Monetise off-gases for low cost ammonia
Cheaper ethane supplies present an opportunity for ammonia production using cracker off-gases
V K ARORA
Kinetics Process Improvements
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Ammonia production using hydrogen rich off-gases has been well known for a long time but practised only in a handful of plants. The dynamics of new feedstock trends in the petrochemicals industry, coupled with several new process options, provide opportunities to source larger volumes of hydrogen rich off-gas streams to produce low cost ammonia. The new sources of hydrogen rich off-gases are large enough to integrate and support a typical world scale ammonia plant to provide economy of scale; even smaller scale operations offer an added environmental benefit. However, sourcing those off-gas streams poses its own challenges which are discussed here.
This article examines various process options to produce ammonia from off-gases, along with case study economics for two locations (US Gulf Coast and Middle East) for different sourcing and process options.
Abundant supplies of ethane from the shale gas boom have positioned the US as the most competitive, low-cost ethylene producer, resulting in increased investments in ethane recovery, pipelines and ethane crackers. Figures 1a and 1b illustrate this excess availability of ethane, along with growth in demand for crackers in the US.
As a result, most ethylene producers in the US have switched to low cost ethane wherever possible and several companies are already progressing with plans to build mega-scale ethylene crackers using ethane.
An ethane cracker produces large amounts of a hydrogen rich gas stream which is conventionally combusted in the cracking furnaces to provide the required heat for cracking. This source of hydrogen rich gas provides a potential opportunity for producers to explore an alternative feedstock option to build world scale ammonia plants with the benefit of lower capital expenditure and energy costs, with improved return on their investments.
Several new ethane based steam crackers with large ethylene producing capacities have been announced in the US (see Table 1).
Total planned new ethylene capacity is ~9.8 MMTPA. Nearly 70% of ethylene in the US is produced from ethane as opposed to 45% just six years ago. Globally, ethane represents 36% of ethylene production as opposed to 26% just 10 years ago.
By the same token, a large number of existing steam crackers in the Middle East use associated gas (ethane and ethane/propane) which provides a similar opportunity for ammonia producers.
In Europe, 90% of ethylene is produced by cracking naphtha, gas oil and condensates while cracking of ethane is primarily carried out in the US, Canada and the Middle East.
Shrinkage of product slate from crackers
A larger shift to ethane based olefins production in the US has also taken its toll on propylene, higher olefins, aromatics and other co-products produced by heavier feed cracking. Propylene supply from refineries has also been curtailed due to sluggish demand for gasoline.
A shortfall in propylene is being made up by on-purpose propane dehydrogenation (PDH) units, another source of hydrogen rich gas streams. PDH capacity of over 3 million t/y of propylene in the US has already been announced (see Table 2).
Excess propane supplies coupled with the high price of oil relative to natural gas prices has driven the demand for PDH units as the main growth engine for propylene supply.
Propylene shortages and growth in demand in China have driven a major wave of projects for new PDH units using imported propane, mostly from the Middle East and some from North America. Nearly 6 million t/y of PDH capacity is already in the engineering and construction phase in China for nine separate projects, and another 2 million t/y of PDH capacity is at the planning stage. This also provides opportunities for ammonia producers in China.
The Middle East was the first region to build several PDH units, with a current operating capacity of nearly 3 million t/y of PDH, again with opportunities for ammonia producers.
Sources of hydrogen rich streams
Table 3 lists potential sources of hydrogen rich streams from various processes. The impurities contained in these streams need to be removed if they are to be used for ammonia production. The purification steps to remove these impurities depend on the nature and amount of impurities present, along with the selected scheme.
For all of the process options described below, there is no need for an expensive and energy intensive primary reformer, which helps to reduce both capex and opex for an ammonia plant.
The following process options are reviewed for the hydrogen recovery and syngas generation in combination with additional natural gas for ammonia production:
• Nitrogen wash (N2-WASH)
• Secondary reforming (SEC-AIR)
• Secondary reforming with enriched air (SEC-ENR)
• Secondary reforming with gas heated reforming (SEC-GHR).
The choice of process option will depend on site specific constraints and the resulting economics.
The PSA option schematic shown in Figure 2 is the simplest option, with relatively lower capital cost and least ammonia production. With this option, full recovery of hydrogen is not possible due to the nature of the PSA system. High purity nitrogen required for the process can be provided via a pipeline or an air separation unit (ASU). Using pipeline nitrogen at a competitive price is usually a better economic option.
The make-up syngas produced in this scheme is very clean with practically no inerts. This allows the ammonia synloop to operate efficiently at a lower pressure and lower refrigeration duty for the same ammonia conversion with the least amount of purge gas, resulting in savings in both the capital and operating costs of the ammonia plant.
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