Jun-2017

Removing liquid-phase contaminants from hydrocarbon liquids

A glass-free liquid/liquid coalescer system provides efficient filtration and liquid contaminants separation from NGL products

THOMAS H WINES Pall Corporation
SAEID MOKHATAB Gas Processing Consultant

Article Summary

Raw natural gas from production wells must be treated and processed in a gas plant to meet the quality standards specified by gas pipeline companies. However, when the feed gas contains a significant amount of natural gas liquids (NGLs) there are economic incentives to recover these liquids that can typically command a higher value than natural gas for the same heating value. Figure 1 illustrates a block flow diagram of a gas processing plant designed for NGL production. As can be seen, the recovered NGL stream is typically sent to a fractionation unit for further processing into individual products. The ethane component can be sold as petrochemical plant feedstocks. The propane and butane liquids are valued as a liquid fuel. Natural gasoline (C5+ condensate) product can be exported to refineries as a blending stock for motor gasoline.

The hydrocarbon liquid products from the NGL fractionation unit sometimes do not meet customers’ specifications without further treatment. In fact, if acidic and sulphur compounds are present in the feed gas and have not been removed prior to NGL recovery, they will end up in the NGL products (see Table 1). These contaminants can lead not only to odour problems but also can form sulphur oxides on combustion. They can cause corrosion of equipment unless they are dehydrated. The presence of significant quantities of carbon dioxide can increase the vapour pressure and lower the heating value of the hydrocarbon liquids. Carbonyl sulphide and carbon disulphide, although not corrosive in liquefied petroleum gas (LPG), will hydrolyse slowly to hydrogen sulphide, resulting in off-spec products.

Both NGL and LPG streams are treated with caustic to remove residual organic sulphur compounds (such as mercaptans) and with amines to remove hydrogen sulphide. In both cases a liquid/liquid contactor is used. Significant amounts of the caustic or amine can be carried over into the product stream in process units that are running at rates above design capacity, are treating high sulphur feedstocks, or have other operational problems. The carried over liquid results in off-spec products, excessive losses of caustic or amine, and can cause operating problems in downstream processes. In addition, water is a significant contaminant which can cause LPG and natural gasoline to be off-spec. The acceptable water content in light hydrocarbon liquid streams varies from no free water present to very low levels of moisture in liquid products.²

In light of the above-mentioned items, there is an essential need to install a high-efficiency liquid/ liquid coalescer system downstream of the NGLs processing unit to properly separate liquid dispersions of caustic, amine, or water from NGL products.

Coalescer principles and materials of construction
A coalescer is a vessel that contains elements such as wire mesh pads that take small droplets in an emulsion and grow them into large drops that are more easily separated by gravity. This process is accelerated over natural coalescing by the fibres present in coalescer media that force the contact of small droplets, thereby promoting the coalescing process. The pore gradient of coalescer medium is constructed so that the inlet medium has fine pore sizes that increase in size with the flow direction (see Figure 2).

Coalescers have been primarily constructed with glass fibre media until recently when polymer and fluoropolymer materials were adopted. Glass fibre works adequately for emulsions with interfacial tensions of >20 dynes/cm. However, fibreglass is not suitable, as amine degrades it. It is also known to disarm and lose efficiency in the presence of surfactants.

High-efficiency liquid/liquid coalescers are the newest generation of coalescers, incorporating the latest in coalescer technology. They are constructed from polymer and fluoropolymer materials that have been optimised to separate the most difficult emulsions with interfacial tensions as low as 0.5 dyne/cm. This coalescer can be used with a broad range of applications. It can process aggressive chemicals and handle demanding operating conditions while providing the highest level of performance.
   
Pall liquid/liquid coalescer system
Pall’s AquaSep liquid/liquid coalescer (developed for separation of water from hydrocarbons) and PhaseSep liquid/liquid coalescer (developed for separation of caustic and amine from hydrocarbons)  provide high coalescing efficiency, capacity, and contaminant removal. These liquid/liquid coalescers, which separate liquids with interfacial tensions as low as 0.5 dyne/cm, consist of three stages (pre- filter, coalescer, and separator) that will be described below.

Pre-filter stage
The pre-filter stage is generally a separate vessel with absolute rated filter cartridges that will be used for separation of solids/preconditioning of the fluid. Solids can increase the stability of an emulsion and removing solids can make coalescing easier. Generally, this step can be achieved by a separate cartridge filter system. In addition, the filtration stage protects the coalescer and increases service life. This step also initiates the coalescence of the hydrocarbon droplets, thereby enhancing the separation capabilities of the system.

Coalescence and separation stages
The coalescing and separation stages can be configured in two possible vertical and horizontal configurations.

The vertical configuration (see Figure 3) is used for the removal of an aqueous dispersed contaminant from a continuous hydrocarbon stream. This configuration has separated aqueous-hydrocarbon systems with interfacial tensions as low as 3 dynes/cm. The system contains coalescer cartridges stacked on top of hydrophobic barrier separator cartridges. The fluid enters the top of the housing and then flows from the inside of the coalescer cartridges radially outward, causing the enlargement or coalescing of the inlet dispersion into large droplets in the outlet stream. The coalesced droplets then flow axially downward with the convective flow and are repelled by a hydrophobic separator barrier cartridge. The purified bulk hydrocarbon fluid then flows from the outside of the separator cartridges to the inside and leaves the vessel as a purified stream. The coalesced aqueous droplets are collected in a sump section at the bottom of the housing.

The horizontal configuration (see Figure 4) is used for the removal of an aqueous dispersed contaminant from a bulk hydrocarbon stream when the interfacial tension is below 3 dynes/cm and can be used to remove non-aqueous dispersed contaminant from a continuous aqueous stream. The system consists of a horizontal coalescer cartridge stage followed by a settling zone that relies on the difference in density for separation of the coalesced droplets. The fluid enters at the side of the housing and flows from the inside of the coalescer cartridges radially outward, causing the enlargement or coalescing of the inlet dispersion into large droplets in the outlet stream. The coalesced droplets then flow axially in the horizontal direction through a settling zone. If the specific gravity of the dispersed phase is greater than the specific gravity of the continuous phase (water from oil separation), the coalesced droplets settle downward by gravity and are collected in a sump located at the bottom of the housing. If the specific gravity of the dispersed phase is less than the specific gravity of the continuous phase (oil from water separation), the coalesced droplets are collected in a sump located at the top of the housing.