CHLORINE GAS LIQUIFICATION

 

CHLORINE GAS LIQUIFICATION

INTRODUCTION:

Chlorine is pale yellow-green gas that has its distinctive strong smell (the smell of bleach) and is a powerful oxidant. Chlorine is the single material on which production of other chemicals mostly depends. It is used in 60% of all Commercial Chemistry, 85% of all Pharmaceutical Chemistry and 95% of all yield enhancing or agrochemical chemistry. Chlorine can be manufactured by electrolysis of a sodium chloride solution (brine). The production of chlorine results in the co-products caustic soda (sodium hydroxide, NaOH) and hydrogen gas (H2). These two products, as well as chlorine itself, are highly reactive.

 There are three industrial methods for the extraction of chlorine by electrolysis of sodium chloride solutions, all proceeding according to the following equations:

Cathode: 2 H⁺ (aq) + 2 e⁻ → H₂ (g)

Anode: 2 Cl⁻ (aq) → Cl₂ (g) + 2 e⁻

Overall process: 2 NaCl + 2 H₂O → Cl₂ + H₂ + 2 NaOH

1.     Mercury cell electrolysis

2.     Diaphragm cell electrolysis

3.     Membrane cell electrolysis

Electrolysis of brine in a diaphragm or membrane cell or in a Mercury cell gives chlorine gas at the anode. This gas leaving at the anode is hot 79 ⁰C to 93⁰C and saturated with water. Since it is wet, it is also very corrosive. It is fed to glass or fiberglass reinforced polyester materials to heat exchangers for cooling.

Four major steps involve in the liquefaction of chlorine gas that are:

1)    Cooling and drying

2)    Chlorine compression

3)    Chlorine liquefaction

4)    Liquid chlorine storage

COOLING AND DRYING:

Chlorine gas exiting the cell line must be cooled and dried since the exit gas can be over 80 °C (176 °F) and contains moisture that allows chlorine gas to be corrosive to iron piping. Cooling process allows for a large amount of moisture from the brine to condense out of the gas stream. Cooling also improves the efficiency of both the compression and the liquefaction stage.

 

After passing through shell and tube heat exchanger, chlorine gas is cooled and fed to chlorine washer. In washer, sulfuric acid is fall from the top and chlorine gas come form bottom (counter current fashion to minimize the consumption of acid).Sulfuric acid is used for drying the gas ,during this process sulfuric acid become hot and send to double pipe heat exchanger and then again to washer(recycle). Same procedure is carrying out for second washer to remove moisture and impurities. Finally this gas is send to demister (remove suspended particles or entrained acid).From the drying system, the chlorine gas is piped to the chlorine gas compressor.

CHLORINE COMPRESSION:

Chlorine compressor is a mechanical device that increases the pressure of a Cl₂ gas by reducing its volume. Compressors are similar to pumps, both increase the pressure on a fluid and both can transfer fluid through a pipe. Several method of compression may be used: liquid ring, reciprocation, or centrifugal.

Main components:

1)    Compressor

2)    Motor

3)    Acid Separator

4)    Heat Exchanger and Surge Drum

5)    Strainer

6)    Inter connecting Pipelines and valves

7)    Safety valve & Instruments

In chlorine compression liquid piston rotary compressor mostly used constructed of iron with concentrated sulfuric acid as the sealing liquid. For larger capacity and higher pressures, centrifugal and non-lubricated reciprocating compressors are chosen. It contains two meshed rotating positive-displacement helical screws to force the gas into a smaller space.

Specification:

Power 2.2kw to 890kw

Pressure > 1200psi or 8.3 Mpa

 

In the process of Compression, chlorine gas is compressed through suction separator and then sent to discharge separator. Here sulfuric acid is used as lubricant in compressor which is recycling for cooling purpose. Three filters are used to protect against dirt particles in the chlorine gas flow and to separate liquid drops from the chlorine gas flow. First two filters are in parallel arrangement and third one is in series. These filters have Teflon filter membranes that can withstand the most severely corrosive condition at temperature as high as 260 0C and down to the cryogenic range. The membrane is hydrophobic and maintains its strength wet or dry.

CHLORINE LIQUIFICATION:

After compression, chlorine gas flows to the liquefiers, where it is cooled enough to liquefy. This cooling is performed by means of a closed-loop compressor based refrigeration system. Non condensable gases and remaining chlorine gas are vented off as part of the pressure control of the liquefaction systems. A typical chlorine gas composition falls in following:

Cl2      97 to 99.5%

O2      0.5 to 2.0%

H2       0.03 to 0.3%

The refrigeration system for actual chlorine liquefaction consists of components such as compressor, condenser, receiver, vertical liquefier, controls, accessories and piping.  Liquefaction of compressed chlorine gas is done in liquefier where Freon (chlorofluorocarbon) gas is used for cooling agent. The gaseous refrigerant (Freon gas) is compressed in centrifugal compressor, liquefied by water cooling and then sends to liquefier where it evaporates after absorbing heat and cooling the chlorine. Freon compressor uses the sudden expansion phenomena that the pressure decreases from 18 to 5 bars and the temperature of Freon gas decreases as low as -142 degree Celsius. Freon gas is use in recycle process, when Freon is evaporated in liquefier then it is send to Freon tank again and then for condensation and expansion.

The main parts of Liquefaction units are:

1.     Evaporator (chlorine condenser) is a vertical shell and tube heat exchanger. Chlorine is on the tube side and liquid Freon on shell side. Chlorine gas transfer heat to Freon gas and converted into liquid. On the other hand Freon vapor formed and send condenser for recycle process.

2.      The compressor compresses the Freon gas generated in the evaporator. The compressor is driven.

3.     Condenser use water to condense the Freon gas from compressor

4.     The economizer flashes of Freon and returns vapor to the compressor in order to sub-cool the liquid Freon.

5.     Purge unit pulls non-condensable out of the Freon system and discharges to the atmosphere

6.     The chiller uses Freon evaporation to chill the water for the chilled water system.

LIQUID CHLORINE STORAGE:

Chlorine is a very active chemical which combines directly with much other. Pure chlorine, both as a gas and as a liquid under pressure, reacts with only a few metals at ordinary temperatures. For this reason, chlorine may be safely stored in metal containers. Moist chlorine, however, corrodes most metals rapidly and a solution of chlorine in water has powerful oxidizing, bleaching and germicidal properties. Glass, ceramics, hard rubbers and some plastic are used to resist the moist chlorine. Liquid chlorine is supplied in steel tanker, cylinder containing 1 ton, and in standard cylinders containing approximately 70kg about 5 atm.

The following are procedures for safely handling chlorine cylinders:

1)    Move cylinders with a properly balanced hand truck, with clamp supports that fasten at leas two-thirds of the way up the cylinder.

2)    Always replace the protective cap when moving a cylinder.

3)    Store cylinders in an upright position. 

Material handling:

Need to wash thoroughly after handling. Avoid breathing the vapors. Vacate poorly ventilated areas as soon as possible. Do not return until the strong odors have dissipated. Open the cylinders with care and do not intake internally.

Application of Chlorine:

1)    Water disinfection and treatment

2)    Paper and board

3)    Cooling towers

4)    Production of plastic, such as PVC

5)    Pharmaceuticals formulation

6)    Agro-Chemical production (insecticides, herbicides & fungicides)

7)    Production of chlorinated solvent, hydrocarbons and derivatives.

 

Posted in: Chlorine liquification