شما می توانید با مراجعه به قسمت آرشیو موضوعي (سمت چپ و پايين)  عنوان  مرتبط با موضوع مورد نظر خود را بیابید.

به امید اینکه مطالب این وبلاگ برای شما مفید باشد.

 نظرات و انتقادها و پيشنهادهاي خود را از من دریغ نکنید.

mohamad27m@mail.com

Compressed Natural Gas (CNG) is a substitute for gasoline (petrol), diesel, or propane fuel. It is considered to be an environmentally "clean" alternative to those fuels and it is much safer than other motor fuels in the event of a fuel spill: natural gas is lighter than air, so it disperses quickly when leaked or spilled. It is made by compressing natural gas (which is mainly composed of methane (CH₄)), by about 75%. It is stored and distributed in hard containers, at a normal pressure of 200–220 bar (20–22 MPa), usually in cylindrical or spherical shapes to maintain equal pressure on the walls of the containers.

In response to high fuel prices and environmental concerns, compressed natural gas is starting to be used in light-duty passenger vehicles and pickup trucks, medium-duty delivery trucks, and in transit and school buses

Technology

A CNG powered high-floor Neoplan AN440A, operated by ABQ RIDE in Albuquerque, New Mexico.

CNG can be used in Otto-cycle (gasoline) and modified Diesel cycle engines. Lean-burn Otto-cycle engines can achieve higher thermal efficiencies when compared with stoichiometric Otto-cycle engines at the expense of higher NOx and hydrocarbon emissions. Electronically-controlled stoichiometric engines offer the lowest emissions across the board and the highest possible power output, especially when combined with EGR, turbocharging and intercooling, and three way catalytic converters, but suffer in terms of heat rejection and fuel consumption. A suitably designed natural gas engine may have a higher output compared with a petrol engine because the octane number of natural gas is higher than that of petrol.

CNG may be refueled from low-pressure ("slow-fill") or high-pressure ("fast-fill") systems. The difference lies in the cost of the station vs. the refueling time. There are also some implementations to refuel out of a residential gas line during the night, but this is forbidden in some countries.

CNG cylinders can be made of steel, aluminum, or plastic. Lightweight composite (fibre-wrapped plastic) cylinders are especially beneficial for vehicular use because they offer significant weight reductions when compared with earlier generation steel and aluminum cylinders, which leads to lower fuel consumption. The CNG cylnders generally follow the ISO 11439 standard. ^[1]

The equipment required for CNG to be delivered to an Otto-cycle engine includes a pressure regulator (a device that converts the natural gas from storage pressure to metering pressure) and a gas mixer or gas injectors (fuel metering devices). Earlier-generation CNG conversion kits featured venturi-type gas mixers that metered fuel using the Venturi effect. Often assisting the gas mixer was a metering valve actuated by a stepper motor relying on feedback from an exhaust gas oxygen sensor. Newer CNG conversion kits feature electronic multi-point gas injection, similar to petrol injection systems found in most of today's cars.

[edit] Drawbacks

Compressed natural gas vehicles require a greater amount of space for fuel storage than convention gasoline power vehicles. Since it is a compressed gas, rather than a liquid like gasoline, CNG takes up more space for each GGE (Gallon of Gas Equivalent). This makes it difficult to design smaller vehicles that look and operate like the vehicles that people are accustomed to.

Still, it is more convenient than traditional fuels, as its price is in a range of 1/3 to 1/2 that of gasoline in Europe.

[edit] CNG compared to LNG

CNG is often confused with liquefied natural gas (LNG). While both are stored forms of natural gas, the key difference is that CNG is in compressed form, while LNG is in liquefied form. CNG has a lower cost of production and storage compared to LNG as it does not require an expensive cooling process and cryogenic tanks. CNG requires a much larger volume to store the same mass of natural gas and the use of very high pressures (3000 to 4000 lbf/in², or 205 to 275 bar).

[edit] Worldwide

[edit] Canada

Canada is a large producer of natural gas, so it follows that CNG is used in Canada as an economical motor fuel. Canadian industry has developed CNG-fueled truck and bus engines, CNG-fueled transit buses, and light trucks and taxis. Both CNG and propane refueling stations are not difficult to find in major centres.

[edit] Europe

In Germany, CNG-generated vehicles are expected to increase to two million units of motor-transport by the year 2020. The cost for CNG fuel is between 1/3 and 1/2 compared to other fossil fuels in Europe.^{[citation needed]}

[edit] South America

Argentina and Brazil are the two countries with the largest fleets of CNG vehicles. Conversion has been facilitated by a substantial price differential with liquid fuels, locally-produced conversion equipment and a growing CNG-delivery infrastructure. A 'Blue-network' of CNG stations is being developed on the major highways of the Southern Cone (including Chile and Bolivia) to allow for long-haul transportation fuelled by CNG.

[edit] Asia

CNG Radio Taxi in New Delhi, India

One of the many CNG propelled autorickshaws on the streets of New Delhi, Delhi. A fleet of twelve also operates in Brighton, England.

A CNG powered Volvo B10BLE bus, operated by SBS Transit in Singapore.

In Asian Economies such as India, CNG costs are at Rupees 19.20(USD $0.48) per kg compared with Rs.50.00 (US$ 1.25) per liter of petrol. The cost saving is immense along with reduced emissions and environmentally friendlier cars.

CNG has been made mandatory for all public transport in the Indian capital city of New Delhi.

CNG has grown into one of the major fuel sources used in car engines in Iran, Pakistan, Bangladesh and India. The use of CNG is mandated for the public transport system of India's capital New Delhi as well as for the city of Ahmedabad in the state of Gujarat. The Delhi Transport Corporation operates the world's largest fleet of CNG buses. The government of Punjab, Pakistan, the most populous province of that country, has mandated that all public-transport vehicles will use CNG by 2007. Today many rickshaws as well as personal vehicles in India and Bangladesh are being converted to CNG powered technology, the cost of which is in the range of $800-$1000. In the Bangladesh capital of Dhaka not a single auto rickshaw without CNG has been permitted since 2003. As of July 2007 Pakistan is the largest user of CNG in Asia, and second largest user in the world.^[2]

According to the International Association for Natural Gas Vehicles, Pakistan has the second-largest number of natural gas vehicles.^[2] In the Middle East and Africa, Egypt is a top ten country in the world with more than 63000 CNG vehicles and 95 fueling stations nationwide. Egypt was also the first nation in Africa and the Middle East to open a public CNG fueling station in January 1996.^[3]

[edit] Oceania

During the 1970s and 1980s, CNG was commonly used in New Zealand in the wake of the oil crises, but fell into decline after petrol prices receded.

Brisbane Transport and Transperth in Australia have both adopted a policy of only purchasing CNG buses in future. Transperth is purchasing 451 Mercedes-Benz OC500LE buses, including 58 articulated buses, while Brisbane Transport has purchased 216 Scania L94UB and 180 MAN 18.310 models as well as ordering up to 30 articulated CNG buses on MAN chassis'.

In the 1990s Benders Buslines of Geelong, Victoria trialled CNG buses for the Energy Research and Development Corporation.^[4] Recently Landi Renzo of Italy has set up a production subsidary in Karachi to cater the growing demand of CNG Kits in Pakistan.OEM's like Toyota Pakistan and Suzuki Pakistan is producing company fitted CNG cars

شما می توانید با مراجعه به قسمت آرشیو موضوعي (سمت چپ و پايين)  عنوان  مرتبط با موضوع مورد نظر خود را بیابید.

به امید اینکه مطالب این وبلاگ برای شما مفید باشد.

 نظرات و انتقادها و پيشنهادهاي خود را از من دریغ نکنید.

mohamad27m@mail.com

Liquefied natural gas or LNG is natural gas that has been converted to liquid form for ease of storage or transport. Liquified natural gas takes up about 1/600th the volume of natural gas at a stove burner tip. It is odorless, colorless, non-corrosive, and non-toxic. When vaporized, it burns only in concentrations of 5% to 15% when mixed with air. Neither LNG, nor its vapor, can explode in an unconfined environment.

The liquefaction process involves removal of certain components, such as dust, helium, water, and heavy hydrocarbons, which could cause difficulty downstream, and then condensed into a liquid at close to atmospheric pressure (Maximum Transport Pressure set around 25 kPa (3.6psi)) by cooling it to approximately −163 °C (−260 °F). The reduction in volume makes it much more cost-efficient to transport over long distances where pipelines do not exist. Where moving natural gas by pipelines is not possible or economical, it can be transported by specially designed cryogenic sea vessels (LNG vessels) or cryogenic road tankers

Basic facts

LNG is principally used for transporting natural gas to markets, where it is regasified and distributed as pipeline natural gas. LNG offers an energy density comparable to petrol and diesel fuels and produces less pollution, but its relatively high cost of production and the need to store it in expensive cryogenic tanks have prevented its widespread use in commercial applications. It can be used in natural gas vehicles, although these are more commonly designed to use compressed natural gas.

The density of LNG is roughly 0.41 to 0.5 kg/L, depending on temperature, pressure and composition, compared to water at 1.0 kg/L. The heat value depends on the source of gas that is used and the process that is used to liquefy the gas. The higher heating value of LNG is estimated to be 24 MJ/L at −164 degrees Celsius. This corresponds to a lower heating value of 21 MJ/L.

The natural gas fed into the LNG plant will be treated to remove water, hydrogen sulfide, carbon dioxide and other components that will freeze (e.g., benzene) under the low temperatures needed for storage or be destructive to the liquefaction facility. LNG typically contains more than 90% methane. It also contains small amounts of ethane, propane, butane and some heavier alkanes. The purification process can be designed to give almost 100% methane.

The most important infrastructure needed for LNG production and transportation is an LNG plant consisting of one or more LNG trains, each of which is an independent unit for gas liquefaction. The largest LNG train in operation is the Train 4 of Atlantic LNG with a production capacity of 5.2 million metric ton per annum (mmtpa),^[1] followed by the SEGAS LNG plant in Egypt with a capacity of 5 mmtpa. The Qatargas II plant, under construction by QP and ExxonMobil, will have a production capacityof 7.5 mtpa for each of its two trains. LNG is loaded onto ships and delivered to a regasification terminal, where the LNG is reheated and turned into gas. Regasification terminals are usually connected to a storage and pipeline distribution network to distribute natural gas to local distribution companies (LDCs) or Independent Power Plants (IPPs).

In 1964, the UK and France were the LNG buyers under the world’s first LNG trade from Algeria, witnessing a new era of energy. As most LNG plants are located in "stranded" areas not served by pipelines, the costs of LNG treatment and transportation were so huge that development has been slow during the past half century. The construction of an LNG plant costs USD 1-3 billion, a receiving terminal costs USD 0.5-1 billion, and LNG vessels cost USD 0.2-0.3 billion. Compared with the crude oil, the natural gas market is small but mature. The commercial development of LNG is a style called value chain, which means LNG suppliers first confirm the downstream buyers and then sign 20-25 year contracts with strict terms and structures for gas pricing. Only when the customers were confirmed and the development of a greenfield project deemed economically feasible could the sponsors of an LNG project invest in their development and operation. Thus, the LNG business has been regarded as a game of the rich, where only players with strong financial and political resources could get involved. Major international oil companies (IOCs) such as BP, ExxonMobil, Royal Dutch Shell, BG Group; and national oil companies (NOCs) such as Pertamina, Petronas are active players. Japan, South Korea and Taiwan import large volumes of LNG due to their shortage of energy. In 2002, Japan imported 54 million tons of LNG, representing 48% of the LNG trade around the world that year. Also in 2002, South Korea imported 17.7 million tons and Taiwan 5.33 million tons. These three major buyers purchase approximately 70% of the world's LNG demand.

In the early 2000s, as more players take part in investment, both in downstream and upstream, and new technologies are adopted, the prices for construction of LNG plants, receiving terminals and vessels have fallen, making LNG a more competitive means of energy distribution, but increasing material costs and demand for construction contractors have driven up prices in the last few years. The standard price for a 125,000-cubic-meter LNG vessel built in European and Japanese shipyards used to be USD 250 million. When Korean and Chinese shipyards entered the race, increased competition reduced profit margins and improved efficiency, reducing costs 60%. Costs in US dollar terms also declined due to the devaluation of the currencies of the world's largest shipbuilders, Japan and Korean. Since 2004, ship costs have increased due to a large number of orders increasing demand for shipyard slots. The per-ton construction cost of an LNG liquefaction plant fell steadily from the 1970s through the 1990s, with the cost reduced approximately 35%.

Due to energy shortage concerns, many new LNG terminals are being contemplated in the United States. Concerns over the safety of such facilities has created extensive controversy in the regions where plans have been created to build such facilities. One such location is in the Long Island Sound between Connecticut and Long Island. Broadwater Energy, an effort of TransCanada Corp. and Shell, wishes to build an LNG terminal in the sound on the New York side. Local politicians including the Suffolk County Executive have raised questions about the terminal. New York Senators Chuck Schumer and Hillary Clinton have both announced their opposition to the project. Several terminal proposals along the coast of Maine have also been met with high levels of resistance and questions. The Government Accountability Office (GAO) is planning to deliver its report to Congress on LNG in the spring of 2006

Trade

LNG is shipped around the world in specially constructed seagoing vessels. The trade of LNG is completed by signing a sale and purchase agreement (SPA) between a supplier and receiving terminal, and by signing a gas sale agreement (GSA) between a receiving terminal and end-users. Most of the contract terms used to be DES or Ex Ship, which meant the seller was responsible for the transportation. But with low shipbuilding costs, and the buyer preferring to ensure reliable and stable supply, there are more and more contract terms of FOB, under which the buyer is responsible for the transportation, which is realized by the buyer owning the vessel or signing a long-term charter agreement with independent carriers.

The agreements for LNG trade used to be long-term portfolios that were relatively inflexible both in price and volume. If the annual contract quantity is confirmed, the buyer is obliged to take and pay for the product, or pay for it even if not taken, which is called the obligation of take or pay (TOP).

In contrast to LNG imported to North America, where the price is pegged to Henry Hub, most of the LNG imported to Asia is pegged to crude oil prices by a formula consisting of indexation called the Japan Crude Cocktail (JCC).

The pricing structure that has been widely used in Asian LNG SPAs is as follows: PLNG = A+B×Pcrude oil, where A refers to a term that represents various non-oil factors, but usually a constant determined by negotiation at a level that can prevent LNG prices from falling below a certain level. It thus varies regardless of oil price fluctuation. Typical figures of ex-ship contracts range from USD 0.7 to 0.9. B is a degree of indexation to oil prices; typical figures are 0.1485 or 0.1558, and Pcrude oil usually denominated in JCC. PLNG and Pcrude oil stand for price of oil in USD per million British Thermal Unit (MMBTU (in the fuel industry, M stands for 1000 and MM for 1 000 000)). Many formula include an S-curve, where the price formula is different above and below a certain oil price, to dampen the impact of high oil prices on the uyer, and low oil prices on the seller. With new demand from China, India and US increasing dramatically, and crude oil price skyrocketing, the LNG price is on the rise too.

In the mid 1990s, LNG was a buyer's market. At the request of buyers, the SPAs began to adopt some flexibilities on volume and price. The buyers had more upward and downward flexibilities in TOP, and short-term SPAs less than 15 years came into effect. At the same time, alternative destinations for cargo and arbitrage were also allowed. By the turn of the 21st century, the market was again in favor of sellers. Sellers now propose rigid SPAs and would like an association similar to OPEC to be established to protect their interests. It is certain that the competition between sellers and buyers will go on.

Until 2003, LNG prices have closely followed oil prices. Since then, LNG prices to Europe and Japan, have been lower than oil prices, though the link between LNG and Oil is still strong In contrast, recent prices in the US and UK markets have skyrocketed then fallen as a result of changes in supply and storage.

Price arbitrage has not yet led to a convergence of regional prices and to a global market For the time being, the market is a seller’s market (hence net-back is best estimation for prices). The balance of market risks between the buyers (taking most of the volume risks through off-take obligations) and the sellers (taking most of the value risks through indexation to crude oil and petroleum products) is changing.

Receiving terminals exist in several countries, including the USA, countries in Europe and in Aisa, allowing gas imports from other areas.

The U.S. Department of Energy's Energy Information Administration provides estimates of LNG trade in 2002 as follows:

In 2005, Egyptian NG production outpaced consumption and it joined the LNG exporting countries.

Global LNG demand is expected to reach 500 bcm/year by 2015 and 635 bcm/year in 2020. The International Energy Agency estimates that European imports of gas from Africa and the Middle East (mainly in the form of LNG) will quadruple by 2030 (source: Economist, 14/4/07, p39

Environmental concerns

Natural gas can be considered as the most environmentally friendly of the fossil fuels, because it has the lowest CO2 emissions per unit of energy^{[citation needed]} and because it is suitable for use in high efficiency combined cycle power stations. Because of the energy required to liquefy and to transport it, the environmental performance of LNG is inferior to that of natural gas^{[citation needed]}, although in most cases LNG is still superior to alternatives such as fuel oil or coal^{[citation needed]}. This is particularly so in the case where the source gas would otherwise be flared.

Some environmental groups argue strongly against the use of LNG^{[citation needed]}. One (unspecified) study^{[citation needed]} concluded that a proposed LNG terminal proposed near Oxnard, California will emit 25 million tons of greenhouse gases per year.^{[citation needed]} On the West Coast of the United States where up to five new LNG importation terminals have been proposed, environmental groups, such as Pacific Environment, Ratepayers for Affordable Clean Energy (RACE), and Rising Tide have moved to oppose them.^[2] Whilst natural gas power plants emit approximately half the carbon dioxide of an equivalent coal power plant^{[citation needed]}, the natural gas combustion required to produce and transport LNG to the plants adds 20 to 40 percent more carbon dioxide than burning natural gas alone.^[3] With the extraction, processing, chilling transportation and conversion back to a usable form is taken into account LNG is a major source of greenhouse gases^{[citation needed]}.

[edit] Safety and accidents

In its liquid state, LNG is not explosive. For an explosion to occur with LNG, it must first vaporize, then mix with air in the proper proportions (the flammable range is 5% to 15%), and then be ignited. Serious accidents involving LNG to date are listed below:

• 1944, 20 October. The East Ohio Natural Gas Company experienced a failure of an LNG tank in Cleveland, Ohio.^[4] 128 people perished in the explosion and fire. The tank did not have a dike retaining wall, and it was made during World War II, when metal rationing was very strict. The steel of the tank was made with an extremely low amount of nickel, which made the tank brittle when exposed to the extreme cold of LNG, and the tank ruptured, spilling LNG into the city sewer system.

• 1973 February, Staten Island, New York. While repairing the interior of an empty storage tank, a fire started.^[4] The pressure increased inside the tank so fast the concrete dome on the tank lifted and then collapsed falling inside the tank and killing the 37 construction workers below. No LNG was involved in this incident.

• 1979 October, Lusby, Maryland, at the Cove Point LNG facility a pump seal failed, releasing gas vapors, which entered and settled in an electrical conduit.^[4] A worker switched off a circuit breaker, igniting the gas vapors, killing a worker and causing heavy damage to the building. National fire codes were changed as a result of the accident.

• 2004, 19 January, Skikda, Algeria. Explosion at Sonatrach LNG liquefaction facility.^[4] 27 killed, 80 injured, three LNG trains destroyed, 2004 production was down 76% for the year. A cold hydrocarbon leak occurred and hydrocarbon gases were drawn into the combustion air for a high-pressure steam boiler. The explosion inside the boiler fire box precipitated a larger explosion of vapors outside the box.

Seaborne LNG transport tankers (including their loading terminals) have not had a major^[5] accident in over 47,000 voyages^[6] since the first test cargo in 1959. There have, however, been several significant incidents with LNG ships, but with only minor spills. In addition to accidents, terrorism experts are concerned that intentional sabotage could lead to unprecedented releases, resulting in massive fires and other damaging effects. The latter may include detonations (producing large blast waves) and deflagration-to-detonation transition phenomena. As the Department of Energy notes in its December 2004 report (Sandia National Labs, SAND2004-6258), the available testing data on LNG spills are based on releases of very small size in comparison to releases expected from intentional attacks. The Sandia report assumes that a ship's tank with a hole in it (up to several square meters in area) from any cause will drain by gravity while a fire burns outside the ship. This may not be a valid assumption for holes in tanks below the waterline of a vessel, or at the waterline. If the hole is below the waterline, the LNG in contact with water may vaporize violently in a rapid-phase transition, possibly pressurizing the tank and forcing LNG and gas to exit the hole under the water. The Sandia report notes the experimentally observed phenomena of RPT causing extensive damage to a marine structure, and gives a corresponding yield of high explosive for a relatively small LNG spill into water. The extent of damage to an LNG tank from RPT phenomena that could lead to a cascading failure of all tanks on an LNG ship remains to be determined. For a hole at the waterline in the presence of a pool fire, the contact of water with the LNG may also cause RPT at the hole, and might cause a dynamic pressure condition within the holed tank, leading to a higher rate of LNG release than from gravity alone. For a hole above the waterline in the presence of fire, the volume of LNG released from an unvented rigid tank must be replaced by an equal volume of some mass entering the tank. In the context of the Sandia report analysis, the available mass to enter the tank above the waterline consists of one or more of the following components: LNG vapor, air, flame, and combustion products. If the tank is rigid and vented while holed, the same components are presumably present to enter the holed tank. This import of mass and corresponding heat energy might be expected to vaporize the LNG in the tank, and may create the conditions needed for a detonation inside the tank. The recent GAO report of February 2007 (GAO-07-316) surveyed experts who were not unanimous in confirming the findings of the original Sandia report. The expert GAO panel concluded that additional research was needed to understand the potential effects of LNG ship accidents and malevents. The Department of Energy has funded Sandia to continue research, but not all of the topics identified by the GAO study are included in the current Sandia research agenda. While natural gas explosions are common in confined spaces such as houses, claims that outdoor releases of natural gas are at low risk of explosion must consider adjacent structures as enclosed spaces. If an LNG ship release occurs, any buildings, vehicles, or escort vessels close to the release constitute enclosed spaces where natural gas explosions may occur. Despite intense local opposition, the Federal Energy Regulatory Commission has approved a site permit for an LNG terminal in Fall River, Massachusetts in a densely populated harbor area.

[edit] Storage

LNG storage tank at EG LNG

LNG above-ground tanks are mainly of double-wall, high-nickel steel construction with extremely efficient insulation between the walls. Large tanks are low aspect ratio (height to width) and cylindrical in design with a domed roof. Storage pressures in these tanks are very low, less than 5 psig. Sometimes more expensive frozen-earth, underground storage is used. Pre-stressed concrete backed up with suitable thermal insulation, are designed to be both under and above ground to suit sites conditions and local safety regulations and requirements. Smaller quantities, 190,000 US gallons (700 m³) and less, are stored in horizontal or vertical, vacuum-jacketed, pressure vessels. These tanks may be at pressures anywhere from less than 5 psig to over 250 psig (35 to 1700 kPa gauge pressure).

LNG must be maintained cold (at least below −117 °F or −83 °C) to remain a liquid, independent of pressure. There will inevitably be some degree of boil-off as a result of heat gained from the outside ambient atmosphere. This gas may be returned to storage by recompression and reliquefaction, or used in the liquefaction process. When gas is cooled to -160°C (-260°F), NG becomes liquid and is much more compact-occupying 1/600 of its gas volume. Where long overseas distances are involved, transporting NG in its liquid state is more economical. The LNG industry is set for a large and sustained expansion as improved technology has reduced transportation costs of formerly stranded NG reserves as a liquid to consumer markets.

[edit] Transportation

Transportation and supply is an important aspect of the gas business, since LNG reserves are normally quite distant from consumer markets. LNG has far more mass than oil to transport, and most gas is transported by pipelines. There is a pipeline network in the former USSR, Europe and North America. LNG, when in its gaseous state is rather bulky. Gas travels much faster than oil though a high-pressure pipeline can transmit only about a fifth of the amount of energy per day. As well as pipelines, LNG is transported using both road/rail truck and ship. LNG will be sometimes taken to cryogenic temperatures to reduce the mass. Recently ship-to-ship transfer (STS) transfers have been carried out by Exmar the Belgian gas tanker owner in the Gulf Of Mexico which involved the transfer of LNG from the LNG regasification vessel (LNGRV) to a conventional LNG carrier. Prior to this commercial exercise LNG had only ever been transferred between ships on a handful of occasions as a necessity following an incident.

[edit] Refrigeration

The insulation, as efficient as it is, will not keep LNG cold enough by itself. As a result, LNG is stored as a "boiling cryogen", that is, kept at its boiling point for the pressure at which it is stored. Because the released vapor carries heat away, the remaining LNG will stay at near constant temperature if kept at constant pressure by letting the vapor off. This phenomenon is called "autorefrigeration"

شما می توانید با مراجعه به قسمت آرشیو موضوعي (سمت چپ و پايين)  عنوان  مرتبط با موضوع مورد نظر خود را بیابید.

به امید اینکه مطالب این وبلاگ برای شما مفید باشد.

 نظرات و انتقادها و پيشنهادهاي خود را از من دریغ نکنید.

mohamad27m@yahoo.com

mohamad27m@mail.com

mohamad27m@gmail.com

mohamad27m@hush.com

mohamad27m@inbox.com

mohamad27m@hushmail.com

mohamad27m@mail2engineer.com

mohamad27m@techemail.com

 گروه بچه های مهندسی پالایش :

group.yahoo.com/group/refining-engineers

بسته به تعريفي كه از بيوتكنولوژي داريم، بيوتكنولوژي مي­تواند به­عنوان يكي از قديمي­ترين تكنولوژي­هاي صنعتي يا يكي از  جديدترين تكنولوژي­ها مورد توجه قرار گيرد. با وجود اين براي مهندسي شيمي مسالة اصلي مقياس عملياتي مي­باشد. در بيوتكنولوژي جديد، اغلب محصولات داراي ارزش بالا بوده و به حجم كمي از آنها نياز است؛ البته صنايع بيولوژيك با حجم توليد زياد نيز همچنان داراي اهميت مي­باشد. اما تفاوت­هاي كليدي بين فريندهاي بيولوژيك و فرآيندهاي شيميايي وجود دارد كه بايستي نقش مهندسي شيمي را با توجه به اين تفاوت­ها مورد بازبيني قرار داد. در اين مقاله كه از مجله  The Chemical Engineering Journal, 50 B9-B16 انتخاب و ترجمه شده است، فرآيندهاي بيوتكنولوژي متداول با فرآيندهاي شيميايي مشابه مقايسه شده و نقش مهندسي شيمي در طراحي و توسعه فرآيند مورد نظر مورد بررسي قرار گرفته است: 

همان­طور كه صنايع شيميايي تا مدت زيادي فقط توسط شيميست‌ها (و نه مهندسان شيمي) مورد بررسي قرار مي­گرفت، فرآيندهاي زيستي نيز هنوز توسط ميكروبيولوژيست‌هاي صنعتي مورد بررسي قرار مي­گيرد. بنابراين بسياري از حوزه­هاي بيولوژيكي وجود دارد كه در آنها مي­توان به­وسيلة كاربرد مفاهيم سادة مهندسي، فرايندهاي بيولوژيكي را توسعه داد.

روند تحول مفهوم بيوتكنولوژي از نظر مهندسان شيمي

اگر چه بيوتكنولوژي يك تكنولوژي نوين محسوب مي­شود، اما مي­توان به­عنوان يكي از قديميترين تكنولوژي‌هاي صنعتي نيز از آن ياد كرد. براي مهندسان بيوشيمي، استفادة صحيح از ميكروارگانيزم­ها براي توليد آبجو، مشروب و پنير، از قديم مطرح بوده است. همچنين تصفية بيولوژيكي پسمانده­ها و پساب­ها نيز مطرح بوده است كه جزو بيوتكنولوژي محسوب مي­شود.

حتي خود كلمه "بيوتكنولوژي" نيز آنطور كه تصور مي­شود جديد نيست. اين كلمه در ابتدا در يك كتاب و در سال 1919 توسط يك مجاري بنام Erkey مطرح شد. در اين كتاب همة خطوط كاري توليد محصولات توسط ميكروارگانيزمها توضيح داده شده است. اين موضوع بطور مشخص براي كشاورزي مطرح شد، اما در حدود همان زمان بود كه chaim wiezman (از دانشگاه منچستر) يك فرايند صنعتي را براي توليد انبوه استون توسعه داده بود كه اين عمل توسط فرمانتاسيون صورت مي­گرفت. اين فرآيند با تعريفي كه به­وسيلة Erkey ارائه شده بود منطبق بود.

با پيشرفت بيوتكنولوژي مفهوم آن نيز تغيير پيدا كرد تا اينكه مترادف با "تكنولوژي تخمير" شد. اين تعريف از بيوتكنولوژي در يك مقالة چاپ شده در مجلة جديد "بيوتكنولوژي و مهندسي زيستي" توسط Elmer Gaden Jr. در سال 1962 مطرح شد. تعريف اساساً مشابهي نيز هنگامي‌كه اتحادية بيوتكنولوژي اروپا تاسيس شد مورد استفاده قرار گرفت. اما درست 1 سال بعد، اين كلمه (بيوتكنولوژي) مجدداً داخل يك نشرية مهندسي ژنتيك تعريف شد تا "توسعة علمي و اقتصادي در زمينة ژنتيك" را تشريح نمايد. تعريف اخير تعريفي است كه مورد توجه كميسيون علائم تجاري آمريكا قرار گرفت و به‌دليل تفاوت زياد با تعريف قبلي به­صورت يك علامت تجاري (Trade Mark) مورد استفادة آن مجله مهندسي ژنتيك قرار گرفت.

رفته­رفته با ارائه نتايج و محصولات مهندسي ژنتيك، تمايز بين اين دو تعريف از بين رفت و بنابراين زماني كه كلمة بيوتكنولوژي مورد استفاده قرار مي­گيرد روشن نيست كه آيا علم "دست­كاري ژنتيكي" مورد نظر است يا "استثمار صنعتي سيستم­هاي زنده" مد نظر است. بنابراين از نظر اقتصادي براي مهندسي شيمي صورت كليدي در فرآيندهاي بيوتكنولوژي صنعتي، "استفاده از ارگانيزم‌هاي زنده براي توليد محصولات مناسب" مي­باشد. با اين وجود تفاوت عمده­اي بين بسياري از محصولات بيوتكنولوژي جديد و محصولات بيوتكنولوژي قديمي وجود دارد. 

تفاوت بيوتكنولوژي جديد و قديم براي مهندسي شيمي: در بيوتكنولوژي جديد اغلب محصولات، داراي ارزش بالا مي­باشند كه به مقادير كمي از آنها نياز است (معمولاً براي اهداف تشخيصي يا پزشكي)؛ در حالي كه در بيوتكنولوژي قديمي عموماً محصولات داراي ارزش كم تا متوسط توليد مي­شوند و مقادير آنها طوري است كه به تجهيزات فرآيندي با مقياس بزرگ نياز است.

محصولات بيوتكنولوژي قديمي، با فرآيندهاي با مقياس نسبتاً بزرگ، نقش نسبتاً قديمي را براي مهندسي شيمي بوجود مي‌آورند. آنها با مسائل مشابهي نظير مكانيك سيالات، انتقال جرم و حرارت و فرآيندهاي واكنش و جداسازي روبرو هستند كه اين مسائل در متن مهندسي شيمي قرار دارد. البته تفاوت اساسي، در سيستم­هاي زنده­اي است كه به­كار مي­رود. بنابراين يك مهندس شيمي كه در اين زمينه مشغول فعاليت است، تنها بايد دانشي از فرآيندهاي زيستي را توأم با دانش خود كند. اين موضوع مختص مهندس بيوشيمي است.

در مقابل، مسائلي كه با فرآيندهاي بسيار كوچك بيوتكنولوژي همراه است، در حوزه­هاي قديمي مهندسي شيمي قرار نمي­گيرند و بسياري از آنها داراي فرآيندهاي منحصر به­فرد هستند. در غالب اين موارد، بدليل كوچك بودن مقياس مورد استفاده، "بازدهي" يك مسأله مهم نيست و لذا نقش مهندسي شيمي در اين موارد خيلي مشخص نيست. تقريباً اغلب اين فرآيندها به بيوتكنولوژيست مربوط مي‌شود تا مهندس بيوشيمي. 

دلايل توسعة آيندة فرآيندهاي بيوتكنولوژي:

هيچ شكي نيست كه صنعت بيوتكنولوژي رو به رشد خواهد بود (اگر چه راهي طولاني براي آن وجود دارد) تا تسلط پيش­بيني شدة آن بر صنايع شيميايي رايج تحقق يابد.

يكي از دلائل اصلي براي اطمينان از اين توسعة مداوم، آن است كه فرآيندهاي بيوتكنولوژي برمبناي منابع تجديد­پذير استوار هستند. در نتيجه اين مورد زماني اهميت پيدا خواهد كرد كه مواد خام تجديدنا­پذير معمولي رو به اضمحلال هستند. بنابراين همة محصولات از مواد هيدروكربني تجديد­پذير توليد خواهند شد؛ به­خصوص آنهايي كه پساب‌هاي صنايع غذايي و كشاورزي را تشكيل مي­دهند (به­عنوان مثال انبوهي از زايدات غذايي). اين پسمانده­ها، سوبستراهاي (منبع غذايي) ايده­آلي براي فرآيندهاي بيولوژيكي مهيا مي­كنند. اقتصادي بودن تبديل اين پسمانده­ها به­وسيلة روش­هاي بيوتكنولوژي بيشتر از فرآيندهاي شيميايي بوده است.

علاوه بر ­اين اخيراً ملاحظات سياست جهاني بر آن بوده است كه تا جائي كه ممكن است محصولات از طبيعت (natural-production) توليد شوند و اين بطور ضمني دلالت بر اين دارد كه توليد از روش بيولوژيكي تقريباً در هر جايي كه شدني و مناسب باشد ترجيح داده شود.

دليل ديگر براي گسترش مداوم بيوتكنولوژي، حوزة روبه­رشد محصولات باارزشي است كه از روش‌هاي بيولوژيكي قديمي و يا دست­ورزي ژنتيكي توليد مي­شود. محدودة كاملي از محصولات ممكن كه از روش بيوتكنولوژي قابل توليد هستند شناخته شده است. به­عنوان مثال، هم­اكنون رشد سلول‌هاي بافت انساني در كشت انبوه و در تجهيزات فرمانتاسيون ساده به­طور روتين انجام مي­شود. محصولات ممكن اين بافت‌ها هم­اكنون تحت بررسي است.  

مقايسة حوزه­هاي مختلف فرايندهاي بيوتكنولوژيكي

جدول 1، حوزه­هاي تقريبي فرآيندهاي بيوتكنولوژيكي را نشان مي­دهد. علاوه بر محصولات حاصل از صنايع بيولوژيكي سنتي نظير مشروبات الكلي، غذاهاي تخميري، آنزيم‌ها، آنتي بيوتيك‌ها و تصفيه پساب‌ها محصولات ديگري نيز وجود دارند. بسياري از محصولات با ارزش بالا كه در جدول 1 آمده است به‌ويژه آنهايي كه مواد شيميايي مورد نياز در آنها مقادير بسيار كمي هستند، اغلب در حد چند كيلوگرم در سال مي­باشند و تقريباً براي توسعة آنها جنبه اقتصادي توليد، يك عامل محدودكننده نمي­باشد.

جايگزين­هاي محصولات طبيعي از جمله پروتئين تك­ياخته (SCP)، در صورت موفقيت، متقابلاً بايد با محصولات پروتئيني كشاورزي معمولي رقابت كنند. در عوض زماني كه دسترسي به نفت آسان است، جايگزين‌هاي توليد سنتزي محصولات شيميايي نظير اسيدهاي آلي و حلال‌ها درصورتي امكان­پذير است كه از نظر اقتصادي پيشرفتي صورت گرفته باشد. در اين مورد بايد احتمالاً منتظر از بين­ رفتن اين مادة خام بود.

همچنين جايگزين‌هاي محصولات پتروشيمي نظير الكل‌هاي سوختي كه در مقياس وسيع توليد مي­شوند بايد در مقياس خيلي بيشتري نسبت به آنچه كه فعلاً براي فرايندهاي بيولوژيكي قابل اجرا است توليد شوند تا اينكه اقتصادي و مقرون به صرفه باشند (به غير از تصفية پسمانده­ها).

آخرين گروه در جدول 1 بيشترين چالش را براي مهندس بيوشيمي ايجاد نموده است و در دهه­هاي قرن اخير هنوز صنعت بيوتكنولوژي در اين مورد قابل مقايسه با صنعت شيميايي بوده است. در اين حوزه عموماً بيوتكنولوژيست‌ها به جاي مهندسان وارد عمل مي­شوند و به شدت برتكنيك‌هاي Black-art (كه تخصص بيوتكنولوژيست­هاست) تكيه مي­شود. اين تكنيك­ها شايد براي محصولات با ارزش بالا و مقياس پايين كه راه ديگري براي توليد به روش بيوتكنولوژيكي ندارند كافي باشد اما براي مواردي كه در آنها فرايندهايي با مقياس بزرگ بكار مي­رود، ناكافي مي­باشد؛ چرا كه مسائل اقتصادي تعيين­كننده بوده و از لحاظ مهندسي تأثير هزينه مي­تواند به معناي تفاوت بين شكست و پيروزي باشد. 

متاسفانه بسياري از مهندسان بيوشيمي با توسعة بيوتكنولوژي به سمت بيوتكنولوژيست­شدن حركت كرده­اند كه در آن تحقيقات تكنولوژيكي بر محور توليد متمركز شده و زمينه­هاي وسيع مهندسي فرايند را رها كرده­اند. البته درحالي كه جايگاه مهمي براي بيوتكنولوژيست‌ها در توسعه صنعت وجود دارد، روشن است كه نقش مهندسي شيمي در بيوتكنولوژي بايد همان نقش مهندسي بيوشيمي باشد، نه بيوتكنولوژيست. 

تفاوت اصلي فرايندهاي زيستي با فرايندهاي شيميايي:

اگر چه برخي فرايندهاي بيوتكنولوژيكي اساساً مشابه با فرايندهاي شيميايي هستند و داراي سه مرحله اصلي يعني آماده­سازي مواد خام، واكنش و بازيافت محصول مي­باشند، اما تفاوت‌هاي بسيار مهمي نيز دارند. مهمترين اين تفاوت‌ها اغلب در تعداد نامحدود محصولاتي مي­باشد كه ممكن است از يك مادة خام به‌دست آيد؛ به‌دليل آنكه اين ماده خام صرفاً يك منبع غذايي (سوبسترا) براي رشد ميكروارگانيزم‌ها است.

معمولاً محصولات مورد نظر، فقط زائدات فرايند رشد ميكروبي هستند. در نتيجه واكنش از پيش تعيين‌شده­اي بر مبناي يك گروه به‌خصوص از واكنش­دهنده­ها وجود ندارد. قانون حاكم اين است كه محصول، تابع ميكروارگانيزم‌هايي است كه براي انجام واكنش انتخاب مي‌شوند. حتي اين نيز ويژگي مورد نظر را تضمين نمي­كند؛ زيرا همان ميكروارگانيزم‌ها كه در يك سوبسترا رشد مي­كنند، ممكن است به­عنوان مثال اتانول، اسيد لاكتيك، آنزيم به‌خصوص يا يك آنتي­بيوتيك توليد كنند. فقط كنترل دقيق شرايط فيزيكي يا انتخاب و زمان‌بندي برخي شرايط اطمينان خواهد داد كه محصول مطلوب همان محصولي است كه توليد مي­شود. جزء كليدي مخلوط واكنش (ميكروارگانيزم)، هم كاتاليزور واكنش است و هم محصول واكنش؛ كه در شروع واكنش به سادگي و به ميزان زيادي فراهم مي­شود.

براي اطمينان از صحيح بودن ميكروارگانيزم انتخاب شده و يا محصولي كه توليد مي­شود، بايد سوبسترا حاوي مقدار كمي از ميكروارگانيزم انتخاب شده در محيط كشت باشد و بنابراين از رقابت ساير ميكروارگانيزم‌ها ممانعت مي­گردد. مهمتر از اين، اشكال ديگر زندگي ميكروبي نيز مي­باشد كه بايد از سوبسترا حذف شوند. زيرا رقابت مستقيمي را با ميكروارگانيزم‌هاي مورد نظر خواهند داشت و گاهي با احتمال و موفقيت بيشتري تكثير مي­يابند. با استفاده از استريليزاسيون سوبسترا، جداسازي آنها از رقابت­كننده‌ها امكان­پذير است كه اين مورد بايد در سرتاسر واكنش دنبال شود.

از آنجا كه عمدة محصولات بيوتكنولوژي جديد، داراي ارزش فوق­العاده زياد و حجم كم مواد بيوشيميايي است، بنابراين فرآيندهاي بازيافت (جداسازي) براي اين محصولات ممكن است به­دليل مقادير كم مورد استفاده، نسبتاً هزينه‌بر و با صرف انرژي زياد صورت گيرد. همچنين براي به حداقل رساندن اتلاف محصول با ارزش فراوان، بايد بازدهي بالايي را در نظر گرفت. اين مورد با فرايندهاي بيولوژيكي قديمي‌تر صنايع غذايي و نوشيدني، آنتي­بيوتيك­ها و محصولات دارويي با ارزش متوسط و تصفية فاضلاب در تضاد مي­باشد. 

تفاوت فرايندي واكنش­هاي شيميايي و بيولوژيكي از نظر شرايط واكنش:

اغلب واكنش­هاي بيولوژيكي به­طور قابل توجهي آهسته­تر از واكنش‌هاي شيميايي انجام مي­شوند. برخلاف صنايع شيميايي كه در آنها فرايندهاي مداوم ترجيح دارد، عموماً اين واكنش‌هاي بيولوژيكي به صورت عمليات ناپيوسته (batch) صورت مي­گيرند. اغلب واكنش‌هاي بيولوژيكي توسط غلظت‌هاي پايين محصول ممانعت مي­شوند و اين خود دليل ديگري براي ارجح­بودن عمليات ناپيوسته (batch) مي­باشد. همچنين برخلاف اغلب واكنش‌هاي شيميايي، سرعت واكنش‌هاي بيولوژيكي نمي­تواند با افزايش دما و فشار افزايش يابد و اغلب بايد در تحت شرايط نسبتاً ملايم و نزديك به دماي محيط انجام شوند. محصولات مورد نظر نيز با گرما ناپايدار هستند و براي جلوگيري از تخريب آنها بايد انجام واكنش در تحت شرايط نسبتاً ملايم صورت گيرد. با وجود اين تفاوت‌ها در فرايندهاي بيولوژيكي و فرايندهاي شيميايي معمولي، بسياري از حوزه­ها وجود دارند كه يك مهندس بيوشيمي براي طراحي و اجراي عمليات فرايندهاي بيولوژيكي مي­تواند نقش داشته باشد. 

نتيجه:

مطالب بالا به برخي از شيوه­هايي متعددي كه در آنها مهندس فرايند يا شيمي در توسعه صنعتي بيوتكنولوژي مي­تواند نقش داشته باشد اشاره نمود. چالش‌هاي فراواني براي مهندس شيمي در بيوتكنولوژي وجود دارد كه بسياري از آنها هنوز بوجود نيامده‌اند. حوزه­هايي وجود دارند كه در آن مهندس و بيوتكنولوژيست بايد با يكديگر همكاري كنند تا اولاً مشكلات را مشخص كنند و ثانياً راه حل­ها را پيدا نمايند.

عموماً در مقايسه با فرايندهاي شيميايي، فرايندهاي بيولوژيكي، در سرعت‌هاي حجمي و غلظت‌هاي توليدي پايين صورت مي‌گيرند. ممكن است بابكار بردن برخي از روش­ها (به‌عنوان مثال، استفاده از تثبيت سلولي) فرآيند را بهبود داد. اما اين مورد نيز داراي محدوديت است زيرا برخي ميكروارگانيزم‌ها ممكن است شامل ويژگي­هاي فيزيولوژيكي و فيزيكي ايده­آل براي تثبيت نباشند؛ به­خصوص در آنجا كه رشد و حيات براي توليد نقش اساسي دارد. ممكن است برخي روش‌ها نيز جهت بهبود سرعت بكار روند؛ مثلاً سلول‌ها بتوانند براي چسبيدن به سطح، يا براي ارائه محصولات داخل سلولي يا خارج سلولي يا براي رهاسازي محصولات بعد از برانگيختن، مطابق با نيازهاي كلي فرايند، مهندسي شوند.

اكثر فرايندهاي تخميري كه از لحاظ تجاري بزرگ­مقياس موفق بوده­اند مقادير نسبتاً كمي را توليد كرده­اند. اين فرايندها، فرايندهايي بوده­اند كه به صورت غيراستريل كار كرده­اند. در بخش استريليزاسيون و نگهداري، هزينه­ها (اعم از عملياتي يا سرمايه­اي) قابل توجه هستند و هر فرايندي كه بتواند اين مراحل را نداشته باشد براي آن يك مزيت بحساب مي­آيد. اين مورد مي­تواند توسط دست­ورزي ژنتيكي صورت گيرد تا مزيت‌هاي مشابهي را به اين گونه­هاي ضعيف­تر ببخشد.

همچنين فرايندهاي با مقياس بزرگ و با موفقيت بيشتر فرايندهايي هستند كه شرايط فرايندي پايين­دستي نسبتاً ساده دارند. اگرچه اخيراً توجهات بسياري بر روي Scale up فرايندهاي جداسازي خاص شده است (بر مبناي تكنيك‌هاي قابل دسترس در آزمايشگاه تجزيه) اما تقريباً گران بوده و بنابراين به محصولات با ارزش بسيار بالا محدود مي­شوند. توسعه فرايندهاي جداسازي كم­هزينه يا از نظر ديگر تخميرهايي كه نياز به فرايندهاي پايين‌دستي كمتري دارند، هنوز براي مهندسي شيمي و بيوتكنولوژيست به طور يكسان به‌صورت يك چالش باقي مانده است.

نهايتاً شايد بيشترين مساله براي مهندسي شيمي در بيوتكنولوژي مواجهه با پسمانده­اي است كه از فرايندهاي تخميري حاصل مي­شود. برخلاف فرايندهاي شيميايي معمولاً محصولات جانبي خيلي كمي از يك محصول تخميري صنعتي به‌وجود مي‌آيد. اغلب بهترين حالت آن است كه بيومس بي­رمق را به صورت يك منبع غذايي حيواني به‌فروش برسانند.

اگر بيوتكنولوژي بخواهد با صنايع شيميايي (بجز در مواردي كه محصولات با ارزش بالا توليد مي‌شود) رقابت كند مشكلاتي خواهد داشت كه بايد تحليل و بررسي شود. 

منبع : شبکه تحليل گران تکنولوژي ايران