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CREATIVE WRITING ON CONCURRENCY
Concurrency is the ability of a database to allow multiple users to affect multiple transactions. This is one of the main properties that separates a database from other forms of data storage like spreadsheets.
The ability to offer concurrency is unique to databases. Spreadsheets or other flat file means of storage are often compared to databases, but they differ in this one important regard. Spreadsheets cannot offer several users the ability to view and work on the different data in the same file, because once the first user opens the file it is locked to other users. Other users can read the file, but may not edit data.
The problems caused by concurrency are even more important than the ability to support concurrent transactions. For example, when one user is changing data but has not yet saved (committed) that data, then the database should not allow other users who query the same data to view the changed, unsaved data. Instead the user should only view the original data.
Almost all databases deal with concurrency the same way, although the terminology may differ. The general principle is that changed but unsaved data is held in some sort of temporary log or file. Once it is saved, it is then written to the database’s physical storage in place of the original data. As long as the user performing the change has not saved the data, only he should be able to view the data he is changing. All other users querying for the same data should view the data that existed prior to the change. Once the user saves the data, new queries should reveal the new value of the data.
In computer science, concurrency is the decomposability property of a program, algorithm, or problem into order-independent or partially-ordered components or units.[1] This means that even if the concurrent units of the program, algorithm, or problem are executed out-of-order or in partial order, the final outcome will remain the same. This allows for parallel execution of the concurrent units, which can significantly improve overall speed of the execution in multi-processor and multi-core systems.
A number of mathematical models have been developed for general concurrent computation including Petri nets, process calculi, the Parallel Random Access Machine model, the Actor model and the Reo Coordination Language.
History
As Leslie Lamport (2015) notes, “While concurrent program execution had been considered for years, the computer science of concurrency began with Edsger Dijkstra‘s seminal 1965 paper that introduced the mutual exclusion problem. (…) The ensuing decades have seen a huge growth of interest in concurrency—particularly in distributed systems. Looking back at the origins of the field, what stands out is the fundamental role played by Edsger Dijkstra”.[2]
Issues
Because computations in a concurrent system can interact with each other while being executed, the number of possible execution paths in the system can be extremely large, and the resulting outcome can be indeterminate. Concurrent use of shared resources can be a source of indeterminacy leading to issues such as deadlocks, and resource starvation.[3]
Design of concurrent systems often entails finding reliable techniques for coordinating their execution, data exchange, memory allocation, and execution scheduling to minimize response time and maximise throughput.[4]
Theory
Concurrency theory has been an active field of research in theoretical computer science. One of the first proposals was Carl Adam Petri‘s seminal work on Petri Nets in the early 1960s. In the years since, a wide variety of formalisms have been developed for modeling and reasoning about concurrency.
Models
A number of formalisms for modeling and understanding concurrent systems have been developed, including:[5]
- The Parallel Random Access Machine[6]
- The Actor model
- Computational bridging models such as the Bulk Synchronous Parallel (BSP) model
- Petri nets
- Process calculi
- Communicating sequential processes (CSP) model
- Tuple spaces, e.g., Linda
- Simple Concurrent Object-Oriented Programming (SCOOP)
- Reo Coordination Language
Some of these models of concurrency are primarily intended to support reasoning and specification, while others can be used through the entire development cycle, including design, implementation, proof, testing and simulation of concurrent systems. Some of these are based on message passing, while others have different mechanisms for concurrency.
The proliferation of different models of concurrency has motivated some researchers to develop ways to unify these different theoretical models. For example, Lee and Sangiovanni-Vincentelli have demonstrated that a so-called “tagged-signal” model can be used to provide a common framework for defining the denotational semantics of a variety of different models of concurrency,[7] while Nielsen, Sassone, and Winskel have demonstrated that category theory can be used to provide a similar unified understanding of different models.[8]
The Concurrency Representation Theorem in the Actor model provides a fairly general way to represent concurrent systems that are closed in the sense that they do not receive communications from outside. (Other concurrency systems, e.g., process calculi can be modeled in the Actor model using a two-phase commit protocol.[9]) The mathematical denotation denoted by a closed system S is constructed increasingly better approximations from an initial behavior called ⊥S using a behavior approximating function progressionSto construct a denotation (meaning ) for S as follows:[10]
-
- DenoteS ≡ ⊔i∈ω progressionSi(⊥S)
In this way, S can be mathematically characterized in terms of all its possible behaviors.
Logics
Various types of temporal logic[11] can be used to help reason about concurrent systems. Some of these logics, such as linear temporal logic and computational tree logic, allow assertions to be made about the sequences of states that a concurrent system can pass through. Others, such as action computational tree logic, Hennessy-Milner logic, and Lamport’s temporal logic of actions, build their assertions from sequences of actions (changes in state). The principal application of these logics is in writing specifications for concurrent systems.[3]
Science city Open Text Book
Science City, Kolkata is the largest science centre in the Indian subcontinent[4] under National Council of Science Museums (NCSM), Ministry of Culture, Government of India, is at the crossing of Eastern Metropolitan Bypass and J B S Haldane avenue, Kolkata. It is considered by some people as the most distinguished landmark in post-independence Kolkata.[5] Saroj Ghose, the first director general of NCSM, who is credited with having conceptualised this centre in 1997.[6] This centre was inaugurated by two parts: the ‘Convention Centre Complex’ was unveiled on 21 December 1996 by Paul Jozef Crutzen in presence of the then chief minister Jyoti Basu and the whole centre was opened by the then prime minister Inder Kumar Gujral on 1 July 1997. On 10 January 2010, prime minister of India, Manmohan Singh laid the foundation stone for the second phase of Science City in presence of the then chief minister of West Bengal, Buddhadeb Bhattacharjee.[7]
Science City has added yet another state-of-the-art facility called “Science on a Sphere”, the first of its kind in the eastern part of the country. This room sized spherical projection system uses computers and video projectors to display planetary data onto a 1.80 meter diameter sphere, analogous to a giant animated globe. This is an effective educational tool to help illustrate Earth’s dynamic processes and associated science to people of all ages. Animated images of Earth’s land, oceans and atmosphere can be simulated on the sphere to explain what are otherwise complex processes, in a way that is simultaneously intuitive and captivating. This system is expected to provide a platform for better understanding of complex environmental processes.
Dynamotion Hall
Hands-on and interactive exhibits on various topics of science encouraging visitors to experience with props and enjoy the underlying scientific principles.
- Illusions. A permanent exhibition on the world of illusions with interactive exhibits, explores how motion and placement make a different in the visual perception.
- Powers of Ten. 43 exhibits unfold the smallest or the biggest of the universe through zooming in or out in the order of ten.
- Fresh Water Aquarium. Variety of fresh water fishes in 26 tanks; provide the bio-diversity of the fish species.
- Live Butterfly Enclave. A colony of live butterflies hatched here and screening of a film Rang Bahari Prajapati on life cycle of butterfly.
- Science On a Sphere. The spherical projection system created by NOAA. Each show of 30 minutes duration for around 70 people at a time.
The Science On a Sphere show at Dynamotion Hall.
Earth Exploration Hall
Inaugurated on 6 December 2008 by Ambika Soni, the then Union Minister for Culture, India. A permanent exhibition on earth is housed in a two storied hemispherical building that displays the details of the southern hemisphere in the ground floor and northern hemisphere in the first floor. Slicing a huge earth globe at the centre of the hall into 12 segments vertically in each hemisphere, important features of each segment such as physical geography, lands and people, flora and fauna and other dynamic natural phenomenon on earth have been highlighted around the central globe with the modern display technologies such as attractive visuals, interactive multimedia, video walls, panoramic videos, tilting tables, computer kiosks and 3-D effects theatre wearing a special Polaroid spectacle.
A full grown butterfly with eggs in controlled environment at Science City, Kolkata.
Transverse Wave Motion inside Science City
Space Odyssey
Comprising Space Theatre equipped with Helios Star Ball planetarium supported by 150 special effect projectors and Astrovision 10/70 Large format Film Projection system housed in a 23-meter diameter tilted dome having unidirectional seating arrangements for 360 person immersive shows on sciences. Now the Astrovision film Adventures in Wild California [8] of 40 minutes duration has been screening from June, 2013.
- 3-D Vision Theater. A show based on stereo back projection system where visitors experience 3D effect by Polaroid spectacles.
- Mirror Magic. There are 35 exhibits based on reflection of light.
- Time Machine. 30-seater motion simulator provides virtual experience of space flight or journey into unknown world sitting in a casual maneuvered by hydraulic motion control system.
Maritime Centre
Depicts maritime history of India, artifacts, dioramas and interactive exhibits on shipping and navigation systems. There is an unmanned quiz corner also.
The Science Park. Dec. 2015
In a tropical country like India, the outdoor is sunny and more inviting than the indoors for most part of the year. In a Science Park, people come closer to plants, animals and other objects in their natural surroundings and also learn about the basic principles of science in an open air learning environment. The park interactive exhibits are engineered so as to tolerate all the weather. Science Park has become the integral part in all the centres of NCSM. It comprises Caterpillar Ride, Gravity Coaster, Musical Fountain, Road Train, Cable Cars, Monorail Cycle, butterfly nursery and several exhibits on physical and life sciences and a maze set up in a lush green ambience. there are many people came in different states
Convention Centre Complex
Convention Centre Complex. 360 degree view.
- Grand Theater: 2232 seating capacity main auditorium with stage for 100 performers at a time is the largest auditorium in eastern India.
- Mini Auditorium: 392 seating capacity, with stage for 30 performers at a time is ideal for smaller conferences and shows.
- Seminar Building: Comprising eleven halls, four with seating capacity of 100 persons, two with seating capacity of 40 persons each, two with seating capacity of 30 persons each, two with seating capacity of 15 persons and a meeting room for 12 persons, is ideal venue for conference, seminars, meeting and workshops.
(2)AMMONIA PRODUCTION BY HABER’S PROCESS
Manufacture of ammonia by Haber’s synthesis method
Ammonia (NH3) is an important compound of nitrogen and hydrogen. It is produced by the natural decomposition of animal and vegetable bodies. The death and decay of plants and animals cause the nitrogen compounds present in them to get decomposed, giving ammonia. Ammonia also occurs in the soil in the form of ammonium salts.
Preparation of Ammonia
Ammonia is prepared by the following methods:
From ammonium chloride
Ammonia gas is usually prepared in the laboratory by gently heating ammonium chloride (NH4Cl) and slaked lime [Ca(OH)2].
Fig: 13.3 – Preparation of ammoniaAmmonia gas is lighter than air, necessitating its collection by the downward displacement of air. Because it is highly soluble in water it cannot be collected over it. Passing ammonia gas over quicklime (CaO) dries it. Being a basic gas, ammonia cannot be dried by passing it through concentrated sulphuric acid or phosphorus pentoxide (P2O5), as it reacts with them to form ammonium sulphate or ammonium phosphate respectively.
Calcium chloride also cannot be used for drying ammonia gas as it forms ammoniates with CaCl2.
By the hydrolysis of metal nitrides
Hydrolyzing metal nitrides like magnesium and aluminium nitrides, with water or alkalies, can also produce ammonia gas.
Manufacture of Ammonia
Haber’s process
The manufacture of ammonia by Haber’s process involves the direct combination of nitrogen and hydrogen.
This reaction is, (a) reversible, (b) exothermic, and (c) proceeds with a decrease in volume. According to the Le Chatelier’s principle, the favorable conditions for the formation of ammonia are,
Low temperature
The temperature should be remain as low as possible, (although at unusually low temperatures, the rate of reaction becomes slow). It has been found that the temperature, which optimizes the yield of ammonia for the reaction, is maximum at about 500°C.
High pressure
Since Haber’s process proceeds with a decrease in volume, it is favored by high pressure. In actual practice, a pressure of 200 – 900 atmospheres is employed.
Catalyst
A catalyst is usually employed to increase the speed of the reaction. Finely divided iron containing molybdenum or alumina is used as a catalyst. Molybdenum or alumina (Al2O3) acts as a promoter and increases the efficiency of the catalyst. A mixture of iron oxide and potassium aluminate has been found to work more effectively.
Fig: 13.4 – Manufacturing plant employed in Haber’s process
Source of raw materials
The nitrogen and hydrogen gases used as the raw material in Haber’s process are obtained as follows.
- Nitrogen is obtained from the liquid air and hydrogen from water by electrolysis.
- Hydrogen may be obtained from water gas (mixture of CO and H2) by Bosch process.
- Water gas can be obtained by passing steam over red hot coke.
By bubbling the mixture through water, CO2 is removed.
- A mixture of nitrogen and hydrogen may be obtained by treating a mixture of producer gas (CO + N2), water gas (CO + H2) with steam in the presence of ferric oxide – chromium oxide catalyst at 450°C.
Carbon dioxide is removed by bubbling through water under pressure.
Plant
The plant, which manufactures ammonia, has the following components and processes.
Compressor
A mixture of nitrogen and hydrogen is compressed to 200-900 atmosphere pressure, in the ratio 1:3 (by volume). The compressed gas is sent to ammonia converter.
Fig: 13.5 – Ammonia converter
Converter
Ammonia converter is made from chrome-vanadium steel. It is usually 1.3 meter high and 1 meter in diameter. The converter is provided with a heat exchanger in the upper portion and the catalyst is packed in the central portion of the converter. There is an arrangement for heating the gas mixture. After the gas mixture enters through the inlet at the bottom, the gases circulate around the catalyst maintained at 450-500°C and then pass through to the heat exchanger. The gases finally enter the catalyst chamber to give ammonia. Before entering the condensers the product as well as the unreacted gases pass through the pipes of the heat exchanger and transfer their heat to the incoming gas mixture containing nitrogen and hydrogen.
Condensers
This cools and liquefies ammonia. The condensed ammonia, called ‘liquor ammonia’ is filled into cylinders under pressure.
Re-circulating pump
Some of the nitrogen and hydrogen gases escape condensation and are re-circulated through the converter.
(1)AMMONIA PRODUCTION BY HABER’S PROCESS
The Haber Process combines nitrogen from the air with hydrogen derived mainly from natural gas (methane) into ammonia. The reaction is reversible and the production of ammonia is exothermic.
A flow scheme for the Haber Process looks like this:
Some notes on the conditions
The catalyst
The catalyst is actually slightly more complicated than pure iron. It has potassium hydroxide added to it as a promoter – a substance that increases its efficiency.
The pressure
The pressure varies from one manufacturing plant to another, but is always high. You can’t go far wrong in an exam quoting 200 atmospheres.
Recycling
At each pass of the gases through the reactor, only about 15% of the nitrogen and hydrogen converts to ammonia. (This figure also varies from plant to plant.) By continual recycling of the unreacted nitrogen and hydrogen, the overall conversion is about 98%.
Explaining the conditions
The proportions of nitrogen and hydrogen
The mixture of nitrogen and hydrogen going into the reactor is in the ratio of 1 volume of nitrogen to 3 volumes of hydrogen.
Avogadro’s Law says that equal volumes of gases at the same temperature and pressure contain equal numbers of molecules. That means that the gases are going into the reactor in the ratio of 1 molecule of nitrogen to 3 of hydrogen.
That is the proportion demanded by the equation.
In some reactions you might choose to use an excess of one of the reactants. You would do this if it is particularly important to use up as much as possible of the other reactant – if, for example, it was much more expensive. That doesn’t apply in this case.
There is always a down-side to using anything other than the equation proportions. If you have an excess of one reactant there will be molecules passing through the reactor which can’t possibly react because there isn’t anything for them to react with. This wastes reactor space – particularly space on the surface of the catalyst.
The temperature
Equilibrium considerations
You need to shift the position of the equilibrium as far as possible to the right in order to produce the maximum possible amount of ammonia in the equilibrium mixture.
The forward reaction (the production of ammonia) is exothermic.
According to Le Chatelier’s Principle, this will be favoured if you lower the temperature. The system will respond by moving the position of equilibrium to counteract this – in other words by producing more heat.
In order to get as much ammonia as possible in the equilibrium mixture, you need as low a temperature as possible. However, 400 – 450°C isn’t a low temperature!
Rate considerations
The lower the temperature you use, the slower the reaction becomes. A manufacturer is trying to produce as much ammonia as possible per day. It makes no sense to try to achieve an equilibrium mixture which contains a very high proportion of ammonia if it takes several years for the reaction to reach that equilibrium.
You need the gases to reach equilibrium within the very short time that they will be in contact with the catalyst in the reactor.
The compromise
400 – 450°C is a compromise temperature producing a reasonably high proportion of ammonia in the equilibrium mixture (even if it is only 15%), but in a very short time.
The pressure
Equilibrium considerations
Notice that there are 4 molecules on the left-hand side of the equation, but only 2 on the right.
According to Le Chatelier’s Principle, if you increase the pressure the system will respond by favouring the reaction which produces fewer molecules. That will cause the pressure to fall again.
In order to get as much ammonia as possible in the equilibrium mixture, you need as high a pressure as possible. 200 atmospheres is a high pressure, but not amazingly high.
Rate considerations
Increasing the pressure brings the molecules closer together. In this particular instance, it will increase their chances of hitting and sticking to the surface of the catalyst where they can react. The higher the pressure the better in terms of the rate of a gas reaction.
Economic considerations
Very high pressures are very expensive to produce on two counts.
You have to build extremely strong pipes and containment vessels to withstand the very high pressure. That increases your capital costs when the plant is built.
High pressures cost a lot to produce and maintain. That means that the running costs of your plant are very high.
The compromise
200 atmospheres is a compromise pressure chosen on economic grounds. If the pressure used is too high, the cost of generating it exceeds the price you can get for the extra ammonia produced.
The catalyst
Equilibrium considerations
The catalyst has no effect whatsoever on the position of the equilibrium. Adding a catalyst doesn’t produce any greater percentage of ammonia in the equilibrium mixture. Its only function is to speed up the reaction.
Rate considerations
In the absence of a catalyst the reaction is so slow that virtually no reaction happens in any sensible time. The catalyst ensures that the reaction is fast enough for a dynamic equilibrium to be set up within the very short time that the gases are actually in the reactor.
Separating the ammonia
When the gases leave the reactor they are hot and at a very high pressure. Ammonia is easily liquefied under pressure as long as it isn’t too hot, and so the temperature of the mixture is lowered enough for the ammonia to turn to a liquid. The nitrogen and hydrogen remain as gases even under these high pressures, and can be recycled.
Haber’s Process For Ammonia Preparation
Image Formation by Ray Diagram
RAY DIAGRAM
Diamonds can be made from graphite in a easiest way.Wants to know HOW???
Diamonds can be made from graphite by applying a temperature of 3000 Celsius and pressure as high as 100,000 atm.
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