The problem of enhancing refrigerating capacity of disk regenerative crystallizer designed for oil dewaxing

and slack wax and petrolatum deoiling processes is studied. A computer model is built to study the heat

exchange process in the crystallizer. The numerical modeling data are compared with the industrial disk

crystallizer test data. It is demonstrated that the proposed model adequately describes the hydro- and

thermodynamic conditions of operation of the industrial apparatus and that the disk crystallizer has

considerable potential for augmenting refrigerating capacity.

Current production of group II and III petroleum base oils is based on two different technologies, namely, hydro-

catalytic (hydrocracking of the oil or fuel profile in combination with hydrocatalytic dewaxing) and combined that includes

a solvent dewaxing unit. The unit treats the raffinate after the selective purification and hydroconversion stages. The slack

wax obtained is submitted to hydroisomerization.

These technologies include units for deasphalting heavy resid with propane to get residual base oil component.

The hydrocatalytic oil production process, in spite of all its virtues, does not help get two costly and scarce prod-

ucts, namely, wax and a high-viscosity oil called bright stock. The reason for this is that hydrocatalytic dewaxing cracks the

wax and turns it into gas. Concomitantly, hydrocracking reduces the viscosity of the residual component to a level below what

is required for bright stock production.

The combined technology, which involves selective purification and solvent dewaxing processes, allows production

of a wide variety of base oils, including bright stock, as well as waxes. The operating costs in this technology are higher

because of use of a solvent dewaxing unit.

The dewaxing unit operating costs in some cases may be more than 40% of the total costs for the whole oil block.

The companies Yutec Technolgies and Petrokhim Engineering have developed a disk-shaped regenerative crystal-

lizer, which helps reduce the solvent dewaxing unit operating cost substantially by way of increased extraction of dewaxed

oil from the raffinate and reduced cost of the base oil, which increases competitiveness of the combined process. Moreover,

this technology is preferable from the point of range of products obtainable.

The disk regenerative crystallizer is a horizontal cylindrical apparatus partitioned by cooling disks into separate sec-

tions (Fig. 1). The filtrate produced upon separation of the wax crystals from the feed mixture, which acts as the coolant, is

injected into the hollow disks. The suspension moves inside the housing, gradually overflowing from one section to another

through the peripheral and central annular gaps (channels). The wax crystals form on the surface of the cooling disks and in

the solution volume.

Chemical and Petroleum Engineering, Vol. 49, Nos. 7–8, November, 2013 (Russian Original Nos. 7–8, July–August, 2013)

REFRIGERATING CAPACITY OF DISK CRYSTALLIZER

IN OIL DEWAXING PROCESS

A. V. Vishnevskiy,1 S. S. Kruglov, Sr.,2

S. S. Kruglov, Jr.,2 and V. O. Shakhovskiy3

Translated from Khimicheskoe i Neftegazovoe Mashinostroenie, No. 8, pp. 19–23, August, 2013.

0009-2355/13/0708-0522 ©2013 Springer Science+Business Media New York522

1 Yutec Technnologies Ltd., Haifa, Israel.2 Gubkin Russian State University of Oil and Gas, Moscow, Russia; e-mail: kruglov-s@yandex.ru.3 Petrochim Engineering Company Moscow, Russia.

The volume of the working zone of this apparatus is 15 times larger than the working space of a tube-in-tube crys-

tallizer, which ensures longer residence of the suspension and allows slowing of the suspension cooling rate, increasing

thereby the efficiency of the crystallization process.

The wax crystals formed are removed by rotating scrapers from the cooled disk surfaces. The scrapers are attached

to a slow-rotating shaft driven by a two-speed geared motor. To the shaft are also attached paddle stirrers which, together with

the scrapers, mix the cooled suspension stream, accelerating thereby heat and mass transfer. Vigorous agitation of the sus-

pension raises the heat transfer coefficient above what is achieved in familiar scraper crystallizers. The general direction of

the coolant stream motion is opposite to the direction of the cooled suspension motion.

The experience of the disk crystallizer operation in an LUKOIL-Nizhegorodnefteorgsintez oil dewaxing unit [1]

confirmed its efficiency in treating both distillate raffinates of various viscosities and residual material.

A disk regenerative crystallizer under start-up and adjustment operation mode on a 39-2 oil dewaxing unit at

LUKOIL-Nizhegorodnefteorgsintez (in Kstovo) is shown in Fig. 2.

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Fig. 1. Disk regenerative crystallizer: cooling surface of crystallizer 34 m2, volume of feed suspension 16.1 m3.

Fig. 2. Disk regenerative crystallizer.

According to industrial test data [1–3], the refrigerating capacity of a disk crystallizer with a 34 m2 cooling surface is

20–30% higher than that of a tube-in-tube crystallizer with a 70 m2 cooling surface. Also, the disk crystallizer design has the poten-

tial for enhancing refrigerating capacity further on account of increased coefficient of heat transfer between the process flows.

In order to assess the feasibility of increasing the refrigerating capacity of the disk crystallizer by raising its heat

transfer coefficient without physical experiment, a computer model of the hydro- and thermodynamic processes was built for

this apparatus employing numerical calculation methods.

The rate of heat transfer depends on thermophysical properties and conditions of flow motion, material, wall thick-

ness, and properties of impurities.

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Fig. 3. Models of crystallizer sections center–periphery–center and periphery–center–periphery.

Fig. 4. Distribution of heat transfer coefficient αm over the disk surface (the shaft rotates

clockwise, and the readings on the scale are in W/(m2⋅K)).

The equation for the coefficient of heat transfer between two flows in the disk crystallizer:

(1)

where αm, αc are the coefficients of heat transfer from the feed mixture flow to the disk wall and from the disk wall to the

coolant flow, W/(m2⋅K); Rwd, Rim are the coefficients of heat resistance of the layer respectively of wax deposits on the outer

surface of the disk and impurities on the inner surface of the disk, m2⋅K/W; and Rw is the coefficient of heat resistance of the

disk wall, m2⋅K/W.

The coefficients of heat transfer from the outer and inner sides of the disk depend upon the geometry of the flow sec-

tion, flow speed, and thermophysical properties of the process flows. The coefficient the heat transfer from the feed mixture

side αm is also a function of the rotation speed of the scraping devices and paddle stirrers that agitate the suspension flow.

In [4], an analysis was made of the equations for calculating the coefficient of heat transfer from the heat carrier to

the cooled wall in apparatuses having stirring devices. It was shown that the hydrodynamic conditions of heat carrier motion

in all the proposed functions are taken account of by only the centrifugal Reynolds number Rec. It is well known that in a

disk crystallizer the hydrodynamics depends not only upon the rotation speed n of the scrapers and stirrers, but also upon the

suspension motion through the interdisk space on account of continuous delivery of the feed mixture into the apparatus. Con-

sequently, the flow conditions in the housing must depend upon two Reynolds numbers:

Rec = nd2ρ /η and Rec = wDeρ /η,

where ρ is the density of the medium (suspension), kg/m3; w is the characteristic flow speed, m/sec; De is the equivalent diam-

eter of the interdisk chamber channel, m; and η is the dynamic viscosity of the suspension, Pa⋅sec.

The heat carrier flow speed w is a parameter that determines the solution residence time in the apparatus, the flow

conditions, mixing criterion, heat flux density, etc. So, for the case under analysis it would be more appropriate to describe

the criterial similarity equation in the following form:

Nu = ƒ(Re, Rec, Pr),

where Nu is the Nusselt number, and Pr is the Prandtl number.

Criterial equations for calculating coefficient of heat transfer in a disk crystallizer have not been reported in the lit-

erature [5, 6], and the error of the available correlations is 150–200% higher than what is permissible [4]. The data obtained

in [4] by investigation of a vertical disk crystallizer are unusable for the apparatus under study because of the differing modes

of feed material and coolant motion through the interdisk and disk spaces as well as of the limited range of application of the

equations for calculating αm using the Reynolds number Rec for which they were derived.

In [7, 8], mathematical models are given for calculating heat transfer processes in tube-in-tube type of crystallizers.

Similar models are not available for disk crystallizer to this day. A mathematical description of heat and mass transfer [4] was

provided for specific conditions of the process and design of the disk crystallizer and cannot be used in practical calculations

of the studied apparatus, so the coefficient of heat transfer from the suspension to the outer disk surface αm in the regenera-

tive disk crystallizer was determined by computer modeling.

The hydro- and thermodynamics of the crystallizer was modeled at the Department of Oil and Gas Processing Equip-

ment of the Gubkin Russian State University of Oil and Gas with the aid of Flow Simulation program complex employing

the finite volumes method usable for computational fluid dynamics (CFD) modeling for solving associated heat transfer prob-

lems. To calculate the coefficients of heat transfer in the apparatus, an analysis of the steady heat transfer processes between

the flows was made.

3-D models of the disk crystallizer sections (Fig. 3) were built using the Solid Works automated design system.

KR R R

=+ + + +

1

1 1/ /,

α αm wd w im c

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In order to determine the properties of the feed mixture and coolant flows, use was made of the experimental data of

the Novo-Ufimskii Oil Refinery laboratory pertaining to distillation for getting raffinate and oil in conformity with GOST 10120

and their density and viscosity values at different temperatures in conformity with GOST 3900 and GOST 33, respectively.

The model is based on the design and process parameters of the disk regenerative crystallizer (DKR-1) of the

LUKOIL-Nizhegorodnefteorgsintez 39-2 oil dewaxing plant in Kstovo. A mixture of the raffinate of selective purification of

350–420°C distillate fraction and the solvent in 1:2.5 vol. ratio was fed into the crystallizer. The filtrate consisting of a mix-

ture of the dewaxed oil and the solvent in 1:3 vol. ratio served as the coolant. The solvent used in the dewaxing process con-

sisted of 60 vol.% of methyl ethyl ketone and 40 vol.% of toluene.

These data were used to calculate the density, specific isobaric heat capacity, heat conductivity, and dynamic vis-

cosity of the feed mixture and the coolant (filtrate) in the 10–50°C range. These thermophysical properties were incorporat-

ed in the Flow Simulation database.

In the modeled crystallizer, the heat liberated upon crystallization of the solid hydrocarbons from the solution

(2 wt.% of raffinate) comprises roughly 6% of the heat transferred by the feed material solution to the filtrate. This allows

one to take account of the effect of heat generation in the phase transition process by corresponding elevation of heat capac-

ity of the feed mixture by 1.06 times.

The chief material of the crystallizer housing and the interior devices is 09G2S structural carbon steel. The specific

heat capacity and heat conductivity of this steel at 10–50°C were incorporated in the Flow Simulation database.

Model preparation for calculation: determining subregions of the working flows, materials of the model compo-

nents, building volume network, and setting initial and boundary conditions.

The most stable solution of the problem by the finite volumes method is achieved by setting the flow rate at the

model inlet and the static pressure of the medium at the model outlet [9].

In the process flow model, the following boundary conditions are set:

• feed mixture at the inlet: temperature 44°C, volume flow rate 94.5 m3/h;

• filtrate at the inlet: temperature 22°C, volume flow rate 88 m3/h;

• feed mixture at the outlet: static (excess) pressure 3.1 MPa; and

• filtrate at the outlet: static (excess) pressure 1.6 MPa.

In the calculation, the outer walls of the model are adiabatic, i.e., without heat exchange with the surroundings.

Since the steady heat exchange condition is modeled, the only heat conducting component of the model structure is the disk

(the other components are taken as ideal insulators). And since the condition of flow motion in the crystallizer is highly tur-

bulent, the heat transfer in the liquid due to free convection and the action of the gravitational forces can be ignored. In the

calculation the shaft rotation speed was taken as 8 rpm. The coefficient of heat resistance of a 0.1 mm thick crystalline wax

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Fig. 5. Heat transfer coefficient K versus shaft rotation speed n.

layer was assigned to the outer disk surface and the heat resistance of the deposit layer was assigned to the inner surfaces of

the disk and the labyrinthine channels.

For monitoring the convergence of the numerical solution of the problem, the criteria based on the laws of mass and

energy conservation at the model inlet and outlet were determined.

The coefficients of heat transfer on the inner and outer disk walls were obtained by calculations. The distribution of

the coefficient of heat transfer from the feed mixture side is shown in Fig. 4. The values of the coefficients αm and αc were

averaged over the disk surface and were 399.34 and 1171.39 W/(m2⋅K), respectively.

Thus, using Eq. (1), we can determine the coefficient of heat transfer between the flows K = 220.16 W/(m2⋅K).

According to the Yutec Technologies Ltd. data, the minimal heat transfer coefficient of the disk crystallizer designed

for substituting the existing 70 m long tube-in-tube type of apparatus should be about 185 W/(m2⋅K).

According to the data obtained by industrial tests on an Orsknefteorgsintez oil dewaxing unit, the heat transfer coef-

ficient in the disk crystallizer varied from 221 to 256 W/(m2⋅K).

Based on a comparison of the calculated coefficient K with the computational-experimental data obtained at

LUKOIL-Nizhegorodnefteorgsintez, it was confirmed that the discrepancy between the calculated and the true values of heat

transfer coefficients is not more than 5%.

Consequently, the CFD model adequately reproduces the hydrodynamics and heat exchange of an industrial disk

crystallizer and can be used for calculating heat transfer coefficients at other drive shaft rotation speeds (Fig. 5).

Thus, it is shown by numerical modeling that the refrigerating capacity of a disk regenerative crystallizer can be

increased substantially by increasing the shaft rotation speed. In practice, however, the optimal suspension mixing intensity

must be chosen in each specific case because excessively high rate of scraping of wax deposit from the disk surface may impair

the crystal structure and size, which will have an adverse effect on the operation of the filtration block of the dewaxing unit.

REFERENCES

1. A. V. Vishnevskiy, A. G. Vikhman, S. I. Nikolaev, et al., “Operating experience with a disk-shaped crystallizer,”

Khim. Tekhnol. Topl. Masel, No. 5, 13–16 (2011).

2. A. V. Vishnevskiy, A. G. Vikhman, and D. Yu. Bychkov, “New types of crystallizer for petroleum oil dewaxing units,”

Khim. Tekh., No. 4, 21–23 (2009).

3. A. V. Vishnevskiy, V. P. Kostyuchenko, and A. G. Vikhman, “Advances in dewaxing and deoiling technology,” Khim.

Tekhnol. Topl. Masel, No. 2, 8–11 (2007).

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Al’yans, Moscow (2004).

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Comp. Fluids, No. 33, 869–880 (2004).

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