Apr-2022

Refinery off-gas in hydrogen production

An off-gas feed to an SMR unit for hydrogen production is economically and environmentally attractive as long as it is carefully monitored.

MARCELO TAGLIABUE
Air Liquide

Article Summary

New hydrogen production plants based on steam methane reforming (SMR) and built within or near refineries or petrochemical complexes are usually designed with the possibility of feeding units with different types of hydrocarbons from different available sources. The range of raw materials goes from natural gas or refinery off-gas to gasoline of different qualities, mainly light naphtha or catalytic cracking naphtha.

Plants with SMR already installed consider studying the alternative of making modifications in their plants to be able to have access to other raw materials cheaper than natural gas. The size and complexity of these modifications are highly dependent on the specification of the substitute raw material, its pollutants, and their availability. It is also necessary to evaluate the impact on the economic equation of a possible decrease in steam production once the basic modifications to the plant have been made.

Refineries, in their desire to be increasingly profitable and efficient, tend to make their processes more flexible to adapt them to the demands of the exploitation of heavier crude oils. This trend increases hydrogen demand to meet the most severe needs of hydrotreatment. This greater severity in hydrotreatment is causing an increase in refinery off-gas production in most refineries, which in many cases exceeds the possibility of sending it to other units or burning it entirely in furnaces and boilers. In addition, the fact of being able to use it as a raw material for the production of hydrogen, avoiding in many cases the need for this stream to be sent to other hydrotreatments, enables higher purity hydrogen, resulting in longer runs in hydrotreatment.

Some 66% of the world’s hydrogen production, estimated at 70 million tonnes, is used as an input in petroleum refining, the production of ammonia, methanol and, in recent years, in the development of engines that run on H2.

By early 2021, 70% of the global economy had made ambitious commitments to carbon neutrality, and hydrogen will play a fundamental role in reducing emissions.

Currently, the best known and most developed hydrogen production methods are:
- Water electrolysis, currently limited to 4% of hydrogen production due to its high cost. It is achieved by dissociating the water molecule into its components (H2 and O2) using renewable electrical energy. Hydrogen produced in this way is known as green hydrogen.
- Reformed natural gas (SMR), which represents 96% of current production worldwide. It is a thermochemical process, which requires high temperatures and a subsequent purification of the final stream, obtaining hydrogen as the main product and 9.3 Kg CO2/kg H2 that is sent to the atmosphere. This is the so-called grey H2. If CO2 is captured before being discharged into the atmosphere (in an SMR up to 95% of the CO2 produced can be captured) in this process we are in the presence of blue hydrogen.
To achieve the goal of the highest possible conversion of crude oil into gasoline, diesel or middle distillates, availability of hydrogen has become a critical issue in modern refineries.

Previous results and reasons for the present work
One of our business units operates two SMRs (SMRI and SMRII), mainly to satisfy refinery hydrogen requirements and, to a lesser extent, for sale as high pressure hydrogen gas. Additionally, part of the CO2 generated in SMRI is captured, purified, and sold to the local market as food grade.

In a modern natural gas reforming plant, up to about 60% of the total CO2 produced is contained in the produced syngas (then the pressure swing adsorption [PSA] tail gas), while the remaining 40% is the combustion product of the additional fuel gas required by the steam reformer.

In our case, to recover CO2 in SMRI we used Point 1 shown in Figure 1. SMRI is a side fire type, fed with natural gas, and consists of a CO2 capture and sequestration unit (CCSU). The capture of CO2 is carried out through an absorption process with activated amines and a CO2 liquefaction plant for later sale.

In previous works, it was mentioned that in addition to maintaining the production of hydrogen in SMRI, it was possible to substantially reduce the delivery of CO2, CO and CH4 that were vented to the atmosphere from the non-condensable gases of the CO2 liquefaction unit, using them as feed for the reformer, thus reducing consumption of natural gas.¹

Secondly, in order to increase the profits of the business unit and mainly to continue reducing CO2 emissions to the environment, a disused asset (a CO2 production plant that burned natural gas to produce it) was used to capture CO2 from the SMRI flue gases that were vented to the atmosphere (see Figure 1, Point 2). Thus, about 25 t/d of CO2 are recovered and sold in the local market, reducing emissions to the atmosphere by the same amount.

As clean fuel standards become more demanding, the hydrogen consumption needs of refineries are increasing. In this sense, our company installed SMRII very close to the first one. In this case, the technology is top fired, which does not have a unit to recover CO2.
In this last stage, the main reason for the present work is to take advantage of the second SMR design that has an adiabatic pre-reformer, giving it greater operational flexibility (to be able to process different types of feeds). The use of an off-gas stream was investigated as the SMRII power supply.^2,3 This was done with the expectation of reducing the consumption of fossil fuel and using off-gas that would otherwise be burned and released to the atmosphere.

To reduce emissions to the atmosphere, many refineries are investing in the electrification of furnaces and boilers, so the off-gas that they currently use in their own boilers to produce part of their steam could be available for any other application from 2021.
The present work will show how the main variables evolve in an SMR, depending on the variation of the quality and quantity of off-gas.

Experimental
Refinery off-gas is a mixture of gases generated during the refining of crude oil. The composition of the off-gas varies, depending on the composition of the crude from which it originated and the processes to which it has been subjected. The most common components that make it up include butanes, butylenes, methane, ethane, ethylene, and hydrogen. Some products found in refinery off-gas are subject to control.

The search for cheaper commodity options is certainly reinforced by significant increases in recent years in crude oil and natural gas prices, although the emergence of shale gas production has reversed this trend in some areas. One feedstock option that hydrogen plant operators associated with oil refineries are increasingly considering is refinery off-gas. Relative to natural gas, it has a comparatively low value. Therefore, it represents an attractive raw material option where extra off-gas is available. It is not widely used as a hydrogen plant feed; this is because its composition can lead to a number of processing difficulties in the purification and steam reforming sections.⁴