Processing schemes for shale gas liquids recovery
The energy efficiency and liquids recovery of processing schemes employing propane refrigeration improve with additional fractionation.
S D RADIA and MARCO GRANDE
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Natural gas is generally considered by private enterprises and governments around the world to be the most environmentally attractive hydrocarbon fuel. Global use of this fuel source is projected to exceed 142 trillion cu ft/y (389 billion cu ft/day) by 2021.1 Natural gas consumption is increasing due to the rising number of power plants modified for this fuel source, the use of natural gas as transportation fuel, and the development of LNG export terminals. The inability of conventional natural gas supplies to meet global demand is becoming one of the principal drivers for the development of unconventional supplies. The types of unconventional natural gas supplies are commonly referred to as shale gas, tight gas, coal bed methane and natural gas hydrates. Unconventional natural gas production refers to the methods employed to extract these hydrocarbons as well as the rock types from which the gas is extracted. It has been estimated that if only 1% of unconventional gas-in-place resources can be economically recovered, this would almost double the current global estimate of remaining recoverable proven natural gas reserves.1
This article evaluates various liquids recovery processing schemes for a typical hydrocarbon feed produced from a shale gas formation. Shale is a sedimentary rock that is predominately composed of consolidated clay size particles. The IEA and US Energy Information Administration (EIA) have separately concluded that some 7500 trillion cu ft of technically recoverable shale gas resources (those that can be produced using current technology without reference to economic profitability) exists worldwide.1 Currently, shale gas comprises approximately 12% of global gas production.1
Shale gas liquids recovery objectives
The article in particular evaluates various schemes for two selected processing objectives of a shale gas feed with the intent of assessing impact on capital scope, major equipment sizing and energy efficiency. The first processing objective evaluated is to achieve the hydrocarbon dew point specification of the sales gas product (hydrocarbon dewpointing). The second processing objective is moderate natural gas liquids (NGL) recovery. NGL is a liquid hydrocarbon product that may contain a mixture of ethane (C2), propane (C3), butanes (C4), as well as pentanes and heavier components (C5+). Although shale gas composition varies widely, the typical shale gas feed composition utilised for these evaluations is summarised in Table 1.
The objective of a hydrocarbon dewpointing plant is to produce sales gas that meets the hydrocarbon dewpoint and heating value specifications. This objective is generally accomplished by removing propane or butanes and heavier material (C3+ or C4+) from the sales gas product. The heavier material (C5+) recovered is generally termed condensate. Condensate may be sold as a refinery blend, as a refinery feed, or as diluent for the transportation of bitumen and heavy oils. Due to the relatively high value of condensate, a segregated condensate product may also be a processing objective when hydrocarbon dewpointing. If a segregated condensate product is produced, a segregated NGL stream (specifically C3 to C5 material) may still be required to achieve the hydrocarbon dew point specification for sales gas product while maintaining the vapour pressure specification for a segregated condensate product. If a segregated condensate product is not produced, a combined liquid product is produced that is treated as C3+ NGL which may be further fractionated elsewhere. The placement of propane in the sales gas product versus the NGL product is also a required evaluation. This evaluation entails the assessed value of this material as NGL (LPG fuel or petrochemical feedstock) versus sales gas fuel as well as the associated processing and product transportation costs. The article considers hydrocarbon dewpointing schemes that produce a segregated condensate product and these schemes are developed to avoid propane material in the NGL product. Typical values for a sale gas specification are summarised in Table 2 and typical specifications for a condensate product are summarised in Table 3.
The objective of an NGL recovery plant, with suitable gas feed compositions, is likely to focus beyond the recovery required to meet sales gas specification with an objective of maximising a C2+ or a C3+ NGL product recovery. NGL recovery is often warranted as NGL commands a higher value than sales gas as a petrochemical feedstock for the C2, C3 and C4 components, as a fuel (liquefied petroleum gas, LPG) for the C3 and C4 components, and as refinery blends, refinery feeds or diluent for the C4, C5 and heavier components. Product evaluations that may be required with this processing objective include the quantity of light end material to be recovered within each of the NGL product categories and again whether a segregated condensate product should be produced. This article evaluates C3+ NGL product recovery with varying amounts of propane and without a segregated condensate product. Typical values for a C3+ NGL specification are summarised in Table 4.
The above are some of the broad and typical considerations of processing objectives. Selecting a contractor that understands the above as it relates to the client’s processing objectives is essential in the early stages of project development. The selected contractor must provide the technical expertise to define scope that will most economically achieve the client’s processing objectives while also working with the client to tailor these objectives to meet their business goals.
Shale gas liquids recovery process
The hydrocarbon dewpointing process and the moderate C3+ NGL recovery process commonly employ a mechanical refrigeration unit (MRU) that consists of a closed loop propane refrigerant. A two stage MRU is often used to minimise refrigerant compressor power. The MRU is typically used for the processing of shale gas as the feed pressures are generally not high enough to employ a Joule-Thompson (JT) plant or a turbo-expander plant process which involves releasing feed pressure to generate the required lower temperature. The use of these other processes is further hindered when the sales gas pressure is high.
In addition to the MRU, the major equipment items required for hydrocarbon liquid recovery include a heat exchanger train, low temperature separator (LTS), sales gas compressor, vapour recovery (VR) compressor as well as a deethaniser and/or debutaniser to recover NGL and/or condensate. An absorber may also be utilised for prefractionation to directly reject lighter material to the sales gas product and to assist with specific recovery targets.
The differences in processing schemes for both hydrocarbon dewpointing and C3+ NGL recovery is illustrated by considering a 5.7 million Sm3/d (200 MMSCFD) gas plant that processes the shale gas composition summarised in Table 1. The feed gas in this example is considered sweet (up to 15 ppm mole H2S) and does not require sweetening facilities. The feed gas is available from the well head through an inlet separator that separates entrained hydrocarbon liquid and water with delivery of the resulting saturated feed gas to the plant site. The gas stream is considered saturated with water and hydrocarbon condensate but with minimal quantities of these liquid phases present in the inlet separator.
Hydrocarbon dewpointing with condensate recovery
The exchanger train for condensate recovery may consist of a feed gas/liquid exchanger and a feed gas/sales gas exchanger to minimise propane refrigeration. Ethylene glycol (typically 80 wt%) is injected to prevent hydrate formation in the exchanger train. The spent glycol separated in the LTS and the absorber flash drum (if present) is sent to regeneration. The remainder of the process configuration may vary depending on the processing objectives and the resulting economics as per the following case studies.
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