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by Alan Zagoria, Solutions & Services
Introduction
Refiners today are finding that hydrogen is one of the most critical challenges facing them as they plan production of clean fuels. In addition, hydrogen management practices significantly impact operating costs, refinery margin, and CO2 emissions.
Therefore, an effective hydrogen management program must address refinery-wide issues in a systematic, comprehensive way. Managing hydrogen more effectively has been found to improve refinery profitability by millions of dollars a year, often enabling the refiner to avoid the capital cost of new hydrogen production.
The hydrogen system consists of producers, purification processes, consumers, and the distribution network itself. Daily operating decisions impact the performance of the hydrogen network and therefore profitability. There are tools and techniques available to manage each of these individual hydrogen network components. However, when you consider the refinery as a whole, instead of individual process units, there is much greater opportunity to impact the refinery profit. The key is to focus on the effect of hydrogen on the performance of hydroprocessing units, and therefore gross margin, to unlock significant profit improvement opportunities.
Hydrogen Producers
The primary sources of hydrogen in a refinery are catalytic reformers, hydrogen plants, and purchased hydrogen.
Catalytic Reformers – Operations
Operating conditions of the catalytic reformers (rates and severities) are typically set by overall refinery economics (the gasoline pool) rather than the need for hydrogen.
Hydrogen yields are primarily a function of the properties of the feed naphtha, severity, catalyst, and operating pressure. Since operating conditions are set by the Planning Department based on refinery-wide economics, there is little opportunity to improve hydrogen production through operating adjustments.
Hydrogen Plants - Operations
Hydrogen plants produce hydrogen primarily through the steam reforming and water gas shift reactions. The optimum operation (temperature, steam to carbon ratio) is unique to each hydrogen plant because the constraints in each unit will be unique. If the refiner's goal is to minimize the per-unit cost of hydrogen rather than maximizing production, there is a different optimum temperature and steam to carbon ratio. Since these optimum setpoints can change daily, as a function of rates and feed compositions, the operator should have the tools to optimize the reformer accordingly.
Increasing Hydrogen Production
In a catalytic reformer, there are a number of methods available to increase hydrogen production. Obviously, hydrogen production may be increased by modifying equipment to enable increased charge rate. Also hydrogen yields can be improved by changing the naphtha feed to one more favorable for hydrogen production; decreasing pressure; or replacing the catalyst charge with one that provides a higher hydrogen yield. Large increases in hydrogen production can be achieved through pressure reduction by converting from fixed bed to continuous catalytic regeneration mode. This type of project can be quite attractive if the alternative is building a new hydrogen plant.
For hydrogen plants, there are a number of approaches to revamp for higher capacity. Increases of up to 25% are common. Debottlenecking may be achieved by mechanical modifications to remove equipment constraints, adding pre-reforming, or adding post-reforming.
Hydrogen Recovery
Hydrogen recovery is typically much less expensive than hydrogen production. Look for hydrogen-containing streams, such as hydrotreater off-gases or “excess” hydrogen streams that are currently being sent to fuel gas or hydrogen plant feed. Hydrogen recovery is typically accomplished using either membrane or PSA technology. The optimum purification scheme takes into consideration feed stream compositions and pressures, required product purity and pressure, and the economic trade off of product purity vs. hydrogen recovery.
Debottlenecking existing purification units is often a very attractive way to increase hydrogen recovery. Debottlenecking of PSAs can be achieved through inexpensive cycle modification, adsorbent change, reduction of tail gas pressure, or additional beds. Membrane purifiers are typically debottlenecked by adding more membrane cartridges or pressure changes.
Hydroprocessing
A minimum hydrogen partial pressure (usually measured as reactor inlet purity or recycle gas purity) is required to operate with a reasonable catalyst life and reactor temperature. The minimum hydrogen partial pressure is not a fixed value. It is a function of current operating conditions – charge rate, feed properties, desired product properties It is critical to think beyond the issue of minimum hydrogen partial pressure. For any set of operating conditions there is an optimum hydrogen partial pressure. Since hydrogen partial pressure drives the reactions, increasing hydrogen partial pressure can enable increased charge rate, improved product properties, or longer catalyst life.
In hydrocrackers, it can enable improved yields, or greater conversion per pass. Therefore, increasing hydrogen partial pressure beyond the minimum can increase the refinery gross margin well above the additional hydrogen cost associated with increasing the hydrogen partial pressure. To maximize the profitability of these units, one must have a good understanding of the process characteristics and refinery economics. Detailed process models that reflect the performance of the units as a function of hydrogen partial pressure are required.
Best Practices for Hydroprocessing Operations
Operators should:
- Regularly monitor the hydrogen partial pressure in key hydrotreaters and hydrocrackers
- Have available hydrogen partial pressure targets that reflect current operating conditions and optimization of refinery gross margin
- Adjust hydrogen partial pressures accordingly
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