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OPERATIONAL FEEDBACK FROM THE
NKOSSA DRIZO GAS DEHYDRATION
UNIT OFFSHORE CONGO
PAPER PRESENTED AT 61ST ANNUAL LAURANCE REID
GAS CONDITIONING CONFERENCE
Oklahoma, USA, 20th – 23rd February 2011
Bernard Chambon & Louis Penel
PROSERNAT
Paris, France
PROSERNAT
Mr Christian Streicher - Vice President Marketing Strategy
Tel. + 33.1.47.67.19.08 Fax. + 33.1.47.67.20.07
E-mail : [email protected]
Van Khoi Vu & Thomas Brenas
TOTAL SA
Paris, France
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Title: Operational Feedback from the NKOSSA Drizo Gas Dehydration Unit Offshore Congo Abstract : For more than 15 years the N'KOSSA barge has been in operation offshore Congo. Design of the facility was to perform deep NGL recovery from the associated gas by a turbo‐expander process before using the treated gas for pressure support in the reservoir. A dehydration technology offering below ppm dry gas water content had to be selected upstream the expanders.
Glycol dehydration with DRIZO™ regeneration was selected as the most cost effective and Best Available Technology thanks to the high glycol regeneration level provided by the DRIZO™ solvent stripping, as well as the most adapted process for offshore installation and operation.
From the start‐up of the unit until today, a successful partnership was established between Operator TOTAL E&P CONGO and Licensor PROSERNAT to perform long term follow‐up of the unit's performances and tackle all upcoming issues. Several problems were overcome mainly due to a weak engineering design. However very good performances were always maintained while lessons learnt enabled to further sharpen the good reliability of the DRIZO™ process.
This paper will develop all the above points and in particular give a detailed description of the unit’s evolution over the last 15 years.
Author’s information:
Van Khoi VU, Process expert consultant for TOTAL, Adress: Tour Coupole, 2 Place J. Millier, 92078, Paris La Défense 6 Cedex, France; Phone: 00 33 (1) 47 44 69 49
Thomas BRENAS, Lead Process Engineer for TOTAL Development Engineering Division, Adress: Tour Coupole, 2 Place J. Millier, 92078, Paris La Défense 6 Cedex, France; Phone: 00 33 (1) 41 35 27 22
Bernard CHAMBON, Gas dehydration expert, PROSERNAT, Adress: Tour Franklin, 100/101 Terrasse Boieldieu, 92042, Paris La Défense Cedex, France; Phone: 00 33 (1) 47 67 19 70
Louis PENEL, Senior Process and Operation engineer, PROSERNAT, Adress: Tour Franklin, 100/101 Terrasse Boieldieu, 92042, Paris La Défense Cedex, France; Phone: 00 33 (1) 47 67 19 82
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Background
The NKP concrete barge was initially designed to process production from the N’Kossa field, situated about 60 km offshore Congo, at a depth of 170m. This field was discovered in 1983 and put in production in June 1996. In its current development stage it consists of 39 production wells, 8 gas injection wells and 6 water injection wells, distributed between 2 well head platforms (NKF‐1 and NKF‐2). It is operated by TOTAL E&P Congo, with Chevron and Société Nationale des Pétroles Congolais as partners.
Since start‐up it has had to adapt first to different production figures than anticipated, and then to the integration of production from new developments. New fields have over the years been routed to the NKP barge for treatment, and it currently treats oil production from the Nsoko and Tchibeli fields and gas from Moho Bilondo field in addition to that from NKossa.
Figure 1 : oil fields associated with the NKP barge
Liquid production of the NKossa field is mainly based on cycling . In this method, reservoir pressure is maintained by substitution of the condensate reservoir gas (rich gas) in the reservoir by lean (poor gas). Lean or poor gas is obtained from condensate reservoir gas coming from production wells, by natural gas liquid extraction (propane +butane + condensate). To obtain a total conservation of reservoir pressure an extra gas from another gas source is added to the produced lean or poor gas. As of September 2009 production figures were:
‐ Oil : 50 000 bbl/day ‐ Reinjected gas : 12.2 Msm3/day ‐ Propane : 7 000 bbl/day ‐ Butane : 4 000 bbl/day
Treatment performed offshore is very thorough, with extraction of NGL from the associated gas prior to its reinjection and fractionation of the NGL to recover LPG, all performed on NKP. In addition to these
Tchibeli
Nsoko
NKossa Moho Bilondo
Djeno
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processes the barge also includes all the more common processes of gas/oil/water separation, water injection and utilities, as well as living quarters.
The concrete barge, 220 m long and 46 m large, is divided into 7 modules in which the functions shown are distributed.
Figure 2 : NKP barge modules repartition
The main processes employed on NKP can be seen on the simplified scheme below, where the oil separation and gas treatment stages can be clearly identified:
M7 : Flare
M6 : DRIZOTM,
NGL extraction & fractionation
M5 : Oil treatment
M4 : Compression
M3 : Power generation
M2 : Water injection & Utilities
M1 : living quarters
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Figure 3 : General view of process units on NKP
The main objective of the gas treatment is to recover as much LPG as possible. For this purpose a cryogenic process is installed, based on a turbo‐expander to reach a temperature as low as ‐60°C. In these conditions the upstream gas dehydration is essential to prevent formation of hydrates and thus allow good operation of the plant. The very low temperatures reached mean thorough dehydration is required down to a water dewpoint lower than the operating temperature of ‐60°C (< 1 ppmV). Why a DRIZOTM ? ‐ General operating principles of the DRIZOTM Process
Only two technologies can achieve this level of drying: DRIZOTM glycol dehydration and Molecular Sieve. However the specifics of the offshore environment made installation of a Molecular Sieve unit problematic: this process requires equipment having a large footprint and heavy weight, both unfavorable characteristics. In addition Molecular Sieves have a significant energy consumption, requiring larger utilities units.
The DRIZOTM unit on the other hand could be integrated to the barge in the form of a compact skid with limited footprint and weight, and has much smaller energy consumption. It was therefore the most adapted and cost effective process for the situation, and consequently selected by TOTAL.
The DRIZOTM process was developed and patented by Dow Chemical in the early 70’s. The technology was bought by the Houston based OPC Engineering company in 1985, which was incorporated by PROSERNAT in 1998. The technology has been since licensed by PROSERNAT.
The principle of DRIZOTM is similar to that of a glycol with stripping gas. However, the stripping agent is not natural gas (i.e mainly light ends such as methane) but a vapourized “solvent” which is in fact a C4 to C9 cut extracted from the wet gas by the glycol.
This has two important favourable impacts. First the solvent cut has an improved stripping efficiency compared to natural gas (“stripping gas”) as it works by reduction of H2O partial pressure (similar to stripping gas) but also by breaking the water‐TEG bonds thanks to the polarity of the aromatic molecules of the solvent. Second, the solvent is largely recovered by condensation after use and therefore dramatically reduces flaring and BTEX emissions.
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The general principles of a DRIZOTM unit can be seen in Figure 4 below:
Figure 4: General principles of DRIZOTM One can see the glycol loop, identical to that of a normal glycol unit with stripping gas, and the specific solvent loop along which the solvent is pumped, vapourized, superheated, used as stripping agent and finally condensed and separated from the produced water. The HP part where the gas dehydration takes place is similar to that of any glycol unit, with its Glycol Contactor, but thanks to deeper glycol purity it allows to reach much lower dry gas specifications.
NKossa process scheme
The design of the NKP DRIZOTM can be summarized as follows:
‐ 9.7 m3/h of lean glycol at 99.99+ %wt ‐ Dehydration of 13 MSm3/d (460 MMSCFD) of water saturated gas at 35°C down to 1 ppmV
The actual NKossa PFD is shown hereunder:
WATER, BTX, & HCABSORPTION
in GAS / GLYCOL CONTACTOR
WET GAS
DRY GAS
RICH GLYCOL + SOLVENT (BTX & HC )
LEANGLYCOLPurity: 99.99+ %wt
GLYCOL REGENERATION WITH DRIZOTM
WATER, BTX, & HCPARTIAL DESORPTION
in GLYCOL REBOILER and STILL COLUMN
WATERCOMPLETE DESORPTION
BTX & HC ABSORPTION
in STRIPPING COLUMN
WATER, BTX, & HCCONDENSATION
in CONDENSER
WATER, BTX, & HCSEPARATION
in WATER / HC SEPARATOR
BTX & HC VAPORISATION
in SOLVENT HEATER
TRACE WATER REMOVAL
in COALESCERDRIZOTM LOOP
GLYCOL LOOP
WATER
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Figure 5 : NKossa DRIZO PFD
The HP section of the unit consists in 4 shell & tube Water/Gas Coolers, an inlet Scrubber fitted with a wire mesh mist eliminator and finally the Glycol Contactor fitted with 250 m2/m3 structured packing. For regeneration, several important features of the DRIZO unit were included:
‐ The Flash Drum overhead system (condenser EC‐608 and drum DS‐603) was designed to reduce solvent losses. The hot vapor from Flash Drum operated between 90 and 110°C are cooled down to 35 °C in the heat exchanger EC608 to condense the heaviest hydrocarbons (solvent) and to recover the entrained glycol. The non condensable components and lightest hydrocarbons are separated in the drum DS 603 and sent to flare.
‐ The liquid solvent is vapourized by heat exchange with the lean glycol from the Reboiler in exchanger EC‐605.
‐ There is an integrated water cooled Reflux Condenser in the Still Column followed by the water/solvent condenser EC‐609, i.e reflux is not external.
The DRIZOTM started operation when NKP was put online in 1996. PROSERNAT has since 1998 supported TOTAL in the follow‐up support of the unit.
Good performance of NKossa DRIZOTM
Performance has been good with dew‐points below 1 ppmV from the beginning, as assessed by day to day operation and Performance Tests.
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Figure 6: Nkossa DRIZOTM Dehydration performance
However despite these good performances the unit has had to overcome several issues. Some were more directly related to the HP part and some more to the regeneration part, in some cases reducing the dehydration performances.
Issues addressed during NKossa operation
Inlet gas temperature fluctuation
Pressure and overall temperature are very important parameters for the correct operation of a glycol unit, as they impact both the water content of the feed gas and the equilibrium between dry gas and lean glycol (i.e. dry gas water dew point). The lower the gas temperature the lower the water content of the dry gas and the lower the dew point that can be reached with a given lean glycol purity.
On NKP the pressure is stable but not so the temperature: prior to its entry in the dehydration unit, the gas is cooled by a cooling water loop, which is in turn cooled by seawater. The gas temperature therefore indirectly depends on the seawater temperature. It has appeared that the temperature of the sea around NKP was following a cycle of variation over the year, more or less centered around 19°C but with an annual very short and very sudden rise to 25/26°C in the middle of October. The amplitude of this increase creates much more unfavorable conditions for the dehydration unit.
It has been envisaged to upgrade the gas coolers. Such a modification turned out to be difficult on the barge because of the congested layout of the module 6. Indeed, the technical solution to upgrade EC 601 resulted in changing the entire heat exchanger with new corrugated tubes. This offshore construction job would require the mobilization of a construction barge with a powerful crane increasing the cost of the project. It has has therefore not been implemented. Although less efficient, the cooling water system is to be revamped to maintain proper cooling of the gas during the hot seawater period.
0,00
0,50
1,00
1,50
2,00
2,50
3,00
3,50
4,00
4,50
5,00
07/06/09 07/16/09 07/26/09 08/05/09 08/15/09 08/25/09 09/04/09
water con
tent (p
pmV)
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Figure 7 : Annual variation of seawater temperature around N’Kossa
Inlet Scrubber poor design
The second problem on the HP section appeared following debottlenecking modifications. The operator quickly wished to go beyond the 13 MSm3/d for which the unit was designed. A progressive increase to 17 Msm3/d was studied, and one bottleneck identified was the Inlet Scrubber. This Scrubber has an Internal Diameter of 2660 mm as required for sizing with a K factor of 0.086 m/s (0.28 ft/s) for a wire mesh (as per good engineering practice).
To allow for target flow of 17 Msm3/d without changing the vessel shell, a vane pack was installed which the supplier claimed could be sized using a K factor of 0.115 m/s (0.38 ft/s). The internals were therefore changed in 2005 for this vane pack. Soon afterwards a degradation of the performance of the dehydration was observed.
This degradation could be traced to a loss of the efficiency of the Contactor packing due to fouling: the operator conducted several chemical cleaning operations of the packing which all had an initial effect of recovering a better performance followed by a gradual degradation, as illustrated by the following scheme:
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Figure 8 : Performance loss caused by poor upstream scrubbing
This implies that the new Scrubber internals were not performing correctly and liquid hydrocarbons could flow through it, then creating a fouling layer on the Contactor packing. This was later corroborated by a CFD study, which also gave insight that at least part of the problem was linked to the geometry of the inlet piping (consecutive elbows very close to the inlet nozzle) and inlet impactor design. The impactor actually promoted reentrainment of liquid from the Scrubber bottom by diverting the gas flow to the bottom of the vessel, leading to high gas speeds just above the liquid surface, all the more so since the impactor was too close to the liquid surface.
Figure 9 : reentrainment of liquid caused by Scrubber inlet device
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Correct design of gas inlet impactors should not allow gas to flow downwards, and above all the impactor must be high enough above the liquid surface.
The cleanings were becoming less efficient over time. Moreover, as different chemicals were used over time to try obtain better performance, undesirable side effects appeared such as formation of jelly like deposits in the Contactor and heat exchangers, which created a risk of making matters worse when cleaning. It was therefore decided to change the packing altogether, using the opportunity to upgrade to state of the art high efficiency and high capacity structured packing (350 m2/m3). This new packing was installed in July 2007 and since then performances have been recovered and are stable. The root cause of the problem, i.e. poor performance of the Inlet Scrubber, is currently being solved by replacement the internals.
As will be seen later, in addition to severe degradation of absorption in the Contactor, this HC carry over also led to operating problems in the regeneration section. It is therefore of paramount importance, for glycol units in general, to prevent HC carry‐over into the Contactor.
The primary means for preventing this is of course correct selection of Scrubber internals. For new developments, it should be standard to use mesh pads. This technology provides the best performance when a single device is installed in the vessel. It also allows more versatility for later modifications as vessels dimensions are larger.
For revamps, as was the case on NKP, recommendation is to use cyclone systems or liquid coalescing cartridges, which offer very good performances. If cyclones are selected however, great care must be taken for proper draining of the liquids they collect. It is not uncommon to see operation jeopardized by reentrainment from poorly designed collection systems. If necessary, external draining can be considered if the height of the existing Scrubber does not allow another solution.
Glycol filtration quality
Regarding the regeneration part of the unit, several problems also had to be overcome over the years. Some were related with loosening of operator attention when the units were working well and operation becomes routine. This causes less attention to be paid to details which do not directly or immediately affect performance, but can on the long run become really detrimental.
One such problem is the performance of glycol particle filtration. It appeared during a performance test carried out in September 2009 that the glycol filters were fitted with 75 µm cartridges instead of the normal 10 µm ones. Because the cartridges all have the same appearance and the filtration rating threshold is often not clearly or explicitly indicated, the rating must be deduced from the cartridge supplier code, this kind of error can easily occur. This is why close attention must be paid to details at all time.
It is difficult to know if improper filtration had a significant detrimental effect on the unit, but it may have played a role in not preventing the pluggings that occurred in the Glycol/Glycol Heat Exchanger.
Still Column outlet temperature
Another such problem was the temperature control of the off gas from the Still Column. A water cooled Reflux Condenser is installed at the top of the NKP Still Column to provide water reflux and limit glycol losses. The unit had been designed so that the flow of cooling water was controlled continuously to maintain a target outlet temperature. Because the off gas outlet temperature can be considered indicative of the amount of glycol lost as vapour from the column: the lower the temperature, the lower the vapor pressure losses.
But what could seem on paper a good idea proved unadapted to site operation: indeed if the outlet temperature is an indication of the glycol content of the off gas, it is also the result of equilibrium at Reboiler pressure between the water and solvent vapours that go out and the liquid water condensed. At
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the outlet of the Still Column, both pressure and composition are fixed independently of operator control: the pressure because it is set by the Flare and the composition because what goes out of the column can only be what goes in, independently of the Reflux Condenser duty.
The outlet temperature is therefore fixed and does not depend on the amount of cooling duty. When this equilibrium is reached, additional cooling water flowrate results only in higher reflux not in lower outlet temperature.
This made the temperature control loop counterproductive: had the set point of the loop been adapted for the initial operating conditions, it would have had to be modified regularly according to Reboiler operating conditions (solvent flowrate, pressure) to stay effective. As operators did not have an understanding of the equilibrium problems, they ended up considering the control system was not effective enough and opened entirely the cooling water control valve and its by‐pass. This generated increased reflux flowrate and in turn increased load in the Reboiler, which was detrimental when Reboiler power was reaching its limit.
Recommendation for this problem is never to implement automatic control of the outlet temperature of the Still Column. This should be adjustable manually by the operator, together with clear recommendations that it is not necessary to try lower further this temperature when increasing cooling has no more impact. Nevertheless a temperature monitoring with high alarm shall be installed at the outlet of the Still Column. Monitoring the Reboiler pressure in the Control Room should be performed, as pressure increase results in higher outlet temperature.
Solvent/water separation
Regarding the solvent loop, i.e purely DRIZO related matters, the NKP unit allowed an opportunity to identify a specific DRIZO design issue. Initial performance of the unit was somewhat below expectations, with dry gas dew points of ‐55°C. This was traced down to abnormally high water content of the solvent in the DRIZO loop, which caused deteriorated stripping.
That high water content was caused by insufficient separation between water and solvent in the Solvent Drum. In DRIZO units up to that date, only decantation had been used as means to separate water and solvent. N’Kossa however was the first to be installed offshore on a floating support, and probably for this reason separation was diagnosed as insufficient on this unit due to negative effects of the barge motion on the separation.
An improvement in the design of the solvent loop was therefore implemented by adding a liquid/liquid coalescer on the loop to extract the remaining free water. This modification was carried out in 2002 and proved immediately very successful, with dew points falling to ‐60°C after subsequent start‐up.
This design has since then been included in all the new DRIZO units, either offshore or onshore.
Contamination of regeneration unit by liquid entrainments from Inlet Scrubber
The inefficiency of liquid separation in the Gas Inlet Scrubber, also had negative effects on the operation of the regeneration unit. These were of two kinds: problems linked to liquid HC carry over and problems linked to liquid water carry over.
The liquid HC entering the Contactor ended up for the most part in the rich glycol stream. The heavy tail is not boiled off in the Reboiler and stays in the Reboiler or is carried with the lean glycol. Some of it is then degraded by the heat, creating fouling deposits that prevent correct operation of the heat exchangers. Chemical cleanings proved necessary on NKP and in one occasion the glycol/glycol exchanger was entirely blocked.
Formation of deposits was also promoted by inadapted pH control operations: the pH of the rich glycol was maintained above 8, which allows naphthenes carried over from the gas to precipitate in the form of
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naphthenates. The pH control injections were also too massive, creating basicity shocks, again promoting naphthenates. These injections should be regular and lightly dosed.
Some of the liquid HC flow forms a separate phase with the lean glycol from the Reboiler and then separates by decantation in the Surge Drum. Regular manual skimming of the Surge Drum is required to prevent the HC layer in this vessel from becoming excessive.
As for the liquid water, the main problem is that it carries salts with it. In NKP, chlorides were imported by this water, and also calcium sulfates. The chlorides generated severe corrosion of the Stripping Column 316L packing, eventually leading to its utter destruction and unplanned shutdown of the unit. This occurred in only 2 years time.
The calcium sulfates were prone to depositing between the plates of the Water/Solvent Condenser. Reduction of the flow area for the overhead product led to repeated increase of the Reboiler pressure, which is a parameter of paramount importance for the quality of the glycol regeneration. Several cleanings were required but experience shows that the Reboiler pressure tends not to be watchfully monitored by the Operator and sometimes the situation was really degraded before action was taken.
The speed at which pressure increase was observed in the exchanger after cleaning and even after replacement for a new exchanger shows the plate and frame technology used is not suitable for the service. The plates on these exchangers are too close and offer easy targets for deposits. Shell & Tubes exchangers should be considered for Still overhead condensers.
Insufficient Reboiler power
The final problem was the supply of sufficient power to the Glycol Reboiler for correct regeneration of the glycol. The duty of the NKP Reboiler is supplied by three electrical bundles. Three bundles are installed, each able to supply 50% of the original design duty (520 kW per bundle). This design allowed a margin on the duty available for operation and a spare bundle in case one would fail entirely for whatever reason. This overall sound and careful design had however one weakness: as at any time one bundle of the three was supposed to be spare, the dedicated power control system of the Reboiler was designed to allow two and only two bundles to be operated at the same time.
This was no problem while gas flowrate through the Contactor was within its original range, but when it was progressively increased after successive debottleneckings the duty required from the Reboiler increased with it. The nominal duty of the Reboiler is 150% of the initial design duty when considering all three bundles in operation, so no problem was identified during the debottlenecking studies. The fact that the control system did not allow it was overlooked.
Consequently when the gas flowrate or its temperature were high it became difficult to maintain the normal operating temperature in the Reboiler. This being the primary parameter for correct glycol regeneration, performance was lower in these situations. To solve this problem the Operator did a self‐designed modification of the Reboiler power distribution wiring that did allow simultaneous operation of the three bundles, but that also disabled entirely the duty control. When this new “3 bundles operation” mode was enabled, all three delivered their maximum duty with no regard for Reboiler temperature. This created a sharp increase in glycol temperature every time it was turned on.
This meant that when needed, this mode was activated in an On‐Off way, manually making the temperature to oscillate around the target 204°C. It could therefore be used only as last resort and not for normal continuous and stable operation of the unit.
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Figure 10 : Inadequacy of existing Reboiler power control system
This can only be solved by careful modification of the power control system in coordination with the Manufacturer.
For new units this feedback shows that large design margins shall be considered for electrical heaters, either by considering a minimum 20% overdesign or by installing 3 x 50% bundles with a control system allowing operation with all the installed power.
Conclusion ‐ Overall results from 14 years of operation of the NKP DRIZO
Overall what the 14 years of operation of the NKP DRIZO show is that despite some problems the unit is giving satisfactory performance. It consistently delivered dry gas water dew points at or below 1 ppmV, corresponding to the ‐60°C needed at Turbo‐expander outlet pressure (27 barg). The Turbo‐expander has actually operated with an outlet temperature between ‐55°C and ‐60°C over the years without problems,
Reboiler Temperature decrease caused by high gas flowrate
Third bundle manually enabled ‐ no more temperature control
Wet gas flowrate
Reboiler temperature
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and without methanol injection (except during the Glycol Contactor packing fouling problems). It allows high LPG recoveries, adding value to NKossa field production.
It should also be noted that the solvent balance of the unit is pretty neutral: periods when solvent was produced in excess by the unit alternate with periods where some limited make‐up is required, this depending on the process gas composition. In any case, make‐up is easy to perform using condensates readily available on the site. An example of one observed solvent composition of NKP is given hereafter:
Figure 11 : NKossa solvent compostion
The unit is also easy to operate, and very stable. In fact, operation has been so trouble free that Operators tended to give it too little attention and allowed some small problems to build up over time. The main issue for day to day operation and performance alike is the carry‐over of liquid hydrocarbons and salty water from the Inlet Scrubber. This requires the frequent skimming of the Surge Drum that hassle Operators, and more importantly it degrades dehydration performance, as from 2005 to 2007 when the packing was fouled and regular chemical cleanings were required. It must be noted that this problem is in no way specific to a DRIZOTM unit.
The case of NKP shows that consistently low dew points can be achieved with DRIZOTM units, despite the negative effect of floating support motions.
Main components : C5 to C8
Carbon number
%Weight