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Gas hydrate based wastewater treatment technology

  • Gas hydrate based wastewater treatment technology

About the blog

Amit Katyal
Amit Katyal, an independent inventor/researcher based in New Delhi, INDIA, has developed 4 technologies for hydrocarbons/water treatment sector and numerous oil and gas softwares. His research work is featured on www.eq-comp.com website.
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Water pollution is a big and ever-growing environmental problem. Highly polluted water coming from various industries like oil and gas industry, pharmaceutical industry, paper and pulp industry, dyeing industry and tanneries is sometimes extremely difficult to treat. Reverse-osmosis based wastewater treatment systems cannot treat this highly polluted water because of requirement of very high pressure for pushing the polluted water through reverse-osmosis membranes. Also, costly pretreatment of polluted water is required to avoid fouling of reverse-osmosis membranes making such systems economically non-viable.

Distillation based wastewater treatment systems require very high energy to treat polluted water due to high latent heat of vaporization of water. Also, separate distillation-based wastewater treatment systems are required for removing dissolved solids and miscible liquids from industrial wastewater with expensive tall vertical distillation columns required to remove miscible liquids from industrial wastewater. Freezing based wastewater treatment systems have been found to be technically infeasible on commercial scale due to difficulty in separation of polluted water from ice. Also, low temperature requirement for operation of freezing based wastewater treatment systems make such systems highly energy consuming and thus economically non-viable. Besides being techno-economically non-viable, no existing wastewater treatment system can be used to treat all types of wastewater coming from different industries universally. Therefore, a techno-economic universal solution for treatment of highly polluted water coming from various industries is actively being sought.

The present article describes a new gas hydrate-based wastewater treatment technology which can fill this void of a universal techno-economic solution for treatment of highly polluted water coming from various industries.

Gas hydrate-based wastewater treatment systems can be used to treat highly polluted water techno-economically without requiring very high pressures or costly pretreatment of water

Gas hydrates are inclusion compounds of low molecular weight gases like Oxygen, Hydrogen, Nitrogen, Carbon dioxide, Methane, Hydrogen sulfide, Argon, Krypton and xenon with water. Some higher hydrocarbons like Ethane, Propane and Butane and some Chloro-fluoro carbons (CFCs) also form gas hydrates. One unit of gas hydrates consists of single molecules of these gases existing in a cage formed of water molecules. Three types of gas hydrates have been observed to be formed namely – S I or Structure one, S II or Structure two and S H or Structure H. Gas hydrates are similar in structure to ice but unlike ice they are stable at temperatures higher than 0 C/ 32 F (freezing point of water) and at pressures above atmospheric pressure. Methane hydrates found in nature burn with a flame when subjected to fire. Because of this, Gas hydrates are also called “Ice that burns”. Gas hydrates are undesirable in subsea crude oil pipelines where suitable temperature and pressure conditions for formation of gas hydrates exist. Gas hydrates form in subsea crude oil pipelines resulting in difficult to remove blockages in the pipeline requiring shut down of the operations and loss of time, money and manpower. However, gases can be stored in the form of gas hydrates as the volume of gases decrease by around 160 times in gas hydrate phase. Also, when formed out of polluted water, gas hydrates expel the pollutants, leaving behind the pollutants in water. It is this property of gas hydrates which is used in the present gas hydrate based wastewater treatment technology to universally treat highly polluted water coming out of various industries like produced water from oil and gas wells, wastewater from refineries, wastewater from pharmaceutical industry, wastewater from paper and pulp industry, wastewater from dyeing industry, wastewater from tanneries etc.

Unlike reverse-osmosis based wastewater treatment systems, gas hydrate-based wastewater treatment systems can be used to treat highly polluted water techno-economically without requiring very high pressures or costly pretreatment of water. Gas hydrate-based wastewater treatment systems can be used to separate both dissolved solids and miscible liquids in a single design which is not possible in distillation-based wastewater treatment systems. Also, gas hydrate-based wastewater treatment systems are much more economically viable than distillation-based wastewater treatment systems as latent heat of formation of gas hydrates is just around 20 % of latent heat of vaporization of water. As gas hydrates are formed at temperatures higher than ice forming temperatures so gas hydrate-based wastewater treatment systems present an advantage over long existing but techno-economically non-viable freezing based wastewater treatment systems.

The present gas hydrate-based wastewater treatment technology uses the concept of formation and dissociation of gas hydrates to effectively treat highly polluted water coming out of any industry. The treatment process consists of the following steps:

  • Formation of gas hydrates
  • Removal of concentrated polluted water
  • Washing of hydrate crystals to remove pollutants on the surface of hydrate crystals
  • Removal of hydrate wash water
  • Dissociation of gas hydrates
  • Removal of less polluted water

The complete treatment process, including all process steps, is carried out in a single pressurized tank avoiding separate tanks for separate steps resulting in very low capital cost. Another tank is used to store hydrate former, to transfer hydrate former to the treatment tank during hydrate formation and to receive the hydrate former back from the treatment tank during hydrate dissociation. It is desirable to store the hydrate former in the liquid phase in the storage tank so that it occupies less volume. After extensive literature survey, it has been found that rapid formation of gas hydrates takes place when microbubbles of hydrate former are made to pass through water. Therefore, microbubbles of hydrate former are formed using a microbubble generator and are released near the bottom portion of the desalination tank. The vapor spaces of the storage tank and the treatment tank are connected using a connecting pipe allowing free flow of hydrate former in vapor phase between the storage tank and the treatment tank and maintaining same pressure in the storage tank and the treatment tank. The hydrate former in vapor phase for formation of microbubbles is taken from the vapor space of treatment tank by the microbubble generation system.

Any type of microbubble generating system capable of producing microbubbles at the desired rate under the operating temperature and pressure conditions of the treatment tank can be used. A venturi-based microbubble generation system takes polluted water from the treatment tank and pumps it through a venturi nozzle creating low pressure in the venturi nozzle. Venturi nozzle is connected to the vapor space of treatment tank through a pipe. Low pressure in the venturi nozzle results in sucking of hydrate former vapors from the vapor space of treatment tank which are bubbled in the polluted water stream flowing in the venturi nozzle through numerous very small tubes. The flow of polluted water shears these bubbles of hydrate former from the tubes before they reach their maximum size resulting in formation of microbubbles of hydrate former. These microbubbles are transferred back into the treatment tank along with the stream of polluted water being pumped through the venturi nozzle.

Ethane is used as the preferred hydrate former for the present gas hydrate-based wastewater treatment technology as it has been found after detailed analysis that ethane hydrates require the most lenient temperature and pressure conditions (high temperature and low pressure) among commonly available hydrate forming gases to form gas hydrates. Constant temperature is maintained in the storage tank so as to maintain constant operating pressure in the storage tank and the treatment tank. This constant temperature is fixed such that the minimum hydrate formation temperature desired in the treatment tank at the constant operating pressure is suitably higher than the constant temperature maintained in the storage tank so that ethane vapors do not condense in the treatment tank, ethane vapors are suitably sucked by the venturi nozzle forming microbubbles and the complete process functions smoothly. For example, for treatment of near eutectic NaCl brine, ethane is stored in liquid phase in the storage tank at around -9 C (264 K) and a corresponding pressure of 19 bars. Ethane hydrate formation temperature for near eutectic NaCl brine at 19 bars pressure is -3 C (270 K) which is suitably higher than the storage tank temperature of -9 C (264 K) ensuring smooth functioning of the process. Ethane liquefaction temperature values and ethane hydrate formation/ dissociation temperature values from NaCl eutectic brine for different pressure values is shown in Table 1 and displayed graphically on Figure 1.

 TABLE 1

                                                                                                                    FIGURE 1

Similarly, storage tank temperature and operating pressure of process can be increased or decreased depending on higher or lower minimum ethane hydrate formation temperature required in the treatment tank. These low temperatures in the storage tank and the treatment tank are maintained using a suitable refrigeration system. Maintaining a high storage tank temperature implies a low energy consumption by the refrigeration system and the process and vice-versa. Other hydrate formers like propane and carbon dioxide or a mixture of hydrate formers can also be used in the process but use of ethane provides the maximum flexibility, simplest operation and best economic benefit for the process. The above described apparatus is shown in Figure 2.

                                                                                                                    FIGURE 2

A unique and innovative temperature control system has been suggested for the gas hydrate-based wastewater treatment system to maintain the temperature of the storage tank at a lower value than the minimum hydrate formation temperature of the treatment tank.

Both the treatment tank and the storage tank have internal or external heat transfer arrangement in the form of internal or external heat transfer coils. Heat transfer liquid at suitable temperature is pumped into these coils to maintain the temperatures of the treatment tank and storage tank at suitable values. For treatment of near eutectic NaCl brine, the temperature of the storage tank is maintained at -9 C by pumping heat transfer liquid in heat transfer coil whereas heat transfer liquid at a different temperature is pumped through heat transfer coil of treatment tank so as to lower the temperature of the treatment tank from ambient temperature to a minimum value of around -3 C during formation of gas hydrates in the treatment tank. As ethane gas is transferred from the storage tank to the treatment tank during formation of gas hydrates in the treatment tank so ethane gas is evaporated in the storage tank requiring heating of storage tank and used to form gas hydrates in the treatment tank requiring cooling of treatment tank. Similarly, for treatment of near eutectic NaCl brine, the temperature of the storage tank is maintained at -9 C by pumping heat transfer liquid in heat transfer coil whereas heat transfer liquid at a different temperature is pumped through heat transfer coil of treatment tank so as to raise the temperature of the treatment tank from around -3 C to ambient temperature during dissociation of gas hydrates in the treatment tank. As ethane gas is transferred to the storage tank from the treatment tank during formation of gas hydrates in the treatment tank so ethane gas is condensed in the storage tank requiring cooling of storage tank and released from dissociation of gas hydrates in the treatment tank requiring heating of treatment tank. Similar quantities of heat are absorbed from treatment tank during formation of gas hydrates and absorbed by treatment tank during dissociation of gas hydrates. Also, similar quantities of heat are absorbed by storage tank during formation of gas hydrates and absorbed from storage tank during dissociation of gas hydrates.

A unique thermal coupling of two gas hydrate based wastewater treatment systems operating out of phase with each other such that when gas hydrates are being formed in one of the gas hydrate based wastewater system, gas hydrates are being dissociating in the other gas hydrate based wastewater treatment and vice-versa, is achieved by using a single heat pump type refrigeration system. The heat pump type refrigeration system consists of a refrigeration compressor and an expansion valve and a four-way valve to change the direction of flow of the refrigeration fluid. The treatment tank of gas hydrate based wastewater system in which gas hydrates are being formed and the storage tank of the gas hydrate based wastewater treatment in which gas hydrates are being dissociated are cooled in series using the same heat pump type refrigeration system whereas the waste heat of the heat pump type refrigeration system is used to heat the storage tank of gas hydrate based wastewater system in which gas hydrates are being formed and treatment tank of gas hydrate based wastewater system in which gas hydrates are being dissociated in series. Four-way valve is used to change the direction of flow of the refrigeration fluid in the heat pump type refrigeration system after the desired amount of gas hydrates are formed in one of the gas hydrate based wastewater system and all of the hydrates are dissociated in the other gas hydrate based wastewater system. The above described arrangement is shown in Figure 3.

                                                                                                                              FIGURE 3

A single heat pump type refrigeration system is required for temperature control of two gas hydrate-based wastewater systems operating simultaneously and out of phase with each other resulting in substantial saving in capital cost of the thermally coupled gas hydrate-based wastewater systems. Also, waste heat of the heat pump type refrigeration system is utilized in dissociation of gas hydrates in the treatment tank of one of the gas hydrate-based wastewater treatment system and evaporation of ethane in the storage tank of the other gas hydrate-based wastewater treatment system resulting in very low operating cost of the thermally coupled gas hydrate based wastewater systems.

Instead of thermally coupling two gas hydrate-based wastewater treatment system, one gas hydrate-based wastewater treatment system can be thermally coupled with an ice tank using a heat pump type refrigeration system. Latent heat of freezing for water is comparable to heat of formation of gas hydrates. Therefore, the treatment tank of gas hydrate based wastewater system can be cooled using heat pump type refrigeration system during formation of gas hydrates and the waste heat of the heat pump type refrigeration system can be used to heat the storage tank to evaporate liquid ethane and melt ice to form water in the ice tank in series. Four-way valve is used to reverse the flow of refrigeration fluid in the heat pump type refrigeration system after desired amount of gas hydrates are formed in the treatment tank of the gas hydrate-based wastewater treatment system. Now, storage tank of gas hydrate-based wastewater treatment system is cooled to condense ethane gas and water is frozen to form ice in the ice tank in series using the heat pump type refrigeration system and the waste heat of the heat pump type refrigeration system is used to dissociate gas hydrates in the treatment tank of the gas hydrate-based wastewater treatment system. The above described arrangement is shown in Figure 4.

                                                                                                                       FIGURE 4

This type of thermally coupled system is low in capital cost but high in operating cost compared to two thermally coupled gas hydrate-based wastewater treatment systems. Therefore, this type of thermally coupled system is used when less expensive infrastructure is required and electricity cost is low. Also, thermal coupling of ice tank with a gas hydrate-based wastewater treatment system can be done during start-up and shut-down of two thermally coupled gas hydrate-based wastewater treatment systems for effective operation.

Also, two gas hydrate-based wastewater treatment system operating out of phase with each other can be connected operationally where two treatment tanks operating out of phase with each other are connected to a single storage tank. In an operationally connected system, when gas hydrates are being formed in one treatment tank, gas hydrates are being dissociated in the other treatment tank and when ethane gas is being transferred from the storage tank to the treatment tank in which gas hydrates are being formed, ethane gas is being transferred to the storage tank from the treatment tank in which gas hydrates are being dissociated. Therefore, a single storage tank and lesser quantity of ethane is required in such a system with the storage tank acting as a buffer to store or release any additional quantity of ethane gas. The above described arrangement is shown in Figure 5.

                                                                                                                           FIGURE 5

This type of operationally connected system is low in capital cost compared to two stand-alone gas hydrate-based wastewater treatment systems operating simultaneously as it will require a single storage tank and lesser quantity of ethane but has the same treatment capacity as two stand-alone gas hydrate-based wastewater treatment systems operating simultaneously.

Two treatment tanks operating out of phase with each other can be directly connected operationally to each other without any storage tank. Initially, specified quantity of liquid ethane is filled in one of the treatment tanks and is transferred to the other treatment tank during formation of gas hydrates till the complete liquid ethane is evaporated. Now, water to be treated is filled in the empty treatment tank and gas hydrates are dissociated in the other treatment tank. This results in transfer of ethane gas to the first tank where this ethane gas is converted to gas hydrates. So, when gas hydrates are being dissociated in one treatment tank, gas hydrates are being formed in the other treatment tank resulting in no requirement of storage tank. The above described arrangement is shown in Figure 6.

                                                                                                                     FIGURE 6

This type of operationally connected system is low in capital cost compared to both two stand-alone gas hydrate-based wastewater treatment systems operating simultaneously and operationally connected system with two treatment tanks and one storage tank as no storage tank is required in this type of system.

A gas hydrate-based water treatment system for treatment of near eutectic NaCl brine as per the above described technology details has been designed and simulated using MS Excel and VBA. The energy consumption for treatment of near eutectic NaCl brine designed and simulated as per the above described technology details is calculated to be around 0.05 kwh per litre of desalinated water produced. A US patent number 9643860 B2 and an Indian patent number 306391 has been granted to the technology.

Reference:

1. Masahoshi Takahashi et al., “Effect of Shrinking Microbubbles on Gas Hydrate Formation”, The Journal of Physical Chemistry, 107, No. 10, 2003

2. Kyeong-nam Park et al., “A new apparatus for sea water desalination by gas hydrate formation and removal characteristics of dissolved material”, Feb 23, 2011, Desalination, Vol 274, pp 91-96

3. E. Dendy Sloan, Jr., Carolyn Koh, Clathrate Hydrates of Natural Gases, Third Edition

4. Phelp et al., US patent publication number 2007/0004945 A1, 1/2007

5. Skejetne et al., US patent number 7,794,603 B2, 9/2010

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