Ammonia Plant Front-End Revamp Using Multi-Functional Air Compressor
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Solution Overview
Problem
The high cost of revamping ammonia plants due to the expensive air separation units required for separating oxygen and nitrogen to achieve the necessary H2:N2 molar ratio for ammonia synthesis, primarily because of the large and costly compressors needed to achieve frontend pressure.
Innovation Solution
The method involves using an oxygen current with a purity of at least 50% molar, fed to the secondary reformer, and compressing a nitrogen current with the existing air compressor to introduce it at a suitable location in the plant, potentially splitting the nitrogen stream to match the compressor capacity, thereby reducing the need for expensive compressors and modifying existing equipment minimally.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If a dedicated air separation unit (ASU) is provided to furnish oxygen and nitrogen, then the required H2:N2 molar ratio for ammonia synthesis is achieved, but the cost increases significantly due to large compressors
Solution Approach 1:
The existing process air compressor is made multi-functional by using it to compress both process air (for the secondary reformer) and nitrogen product (for the purification section). This eliminates the need for a dedicated nitrogen compressor, reducing the number of large expensive compressors in the ASU and thereby reducing revamp costs while maintaining the required H2:N2 molar ratio
Solution Approach 2:
The functions of compressing process air and compressing nitrogen product are merged into a single compressor unit. The compressor is configured with a common suction line receiving both process air and nitrogen, and a common discharge line delivering both streams to their respective destinations, thereby consolidating equipment and reducing overall system cost
2Stress or pressure
If three large compressors are installed in the ASU to deliver oxidant and nitrogen at frontend pressure, then the required delivery pressure (20-50 bar) is achieved, but the device complexity and cost increase greatly
Solution Approach 1:
The process air compressor is designed to perform multiple compression functions simultaneously - compressing both the air stream for the secondary reformer and the nitrogen stream for the purification section. This multi-functionality reduces the total number of compressors from three to one or two, significantly simplifying the device complexity while maintaining required delivery pressures
Solution Approach 2:
The compression system is segmented into strategic points where compression is applied - the process air compressor compresses at the ASU outlet, and nitrogen compression is integrated at specific delivery points. This segmentation allows pressure to be achieved at critical locations without requiring three separate full-capacity compressors throughout the system
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach significantly reduces revamp costs by avoiding the need for high-pressure compressors and minimizing modifications to existing equipment, while achieving a substantial increase in plant capacity with only a small increase in flow rate through the process air compressor.
Implementation Method 1
The cold box operates at cryogenic temperatures, wherein the air is separated in their components with the required purities. The cold box mainly includes separation columns and heat exchangers.
Implementation Method 2
The oxidant and the nitrogen can be compressed before entering, respectively, the secondary reformer and the purification unit, inside the ASU or in a dedicated compressor.
Data Source
AI summary
A method for revamping a front-end of an ammonia plant, said front-end comprising a reforming section (1, 2) with air-fired secondary reformer or autothermal reformer (2), a treatment section (3) of the effluent from said reforming section, and an air feed compressor (6), wherein an O2-containing stream (8) is directed to said reforming section (2) for use as oxidant, at least one nitrogen stream (9) is introduced at a suitable location of the front-end, to provide a desired molar ratio between hydrogen and nitrogen in the product gas, and at least part of said nitrogen stream (9) is compressed via said feed compressor (6).