linear and branched polymers overview

    Linear polymer is a polymer in which the molecules form long chains without branches or cross-linked structures. The molecular chains of a linear polymer may be intertwined, but the forces tending to hold the molecules together are physical rather than chemical and thus can be weakened by energy applied in the form of heat. Such linear polymers are thermoplastics. The simplest polymer is a linear polymer. A linear polymer is simply a chain in which all of the monomers exist in a single line. An example of a linear polymer is Teflon, which is made from tetrafluoroethylene. It is a single strand of units made from two carbon atoms and four fluorine atoms. When formed, these linear polymers can create strands of fibers or form a mesh that can be very strong and hard to break through.

    These linear polymers are well packed and have high magnitude of intermolecular forces of attraction and therefore have high densities, high tensil (pulling) strength and high melting points. Some common example of linear polymers are high density polyethylene nylon, polyester, PVC, PAN etc.

Linear, Branched, and Cross-linked Polymers:

 

Polyethylene is called a linear or straight-chain polymer because it consists of a long string of carbon-carbon bonds. These terms are misleading because the geometry around each carbon atom is tetrahedral and the chain is neither linear nor straight, as shown in the figure

As the polymer chain grows, it folds back on itself in a random fashion to form structures such as the one shown in the figure below on the left.  Straight chains can sometimes fold tightly enough to make crystal structures (on the right below) even though the molecules are very long!

Neatly packed straight chains can make crystals!

Randomly oriented straight chains can be pretty messy

      

   Polymers with branches at irregular intervals along the polymer chain are called branched polymers (see figure to the right).   These branches make it difficult forthe polymer molecules to pack in a regular array, and therefore make the polymer less crystalline and less dense.  The amount and type of branching also affects physical properties such as viscosity and elasticity (see below).  Branches often prevent chains from getting close enough together for intermolecular forces to work effectively. they have low tensile strength, low density, boiling point and melting points than linear polymers. Some common examples are low density polythene, glycogen, starch etc. (Amylopectin).

         Cross-linked polymers contain short side chains (cross links) that connect different polymer chains into a “network” as shown in the figure to the right.  At first, adding cross-links between polymer chains makes the polymer more elastic (they can stretch and return to their original form.)    The links can “pull” the chains back together when they are stretched!  The vulcanization of rubber, for example, results from the introduction of short chains of sulfur atoms that link the polymer chains in natural rubber.  Cross-linking also decreases the viscosity (the resistance to flow) of polymers.  In order for polymers to flow, the chains must move past each other and cross-linking prevents this.  Elastomers are elastic polymers created by limited cross-linking.  As the number of cross-links increases, however, the polymer becomes more rigid and cannot stretch as much; the polymer will become less viscous and less elastic and might even become brittle. e.g., Bakelite, malamine formaldehyde resin etc.

cross linking

Cross-link is a bond formed between polymer chains, either between different chains or between different parts of the same chain. Cross-links can be covalent bonds or ionic bonds. Polyethylene consists of many random molecular

chains with no particular orientation and no chemical bonds existing between chains. When heat is applied to such a material, the chains are free to slip and flow under relatively small outside force. Such a material is called a thermoplastic. If we are able to introduce cross-linking bonds between adjacent molecular chains, this adds form stability at higher temperatures. There will still be some loss of strength at elevated temperatures, but the cross-linked molecular chains are much more resistant to flow when stress is applied. The chemical process of vulcanization is a type of cross-linking and it changes the property of rubber to the hard, durable material we associate with car and bike tires. This process is often called sulfur curing, and the term vulcanization comes from Vulcan, the Roman god of fire. However, this is a slow process taking around 8 hours. A typical car tire is cured for 15 minutes at 150°C. However, the time can be reduced by the addition of accelerators such as 2-benzothiazolethiol or tetramethylthiuram disulfide. Both of these contain a sulfur atom in the molecule that initiates the reaction of the sulfur chains with the rubber. Accelerators increase the rate of cure by catalyzing the addition of sulfur chains to the rubber molecules. The resulting modification of mechanical properties depends strongly on the cross-link density. Low cross-link densities decrease the viscosities of polymer melts. Intermediate cross-link densities transform gummy polymers into materials that have elastomeric properties and potentially high strengths. Very high cross-link densities can cause materials to become very rigid or glassy, such as phenol-formaldehyde materials. Cross-links are the characteristic property of thermosetting plastic materials. In most cases, cross-linking is irreversible, and the resulting thermosetting material will degrade or burn if heated, without melting. Especially in the case of commercially used plastics, once a substance is cross-linked, the product is very hard or impossible to recycle. In some cases, though, if the cross-link bonds are sufficiently different, chemically, from the bonds forming the polymers, the process can be reversed. Permanent wave solutions, for example, break and re-form naturally occurring cross-links (disulfide bonds) between protein chains in hair. When polymer chains are linked together by cross-links, they lose some of their ability to move as individual polymer chains. For example, a liquid polymer (where the chains are freely flowing) can be turned into a "solid" or "gel" by cross-linking the chains together. Chemical covalent cross-links are stable mechanically and thermally, so once formed are difficult to break. Therefore, cross-linked products like car tires cannot be recycled easily. A class of polymers known as thermoplastic elastomers rely on physical cross-links in their microstructure to achieve stability, and are widely used in non-tire applications, such as snowmobile tracks, and catheters for medical use. They offer a much wider range of properties than conventional cross-linked elastomers because the domains which act as cross-links are reversible, so can be reformed by heat. The stabilising domains may be non-crystalline (as in styrene-butadiene block copolymers) or crystalline as in thermoplastic copolyesters.

crude oil desalting

   When crude oil is recovered it contains salts, sand, water and small contents of metals such as copper, nickel, and vanadium. Crude oil often contains water, inorganic salts, suspended solids, and water-soluble trace metals. As a first step in the refining process, to reduce corrosion, plugging, and fouling of equipment and to prevent poisoning the catalysts in processing units, these contaminants must be removed by desalting (dehydration).

  The desalting of crude oil is a process that does not have a high profile, but is vital to the operation of the modern petroleum refinery. Desolaters provide more protection to costly refinery equipment than any other single piece of process hardware. The salts that are most frequently present in crude oil are calcium, sodium and magnesium chlorides. If these compounds are not removed from the oil several problems arise in the refining process. The high temperatures that occur downstream in the process could cause water hydrolysis, which in turn allows the formation of corrosive hydrochloric acid. Sand, silts and salt cause deposits and foul heat exchangers. The need to supply heat to vaporize water reduces crude pre-heat capacity. Sodium, arsenic and other metals can poison catalysts. By removing the suspended solids, they are not carried into the burner and eventually flue gas, where they would cause problems with environmental compliance such as flue gas opacity norms 

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Factors affecting crude oil desalting :

The objective of oil desalting is to remove water-soluble salts and the entrained water, which normally contains dissolved salts. Formation water flows with crude in two types: free and emulsified. The free water is not intimately mixed in the crude and is found in larger drops scattered throughout the oil phase. This kind of water is easy to remove by gravity oil-water separators, surge tanks (wet tanks), and desalting vessels. On the other hand, emulsified waters are intimately mixed and found scattered in tiny drops in the oil phase. This kind is hard to remove by simple settling devices, so further treatment such as chemical injection, freshwater dilution, mixing, heating, and electricity are required. The addition of diluent water, heating, and applying electricity can enhance gravity separation.

Factors affecting desalting performance are:

Types of pumps

      A pump is a device used to transport fluids, such as liquid (pump) , gases (compressor) or slurries . A pump displaces a volume by physical or mechanical action. Pumps fall into three major groups: direct lift, displacement, and gravity pumps.Their names describe the method for moving a fluid.

Types

• Positive displacement pump

MORE     A positive displacement pump moves a fluid by trapping a fixed amount of it then forcing (displacing) that trapped volume into the discharge pipe.

Some positive displacement pumps work using an expanding cavity on the suction side and a decreasing cavity on the discharge side. Liquid flows into the pump as the cavity on the suction side expands and the liquid flows out of the discharge as the cavity collapses. The volume is constant given each cycle of operation. positive displacement pumps are in hydraulic systems to a pressure up to 34500 bar.

Positive displacement pump behavior and safety