New Realization of Periodic Cycled Separation - Industrial

Article pubs.acs.org/IECR

New Realization of Periodic Cycled Separation Bjarne Toftegård,‡ Charlotte H. Clausen,§ Sten B. Jørgensen, and Jens Abildskov* CAPEC-PROCESS, Department of Chemical and Biochemical Engineering, Technical University of Denmark, Building 229, DK-2800 Kgs. Lyngby, Denmark S Supporting Information *

ABSTRACT: A new realization of periodic cycled gas/liquid separation is presented. Separation factors and column efficiencies are compared for a column stripping ammonia from water with air, using three different sets of internals: conventional sieve trays, Sulzer BX gauze packings, and periodically cycled trays. The proposed new periodic trays are shown to be advantageous compared to their continuous alternatives. It is demonstrated experimentally that periodic tray efficiencies of up to 300% are achievable. With the proposed new tray design, a new operation form is also introduced in which the trays are drained sequentially rather than simultaneously, such that the vapor flow is not interrupted during the liquid drainage. For different ratios of counter-current vapor/liquid flow rates, column efficiencies for periodically cycled columns are shown experimentally to be two times greater than those for columns with sieve trays.

1. INTRODUCTION Distillation has long been the separation method of choice for systems that have coexisting vapor and liquid within the allowable temperature range because of its simplicity of equipment, robustness of operation, and ease of maintenance. At the same time, however, conventional distillation is highly energyintensive. There is therefore a significant interest in identifying, developing, and implementing efficient separation methods for isolated separation schemes as well as for schemes integrated into the entire surrounding process. There has been a renewed interest in separations based on periodically operated trays1,2 in recent years. Here we define periodic cycled gas/liquid separation as an operation form, where mixing of liquid phases from stages with different composition is reduced or eliminated through usage of (forced) periodic movement of the phases. Reduction of mixing enables increased separation efficiency of periodic cycled operation, compared with conventional operation. In this respect, periodic cycled separation differs from other periodically operated processes, such as reactors, in which increased efficiency may be obtained through exploiting process nonlinearities. Lewis3 concluded that the ratio of tray efficiency to point efficiency of 2 would be the result of a realization of his so-called Case 2, for trays with a Murphree efficiency of unity. Analogies4 between periodic operation and the Lewis Case 2 operation of a tray column suggests that if periodic operation could be realized, due to its analogy to the Lewis Case 2, tray efficiencies of 200% of sieve trays should be within reach. Therefore, realization of periodic operation has caught the attention of a number of industrial and academic research groups since the late 1950s. A number of realizations have been presented over the decades, and several improvements have been made. As pinpointed below, however, these are mostly based on simultaneous drainage of the liquid from the trays in the column, which may represent a serious disturbance to most separations. In this paper a new operational principle based on sequential liquid drainage from the trays is introduced. © XXXX American Chemical Society

This operational principle requires a new tray design as described below. In this paper we will outline the development of a realization of periodic cycled separation developed at our department over many years. In chronological order we • summarize the background for the developments, • summarize the performance measures of conventional and periodic tray columns, • describe the experimental methods (setup, measurements, and data manipulations) used to obtain experimental column data for a stripping column, • present the column and tray efficiency results obtained. Finally, we will discuss a series of next experimental efforts to support further developments.

2. BACKGROUND A set of important events in developing realizations of periodic separations up to the mid-1980s are outlined. Early designs and the problems with these are summarized. This is to give the motivation behind the trays developed for this work. 2.1. Periodic Operation. Concepts, Realizations, and Tray Designs. 2.1.1. Survey of Early Realizations. Toftegård and Jørgensen5 reviewed the literature on realizations of periodic separation covering the period from 1960 to the mid1980s. Studies of the cyclic operation mode of separation appeared in the chemical engineering literature in 1961, when the Cannon group at Penn State published three articles6−8 describing experimental studies of cyclic distillation made during the late 1950s. With their realization (Figure 1), all vapor/liquid valves are manipulated periodically during the distinctly different vapor and liquid flow periods. During the vapor flow period, rising Received: October 17, 2015 Revised: January 17, 2016 Accepted: January 18, 2016

A

DOI: 10.1021/acs.iecr.5b03911 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

• Hentrich and Vogelpohl’s design might lead to very large columns, if the space occupied by storage containers were to account for 10−25% of the tray area. • With the TD tray of Furzer, it might be difficult to obtain sufficiently long delays. • Matsubara’s approach could result in high pressure drops, long LFPs, and a tall column. Review of these and the other preceding published experimental work led to two design criteria and an operational principle: Periodic Cycling Tray Design Criteria. The periodic cycled separation tray should be constructed such that (1) all the liquid leaves the tray and enters the tray below without being mixed with any other liquid during the liquid drainage and (2) the vapor flow can continue during liquid drainage. To minimize mixing with liquid from other trays, it is advantageous to enforce a special operational principle for periodic cycling: Sequential Tray Draining. The trays are emptied sequentially rather than simultaneously, while the vapor flow is maintained throughout. The sequential tray draining is illustrated for a column with three trays in Figure 2. Two advantages of the sequential

Figure 1. Realization of Cannon and co-workers.6−8

vapor prevents liquid down-flow. During the liquid flow period, where vapor flow is interrupted, liquid can simultaneously flow down from all trays. Thus, no down-comers are needed on the trays because the vapor and liquid phases travel the same avenues in opposite directions in the two different periods. This operation form may be labeled “simultaneous tray draining”, where the total period time is the sum of the vapor flow period and the liquid flow period: TP = VFP + LFP. In spite of a set of problems with obtaining the theoretically predicted improvement, their realization of “controlled cycling distillation” (Figure 1) formed the basis of the realizations that appeared during the 1960s and 1970s. However, neither Cannon’s realization nor its modifications caught on. Two problems were apparent: One was obtaining uniform liquid draining of all trays during the liquid flow period (LFP). While liquid mixing must be avoided to achieve the higher efficiencies possible with cycling operation, early experimental papers9,10 described the nonuniform flow of liquid from the trays during the LFP. This behavior was later rationalized by simulations.11 Attempts to deal with this problem included installation of pressure-equalizing manifolds on columns for use during the LFPs and venting the bottom of the column to atmosphere during the LFPs. Also, under certain conditions liquid on the trays would oscillate and cause dumping,10,12 though introducing baffles12 could circumvent this. The most successful realizations in terms of achieving theoretical performance were the following: • “Stepwise periodic operation” of Baron et al.13 • The trays designed by Hentrich and Vogelpohl,14 where the liquid was led into a storage tube; when all trays were empty, the liquid was released into the next tray. This was accomplished using a central rod in the column. That solution was considered too expensive later on. Instead, a sieve tray with packing material below it was investigated. This was to make the liquid drain more slowly during the LFP and only reach the next stage when this was emptied. • The (TD) time delay tray of Furzer,15 which used gutter channels to delay the liquid draining. • Matsubara’s16 approach, which involved collection of liquid in a store prior to transfer to the tray below. Also, the vapor was transferred in tubes from tray to tray. These approaches, however, were also anticipated to possess the following disadvantages: • Stepwise periodic operation might lead to gas distribution problems if scaled up.

Figure 2. Sequential tray drainage operation for a column with three trays. When the top tray is empty a load of liquid is fed to the top tray. The column remains in state e for the remainder (if any) of the cycling period time until a new liquid drainage sequence is initiated at the start of the next period.

tray draining are obvious. By sequential tray emptying, the liquid from a tray is drained onto an empty tray below; therefore, no or minimal liquid mixing occurs. Second, the vapor flow is maintained throughout the operation; thus, only rather small column pressure variations are introduced, and the liquid is kept on all the nondraining trays. In sequential tray draining the total period time is simply the time between the start of the sequential tray draining because the vapor flow is continuous. To reduce horizontal liquid movement and wave formation on a tray, it is advantageous to insert a layer of packing material on each tray. This layer should be relatively small to ensure a short drainage time. The trays should have a small free area (e.g., 6%) during the vapor flow and operate as ordinary sieve-trays, although without down-comers. However, they should also be able to extend the free area substantially so that the liquid can drain B


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Figure 3. Tray design with two half circular perforated sheets, with structured packing above the tray to improve the weeping characteristics of the trays. The sheet movement is enforced by a pneumatic cylinder.

With this type of tray, the liquid drainage time (LDT) is a few seconds, and the tray is completely drained independently of the holdup (the liquid simply dumps down). The placement of packing materials on the trays helps prevent oscillations. As described by Duffy and Furzer,12 it is important to have some liquid oscillation-damping device on the trays. They used 19 mm spaced 76 mm high parallel baffle plates to prevent oscillations. Here we have employed packing materials. In addition to preventing oscillations, packings also have the desirable property that the tray efficiency is enhanced. Structured packing with the same diameter as the column and the height of 100 mm has proven adequate. The tray distance is 500 mm. The present realization solves some problems of the predecessors. With this tray, liquid will not drain more than one stage during the liquid drainage period. The other key advantages are that

down through the tray at the same time as the vapor rises up through the tray uninterrupted when the tray is “opened”. 2.1.2. Present Tray Design. The proposed design allows (nearly) 100% open area in the column for liquid drainage. This is achieved by use of perforated sheet type trays, which in the present case are two half circular perforated sheets (Figure 3). The perforation follows the design guidelines for conventional sieve trays. When the sheets are horizontal, i.e., closed, the free area is determined by the perforation, in the actual case 6%. Then the vapor rises through the perforated sheets providing good contact and mass transfer with the liquid phases on the trays. With the structured packing above the trays, a good approach to equilibrium should be achieved. When the sheets are opened, they form a large angle to horizontal, such that the liquid flow area is very large (nearly 100%). The rotational axis of the perforated sheets is one of the diameters of the half circles, such that no part of the sheets is turning above the tray level when the tray is opened. This allows packing material to be placed immediately above the tray. In the following, this tray design will be labeled cyclically operated perforated sheet (COPS) tray. The operation of the COPS tray occurs as follows: A pneumatic cylinder is mounted inside the column to control the sheet movement via two rods connected to the cylinder plug. The cylinder is connected to a pressurized air supply (at the two blue connection points in Figure 3), controlled by on/off pneumatic valves (not shown). In Figure 3a, the bottom connection line is pressurized while the top line is not; thus, the cylinder plug is in the up position, which means that the tray is closed. When the on/off pneumatic valves are switched, then the bottom connection is depressurized while the top connection is pressurized and the cylinder plug is pushed down, as seen in Figure 3b where the sheets are opened. The trays operate following the principle of successive tray draining (Figure 2). In this realization, the vapor/gas will flow upward at all times. When a tray is opened, liquid drains rapidly and completely, reaches the empty (closed) tray below, and remains there until that tray is opened during the next cycle.

• vapor flow does not need to be interrupted during liquid drainage from a tray; • the column pressure profile is nearly constant during operation; • the liquid drainage time becomes quite short (a few seconds); • the layer of structured packings on the trays increases the mass-transfer area and extends the weeping limit. 2.2. Performance Measures. Efficiency measure definitions are outlined before presenting measurements made to examine these. The present realization has been tested on stripping of ammonia from aqueous solution. Therefore, certain concepts are detailed in a way that closely addresses studies of this system. 2.2.1. Tray and Point Efficiencies. The degree of equilibrium reached at any local position on a tray can be expressed by a point ef f iciency

εp = C

yout − yin * −y yout in

(1) DOI: 10.1021/acs.iecr.5b03911 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research If one can average the compositions over the whole tray, one obtains the tray ef f iciency y ̅ − yin εt = out y* − y (2) out

Note that the ideal number of stages is the number of equilibrium stages in a counter-current conventional column. 2.2.3. Separation Measures. A separation factor for a stripping process can conveniently be defined as

in

S=

When the liquid and gas phases are completely mixed and have no concentration gradients, the tray and point efficiencies become identical. When a molar balance is made over a stripping tray, with a single vapor inlet and a single vapor outlet (H is the liquid holdup on a tray) H

dx = G(yin − yout ) dt

(3)

dx G dx G = −εt m ·x ⇒ + εt m ·x = 0 dt H dt H (4)

εt =

(5)

G is the vapor flow rate, m a solute K-factor (here for ammonia), x the solute mole fraction in the liquid, and t time. To determine a value for εt, one can plot the value of concentration versus time.10 However, it is not convenient to determine concentrations at several points in time. For the construction of a line, it is much more convenient to measure and log values of pH versus time. If the solute is a weak base, then pH(t ) = pK w −

1 [pK B − log c(t )] 2

(6)

This requires that KB ≪ c. Because pKB of ammonia in water has a value of 4.75, this corresponds to a molar concentration of 1.8 × 10−5. The feed stream concentrations applied in this work exceed this value by orders of magnitude. Thus, one can rearrange the efficiency into H [log c(t = 0) − log c(t )]ln 10 G·m·t 2·H [pH(t = 0) − pH(t )]ln 10 = G·m·t

εt =

(7)

This is not entirely uncontroversial. The presence of carbon dioxide in air may consume hydroxy ions. If this happens to a significant extent, pH is not necessarily a fully adequate measure of ammonia concentration. At the concentration levels considered here, this effect is not expected to have a substantial impact on slopes. However, for the column efficiency measures where concentrations of feed and steady-state product streams have to be known accurately, all stream concentrations have been checked by titration with hydrogen chloride. 2.2.2. Column Efficiency Measures. The measure of column efficiency adopted here is the ratio of the number of ideal stages to the number of real stages accomplishing the same separation. Thus, for the periodic (periodic trays) and conventional (sieve trays) columns, respectively CEp =

Nid N , CEc = id Np Nc

(9)

3. METHODS The present realization is described more comprehensively with its piping and instrumentation diagram. The column strips ammonia from water. Periodic columns performing this separation have also been described by Gerster and Scull10 and Furzer and Duffy.17 3.1. Setup. A piping and instrumentation diagram of the present realization is shown in Figure 4. The column is 3500 mm tall. It has a diameter of 470 mm and is made in stainless steel. Liquid can be supplied from the feed tank to the column on any of the three trays. When the column is operated as a stripper, the liquid is supplied to the top tray. The volumetric rate of liquid may be varied up to about 40 L/min. Air is supplied by an 11 kW centrifugal blower, producing a maximum volumetric flow of 0.4 m3/s. It is transferred to the humidifier through 103 mm PVC pipes. The maximum air velocity is 3.2 m/s because the measurement is best at an air velocity of 2−3 m/s. A Danfoss VLT series 3500 HV-AC frequency converter is implemented on the blower to control the gas flow rate to the column.18 The humidifier is a 1550 mm high, 490 mm diameter stainless steel cylinder with a 300 mm BX-SULZER packing placed inside of it. A sieve tray distributes water in the humidifier. The heated water supply to the humidifier is recirculated. Feed mixtures are prepared in a 400 L feed tank. One liter of 25% ammonia solution is poured into 400 L of distilled water, under stirring, to give a feed concentration of 0.03 M. The column may be equipped with either three sieve trays or three periodic trays. In both cases, the spacing is 500 mm. Alternatively, 1500 mm structured BX Sulzer gauze packing can be employed. 3.2. Measurements. The measurements are logged by a computer, which also is used for controlling the process and collecting data. An in-house developed Instrumentation eXpert software is used for this purpose. When the column is operating, the humidity of air entering the column is controlled to produce a constant humidity of 100% in the column. The control is done by three controllers in a cascade. The controller set points are primary loop, column top temperature (T6); secondary loop, temperature of air leaving the humidifier (T4); and tertiary loop, temperature of water recirculating in the humidifier (T5). The configuration of controllers are (from the outside) PI, P, and P. The controller gains were found by making step responses to produce Ziegler−Nichols settings. The air humidity is calculated

whence H [ln x(t = 0) − ln x(t )] G·m·t H = [ln c(t = 0) − ln c(t )] G·m·t

xp(1 − xf )

where xf is the liquid mole fraction in the feed stream and xp is the liquid mole fraction in the product stream. This separation measure is preferable to others employing vapor compositions because it is not as convenient to measure the vapor concentration of ammonia-containing air exiting the system. Inlet liquid and outlet liquid (product) compositions are more conveniently collected and analyzed.

Using yin = 0 (clean air), integration of this equation gives

⇒ x(t ) = x(t = 0) ·e−εtG / Hm·t

xf (1 − xp)

(8) D


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Figure 4. Piping and instrumentation diagram for the stripping process.

from the dry and wet thermometer temperatures T1, T2, T3, and T4. Because the pressure drop across the orifice (P1, P2) is a measure of the gas flow rate, its value is used in a PI-control loop manipulating the frequency of the frequency converter. The orifice is designed after the ISO Standard 1952 (August 1971). The tube internal diameter is 103.5 mm. The orifice diameter is 77.6 mm, and its thickness is 1.5 mm. The volumetric gas flow rate (before the humidifier), Gbh, can be determined from the following relation based on the Bernoulli equation: G bh = 1.5435 × 10−2

n MV

Δp

M=

(1 + X w )10−3 1 29

+

Xw 18

(11)

where Xw is the humidity of air before the humidifier in mH2O/mdry air. The gas is saturated in the humidifier before entering the column to avoid temperature gradients in the column. The gas flow rate in the column is calculated from G = G bh

1 + Xs 1 + Xw

(12)

where G is the gas flow entering the column and Xs is the humidity of air after the humidifier in mH2O/mdry air. A single pH sensor is available. pH can be measured at three positions (PH1, PH2, and PH3). Commonly, PH1 is used. The pH of the product stream is measured to detect when the column is operating at steady state. When pH is constant,

(10)

where n/V is the moles per cubic meter found from the ideal gas law, Δp the pressure drop across the orifice, and M the (average) molecular weight of air (kg/mol) E


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Industrial & Engineering Chemistry Research the liquid product is titrated to find the product concentration of ammonia. The liquid concentrations of ammonia are also measured for the feed solutions by titration with hydrogen chloride using methyl red as indicator. When two titrations with 10 min interval show the same concentration in the feed, the feed is assumed to be well-mixed. The inlet concentration of ammonia in the gas phase is zero. The outlet concentration is found from a molar balance yout =

L (x in − xout) G

Kh has units of mmHg divided by molarity of ammonia in aqueous solution. T is the absolute temperature in Kelvin. For dilute concentrations, the following relation exists between the slope of the equilibrium curve y* = mx and Kh m = Kh

m = 0.073·106.695 − 1665/ T

Nid =

=

F ρG

(15)

for the limiting values ⎛ mol ⎞ ⎟ < 16.3 13 < G⎜ ⎝ s ⎠

(16)

The later BXPlus gauze packings extend the lower limit of this range to 6.5 mol/s. Thus, as will be apparent from the experimental results, the gas ranges covered in this paper are somewhat lower than the optimum range for the packing employed. 3.5. Properties and Phase Equilibria. The concentration ranges covered here remain well within the region where the equilibrium curve is linear. From equilibrium data on the ammonia−water−air system, the following relation based on Henry’s law has been found20 log Kh = 6.695 −

1665 , T

283 < T (K) < 303

⎡ ln⎢ 1 − ⎣

(

⎡ ln⎢ 1 − ⎣

(

L mG

*

⎤

) xx −− xx* ⎥⎦ + mGL f

out

p

out

ln(mG /L) L xf ⎤ + mG xp ⎥ ⎦

)

ln(mG /L)

L mG

(20)

4. RESULTS To test the performance of the present realization, different experiments have been made. Draining times of about 3 s were established at an early stage. The weeping limits and efficiencies of periodic trays are determined. Also, column efficiencies when operated with different internals are given. This section provides an overview of all results to arrive at a set of design considerations for gas rates and vapor flow periods. 4.1. Weeping Limits for Periodic Cycling Trays. For a given tray, the weeping limit depends on the liquid height and the vapor velocity. For a given height, weeping occurs when the vapor velocity is below a certain critical value. Weeping limit experiments were made on the top tray, where it is possible to measure the pressure difference over the tray. With a certain vapor velocity, experiments were made with different holdups. The pressure differences over the top tray and the orifice were measured between 200 and 500 s. During that time, the amounts of weeping liquid were collected in a bucket and weighed after the experiment was over. The tray was then emptied, and this amount of liquid was also weighed. Changes in pressure drop over the top tray and any weeping liquid collected were taken as indications of weeping. These examinations suggest that the weeping limit is at a ratio of tray holdup to gas rate, H/G, slightly greater than 80 s. To avoid weeping, the liquid height (holdup) should be small. However, as shown in the next paragraph, the tray efficiency decreases when holdup is decreased. Therefore, an optimal holdup must produce large tray efficiency and at the same time avoid weeping. 4.2. Tray Efficiencies. There are in general three variables which affect tray efficiency when entrainment is negligible: (1) the rate of mass transfer in the gas phase, (2) the rate of mass transfer in the liquid phase, and (3) the degree of liquid mixing on the tray. For the ammonia−water system, the slope of the equilibrium curve is quite small (about 0.8−0.9). Therefore, the difference between tray and point efficiency is often only a few percent. For the same reason, this system has essentially no

(14)

RT 1 · · P A

(19)

The Kremser−Souders−Brown equation requires that the vapor−liquid equilibrium is linear. As noted, for the present system, at the conditions of interest, this is a valid assumption.

This corresponds to a gas flow of RT 1 · · ρG ⇒ G = P A

(18)

The number of ideal stages is found from the Kremser− Souders−Brown equation.21 Because the liquid at equilibrium with the entering vapor has a composition of zero

When the column operates with sieve trays installed, or with packings, the liquid flow rates can be measured with a flow meter, based on the pressure drop (P3, P4) across the top tray. This information is used to control the liquid holdup. The feed pump is stopped when the measurement has reached the set point for the holdup. For flow rates less than 9 mol per second, this is not very accurate. In such cases, the flows are doublechecked by collecting outlet liquid in buckets. When the column operates with periodic trays, the liquid flow rates are found by weighing the holdup and dividing the weighed amount by the period time. 3.3. Trays. The periodic cycling sieve trays employed have hole diameters of 2 mm. The three sieve plates in the column have spacing of 500 mm. The free area of the trays is 6% when closed. The weir height is 5 cm. The 100 mm of Sulzer BX gauze packing with a porosity of 92% and a specific area of 500 m2/m3 is attached above each tray. The ordinary sieve trays also have holes of 2 mm diameter and a weir height of 5 cm. 3.4. Packings. When operating as packed column, Sulzer BX gauze packings are used. This packing has a minimum liquid load of 0.2 m3/h·m2. Substantially lower values are specified for the later BXPlus Gauze Packings; in fact, 0.05 m3/m2h is found from Sulzer.19 Because the present column has a cross-sectional area of 0.1735 m2, the minimum liquid load for this column is 0.535 mol per second. The experiments described below are well above this limit. The most economical load range is when the F-factor is in the range

F = G·

P[ = ]mmHg

At atmospheric pressures, the following relation can be used

(13)

2 < F( Pa ) < 2.5

55.56 , P

(17) F


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Industrial & Engineering Chemistry Research liquid-phase mass-transfer resistance. It has mass-transfer resistance only in the gas phase. 4.2.1. Periodic Tray Efficiencies. For the periodic trays, efficiencies were determined for different liquid holdups and vapor flows. The tray efficiency for the periodic column depends upon the ratio between the liquid holdup and the vapor flow, as shown in Figure 5. This relationship is determined using a

Figure 6. Separation factor as a function of flow parameter, FP, for the three types of internals.

Figure 6 shows the separation factor as a function of flow parameter Figure 5. Tray efficiency as a function of holdup, H, and gas flow rate, G.

FP =

ρG ρL

(23)

For low FP-values ( TP/LDT. However, such a realization should not pose any practical difficulties. Clearly, if the LDT is of the same magnitude as the TP for the perforated venetian blind type trays, then capacity is increased at the expense of the separation performance. Clearly, industrial application of the COPS trays will be highly interesting. Such applications also necessitates using an actuator per tray which in the present design is a pneumatic cylinder mounted inside the column. Clearly, other actuation principles could be feasible for adaptation to the properties of the particular mixture to be separated.

The new tray design may prove especially advantageous for reactive separations in which longer tray residence time may be desirable and catalyst(s) may be impregnated into the packing layer on the desired trays.23

6. CONCLUSIONS A new tray design with COPS trays has been demonstrated and evaluated in a stripping column. The point efficiency shows clearly the advantage of the periodic operation over conventional sieve tray efficiencies in that a tray efficiency of up to 300% is demonstrated. Also, the capacity improvement is demonstrated by the separation factors achieved. The column efficiency obtained by the periodic cycling column is a factor of 2 above that obtained with conventional sieve trays at conditions where the potential of the tray efficiency was not fully exploited. Finally, an optimum cycling operation period (TP) selection is shown for the specific demonstration stripping column. The determined efficiency improvements suggest substantial benefit over conventional separation internals. The introduction of the COPS tray type has enabled the introduction of a new operation type for periodic cycling separation in which the trays are drained sequentially rather than simultaneously. This leads to a more steady operation of a periodic cycling separation column where the column pressure is not much affected by the tray drainage because the vapor flow can be maintained continuously. The new tray design will clearly benefit reboiler operation in distillation columns.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b03911. Tray efficiency data for a single tray as a function of holdup and gas flow rate (Table S1); experimental column data for periodic operation (Table S2), column with sieve trays (Table S3), and column with Sulzer BX gauze packings (Table S4) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Addresses ‡

B.T.: Havneholmen 80, 4.tv., 1561 København V, Denmark. C.H.C.: Novo Nordisk A/S, Hagedornsvej 1, 2820 Gentofte, Denmark. §

Notes

The authors declare no competing financial interest.

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J

NOMENCLATURE c = Molar concentration t = Time P = Pressure, bar absolute T = Temperature, °C R = Universal gas constant, 8.31441 J/(mol K) S = Separation factor L = Liquid flow rate (mol/s) G = Gas flow rate (mol/s) M = Molar mass X = Humidity m = Slope of equilibrium curve (dy/dx) DOI: 10.1021/acs.iecr.5b03911 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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(12) Duffy, G. J.; Furzer, I. A. Mass Transfer on a Single Sieve Plate Column Operated With Periodic Cycling. AIChE J. 1978, 24, 588. (13) Baron, G.; Wajc, S.; Lavie, R. Stepwise Periodic Distillation-I. Total Reflux Operation. Chem. Eng. Sci. 1980, 35, 859. (14) Hentrich, P.; Vogelpohl, A. Untersuchung von Verweilzeitverhalten und Rückvermischung der Flüssigkeitsströmung intermittierend betriebener Gegenstromkolonnen. Verfahrenstechnik 1980, 12, 806. (15) Furzer, I. A. Distillation Columns. U.S. Patent 4,381,974 A, 1983. (16) Matsubara, M.; Watanabe, N.; Kurimoto, H.; Shimizu, K. Binary Periodic Distillation Scheme with Enhanced Energy Conservation − II: Experiment. Chem. Eng. Sci. 1985, 40, 755. (17) Furzer, I. A.; Duffy, G. J. Mass Transfer on Sieve Plates Without Downcomers. Chem. Eng. J. 1977, 14, 217. (18) Clausen, C. H. Operation and Regulation of Separation Processes. M.Sc. Dissertation, Technical University of Denmark, Kgs. Lyngby, DK, 1994. (19) Structured Packings − Energy-efficient, Innovative and Profitable (Document 22.13.06.40 V.15), Sulzer Chemtech Ltd Home Page. http://www.sulzer.com/ (accessed October 6, 2015). (20) Gerster, J. A.; Hill, A. B.; Hochgraf, N. N.; Robinson, D. G. Tray Efficiencies in Distillation Columns; AIChE Research Committee; The Science Press Inc.: New York, 1958. (21) King, C. J. Separation Processes; McGraw-Hill: New York, 1971. (22) Maleta, V. N.; Kiss, A. A.; Taran, V. M.; Maleta, B. V. Understanding Process Intensification in Cyclic Distillation Systems. Chem. Eng. Process. 2011, 50, 655. (23) Pătrut, C.; Bîldea, C. S.; Kiss, A. A. Catalytic Cyclic Distillation − A Novel Process Intensification Approach in Reactive Separations. Chem. Eng. Process. 2014, 81, 1.

KB = Base equilibrium constant (for ammonia in water) KW = Waters ionic product, equal to [H+]·[OH−] in aqueous solutions V = Valve (Figure 1) x = Liquid mole fraction y = Vapor mole fraction H = Holdup (moles) Sc = Schmidt number W = Weir height N = Number of stages Subscripts

id = Ideal number of stages L = Liquid G = Gas t = Tray f = Feed stream p = Point or Periodic or Product Superscripts

* = Value at equilibrium Greek Symbols

ε = Efficiency Δ = Change ρ = Density Abbreviations

FP = Flow parameter COPS = Cyclically operated perforated sheet LDT = Liquid drainage time LFP = Liquid flow period duration VFP = Vapor flow period duration TP = Total cycling period duration TD = Time delay bh = Before humidifier log = Base 10 logarithm ln = Natural logarithm

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REFERENCES

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