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by Thomas K. Gaskin, P.E. Advanced Extraction Technologies, Inc. and Yuv R. Mehra, P.E., formerly President & COO, Advanced Extraction Technologies, Inc.,
Ever since the discovery of natural gas and recognition of its use as a desirable fuel, the need for its transportation to markets has led to the development of treating and processing technologies. Worldwide, the gas processing industry meets a wide variety of economic and recovery objectives. These range from simply meeting a gas transportation specification to achieving extremely high ethane recovery for providing feed to an ethylene facility. Just as local markets for natural gas liquids (NGL), gas usage, and infrastructure vary, the inlet conditions and contaminants of the available natural gas to be processed can also vary. Every processing application must meet specific criteria for profitability depending upon the combination of characteristics of the available gas and the markets for gas and NGL products. The recovery of NGL generally falls into three categories: propane-plus recovery, ethane-plus recovery, and flexible ethane recovery or rejection. Inlet gas characteristics including its pressure, CO2 content, degree of water saturation, heavier liquid content, and other contaminants can impact the selection of the best technology to treat and process gases. Production objectives such as liquid recovery percentages, liquid and gas product specifications, and type of liquid products to be produced also have significant impact. This article provides an overview of the available technologies for each of the three processing categories and provides, via charts and graphs, the relative ranges of applicability of each with regard to accommodating the wide range of inlet conditions and production requirements. Based on many studies and analyses carried out to date, we believe that for propane-plus recovery applications, the use of a cryogenic "two-tower" type design offers intrinsic advantages at inlet gas pressure levels above 1,200 psig. At lower pressures, the enhanced absorption process has evolved to a level where the total energy requirement for heat and compression is now equal to that of a cryogenic process, and requires substantially lower compression horsepower. For ethane-plus recovery applications, cryogenic processes are most efficient in the range of 60-85% ethane recovery. On the other hand, the enhanced absorption system offers distinct advantages with regard to CO2 tolerance and allows the processor the flexibility in choosing which product streams contain the CO2 that is present in the inlet natural gas. For all other areas of applications, both cryogenic and enhanced absorption processes must be carefully considered because relative value and importance of other variables may dictate the choice for a most cost-effective facility. It is rare to have a project that is ideally-suited for only one technology. The objective of the provided information is to allow rapid evaluation of how well a potential project's design basis fits within the available technology options; thereby identifying which hurdles to overcome and evaluations to complete prior to moving forward with building a cost-effective plant for the specific project objectives.
Propane-plus
Recovery Cryogenic turbo-expander processes entered the commercial market in the 1960's with the initial designs having minimal heat integration and little or no reflux. Introduction by Ortloff Engineers, Inc. (Ortloff) of the Gas Subcooled Process (GSP) in the 1970's (Figure 4) and ABB Randall Corporation's use of dephlegmators (Figure 5) were significant milestones in efficiency improvements. Even though the cryogenic turbo-expander processes were introduced primarily for the recovery of ethane, improvements and options for propane-plus recovery have continued through the introduction of residue reflux systems (Figure 6), two-tower systems (Figure 7), enhancements to the original GSP process as Recycle Split-Vapor (RSV) process (Figure 8) and the DELPRO Process (Figure 9). In 1997, Advanced Extraction Technologies, Inc. reintroduced through commercial utilization in Canada the enhanced solvent-based absorption, as part of the patented AET Process technology portfolio, incorporating improvements in the presaturation and chilling locations, thereby allowing the use of lighter C5+ NGL components as the preferred solvent (Figure 10).In the past twenty years,
advances in the mechanical equipment utilized by all processes have
led to improved efficiencies, including the use of plate-fin exchangers,
higher efficiency expander-compressors, improved tray and packing
designs, and also the use of more flexible and accurate process
simulation programs. Unquestionably, all of the above processes
do work; however, most of them have specific areas where they hold
quantifiable advantages. Primary variables that affect the choice of the most cost-effective process for a given application include: Inlet Conditions (gas pressure, richness, and contaminants), Downstream Conditions (residue gas pressure, liquid products desired, and liquid fractionation infrastructure), and Overall Conditions (utility costs and fuel value, location, existing location infrastructure, and market stability). Inlet Pressure (Table 1) Expander-based cryogenic processes require a high inlet pressure to produce a desired tower top temperature for achieving optimal propane recovery. As such, in order to avoid installation of a refrigeration system, the low-pressure gases must be compressed to a minimum pressure dictated by gas composition and the desired recovery. For marginal pressure cases, inlet gas compression can be substituted via the use of pre-boost with expander energy, thereby eliminating a separate compressor service. Typically, an inlet pressure of above 750 psig is desired for most expander processes. For inlet gas pressures above 1,200 psig, the options for expander processing are reduced, i.e. the two tower system has an intrinsic advantage of initial separation at higher pressures, and subsequent deethanizing at lower pressure, thereby reducing the overall gas recompression requirement.
T he enhanced absorption system performs well with a lower minimum inlet pressure (< 450 psig) than the cryogenics. In fact, operation at the lower end of its range improves performance by providing higher relative volatilities. For inlet gas pressures above 450-500 psig, the enhanced absorption process requires a dual pressure absorber column comprised of high and low pressure sections. Doing so allows deethanization without exceeding critical conditions at the tower reboiler. For inlet gas pressures above 1,200 psig, the enhanced absorption system can still avoid critical conditions, but reduced relative volatility will reduce system efficiency.Propane Recovery (Table 2) Even with its low propane recoveries (typically 20% to 50%), a refrigerated low temperature separator (LTS) system (Figure 2) meets the minimum transportation needs of hydrocarbon dew point control for most gas pipelines. For higher propane recoveries, cryogenic temperatures or solvent-based absorption is required. Over 90% of propane is easily recovered from a typical gas stream. Propane recovery above 99% is possible using solvent systems or a two-tower cryogenic system. Losses associated with using inlet gas for reflux typically prevents a GSP plant from achieving 99% recovery; however, recovery of butane-plus will be essentially 100%. Table 2 - Desired Propane Recovery
The enhanced absorption system will achieve the 99%+ propane recovery, but with varying recoveries of heavier components based on the process configuration utilized. Utilization of plate-fin exchangers and an inlet expander can allow an absorption plant to deethanize in a single absorber tower even with high pressure inlet gas, thereby increasing energy efficiency and increasing solvent retention because of the lower C5+ k-values at lower operating pressure. The enhanced absorption process will achieve relatively high C5+ recovery at lower energy consumption than a conventional refrigerated lean oil plant. Richness of the inlet gas affects energy usage of plants that employ refrigeration, typically the LTS and the absorption-based processes. Expander-based processes can be adversely affected by rich gas when the process flow scheme requires the addition of an external refrigeration system. This can occur when there is an abnormally large energy removal from the system in the liquid product, and when gas cooling curves deviate substantially from a straight line. Other Factors (Table 3) Desired liquid product slate can be important when more than one product is desired. Without additional towers, the LTS and cryogenic plants will produce a single C3+ product, while the enhanced absorption system will typically produce separate fractionated C3/C4 mixed and C5+ products. The enhanced absorption plant is additionally capable of producing an HD-5 propane stream at the top of the regenerator column. This can help unload a downstream fractionator and produce a fractionated propane product for local market thereby increasing netbacks. Table 3 - Impact of Other Factors in Propane Recovery
For propane-plus recovery, CO2 freezing will not be a problem for any system. Removal of mercury, when present, will be required for systems using aluminum plate-fin exchangers. Historically, this has only been a requirement for the cryogenic plants. Remote locations often favor use of the simplest components and construction materials. As such, the requirement for stainless steel metallurgy, high speed expanders, and molecular sieves associated with cryogenic processes can be disadvantageous, e.g. especially with the use of stainless metallurgy in a saltwater environment. Provisions for future ethane recovery are sometimes included in the design basis for a propane-plus recovery unit. Among deep propane recovery designs, the two-tower system is the least suitable for incidental ethane recovery, with only 20-25% ethane recovery possible. The GSP, residue reflux, and enhanced solvent-based designs are better suited for either intrinsic recovery capabilities or recovery through debottlenecking. Utility and shrinkage cost bases are the same when gas-fired rotating equipment is used. When residue gas value is high, process efficiency can be critical. Since conventional refrigerated lean oil plants are inefficient in comparison to the cryogenic expander designs, absorption-based systems have been perceived to be high fuel users. With the improvements inherent to the enhanced solvent absorption system, the total fuel consumption is essentially identical to that of a most efficient two-tower cryogenic system, whether on a fuel vs. percent C3 recovery or propane-plus product recovered (Figure 11). However, the total compression required for identical propane-plus recovered (Figure 12) or percent propane recovery (Figure 13) is significantly lower for the enhanced absorption system when compared to the two-tower cryogenic system. Naturally then, from initial capital and ongoing maintenance standpoints, a process requiring lower compression will generally be preferred. In Figures 11, 12 and 13, the comparison is based for an inlet and residue gas pressure of 850 psig, with inlet gas containing 7.09 mol% (or 2.18 gallons /1,000 scf) propane-plus liquid content. The two curves for the enhanced absorption process represent different molecular weight solvent streams derived from the inlet gas.Application Areas Choosing the best process system for propane-plus recovery begins with obtaining a complete design basis with values for all streams and utility costs, along with the economic criteria used to evaluate the options. Potential variability in the feed gas and pricing is equally important. Although each case is different, Table 4 provides a summary of ranges of variables discussed and the processes to which they offer advantages. This table can be used for initial screening and to learn what factors need further development prior to selecting a process. Table 5 provides similar information, but in the form of presenting the case deemed ideal for installation of a plant of each design. able 4 - Variable AdvantageRange Summary - Propane Recovery
Table 5- Most and Least Favorable Conditions for Propane Recovery
In contrast to the propane-plus recovery operations, ethane recovery is generally driven by product over fuel price differential economics. Sometimes, there is a need to either purify methane as a chemical plant feedstock or to produce ethane as a dedicated feed-stock for an ethylene plant. These uses demand very different design considerations when compared to those for recovering propane. Processes The slate of processes changes for ethane-plus recovery. The two-tower system and the propane-refrigerated LTS are not suitable for ethane recovery. The residue reflux system is much better suited for the ethane-plus application as opposed to its use for the propane-plus recovery. Both the GSP and the enhanced solvent systems are suitable for ethane-plus recovery. Although a conventional lean oil plant could also accomplish higher ethane recoveries, it is too inefficient to consider building one under the prevailing economic environment. Evaluation Variables All variables previously presented for propane-plus recovery equally apply for ethane-plus recovery. Specific to ethane-plus recovery designs, impact of the presence of carbon dioxide in the feed gas, in the residue gas, and in the recovered ethane can be very significant. In a relatively small number of cases, presence of nitrogen and/or helium can significantly impact the ability to cost-effectively recover ethane. Inlet Pressure (Table 6) The significance of inlet pressure for ethane-plus recovery is very similar as for propane recovery. For inlet gas pressures above 550 psig, a lower pressure stripping section is required to complement the absorber in the enhanced absorption process. Doing so avoids operating at critical conditions within the bottom reboiler. The two-tower cryogenic system held an advantage at pressures above 1,200 psig for propane-plus recovery since propane can be substantially removed from natural gas in low temperature-high pressure separators operating in the range of 800 psig after initial expansion. This is not possible for ethane recovery because a lower temperature is required and the remaining gas (primarily methane) would become supercritical prior to substantial liquefaction of ethane. Therefore, any perceived advantage from high pressure inlet gases held by the cryogenics over the enhanced solvent absorption system is no longer valid. Carbon Dioxide (Table 7) Carbon dioxide in the feed gas will normally split between the recovered ethane and the residue gas, potentially affecting specifications for both products. It can also freeze in the ethane recovery process. Any process for recovery of ethane when carbon dioxide is present in the inlet gas must either operate in a region that will avoid freezing and/or off-spec products, or provide for carbon dioxide removal from one or more streams as required to ensure proper operability. Freezing within the recovery process can be
reasonably predicted using a combination of GPSA data, process
simulators, and related experience. For high ethane recovery (
concentrations at the top of the demethanizer -- the coldest temperature point. Additional reflux, either via higher residue gas flow rates for the residue reflux designs or additional cold separator vapor and some cold separator liquid in the case of a GSP-type configuration, helps to avoid CO2 freezing, with only a minor efficiency loss. Potential freezing at the upper side reboiler should also be checked. For absorption processes, such as the enhanced absorption process, CO2 freezing is not an issue since the minimum temperature within the process is above the freeze point of even pure carbon dioxide. Processes recovering ethane will typically recover more than half of the inlet carbon dioxide in the recovered liquid when designed to minimize capital investment and operating costs for ethane recovery. As such, even minimal inlet CO2 can lead to the need for liquid product treating. In a cryogenic expander plant, the demethanizer can be designed to reject CO2 overhead by using a warmer than optimal bottoms temperature provided that: (1) the tower does not freeze; (2) the residue gas specification is still met; and (3) the added inefficiency within the system can be tolerated while maintaining ethane recovery and preferably avoiding the need for adding a refrigeration system. Within the enhanced absorption process, the CO2 can be rejected overhead without the concern of tower freezing. The additional heat will represent as an inefficiency to the absorption system, but with the refrigeration system already in place to supplement chilling and/or the opportunity to increase solvent circulation, additional new equipment is not required. At the same time, the ethane recovery can be maintained. Mild carbon dioxide removal from natural gas to meet a residue gas specification can also be accomplished with the enhanced absorption process in conjunction with ethane recovery. This purposefully drives CO2 into the ethane product, thereby allowing liquid rather than gas treatment. This purification with greater than 2 mol% CO2 in the inlet gas is not possible within a cryogenic plant, as freezing will result at either the top of the demethanizer tower or at the side reboiler. As indicated in Table 7, the enhanced absorption process offers the highest flexibility in the presence of CO2 by making its freezing a non-issue, and by allowing the choice to either recover or reject any amount of CO2. Doing so eliminates the need for treating or at least allows the designer to choose which location is best for installation of a treater-inlet gas, residue gas, or liquid product. Conversely, a cryogenic ethane recovery facility may require treating at more than one location. Table 7 - Impact of CO2 on Ethane Recovery
Ethane Recovery (Table 8) An enhanced absorption plant designed for high propane recovery can also recover incidentally about 45% of the contained ethane. In other words, the ethane will be co-absorbed without an increase in solvent circulation. A cryogenic plant can also recover this low percentage, but doing so will be inefficient because the same low pressure required for high ethane recovery must be used to allow the demethanization of the recovered liquid while using inlet gas heat for tower reboiling . Ethane recovery in an intermediate range (60-85%) is the "natural" range for a cryogenic expander plant, and the process is at its best efficiency. While the enhanced absorption process can also recover in this range, such an operation is not optimal. Somewhat higher ethane recovery (85-93%) is also possible with both processes. However, at these recovery levels, the cryogenic processes are approaching a more asymptotic energy region, and the residue reflux approach in most cases becomes more efficient than a standard GSP. Above 93% ethane recovery, cryogenic processing becomes much more difficult due to: (1) the CO2 content; (2) gas recompression horsepower will increase significantly; and (3) addition of a refrigeration loop often becomes preferable to the asymptotic increase in recompressor horsepower. At these high ethane recovery levels, the enhanced absorption process also has increased power requirements; however, with the enhanced absorption process it can be done without the addition of new equipment services and, of course, there are no CO2 freezing concerns--which becomes a more increasingly difficult problem for cryogenic processing. Table 8 - Desired Ethane Recovery
Inlet Gas Richness (Table 9) Lean gas is ideal for cryogenic turbo-expander processes and rich gas is ideal for the absorption process. With lean gas, the cryogenic processes see almost straight-line cooling curves, enabling efficient heat exchange and no potential need for external refrigeration. As the richness of inlet gas increases, exchanger pinch-points appear, initially requiring only additional recompressor horsepower. With rich gas, an external refrigeration system will be required to complement cryogenic processing to avoid the exchanger pinch-points and to provide the energy to compensate for the relatively large amount of energy leaving the system as liquid product. The efficiency of the enhanced absorption process is quite the inverse: rich gas simply makes the recovery easier and the required refrigeration system is part of the infrastructure. With exceptionally lean gas (such as gas exiting from a propane recovery plant and with the need for ethane recovery desired to feed an ethylene facility), solvent make-up would be required, a debit for the enhanced absorption system. Table 9 - Impact of Inlet Gas Richness on Ethane Recovery
Other Factors (Table 10) The slate of desired liquid products, water content and compositional variability of inlet gas, plant location, and feed contaminants impact is quite similar for ethane recovery as for propane recovery. However, the presence of large amounts of light inerts will affect ethane recovery more strongly in a cryogenic plant than in an absorption plant. The light components interfere with the efficiency of and the ability to condense the reflux stream within the cryogenic process. For the absorption process, the lighter components are no different than absorption of ethane away from methane. Application Areas For the
ethane-plus recovery, the enhanced
absorption process offers the best flexibility for feed gases
containing
Ethane Recovery - Rejection When ethane recovery economics are based solely on a varying value margin between ethane's fuel value and its value as a chemical feedstock, the ability to reject or recover ethane can become very important to annual profitability of a gas processing facility. The ability to respond to changing markets is sometimes necessary simply to justify the construction of a new facility. As the objective is to increase profitability, providing the ability to vary the ethane recovery level cannot add extensively to the cost or complexity of the facility. The US Gulf Coast is the primary world market that fosters the demand for ethane flexibility because of the extensive infrastructure that allows ethylene producers to switch away from ethane feedstock to a variety of readily available feedstocks. Processes The processes used for ethane recovery are all suitable for recovering less ethane when required. The degree of rejection available depends on the flexibility incorporated into the original design. For cryogenic facilities, the ability to realign side reboilers is typically required - this allows reheat of cold separator liquids prior to their entry into the demethanizer tower, while still providing some tower reboiling from the inlet gas. This alignment capability can be added to an existing facility with appropriate care given to line expansion and access in the tight quarters around reboilers. Addition of external heat is often required, as the tower pressure typically cannot be lowered to aid in ethane rejection while still reboiling solely with inlet gas. This is due to the higher volume of residue gas that needs to be recompressed during ethane rejection. An underloaded existing plant can have more recompression available, just as compression can be added after initial start-up of a facility, and either situation will enhance ethane rejection capability. Waste heat from recompressor turbines is the most common source of heat for cryogenic processes. Often, demethanizer tower diameter in the bottom section limits the ability to reject ethane. This is the most difficult bottleneck to retrofit around. With the above items included in a design, ethane recovery-rejection can often swing from the design high recovery levels to about 10% of the inlet ethane with some loss of propane recovery. Typically, the rejection possible is not enough so as to allow the NGL product to meet ethane content of propane specification without downstream deethanizing. At the 1998 Annual Convention of the Gas Processors Association, Ortloff engineers presented additional modifications to their processes by adding residue reflux on top of use of inlet gas as reflux to improve propane-plus recoveries while rejecting ethane.
Table 11
- Variable Advantage
Application
Areas Conclusions A broad
range of factors have been presented above that impact the choice of
technology which will result in the most cost-effective gas
processing facility. With the recent commercialization of the
evolutionary, enhanced solvent absorption process, it is timely to
question the notion that "the cryogenic
processes have replaced the absorption processes." For
propane-plus recovery applications, the use
of a cryogenic "two-tower" type design has intrinsic advantages at
inlet gas pressure levels above 1,200 psig. For the lower pressure
gases, the total energy requirement for heat and compression for the
enhanced absorption process is now equal to that of a comparable
cryogenic process, and the enhanced absorption requires
substantially lower compression. For ethane-plus recovery applications, cryogenic
processes are most efficient in the range of 60-85% ethane recovery.
The enhanced absorption system offers distinct advantages with
regard to
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