223 - Technical scheme for the vane separator dedicated to heavy hydrocarbon separation in LNG plants

This post was last edited by luoli519 on 2024-1-29 at 15:36. As is well known, whether LNG is produced using coal-derived natural gas as a raw material, or using wellhead gas or pipeline natural gas, the process involves the separation of heavy hydrocarbons. In some process flows involving heavy hydrocarbon separation, a two-step separation is employed: namely, heavy hydrocarbon separation via ambient-temperature precooling and heavy hydrocarbon separation within a cold box. Whether it is one-step heavy hydrocarbon separation or two-step dual separation, the core equipment used is still the heavy hydrocarbon separator. In traditional LNG plants, the heavy hydrocarbon separator often employs a wire mesh-type heavy hydrocarbon separator.

Regarding heavy hydrocarbon separators with traditional screen internals, they are quite common; please refer to the attached diagram:

Below, we will conduct an analysis and discussion using actual projects we have handled. We have received a client document requesting the upgrade of the traditional wire mesh separators in the heavy hydrocarbon separator of a natural gas liquefaction project with a capacity of 200 million standard cubic meters per day, by replacing them with vane separators. The operating temperature of this equipment is -53 to -75°C, and the operating pressure is 4.9 MPaG. The gas stream containing liquid passes through an 8\" inlet pipeline into the heavy hydrocarbon separator V-0304A/B for gas-liquid separation. The natural gas after liquid removal is sent downstream to the cryogenic unit via an 8\" natural gas outlet pipe located at the top of the heavy hydrocarbon separator V-0304A/B, while the separated condensate is discharged from the heavy hydrocarbon separator through a 2\" liquid phase outlet pipe located at its bottom. The heavy hydrocarbon separator initially provided by the customer to the owner was a screen-type heavy hydrocarbon separator based on traditional technology.

The attached diagram is a schematic illustration of the screen-type heavy hydrocarbon separator provided by the client to the owner initially:

This post was last edited by luoli519 on 2024-5-31 15:27. The fluid inside this screen-type heavy hydrocarbon separator is a gas-liquid mixture consisting of a gas phase with a volume flow rate of 405.5 m3/h and a liquid phase with a volume flow rate of 2.648 m3/h.

Heavy hydrocarbon separation falls under gas-liquid separation technologies, which in turn belong to the category of precise kinetic separation technologies. For heavy hydrocarbon separators, whether traditional screen separation technology or advanced vane separation technology is used, it is necessary to develop a system platform through precise dynamic separation technology calculations and configuration design, based on the specific parameters of different operating conditions, in order to design the appropriate separation equipment. The surface tension of the liquid phase in the mixed flow, an important parameter related to separation characteristics, is not included in the customer data sheet shown in the accompanying figures. A separation solution can be designed only after it is supplemented by a professional dynamics separation technology company from its database. Therefore, please be sure to pay attention to this important datum—the surface tension of the liquid phase.

In the actual operation of natural gas liquefaction plants, complex and variable operating conditions often arise due to significant fluctuations in parameters such as the gas-liquid composition under operating conditions, temperature, pressure, and volumetric flow rate. Traditional screen separators are unable to cope with these complex conditions, resulting in large variations in separation efficiency; moreover, the condensed liquid tends to escape and flow back into subsequent cryogenic pipelines, causing frequent blockages by ice and making it difficult for the plant to operate stably at its designed capacity. This leads to high costs associated with ice removal and pipeline clearance tasks, as well as elevated operational and maintenance expenses.

Based on the process operation data for the heavy hydrocarbon separators V-0304A/B provided by the client, NOVEL utilized its precise kinetic separation technology computing platform to evaluate the performance of these separators when using conventional wire mesh separation technology under the aforementioned operating conditions. The results showed that even when utilizing the imported YORK 431 wire mesh separation elements—which are recognized internationally as having the highest overall performance in terms of separation efficiency and operational flexibility—the precision of liquid removal from the gas stream under these conditions could only reach 211.82 micrometers. As a result, a large number of condensed liquid droplets escaped and entered the subsequent cryogenic pipelines. This level of performance falls far short of the 10-micrometer separation precision requirement specified in the MR document.

Analyzing the main reasons, on the one hand, in a cryogenic tank the higher pressure conditions in the heavy hydrocarbon separator result in an increased gas phase density of the natural gas mixture, while the liquid phase density of the hydrocarbons is low; this leads to a small density difference between the gas and liquid phases, thereby reducing the efficiency of gravity-based separation of gas and liquid and making separation difficult； On the other hand, the poor separation performance of traditional screens is due to performance defects resulting from inherent structural flaws in the internal system of the screens. The structural design of the screen separator system determines its separation performance.

This post was last edited by luoli519 on 2024-5-31 at 15:29. Firstly, the mesh separation element forms \"cells\" through the mutual \"bridging\" of its mesh fibers, and separation is achieved by using these \"cells\" to block and intercept particles. Large-sized carriers, when encountering small-sized “pores,” are similar to large fish caught in a fine-mesh net, and can be blocked and intercepted. However, the size of the \"cells\" formed by the mutual \"bridging\" of the metal wire fibers in the screen separation element is uneven; their sizes typically range from a few micrometers to several hundred micrometers, following a Gaussian distribution. Particles of a few micrometers in size within an air current can be intercepted when they encounter even smaller \"pores\" ； At the same time, however, these tiny charged particles with a size of just a few micrometers can encounter “pores” that are hundreds of micrometers in size; they can then directly pass through such large pores and escape. Furthermore, the liquid droplets and mist carried in the airflow act like cunning “fish.” As these droplets enter the pores of the mesh, their shape changes from that of a water droplet to a dumbbell or spindle shape, which helps them escape through those pores. Under the effect of surface tension after passing through the pores, they return to their original droplet shape and reappear in the airflow within the downstream pipeline equipment ； The solid particles carried in the airflow are like foolish “fish”; as the airflow transports these particles through the pores of the mesh, their size does not change significantly, making them easy to be intercepted. This is why using a screen to intercept and separate solid particles carried by airflow is generally more effective, while it turns out to be less effective for intercepting and separating liquid droplets and foam carried by airflow.