NATURAL GAS PROCESSING&

LNG

By

Muhammad Sharique Khan26, September

2009

Introduction

Objectives of Natural Gas Processing & LNG :

Purification of CO2, H2S, H2O, etc

Hydrocarbon recovery like liquefied petroleum gas (LPG), ethane.

Upgrading N2 rejection and Helium recovery.

Huge reduction in Volume, a factor of 650 and decreasing the size and cost of storage and transportation.

Composition of some Natural Gases

Maximum Allowable Impurities Levels in Natural gas.

Purification A Purification is the process where the material

removed from the gas stream is discarded e.g. water.

Recovery process is one where the material removed has economical value e.g. helium or ethane.

Impurities are removed to: Obtain a desired product purity. Prevent the deposition of solids. Remove an explosion hazard. Prevent the blanketing of heat transfer surfaces by non

condensable gases. Prevent Corrosion.

Methods of Impurity Removal The methods used to remove specific impurities could be

divided into two classes:

External : Processes that are carried out at near-ambient temperatures or ahead of the cold box.

Internal: Purification steps that take place after the feed gas has entered the low-temperature system.

On the Basis of Volatility of impurity:

High boiling impurities like high boiling HC, organic sulfur compounds like CS2, mercaptanes , SO2, Water.

Low boiling impurities like N2, CO, Ar and Methane

Classification of Impurity Removal Process

Condensation to a liquid by increase of pressure at constant temperature

Condensation to liquid or solid by cooling at constant pressure

Absorption by a liquid Purely physical With chemical reaction

Adsorption by a solid Physical With chemical reaction

Chemical reaction.

Permeation through a membrane.

Mechanical separation.

Water Removal- Dehydration

Natural feed to baseload plants can contain as much as 1600 kg H2O/106m3.

The removal of water vapor is accomplished either by absorption in a liquid or by adsorption on a solid.

The absorption processes generally use diethylene glycol or triethylene glycol as the drying liquid in a countercurrent absorber.

Glycol Water Absorption Process

Molecular Sieve Dehydration Process

Sieves Adsorption is used to produce very low water content in the natural gas due to the greatest affinity for water due to their natural structure.

Sieves can effectively remove acid gases like H2S and CO2 with water.

Over 75% of all Cryogenic hydrocarbon recovery plants rely on molecular sieve for dehydration.

Dehydration to dew points of 200K(-100-105 F) is accomplished through the use of molecular sieve.

Disadvantages of Sieves dehydration: High capital costs, limitation of maximum economical throughput Danger of adsorbent contamination and undesirable co adsorption Need for source of clean, dry reactivation gas Possibility of adsorbent dusting

Molecular Sieve Dehydration Process

Two-Stage Adsorption &

Purification

Four-tower molecular sieve system is used

Two towers (dryers) for removing water from the total inlet natural gas stream.

Two towers (purifiers) for removing carbon dioxide from the portion of gas being liquefied

Advantages of four-tower, two-stage system

The first-stage absorbers can be efficiently designed for water and H2S removal, taking advantage of the high adsorption capacity of the adsorbent for these impurities.

Second-stage absorbers can be designed specifically for the higher concentrations and lower adsorptive capacity of CO 2. The adsorbed thus handles a smaller quantity of feed gas, thereby reducing overall system requirements

Four-tower Molecular Sieve system for Natural Gas Dehydration & CO2 removal.

Removal of CO2 & H2S Inlet natural gas typically contains 0.5–2.0 vol % CO2.

Local freeze out of CO2 occur at low temp.

CO2 content of the feed gas usually kept below 50 ppm.

Hydrogen sulfide is removed for two reasons To eliminate an undesirable component in the gas stream. To recover the H2S as elemental sulfur.

The H2S and CO2 removal operations may be roughly grouped into three categories: Processes using a reversible chemical reaction. Processes using physical absorption. Processes employing fixed beds of solids.

Amine system for natural gas CO2& H2S removal

Mercury Removal Mercury concentrations ranging from 0.001 to 180 mg/m3

exist in all natural gas streams.

Mercury is responsible for aluminum corrosion so it should be removed from the feed gas.

Mercury is first converted to organic and inorganic form because their presence has no problem in the plants and removed easily.

Mercury removal in this process is based on the high reactivity of mercury with sulfur and its compounds; less than 10 ng/m3 is obtained with a high space velocity.

LIQUEFACTION OF NATURAL GAS

After the Natural Gas has been treated to remove impurities the next step is liquefaction.

Three basic liquefaction cycles are:

The classical cascade cycle

The expander cycle

The mixed-refrigerant cascade cycle

Cascade Process Flowsheet

The Classical Cascade Cycle

Typically, a propane–ethylene–methane cascade combination is used with the natural gas being cooled through the vaporization of each component in turn.

By vaporizing each refrigerant at two or three pressure levels, the efficiency of the natural gas cooling process is increased, but so is the complexity of the entire liquefaction cycle.

The rapid increase in power requirement with decreasing temperature is a key consideration in the design of natural gas liquefaction units.

Expander Cycle

The Expander Cycle

Expander cycle uses the cooling effect obtained by expansion of the natural gas to liquefy a portion of the natural gas.

Expansion turbines are used in this process, with liquefaction occurring upon expansion through a Joule–Thomson valve.

Closed-cycle mixed-refrigerant cascade

Mixed-Refrigerant Cascade Cycle MCR Cascade cycle based on the same principle as the

classical cascade cycle.

A single refrigeration loop complete with their own compressor is used .

If composition of feed changes the composition of the mixed refrigerant must also be changed to reduce thermodynamic irreversibility.

The mixed refrigerant is repeatedly condensed, vaporized, separated and expanded.

Major problem with MCR cycle is that problem occur in handling two-phase multicomponent mixtures in the heat exchangers.

Thank you

For Details contactShariq1petro@gmail.com