Production systems with interruptions, arbitrary topology and finite buffers

We consider a production system with finite buffers and arbitrary topology where service time is subject to interruptions in one of three ways, viz. machine breakdown, machine vacations or Npolicy. We develop a unified approximation (analytical) methodology to calculate the throughput of the system using queueing networks together with decomposition, isolation and expansion techniques. The methodology is rigorously tested covering a large experimental region. Orthogonal arrays are used to design the experiments in order to keep the number of experiments manageable. The results obtained using the approximation methodology are compared to the simulation results. The ttests carried out to investigate the differences between the two results show that they are statistically insignificant. Finally, we test the methodology by applying it to several arbitrary topology networks. The results show that the performance of the approximation methodology is consistent, robust and produces excellent results in a variety of experimental conditions.     

Selection of suitable operating conditions to minimize the gradient equilibration time in the separation of drugs by Ultra-High-Pressure Liquid Chromatography with volatile (mass spectrometry-compatible) buffers

Reversed phase gradient elution is the method of choice for pharmaceuticals analysis since it allows reducing the analysis time while improving both the quality of the separation and the detection limits. The current trends are towards faster separations which can be achieved thanks to equipments withstanding ultra-high pressures and/or high temperatures. Under such conditions, gradient separations can be carried out within a few minutes or even a few tens of seconds. A long equilibration time in addition to the gradient time can be therefore very detrimental. In this work, we investigated the extent to which the gradient equilibration time can be reduced and which parameters mainly affect the retention variability of ionizable compounds when using volatile buffers. We first found out an excellent repeatability between run-to-run experiments whatever the equilibration time and the operating conditions. We then pointed out the key operating parameters which allow achieving reproducible runs when varying the equilibration time between runs. With a view of reducing the equilibration time, the effects of various conditions were examined. The latter include the type of additive for mobile phase pH adjustment, the initial eluent composition, the type of stationary phase, the temperature and the flow-rate. Although much remains to be understood about the equilibration process, our study allows making progress in the knowledge of this phenomenon. Based on the present results, a beneficial effect of both temperature and flow-rate was highlighted and operating conditions leading to faster column equilibration are suggested.     

Kinetic Buffers

This paper proposes a new type of molecular device that is able to act as an inverse proton sponge to slowly decrease the pH inside a reaction vessel. This makes the automatic monitoring of the concentration of pH‐sensitive systems possible. The device is a composite formed of an alkyl chloride, which kinetically produces acidity, and a buffer that thermodynamically modulates the variation in pH value. Profiles of pH versus time (pH–t plots) have been generated under various experimental conditions by computer simulation, and the device has been tested by carrying out automatic spectrophotometric titrations, without using an autoburette. To underline the wide variety of possible applications, this new system has been used to realize and monitor HCl uptake by a di‐copper(II) bistren complex in a single run, in a completely automatic experiment.     

Solvation consequences of polymer PVP with biological buffers MES,MOPS, and MOPSO in aqueous solutions

Densities and viscosities were measured for the aqueous buffer (MES, MOPS, or MOPSO) solutions containing different concentrations of polyvinylpyrrolidone (PVP) (5, 10, 15, 20 and 30) mass% at temperatures from (298.15 to 318.15) K under atmospheric pressure. The DFT calculations were also performed and the binding energies of the possible (PVP + buffer) complexes were obtained. The experimental and computational results reveal the interactions of the PVP with the constituent compounds in the aqueous buffer solutions. Additionally we have explored the solvation behavior of the buffers by measuring the densities and the viscosities data of the aqueous buffer solutions from (0.0 to 1.0) mol · kg⁻¹ at temperatures from (298.15 to 318.15) K. The viscosity results were correlated with the Jones–Dole equation. The correlated results confirmed that all the investigated buffers behave as Kosmotropes (structure makers).     

Partial Pooling in Tandem Lines with Cooperation and Blocking

For a tandem line of finite, single-server queues operating under the production blocking mechanism, we study the effects of pooling several adjacent stations and the associated servers into a single station with a single team of servers. We assume that the servers are cross-trained (so that they can work at several different stations) and that two or more servers can cooperate on the same job. For such a system, we provide sufficient conditions on the service times and sizes of the input and output buffers at the pooled station under which pooling will decrease the departure time of each job from the system (and hence increase the system throughput). We also show that pooling decreases the total number of jobs in the system at any given time and the sojourn time of each job in the system if the departure time of each job from the system is decreased by pooling and there is an arrival stream at the first station. Moreover, we provide sufficient conditions under which pooling will improve the holding cost of each job in the system incurred before any given time, and extend our results to closed tandem lines and to queueing networks with either a more general blocking mechanism or probabilistic routing. Finally, we present a numerical study aimed at quantifying the improvements in system performance obtained through pooling and at understanding which stations should be pooled to achieve the maximum benefit. Our results suggest that the improvements gained by pooling may be substantial and that the bottleneck station should be among the pooled stations in order to obtain the greatest benefit. AMS subject classification: 90B22     

Hierarchical controls in stochastic manufacturing systems with machines in tandem

This paper presents an asymptotic analysis of hierarchical production planning in a manufacturing system with serial machines that are subject to breakdown and repair, and with convex costs. The machines capacities are modeled as Markov chains. Since the number of parts in the internal buffers between any two machines needs to be non-negative, the problem is inherently a state constrained problem. As the rate of change in machines states approaches infinity, the analysis results in a limiting problem in which the stochastic machines capacity is replaced by the equilibrium mean capacity. A method of “lifting” and “modification” is introduced in order to construct near optimal controls for the original problem by using near optimal controls of the limiting problem. The value function of the original problem is shown to converge to the value function of the limiting problem, and the convergence rate is obtained based on some a priori estimates of the asymptotic behavior of the Markov chains. As a result, an error estimate can be obtained on the near optimality of the controls constructed for the original problem.     

Critical evaluation of buffering solutions for pKa determination by capillary electrophoresis

The performance of the most common and also some other less common CE buffers has been tested for the pKa determination of several types of compounds (pyridine, amines, and phenols). The selected buffers cover a pH ranging from 3.7 to 11.8. Whereas some buffers, like acetic acid/acetate, BisTrisH+/BisTris, TrisH+/Tris, CHES/CHES-, and CAPS/CAPS- can be used with all type of analytes, others like ammonium/ammonia, butylammonium/butylammonia, ethylammonium/ethylammonia, diethylammonium/diethylammonia, and hydrogenphosphate/phosphate are not recommended because they interact with a wide range of compounds. The rest of the tested buffers (dihydrogenphosphate/hydrogenphosphate, MES/MES-, HEPES/HEPES-, and boric acid/borate) can show specific interactions depending on the nature of the analytes, and their use in some applications should be restricted.     

Use of quasi-isoelectric buffers to limit protein adsorption in capillary zone electrophoresis

The use of quasi-isoelectric buffers consisting of narrow pH cuts of carrier ampholytes (NC) has been investigated to limit protein adsorption on capillary walls during capillary zone electrophoresis experiments. To quantify protein adsorption on the silica surface, a method derived from that of Towns and Regnier has been developed. alpha-Lactalbumin (14 kDa, pI 4.8) and alpha-chymotrypsinogen A (25 kDa, pI 9.2) have been used as model proteins. Acidic narrow pH cuts of carrier ampholytes (NC, pH 3.0) obtained from fractionation of Serva 4-9 carrier ampholytes were used as BGE in bare-silica capillaries, and allowed to decrease significantly protein adsorption, as compared to experiments performed with classical formate buffer. The use of NC as BGE appeared to be as efficient as the use of polydimethylacrylamide coating to prevent protein adsorption. This increase of protein recovery when using NC was attributed to the interaction of carrier ampholytes with the silica surface, leading to a shielding of the capillary wall.     

Towards minimum loss job routing to parallel heterogeneous multiserver queues via index policies

This paper considers a Markovian model for the optimal dynamic routing of homogeneous traffic to parallel heterogeneous queues, each having its own finite input buffer and server pool, where buffer and server-pool sizes, as well as service rates, may differ across queues. The main goal is to identify a heuristic index-based routing policy with low complexity that consistently attains a nearly minimum average loss rate (or, equivalently, maximum throughput rate). A second goal is to compare alternative policies, with respect to computational demands and empirical performance. A novel routing policy that can be efficiently computed is developed based on a second-order extension to Whittle’s restless bandit (RB) index, since the latter is constant for this model. New results are also given for the more computationally demanding index policy obtained via policy improvement (PI), including that it reduces to shortest queue routing under symmetric buffer and server-pool sizes. A numerical study shows that the proposed RB index policy is nearly optimal across the instances considered, and substantially outperforms several previously proposed index policies.     

硫敏化中心中的银原子簇的增感作用

采用一定电位范围内的氧化还原缓冲液处理单分散立方溴化银乳剂颗粒表面上的硫敏化中心和由曝光生成的潜影中心,考察银原子簇的漂白和再生后的曝光响应曲线.实验结果再一次确认硫敏化中心中存在的银原子簇对硫敏化起重要作用,而且敏化效果也与银原子簇的含量有关.为此,提出硫敏化中心是由硫化银簇与银簇组成,它们在潜影形成的过程中各起不同的作用,但都有利于提高潜影的生成效率,因而共同产生敏化效应.