ASPEN-0-16-VCM Report

AspenTech Report

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VCM P LANT M ODEL

SINOPEC Qilu Petrochemical

Company

May 2002

This report contains proprietary information of

SINOPEC Qilu Petrochemical Company and Aspen Technology, Inc.

Table of Contents

1Summary (3)

2VCM Manufacture and Project Goals (4)

3Section 100 – Direct Chlorination (7)

4Section 200 – Oxychlorination (9)

5Section 300 – EDC Purification (16)

6Section 400 – EDC Pyrolysis (18)

7Section 500 – VCM Purification (22)

References (26)

1 Summary

This report documents the Aspen Plus models for SINOPEC Qilu Petrochemical Company’s balanced vinyl chloride monomer (VCM) plant developed by AspenTech with assistance of three engineers from the Qilu Petrochemical Company (QPC). The three QPC engineers are ZhengHua Cai, YuChu Guo and WeiQiang Xu.

Effective models for VCM (and other chemical) facilities require accurate and reliable foundation models (for thermodynamic properties, reaction kinetics and heat transfer), flexible and robust models for the process equipment (for example, reactors, heat exchangers, flash separators and distillation columns) and robust schemes to converge the process flowsheets. The VCM process is especially challenging because it contains nonideal mixtures, electrolytes, complex reactors and integrated flowsheets. This project used the combination of the deep process-modeling knowledge of AspenTech personnel and the intimate knowledge of the VCM plant of the QPC process engineers to develop effective steady-state models of the QPC VCM plant. The joint project also served as the basis for AspenTech to train the QPC engineers to model VCM and other chemical plants. The QPC VCM plant uses the balanced process and is divided into five sections: direct chlorination, oxychlorination, ethylene dichloride (EDC) purification, EDC pyrolysis and VCM purification sections (see Figure 1). All these five sections have been modeled in this project and the models are documented in this report.

This report is organized in the following way. Chapter 2 provides background information on VCM manufacture and the broad approach taken in the project. Chapters 3-7 describe the simulation models developed for the five plant sections. Chapter 8 provides instructions on running the AspenTech VCM models using Aspen Plus. A chapter called References provides citations for the technical literature on the VCM process that are cited in this report.

The VCM models will enable analysis to understand, trouble-shoot and improve plants, and will serve as the basis for planning and executing future plant expansions.

2 VCM Manufacture and Project Goals

The QCP VCM plant uses the balanced process based upon ethylene and chlorine (Cowfer and Gorensek, 1996; Ullmann’s, 1999). The reac tions for each component process are shown in equations 1-3 and the overall reaction is given by equation 4:

Direct Chlorination CH2=CH2 + Cl 2 → ClCH2CH2Cl (1) EDC Pyrolysis 2ClCH2CH2Cl → 2CH2=CHCl + 2HCl (2) Oxychlorination CH2=CH2 + 2HCl + ?O 2 → ClCH2CH2Cl + H 2O (3) 222

In a typical balanced plant producing VCM from EDC, all the HCl produced in the EDC pyrolysis is used as the feed for the oxychlorination. On this basis, EDC production is about evenly split between direct chlorination and oxychlorination, and there is no net production or consumption of HCl. The three principal reaction steps and the associated separation and heat-integration steps are shown in Figure 1.

Vinyl Cl C 2H O 2

HCl Recycle

Figure 1. Principal Steps in a Balanced Vinyl Chloride Process

In the QPC plant, the principle steps are designated as section numbers as follows:

Section Process Step

Section 100 Direct Chlorination Section 200 Oxychlorination

Section 300 Ethylene Dichloride (EDC) Purification Section 400 EDC Pyrolysis

Section 500 Vinyl Chloride (VCM) Purification

The goal of the project was to develop rigorous kinetic models for the Oxychlorination (Section 200) and EDC Pyrolysis (Section 400) reactors and a stoichiometric conversion model for the Direct Chlorination (Section 100) reactor, to develop effective models for the various flash separation, heat exchange and distillation models used in the process and to combine these building blocks into flowsheet models for each of the five sections of the QPC VCM plant.

The modeling approach taken was, as far as possible, to use standard models within Aspen Plus. Thus, the reactor kinetics were described using POWERLAW (power law) or LLHW (Langmuir-Hinschelwood-Hougen-Watson) kinetics. Because of this approach, most of the models developed in this project are described by the standard Aspen Plus documentation. The only exception to this is the model developed for the radiative heat transfer in the EDC Pyrolysis reactor. In this case a user subroutine needed to be developed. The use of this model, together with general comments on running Aspen Plus models is presented in Chapter 8.

In this report it is assumed that the reader is familiar with the use of Aspen Plus, and especially advanced features like electrolyte thermodynamics, reactor models and advanced distillation. Brief training was provided to QPC engineers in the use of these advanced features. It is recommended that engineers who use and further develop the VCM simulation models take the appropriate courses from AspenTech.

It is expected that the VCM models will achieve the following objectives:

?Design optimal process configurations and plant operations to maximize production rate and minimize production costs.

?Assist in scaling up and debottlenecking of existing production lines, including reactor and flowsheet modifications.

?Explore operating conditions for new catalysts.

?Train operators to better operate the plant and handle plant upsets.

?Support the study of capital investment projects to design and revamp the plant. Based upon AspenTech’s past experience, expected benefits of the VCM modeling project will be as follows:

?Material consumption will be reduced by 2%.

?Utility consumption will be reduced by 2%.

In addition to the direct benefits, several indirect benefits are expected to be achieved:

?Production rate will be increased.

?Engineers will be trained to better understand the technology of building process models and to apply process models.

?Operators will be trained to better operate plants and to understand the process.

3 Section 100 – Direct Chlorination

Direct chlorination of ethylene is conducted in liquid EDC in a bubble column reactor. Ethylene and chlorine dissolve in the liquid phase and combine in a homogeneous catalytic reaction to form EDC. Under typical process conditions, the reaction rate is controlled by mass transfer, with absorption of ethylene as the limiting factor. The catalyst, ferric chloride, is highly efficient for the chlorination reaction. The product is extracted from the vapor phase. In the Aspen Plus model developed in this project the reactor is modeled simply as an RSTOIC model in which the conversion to EDC and the various side products is specified.

The components used in the Section 100 model are as follows:

ID Component name Formula

C2H4 ETHYLENE C2H4

C2H6 ETHANE C2H6

C3H6 PROPYLENE C3H6-2

CL2 CHLORINE CL2

O2 OXYGEN O2

HCL HYDROGEN-CHLORIDE HCL

H2 HYDROGEN H2

N2 NITROGEN N2

LIGHTS ETHYL-CHLORIDE C2H5CL

EDC 1,2-DICHLOROETHANE C2H4CL2-2

C2H3CL3 1,1,2-TRICHLOROETHANE C2H3CL3

HEAVIES 1,1,2,2-TETRACHLOROETHANE C2H2CL4-D2

H2O WATER H2O

C2CL4 TETRACHLOROETHYLENE C2CL4

The physical property method NRTL-RK is used. This method uses the NRTL activity-coefficient model to describe the nonideality of the liquid phase and the Redlich-Kwong model to describe the nonideality of the vapor phase. It is appropriate for the mixtures and conditions of the Direct Chlorination section.

The Direct Chlorination reactor has been described using an RSTOIC model with the following reactions and specified conversions:

Rxn

No. Stoichiometry Key

Component

Fractional

Conversion

1 C2H4 + CL

2 --> EDC CL2 0.9984

2 C2H4 + 4 CL2 --> C2CL4 + 4 HCL CL2 0.0002

3 EDC + CL2 --> C2H3CL3 + HCL CL2 0.001

4 2 C2H4 + 2 CL2 --> LIGHTS + C2H3CL3 CL2 0.0004

The first reaction in the above table is Equation (1) in Chapter 1. The other reactions describe the main side reactions. The reactions and conversions were chosen based upon the information published in VCM references (Cowfer and Gorensek, 1996; Ullmann’s, 1999) and the experience of the AspenTech and QPC personnel.

Using the Direct Chlorination reactor described above, an Aspen Plus model for Section 100 of the plant was developed as shown in Figure 2.

The reactor described above has been implemented in Aspen Plus as DC101. The other blocks in Section 100 are standard Aspen Plus models and will not be described further.

4 Section 200 – Oxychlorination

In oxychlorination, ethylene reacts with dry HCl and either air or pure oxygen (the QPC VCM plant uses pure oxygen) to produce EDC and water (Equation (3) of Chapter 1). The catalyst contains cupric chloride as the active ingredient. The reaction is highly exothermic and thus good temperature control is very important. The QPC plant uses a fluidized bed with internal cooling coils containing hot water for heat removal. The by-products from the reactor include unreacted HCl and also CO2, and this section contains distillation columns to remove them. Since these compounds and the absorbing chemicals form electrolytes, an ionic model is necessary for the development of an effective model.

The components used in Section 2 are as follows:

ID Component name Formula

HCL HYDROGEN-CHLORIDE HCL

C2H2 ACETYLENE C2H2

VCM VINYL-CHLORIDE C2H3CL

EDC 1,2-DICHLOROETHANE C2H4CL2-2

C4H5CL CHLOROPRENE C4H5CL

LB ETHYL-CHLORIDE C2H5CL

TEC 1,1,2-TRICHLOROETHANE C2H3CL3

HB 1,1,2,2-TETRACHLOROETHANE C2H2CL4-D2

NAOH SODIUM-HYDROXIDE NAOH

H2O WATER H2O

NA+ NA+ NA+

H3O+ H3O+ H3O+

OH- OH- OH-

CL- CL- CL-

NACL SODIUM-CHLORIDE NACL

NA2CO3 SODIUM-CARBONATE NA2CO3

NACLO3 SODIUM-CHLORATE NACLO3

NA2SO4 SODIUM-SULFATE NA2SO4

H2SO4 SULFURIC-ACID H2SO4

CO2 CARBON-DIOXIDE CO2

HSO4- HSO4- HSO4-

HCO3- HCO3- HCO3-

CLO3- CLO3- CLO3-

CO3-- CO3-- CO3-2

SO4-- SO4-- SO4-2

N2 NITROGEN N2

O2 OXYGEN O2

CO CARBON-MONOXIDE CO

C2H4 ETHYLENE C2H4

C3H8 PROPANE C3H8

C3H6 PROPYLENE C3H6-2 H2 HYDROGEN H2 C2H6 ETHANE

C2H6 CO2-IN

CARBON-DIOXIDE

CO2

Three physical-property models are used in this section: NRTL-RK, ELECNRTL, and STEAMNBS. NRTL-RK uses the NRTL activity-coefficient model to describe the

nonideality of the liquid phase and the Redlich-Kwong model to describe the nonideality of the vapor phase. It is appropriate for the mixtures and conditions of the

Oxychlorination section containing non-electrolyte mixtures. For mixtures containing electrolytes, the ELECNRTL option is used. This option is similar to the NRTL-RK model, but is also able to describe mixtures containing ions. ELECNRTL defaults to NRTL-RK when the concentration of ionic species becomes zero. The STEAMNBS model is a highly accurate model for pure water and is used for the cooling circuit of the reactor.

The main reaction of oxychlorination is given by Equation (3) of Chapter 1. Several other by-products are also formed in this reactor. We have chosen the following seven reactions based upon the recent literature (Cowfer and Gorensek, 1996; Ullmann, 1999; Carrubba and Spencer, 1970; Wachi and Asai, 1994; Prasad, Prasad and Ananth, 2001; Al-Zahrani, 2001):

1. C2H4 + 2 HCL + .5 O2 → EDC + H2O

2. C2H4 + 3 HCL + O2 → TEC + 2 H2O

3. C2H4 + HCL → LB

4. C2H4 + 4 HCL + 1.5 O2 → HB + 3 H2O

5. C2H4 + 3 O2 → 2 CO2 + 2 H2O

6. C2H4 + 2 O2 → 2 CO + 2 H2O

7. C2H4 + HCL + .5 O2 → VCM + H2O

Reaction 1 is the main oxychlorination reaction. Reactions 2, 3, 4 and 7 produce undesired chlorinated compounds and Reactions 5 and 6 produce CO2 and CO, respectively. For Reaction 1, we have adopted a rate expression similar to the one proposed by Wachi and Asai (1994):

()4

2038

.02

421*1****H C a O HCl H C a C K C C C K T k R +=

(5)

where, C refers to composition in terms of molarity (kmol/m 3 in SI units) and Ka is equal to 0.63 m 3/mol, according to Wachi and Asai (1994).

Actually, Wachi and Asai (1994) did not use the dependence on the concentrations of HCl and O2 shown in Equation (5). Wachi and Asai did propose a linear dependence on the

concentration of the catalyst, cupric chloride, which we have assumed to be equivalent to the concentration of HCl. We added a weak dependence on the concentration of O 2 (exponent = 0.038). Essentially all the oxygen fed to Section 200 is consumed in the Oxychlorination Reactor, but there is a small amount of oxygen in the exit stream from the reactor. The weak dependence on the composition of O 2 is a semi-empirical way to capture this effect. We note that the work of Carrubba and Spencer (1970) indicates that the oxychlorination reaction does depend on the oxygen concentration.

The temperature dependence of the rate constant is given by the usual Arrhenius rate expression:

()????

?

???????--=0011exp *T T R E k T k (6) where,

T o - Convenient reference temperature, chosen here to be 150?C (or 423.15 K)

E - Activation Energy, determined to be 37,800 kJ/kmol by Wachi and Asai (1994) K 0 - Reaction rate constant at T 0

The other reactions are assumed to have similar rate expressions. All seven reactions have been implemented into the LLHW (Langmuir-Hinschelwood-Hougen-Watson) model in Aspen Plus, with the following general expression:

()[]{}Adsorption ion Concentrat T T R E k T k +????

????????--=1/*11exp *00 (6)

The implementation for each term is described in the following table:

The values of k0 were determined through data supplied by QPC engineers, and may need to be adjusted as the catalyst activity changes. The reaction set has been implemented into an LHHW model named OXYCHLOR.

A schematic of the Oxychlorination reactor is presented in Figure 3.

Figure 3. Oxychlorination Redactor

Several schemes were investigated for modeling the reactor. It was finally decided to model the reactor simply as a constant temperature plug-flow reactor with 3.84712 m and length

3.415 m. The constant temperature was set to 230?C and the heat produced in the reactor is used to heat the cooling water through the use of a heat stream.

The Oxychlorination reactor has cyclones to recover the catalyst particles. AspenTech did supply QPC engineers with approximate models to describe the performance of the cyclones, but this unit is not included in the formal models developed for this project.

DC201. A heat stream from the reactor DC201 is used to determine the temperature rise in the cooling-water system.

The outlet from DC201 enters distillation column DA201 in which the aqueous HCl is separated from the other components in the stream. This column has been modeled using the Radfrac block. The column has a rather complex heat-exchanger network, which has been modeled using pumparounds. Aqueous HCl must be modeled using an electrolyte model. A single reaction describing the ionic dissociation of HCl has been used in a Chemistry model named HCL:

HCL + H2O ? H3O+ + CL- (7)

The Radfrac simulation of DA201 has been done in the apparent mode. The simulation allowed the presence of two liquid phases, but only a single liquid phase was found.

The vapor from DA201 is further processed in another distillation column, DA202, in which the CO2 is absorbed in a sodium hydroxide solution. A more complete Chemistry model, named GLOBAL is required to describe the acid-base chemistry that occurs in DA202:

CO2 + 2.0 H2O ? H3O+ + HCO3- (8)

HCL + H2O ? H3O+ + CL- (9)

NAOH ? NA+ + OH- (10)

2. H2O ? H3O+ + OH- (11)

HCO3- + H2O ? CO3-- + H3O+ (12)

Similar to DA201, DA202 is modeled as a Radfrac block in the apparent mode and allowing the presence of two liquid phases. In this case, two liquid phases are predicted in the simulation and this is in accordance with plant observation.

The amount of caustic is sufficient to completely remove the CO2, but in practice a rate-based process, probably gas-phase mass transfer, limits the CO2 removal. Radfrac does not have the capability to describe processes that are controlled by mass-transfer limitations and hence we have implemented an approximate, but effective, approach. It is assumed that the CO2 is initially in the form of an “inert CO2” (CO2-IN) that does not participate in the Chemistry GLOBAL (Equations (8) – (12)); CO2-IN has also been forced to be only sparingly soluble in the liquid phase by declaring it to be HENRY component with a large Henry’s Law constant. Now a REACTIONS model, named CO2-MT converts CO2-IN. The scheme has been implemented through an RSTOIC block CO2-C1 that converts all the incoming CO2 to CO2-IN. In the distillation column DA202, CO2-MT converts CO2-IN to CO2, but its finite rate limits the amount of absorption of CO2. The vapor phase from the top of DA202 converts any remaining CO2-IN back to CO2 and thus the presence of CO2-IN is limited to the region around DA202. The formulation adopted to describe the mass-transfer of CO2 is similar to one proposed by Miller (1987) for nitric-acid columns. Finally, we note that his scheme provides results that are in good agreement with plant observations. The vapor effluent from DA202 goes through a series of condensers to remove the heavier components and finally to produce the recycle gas (Stream 229) that returns to the reactor. These pieces of equipment have been modeled as HEATER, FLASH2, COMPR and FSPLIT blocks as appropriate.

The liquid effluent from DA202 is processed by a series of decanters and washes to produce the EDC product and waste water streams. These pieces of equipment have been modeled as PUMP, MIXER, DECANTER and FSPLIT blocks as appropriate.

Convergence of this plant section is difficult because of the interdependency and complex connections of the various streams and slow convergence of the blocks (caused by electrolyte chemistry and liquid-liquid equilibrium. However, convergence can be reliably

obtained by providing increasingly good guesses for the tear streams (using the Reconcile capability in Aspen Plus).

The results of the Aspen Plus simulation of Section 200 have been judged by QPC engineers to be in good agreement with the plant observations.

5 Section 300 – EDC Purification

Section 300 consists of three distillation columns and associated pumps, heat exchangers and flash separators designed to produce purified EDC as a feed for the EDC Pyrolysis unit. The components used in the Aspen Plus model for Section 300 are:

ID Component Name Formula

C2H4 ETHYLENE C2H4

C2H3CL VINYL-CHLORIDE C2H3CL

C2H5CL ETHYL-CHLORIDE C2H5CL

P-CCL2 1,1-DICHLOROETHYLENE C2H2CL2-D1

F-CCL2 TRANS-1,2-DICHLOROETHYLENE C2H2CL2-D3

C4H5CL CHLOROPRENE C4H5CL

1,1EDC 1,1-DICHLOROETHANE C2H4CL2-1

1,1,1-01 1,1,1-TRICHLOROETHANE C2H3CL3-D0

CHCL3 CHLOROFORM CHCL3

CH2CL2 DICHLOROMETHANE CH2CL2

CCL4 CARBON-TETRACHLORIDE CCL4

CIS-1-01 CIS-1,2-DICHLOROETHYLENE C2H2CL2-D2

C6H6 BENZENE C6H6

C2HCL3 TRICHLOROETHYLENE C2HCL3

1,2-EDC 1,2-DICHLOROETHANE C2H4CL2-2

C2CL4 TETRACHLOROETHYLENE C2CL4

1,1,2-02 1,1,2-TRICHLOROETHANE C2H3CL3

H2O WATER H2O

HCL HYDROGEN-CHLORIDE HCL

N2 NITROGEN N2

The physical property method NRTL-RK is used. This method uses the NRTL activity-coefficient model to describe the nonideality of the liquid phase and the Redlich-Kwong model to describe the nonideality of the vapor phase. It is appropriate for the mixtures and conditions of the EDC Pyrolysis section.

The Aspen Plus flowsheet developed for Section 300 is presented in Figure 5.

which have all been modeled using the Radfrac model in Aspen Plus. DA301 is specified to allow two liquid phases, but a second liquid phase was not found. DA302 has a design-spec to vary the distillate rate in order to obtain a top tray temperature of 60 C. Columns DA303 and DA304 do not have any special features. All four Radfrac blocks converge robustly.

All other Aspen Plus blocks chosen to represent the process equipment in this section are straightforward application of standard Aspen Plus blocks.

The Broyden method has been chosen to converge this Aspen Plus model. Due to the integrated nature of the flowsheet is important to ensure tight tolerances on the individual blocks to obtain overall convergence of the Aspen Plus simulation. Hence the flash tolerance under Setup/Simulation Options/Flash Convergence has been set to 1.E-6, and the Radfrac tolerance has also been set to the same value (i.e., 1.E-6). It is also important to start with good guesses for the tear streams (308 and 325). With these practices, the flowsheet convergence is robust. It is recommended that QPC engineers follow these practices and the simulation model is modified in the future.

The results of the Aspen Plus simulation of Section 300 have been judged by QPC engineers to be in good agreement with the plant observations.

6 Section 400 – EDC Pyrolysis

EDC decomposes on heating to VCM and HCl. The reaction is endothermic and thus the reactor is a direct-fired tubular furnace. In the QPC plant, the reactor is fired using butane as the fuel. A schematic diagram of the reactor is presented in Figure 6.

VCM

Figure 6. EDC Pyrolysis Reactor

The components used in the Aspen Plus model for Section 300 are:

ID Component name Formula

HCL HYDROGEN-CHLORIDE HCL

C2H2 ACETYLENE C2H2

VCM VINYL-CHLORIDE C2H3CL

EDC 1,2-DICHLOROETHANE C2H4CL2-2

C4H5CL CHLOROPRENE C4H5CL

LB ETHYL-CHLORIDE C2H5CL

TEC 1,1,2-TRICHLOROETHANE C2H3CL3

HB TETRACHLOROETHYLENE C2CL4

H2O WATER H2O

CO2 CARBON-DIOXIDE CO2

N2 NITROGEN N2

O2 OXYGEN O2

CO CARBON-MONOXIDE CO C2H4 ETHYLENE C2H4 C3H8 PROPANE C3H8 C3H6 PROPYLENE C3H6-2 H2 HYDROGEN H2 C2H6 ETHANE C2H6 BENZ BENZENE

C6H6 CARBON CARBON-GRAPHITE C

NC4 N-BUTANE C4H10-1 IC4

ISOBUTANE C4H10-2 ARGON

ARGON

AR

A composite model has been developed to describe the reaction and heat exchange depicted in Figure 6. The POWERLAW model describes the pyrolysis reactions that occur inside the tubes. In the POWERLAW model, the rate expression is as follows:

{}essure Partial T T R E k Rate Pr *11exp *00????

?

???????--= (13)

In the implementation for EDC pyrolysis, T0 has been set to 500 K and E has been set to 93,600 kJ/kmol for all reactions. A set of seven reactions has been used to describe the pyrolysis kinetics, as described in the table below.

In the above table, the partial pressures are in SI units, i.e., Pa.

The choice of the reactions, the form of the POWERLAW correlations and the kinetic parameters were based upon information in the literature (for example, Cowfer and

Gorensek, 1996; Ullmann’s, 1999; Holbrook, Walker and Watson, 1971; Incavo, 1996), as well as the experience of AspenTech personnel and the QPC engineers. The POWERLAW model has been implemented in Aspen Plus as a Reactions model named EDCPR.

Radiative heat transfer of the combustion of butane heats the endothermic EDC pyrolysis reaction. Aspen Plus does not have a radiative heat-transfer model and thus the mechanism has been implemented as a user heat-transfer model.

The overall heat-transfer process may be approximated as radiative heat transfer from the flame to the tube wall (proportional to the fourth power of the temperature) and convective heat transfer from the tube wall to the process fluid.

{}{}()

)(4

4

P W C W F R T T U T T U Q -=-= (14)

where,

Q - Heat-transfer flux, J/m 2.K

U R - Radiative heat-transfer coefficient, J/m 2.K 4 U C - Convective heat-transfer coefficient, J/m 2.K T F - Temperature of flame, K T W - Temperature of wall, K

T P - Temperature of process fluid, K

The above model has been implemented in a user Fortran heat-transfer subroutine named EDCHT that has three real input variables as follows: REAL(1) - Wall temperature, T w REAL(2) - Convective heat-transfer coefficient (J/m2.K), U C REAL(3) - Radiative heat-transfer coefficient (J/m2.K 4), U R

EDCHT first solves Equation (14) to find the wall temperature and then computes the heat flux, Q.

The combination of reactor and flame is simulated by using an RPLUG block with the EDCPR kinetic model and an RSTOIC model for the butane combustion. A Calculator block named EDCPYR passes the temperature of the flame to REAL(1) of the RPLUG model and a design-spec named EDCREACT varies the temperature of the flame such that the heat generated in the combustion equals the heat transferred to the reactor.

The physical property method NRTL-RK is used. This method uses the NRTL activity-coefficient model to describe the nonideality of the liquid phase and the Redlich-Kwong model to describe the nonideality of the vapor phase. It is appropriate for the mixtures and conditions of the EDC Pyrolysis section.

The Aspen Plus flowsheet developed for Section 400 is presented in Figure 7.