of existing plants
• Lower investment costs, particularly in plant expansions
• Reduced steam utility size and cost
• The complete unit is transported to the processing plant for installation, which saves costs when the plant is located far from infrastructure or in a high wage country
• Debottlenecks the existing reforming section.