Thermal Processing of Zeolites for Catalytic Cracking



by David Phillips

Heyl & Patterson, USA

Zeolite refers to a family of more than 200 different minerals with a variety of interesting uses, ranging from water softeners to cat litter to industrial process catalysts. Zeolites are aluminosilicate minerals, made from interlinked alumina and silica tetrahedrons. Their porous crystalline structure is built from aluminum, oxygen and silicon, and water molecules and alkaline metals such as sodium, potassium and magnesium are trapped in the gaps between their lattice-like components. The framework of zeolites is what makes them useful, allowing them to trap other molecules inside, where their bonds can be broken down.

The word "zeolite" comes from the Greek words "zeo" meaning "to boil" and "lithos" meaning "stone," because of early observations that these types of rocks released water when heated. The minerals are resistant to high temperatures and pressures, do not dissolve in water or oxidize in air, and have no negative health or environmental impacts. Naturally occurring zeolites are mined using a conventional open mine technique, and deposits may be blasted or stripped for further processing. The ore is dried, then crushed or milled to prepare it for market. Milled zeolite may be air-classified down to particle size and sold in bags or in bulk, while crushed ore can be run through a screen to remove any fine material and separate it from the granular product. The fine material can go on to be granulated, and then sold.

These minerals can also be synthetically manufactured, and actually make up the majority of zeolites available. Their creation involves heating water-based solutions of silica and alumina with sodium hydroxide, which can be sometimes be substituted with sodium silicate and sodium aluminate.

Cracking is a term used for the process of breaking large hydrocarbon molecules into smaller, more manageable pieces. There are two ways this can be done -- at high temperatures and pressures without the use of a catalyst, or at low temperatures and pressures using a catalyst. Zeolites can act as pharmaceutical and petrochemical catalysts due to their negative charge and large lattice structure. Due to their porosity, they are ideal for use in catalytic cracking, helping to break down heavy hydrocarbons into gasoline, kerosene, diesel fuel and other petroleum byproducts. Reactions occur faster in catalysts, and they require less energy and yield high percentages of hydrocarbons with between five and ten carbon atoms, which is a desired effect. Only small amounts of the mineral are needed, and as catalysts, they can be recycled and reused.

When used in catalysis, zeolites must undergo a controlled calcination process. The minerals are fed through a rotary calciner in a nitrogen atmosphere at 752ºF (400ºC) for four hours, followed by a second calcination treatment at 1022ºF (550ºC) for five hours in an atmosphere of plain air. This controlled catalyst processing results in a product that gives higher activity and better yields than zeolite that is not calcined, or only calcined under a one-step process. By controlling the calcination in this way, the acid from the aluminum present in the zeolite decreases and has a positive effect on zeolite for use as a catalyst.

The micro-porous abilities of zeolites to capture some ions while allowing others to pass freely makes it popular in the nuclear industry, specifically for removing fission products from nuclear waste. Once its pores are full of trapped fission products, the zeolite can be hot-pressed into highly durable ceramic bricks, permanently sealing the pores and trapping the waste particles inside. For this reason, zeolites are excellent at controlling nuclear spills by adsorbing the waste. In fact, after the Fukushima nuclear disaster in the wake of the 2011 Japan earthquake and tsunami, sandbags filled with calcined and milled zeolite were dropped into the seawater near the plant to filter away radioactive caesium.

Synthetically-produced zeolites are widely used in the pharmaceutical industry for their properties as catalysts. Zeolites can confine molecules to small spaces, and this alters their structure and reactivity. The hydrogen form of zeolites are powerful solid state acids which can make a wide range of acid-catalyzed reactions possible.

Synthetic zeolites are often used as an additive in the production process of warm mix asphalt. By doing so, the temperature is lowered during production and the laying of concrete, lessening the consumption of fossil fuels. When used in hot mix asphalt, zeolite helps lead to easier compaction, longer hauls and cold weather paving.

Zeolites are also used as solar thermal collectors and adsorption refrigeration. For these uses, their high heat of adsorption and dehydration/rehydration while still maintaining their structural stability is exploited. These properties coupled with an energy-releasing reaction when making the transition from hydrated to dehydrated makes zeolite ideal for harvesting solar energy and waste heat.

Zeolite-based oxygen concentrator systems are used in the industry to create pure medical grade oxygen. Their filtering properties make it an excellent sieve for filtering out unwanted gas contaminants like nitrogen. This results in highly potent concentrated oxygen, which can be up to 5% argon.

Their single largest use is the worldwide production of detergents, accounting to 1.44 million tons per year. In addition, non-clumping pet litter is often made from milled zeolite due to its ability to trap molecules within its pores.

Heyl & Patterson manufactures rotary calciners to thermally process zeolite and a wide range of other catalysts. Their versatility makes them an ideal choice for drying, calcining, catalysis and thermal desorption applications. In addition to efficient high-temperture processing, Heyl & Patterson calciners provide high heat transfer rates, minimized equipment size and cost, improved product quality due to the control of the processing atmosphere, and reduced environmental risks due to low, treatable off-gas volumes. This efficiency and versatlity make this equipment an ideal choice for the catalyst market.

If you would like to know more about how Heyl & Patterson rotary calciners can be used to process zeolite, contact us or click here.

Google Search - Web

Google Search - Images

Heyl & Patterson on the Portal ■