Waste Water Minimisation in a Refinery Plant

7/30/2019 Waste Water Minimisation in a Refinery Plant

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35th International Conference of Slovak Society of Chemical Engineering,

Tatranske Matliare, May 26-30 2008, pp135,

Slovak University of Technology, Bratislava,

WASTE WATER MINIMISATION IN A REFINERY PLANT BY USING

PROCESS SIMULATION AND PROCESS INTEGRATION

INSTRUMENTSGheorghe Bumbac

1, Ciornei Cristian

1, Adrian Turcu

1, Aurelian Toma

2

1University Politehnica of Bucharest, Centre for Technology Transfer in the Process Industries,

1, Polizu Street, Building A, Room A056, Sector 1, RO-011061, Bucharest, Romania, Phone:

+40-21-2125125, Fax: +40-21-2230797, email: [email protected] S.A., INCERP Ploiesti Subsidiary, 291A, Republicii Blvd., RO-100072 Ploiesti,

Romania, Phone: +40-244-198738, fax: +40-244-198732, [email protected]

Present tendencies, for the environment protection, long lasting development, pollution prevention and

those connected to the increase of the prices of raw materials resources and wastes treatment encourage the

process industries to find solutions for diminishing the raw material consumption and the amount of waste

produced. According to the systematic methodologies of process synthesis, the topologic process schemes are

realized taking in account that they realize a minimum consumption of resources, including the reusing and

recirculation of the untransformed raw material by a single pass through the installation concurrent with the

achievement of a minimum of the utilities consumption. These thingslead to the diminishing of the level of

waste generation and material emissions to the environment, as to a diminishing of the production costs and

the assurance of a sustained growth of the economic activity. In the last decade there were significant

progresses concerning the recirculation and reusing material resources and utilities optimization. Specifically

analysis and process synthesis techniques (including retrofit) based on process integration has evolved and

developed as an effective instrument in handling and solving problems concerning the resources

consumption minimization (both material and energetic). In the present paper is shown the use of simulationand process integration methods for the identification ofretrofit solutions for the minimization of fresh water

consumption used on a crude oil refining site (only three processing unit were taken in account: crude

distillation unit CDU, Fluid catalytic cracking unit FCCU and Delayed Coking Unit DCU) as the

reduction of the waste water discharged by this industrial processing site. The paper presents the identified

topological solution as well as the performances of this solution compared with those already existing.

1. IntroductionThe physical and chemical processes specific to the processing units from a refining oil

site implies the presence of water. Water acts both as a chemical species that prevents the

process of undesirable chemical reactions, specially during the thermal treatments for the rawmaterial processing the crude oil (for instance it prevents carbon deposition on the coil piping

inside the super-heating furnaces) and as an energy carrier when it is used as a stripping and/or

stripping agent in separation processes. Huge amounts of fresh water are implied and also large

quantities of waste water that must be discharged from the processes to the environment. The

costs and the efforts for the waste water treatment are very large but the most important problem

is that in spite all the efforts the environment is affected because the waste water treated and

discharged has not the same quality as the fresh water used. Therefore the more efficient use of

water inside the mentioned industrial site is the redeemer solution for reducing operating costs

and for fulfilling environment protection legislation.

The main idea we developed in this paper was to solve the interconnections between the

process units of the refining total site in order to increase the utilization degree of the waterinside the site by coupling the water streams (sources and sinks) of its main processing units. As

the increase of the degree of utilization of water streams can be realized only by the water


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recycle and reuse inside each processing units as well as between them, we have to limit to the

choice of process units that have a compositional compatibility of the water streams implied into

the respective processes. In the present paper we choose for the integration analysis and the

retrofit scheme synthesis the crude distillation unit CDU the fluid catalytic cracking unit

FCCU and the Delayed Coking Unit DCU. The water streams of these processes contain

approximate the same components (hydrocarbons) that means that there is a compositional

compatibility and that can be realized the coupling between the currents with a water shortagefrom a unit, the currents with water over from the others.

For the identification of the specified solution were used the systematic procedures of

process analysis and synthesis based on the process simulation and integration. Process

integration represents a process analysis and synthesis instrument, used either for grass root

design or for retrofit (El-Halwagi, 1997). In the context of pollution prevention, process

integration is an element of analysis and synthesis of process schemes that plays an important

role in obtaining solutions for process schemes able to fulfill certain performance objectives.

In process synthesis, from the point of view of the application methodology two domains

are relatively better developed, that is the synthesis methodology of heat exchangers networks

(HEN) and mass exchangers networks (MEN). Both domains of mass and heat integration are

reported in detail in the literature sources (v. Douglas, 1988; Biegler et al. 1997; Turton et al.1998; Allen and Shonnard, 2002; Seider at al. 2003; Smith 1995, 2005). Heat and mass

integration and more recently the new methodology of property integration (Shelley and El-

Halwagi, 2000; Kazantzi Vasiliki and El-Halwagi, 2005; El-Halwagi, 2006) have become

together three practical instruments of process integration application,

In the case study from the present work, process simulation for the three process units

was realized, using the simulator HYSYS

, the extraction of the data concerning the water

streams, and as instruments for applying Pinch technology for water was first used the Program

Table Algorithm of Water Cascade, taking in account a single contaminant and then the

procedure based on the optimization, case in which two contaminants in the water streams were

considered.

2. Site Process Units description

a. The CDU- Crude oil often contains water, inorganic salts, suspended solids, and water-

soluble trace metals. As a first step in the refining process, to reduce corrosion, plugging, and

fouling of equipment and to prevent poisoning the catalysts in processing units, these

contaminants must be removed by desalting (dehydration).

The two most typical methods of crude-oil desalting, chemical and electrostatic

separation use hot water as the extraction agent. In chemical desalting, as in our case study,

water and chemical surfactant (desemulsifiers) are added to the crude, heated so that salts and

other impurities dissolve into the water or attach to the water, and then held in a tank where they

settle out. The desalting device is operated continuous. The feedstock crude oil is heated to

between 150 and 350C to reduce viscosity and surface tension for easier mixing and separation

of the water. The temperature is limited by the vapor pressure of the crude-oil feedstock. In both

methods other chemicals may be added. Ammonia is often used to reduce corrosion. Caustic or

acid may be added to adjust the pH of the water wash. Wastewater and contaminants are

discharged from the bottom of the settling tank to the wastewater treatment facility. The desalted

crude is continuously drawn from the top of the settling tanks and sent to the crude distillation

(fractionating) tower. Figure 1 gives a simplified CDU process scheme. The first step in the

refining process is the separation of crude oil into various fractions or straight-run cuts bydistillation in atmospheric and vacuum towers. The main fractions or "cuts" obtained have


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specific boiling-point ranges and can be classified in order of decreasing volatility into gases,

light distillates, middle distillates, gas oils, and residuum.

At the refinery, the desalted crude feedstock is preheated using recovered process heat.

The feedstock then flows to a direct-fired crude charge heater where it is fed into distillation

column just above the bottom, at pressures slightly above atmospheric and at temperatures

ranging from 340 to 370 C (heating crude oil above these temperatures may cause undesirable

thermal cracking). To prevent thermal cracking, steam is injected in the crude oil before furnaceheater. All but the heaviest fractions flash into vapor. As the hot vapor rises in the tower, its

temperature is reduced. Heavy fuel oil or asphalt residue is taken from the bottom. At

successively higher points on the tower, the various major products including lubricating oil,

heating oil, kerosene, gasoline, and uncondensed gases (which condense at lower temperatures)

are drawn off as atmospheric column lateral products. The evaporation, condensing, and

scrubbing operation is repeated many times until the desired degree of product purity is reached.

Then side streams from certain trays are taken off to obtain the desired fractions. Products

ranging from uncondensed fixed gases at the top to heavy fuel oils at the bottom can be taken

continuously from a fractionating tower. Steam is often used in towers to lower the vapor

pressure and create a partial vacuum. The distillation process separates the major constituents of

crude oil into so-called straight-run products. Sometimes crude oil is "topped" by distilling off

only the lighter fractions, leaving a heavy residue that is often distilled further under high

Vacuum Distillation Tower. In order to further distill the residuum or topped crude from the

atmospheric tower at higher temperatures, reduced pressure, and steam injection into system is

required to prevent thermal cracking. The process takes place in one or more vacuum distillation

towers. The principles of vacuum distillation resemble those of fractional distillation and, except

that larger-diameter columns are used to maintain comparable vapor velocities at the reduced

pressures, the equipment is also similar. The internal designs of some vacuum towers are

different from atmospheric towers in that random packing and demister pads are used instead of

trays. A typical first-phase vacuum tower may produce gas oils, lubricating-oil base stocks, and

heavy residual for propane deasphalting. A second-phase tower operating at lower vacuum maydistill surplus residuum from the atmospheric tower, which is not used for lube-stock processing,

and surplus residuum from the first vacuum tower not used for deasphalting. Vacuum towers are

typically used to separate catalytic cracking feedstock from surplus residuum. The areas in which

the presence of water is implied are stated in Figure 1..

Figure 1 Simplified scheme of CDU process

b. DCU - In delayed coking the heated charge (typically residuum from atmospheric distillationtowers) is transferred to large coke drums which provide the long residence time needed to allow

the cracking reactions to proceed to completion. Initially the heavy feedstock is fed to a furnace


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which heats the residuum to high temperatures (460-495 C) at low pressures and is designed

and controlled to prevent premature coking in the heater tubes. For this reason steam is injected

before heater feeding into processing stream. The mixture is passed from the heater to one or

more cocker drums where the hot material is held approximately one day (delayed) at low

pressures, until it cracks into lighter products. Vapors from the drums are returned to a

fractionator where gas phase species, naphtha, and gas oils are separated out. The heavier

hydrocarbons produced in the fractionator are recycled through the furnace.A simplified process scheme is given in the figure 2. After the coke reaches a predetermined

level in one drum, the flow is diverted to another drum to maintain continuous operation. The

full drum is steamed to strip out uncracked hydrocarbons, cooled by water injection, and

decocked by mechanical or hydraulic methods. The coke is mechanically removed by an auger

rising from the bottom of the drum. Hydraulic decoking consists of fracturing the coke bed with

high-pressure water ejected from a rotating cutter.

Figure 2 Simplified process scheme of DCU.

c.) FCCU - FCC process involves mixing a preheated hydrocarbon charge with hot, regeneratedcatalyst as it enters the riser leading to the reactor. The charge is combined with a recycle stream

within the riser, vaporized, and raised to reactor temperature (480-530 C) by the hot catalyst.

As the mixture travels up the riser, the charge is cracked at low pressure. In the more modern

FCC units, all cracking takes place in the riser.

Figure 3. FCCU a simplified process scheme.The "reactor" no longer functions as a reactor; it merely serves as a holding vessel for the

cyclones. This cracking continues until the oil vapors are separated from the catalyst in the


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reactor cyclones. The resultant product stream (cracked product) is then charged to a

fractionating column where it is separated into fractions, and some of the heavy oil is recycled to

the riser. Spent catalyst is regenerated to get rid of coke that collects on the catalyst during the

process. Spent catalyst flows through the catalyst stripper to the regenerator, where most of the

coke deposits burn off at the bottom where preheated air and spent catalyst are mixed. Fresh

catalyst is added and worn-out catalyst removed to optimize the cracking process. In figure 3 a

simplified process scheme for FCCU considered in our case study is given.3. Results and discussions

Water stream data characteristics

After achieving the simulation modules (more detailed information it can find in source Turcu A,

2008) we extracted the data concerning the water currents of the 3 processes. Thus, Tables 1, 2

and 3 present the data of the water currents for CDU, DCUand, respectively FCCU.

Table 1 CDU water streams dataSource currents

Name Flow rate, kg/h Hydrocarbons concentration,

ppm

Water separated from the heavy

gasoline produced in the CDU

2.5 10

Water separated from the diesel

produced in the CDU

6.3 10

Water separated from the

vacuum system of the DCU

4857 15

Water after the desalting 32870 5000

Fresh water used for desalting To calculate 0

Currents in the red


ppmDesalting 33750 3000

Table 2 DCU water streams dataSource currents


ppm

Current DW1+DW2+DW3 374 500

Table 3. FCCU water streams data

Source currents


ppm

Water separated from the D 104

vessel

2732 1928

Water separated from condenser

of the main column

9091 123

Fresh water To calculate 0

Currents in the red

Name Flow rate, kg/h Hydrocarbons concentration,ppm

Injection in the main column

condenser

9602 2000


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In Table 4 are presented the results of the Program table algorithm of water cascade, considering

together all the source currents, respectively all the shorts currents of the three processing units

of the refining site.

Table 4 Program table algorithm of water cascade for CDU, DCU and FCCU

The initial flow rates of fresh water and those discharged from the three units according theprocess data (emphasized from the simulation modules) are given in Table 5.

Table 5 Water import-export flows in the existing units we considered

Unit Fresh water, t/h Discharged water, t/h

CDU Desalting: 33.75 Desalting :32.87

Lateral products : 0.009

Vacuum system : 4.86

DCU Condenser injection: 9.602 Separator of the main column condenser : 9.091

Water after D104 separator: 2.732

FCCU DW1 + DW2 + DW3 : 0.374TOTAL Fresh water: 43.352 Discharged water: 49.936

Carrying outthe Program Table Algorithm of Water Cascade for the water currents of the CDU,

DCU and FCCUprocesses (see Table 4) provides information concerning the target values for

the necessary of fresh water for the three units: 6.40 t/h as well as the target flow rate of water

discharged from the three units : 12.99 t/h. Comparing the target values with those existing

nowadays for the three units a serious potential can be observed for reducing the flow rates, by

achieving the integration of water resources between the three units, more precisely : for fresh

water 43.352 6.40 = 36.952 t/h (meaning a saving of 85.24%) and for the discharged water:

49.93 12.99 = 36.94 t/h. (meaning a saving of 73.98%).The savings obtained are but objectionable due to the fact that they are taking in account only

one contaminant. Therefore, in the following analysis, were used the data from the mentioned


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simulation modules, taking in account a hydrocarbon(n-C5)and a salt (NaCl).As an analysis

instrument was used the procedure based on taking in account the mass transfer, implemented in

a numerical algorithm in a software instrument called WATER(elaborated in the Department

of Process Integration at the University of Manchester - UMIST).In Figures 4 and 5 are shown

the data of the water sources from the three refinery units used in the analysis with two

contaminants.

Figure 4. Water source streams data for CDU, DCU and FCCU in analysis with twocontaminants

Figure 5. Water sink streams data for CDU, DCU and FCCU in analysis with two

contaminants


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Using as analysis option water reuse and minimum source water flow rate, according the

integrating analysis, taking in account 2 contaminants, the necessary of fresh water for the three

units is 36.79 t/h. The differences (savings) of flow rates obtained by integration are: fresh water:

43.352 19.755 = 23.597 t/h; (54.43 % savings) discharged water: 49.936 - 36.79 = 13.146 t/h;

(26.32 % savings), obtaining also the topological scheme of interconnections as they are shown

in Figure 6.

Figure 6. The topological scheme of the interconnections between the source waterstreams and short streams arisen for the case when there are taken in account twocontaminants (DW1+DW2+DW3 refers to water separated from DCU separation

column products)

In conclusion, the economies obtained for the case calculated with ATCA with a single

contaminant were of 85.24 % for the fresh water and, respectively, 73.98 % for the waste water

discharged from the processes. Due to the fact that the identified economies were not reliable,

by considering a single contaminant, the problem rises to verify the method with another method

that can use two or more contaminants. The savings achieved for the case calculated with

WATER, were of 54.43% for the fresh water and, respectively, 26.32 % for the waste water

discharged from the processes. Based on these objectives we synthesized a water network

common to the three processes considered. The solution of the joint reuse between the three

fabrication processes analyzed from an industrial crude oil refinery site is realistic, because the

waste water issued both from the fluid catalytic cracking unit and from the delayed coking unit is

reused inside the cracking process and in the crude distillation unit, having no incompatibilities

between the chemical species recycled in the process and the existing ones.

The practical implementation of the integration solution leads to an improvement of the process

performances measured by the diminishing of fresh water consumption and waste water

discharge in the environment. The new topology with new technological relations between the

site processes does not affect their good functioning and this creates the conditions of feasibility

from the economic point of view. In this paper we did not use operating costs or capital costsbecause this paper has not as an objective to determine some economic indicators.


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Waste Water Minimisation in a Refinery Plant

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