Sharif Digital Repository

Economic loss due to wax precipitation in oil exploitation and transportation has reached several billion dollars a year recently. Development of amodel for better understanding of the process of wax precipitation is therefore veryimportant to reduce the loss.Currently, there are two types of soild models. One is the solid-solution model, and the other is the multisolid-phase model.In this work, a new thermodynamic model that combines the two methods mentioned above, has been presented. The PC-SAFT EOS is used to describe the vapor, liquid and wax phase.SAFT equations describe the phase behavior of large molecules accurately. Intermolecular forces in wax phase are caused by London and Van-... 

gas adsorption in porous solids and adsorbent,is the important cases in gas processing industries. Including the removal of compounds such as carbon dioxide, hydrogen sulfide and mercaptans from sour gas in natural gas sweetening industry is of great importance.Also in solving the greenhouse phenomena extraction of gases such as methane and carbon dioxide of important environmental issues.To understand and predict the behavior of the absorption capacity of various adsorbents and thus the possibility of absorption systems, a mathematical model is needed.Library resources available in order to determine the kinetics and thermodynamic processes of gas adsorption on solid absorbents use... 

One of the main problems associated with the oil production is asphaltene precipitate in the reservoirs. In order to plan for the EOR methods, production scenario, and selection of completion technology, it is necessary to study the initiation of asphaltene precipitation called onset. Since the experimental methods is not only is so expensive and time consuming, the required materials are not available. Consequently, modeling approaches as new methods are proper substitutes for laboratory procedure. These methods need less costs and time and higher accuracy compared to the experimental investigations which are the advantages of these methods. Computational operations for onset determination... 

This study investigates the dynamic adsorption of asphaltene-in-toluene solutions on pack of wet calcite. All experiments were performed on mineral packs in sand-pack with mesh sizes between 100 to 140 (105 to 150 micron) and porosities between 40 to 46 percent. A UV–vis spectrophotometer was used to monitor the outlet concentration of asphaltenes. Experiments were performed for seven ionic strength of sodium chloride (6 to 1500 mM), five ionic strength of sodium sulfate (6 to 2400 mM) and four ionic strength of these salts mixture (12 to 3300 mM). In previous experiments the asphaltene concentration is 500 mg/l. Also two experments were performed with asphaltene concentration of 2000... 

During the over balanced drilling operation in high permeability or fractured formations, drilling fluid loss and in more severe occasions fluid lost occurs which causes many problems and dangers and delayed the drilling operation until it was repaired. On the one hand, because of the high value of this material, and on the other hand, due to the increase in rig/day costs resulting from lost circulation control, leads to a significant increase in drilling costs. In this research, polymer-fiber composites have been designed and manufactured using extrusion method and have been investigated as fluid lost control agent in the laboratory scale for the first time in Iran. The effective parameters... 

In this study a novel, comprehensive and applicable hybrid radial basis function (RBF) neural network model optimized by particle swarm optimization (PSO) has been developed to predict MMP of pure and impure Co2 in miscible injection processes. 178 data sets from open literature used to validate and verify the proposed model. Also, slim tube test using Omani crude oil sample was carried out to confirm the model performance and its generalization Reservoir temperature, volatile to intermediate ratio and molecular weight of C5+ in the reservoir oil, composition of the Co2 and contaminants in the injected gas, considered as the input parameters. Performance of the proposed model was compared... 

Wax precipitation may occur in production or transportation of crude oil form field which is a serious problem in petroleum industry. Flow assurance issues concerning wax precipitation make it necessary to develop a precise thermodynamic model to predict the wax appearance temperature and amount of precipitation at different conditions. In this work a new procedure has been proposed to characterize crude oil based on the SARA test considering the wax and asphaltene as single pseudo components. Two scenarios have been investigated for the survey of the crude oil characterization, with and without asphaltene pseudo component. Also, in this work, the Perturbed ... 

Wax precipitation is one of the major problems in the transfer of crude oil from field production to refineries. This wax with asphaltene compounds and some water and solids, closes the oil pipeline. One of the ways to reduce the amount of wax in pipes is to use additives (flow improvers) to crude oil and improve its rheological behavior. In this research, Five Iranian crude oils were characterized with SARA test and then the effects of various additives including ethylene vinyl acetate (EVA) in concentrations of 500 and 1500 ppm, alkyl phenol ormaldehyde resins (NPFR) with 1500 ppm and styrene-(ethylene-cobutylene)- styrene (SEBS) with 1500 ppm on viscosity, wax appearance temperature... 

Secret sharing scheme is a method for distributing the secret (secret information) among a set of participants in such a way that only the authorized sets can recover the secret and the unauthorized sets could not. In a perfect secret sharing scheme, unauthorized sets cannot get any additional (i.e. a posteriori) information about the possible value of the secret. In these schemes, to prevent information leakage and increase communication efficiency, the size of the share should be as close to the secret size as possible. In other words, finding the shares with reasonable size which results in an optimal information rate for a given access structure, improves the efficiency of the scheme.... 

Ever-encreasing progress in the field of wireless communication has paved the way for utilisation based on radio frequency authentication systems. In recent years, ITU started the scenario of "Internet of Things". RFID systems have an exclusive role in development ofthis idea, since they can make it possible for things which have an ID to be identified automatically in a large scale. The necessity of identification and tracing people and things has caused the deployment of RFID technology. Onthe other hand, these systems communicate via a wireless channel to transmit data, therefore they are vulnerable to eavesdropping and interception. Thus, one of the most important issues in using such... 

Wide use of communication systems and transmission of huge data has increased the need for cryptographic and encoding systems with high speed operations and lower energy consumptions. Code based and joint encryption encoding schemes are examples of such systems, which enjoy simultaneously high performance and security against quantum computers. So far, there has not been developed any quantum algorithm which can solve any of the hard instances of coding theory based on which these cryptographic schemes have been designed. That is why such systems are called post quantum cryptosystems. The main disadvantage of such cryptosystems is large size the key which is to be stored or shared. In this... 

Quantum computers can be used to process complex computations much more efficient than their classical counterparts. Cryptanalysis of classical ciphers is no exception to this rule. Most quantum cryptanalysis of symmetric ciphers is based on finding the periodicity, linear structure, and confidential information of the target algorithm. In this thesis, assuming that the attacker has access to the quantum computer, we propose a quantum method for deriving the appropriate differential characteristic of block ciphers, using Simon's quantum algorithm. It is worth noting that, unlike classical methods, where increasing the number of rounds of the target block cipher increases the complexity of... 

Secret sharing schemes are used for storage and management of sensitive data such as cryptographic keys and bank account numbers. These schemes provide both confidentiality and reliability. Advances in quantum computers have threatened the security of currently used public-key cryptographic algorithms. In this thesis, for designing quantum resistant cryptographic schemes, we construct a lattice based threshold multi-stage secret sharing (TMSSS) scheme. Using a lattice based one-way function, even after some secrets are revealed, the computational security of the unrecovered secrets is provided against quantum computers. The scheme is multi-use and verifiable meaning that the participants can... 

With the dramatic advancements in information technology and the industry requirements for security and privacy, proof systems play a crucial role in cryptography. Among the vast variety of proof systems, succinct non-interactive arguments (SNARG) seem to be the most appealing class of proof systems, due to their attractive properties. SNARGs are usually made up of two constructive components, namely the information theoretic part and the cryptographic part. The GKR protocol was introduced as a proof system for a tractable family of languages called “log-Space Uniform Circuits”. The log-space uniformity is a necessary condition for the protocol to be succinct. Hence, it is not possible to... 

With the dramatic advancements in information technology and the industry requirements for security and privacy, proof systems play a crucial role in cryptography. Among the vast variety of proof systems, succinct non-interactive arguments (SNARG) seem to be the most appealing class of proof systems, due to their attractive properties. SNARGs are usually made up of two constructive components, namely the information theoretic part and the cryptographic part. The GKR protocol was introduced as a proof system for a tractable family of languages called “log-Space Uniform Circuits”. The log-space uniformity is a necessary condition for the protocol to be succinct. Hence, it is not possible to... 

Stream ciphers are one of the cryptography primitives which have high speed in encryption process and less amount of hardware which they used. Guess and determine attacks are general attacks on stream ciphers which have often been implemented in ad hoc manner. Finding the minimal basis for internal state of a cryptosystem based on connection equations is the main challenge in stream ciphers. In recent years, some efforts for applying this attack in the systematic manner are started. First work in this area is the Simple Guess and Determine attack. The dvanced and Heuristic Guess and Determine attacks then presented, which all of them have heuristic structure. In this thesis, after reviewing... 

Many lattice-based public key encryption schemes are forced to use long keys. To deal with this problem, structured lattices such as NTRU can be used. Lattice-based searchable public key encryption schemes are no exception. Furthermore, in most existing searchable public key encryption schemes, it is assumed that the user searches documents with only one keyword. However, using a conjunctive keyword search, we can search for documents containing several keywords by executing the trapdoor generation algorithm only once. On the other hand, in most of the published searchable public key encryption schemes, storing and searching encrypted keywords are responsibility of a central entity called... 

Secret sharing is a method for sharing a secret among a number of participants in a manner that an authorized subset of participants can recover the secret using some information, called share, and some public values. One type of secret sharing schemes is Multi-stage Multisecret sharing scheme (MSSS). In such schemes, assigning one share to each participant, an authorized subset of participants is able to recover the secrets in different stages without any information leakage from the recovered secret(s) to the other one(s), using some public information.The number of public values is one of the most important parameters in secret sharing schemes. In this thesis we show that the number of... 

Blockchain systems are considered a subclass of distributed systems and have drawn a lot of attention from academia in recent years. The deployment of these systems in various contexts has been the focus of extensive studies. Because of their design, which places a strong focus on security and decentralization, blockchain systems have some challenges. The most prominent challenge is the one that involves scalability. A blockchain's scalability refers to its capacity to achieve a target throughput and solid performance in confirming transactions with an ever-increasing workload or number of nodes in the network. A solid performance suggests that the blockchain system can sustain its... 

Nowdays, because of the promotion and developments of Information technology (IT) and the need for powerful resources for computation and storage, we are the witness of migration from existing computing schemes to the cloud computing and cloud storage environment. As a result, we can see that the client and the IT users are the beneficiaries of the services which are provided by the existing cloud providers. However, the cloud providers are not usually fully trusted and we need to protect the privacy of our sensitive documents. In this case, we should use cryptographic primitives to encrypt our data before outsourcing to the cloud. Searchable encryption and attribute based encryption are two...