Introduction
Aromatic hydrocarbons (Benzene, Toluene, Xylenes or BTX) are some of the most important intermediate products in the petrochemical industry. They are high value by-products of refinery and petrochemical processes and significant financial incentives exist for them to be extracted, purified and sold as finished petrochemical products rather than be sold as fuel. The demand for high purity BTX products has been steadily increasing over the past several decades and is projected to grow even more in the near future. A process unit designed to extract aromatic compounds from a mixture of hydrocarbons is called an Aromatics Extraction Unit (AEU).
Sources of aromatic hydrocarbons
Aromatic rich streams are formed in a variety of refinery and petrochemical processes as by-products, but the most common sources are catalytic reformers units and steam crackers. The feed streams obtained from these sources are called reformate and pygas (short for pyrolysis gasoline) respectively. Other aromatic rich streams are coke oven light oil (COLO) from coke oven plants. The BTX content of these streams can range from low to high (15% to over 90%). A typical AEU feed stream consists of C5-C10 hydrocarbon compounds with a mixture of paraffins (P), olefins (O), naphthenes (N) and aromatics (A). Of course, C6-C8 aromatics compounds form the biggest fraction in the feed stream.
What is extractive distillation?
Under normal conditions, the boiling points of different aromatic and non-aromatic hydrocarbons are so close that they cannot be separated by simple distillation alone, or the required product purity levels cannot be achieved simply by distillation. Extractive distillation is the technique of separating the aromatics hydrocarbons (C6-C8 A) from non-aromatic hydrocarbons (C5-C10 paraffins, olefins and naphthenes) via the use of a solvent. By introducing a solvent, the relative volatility of the components in the mixture is altered enough that they can be separated by simple distillation. The high boiling solvent preferentially ‘dissolves’ the aromatics hydrocarbons in itself and the non-aromatic hydrocarbons are easily separated by distillation. The ‘extracted’ aromatic compounds are then recovered from the solvent by steam stripping. The non-aromatic product stream is called the ‘raffinate’ has extremely low aromatic content (less than 1%) and can either be routed back to olefin block (for pygas feed) or sent for fuel blending (for refineries).
Product specifications
Benzene’s product purity requirements are usually very high – 99.9% min. Toluene has a product purity requirement of 99.5% min. although, nitration grade Toluene can have lower purity requirements of 98.5% min. C8 aromatics are usually as a mixture of ortho, meta & para-xylenes (called mixed xylenes) and Ethyl benzene (EB). Their product purity requirements can range from 96.5 – 98% min depending on feed composition.
Solvent Characteristics
The solvent used in aromatic extraction, therefore, must have the following characteristics:
- High boiling point (to be thermally stable at operating temperatures)
- Miscible with the mixture
- Easily separable from the dissolved components
- Should not form an azeotrope with any component in the feed mixture
Technologies and Licensors for aromatic extraction
Many commercial processes are available for extracting aromatic compounds, depending on type of technology or solvent. Typical examples of technology are Liquid-liquid extraction (LLE) and extractive distillation (ED). Types of solvents include Sulfolane, N-Methyl Pyrrolidone (NMP), Ethylene Glycol & other glycol based solvents (DEG, DGA, TEG and Carom), N-Formylmorpholine (NFM), etc. The most widely solvent used is Sulfolane.
These technologies, along with their respective solvents, have been successfully marketed and implemented worldwide by licensors like UOP (UOP ED Sulfolane), Uhde (Morphylane), Sulzer GTC (GT-BTX) and a large number of their units are in operation worldwide. These technologies differ primarily by the CAPEX requirements (total number of equipment), operating cost (energy consumption), solvent selectivity, stability and degradation, corrosion issues faced by operators, and aromatic recovery guaranteed by the licensors.
GKPE’s offering
Aromatic Extraction Units are rarely stand-alone units and are typically part of a larger complex in refinery or petrochemical plants. No two AEUs are the same due to wide range of feeds used in upstream units (for example liquid or gas or mixed feed for steam crackers) and their different modes of operation (for example, ethylene mode or propylene mode ). This results in wide variations in flowrate, aromatic content and even contaminants in AEU feed streams. This sometimes necessitates the pre-treatment of feed before it can be fed to the extractive distillation section. Examples include pygas hydrotreating and clay treatment of benzene. All these factors contribute to the overall cost of building and operating an Aromatic Extraction Unit.
GK Process Engineering LLC can help its customers understand the pros and cons of all available technologies and select the best one suited for their specific purposes – in both technical and commercial terms.
Amit Kanda
GK Process Engineering LLC