Separation Units

Nevertheless, the very first process in oil processing historically is still the core process of any refinery, the physical separation. All distillations fall into this class, atmospheric, superatmospheric and vacuum distillation, as also any light ends recovery or specialty separations, like LPG specification or propane-butane separation. In this rank are included any solvent extractions, like crude oil desalting and dewatering.

Distillation and Fractioning Units Petroleum is a mixture of different hydrocarbon compounds with diverse boiling points ranging from 35 °C to over 450 °C. Separating it into its various fractions is the first operation in the refinery, and is accomplished by distillation. It is performed in a column equipped with internal contact elements, like fillings and trays to improve contact and separation between the liquid and vapor phases. Hydrocarbon molecules vaporize and leave the liquid mix, as their boiling point is reached, being then withdrawn from the distillation tower, condensed and collected in streams, the oil fractions. Lighter fractions with boiling point lower than 420 °C are separated in an atmospheric pressure distillation. Superatmospheric distillation is usually a primary step whenever the crude oil is very light, to favor the separation of gases and lighter products and reduce atmospheric tower duty. Heavier fractions with boiling points higher than 420 °C must be distilled in a vacuum tower at subatmospheric (vacuum) pressure, because at temperatures above this point under atmospheric conditions, hydrocarbon molecules degrade by thermal cracking reaction. To enhance separation of the molecules, steam is injected into side columns to reduce partial vapor pressure that helps lighter fractions to flow to the vapor phase. So the distillation might be described as a process that performs a huge heat exchanging, receiving and withdrawing energy to perform continuous and correlated evaporation and condensation processes.

Regardless of the feedstock, any distillation or fractioning process works basically in this way, either for crude oil, propane-butane mixtures, catalytic cracking products, whatever the liquid mix composed of different boiling temperatures compounds. The following approach addresses crude oil distillation.

Heat is primarily delivered to crude oil in a fired heater that is then fed to the column. Before being sent to the heater, crude oil is preheated in a series of heat exchangers that cool down distillation product flows, recycling energy to optimize efficiency. Additional cooling is required to allow distillation products to reach ambient temperature and it is usually supplied by water from cooling towers or air heat exchangers.

Vacuum for the secondary distillation column is provided by means of steam ejectors and vacuum pumps. Steam requirements for this unit are usually obtained through steam turbines that move internal equipment like pumps and blowers. Some medium and low pressure steam might be generated in this unit and used as stripping steam.

Regular measures to improve energy efficiency in these units are mostly related to heat containment and recovery, like heat integration between atmospheric and vacuum tower product flows in the crude preheat exchangers train. Since heat exchangers are an important piece of equipment, fouling control may be a significant option. The vapor-liquid contact element inside the distillations towers also plays an important role in heat transfer and conservation. Distillation column energy optimization can be done by tower pressure reduction when applicable, because low pressure diminishes the temperature profile to evaporate the same fluids. Reducing reflux rates and pumparound duties are important opportunities, less reflux decreases overall energy flow through the column. Pumparound is an operation where liquid is removed from the tower, cooled by a heat exchanger, and returned to the tower. Both intend to reduce the duty of the top condenser while providing enough reflux for optimal distillation, but if used in excess, they withdraw too much energy from the tower, implying a bigger fuel demand in the fired heater. Improving tray design or even substitution of traditional trays by packing is another possibility. The use of heat pumps should also be considered, since heat is removed from column top and injected at its bottom, there is an opportunity to recycle the heat rejected at the top to the bottom by means of a heat exchange fluid and compression energy, reducing net energy demand. Adjusting and controlling the steam stripping flow is also a good practice, which not only saves energy but reduces the load on the wastewater treatment system, since less oily water will be produced.

Since the distillation unit is at the heart of the refinery and receives the biggest feed in the whole complex, these units are the main energy demand and deserve special attention at any time in a refinery.

Solvent Extraction and Similar Units 'Deasphalting' removes asphaltenes from heavy vacuum residue to prepare feedstock for catalytic conversion units and for the production of lube oils. In conversion units, catalyst performance is impaired by heavy metals and high residual carbon and both are related to asphaltene concentration. In lube oil production the extracted light liquid phase makes excellent base lube oils, called 'bright stock' that can be further processed to meet specifications for lube blend stocks.

Vacuum residue, normally straight from the vacuum distillation tower, is heated and fed to the top of a trayed extraction tower at a pressure around 30 atm, while liquid propane is loaded from the bottom. Propane solvent moves up counter current to the asphalt, which is removed from the bottom after extraction. This stream, after being heated in a fired heater, enters the top tray of a baffle trayed separator. Side flashing and steam stripping towers separate deasphalted oil, asphalt and recover propane. After cooling, asphalt and deasphalted oil are ready for storage and blending. Recovered propane is compressed, cooled and drained free of the water from the stripping steam in an accumulator drum. Dry propane is recycled to extraction tower.

Other significant solvent extractions are strictly related to lube oil production. The furfural extraction process is designed to produce base lube oil with high viscosity indices and other desirable qualities, like color and stability. This process removes aromatic and naphthenic compounds from feedstock. Liquid SO2 or phenol may be used as a solvent for this purpose. Feedstock enters an extraction tower, that can be either a trayed packed tower or rotating disc contactor, from below the bottom tray and a dry furfural stream enters from the top tray flowing counter current to the oil feed. Aromatic compounds are removed from the bottom as a rich furfural stream and treated base lube oil, the raffinate, leaves the top. Stripping towers that use inert gas like CO 2, recover furfural from the raffinate and extract. The entrained inert gas is flashed off from recovered furfural and can be either vented to the atmosphere or returned to inert gas system.

Lube oil dewaxing is necessary to ensure proper viscosity at low ambient temperatures. In solvent dewaxing, oil feedstock is diluted in solvent to lower its viscosity, chilled until the wax is crystallized, and then solvent is removed by vacuum filtration. Commonly methyl ethyl ketone (MEK) with methyl isobutyl ketone (MIBK) or MEK with toluene are used as solvents. Solvent recovery from oil and wax is performed through flashing and steam stripping.

Basically all these processes consist of blending, mixing, distilling and stripping stages.

A special extraction is the crude oil desalting, which is a process to wash the crude oil with fresh water at high temperature and pressure to dissolve, separate and remove the salts and solids. Crude oil and fresh water are mixed together to produce washed desalted crude oil and contaminated water. The main energy input is power for pumping, while mixing is provided by design. Although electrical energy is injected directly on the desalting drum to promote the coalescence of dispersed water droplets in the crude oil, its effective consumption is negligible compared with power pumping demand.

Good practices to improve energy efficiency in these units are again heat containment and recovery. Attention to solvent recovery stages is very important because solvent recycling reduces cost and improves heat conservation. Some of the same considerations for distillation apply here, specially adjusting and controlling of steam stripping for same reasons and additionally interference with solvent recovery.

Crude oil desalting has a particular relation with energy efficiency, not by its own energy consumption but for implications on the distillation tower. Poorly desalted crude oils when distilled liberate hydrochloric acids that attack the top condenser and produce rust and fouling, reducing heat transfer and energy efficiency. And the top condenser is a very sensitive piece of equipment for energy performance of the distillation unit, a main energy consumer for the refinery.

Generally extraction units represent a small share of the whole refinery energy consumption, but they deal with a small volume share of crude oil throughput, that happens to be among the higher value products of the refinery like lube oils. Any improvement in energy efficiency in these streams may be valuable.

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