(80f) Development of Heat Exchangers for Use As Replacement Plates

Development of Heat
Exchangers for Use as Replacement Plates

In Distillation Columns:
Proof of Concept and Collection of Design Information

²Graham Thomas
Polley, ¹Edgar Enrique Vázquez Ramírez ¹Antonio Alberto
Aguilar Moreno, ¹Jose Manuel Riesco Ávila, ¹Carlos Omar
Ríos Orozco.

¹Departamento de Ingeniería
Mecánica, Universidad de Guanajuato,

Carretera Salamanca?Valle
de Santiago km. 3.5 + 1.8 km, Comunidad de Palo Blanco, Salamanca, Gto., México

Teléfono: 01 464 64
79940 ext. 2306

² Departamento
de Ingeniería Química, Universidad de Guanajuato

Introduction

Separation of chemical species by
distillation is an energy intensive process. Whilst it has been recognized for
many years that the extraction of heat from intermediate stages within a
distillation column can result in significant energy savings the equipment
required to do this is generally expensive. For instance, in columns used for
the distillation of crude oil small individual columns (side strippers) are
often installed alongside the main distillation column.

In more recent years the option of actually
undertaking the distillation inside heat exchangers has been examined. However,
this usually results in significant reduction in the concentration driving
forces used in the separation with significant effect upon column size and
cost.

Work being undertaken at FIMEE is directed
at the examination of an inexpensive form of heat exchanger (fabricated from
parts manufactured for use in the motor industry) that can be installed as a
replacement for a plate within an existing distillation column.

Exchanger
Design

A distillation column consisting of a
number of sieve plates is illustrated in Figure 1. A sieve plate is simply a
flat plate through which a number of small diameter holes have been drilled.
The vapor traveling upward through the column has sufficient velocity to
prevent liquid flowing through these holes. As a consequence, a two phase
mixture is produced on top of the plate. It is maintained on the plate by a
weir. Whilst the vapor velocity is sufficient to prevent liquid flowing through
the holes it is not high enough to carry liquid to upward through the column.
Consequently, liquid and vapor separate above the reservoir formed by the weir
with the liquid flowing down the plate down-comer that feeds the next plate
down the column.

The new exchanger fits in the space of two
existing plates. The upper outlet then acts as a sieve plate. The liquid over
flows the weir and feeds the next plate down the column.

Figure 1. Distillation Column
Arrangement and New Exchanger

The proposed design has two significant
features. The first is a liquid feeder plate that distributes the liquid from
the feed down-comer across the heat exchanger. This is illustrated in Figure 2.

Figure 2. Liquid Feeder

This base of this plate is sealed. The
upper plate is perforated.

The second feature is the ?flow inducer?
and is illustrated in Figure 3.

Figure 3, Flow Inducer

Vapor flowing from the sieve plate below
the exchanger passes through tubes installed in the base of the liquid feed
plate. These tubes protrude into the individual heat exchanger tubes. Liquid
from the feed down-comer is then entrained by this vapor flow and drawn through
the heat exchanger.

Experimental
Apparatus

The apparatus used in this study only
considers hydraulic performance of the heat exchanger. It consist consists of a
single transparent tube (in order that flow patterns can be observed using high
speed photography) that represents a tube within the heat exchanger that is
fitted with flow inducer (as described above - Figure 3).

The rig (both schematic and photograph) is
shown in Figure 4.

This tube sits inside a large diameter tube
that provides a reservoir of liquid. The reservoir has gauge marks that are
used to measure the height of liquid above the flow inducer.

Liquid is fed to the feed reservoir from
tank A. The flow being controlled by a valve positioned in the feed leg.

Air is injected into the tube at a
controlled and measured rate (measured using rotameters).

The two-phase mixture leaving the exchanger
tube is separated in a tank (tank C), with liquid flowing to a collection
vessel (tank D). This vessel is used to measure the rate of flow of liquid
through the exchanger tube (through measuring the time to collect a given
quantity of liquid). On completion of the measurement this liquid is emptied
into tank B. Tanks A and B operate on a constant recycle in order to minimize
pressure variation in the feed leg and provide good flow control (confirmed by
differential height calculations during collection in tank D).

Figure 4. Experimental Rig

Analysis
of Experimental Data

Data obtained in the experiments are given in Table 1.

The head required
to provide the recirculation is assumed to
consist of three terms: gravitational head (a function of the voidage within the tube), frictional
losses and a kinetic energy term. This
kinetic energy ter mis the difference between that of the two phase
flow leaving the tube and that
of the liquid and vapor flow entering the
tube. The kinetic energy of the vapor entering the tube is
calculated from the measured air flow. The kinetic
energy of the liquid entering the tube is
assumed to be zero.

A ?homogeneous? model is found
to provide good correlation of the measurements for conditions in which frictional lossses dominate. Accuracy of prediction falls as the % of overall pressure drop due to
kinetic effects increases. This suggest that assumption
that the incoming liquid has zero kinetic energy
is incorrect.

Further experimentation
is necessary to determine the significance of kinetic energy effects within the wáter injection system.

Table 1. Experimental Data and Predictions of Homogeneous Flow Model

Evaluation
of Thermal Performance

Heat transfer within the tubes of the
exchanger will be controlled by the balance of two mechanisms: two-phase
convective heat transfer and nucleate boiling heat transfer.

The convective heat transfer coefficient is
given by:

The predictions of this equation have been
compared with a large data base of measured heat transfer coefficients during
upward flow boiling within circular tubes. The data are well correlated.

The principle problem that has been
identified is that under many circumstances the liquid Reynolds Number falls in
the laminar flow region. But this leads to the identification of two potential
improvements in the exchanger design. These are the provision of a liquid
recycle and the use of tube inserts to promote heat transfer within the
exchanger.

Linking
Column Design, Exchanger Design and Integration

The vapor flow thru the column is set by
the column duty.

The maximum cross section of the heat
exchanger is dependent on the area taken up by the down-comers. This cross
section can be related to the number of tubes installed in the heat exchanger
allowing calculation of the minimum mass flux of vapor flowing through an
exchanger handling the specified column duty. Higher mass fluxes are obtained
by reducing the number of tubes in the exchanger.

The mass flux of liquid flowing through the
exchanger can be set independently by adjusting the positioning of recycle
vanes installed in the liquid down-comer.

So, it is possible (using a quite simple
spreadsheet) to determine the heat transfer coefficient achieved in the heat
exchanger for different combinations of vapor and liquid mass fluxes. The
homogeneous pressure drop model will then allow calculation of minimum tube
length for the heat exchanger.

Knowing the overall heat transfer
coefficient for the heat exchanger the heat load obtained for a given
temperature difference between the column and the fluid with which it is
integrated.

Conclusions

The concept of an integrated internal heat
exchanger that takes position previously occupied by distillation plates has
been demonstrated using a simple apparatus.

It has been found that the hydraulic
behavior of such an exchanger is well modeled by a homogeneous flow model.

There is significant scope to improve the
heat transfer performance of such exchangers by installing tube inserts.

The flexibility of such exchangers can be
improved by installing vanes in the liquid down-comers in order to provide
liquid recycle and increased liquid flow through the exchanger.

A procedure that allows the engineer to
link column duty, heat exchanger design and process integration opportunity has
been developed.