TOPIC II

HYDRATES- INHIBITION-DEHYDRATION

Controlling and/or Removal of Waterin Natural Gas

Hydrates Inhibition Hydrates Removal

Glycol Absorption (high quantity of water) Description of process Field of application Unit design:

a. foaming; b. glycol degradation/corrosion

Adsorption (low to moderate amount of water) Description of process Adsorbent characteristic Unit design:

a. choice of adsorbentb. Dryer arrangementc. Restricting the quantity of adsorbent required

HydratesHydrates are: crystal (solid) formed. physical combination of water and light hydrcarbon

molecules.

Characteristics resemble packed, wet snow flakes density = 0.9 kg/dm3 regular cubic structure in shape only small gas molecules can form hydrates

(i.e: CH4,C2H6,C3H8,C4H10, CO2,H2S and N2)- hydrocarbon tend to be trapped in cavities of water molecule structures

large proportion of water (CH4.6H20 methane hydrate) Hydrates contain a very large proportion of water (CH4.6H2O for methane hydrate;

C3H8.17H2O for propane hydrate) high concentration entails presence of liquid water

Conditions for Hydrates Formation : Primary cause in presence of liquid water temperature does not exceed the water dew point specification

(dew point= lowest temperature at which vapour starts to condense/liquefy)

Effects of hydrate formation- increase processing difficulty; caused disruption of process during natural gas treatment- blockage; accumulation of hydrates at orifice plates or valves, reduce the cross-sectional area.- corrosion of equipment and lines by acid gases (CO2, H2S) in presence of water

How to prevent hydrates formation1. Operating Condition

- modify the operating condition of natural gas treatment. at P=60 bar, hydrates formed at T

HYDRATES INHIBITIONMethanolAdvantages: effective in case of polar products especially water. attractive due to low crystallization temperature, low viscosity

Disadvantages: not economical volatile product high losses of vaporization in gas

Ethylene GlycolAdvantages: more preferred than methanol. used on intermittent basis as curative rather than preventive measurement. important in drying natural gas. loss through vaporization in natural gas can be neglected

Types of Glycols widely used: monoethylene glycol, MEG (HOC2H4OH) for low T diethylene glycol, DEG (HO(C2H4O)2H) triethylene glycol, TEG (HO(C2H4O)3H) for high T

* depend on operating temperature that influence on viscosity

Properties of Hydrate Inhibitors

Quantity of Inhibitor to InjectEthylene GlycolProperties: MEG used for temperature below 5 to 10oC TEG too viscous at T = -5 /-10oCHammerschmidt FormulaQuantity of inhibitor:W = weight percentage of inhibitor in aqueous phasedx = decrease in temperature of hydrate formationM = molecular weight of inhibitorKi = constant related inhibitorOperation remove free water from circuits first before start up process drain the lower points blow the circuits with dry air / nitrogen monitor the variation in operating parameters (i.e P, T,delta P)

DRYING GAS BY HYDRATE-INHIBITION

Type: MEG, DEG

T= -5 to -10oC

P=

T=

DEHYDRATION

Definition : Dehydration is a process of water removal or drying of natural gas. compliance with a water dew point specification.

Purposes to avoid the risk of condensation of water in the pipeline. to avoid the formation of liquid slugs. to prevent gradual plugging of the circuits by ice.

Types of dehydration processes1. Glycol absorption dehydration most suited is TEG (Triethylene Glycol) TEG has low vapor pressure hence minimum losses in gas high concentration of TEG give low gas dew point

temperature

2. Dehydration by adsorption used adsorbent in dehydration process principle based on porous solid with specific property,

able to fix water molecules on the surface of pores where water vapor condensed.

Glycol Absorption DehydrationPrinciple: wet gas dehydrated by glycol in absorber water contained glycol reconcentrated in regenerator water removed from natural gas, recycled back at top of the column

Process Description (refer to Fig 2): wet natural gas (NG) pass through separator on leaving the separator, the gas is fed into bottom of absorber lean glycol solution is fed on top of absorber rich glycol expanded in expansion drum to degas the light

hydrocarbons, H2S and CO2 entering regenerator and reboiler to minimize risks of corrosion exchanger is used to condense the regenerator reflux the gas free water leaves the top of the column

DRYING GAS BY DEHYDRATION: GLYCOL ABSORPTION PROCESS

T glycol= -6 to -10oC(low dew point)

T regenerate = 204oC

P = high

Glycol Absorption DehydrationMain Operating Parameter: Glycol concentration has to be highly concentrated to lower the dew point The lower the dew point to be reached, the greater the degree of regeneration

of glycol required The actual dew point reached in the absorber is bet. 6-10 deg C higher the

equilibrium dew point. The gap is called the approach cannot obtain directly by reboiling-rectification, need to regenerate at high

temperature (204oC) high T can caused glycol breakdown and formed of corrosive compound

Alternative Solution to Operate at T>204 deg low pressure by vacuum operation (risk of oxygen inducing flammable

mixture) fuel gas or natural gas (to decrease the water partial pressure , thus >ing

vaporisation iso-octane (non miscible substance)

Phenomena for Optimal Dehydration Unit Design Operation: Foaming Glycol Degradation

Phenomena for Optimal Dehydration Operation1. Foamingi. Glycol has a tendency to foam when in contact with foaming promoters such as

liquid HC, solid particles (compound produced by thermal degradation of glycol)

ii. develop in absorber, and can fill the entire columniii. top of column no effect on foam iv. the onset of foaming in absorber reflected by increase pressure drop in absorber

Prevention of foamingi. Installation of separator (to remove free water, liq. HC, solid particles)ii. Lean glycol inlet temperature need to be higher than temperature of natural gas

to prevent condensation of HCiii. TEG regeneration not exceeding its thermal degradation, 206 oCiv. Glycol filtering to extract foaming promoters:

i. Catridge filter (polypropylene) to remove solid particlesii. Activated carbon filter to remove HC and products of glycol degration

v. pH of solution maintain between 6 to 8; >8 there is a risk foamingvi. Injection of small quantity of an anti-foaming agent

Phenomena for Optimal Dehydration Operation2. Glycol degradation thermal degradation of TEG at 206oC onwards produce highly corrosive organic acid glycol solution absorbs acid gases, lead to decrease pH corrosion occur caused by presence of salt water lead to give erosion-corrosion that caused by high rate of glycol

circulation in piping/bends. Injection of a small amount of corrosion inhibitor, e.g. amines

Prevention of glycol degradation limit the temperature in reboiler to 204oC prevent air from enter unit pH need to be maintained above 6 to avoid TEG breakdown ensure satisfactory separation of free water injection a small amount of corrosion inhibitor

Dehydration by AdsorptionPrinciple: A physical process whereby a suitable porous solid with specific property is

able to fix water molecules on the surface of pores where water vapor condensed

Is the fixation of molecules by reversible reaction on the surface of a solid. Three different phenomena of adsorption:

Chemisorptionforming the first layer at low partial pressures.

Physisorptiondue to the formation of multiple layers by hydrogen bonding in the adsorbent pores.

Capillary condensationwhere localized condensation takes place at temperatures above that of the bulk fluids dew point.

Characteristic of Adsorbents have very large internal contact, 250-850 m2/g Possess a strong affinity for water vapor and a high capacity for adsorption Be easily and economically regenerable Undergo slight pressure drop under flow of gas Possess good mechanical strength

Adsorption Process Description Water adsorbs and condenses on the surface of adsorbent

Beyond the pure surface adsorption, a secondary mechanism, capillary condensation kicks-in when pore diameter is comparable to molecular diameter.

Pores in the adsorbent are asymmetrical, i.e. further down into the gel, the narrower the pore becomes (like a volcano crater).

This capillary condensation is driven by differences of partial pressure outside and inside of the pore.

To remove the water from the adsorbent surface, energy is used

After regeneration, the adsorbent can be put to use again after cooling to ambient temperature.

Schematic Representation of Capillary Condensation

ADSORBENT TYPES Activated alumina Alumina is the most widely used adsorbent because of:

The chemical properties of its surface Its ability to be shaped with well-defined pores defined as follows:

(Ultra) Macropores (> 1000 ) to enhance diffusion into the pore system. Mesopores (30 to 1000 ) to accommodate medium size molecules. Micropores (< 30 ) to accommodate small molecules like water.

Zeolite/Molecular Sieve Type A zeolite

With sodium cations has a pore width of 4 , called MS4A. Replacement of sodium by calcium cations leads to 5 pores, called MS5A. Replacement of sodium by potassium leads to molecular sieve MS3A.

Type X Zeolite Gives pores of 10 and the calcium type corresponds to MS13X.

Silica gel Activated carbon.

Types of Adsorbent activated alumina

SA=280 m2/g; pore volume=0.4 m3/g; pore diameter=2-4 nm; density= 720-820 kg/m3)

Reduce content by 1 ppmv Regeneration T= 150-220 oC

silica gel SA=550-800 m2/g; pore volume=0.35-0.5 m3/g; pore diameter=2.5 nm; density=

720-800 kg/m3) Reduce content by 10 ppmv Regeneration T= 150-250 oC

molecular sieves (or zeolites) Composition oxides of (Si, Al) and Na or K or Ca Zeolite 3A (K), Zeolite 4A(Ca); Zeolite 5A(Na); Zeolite X (10 A diameter) SA=650-800 m2/g; pore volume=0.27 m3/g; pore diameter=3-5 nm; density= 690-720

kg/m3) Reduce content by 1 ppmv Regeneration T= 200-300 oC

activated carbon

Adsorption of Water to the Alumina Surface

MOLECULAR SIEVES

Dehydration by Adsorption

Characteristic of a Good Adsorbent: should posses strong affinity for water vapor high capacity for adsorption process

capacity of adsorbents depends on their nature low value of relative humidity gives high capacity of molecular sieve

(ex: adsorbent) easily and economically re-generable undergo little drop in pressure posses good mechanical strength low dew point is required for cryogenic treatment of natural gas ability to adsorb heavy HC can show the selectivity of adsorbent

DEHYDRATION UNITS

ADSORPTION PHASE:

Gas Flow: From top to bottom

Adsorbent : Saturated with H2O

Polarity of H2O: Stronger

HC Molecule: Less Stronger

REGENERATION PHASE:

T: 200-300oC

P: Low

Adsorbent : Activated Al, Si Gel, Mol SievesStages of Regeneration: Heating Phase

Cooling Phase

Dehydration by AdsorptionProcess Description: using 2 columns called dryers packed with solid adsorbent involve 2 phase: adsorption phase & regeneration phase

1. Adsorption Phase gas flow through drier from top to bottom adsorbent saturated with water halted phase first before reach breakthrough point polarity of water much stronger attraction on adsorbent ejects hydrocarbon molecules that is less stronger

2. Regeneration Phase regenerate bed of adsorbent performed operation by increasing in T (200-300 oC) or lowering the P, or by a

combination of both can be performed by heating bed of adsorbent 2 stage of regeneration of drier: heating phase & cooling phase

Heating Phase: Hot air desorb the water from the adsorbent Cooling Phase: Drier is cooled at the end of the heating phase to the initial condition

Picture of EZ (Equilibrium Zone) & MTZ (Mass Transfer Zone)

Adsorption and Regeneration for PSA and TSA Processes

Pressure Swing Adsorption (PSA)

Temperature Swing Adsorption (TSA)

Block Diagram of Molecular Sieve Process

Field of Applications

Aging/Deactivation of Solid Adsorbents

A gradual reduction in adsorption capacity is caused by aging of the adsorbent.

Two types of aging exist;1. Hydrothermal aging

- an irreversible change of adsorbent structure caused by hydrothermal treatment during regeneration, resulting in reduced active area.

2. Aging from contamination- caused by co-adsorption of undesired species and coke formation on the active surface of the adsorbent. This phenomenon is not completely reversible, and carbon deposits increase with each regeneration

Isotherms of Activated Alumina, Silica and Molecular Sieves

Combination of Activated Alumina and Molecular Sieves

Isotherm Activated Alumina and 4A Molecular Sieves

Life Factor vs Number of Regenerations for Al2O3/ MS 4A /MS 4A in Natural Gas Drying

Industrial Adsorbent Problems

Deep dehydration Upstream process upsets, leading to dessicant

degradation (reduce adsorption capacity & pressure drop, hence frequent regeneration)

Causes: liquid (free) water or entrainment of amine deposition on the adsorbent, high CO2 concentrations, and heavy metal adsorption

Free water entrainment can lead to caking and powdering, which leads to >ed pressure drop and poor gas distribution