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Jul-2017

Controlling the hydrocarbon dew point of pipeline gas

A key factor in selecting technology to control the hydrocarbon dew point of natural gas is the quantity of heavy hydrocarbons in the feed stream

SAEID MOKHATAB, Gas Processing Consultant
SCOTT NORTHROP, ExxonMobil Upstream Research Company
MICHAEL MITARITEN, Air Liquide
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Article Summary
Recovery of heavy hydrocarbon components from natural gas is required to avoid the unsafe formation of a liquid phase during transport, which can lead to operational problems in gas transmission pipelines. To avoid liquid dropout, most current operating specifications for gas transmission pipelines require  pipeline operation above the hydrocarbon dew point temperature at which liquid begins to appear at a given pressure. More commonly, the carrier will specify a maximum hydrocarbon dew point temperature to suppliers, who often use low temperature separation processes to drop out heavy hydrocarbons to meet the specified dew point. Silica gel adsorption and membrane separation are commercially available competitive hydrocarbon dew point control technologies for feed compositions with a relatively low heavy hydrocarbon content over a certain range of operating pressures and flow rates. This is the case for a number of shale gas resources. Silica gel or membranes may also reduce the BTU content of a rich gas that will reduce foaming tendency in a downstream amine unit, or remove the ‘tail’ of heavy hydrocarbons from a pipeline gas before it is converted to a liquefied natural gas (LNG). This article describes the basic principles of silica gel and membrane systems as well as some of the recent developments in these fields.

Hydrocarbon distribution
Wellhead natural gas contains hydrocarbons and water, and commonly other impurities. The hydrocarbon distribution of a raw natural gas stream depends on its source and can change over the life of the production well. Some gases are quite lean, while associated gases, including certain shale gas sources, can contain significant concentrations of heavy hydrocarbons. The hydrocarbon dew point is sensitive to small quantities of C6+ components. As little as 450 ppm of C8 hydrocarbon added to a lean gas can give it a cricondentherm of 50°F, for example.

Pipeline gas is typically an aggregate of many sources, some of which may have already have had their natural gas liquids (NGLs) recovered. An example of a North American pipeline gas composition is shown in Table 1. To use such a gas for LNG plant feed could be problematic. While most of the NGL components are typically recovered prior to the introduction of the gas into the pipeline, the low levels of residual C6+ components could freeze in the main cryogenic heat exchanger of the LNG facility. Many  LNG processes (except Prico SMR, for example) rely on a scrub tower for heavy hydrocarbon removal, and US pipeline gas typically has too few C3+ components to effectively operate a scrub tower without supplementing with purchased NGLs. Therefore, alternative methods are needed to remove the heavy components from pipeline quality gas prior to LNG manufacture.

Certain contaminants (such as acid gases) require treatment of the gas to make it suitable for a given application.2 If excess heavy hydrocarbons are present, processing of the gas is also necessary. The choice of gas conditioning plant configuration and its complexity depend on the feed gas composition, economic desirability of extracting NGLs and the level of processing required to deliver product gas meeting specifications and emission limits. A typical approach for many gas conditioning plants designed to produce pipeline quality gas from a sour gas feed is shown in Figure 1. Typical hydrocarbon dew point specification for pipeline gas is 14°F (-10°C). If a gas stream is near this dew point specification, usually all that is required is removal of a small amount of heavy hydrocarbons,2 which can be performed using adsorption or membrane processes instead of the more typical refrigeration processes.
   
Hydrocarbon dew point controlling technologies
There are several commercial routes to obtain the desired hydrocarbon dew point temperature. Reducing the dew point by Joule-Thomson (JT) expansion or mechanical refrigeration is common for smaller flow rates of feed gas, and turbo-
expander based units are often used for higher flow rates. These processes are covered in many technical papers presented for the natural gas industry and are not the focus of this article.

Table 2 shows the common low temperature routes and includes less commonly used technologies employing adsorption or membranes. The advantages and disadvantages of these processes are compared below. Twister,4 membranes,5 and silica gel,6 can all achieve water and hydrocarbon dew point in a single unit. The more traditional low temperature processes are primarily focused on recovering larger volumes of heavy hydrocarbons. Due to the low temperatures achieved in these processes, upstream dehydration or hydrate inhibition by methanol or monoethylene glycol (MEG) injection is usually required.

The Twister Supersonic Separator is a mechanical flow device that operates by introducing a high velocity swirl in the feed gas stream followed by near isentropic pressure expansion of the gas at supersonic velocity to achieve low temperatures. The combination of swirl and pressure reduction causes condensation of water and heavy hydrocarbons. The liquids are separated and the residual gas is available with a nominal pressure reduction.

Twister shares some of the benefits of simplicity, robustness and ease of operation with the low temperature JT separation (LTS) process, while requiring an engineered solution  for specified turndown or dew point requirements. Two studies have shown that Twister can recover more hydrocarbons than the JT valve for the same pressure drop.4,8 Therefore, it can potentially be operated at a reduced pressure drop for the same performance as a JT valve, thus reducing the sales gas compression power and cost. Twister technology offers environmentally friendly, chemical-free operation within a small footprint. It can be particularly interesting for remote offshore applications due to its small footprint and low maintenance requirements. An additional benefit of Twister is the ability to remove water and hydrocarbons simultaneously.

Twister BV introduced the Twister SwirlValve, which improves hydrocarbon dew point performance of existing LTS units by improving the separation of two-phase flow across a pressure reduction valve, such as a choke valve, JT valve or control valve.9 This in turn significantly improves the liquid separation efficiency of downstream separators. This improved separation can be used either to increase the flow capacity of existing LTS units, or to reduce the pressure drop required for JT cooling, or to lower the hydrocarbon dew point and also to reduce glycol carry-over if upstream dehydration is used. A further step is taken by integrating the SwirlValve and an inline separator, resulting in the SwirlSep, which is a compact two-phase separator with a performance independent of flow rate. Aimed at compact offshore/subsea and mobile well testing applications, this concept results in considerably lower installation and maintenance costs (J Young, Twister BV, personal communication, 6 March 2017).

Quick-cycle silica gel units have been used for decades to achieve simultaneous reduction of  hydrocarbon and water dew points. More than 200 silica gel units are installed in natural gas applications under different conditions worldwide, both on- and offshore (J Schulze-Schlarmann, BASF Catalysts, personal communication, 20 April 2009). Usually, an adsorptive hydrocarbon recovery unit consists of three or more vessels, where adsorption and regeneration takes place in parallel. The cycle chosen generally includes one vessel heating and one vessel cooling while one or more vessels are on the adsorption step. Since the impurity breakthrough increases over the length of the adsorption step, two or more vessels are often used where the adsorption step is staggered to blend the produced product to a more consistent product purity level. The required adsorbent quantity directly affects installation costs, and the adsorbent quantity depends on the gas composition, flow rate and required product dew point.
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