73 - Liquid-liquid separators are used for the efficient separation of hydrocarbons from water, alkylates from acids, light hydrocarbons from MDEA solution, and hydrophobic phases from hydrophilic phases

In refining, coal chemical, coking, and large-scale chemical plants, MDEA or low-temperature methanol is often used to remove acidic gases from the process gas. However, process gas contains various light hydrocarbons, such as C3, C4, C5, and C6 hydrocarbons, which are stripped by MDEA, low-temperature methanol, etc., gradually accumulating to form a “biphasic liquid system of polar and non-polar components”. Non-polar hydrocarbons must be promptly removed from polar liquids such as MDEA, methanol (and even the dehydration solvent TEG) to ensure that acidic gases come into full contact with the polar liquids for efficient absorption and removal. The separated light hydrocarbons such as C3-C6 still contain a significant amount of polar dispersed phases; therefore, efficient liquid-liquid separation is necessary to remove these polar substances from the light hydrocarbons, in order to meet the requirements for their recovery and use.

This post was last edited by luoli519 on 2017-1-14 19:34. In alkylation units, the alkylation products must undergo acid washing, alkali washing, and even water washing processes in order to reduce the dispersed phases of acids and bases present in those products. However, even after several washes, the alkylation product still contains a considerable amount of acid and alkali droplets. It must undergo further removal of these droplets using a high-efficiency liquid-liquid coalescer, in order to meet the requirements regarding the residual acid and alkali content in the feedstock for the distillation column system. Prevent the acids and bases carried by the alkylated products from entering subsequent distillation column units, thereby avoiding rapid corrosion of the feed preheater, top condensers (water-cooled or air-cooled), and pipelines. However, many alkylation units have significant problems with their product caustic wash separators, resulting in severe corrosion of the equipment and pipelines in the subsequent fractionation column system by the alkylation products.

Some petrochemical companies also face the challenge of achieving thorough separation between organic phase esters and acid solutions in their MMA plants. Even after undergoing traditional processes such as water washing, alkali washing, and gravity sedimentation, the residual amount of acidic and alkaline solutions in MMA still fails to meet the requirements of subsequent processes. This also requires the use of efficient liquid-liquid coalescing separators to help companies meet higher quality requirements for MMA.

In offshore oil and gas platform projects, the wastewater and crude oil that have been preliminarily separated by conventional, simple three-phase separators at the wellheads contain numerous dispersed water droplets in the oil, while the wastewater contains a significant amount of crude oil. In particular, the wastewater fails to meet the discharge standards for marine discharge. Therefore, it is necessary to employ an efficient liquid-liquid coalescer to remove the dispersed crude oil from the wastewater; only after meeting the standards can it be discharged. However, many wastewater treatment units on offshore platforms have serious problems with liquid-liquid separators, necessitating technical upgrades and renovations.

In coke ovens, similar to the MDEA process, sodium salt, potassium salt, and cobalt phthalocyanine processes are also used for the wet desulfurization of coke oven gas. The non-polar substances of coal tar eluted by wet method form a two-phase liquid system with the polar alkali saline solution. Among them, the coal tar separated by gravity sedimentation still contains a considerable amount of saline-alkali solution, which requires further removal treatment. However, many coking enterprises, as well as those involved in the upgrading of low-grade coal and the deep processing of coal tar, face significant problems in the desalination and alkaline removal of coal tar, and are unable to meet the specifications required for coal tar for sale or for use in deep hydrogenation processes.

In many domestic LNG production facilities that utilize feed gases such as coke oven gas, coalbed methane, and methanol off-gases, the light hydrocarbons and wastewater generated by the regeneration condenser of the molecular sieve dehydration unit also require liquid-liquid separation. In particular, the dispersed-phase light hydrocarbons contained in wastewater need to be further separated to meet the influent specifications of downstream wastewater treatment facilities. However, in currently installed LNG process packages and facilities in China, the liquid-liquid separators for wastewater generated by the regeneration condensers in molecular sieve dehydration units have numerous problems, placing owners under tremendous pressure regarding wastewater treatment and environmental protection.

Furthermore, in fine chemical plants such as **plant and caprolactam plants, the oxidation-hydrolysis and oxime-hydrolysis process units require efficient liquid-liquid coalescing separators. Currently, in these process packages or the process designs developed by design institutes, traditional and inefficient liquid-liquid separation methods are still being used, involving multiple sedimentation tanks of large size as well as complex, difficult-to-stabilize serial and parallel liquid-liquid sedimentation processes. Basically, all enterprises face low liquid-liquid separation efficiency and high operation and maintenance costs. Moreover, the online water analyzers at the outlet of the organic phase from the liquid-liquid separators are frequently in an alarm state, forcing them to reduce their processing load. The low-output organic phase results in a dilemma where the upstream processes suffer from “indigestion”, while the downstream units are left “underfed”. There is an urgent need to technically upgrade and reform the existing liquid-liquid separation unit.

Currently, many petrochemical, coal chemical, large-scale chemical, fine chemical, and even energy companies are installing VOCs treatment systems. The wastewater generated from the condensate of the regeneration gas for the molecular sieve also contains numerous organic dispersed droplets and foam that need to be further removed. However, in these VOCs treatment devices that are part of the environmental protection industry, the wastewater generated from the condensation of gas used for molecular sieve regeneration often fails to meet the inlet quality standards for wastewater treatment facilities. This is due to issues related to the design and selection of the liquid-liquid separation equipment; needless to say, it certainly cannot be discharged in compliance with regulatory standards.

This post was last edited by luoli519 on January 14, 2017, at 21:18. Currently, the main liquid-liquid separation devices and technologies are as follows: 1. Chemical demulsification or thermochemical demulsification； 2. Pure gravity sedimentation separation or flotation separation ； 3. Extraction and separation ； 4. Adsorptive separation ； 5. Separation by coalescer filter or dual coalescer filters ； 6. Shallow-pool separation and blade coalescence separation technologies ； 7. The aforementioned combined techniques. Among them, there are technical equipment of types 2, 5, and 6, which fall under dynamic separation technology equipment. Reliable designs and operational performance of such separation equipment can only be achieved through a precise dynamic separation system calculation and design platform. This is precisely where China’s technological weaknesses lie.

Since the viscosity of the organic phase (or oil phase or light phase) is higher than that of the inorganic phase (or aqueous phase or heavy phase), the viscous drag force acting on the tiny dispersed droplets of the inorganic phase (or aqueous phase or heavy phase) is greater; thus, these inorganic dispersed droplets have difficulty separating from the continuous organic phase. Therefore, the technical difficulty of separating and removing droplets of an inorganic dispersed phase from an organic continuous phase is often greater than that of separating and removing droplets of an organic dispersed phase from an inorganic continuous phase.