analyzers moisture dew points

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GAS PROCESSING DEVELOPMENTS SPECIALREPORT

HYDROCARBON PROCESSING JANUARY 2011 I

49

Use online analyzers for

successful monitoringImproved analytics measure moisture and dew points for natural gas components

A. BENTON, Michell Instruments Ltd., Ely, Cambridgeshire, United Kingdom; andC. VALIZ, PDVSA, Jose Complex, Venezuela

U

sing advanced online water and hydrocarbon dew-pointanalysis techniques is critical to the efficient and reliable

operation of natural gas liquid-extraction processes, pro-ducing valuable light alkane liquids while the remaining gas issuitable for sales distribution. At its San Joaquin facility, Petróleosde Venezuela, S.A. (PDVSA) initially processed raw wellheadnatural gas by separating residual hydrocarbon (HC) condensates,followed by glycol dehydration to reduce the water dew point.Reduced temperature separation was used to decrease the HCdew point prior to molecular sieve dehydration. This providedfeed gas with moisture controlled to trace levels into the primary liquid-extraction process. Maintaining moisture concentrationsto less than 0.1 parts per million (ppm) by volume is essential forreliably operating turbo expanders. The turboexpanders’ functionis to use the depressurization of natural gas to achieve deep cooling

of the process flow to –80°C. Avoiding ice formation within theseparation process—particularly the turboexpander—is critically important for continuous plant operations and to prevent astro-nomical maintenance costs. Online measurement of dew pointtemperature within the feed gas, containing water precipitateand hydrocarbon condensate, enabled PDVSA process operatorsto extend the lifetime of the desiccant beds while protecting theturboexpanders from risk of damage.

The San Joaquin plant (Fig. 1) started operation in 1985 andcurrently provides a third of the total Venezuelan production of natural gas liquids, roughly 43,000 barrels per day, transferred

via pipeline to the PDVSA Jose Complex, near Barcelona. The Jose fractionation plant extracts butane, iso-butane and other

individual alkanes. Residual gas production at 1,000 millionstandard cubic feet per day (MMf 3d) enters the Venezuelan mar-ket network.

The extraction process. The production train at the San Joaquin process plant is shown in Fig. 2, while Fig. 3 illustrates aschematic of an individual train.

Unprocessed wellhead natural gas (two-phase gas flow withentrained hydrocarbon liquids and water) enters the San Joaquinfacility. It is initially processed using separation vessels to collectthe bulk-entrained natural gas liquids that flow in the feed pipe-line. The gas at this point is highly corrosive and saturated withmoisture, and is prone to hydrate formation. Hydrate formation

is crystalline solids that form from condensed liquid water incombination with methane under pressurized conditions, evenat temperatures above freezing. Initial dehydration is carried outat the earliest stage by a glycol contactor where liquid triethyleneglycol (TEG) is spray injected as a desiccant into the gas flow ris-ing through a process column contactor (Fig. 4). Moisture-ladenTEG is collected from the contactor for heat regeneration greaterthan 200°C, so boiling off the adsorbed moisture is done in acontinuous circulation process.

At this stage, the gas is dry—the water dew-point temperatureis lower than 0°C, below the process temperature conditions at

PDVSA San Joaquin extraction plant.FIG. 1 Production train at San Joaquin.FIG. 2



GAS PROCESSING DEVELOPMENTSSPECIALREPORT

50 I JANUARY 2011 HYDROCARBON PROCESSING

the prevailing process pressure. Therefore, while not entrained with liquid water, the gas is still heavily laden with HC liquidsas a potential two-phase flow. The liquid content of the gas is

suppressed through two stages of separation, before and after a Joule-Thomson (J-T) expansion valve that generates moderate

cooling of the gas by approximately 7 Kelvin through partialpressure reduction at 8 bars. This yields part of the natural gasliquid production while reducing the HC dew-point gas tempera-ture. The effect of this separation process is illustrated in Fig. 5.

Reducing the HC dew point is important for the next stage of theprocess—dehydration to trace moisture concentration throughmolecular sieve columns (Fig. 6).

Reduction in the liquid loading of the gas is critical to themolecular sieves’ drying efficiency. If liquids are present, itadversely affects the moisture adsorption properties of the mate-rials’ lattice structure and the overall operation lifetime of thesieves. Reduction in the water dew point below –80°C (at processpressure equating to a moisture concentration of less than 0.1ppmV ) is critically important for operating the turboexpanders.

Turboexpanders, (Fig. 7) or expansion turbines, recover use-ful work from the expansion of a gas stream while lowering theprocess temperature, resulting in partial liquefaction of the bulk

stream. As the expansion nears isentropic, the turboexpanderreduces the process gas temperature significantly more than

Wellheadnatural gas

Natural gas liquids

Glycol (TEG)dehydrationcontactor

Glycolregeneration

system

S e p e r a t o r

J - T i n l e t

S e p a r a t o r

T - E o u t l e t

S e p a r a t o r

T - E i n l e t

SeparatorJ-T outlet

Molecular sievecolumns x 4

12

2

34

1

Silicagel x 2 55 bar

-33ºC

17 bar-79ºC

E x p a n s i o n

C o m p r e s s i o n

Turbo-expander

Residual gas tonational distribution

system 69 barg

DeethanizerLiquidstabilizer

Natural gas liquids

Joule-Thomson valve

66 bar18ºC

58 bar11ºC

Process train schematic.FIG. 3

Glycol contactor tower.FIG. 4

40

60

80

100

120

140

P r o c e

s s p r e s s u r e ,

b a r

Hydrocarbon dew-point reduction at process stages

0

-100 -80 -60 -40 -20 0 20 40 60 80

20

Hydrocarbon dew-point temperature, °C

Gas entering plantOutlet J-T seperatorResidual gas

Phase envelop (HC dew point curves) at stages throughoutthe liquid extraction process.

FIG. 5



GAS PROCESSING DEVELOPMENTSSPECIALREPORT

52 I JANUARY 2011 HYDROCARBON PROCESSING

expansion across a J-T valve for the same 38-bar pressure drop.The consequences of higher moisture concentrations within theprocess gas can be catastrophic, as the lowest temperatures in theliquid extraction process approaches –79°C. If the gas water dew point rises above that temperature, then rapid ice formation willbring a high risk of physical damage to the rotating blades on theturboexpander. The deep cooled natural gas goes through multiplestages of liquid separation as the temperature cascades downwardinto a deethanization column. The result is a lean natural gas withonly C1 and C2 hydrocarbons and some residual CO2 (Fig. 5).Compression of this remaining gas into the national distributionsystem is carried out by a centrifugal compressor linked to theshaft of the turboexpander, therefore capturing useful work from

the gas expansion. A rich blend of propane, butane and heavierhydrocarbons forms the natural gas liquids transferred by pipelineto the fractionation plant.

Online dew-point analysis. Close monitoring of the dew-point levels at critical points across the San Joaquin extractionfacility requires 12 combined hydrocarbon and water dew-pointanalyzers (Fig. 8). PDVSA installed two analyzers on the glycoldehydration contactors and five on each production train. Eachtrain had four individual molecular sieve columns that needed tobe monitored continuously along with the common header outletfeed to the turboexpander plant. Dew point is the temperature where a vapor or the combination of vapors condense to form

a liquid, seen as dew drops, when the gas is cooled. When thedew-point temperature is below freezing, ice crystals form. HC

dew point is the condensation temperature of the heaviest HCcomponents. Often, molecules with greater than 10 carbon atoms,present in ppm and sub-ppm concentrations, will condense toform HC condensates.

The analyzer (Fig. 7) will simultaneously measure HCs and water dew point using two discrete sensor cells applying dedi-

cated sensor technologies (Fig. 9). A direct fundamental principleadapted from the optical cooled mirror technique is applied forthe HC dew-point measurement. An abraded optical surface witha conical depression profile has been developed specifically for thedetection of the low surface tension films that are characteristic of the precipitation that occurs at the HC dew point. A visible redspectrum source directed onto the abraded surface scatters thelight. A photo detector is positioned so that it captures the scat-tered light. It diminishes rapidly as the condensate film forms onthe surface, cooling down to the temperature region of the HC dew point by a heat pump under the automatic control of the analyzerfirmware. As the condensate film forms, a ring of light develops asthe surface within the conical depression becomes more reflective.

However, it is the secondary effect of reduction in scattered lightintensity that enables the analyzer to detect the HC dew point to asensitivity of 5 mg/m3 condensate mass per volume of gas.

Unlike the HC dew point, an optical condensation techniquecannot be applied to the water dew point measurement in thisapplication. The water dew point temperature is significantly lower than the HC dew-point, such that any cooled surface would be flooded with condensate at a temperature far higherthan the water dew point. For such measurements, a ceramicmoisture sensor is used. Working on a moisture adsorption prin-ciple, the sensor adsorbs moisture into a micron-thin hygroscopicmetal film in equilibrium with the flowing gas stream. The sensorexhibits an impedance change in proportion to the partial pres-

sure of moisture vapor, the most elementary hygrometric unit.This is directly related to dew-point temperature, the measure-

Molecular sieve dehydration columns.FIG. 6

Turboexpander and a deethanizer column.FIG. 7

Hydrocarbon and water dew-point analyzer with asampling system.

FIG. 8



GAS PROCESSING DEVELOPMENTS SPECIALREPORT

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53

ment unit where each sensor is calibrated.These sensors have an extremely wide sen-sitivity range, being calibrated traceable tonational metrology standards from –100°Cto +20°C dew point. They also have thecapability to measure directly at processpressure conditions representing greater

than 20,000 ppmV moisture content at fullscale compared to less than 0.01 ppmV atthe low end—this is important for measur-ing the turbo expander feed gas.

In-situ measurement verification. Al though the sensor s pe rforming theonline water dew point measurement arefactory-calibrated traceable to nationalmetrology standards, the critical natureof the water dew-point measurement forthe turboexpander feed gas demands thatPDVSA site personnel carry out periodic

verification of the online analyzers duringfield operation. This is done in-situ usinga portable dew point analyzer. Given theextremely low level of water dew point within the process (< –80°C dew point) andthe process pressure of 60 barg, equatingto less than 0.1 ppmV , it is advantageousto carry out the verification measurementby installing a reference dew-point sensor within the online analyzer on a temporary basis. This enables both online andreference sensors to be in total equilibrium with the process gas sample under the same

conditions over the duration of some hours.Through this method, PDVSA was able tomaintain the highest level of confidence inthe measurements.

Process plant proving field trials. During June and July of 2009, PDVSA carried out trials to determine the operationalstatus of the molecular sieve desiccant within the four drying col-umns on Train B at the San Joaquin facility. During normal plantoperation, the columns were operated in an overlapping sequencefor 36 hr before going offline for regeneration by heating to300°C and back-flushing during an 8-hr period. At least threeof the columns were in process operation in parallel at one time.

During the plant-proving trials, the total adsorption capacity of each column was assessed through continuous operation untilmoisture breakthrough was detected by the online monitoringof dew point analyzers. After three years of use for the currentmolecular sieve material and an expected operational lifetimeprediction of 3½ yrs, the purpose of the test was to determinethe capacity of each bed and the urgency of scheduling whento replace each desiccant. Furthermore, the regeneration cycletimes were reassessed to optimize the sequence throughout theremaining lifetime of the existing bed material. Fig. 10 illus-trates the moisture breakthrough on two individual desiccantbeds occurring after 30 hr and 32 hr of continuous adsorption.The red trace shows the increase in the gas’ dew point in the

common header resulting from these high moisture excursions.Given similar findings on the other two desiccant beds, PDVSA

reduced the operational adsorption period for each column to 27hr, with a 9-hr regeneration sequence, to extend the useful life of the molecular sieve material through the intended replacementdate of February 2010.

The proving trials served to reaffirm to the PDVSA plantoperations and laboratory staff. It was shown that the perfor-mance of the analyzers after the first 18 months of success-fully monitoring the dew-point conditions was critical to thecontinuous process operation and the reliability of key plant

equipment. HP

Permeable gold film

Hygroscopic mono-layer

Base electrode

Ceramic substrate

Moisturemolecules

Schematic diagrams of ceramic impedance moisture (left) and HC dew-point detectiontechnique (right).

FIG. 9

Traces from molecular sieve capacity tests. Note: the Y-axis is scaled in °Fdew-point temperature.

FIG. 10

Carlos Valiz joined PDVSA Gas 15 years ago as senior chemist

specializing in laboratory analysis. His current position is quality

control supervisor for the complete Eastern Venezuela division ofPDVSA Gas.

Andy Benton joined Michell Instruments Ltd in 1983, initially

in the R&D section. Throughout his career he has specialized in

devising innovative solutions to applications for online process gas

and liquid analysis. Most particularly for the natural gas industry,

Mr. Benton has advised best practice for water and hydrocarbon

dew-point control and measurement to producers, pipeline operators and large scale

end users covering six continents.

analyzers moisture dew points

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