Unit Clauses Research Articles

Recovery of sulfur from sour acid gas: A review of the technology

AbstractThe modified Claus process is the major technology for the recovery of elemental sulfur from H2S and SO2. A number of commercial technologies for the recovery of sulfur from acid gases are also highlighted here. A Claus tail‐gas clean‐up treatment is essential to give high sulfur recovery efficiency from sour acid gases. Generally, the existing tail‐gas clean‐up technologies can be classified into two groups: those that attain 99% overall sulfur recovery efficiency, and those that achieve 99.9% efficiency, including the sulfur recovered in the Claus unit. These processes are the Amoco's Cold Bed Adsorption (CBA), the SNPA/Lurgi Sulfreen, the IFP, the SCOT, the Beavon, and the Wellman‐Lord processes. The SCOT process is generally the most reliable and flexible technology. Process comparisons are also summarized in terms of the sulfur recovery efficiency, hazards and disadvantages, reliability and advantages, plant capacity and ecological impacts. Several changes and new trends are also highlighted here, such as the introduction of non‐permselective catalytic membrane reactors for the Claus reaction, and the in situ adsorption of water inside the Claus catalytic reactor. The successful utilization of H2S by converting it to sulfur and H2 attains the triple objectives of waste minimization, resource utilization, and environmental pollution reduction. Photochemical and plasmochemical methods are still in the development stage. Application of electrochemical technology to H2S requires further development. Research for an optimum porous catalyst structure is ongoing for obtaining a relation of micropores and macropores which would provide effective conversion of H2S and SO2.

Influence of fractal pore structure in Claus catalyst performance

The Claus process is an efficient way of removing H 2S from acid gas streams and it has been widely practised in industries such as natural gas processing, oil refining and metal smelting. Catalytic stage plays a paramount role in Claus units performance. Different samples of Claus reaction alumina catalyst were characterised by the fractal dimension parameter employing nitrogen adsorption porosimetry. The influence of such a dimension over Claus reactor performance was studied employing a reactor model. Significant differences between fractal and smooth catalysts performance were found as well as implications in reactor design. Deactivated catalyst samples were also characterised and its behaviour as Claus catalyst shows influence of the fractal dimension value. Fractal dimension appears as a useful parameter to the Claus catalyst selection procedure.

Chain programs for writing deterministic metainterpreters

Many metainterpreters found in the logic programming literature are nondeterministic in the sense that the selection of program clauses is not determined. Examples are the familiar ‘demo’ and ‘vanilla’ metainterpreters. For some applications this nondeterminism is convenient. In some cases, however, a deterministic metainterpreter, having an explicit selection of clauses, is needed. Such cases include (1) conversion of OR parallelism into AND parallelism for ‘committed-choice’ processors, (2) logic-based, imperative-language implementation of search strategies, and (3) simulation of bounded-resource reasoning. Deterministic metainterpreters are difficult to write because the programmer must be concerned about the set of unifiers of the children of a node in the derivation tree. We argue that it is both possible and advantageous to write these metainterpreters by reasoning in terms of object programs converted into a syntactically restricted form that we call ‘chain’ form, where we can forget about unification, except for unit clauses. We give two transformations converting logic programs into chain form, one for ‘moded’ programs (implicit in two existing exhaustive-traversal methods for committed-choice execution), and one for arbitrary definite programs. As illustrations of our approach we show examples of the three applications mentioned above.

A probabilistic model for the modified claus process

A model is proposed for simulation of an industrial Claus unit with a straight-through configuration and two catalytic reactors. Process design evaluation based on deterministic calculations does not take into account the uncertainties associated with the input variables. The probabilistic simulation method was applied in the Claus model to obtain an impression of how some of these inaccuracies influence plant performance. The application was tested experimentally to confirm its validity, then used for control purposes.

Variant of Oxidative Utilization of Hydrogen Sulfide-Containing Gases

Direct oxidation into sulfur in a pseudo-fluidized bed of spherical catalyst is a selective method of treating hydrogen sulfide-containing gases (HSCG) [1]. In contrast to the Claus method, it can be used for treating gases with a wide range of hydrogen sulfide contents [2]. “Acid” (as in Claus units) and hydrocarbon-containing (casing-heated petroleum, natural, refinery) gases can be treated with this method [3].

Sulphur condensation influence in Claus catalyst performance

The Claus process is an efficient way of removing H 2S from acid gas streams and this is widely practised in industries such as natural gas processing, oil refining and metal smelting. Increasingly strict pollution control regulations require maximum sulphur recovery from the Claus unit in order to minimise sulphur-containing effluent. The most widely used Claus catalyst in sulphur recovery units is non-promoted spherical activated alumina. Properties associated with optimum non-promoted Claus catalyst performance include high surface area, appropriate pore size distribution and enhanced physical properties. The objective of this paper is to outline a procedure in order to estimate Claus catalyst effectiveness after pore plugging due to sulphur condensation. Catalyst deactivation due to pore plugging by sulphur is modelled employing a Bethe lattice and its corresponding performance is described by means of a modified effectiveness factor. Model results show an improvement in the modified effectiveness factor due to modifications in catalyst porous structure.

Revising the logical foundations of inductive logic programming systems with ground reduced programs

In this paper we revise Muggleton’s theory of inverse entailment, which is the logical foundation of Progol, one of the most famous ILP systems. We first point out that the theory is incomplete in general. Secondly we prove that the theory is complete if the background knowledge given to the system is a ground reduced program, every training example is a ground unit clause, and the hypothesis space is the set of all definite clauses. The proof is obtained by showing that every ground reduced logic program is logically equivalent to the conjunction of all atoms in its least Herbrand model. As a corollary to this equivalence, we are finally able to improve the logical foundation of the GOLEM system.

The horn basis of a set of clauses

We formalize the idea that a set of propositional clauses that is not Horn-renamable can still be partially so. We show that for any finite set of clauses S defined on a set of variables V, there exists a largest subset U of V, with regard to inclusion, such that S is Horn-renamable with respect to U; we call it the Horn basis of S. If the Horn basis is not empty and S contains no unit clause, then deciding whether S is satisfiable reduces to deciding whether a proper subset of S is satisfiable. We show that the Horn basis can be computed in linear time.

LINUS - A Link Instantion Prover with Unit Support

LINUS is a theorem prover for clause logic based on hyperlinking. The main new feature of the system is the emphasis on the preference for unit clauses, which are generated as a by-product of the hyperlinking method. The mechanism of unit support also motivates a slightly different interpretation of hyperlinking, namely, as mate saturation. Mate saturation can be viewed as a generalization of unit-resulting resolution that is compatible with any set-of-support strategy. We give a survey on the theoretical background and the architecture of the system, and decribe its performance.

Linear and unit-resulting refutations for Horn theories

We present a new transformation method by which a given Horn theory is transformed in such a way that resolution derivations can be carried out which are bot h linear (in the sense of Prologs SLD-resolution) and unit-resulting (i.e. the resolvents are unit clauses). Thi s is not trivial since although both strategies alone are complete, their nacombination is not. Completeness is recovered by our method through a completion procedure in the spirit of Knuth-Bendix completion, however with different ordering criteria. A powerful redundancy criterion helps to find a finite system quite often . The transformed theory can be used in combination with linear calculi such as e.g. (theory) model elimination to yield sound, complete and efficient calculi for full first o rder clause logic over the given Horn theory. As an example application, our method discovers a generalization of the well-known linear paramodulation calculus for the combined theory of equality and strict orderings. The method has been implemented and has been tested in conjunction with a model elimination theorem prover.

A Variational Method for Analysing Unit Clause Search

We expose a variational method for analysing algorithms, as applied to analyse the algorithm $\textbf{UC}$, which is the Davis-Putnam procedure for a set of clauses of three literals. The variable from a unit clause or, if there is none, the first remaining variable from a fixed list is chosen. The algorithm $\textbf{UC}$ finds all the solutions as a set of cylinders. Following the nomenclature of Purdom [J. Inform. Process., 13 (1990), pp. 449–455], we call this algorithm “unit clause backtracking with cylinders of solutions.” First we give an expression for the number of nodes of the calculation trees of all the inputs. Then we use a variational method to calculate the base $\beta $ of the principal exponential part of the average time of calculation T. This “exponential base” is the maximum of three elementary functions $f_{i}$ of four real variables, These functions are defined on the product of the half positive real line by the 3-dimensional unit real cube. We finally obtain the following short statement. Let v be the number of the variables. Let c be the number of the clauses. Let $\gamma = \frac{c}{v} > 1$. Let $\gamma $ be constant when v grows to infinity. The principal exponential part of the average time of $\textbf{UC}$ is $\beta^{v}$ where \[ \beta = \max\limits_{0 \leq \lambda \leq 1} 2^{\lambda} \left( 1 - \frac{3\lambda ^{2}}{8} + \frac{\lambda ^{3}}{4} \right)^{\gamma}. \] We mean that $\lim_{v \rightarrow \infty} T^{1/v} =\beta $. As a first consequence of our method we match $\textbf{UC}$, with the algorithm $\textbf{B}$ without rearrangement (i.e., with a fixed order for introducing the variables). This gives a proof to a conjecture of P W. Purdom [Artif. Intell., 21(1983), pp. 117–133].

Microbial reduction of sulfur dioxide with anaerobically digested municipal sewage biosolids as electron donors.

A concentrated stream of sulfur dioxide (SO2) is produced by regeneration of the sorbent in certain new regenerable processes for the desulfurization of flue gas. We have previously proposed that this SO2 can be converted to elemental sulfur for disposal or byproduct recovery using a microbial/Claus process. In this process, two-thirds of the SO2-reducing gas stream would be contacted with a mixed culture containing sulfate-reducing bacteria (SRB), where SO2 would act as an electron acceptor with reduction to hydrogen sulfide (H2S). This H2S could then be recombined with the remaining SO2 and sent to a Claus unit to produce elemental sulfur. The sulfate-reducing bacterium, Desulfovibrio desulfuricans, has been immobilized by coculture with flocforming heterotrophs from an anaerobic digester, resulting in a SO2-reducing floc that may be collected from the effluent of a continuous reactor for recycle by gravity sedimentation. The carbon and energy source for these cultures was anaerobically digested municipal sewage solids. The maximum specific activity for SO2 reduction in these cultures, in terms of dry weight of D. desulfuricans biomass, was 9.1 mmol of SO2/h.g. The stoichiometry with respect to the electron donor was 15.5 mg of soluble COD/mmol of SO2 reduced.

Discourse grammar and verb phrase anaphora

We argue that an adequate treatment of verb phrase anaphora (VPA) must depart in two major respects from the standard approaches. First of all, VP anaphors cannot be resolved by simply identifying the anaphoric VP with an antecedent VP. The resolution process must establish a syntactic/semantic parallelism between larger units (clauses or discourse constituent units) that the VPs occur in. Secondly, discourse structure has a significant influence on the reference possibilities of VPA. This influence must be accounted for. We propose a treatment which meets these requirements. It builds on a discourse grammar which characterizes discourse cohesion by means of a syntactic/semantic matching procedure which recognizes parallel structures in discourse. It turns out that this independently motivated procedure yields the resolution of VPA as a side effect.

Probabilistic analysis of a generalization of the unit-clause literal selection heuristics for the k satisfiability problem

Two algorithms for the k-satisfiability problem are presented and a probabilistic analysis is performed. The analysis is based on an instance distribution which is parametrized to simulate a variety of sample characteristics. The algorithms assign values to literals appearing in a given instance of k-satisfiability, one at a time, until a solution is found or it is discovered that further assignments cannot lead to finding a solution. One algorithm chooses the next literal from a unit clause if one exists and randomly from the set of remaining literals otherwise. The other algorithm uses a generalization of the unit-clause rule as a heuristic for selecting the next literal: at each step a literal is chosen randomly from a clause containing the least number of literals. The algorithms run in polynomial time and it is shown that they find a solution to a random instance of k-satisfiability with probability bounded below by a constant greater than zero for two different ranges of parameter values. It is also shown that the second algorithm mentioned finds a solution with probability approaching one for a wide range of parameter values.

Integration of prolog and databases in both evaluational and non-evaluational approaches

DB-Prolog is an extended version of Prolog which allows users to access relational databases in both evaluational and non-evaluational approaches. Using the former approach, DB-Prolog offers some built-in predicates for retrieving relational databases. Using the latter approach, DB-Prolog enables relational databases to store facts (ground unit clauses) which may have compound terms in their arguments. As a result, DB-Prolog can execute Prolog programs which contain both a large set of facts, and conventional data (both numerical and character) efficiently. These Prolog programs are required for the construction of practical expert systems. DB-Prolog implementation techniques are described, and some of its available functions are discussed.

Integrated Air Separation Facilities For Hydrocarbon And Sulphur Recovery

Abstract The concept of using an air separation plant to generate nitrogen jar enhanced hydrocarbon recovery and oxygen for sulphur production is reviewed. Nitrogen generation (for injection) and nitrogen rejection processes are discussed and cost curves are presented. Conventional oxygen enrichment and the COPE ™ (Claus Oxygen-based Process Expansion) sulphur recovery processes are compared. Locations where an air separation facility can be designed to produce both nitrogen jar injection and oxygen jar sulphur recovery are identified. Having identified potential sites for integration, oxygen and nitrogen economics are presented. Introduction North America contains vast reserves of conventional crude oil, heavy oil, oil sands, and natural gas containing sulphur (as hydrogen sulphide). In order to produce these reserves economically, industry is constantly investigating the application of new methods to enhance production. Some of these methods use industrial gases such as nitrogen (in secondary and tertiary hydrocarbon recovery projects) and oxygen (in situ combustion for heavy oil recovery and to displace air in sulphur recovery units). These gases are co-produced in cryogenic air separation facilities. However, to date, the application of air separation technology has been very site/project specific and designed to recover only one industrial gas. Many air separation facilities have been installed to deliver nitrogen for enhanced hydrocarbon recovery (EHR). At these locations the low-purity oxygen by-product is vented to the atmosphere even though it could be used as an efficient source of oxygen for combustion processes, such as sulphur recovery from a Claus unit. Hydrocarbon Recovery- Nitrogen Injection/Rejection The use of cryogenically produced nitrogen from an air separation unit (ASU) was initiated in the late seventies as a cost-effective alternative to natural gas injection(I.2). Its use for EHR has grown dramatically and has been applied primarily in the "lower 48" United States. In their June 1985 paper, Clancy et al.(2) provide a list of 33 reservoirs which have used nitrogen injection technology. Figure 1 locates the sites (illustrated by triangles) from this list which have used cryogenic air separation facilities to produce nitrogen. Sites where a nitrogen rejection unit (NRU) is installed as well are located by a darkened triangle in Figure 1. One of the eight sites where both nitrogen injection and rejection and are practiced is Dome's Willesden Green Field in Alberta(4). Exxon operates an integrated injection/ rejection project at Jay-Little Escambia Creek Field near the Florida/Alabama border(5). The remaining six sites are located at University Block 31 (Texas), Anschutz Ranch East Field (Wyoming), Andector-Ellenberger Field (Oklahoma), Fordoche Field (Louisiana), and two at the Painter Field (one triangle shown in Wyoming). Sulphur Recovery The primary method of producing elemental sulphur from natural gas is with the use of the Claus process. A typical modified Claus sulphur recovery unit (SRU) with two catalytic conversion stages and a modem reheat scheme uses air as the course of oxygen to recover 93% to 96% of the sulphur from a relativelyrich acid gas feed.

A high-performance low risc machine for logic programming

This paper describes a reduced-instruction-set computer ( RISC) architecture for PROLOG and gives examples of Warren-machine ( WAM) instructions, built-in functions, and unit clauses written using its instruction set. Two optimizations that allow dereferencing and trailing to be omitted frequently are applied to the RISC code, allowing it to execute 30% faster than unoptimized macro expansions of Warren-machine instructions. Using an instruction cache and a data cache, hand-optimized unit clauses are estimated to run at more than 1,700,000 logical inferences per second ( LIPS), while a mixture of hand-optimized and macro-expanded RISC code should execute in the range of 200,000 to 700,000 LIPS. Hand-optimized RISC code is four times the size of corresponding WAM code; macro-expanded RISC code is seven times large.

CLINTOX Claus tailgas treatment

CLINTOX is an innovative Claus tail gas clean-up process. It comprises incineration of the Claus tail gas, physical scrubbing of the resulting SO 2 and the recycle of the SO 2 fraction to the Claus unit front end. This process can achieve > 99.9% sulphur recovery as a long-term average. The main advantage of CLINTOX is that it allows treatment of very dirty gases and that it simplifies, and makes cheaper, the operation of the upstream Claus unit, at reasonably low investment and operating cost.

Probabilistic Analysis of Two Heuristics for the 3-Satisfiability Problem

An algorithm for the 3-Satisfiability problem is presented and a probabilistic analysis is performed. The analysis is based on an instance distribution which is parameterized to simulate a variety of sample characteristics. The algorithm assigns values to variables appearing in a given instance of 3-Satisfiability, one at a time, using the unit clause heuristic and a maximum occurring literal selection heuristic; at each step a variable is chosen randomly from a subset of variables which is usually large. The algorithm runs in polynomial time and it is shown that the algorithm finds a solution to a random instance of 3-Satisfiability with probability bounded from below by a constant greater than zero for a range of parameter values. The heuristics studied here can be used to select variables in a Backtrack algorithm for 3-Satisfiability. Experiments have shown that for about the same range of parameters as above the Backtrack algorithm using the heuristics finds a solution in polynomial average time.

A note on sorting Prolog assertions

In Prolog programs, arguments are often used to exchange information between relations. This kind of communication is tedious when a large amount of information has to be passed around. One way of solving the problem is to add assertions (unit clauses) to the knowledge base in execution time. These assertions are maintained as "global data structures." They are typically managed by the addition and deletion of clauses. However, most of the Prolog implementations offer little support for these operations. In C-Prolog [1], for example, a clause can only be added as the first or last clause of a relation. It would be useful if one could manipulate clauses in the same way as one manipulates lists. For example, sorting the assertions in the working memory of an expert system may lead the problem solver to follow a "better" path in the search space. In many applications, the working memory is build up incrementally. Thus, the insertion sort is a natural sorting method. The sorting may be combined with the addition of assertions using an assertcl(C) relation. A C-Prolog implementation of the articl(C) relation given below.