Design and control of dual condensers in distillation columns

Most distillation columns use water-cooled condensers cooling water that is inexpensive. With cooling water supplied at 305 K, a reflux-drum temperature of 322 K is normally used for column design. This temperature and the distillate composition set the require column pressure.This paper demonstrates that the use of refrigeration in a second condenser in series with the primary water-cooled condenser has economical advantages in some separations. The dual-condenser process can be less expensive than the single-condenser process in cases in which a lighter-than-light-key component is present in the feed and in which the separation between the key components is difficult (low relative volatility systems). Using a small refrigerated condenser permits the column to operate at a lower pressure, which reduces reboiler duty enough to compensate for the small amount of refrigeration required.The dynamic control of the dual-condenser process is also studied, and an effective control structure is developed.     

Energy Saving in Crude Oil Atmospheric Distillation Columns by Modifying the Vapor Feed Inlet Tray

Optimization of a typical crude oil atmospheric distillation unit and reduction of energy conservation were carried out through modifying the implementation and change in the flash zone of the tower. A conventional procedure in such units involves the combination of liquid and vapor product of the prefractionation train surge drum upon introduction to the tower. However, it is theoretically illustrated and represented by simulation means that introducing the vapor feed into the upper stages of the distillation column separately can lead to an energy saving of 12.6 % in the condenser duty, an increased liquid‐to‐gas flow (L/G) at certain points of the column, and hence to a reduction in diameter and investment costs of new tower designs of approximately US$ 0.7 million a^–1. The proposal can be put into practice without the need of additional equipments or additional cost of difficult rerouting the streams. An industrial case study of a steady‐state crude oil distillation unit is given by simulation provision of AspenHysys™.     

Feasible separation regions for distillation I: Structure

A feasible separation region is determined for only four special combinations of a saturated vapor/liquid feed and total/partial condenser or reboiler. This work addresses the construction of a feasible separation region for a general case where the feed is a mixture of vapor and liquid in equilibrium and where the column is equipped with a partial/total condenser and reboiler. The analysis reveals that the product composition sets (which are defined for various reflux and reboil ratios and a fixed number of stages in each column section) are the main elements of the feasible separation region. The application of the geometric model of the column in combination with the shape of the distillation line led to the conclusion that the feasible separation region is the union of two product composition sets for both enriching and stripping columns both with an infinite number of stages. The boundary of the feasible separation region consists of several curves related to specific types of operating modes in the column. Some of these curves create a well‐known product composition multitude, whereas other curves form a generalized distillation limit. © 2017 American Institute of Chemical Engineers AIChE J, 2017     

混联式冷凝器的可视化及传热实验分析

在分析混联式冷凝器中强化凝结传热机理的基础上,选用相同尺寸的平行流冷凝器为基准对比样件,通过高速摄影可视化实验及冷凝器性能测试方法, 对分流孔径为2.5、4.5 mm的两种混联式冷凝器进行可视化分析表明,类似于蒸气喷射流的过热蒸气与气液两相流在集流管中强烈扰动混合,改善了进入下一冷却流道的两相流质量分布的均匀性,扩展了两相流传热区域在混联式冷凝器中的相对占有面积,这使其传热性能比平行流冷凝器可提高9.6%; 同时,制冷系统中的制冷剂质量流量则提高13.43%,为设计更紧凑的空调冷凝器提供了一种新的途径。     

Effects of Thermal Feed Quality on the Performance of a Divided Wall Distillation Column

The present work deals with the effect of thermal feed quality on the performance of a divided wall distillation column (DWC). The thermal condition of the feed alters the pressure profile across the two sides of the dividing wall, thereby affecting not only the mass transfer characteristics but also the hydrodynamics of a DWC. It was observed that the natural (feasible) vapor split ratio does not depend on the liquid split ratio and the reflux flow rate when the feed is saturated liquid or sub-cooled liquid (q ≥ 1). However, for q < 1, that is, for two phase (vapor-liquid), saturated vapor, or superheated vapor feed, the liquid split ratio and the reflux flow rate have profound effect on the feasible vapor split ratio, and the pressure profiles are altered significantly. For the stable operation of a DWC, the feed should be either saturated liquid or sub-cooled liquid or the feed quality may be manipulated to adjust the vapor split ratio.     

FIBONACCI SEARCH FOR OPTIMAL FEED LOCATION

Abstract

A Fibonacci search technique is used in conjunction with a rigorous mullicomponenl distillaiion computer module to find the optimal feed location within a section of a distillation column. The function lo be minimized can be one of the following:

?key component ratio difference

?reflux ratio or reboiler ratio

?condenser duty or reboiler duty

This technique has been used successfully in the relocation of feed stages of many existing distillation columns and thus saved energy consumption. We shall describe this technique with a sample problem.     

Balance Equations for Molecular Distillation

Abstract

We have derived balanced equations for continuous molecular (short-path) distillation of a binary mixture in a falling film evaporator. The relation of heat and mass transfer in liquid films on both the evaporating and condensing surfaces were taken into account, as well as the mass transfer in the vapor phase in the distillation gap. Individual balance equations were coupled by boundary conditions into a closed set of partial differential equations. By introducing the streams, we have transformed partial differential equations into the simple form of ordinary differential equations. These were solved numerically or approximately for some selected problems in isothermal (e.g., for turbulent film on evaporation surface) and nonisothermal regimes and for the influence of reevaporation from the condenser. These developed mathematical formulations can be used to model various operational situations in a molecular evaporator or to evaluate the influence of some parameters on the molecular distillation process.     

Effect of Feed Location on the Performance of Single-Stage Membrane Permeators

Abstract

The effect of feed location on the performance of single-stage membrane permeators was determined based on the minimum unit compressor load (recycle ratio). Since certain feed locations correspond to several well-known permeator configurations (e.g., simple recycle permeator, continous membrane column), it is possible to characterize the relative performance of these configurations for separating binary gas mixtures. For separations involving oxygen, nitrogen, and carbon dioxide, it was found that the location of feed introduction was related to the apparent difficulty of separation. For binary seprations of low to moderate difficulty, the optimum feed location was at a dimensionless axial distance of 0.6 to 0.75 from the top of the column. This feed location corresponded to the continuous membrane column configuration. For difficult separations, the optimum feed location was at the top of the column which corresponded to the simple recycle permeator. Based on this study, the simple recycle permeator configuration outperforms the continuous membrane column for the most difficult separations such as in the separation of oxygen from air. However, the continuous membrane column configuration can be used effectively for less difficult gas separations which cannot be accomplished by a membrane permeator without recycle, but do not require high recycle ratios to achieve the desired separation.     

Feasible separation regions for distillation II: A generalized distillation limit

The boundary of the feasible separation region consists of several curves that are related to specific types of operating modes of the column. Some of these curves create a well‐known product composition multitude, whereas other curves form a generalized distillation limit. The generalized distillation limit demarcates the sloppy splits (i.e., separations in which the composition of at least one product lies inside the composition space) from regions not accessible by distillation and depends on the thermodynamic state of the feed (a mixture of vapor and liquid in equilibrium or saturated vapor/liquid) and column equipment (total/partial condenser and total/partial reboiler). The mathematical equations describing the generalized distillation limit are obtained based on the relationships between the curves (which form the generalized distillation limit) and specific types of operating modes of the column as well as the material balances for the enriching and stripping columns. Furthermore, the vapor and liquid pinch‐point curves, which go through the feed composition point, are not dependent on the thermodynamic state of the feed and column equipment. In addition, an algorithm for determining the generalized distillation limit is obtained. © 2017 American Institute of Chemical Engineers AIChE J, 2017     

Simulation of Moving Feed Port Chromatography by Rate Model with Mass Transfer Effect

Abstract

Moving feed port chromatography (MFPC) was simulated by a rate model with mass transfer effect, and two criteria are presented for selection of the feed port velocity to obtain the best separation. When the moving feed port velocity simultaneously satisfied these two criteria, the resulting concentration profiles for MFPC had narrower bandwidth, higher concentration, and improved resolution than those of conventional chromatography. In addition, there existed an optimum number of feed ports (or column length for feed injection) to obtain the best resolution at a fixed total feed time.     

Argon production from air distillation: Use of a heat pump in a ternary distillation with a side rectifier

A ternary feed mixture ABC can be separated into individual components through the use of a main distillation column with a thermally linked side rectifier. To enhance such a separation, a heat pump can be implemented to transfer heat from the condenser at the top of the side rectifier to the reboiler at the bottom of the main column. In this paper, one such heat pump is described and applied to an air distillation system separating the ternary mixture containing nitrogen, oxygen and argon. The separation is performed by a conventional double column with a crude argon side column. When this system is operated at an elevated pressure to obtain higher product pressures, the separation of oxygen and argon becomes very difficult and leads to reduced argon recovery. The proposed heat pump enhances the separation by providing a supplementary crude argon condensing duty through the vaporization of a liquid oxygen stream from the bottom of the low pressure (LP) column. This scheme improves the liquid/vapour ratio (L/V) in the bottom section of the LP column and, more importantly, increases the vapour feed to the crude argon column. This increased feed rate leads to a substantial increase in argon recovery for the elevated pressure air distillation process.     

Retrofit of Distillation Columns Using Thermodynamic Analysis

Abstract

Thermodynamic analysis provides the column grand composite curves and exergy loss profiles, which are becoming readily available for a converged distillation column simulation. For example, the Aspen Plus simulator performs the thermodynamic analysis through its Column–Targeting tool for rigorous column calculations. This study uses the column grand composite curves and the exergy loss profiles obtained from Aspen Plus to assess the performance of the existing distillation columns, and reduce the costs of operation by appropriate retrofits in a methanol plant. Effectiveness of the retrofits is also assessed by means of thermodynamics and economics. The methanol plant utilizes two distillation columns to purify the methanol in its separation Section. The first column operates with 51 stages, has a side heat stream to the last stage, a partial condenser at the top and a side condenser at stage 2, and no reboiler. The second column operates with 95 stages, has a side heat stream to stage 95, a total condenser, and high reflux ratio. Despite the heat integration of the columns with the other Sections and a side condenser in column 1, the assessment of converged base case simulations have indicated the need for more profitable operations, and the required retrofits are suggested. For the first column, the retrofits consisting of a feed preheating and a second side condenser at stage 4 have reduced the total exergy loss by 21.5%. For the second column, the retrofits of two side reboilers at stages 87 and 92 have reduced the total exergy loss by 41.3%. After the retrofits, the thermodynamic efficiency has increased to 55.4% from 50.6% for the first column, while it has increased to 6.7% from 4.0% for the second. The suggested retrofits have reduced the exergy losses and hence the cost of energy considerably, and proved to be more profitable despite the fixed capital costs of retrofits for the distillation columns of the methanol plant.     

A Chemical Exchange System for Isotopic Feed to a Nitrogen and Oxygen Isotope Separation Plant

Abstract

A process has been developed to provide isotopic feed to a nitric oxide isotope distillation plant. Central to the process is the isotopic chemical exchange of NO and nitric acid in countercurrent flows in a 3-in. diameter packed column. An isotopically depleted stream of NO is reenriched to natural isotopic abundances by the exchange and is recycled as feed back to the distillation columns. Makeup NO is generated in another column from sulfur dioxide and nitric acid. Multistage gas purifiers reduce condensible impurities in the nitric oxide below 10 ppm. The process operates unattended at flow rates of 0.5 to 2 mol/min. The new NO recycle-enrichment and generation processes have successfully provided the feedstock for the NO isotope separation columns for over 6 years.     

An experimental study on the performance of a condensing tumbler dryer with an air-to-air heat exchanger

The performance of energy consumption in the closed-loop tumbler dryer with a condenser for clothes drying is evaluated as a function of the heater capacity, the drying air flow rate inside the dryer, and the cooling air flow rate. The clothes dryer in laundries used in this study consists of a tumbling drum, condenser for condensing the humid and hot air flowing out the rotating drums, and electric heater for heating the circulating drying air. Tests were performed at the heating capacity of 1.9 kW to 2.7 kW, the drying air flow rate of 60 m³/h to 140 m³/h, and the cooling air flow rate of 100 m³/h to 240 m³/h. The total energy consumption, the drying time, and the condensate water rate were also investigated. Parametric results showed that a larger heater power resulted in shorter drying time. With increasing heater power, the air temperature and the condensate rate increased due to the higher humidity ratio in the air. The drying air flow rate and the cooling air flow rate did not have a significant effect on drying performance.     

Comparison of various radiation-cooled dew condensers using computational fluid dynamics

Radiation-cooled dew water condensers can serve as a complementary potable water source. In order to enhance passive dew collection water yield, a Computational Fluid Dynamics (CFD) software, PHOENICS, was used to simulate several innovative condenser structures. The sky radiation is calculated for each of the geometries. Several types of condensers under typical meteorological conditions were investigated using their average radiating surface temperature. The simulations were compared with dew yield measurements from a 1 m² 30°-inclined planar condenser used as a reference. A robust correlation between the condenser cooling ability and the corresponding dew yield was found. The following four shapes were studied: (1) a 7.3 m² funnel shape, whose best performance is for a cone half-angle of 60°. Compared to the reference condenser, the cooling efficiency improved by 40%, (2) 0.16 m² flat planar condenser (another dew standard), giving a 35% lower efficiency than the 30° 1 m² inclined reference condenser, (3) a 30 m² 30°-inclined planar condenser (representing one side of a dew condensing roof), whose yield is the same as the reference collector, and (4) a 255 m² multi-ridge condenser at the ground surface provided results similar to the reference collector at wind speeds below 1.5 m s^− 1 but about 40% higher yields at wind speeds above 1.5 m s^− 1.     

Effect of Inert Gas Pressure on the Molecular Distillation Process

Abstract

A one-dimensional model of a short-path (molecular) distillation for two-component systems in the presence of an inert gas was developed based on the weight-not-time-counter Direct Simulation Monte Carlo method. The effect of the composition of the test liquid, the evaporating temperature, the condensing temperature, the distance between evaporator and condenser, the arrangement of distillation space, and the inert gas pressure on mass transfer and separation efficiency is discussed.     

Exergy analysis of distillation processes

Distillation is a unit operation in which two main processes are involved: heat transfer for vaporizing and condensing and mass transfer for the separation of the mixture. A distillation unit can be described as an exergy converter: that is a unit which converts thermal exergy into chemical exergy. To obtain a complete graphical representation of mass, enthalpy and exergy balances in a fractional distillation, we propose the use of a diagram: the specific enthalpy (in KJ/ Kg of mixture) is plotted against the Carnot factor θ to show the heat and mass transfer effects simultaneously. The application of the methodology to the distillation of an ammonia-water mixture is presented. Conventional fractional distillation consumes a lot of exergy. We propose a new type of distillation in which the reboiler and the condenser normally located at the bottom and at the top of distillation column, are replaced by two heat exchangers integrated in the column itself. The exchanger integrated in the bottom of the column inputs heat to the column, and the other exchanger located in the upper part of the column removes heat. This arrangement minimizes the creation of entropy in the column and therefore maximizes the exegetic effectiveness. This new process, called:“diabatic” or “quasi-reversible” distillation should bring important improvement to conventional distillation, not only from the point of view of energy use (large reduction in the consumption of heat carriers fluids for heating and cooling), but also in terms of capital investment as the cost of fitting a heat exchanger in the destillation column will probably be less than the cost of reboiler or condenser and reflux head.     

精馏塔连续放空原因剖析与对策

用脱水塔改造的实际案例,陈述不凝气积聚的观点,及进料中的不凝气在冷凝器中发生积聚是治理精馏塔连续放空的理论依据。文中在多个实际例子中开展了对冷凝工艺的讨论,包括对冷凝器、冷凝流程和压力控制方法的讨论。指出石化工业引进的冷凝工艺,由于采用气液相共用一个出口的冷凝器,造成不凝气积聚,导致压力失控而发生连续放空。文中介绍了传统的气相串联式冷凝工艺的特点和原理,以及在老塔改造中逐步淘汰引进的气液共用一个出口冷凝器的工艺,并在新塔设计中积极推广和应用气相串联式冷凝工艺。     

精馏惰气升压冷却方式的模拟与基础试验研究

针对普通精馏塔塔顶冷凝系统的缺点与不足,在节能环保的前提下对原有操作方式进行了改进:利用惰性气体作为压缩介质将塔顶高温气体加压升温后,在换热器中进行热交换,合理利用凝冷潜热的同时达到了冷却塔顶蒸汽的目的.该方式降低了精馏系统中冷凝器的热负荷,同时避免了冷凝系统中冷冻盐水的高成本及腐蚀性.首先应用Aspen Plus软件对该新工艺流程进行了模拟计算,以SRK方程为热力学模型,通过灵敏度分析模块考察了混合气体压缩比、氮气初始温度、混合气体物质的量之比等因素对该过程的影响.之后进行了该过程的小试试验,试验结果与模拟结果符合良好.     

ENERGY SAVING IN INDUSTRIAL DRYING PLANTS BY PARTIAL RECOVERY OF THE LATENT HEAT OF THE EXHAUST AIR

ABSTRACT

Energy saving in industrial drying by recovering part of the latent heat of the vapor exiting the drier is considered in this paper. This heat recovery is accomplished through a two-stage heat exchanger system, wherein the first stage brings the primary circuit to vapor saturation conditions, while the second stage acts as a condenser. Optimal heat recovery is assured by intelligent, fully automated process control.