Acetylene

Background

Acetylene is a colorless, combustible gas with a distinctive odor. When acetylene is liquefied, compressed, heated, or mixed with air, it becomes highly explosive. As a result special precautions are required during its production and handling. The most common use of acetylene is as a raw material for the production of various organic chemicals including 1,4-butanediol, which is widely used in the preparation of polyurethane and polyester plastics. The second most common use is as the fuel component in oxy-acetylene welding and metal cutting. Some commercially useful acetylene compounds include acetylene black, which is used in certain dry-cell batteries, and acetylenic alcohols, which are used in the synthesis of vitamins.

Acetylene was discovered in 1836, when Edmund Davy was experimenting with potassium carbide. One of his chemical reactions produced a flammable gas, which is now known as acetylene. In 1859, Marcel Morren successfully generated acetylene when he used carbon electrodes to strike an electric arc in an atmosphere of hydrogen. The electric arc tore carbon atoms away from the electrodes and bonded them with hydrogen atoms to form acetylene molecules. He called this gas carbonized hydrogen.

By the late 1800s, a method had been developed for making acetylene by reacting calcium carbide with water. This generated a controlled flow of acetylene that could be combusted in air to produce a brilliant white light. Carbide lanterns were used by miners and carbide lamps were used for street illumination before the general availability of electric lights. In 1897, Georges Claude and A. Hess noted that acetylene gas could be safely stored by dissolving it in acetone. Nils Dalen used this new method in 1905 to develop long-burning, automated marine and railroad signal lights. In 1906, Dalen went on to develop an acetylene torch for welding and metal cutting.

In the 1920s, the German firm BASF developed a process for manufacturing acetylene from natural gas and petroleum-based hydrocarbons. The first plant went into operation in Germany in 1940. The technology came to the United States in the early 1950s and quickly became the primary method of producing acetylene.

Demand for acetylene grew as new processes were developed for converting it into useful plastics and chemicals. In the United States, demand peaked sometime between 1965 and 1970, then fell off sharply as new, lower-cost alternative conversion materials were discovered. Since the early 1980s, the demand for acetylene has grown slowly at a rate of about 2-4% per year.

In 1991, there were eight plants in the United States that produced acetylene. Together they produced a total of 352 million lb (160 million kg) of acetylene per year. Of this production, 66% was derived from natural gas and 15% from petroleum processing. Most acetylene from these two sources was used on or near the site where it was produced to make other organic chemicals. The remaining 19% came from calcium carbide. Some of the acetylene from this source was used to make organic chemicals, and the rest was used by regional industrial gas producers to fill pressurized cylinders for local welding and metal cutting customers.

In Western Europe, natural gas and petroleum were the principal sources of acetylene in 1991, while calcium carbide was the principal source in Eastern Europe and Japan.

Raw Materials

Acetylene is a hydrocarbon consisting of two carbon atoms and two hydrogen atoms. Its chemical symbol is C₂H₂. For commercial purposes, acetylene can be made from several different raw materials depending on the process used.

The simplest process reacts calcium carbide with water to produce acetylene gas and a calcium carbonate slurry, called hydrated lime. The chemical reaction may be written as CaC₂ + 2 H₂O → C₂H₂ + Ca(OH)₂.

Other processes use natural gas, which is mostly methane, or a petroleum-based hydrocarbon such as crude oil, naphtha, or bunker C oil as raw materials. Coal can also be used. These processes use high temperature to convert the raw materials into a wide variety of gases, including hydrogen, carbon monoxide, carbon dioxide, acetylene, and others. The chemical reaction for converting methane into acetylene and hydrogen may be written 2 CH₄ → C₂H₂ + 3 H₂. The other gases are the products of combustion with oxygen. In order to separate the acetylene, it is dissolved in a solvent such as water, anhydrous ammonia, chilled methanol, or acetone, or several other solvents depending on the process.

The Manufacturing
Process

There are two basic conversion processes used to make acetylene. One is a chemical reaction process, which occurs at normal temperatures. The other is a thermal cracking process, which occurs at extremely high temperatures.

Here are typical sequences of operations used to convert various raw materials into acetylene by each of the two basic processes.

Chemical reaction process

Acetylene may be generated by the chemical reaction between calcium carbide and water. This reaction produces a considerable amount of heat, which must be removed to prevent the acetylene gas from exploding. There are several variations of this process in which either calcium carbide is added to water or water is added to calcium carbide. Both of these variations are called wet processes because an excess amount of water is used to absorb the heat of the reaction. A third variation, called a dry process, uses only a limited amount of water, which then evaporates as it absorbs the heat. The first variation is most commonly used in the United States and is described below.

1. Most high-capacity acetylene generators use a rotating screw conveyor to feed calcium carbide granules into the reaction chamber, which has been filled to a certain level with water. The granules measure about 0.08 in x 0.25 in (2 mm x 6 mm), which provides the right amount of exposed surfaces to allow a complete reaction. The feed rate is determined by the desired rate of gas flow and is controlled by a pressure switch in the chamber. If too much gas is being produced at one time, the pressure switch opens and cuts back the feed rate.
2. To ensure a complete reaction, the solution of calcium carbide granules and water is constantly agitated by a set of rotating paddles inside the reaction chamber. This also prevents any granules from floating on the surface where they could over-heat and ignite the acetylene
3. The acetylene gas bubbles to the surface and is drawn off under low pressure. As it leaves the reaction chamber, the gas is cooled by a spray of water. This water spray also adds water to the reaction chamber to keep the reaction going as new calcium carbide is added. After the gas is cooled, it passes through a flash arrester, which prevents any accidental ignition from equipment downstream of the chamber.
4. As the calcium carbide reacts with the water, it forms a slurry of calcium carbonate, which sinks to the bottom of the chamber. Periodically the reaction must be stopped to remove the built-up slurry. The slurry is drained from the chamber and pumped into a holding pond, where the calcium carbonate settles out and the water is drawn off. The thickened calcium carbonate is then dried and sold for use as an industrial waste water treatment agent, acid neutralizer, or soil conditioner for road construction.

Thermal cracking process

Acetylene may also be generated by raising the temperature of various hydrocarbons to the point where their atomic bonds break, or crack, in what is known as a thermal cracking process. After the hydrocarbon atoms break apart, they can be made to rebond to form different materials than the original raw materials. This process is widely used to convert oil or natural gas to a variety of chemicals.

There are several variations of this process depending on the raw materials used and the method for raising the temperature. Some cracking processes use an electric arc to heat the raw materials, while others use a combustion chamber that burns part of the hydrocarbons to provide a flame. Some acetylene is generated as a coproduct of the steam cracking process used to make ethylene. In the United States, the most common process uses a combustion chamber to heat and burn natural gas as described below.

1. Natural gas, which is mostly methane, is heated to about 1,200° F (650° C). Preheating the gas will cause it to self-ignite once it reaches the burner and requires less oxygen for combustion.
2. The heated gas passes through a narrow pipe, called a venturi, where oxygen is injected and mixed with the hot gas.
3. The mixture of hot gas and oxygen passes through a diffuser, which slows its velocity to the desired speed. This is critical. If the velocity is too high, the incoming gas will blow out the flame in the burner. If the velocity is too low, the flame can flash back and ignite the gas before it reaches the burner.
4. The gas mixture flows into the burner block, which contains more than 100 narrow channels. As the gas flows into each channel, it self-ignites and produces a flame which raises the gas temperature to about 2,730° F (1,500° C). A small amount of oxygen is added in the burner to stabilize the combustion.
5. The burning gas flows into the reaction space just beyond the burner where the high temperature cause about one-third of the methane to be converted into acetylene, while most of the rest of the methane is burned. The entire combustion process takes only a few milliseconds.
6. The flaming gas is quickly quenched with water sprays at the point where the conversion to acetylene is the greatest. The cooled gas contains a large amount of carbon monoxide and hydrogen, with lesser amounts of carbon soot, plus carbon dioxide, acetylene, methane, and other gases.
7. The gas passes through a water scrubber, which removes much of the carbon soot. The gas then passes through a second scrubber where it is sprayed with a solvent known as N-methylpyrrolidinone which absorbs the acetylene, but not the other gases.
8. The solvent is pumped into a separation tower where the acetylene is boiled out of the solvent and is drawn off at the top of the tower as a gas, while the solvent is drawn out of the bottom.

Storage and Handling

Because acetylene is highly explosive, it must be stored and handled with great care. When it is transported through pipelines, the pressure is kept very low and the length of the pipeline is very short. In most chemical production operations, the acetylene is transported only as far as an adjacent plant, or "over the fence" as they say in the chemical processing business.

When acetylene must be pressurized and stored for use in oxy-acetylene welding and metal cutting operations, special storage cylinders are used. The cylinders are filled with an absorbent material, like diatomaceous earth, and a small amount of acetone. The acetylene is pumped into the cylinders at a pressure of about 300 psi (2,070 kPa), where it is dissolved in the acetone. Once dissolved, it loses its explosive capability, making it safe to transport. When the cylinder valve is opened, the pressure drop causes some of the acetylene to vaporize into gas again and flow through the connecting hose to the welding or cutting torch.

Quality Control

Grade B acetylene may have a maximum of 2% impurities and is generally used for oxyacetylene welding and metal cutting. Acetylene produced by the chemical reaction process meets this standard. Grade A acetylene may have no more than 0.5% impurities and is generally used for chemical production processes. Acetylene produced by the thermal cracking process may meet this standard or may require further purification, depending on the specific process and raw materials.

The Future

The use of acetylene is expected to continue a gradual increase in the future as new applications are developed. One new application is the conversion of acetylene to ethylene for use in making a variety of polyethylene plastics. In the past, a small amount of acetylene had been generated and wasted as part of the steam cracking process used to make ethylene. A new catalyst developed by Phillips Petroleum allows most of this acetylene to be converted into ethylene for increased yields at a reduced overall cost.

Where to Learn More

Books

Brady, George S., Henry R. Clauser, and John A. Vaccari. Materials Handbook, 14th edition. McGraw-Hill, 1997.

Kroschwitz, Jacqueline I. and Mary Howe-Grant, ed. Encyclopedia of Chemical Technology, 4th edition. John Wiley and Sons, Inc., 1993.

Other

Acetylene Pamphlet G-1. Compressed Gas Association, 1990.

Compressed Gas Association. http://www.cganet.com.

—ChrisCavette

Hair Remover

Hair removers, or depilatories, are products designed to chemically or physically remove undesirable hair from areas on the body. Hair removers are made by mixing together the appropriate raw materials in large stainless steel tanks and then filling them into individual packages. In use for thousands of years, they continue to be an important part of many people's everyday hygiene. Currently, new hair removers are being investigated which are less irritating, more effective, and longer lasting.

Background

Epilatories were the first type of hair removers. The most common of these products is an epilating wax. This product is heated and spread on the skin in the desired area. It is then allowed to cool and harden. The mass of wax is then rapidly removed, pulling with it about 80% of the hairs. It is a slightly painful procedure and a mild anti-+ septic is typically applied to protect against skin irritation. Epilatories have not been widely used by individual consumers, how-ever they are popular in beauty salons.

While epilatories continue to be an important method of hair removal, depilatories are much more common in personal care. Depilatories rely on a chemical reaction between materials in the formula with components of the hair. When the depilatory is applied to the skin, a component in the product, such as thioglycolic acid, reacts with the protein in the hair and weakens it. The hair can then be removed from the skin by gentle wiping, scraping, or rinsing. This is effective on any part of the hair structure that is above the level of the skin.

The compounds in the depilatories, which react with hair, also react with protein in the skin, albeit at a much slower rate. For this reason, depilatories must be left on the skin for only a short while. The manufacturers of depilatories realize this and strive to develop formulas, which have only minimal negative effects on skin. Typically, if a consumer follows the directions as stated on the package, no problems will arise. Epilatories generally will not have a negative effect other than physical irritation on skin since they do not rely on a chemical reaction to function properly.

The first step in producing a hair remover, or any personal care product for that matter, is developing a formula. Cosmetic chemists use their knowledge of standard cosmetic ingredients, consumer research information and various other types of information to construct their formulas. Since hair removers can be sold in many different forms including creams, gels, lotions, and aerosols, the formula must be adapted to the product form. The formulas are first prepared in small beakers in the lab so aspects of the formula can be evaluated. Tests for product effectiveness, stability, and safety are all completed at this point. Other studies such as consumer acceptance testing may also be completed.

History

Hair removal from various parts of the body has been an important part of beauty for thousands of years. The earliest recorded use of hair removers is found in ancient India where hair removal was highly desirable. This society frequently used abrasive pastes and resinous plasters to physically remove hair. In the Middle East, a lime mixture was used for a similar purpose. Other materials, such as antimony and arsenic compounds, were also used; however, it is now known that these materials are quite toxic, and their use has been discontinued.

The earliest hair removers used a method of hair removal known as epilation, or physically pulling hair out. Common procedures included using devices like tweezers to pull hair out selectively or waxes which pulled hair out in large masses. Since hair removal by physical means was often a painful experience, scientists worked on developing formulas, which would chemically remove hair. Little progress was made in this area until about 100 years ago when it was first reported that barium sulfide was used for this purpose. A few years later a similar idea was patented in the United States. Cream depilatories were first introduced in the 1920s and many more patents were issued during the 1930s. Thioglycolate depilatories, which were first introduced in 1938, have become the most important hair removers.

Raw Materials

There are many different materials that have been used in hair remover formulas. Some of these materials are responsible for the hair removing properties of the product while others are needed to improve the product's aesthetics.

As suggested, depilatories and epilatories remove hair in a very different manner. Obviously, they then require different compounds to function. A standard epilatory may be composed of a wax such as beeswax and a sticky, polymeric resin. The wax provides the setting action needed for peeling the product off the skin and the resin helps bind the material to the hair. The active ingredients used for depilatories include thioglycolate salts and sulfides. Thioglycolate salts include materials such as calcium thioglycolate and potassium thioglycolate. In an aqueous solution at the proper pH, they are converted to an acid, which then affects the hair. Sulfides such as barium sulfide or strontium sulfide are also used because they react more rapidly than thioglycolates how-ever, they have other characteristics which make them less appealing. Since pH is critical to the proper performance of depilatories, ingredients such as sodium hydroxide or calcium hydroxide, which adjust the pH, are included.

In addition to the hair removing ingredients, other compounds are necessary to complete formulation. This includes diluents, emollients, thickeners, fragrances, and colorants. Water is used most often as a diluent for depilatories because it is compatible with a large range of raw materials, non-irritating, and inexpensive. Since epilatories are waxes, they are not compatible with water so mineral oil is typically used as the diluent. Emollients are included in formulations to reduce the harshness of the formula and improve the feel. Materials like oils, silicones, and esters are all examples of commonly used emollients. Depending on the product's form, a thickening agent may be required. These materials are typically polymers, surfactants, or modified clays. For aerosol products, a propellant is needed.

To improve the aesthetics of the formula, fragrances are included. These fragrances must be specially designed to overcome the generally offensive odor of the hair removal ingredients. For cream or lotion products, emulsifiers are needed and dyes are used to modify the color. Various other ingredients such as preservatives, antioxidants, extracts may also be included.

Beyond the ingredients that go into the hair remover formula, packaging components are another important raw material. Bottles are primarily used and are made of plastics such as polyvinyl chloride (PVC) or high-density polyethylene (HDPE). For aerosol products, a steel or aluminum can is used. The outer graphics can be either directly silk screened on to the package or an adhesive label can be applied.

The Manufacturing
Process

The process for making a hair remover can be divided into two steps. First, a large batch of the product is made, and then it is filled into the individual containers. While there are many different product forms the hair remover may take such as creams, aerosols, or waxes, the following description will only outline the method for making a lotion depilatory.

Compounding

• 1 The bulk batches of depilatories are produced in a designated area of the manufacturing plant. Plant workers follow a standard formula to make batches, which can be over 3,000 gal (11,355 1). The tanks, which are stainless steel, are equipped with a large mixer and a heating and cooling system. The temperature and the mixer speed are both computer controlled, so the compounder can modify them who is making the batch.

  When the controls are set, the raw materials can be added. In most formulas, water is added first by being pumped in at the appropriate volume. The mixer is then turned on and the other raw materials are added in the order called for in the formula. These raw materials are either poured into the batch from bags or 55 gal (208.2 1) drums. As each goes into the tank, it is thoroughly mixed. Depending on the formula, the batch is heated and cooled as necessary to help the raw materials combine more quickly. A typical 3,000 gal (11,355 1) batch of depilatory may take anywhere from two to five hours to make.

Checking quality

• 2 After the batch is finished, the quality control department must test it before it can be sent along for filling. Physical characteristics are examined to ensure the batch conforms to the specifications outlined in the formula instructions. Typical tests done on a depilatory batch include viscosity checks, pH determination, and appearance and odor evaluations. They may also check the activity of the thioglycolate. If the batch does not meet the ranges set for the specifications, sometimes adjustments to the formula can be made. For instance if the pH is too low, a depilatory will not function properly. Therefore, a certain amount of a base such as sodium hydroxide could be added to adjust the pH. Salt can be added to increase the thickness of many of these products. Fragrance and color may also be adjusted at this point. After quality control approves the batch, it is pumped out of the main batch tank into a holding tank where it can be stored until it gets filled. From the holding tank it may be pumped into a carousel-style, piston head filler.

Filling

• 3 The depilatory is filled on a filling line, which is a series of machines that connected, by a conveyor belt system. At the start of the filling line, empty bottles are put in a hopper. This hopper is a large bin, which contains a device that can physically manipulate the bottles so they are standing upright when they come out. From this bottle-sorting hopper, they are moved along a conveyor belt to the filling machine.
• 4 The filling machine contains a finished batch of depilatory. It is a carousel is made up of a series of piston filling heads that are programmed to deliver the correct amount of product into the bottles. When a bottle passes under the filling machine, product is pumped into it.
• 5 After being filled, the bottles are moved to the capping machine. Just like the bottles, the caps are stored in a hopper that is designed to physically align them in the right order. As the bottles move by, the capping machine automatically attaches and tightens the caps.
• 6 Next, the bottles move to the labeling machines (if necessary) and then on to the boxing area. In the boxing area, the products are lined up and put into boxes. The boxes are stacked onto pallets and hauled away in large trucks to distributors. The entire filling process can produce more than 500 bottles per minute or more.

Quality Control

To produce a consistent product, quality control inspections are done throughout the manufacturing process. At the start, the raw materials are checked to ensure that they meet the manufacturer's specifications. Typically, quality control chemists sample incoming raw materials and run numerous tests before qualifying them. These tests may include checks for appearance, pH, odor, or viscosity. More complex testing may also be performed. During manufacture, the batch of hair remover is periodically tested to make sure that a functional product will be produced.

While the product is being filled, quality inspectors are stationed along the entire filling line. These workers watch the containers as they pass by and pull off any which are defective. This includes those that are inadequately filled, have misplaced labels, or otherwise damaged. Regulations also require that samples be periodically checked for microbial contamination during filling.

The Future

The market for hair removers is relatively small compared to other personal care products such as shampoos or conditioners, so only minimal research is currently being pursued. The focus of this research has been on making products that are less irritating and more moisturizing to the skin, lower in odor and more effective. Irritation is likely to always be a problem for all depilatories that chemically alter proteins so compounds, which weaken hair in other ways, may be developed. Beyond depilatories and epilatories, new drugs could be developed which can inhibit the growth of hair from follicles. This might represent a kind of permanent hair remover.

Where to Learn More

Books

Knowlton, John and Steven Pearce. The Handbook of Cosmetic Science and Technology. Oxford: Elsevier Science Publishers, 1993.

Umbach, Wilfried. Cosmetics and Toiletries Development, Production, and Use. New York: Ellis Horwood, 1991.

Periodicals

Breuer, Hans. "Depilatories." Cosmetics & Toiletries 105 (April 1990): 61-66.

—Perry Romanowski

acetylene See ethyne