General Features & Process Description
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Heavy Water Plant Thal is the first of second-generation plants in India and is made completely with indigenous efforts. The vast technical resources available with Heavy Water Board with the experience gained by commissioning and sustained operation of the earlier plants were utilised for setting up of the plant. It is located at Thal-Vaishet village in Raigad district of Maharashtra and is about 100 kms south of Mumbai on National Highway No.17 It is about 20 kms away from Pen railway station of Panvel - Roha section. The site is also accessible from Gateway of India, Mumbai by Catamaran services. HWP Thal was commissioned in 1987. Setting up of the plant was a major task as a number of components were to be developed for the first time indigenously. Some of them were special tower internals, canned motor pumps, glass seals, mass spectrometers, multilayer vessels etc. and this involved considerable pre-project effort by Heavy Water Board.

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Process Description

The plant comprises of two streams consisting of two separate isotopic exchange units, final enrichment units, final production units & cracker units, but a common ammonia synthesis unit. Feed synthesis gas (a mixture of one part of nitrogen and three parts of hydrogen containing deuterium from the Ammonia plant is routed through the plant at a flow rate of about 96 T/Hr. at a pressure of about 180 kg/cm².

The pressure of the gas is first raised by 40 kg/sq.cm. by a booster, to take care of pressure drop. It is then cooled in a heat exchanger by the outgoing cold gas returned from the plant. The gas is thereafter passed through a purification unit. In this unit the oxygenated impurities contained in the gas are removed and the gas is saturated with ammonia.

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The purified synthesis gas saturated with ammonia is then passed through the first isotopic exchange tower working at –25°C where deuterium in the gas is transferred to a counter current stream of liquid ammonia containing potassium amide catalyst fed from the top of the tower. The deuterium enriched ammonia from the bottom of the exchange tower is then fed to the second isotopic exchange tower where it gets further enriched by coming in contact with the enriched synthesis gases obtained by cracking of enriched ammonia. A part of the enriched gas and liquid from the second isotopic exchange tower is then taken to the final enrichment section where the concentration of deuterium in the ammonia can be further increased as desired upto 99.8%. Finally, the enriched ammonia so obtained is made free of the catalyst and is cracked. A portion of this enriched synthesis gas is burnt to produce heavy water. However, for reasons of better recovery efficiency the concentration of deuterium in ammonia in the final enrichment section is kept low so as to produce heavy water of about 60% which is then vacuum distilled to produce heavy water of nuclear grade.

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The cold synthesis gas depleted in deuterium from the first exchange tower, before its return to Ammonia Plant, is heated by the incoming feed gas from the same plant and is passed through an ammonia synthesis unit. In this unit, a portion of the synthesis gas equivalent to the amount of the ammonia cracked in the crackers is converted into liquid ammonia and fed to the top of first exchange tower. The rest of the depleted synthesis gas equivalent to the feed gas supplied is returned to the ammonia plant. The power to the plant is supplied by Maharashtra State Electricity Board from their 100 KV grid. The fuel for heating of the crackers of the plant is natural gas, which is supplied by GAIL.This plant is operated and maintained under an O & M agreement.

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Some of the special features of the plant are:
  • HWP (Thal) is the first heavy water plant in the country based on the ammonia-hydrogen exchange process, which has been set up without foreign collaboration. The basic design of isotopic exchange unit was developed in-house
  • The plant adopts the latest technology using micro-processor based distributed digital control system for instrumentation and process control instead of the conventional pneumatic analogue system. The system provides supervisory and monitoring functions for feed control stations. It displays and records data acquired from the plant system and has facilities for data logging and printing alarm messages.
  • This plant comprises of two streams with some common units like synthesis unit, sub-station, inert gas unit, instrument air unit, catalyst preparation unit, cooling towers etc.
  • Completion of the project has been successfully achieved without any time overrun
Salient Data of Heavy Water Plant (THAL)
Process Used Ammonia-Hydrogen Mono-thermal Exchange
Process Know How Departmental
Capacity of the Plant 78 MT / Year
Date of Commissioning February 1987
Capital Cost of the Plant Rs.15600 lakhs
Operating Pressure 220 kg/cm²
Synthesis Gas Flow Rate 96 Te/Hr (48 Te/Hr in each stream)
Total Weight of Structure 7600 Te
Total Weight of Equipments 4400 Te
Weight of Heaviest Equipment 425 Te approx.
Maximum Height of Towers 40 M approx.
Total Length of Electrical Cables 330 KM

Last updated on: 07-Feb-2019