Gas hydrates carbon dioxide

FORMATION OF CARBON DIOXIDE GAS HYDRATES IN FREEZING SEDIMENTS AND DECOMPOSITION KINETICS OF THE HYDRATES FORMED... 

Precipitated Calcium Carbonate. Precipitated calcium carbonate can be produced by several methods but only the carbonation process is commercially used in the United States. Limestone is calcined in a kiln to obtain carbon dioxide and quicklime. The quicklime is mixed with water to produce a milk-of-lime. Dry hydrated lime can also be used as a feedstock. Carbon dioxide gas is bubbled through the milk-of-lime in a reactor known as a carbonator. Gassing continues until the calcium hydroxide has been converted to the carbonate. The end point can be monitored chemically or by pH measurements. Reaction conditions determine the type of crystal, the size of particles, and the size distribution produced. 

One of the major uses of DTA has been to follow solid-state reactions as they occur. All decomposition reactions (loss of hydrates, water of constitution, decomposition of inorganic anions, e.g.- carbonate to carbon dioxide gas, etc.) are endothermic and irreversible. Likewise are the synthesis reactions such as... 

Linga, P. Kumar, R. Englezos P. (2007a). Gas Hydrate formation from Hydrogen/carbon dioxide and nitrogen/carbon dioxide gas mixtures, accepted for publication in Chem. Eng. Sci., April 26, 2007... 

Carbon dioxide gas dissolves readily in water and is spontaneously hydrated to form carbonic acid, which rapidly dissociates to a proton and a bicarbonate ion ... 

Clathrate hydrates have been found to occur naturally in large quantities. Around 120 X 10 m (at STP) of methane is estimated to be trapped in deposits of the deep ocean floor [10]. Clathrate hydrates are also suspected to occur in large quantities on some outer planets, moons, and trans-Neptunian objects [11]. In the petroleum industry, hydrocarbon clathrate hydrates are a cause of problems because they can form inside gas pipelines, often resulting in plugging. Deep sea deposition of carbon dioxide clathrate hydrate has been proposed as a method to remove this greenhouse gas from the atmosphere and control climate change [12]. 

The other metal oxides, which are basic, remain as solids. The solution containing the aluminate ion (A102 ) is separated from the sludge of other oxides and is acidified with carbon dioxide gas, causing the hydrated alumina to reprecipitate ... 

Figure 3.9 Behaviour of carbon dioxide in sea-water, (a) Zones of stability of carbon dioxide gas, liquid and hydrate as a function of temperature and depth. The liquid is stable at pressures (depths) below the dotted line and the hydrate is stable in the shaded area, (b) Density of liquid carbon dioxide and of sea-water at great depths.

Figure 3.10 Schematic representation of ocean storage options. In dissolution type ocean storage, the carbon dioxide rapidly dissolves in the ocean water, whereas in lake type ocean storage, the carbon dioxide is initially a liquid on the sea floor, soon crystallizing as a hydrate. Given sufficient time, all forms of carbon dioxide - gas, liquid, hydrate - will dissolve in the water.

The flow diagram for the Solvay process is shown in Figure 21.4. Limestone calcined with coke is used to produce carbon dioxide (Equation 21.1) and calcium oxide for the recovery of ammonia (Equations 21.1, 21.3, 21.9). The brine solution is then saturated with ammonia and carbon dioxide gas to produce ammonium bicarbonate, which then reacts with the salt to form sodium bicarbonate and ammonium chloride. The sodium bicarbonate, which precipitates, is filtered and calcined at 175-225°C to produce light soda ash, having bulk density in the range 0.51-0.62 g/mL. Dense soda ash, with bulk density of 0.76-1.06 g/mL can be produced by hydrating light soda ash. 

Precipitation To the filtered sodium aluminate solution, fine gibbsite crystals are added. Gibbsite is a naturally occurring hydrated alumina [a-Al(OH)3], and its crystals act as seeds for the precipitation of A1(0H)3 from the solution. The pH of the solution is increased by bubbling carbon dioxide gas through it. This increases the precipitation. Washing The precipitate is separated from the solution by filtration. This is then washed to reduce its sodium content. 

Ordinary sodium silicate and calcium silicate are, in fact, metasilicates. Sodium silicate may be made by heating sodium carbonate, Na2C03, with pure sand, Si02, in a furnace to form a glass. Window glass (soda glass) is made by a continuous process in which sand reacts at 1 400 °C with fused carbonates of sodium, potassium and calcium (limestone and some sodium sulfate). At these temperatures the carbonates behave as a mixture of the oxide and carbon dioxide gas (for instance, CaO + CO2). The liquid is stirred by the evolution of CO2, H2O (from the hydrated salts) and SO3. 

Catacarb process An extraction process used to remove carbon dioxide from process gases by scrubbing the hot gases with potassium carbonate solution containing additives which increase the hydration rate of the gas in the solution. The Vetrocoke process is similar. See Benfield process. 

To prepare gas for evacuation it is necessary to separate the gas and liquid phases and extract or inhibit any components in the gas which are likely to cause pipeline corrosion or blockage. Components which can cause difficulties are water vapour (corrosion, hydrates), heavy hydrocarbons (2-phase flow or wax deposition in pipelines), and contaminants such as carbon dioxide (corrosion) and hydrogen sulphide (corrosion, toxicity). In the case of associated gas, if there is no gas market, gas may have to be flared or re-injected. If significant volumes of associated gas are available it may be worthwhile to extract natural gas liquids (NGLs) before flaring or reinjection. Gas may also have to be treated for gas lifting or for use as a fuel. 

If produced gas contains water vapour it may have to be dried (dehydrated). Water condensation in the process facilities can lead to hydrate formation and may cause corrosion (pipelines are particularly vulnerable) in the presence of carbon dioxide and hydrogen sulphide. Hydrates are formed by physical bonding between water and the lighter components in natural gas. They can plug pipes and process equipment. Charts such as the one below are available to predict when hydrate formation may become a problem. 

In a typical gas oil design, the lighter products overhead from the quench tower/primary fractionator are compressed to 210 psi, and cooled to about 100°F. Some Q plus material is recovered from the compressor knockout drums. The gases are ethanolamine and caustic washed to remove acid gases sulfur compounds and carbon dioxide, and then desiccant dried to remove last traces of water. This is to prevent ice and hydrate formation in the low temperamre section downstream. 

Gas hydrates are non-stoichiometric crystals formed by the enclosure of molecules like methane, carbon dioxide and hydrogen sulfide inside cages formed by hydrogen-bonded water molecules. There are more than 100 compounds (guests) that can combine with water (host) and form hydrates. Formation of gas hydrates is a problem in oil and gas operations because it causes plugging of the pipelines and other facilities. On the other hand natural methane hydrate exists in vast quantities in the earth s crust and is regarded as a future energy resource. 

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